Effects of Salt Dome Dissolution on Sediment Diagenesis: an Experimental and Field Study

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1995 Effects of Salt Dome Dissolution on Sediment Diagenesis: An Experimental and Field Study. William Lee Esch Louisiana State University and Agricultural & Mechanical College

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EFFECTS OF SALT DOME DISSOLUTION ON SEDIMENT DIAGENESIS: AN EXPERIMENTAL AND FIELD STUDY

A Dissertation

Submitted to the Graduate Faculty of the Louisiana State University and Agricultural and Mechanical College in partial fulfillment of the requirements for the degree of Doctor of Philosophy

The Department of Geology and Geophysics

by William Lee Esch B.A., University of Colorado, 1990 December 1995 UMI Number: 9618285

This microform edition is protected against unauthorized copying under Title 17, United States Code.

UMI 300 North Zeeb Road Ann Arbor, MI 48103 ACKNOWLEDGEMENTS

I would like to extend my appreciation and thanks to Jeff Hanor, Darrell Henry,

Gary Byerly, Jeff Nunn, and Oscar Huh for serving as members of my dissertation

committee. I am especially grateful to Jeff Hanor, my major advisor, for his constant

interest and support for this research. Jeffs tireless encouragement, wise tutelage, and

patient reviews of my work are greatly appreciated and have given me a far greater

knowledge of the diverse chemical processes that operate in sedimentary environments.

Discussions with Darrell Henry concerning experimental geochemistry were

particularly helpful in developing the experiments in Chapter 2, as well as in

understanding the general challenges and problems encountered in laboratory

experimentation. Gary Byerly's enthusiastic guidance on sample preparation and

analytical work with SEM-EDS and ICP-AES gave me a greater appreciation for the power of these techniques and the potential difficulties encountered in their use. I would also like to thank Barbara Dutrow for her careful review and helpful comments on the text, and Chad McCabe who served as an early member of the committee. I was supported in part by an LSU Board of Regents Fellowship and grants fromth e Gulf

Coast Association of Geological Societies Financial-Aid-to-Students program, and

Sigma Xi, The Scientific Research Society.

I am grateful to the Shell Development Company for the funds that supported the analytical work in this dissertation, the purchase of equipment, and travel support. I would like to thank Shell Offshore, Inc. for its support which included numerous

ii sidewall core samples, fluidanalyses , and well logs fromth e Eugene Island 128A field.

I would like to thank Texaco, Inc. for providing sediment cuttings from the Iberia field, as well as Audrey Workman and Dan Snow for collecting the fluid samples used in the

Iberia study.

Thanks also to Xiaogang Xie who was instrumental in teaching me the operation of the SEM and microprobe, and to Wanda LeBlanc who acquainted me with the XRD equipment and preparation of clays.

The patient support of my wife, Toni, and my children, Andrea and Orion will

not be forgotten. To them I am deeply grateful for the chance to pursue a dream.

iii TABLE OF CONTENTS

ACKNOWLEDGEMENTS ii

LIST OF TABLES ix

LIST OF FIGURES x

ABSTRACT xvi

CHAPTER 1) INTRODUCTION 1 BACKGROUND 1 Flank sediments, dissolved salt, and diagenesis 2 Fluid flow in flank sediments 2 OVERVIEW OF RESEARCH 3 CHAPTER 2) THE EXPERIMENTAL REACTION OF NaCl SOLUTIONS WITH SILICICLASTIC AND MIXED SILICICLASTIC-CARBONATE SEDIMENTS 6 INTRODUCTION 6 EXPERIMENTAL TECHNIQUES 8 Sediments 8 NaCl Solutions 11 ANALYTICAL TECHNIQUES 13 ICP Analyses 13 pH, Alkalinity, and Chloride 16 Cation Exchange Characteristics 17 Thermodynamic Modeling 17 XRD and SEM Analysis 18 RESULTS 19 Exchangeable Cations 19 Variations in Fluid Composition, Carbonate-Free Systems 20 Charge balance problems 20 pH 22 Anions 23 Dissolved Silica 25 Major Cations 27 Minor Species 27 Variations in Fluid Composition, Systems With Carbonate 32 pH and alkalinity 32 Dissolved silica 32 Major species 33 Minor species 33 XRD Results 33

IV SEM Results 36 Thermodynamic Modeling 39 DISCUSSION 44 Exchange reactions versus mineral hydrolysis reactions 46 Dissolution-Precipitation Reactions 48 CONCLUSIONS 51 Recommendations for future experiments 53 CHAPTER 3) SEDIMENT DIAGENESIS NEAR THE SALT-SEDIMENT INTERFACE AT THE EUGENE ISLAND 128 SALT DOME 56 INTRODUCTION 56 PURPOSE OF STUDY 57 EUGENE ISLAND 128A FIELD 58 MATERIALS AND METHODS 59 Sidewall cores 59 Electron microscopy 62 Thin-section and grain-mount preparation 62 RESULTS 63 Diagenetic minerals and textures 63 Identification of drilling mud contamination 65 Clastic sediments 68 Grain size, distribution, shape, and related modification of detrital grains by diagenesis 68 Mineralogy 73 Porosity and permeability 74 Secondary porosity 75 Mechanical deformation 79 Halite 81 Crystal size 81 Composition 81 Porosity in halite 82 Anhydrite, celestite, and barite 85 Morphology 85 Composition 89 Textural relations of sulfate minerals with surrounding sediments 91 Anhydrite dissolution 93 Barite dissolution 93 Sulfides 94 Pyrite 94 Sphalerite 96 Galena, chalcopyrite, and pyrrhotite 97 Carbonates 98 Analcime 98 Other diagenetic phases 99 Formation water compositions 99

v Ca 100 Mg 101 Alkalinity 103 Ba and sulfate 105 DISCUSSION 106 Mechanical controls on diagenesis 107 Effect of crushing on reaction rates 107 Effect of crushing on stability of geochemical system 108 Further aspects of mechanical diagenesis 111 Diagenesis within the evaporites and salt-sediment interface 112 Chemical diagenesis 112 Diagenesis within the flank sediments 115 Calcite cements 115 Redox reactions 116 Sulfates 119 Silicates and aluminosilicates 119 Sulfides 121 Diagenetic sequence 121 Diagenetic zones around the EI 128 salt-sediment interface 122 CONCLUSIONS 125 CHAPTER 4) FAULT AND FRACTURE CONTROL OF FLUID FLOW AND DIAGENESIS AROUND THE IBERIA SALT DOME, IBERIA PARISH, LOUISIANA 129 INTRODUCTION 129 IBERIA FIELD 131 METHODS 133 Flow path identification 133 Formation water analysis 134 Drill-cutting analysis 136 Well logs 137 Thermodynamic modeling 138 RESULTS 139 Formation waters 139 Spatial distribution of dissolved solutes at the Iberia field 139 Dissolved solutes versus chloride for the Iberia field and seven offshore Louisiana fields 145 Diagenetic minerals 151 Faults 157 Fractures and fracture dilation 158 Thermodynamic modeling 158 DISCUSSION 161 Fault influence on the concentration of dissolved species 161 High angle fault acting as a barrier to lateral fluid migration 164 High angle fault open to lateral fluid migration 164

vi High angle fault acting as a barrier to vertical fluid migration 165 High angle fault acting as a conduit for vertical fluidmigratio n 165 Fault influence on flowpath s and dissolved species in the Iberia field 165 Fractures and fracture dilation 167 Accommodation of fluid flowi n intergranular porosity 170 Fluid-mineral equilibria in the presence of dissolved NaCl 170 CONCLUSIONS 171 CHAPTER 5) SUMMARY 173 EXPERIMENTAL WORK 173 Analcime and carbonate minerals 173 Salinity, kaolinite,and analcime 174 Spatial distribution of diagenetic minerals 175 EI 128 SALT DOME 176 Crushing, dissolution rates, and stability of geochemical system 176 Salt dome dissolution and sulfate minerals 176 Redox reactions 177 IBERIA SALT DOME 178 Slopes on log cation-log chloride plots: A measure of equilibrium 179 FUTURE RESEARCH 179 BIBLIOGRAPHY 181 APPENDICES 191 A.l. pH ANALYSES 192 A.2. TOTAL ALKALINITY as mg(HC03')/L 193 A.3. CI" ANALYSES (mg/L) 194 A.4. Ca ANALYSES (mg/L) 195 A.5. K ANALYSES (mg/L) 196 A.6. Mg ANALYSES (mg/L) 197 A.7. Na ANALYSES (mg/L) 198 A.8. DISSOLVED Si02 (mg/L) 199 A.9. Ba ANALYSES (mg/L) 200 A.10. Fe ANALYSES (mg/L) 201 A.ll. Mn ANALYSES (mg/L) 202 A.12. Sr ANALYSES (mg/L) 203 A.13. XRD PEAK-HEIGHT RATIOS 204 A.14a. CALCULATED ACTIVITY DATA 205 A.14b. CALCULATED ACTIVITY DATA 206 A.15. CARBONATE SATURATION INDICES 207 A.16. SILICA SATURATION INDICES 208 B.l. EI-128, WELL 12ST GENERAL LITHOLOGY, POROSITY, AND PERMEABILITY 209 B.2. EI-128, WELL 23 GENERAL LITHOLOGY, POROSITY, AND PERMEABILITY 210

vii B.3. EI-128, WELL 12ST GRAIN-SIZE DISTRIBUTION 211 B.4. EI-128, WELL 23 GRAIN-SIZE DISTRIBUTION 212 B.5. EI-128 FORMATION WATER ANALYSES 213 B.6. CALCULATED BOTTOM-HOLE TEMPERATURES (BHT) AND CALCULATED MAXIMUM TEMPERATURES, EI 128 214 CIA. IBERIA FIELD FORMATION WATER ANALYSES: pH, ACETATE, ALKALINITY 215 C.IB. IBERIA FIELD FORMATION WATER ANALYSES: Na, K, Mg, Ca, Sr, Ba, Si02° 216 C.IC. IBERIA FIELD FORMATION WATER ANALYSES: B, Cu, Fe, Pb, Zn,Mn,Cl 217 C.2A. CALCULATED ACTIVITY RATIOS, Pc02, AND SI'S FOR IBERIA FIELD 218 C.2B. CALCULATED ACTIVITY RATIOS FOR LAND et al. (1988a) DATA 219 VITA 220

viii LIST OF TABLES

Table 2.1. Standards for ICP-AES analysis 14 Table 2.2. Dilution schedule for ICP-AES analysis 15 Table 2.3. Spectral lines, limits of detection, and limits of detection with dilution 15 Table 2.4. Results of cation exchange experiments 19 Table 3.1. Key to Table 3.2 and Table 3.3 63 Table 3.2. Sidewall core summary for well 12ST 64 Table 3.3. Sidewall core summary for well 23 65 Table 4.1. List of wells and fluid sample depth at the Iberia field 134 Table 4.2. Spectral lines and detection for elements in ICP-AES analyses 135 Table 4.3. Key to Table 4.4 151 Table 4.4. Drill cutting lithology and diagenetic mineral summary, well 19 152

ix LIST OF FIGURES

Figure 2.1. Plot of pH versus CI at 25°C. Abbreviations in key: si = carbonate-free siliciclastics; asi = aragonite plus siliciclastics; dsi = dolomite plus siliciclastics; 30d, 60d, 90d, 270d duration of experiment in days; 25° and 90°, temperature of experiments in degrees Celsius 22 Figure 2.2. Plot of pH versus CI at 90°C 23 Figure 2.3. Alkalinity versus CI at 25°C 24 Figure 2.4. Alkalinity versus CI at 90°C 25 Figure 2.5. Dissolved silica versus CI at 25°C 26 Figure 2.6. Dissolved silica versus CI at 90°C 26 Figure 2.7. Na versus CI at 25°C. NaCl line represents line along which Na versus CI compositions plot fromhalit e dissolution 28 Figure 2.8. Na versus CI at 90°C 28 Figure 2.9. K versus CI at 25°C 29 Figure 2.10. K versus CI at 90°C 29 Figure 2.11. Mg versus CI at 25°C 30 Figure 2.12. Mg versus CI at 90°C 30 Figure 2.13. Ca versus CI at 25°C 31 Figure 2.14. Ca versus CI at 90°C 31 Figure 2.15. X-ray diffractograms for unreacted sediments (upper diagram) and for sediments from 90-day, 90°C carbonate-free experiments at high salinities (lower diagram). The 28 A peak likely corresponds to the development of a Na-rectorite phase in the sediments. Abbreviations on plot: I = illite plus detrital muscovite; K = kaolinite; S = smectite; and Q = quartz 34

Figure 2.16. XRD peak height ratios in both air-dried and ethylene glycolated states for quartz/kaolinite and quartz/illite in the 90-days, 90°C experiments. The ratios suggest that quartz is destroyed in the experiments. Abbreviations in key: AD = air dried; EG = ethylene glycol saturated 36

x Figure 2.17. SEI images of reacted and unreacted sediments: (a) unreacted sediments, (b) sediments reacted for 90 days in 100 mg/L NaCl solution, (c) sediments reacted for 90 days in 297,300 mg/L NaCl solution 37 Figure 2.18. SEI image of corroded surface on quartz grain after reaction with high- salinity fluids 38

+ + Figure 2.19. Activity diagram for log (aNa /arH ) versus log (aH4Si04) at 25°C (modified from Drever, 1988). The three vertical dashed lines represent saturation values at 25°C, Psat for quartz (qtz sat), chalcedony, and amorphous silica (am silic) 39

+ + Figure 2.20. Activity diagram for log (aK /ali ) versus log (aH4Si04) at 25°C (modified from Drever, 1988) 40

2+ + 2 Figure 2.21. Activity diagram for log (aCa /(aH ) ) versus log (oH4Si04) at 25°C (modified from Drever, 1988) 40

+ + Figure 2.22. Activity diagram for log (crNa /aH ) versus log (aH4Si04) at 100°C (thermodynamic data fromBower s et al., 1984) 41 Figure 2.23. Activity diagram for log (aNa+/aH+) versus log (oK+/aH+) at 75°C. The parag-musc xxx°C lines are phase boundaries between paragonite and muscovite at 75°C and at 100°C 42 Figure 2.24. Activity diagram for log (aCa2+/(oH+)2) versus log (aMg2+/(oH+)2). The four lines marked calcite 25°, calcite 100°, aragonite 25°, and aragonite 1001°0 represent the stability boundaries between calcite and dolomite, and aragonite and dolomite at the given temperature 43 Figure 3.1. Map of Louisiana and offshore Louisiana showing the location of the Eugene Island Block 128 Field 58 Figure 3.2. Map of the southeast quadrant of the Eugene Island Block 128 salt dome showing approximate depth to top of salt, cross-section lines A-A', A'-A", and the location of wells 1, 5,12ST and 23 in the 128A field 59 Figure 3.3. Cross sections A-A' and A"-A' for the EI 128 salt dome and southeast flank sediments. Well penetrations of salt overhangs and sampled intervals are shown for well 12 ST and well 23, and structural features are shown for section A-A'. No vertical exaggeration. Lines K, Kl, P2, etc., represent the top of sands taken from structure maps supplied by Shell Offshore, Inc 61

Figure 3.4. BSE image of drill-mud contamination from well 12ST, 2697 m (8850 ft) 66

xi Figure 3.5. BSE image of drilling-mud contamination occupying secondary porosity in diapiric halite. Well 12ST, 1874 m (6149 ft) 67

Figure 3.6. BSE image of crushed-grain texture, average grain size reduction, and modification of detrital grain roundness to very angular. Well 12ST, 3048 m (9999 ft) 69

Figure 3.7. BSE image from well 23,2266 m (7436 ft) showing dissolution of massive calcite cements that have precipitated in both primary and secondary porosity 71

Figure 3.8. BSE image of crushed sandstone from well 23,2284m (7494 ft) 72

Figure 3.9. Image of floating sand-grain texture from well 23,2030 m (6659 ft) 73

Figure 3.10. BSE image of linear fractures and irregular fissures. The latter may represent accretion surfaces in the clay rich sediments. Well 23,2103 m (6898 ft) 75

Figure 3.11. BSE image of diagenetic barite and pyrite with euhedral habit adjacent to a potential accretion or fracture boundary in a sample from well 23, 2103 m (6898 ft) 78

Figure 3.12. SEI image of micro-pores and barite, well 12 ST, 1882 m (6176 ft) 79

Figure 3.13. SEI image of chloritized detrital biotite with bent ends that are consistent with post-depositional shearing of sediments. Well 12ST, 1905 m (6249 ft) 80

Figure 3.14. BSE image of barite fragments outlining accretionary snowball texture. Rotation is likely due to shearing of sediments. Well 12ST, 1876 m (6155 ft) 80

Figure 3.15. BSE image of well-developed secondary porosity from halite dissolution in a sample from well 12ST, 1874 m (6149 ft) 82

Figure 3.16. BSE image of halite and barite residua from salt dissolution mixed with fine-grained siliciclastics at the diapir margin. Well 12ST, 1878 m (6163 ft) 83

Figure 3.17. SEI image of barite pseudomorphs after anhydrite in halite. The halite is being mechanically mixed with siliciclastic sediments. Well 12ST, 1881 m (6171ft) 84

Figure 3.18. BSE image of barite pseudomorphs after anhydrite accreted to fine grained sediments at the margin of salt. The barite is left as an insoluble residue from evaporite dissolution. Sample 12ST, 1876 m (6155 ft) 86

xii Figure 3.19. BSE image of barite and sphalerite vein-filling and grain-replacing cements in a sandstone from well 12ST, 3048 m (9999 ft) 87 Figure 3.20. BSE image of celestite precipitating next to anhydrite and secondary porosity fromhalit e dissolution (black areas). Well 12ST, 1878 m (6162 ft) 88 Figure 3.21. BSE image of diagenetic celestite crystals with acicular habit in fine grained siliciclastic sediments. Sample 12ST, 2177 m (7143 ft) 89 Figure 3.22. EDS spectra of zoning in barite, Well 12ST, 1881 m (6171 ft)(Fig. 3.17) 90 Figure 3.23. BSE image of regular accretion boundary containing fragmentedbarit e and no halite. Well 12ST, 2802 m (9195 ft) 92 Figure 3.24. BSE image of corroded anhydrite. Well 12ST, 1874 m (6149 ft) 94 Figure 3.25. BSE image of pyrite framboids and analcime in microfossil test, sample 23, 2622 m (8603 ft) 95 Figure 3.26. BSE image of octahedral pyrite and Ca-Fe carbonate overgrowth (probably ankerite) on dolomite in a sandstone from well 12ST, 2832 m (9290 ft) 96

Figure 3.27. BSE image of complexly intergrown diagenetic pyrite and Ti02 in a sandstone sample from well 23,1941 m (6369 ft) 97 Figure 3.28. Ca versus depth for wells on the southeast flank of the EI 128 dome. Ca appears to increase with depth 100 Figure 3.29. Ca versus CI for formation waters on the southeast flank of EI 128 101 Figure 3.30. Mg versus depth showing decrease of dissolved Mg with depth 102 Figure 3.31. Mg versus CI for formation waters on the southeast flank of EI 128 102 Figure 3.32. Alkalinity versus depth for formation waters in flank sediments at EI128 103 Figure 3.33. Alkalinity versus CI for formation waters in flank sediments at EI 128...104 Figure 3.34. Log activity of Ba versus log activity of sulfate calculated in SOLMINEQ.88 (Kharaka et al, 1988) for formation waters in three wells in the flank sediments at EI 128. The barite stability line has been calculated fromdat a presented in Bowers et al. (1984) for 75° C, Psat 105 Figure 3.35. Graph of the surface area to volume ratio for a cube as a function of d. ..109

xiii Figure 3.36. Sequence of diagenetic events at the Eugene Island Block 128 salt dome, offshore Louisiana 122 Figure 3.37. Schematic diagram for EI 128 defining and describing the spatial relations between the flank sediment zone, shale sheath zone, the accretion zone, the salt-sediment interface, the halite dissolution zone, and the unaltered evaporite zone. Not to scale 124 Figure 4.1. Map of Louisiana showing the location of the Iberia field 132 Figure 4.2. Map of study area on the southwest flank of the Iberia dome 132 Figure 4.3. Detailed cross section along section line A-A' in Figure 2 137 Figure 4.4. Spatial variations in dissolved Ba 140 Figure 4.5. Spatial variations in dissolved B 141 Figure 4.6. Spatial variations in dissolved Ca 141 Figure 4.7. Spatial variations in dissolved Fe 142 Figure 4.8. Spatial variations in dissolved Sr 142 Figure 4.9. The arrow marks the core of the tongue of low Ca-Ba-Fe and high B-Sr water thus demarcating the core of the apparent flowpat h 143 Figure 4.10. Ca versus depth for the Iberia field and data presented in Land et al. (1988a). The trend lines suggest that Ca is introduced into formation waters at depth 144 Figure 4.11. CI versus depth for the Iberia field and data presented in Land et al. (1988a). The trend lines suggest that CI is introduced into formation waters below 2600 m. A likely source is from halite dissolution 144 Figure 4.12. Ba versus CI for Iberia data and Land et al. (1988a) data 146 Figure 4.13. B versus CI for Iberia data and Land et al. (1988a) data 146 Figure 4.14. Ca versus CI for Iberia data and Land et al. (1988a) data 147 Figure 4.15. K versus CI for Iberia data and Land et al. (1988a) data 147 Figure 4.16. Mg versus CI for Iberia data and Land et al. (1988a) data 148 Figure 4.17. Na versus CI for Iberia data and Land et al. (1988a) data 148

Figure 4.18. Dissolved Si02 versus CI for Iberia data and Land et al. (1988a) data 149

xiv Figure 4.19. Sr versus CI for Iberia data and Land et al. (1988a) data 149 Figure 4.20. Total alkalinity versus CI for Iberia data and Land et al. (1988a) data 150 Figure 4.21. SEI image of diagenetic analcime overgrowths on detrital framework grains, 3094 m (10150 ft) 153 Figure 4.22. SEI image of diagenetic octahedral pyrite, 2923 m (9590 ft) 154 Figure 4.23. SEI image of diagenetic vermiform kaolinite, 2905 m (9530 ft) 154 Figure 4.24. EDS spectra for (a) analcime at 3094 m (10150 ft);(b ) barite at 2923 m (9590 ft); and (c) sphalerite at 3130 m (10270 ft). Au peaks are due to sample coating 155 Figure 4.25. SEI image of Al-Cu-Zn vein-filling in fracture, 3194m (10480 ft) 156 Figure 4.26. EDS spectra of Al-Cu-Zn mineral shown in Figure 4.23. Spectrum (a) is fromth e center of the fracture and spectrum (c) is from the margin of the fracture. Spectrum (b) was in the traverse from a to c. Au peaks are due to sample coating 157

+ + Figure 4.27. Plot of log(aNa /oH ) versus log(aH4Si04) for the Iberia data and the Land et al. (1988a) data. The phase boundaries were calculated for 75° C, Psat with thermodynamic data from Bowers et al. (1984) assuming aH20 = 1, and mineral activities =1 159

Figure 4.28. Plot of log(oNa+/aH+) versus log(aK+/aH+) for the Iberia data and the Land et al. (1988a) data. The phase boundaries were calculated for 75° C, Psat with thermodynamic data fromBower s et al. (1984) assuming aH20 = 1, quartz saturation, and mineral activities = 1 160 Figure 4.29. Hypothetical spatial variations in fluid compositions across faults in alternating sand-shale sequences which are either transmissive or barriers to fluid flow 163

xv ABSTRACT

The introduction of high-salinity or high-ionic strength fluidsint o sedimentary

formations has been hypothesized to induce diagenetic reactions. Such reactions are potentially important because they alter porosity and affect the migration of fluidssuc h

as petroleum, or hazardous liquid wastes disposed of by deep-well injection. An experimental and fieldinvestigatio n of the controls on diagenesis in high and variable salinity environments was undertaken regarding Gulf Coast salt domes because ambient formation waters and sediments around salt domes are influenced by the dissolution of evaporites.

Laboratory experiments were performed with carbonate-free and carbonate- bearing siliciclastic sediments in aqueous NaCl solutions with concentrations from 1 mg/L to halite saturation (ca. 350,000 mg/L). The experiments were performed at 25°C and 90°C at approximately 1 bar in runs up to 90 and 270 days. In the reacted fluids, pH and alkalinity decreased with increasing salinity, and the concentrations of Ca, Mg, and

Sr increased with increasing salinity. K increased with salinity only in the carbonate- free experiments. Cation exchange reactions between dissolved Na and adsorbed species on mineral surfaces account for much of the Ca, Mg, and Sr released to solution, however, the presence of dissolved Si and K after reaction is attributable to hydrolysis reactions. X-ray diffraction data show the development of Na-rectorite in the high- salinity carbonate-free experiments and also suggest that quartz is increasingly attacked at high salinities.

xvi Sediments were examined fromth e Iberia salt dome, south Louisiana, and the

Eugene Island 128 salt dome, offshore Louisiana. Diagenetic pyrite, calcite cements, and analcime are present in flank sediments of both. At EI-128, barite pseudomorphs after anhydrite develop where halite is dissolved. Evidence was found at both domes for sediment and grain fracturing which can directly influence fluid transmission, solute transport, and mineral dissolution in flank sediments. At the Iberia dome, spatial variations in pore-fluid composition suggest that fluids are transmitted through faults and fractures as well as through the intergranular porosity of detrital sediments. Thus, fracturing, fluid movements, and redox reactions influence diagenesis in addition to evaporite dissolution.

xvii CHAPTER 1) INTRODUCTION

BACKGROUND

In the northern Gulf of Mexico, the production of petroleum, rock salt, and native sulfur from salt domes has motivated a substantial amount of associated research.

As a result of this activity, a sizable body of published literature on salt domes has been produced. Prior to 1980, much of this work focused on the physical process of salt movement and the associated structural features which develop in sedimentary strata as a result of salt diapirism (see Ingram, 1991, and references therein). In a few early studies, the diverse trace and minor minerals that are present in diapiric salt and caprocks were documented (Hanna and Wolf, 1934; Taylor, 1938; Murray, 1966).

A renaissance in salt dome research occurred during the late 1970s and 1980s. A number of studies explored the formative processes and nature of the anhydrite-gypsum- calcite caprock complex which is present on the top of many salt domes (Kyle and

Agee, 1988; Hallager era/., 1988; Prikryl Texas (Price et al., 1983). Salt domes were also studied as potential storage sites for petroleum and were subsequently utilized as part of the Strategic Petroleum

Reserve (United States Strategic Petroleum Reserve Office, 1977). In the search for safe repositories for radioactive nuclear wastes, salt domes were again considered. However, for this role they were rejected (Posey and Kyle, 1988). During this same period of time, continued research related to petroleum exploration and production yielded new

1 2

findings regarding fluid movement and mineralization in flank sediments around salt

domes. Evidence was produced for large-scale fluid-flow systems in flank sediments

which traversed kilometers of vertical section (Hanor, 1987; Hanor and Workman,

1986; Workman and Hanor, 1985; Leger, 1988), and diagenetic kaolinite, pyrite,

analcime, and massive calcite cements were also found in flank sediments with

increasing abundance near salt (McManus and Hanor, 1988,1993; Leger, 1988).

Flank sediments, dissolved salt, and diagenesis

The observation that diagenetic mineralization increases with proximity to salt

led Hanor (1988) to propose that the introduction of Na+ and CI" into formation waters

from halite dissolution might influence or induce diagenetic reactions in ambient

siliciclastic minerals. A conceptual development for diagenesis induced by salt

dissolution is presented by Hanor (1994a and 1994b) along with supporting evidence for

diagenesis in response to halite dissolution fromth e compositional characteristics of

saline formation waters from sedimentary basins around the world. Arguments diat

support the process of diagenesis induced by halite dissolution can also be made from

the basis of theoretical thermodynamics (Hanor, in press).

Fluid flow in flanksediment s

Fluid movement and the pathways which transmit those fluids are important

factors that can also influence diagenesis and petroleum migration around salt domes.

Mobile formation waters have greater potential for accommodating mass transfer in dissolution-precipitation reactions than do static fluids, provided that rates of flowar e great enough to prevent reactions frombecomin g diffusion controlled. Mobile fluids 3

also have a greater capacity to convey mass through the diagenetic system. For instance,

at the West Hackberry salt dome in southwest Louisiana, it has been calculated that the

presence of pore-occluding calcite and pyrite cements in flank sands is attributable to

•J the dissolution of approximately 0.8 km of salt dome evaporites and the transfer of that

dissolved mass into nearby sediments by formation waters.

Equally important to fluidmovemen t are the migration pathways which transmit

those fluids. The spatial distribution of such pathways in part influences the spatial

distribution of diagenetic mineralizations that are found around salt domes. The

pathways are also responsible for bringing fluidsint o contact with salt at the margin of

the diapir. Although evidence on a larger scope has been produced for kilometer-scale vertical fluid movements around salt domes (Workman and Hanor, 1985 and Hanor

1987), little is actually known about the details of the pathways which accommodate such flow. OVERVIEW OF RESEARCH As existing oil and gas fields around salt domes have become depleted, field operators have been motivated to drill in closer proximity to the salt in order to tap potential "attic accumulations" of hydrocarbons located up-dip from depleted reservoirs.

Diagenetic processes around salt domes, such as the precipitation of pore-occluding cements, can drastically limit the quality of potential reservoirs near the salt. There is thus a critical need to understand the processes that operate near the margin of the salt dome which control diagenesis. Very little field or experimental research has been done on the effects of high and variable salinities on diagenesis in sedimentary basins, and 4

this study was designed, in part, to investigate the influences of dissolved halite on

diagenesis both from an experimental approach as well as from a field approach. A

number of previous experimental studies exist in which natural or artificial sediments

had been reacted with a limited number of NaCl salt solutions, but none of these have used the broad range of salinities generally found around salt domes. The experiments performed in this study used a series of artificial aqueous NaCl solutions that range from 1 mg/L NaCl to halite saturation (ca. 350,000 mg/L NaCl). The experimental study is presented in Chapter 2.

The above concentration range represents the extremes of salinity that can be encountered within a few kilometers of salt domes in the Gulf Coast region of Louisiana

(see Hanor et al, 1986; Bray and Hanor, 1990; Chapter 3, this dissertation). Hence, sediments from around salt domes are ideal natural laboratories for investigating the influences of salt dissolution and variable salinity on diagenesis. The occurrence of petroleum near salt domes in the Gulf Coast has also made access to sediment samples from around salt domes possible. Sediment samples have been obtained from the Iberia salt dome in Iberia Parish, Louisiana, and fromth e Eugene Island 128 salt dome, offshore Louisiana. The sediments from the Eugene Island 128 salt dome are especially important in that they include samples from the salt-sediment interface, a region on the salt dome flanktha t has not been closely investigated in published studies. The Eugene

Island 128 study is presented in Chapter 3.

The sample set obtained fromth e Iberia fieldi s unique in that it was collected in conjunction with electric logs and formation water samples from eighteen wells on the 5

southwest flank of the Iberia salt dome. The density of sampling and diversity of the

sampled materials allows for identification of microscopic and macroscopic features

that accommodate fluid movement around the Iberia salt dome, as well as for the

identification of diagenetic minerals. Because our knowledge of specific flowpaths for

formation waters around salt domes is very limited, the Iberia materials offered the

uncommon opportunity to identify specific features which accommodate fluid

transmission. This portion of the research is presented in Chapter 4.

The ultimate goal of the three studies is to gain a better understanding of the

physical and chemical processes that control diagenesis around salt domes. These

findings can be extended into other similar geologic settings as well, such as in subsalt petroleum exploration targets in the Gulf of Mexico which are characterized by similar physical and chemical conditions found around salt domes. CHAPTER 2) THE EXPERIMENTAL REACTION OF NaCl SOLUTIONS WITH SILICICLASTIC AND MIXED SILICICLASTIC-CARBONATE SEDIMENTS

INTRODUCTION Many sedimentary basins contain waters having salinities greatly in excess of

sea water (Hanor, 1994a and Land 1995). The generation of these subsurface brines is thought to have occurred primarily by infiltration of subaerially-evaporated marine

waters, subaerially-evaporated continental waters, or by the subsurface dissolution of evaporites, principally halite (Hanor, 1979; 1988; 1994a). While such processes can account for the elevated levels of chloride observed in these waters, the cation composition of these brines is distinctly different from the composition of fluids produced by either evaporation or by evaporite dissolution. It is clear that the processes of water-rock interaction during diagenesis substantially alter the cation composition of these fluids (Hanor, 1994b).

There have been numerous field and thermodynamic studies conducted in the attempt to identify reactions which control the composition of subsurface fluids. Savage et al. (1993) reacted granite at 200°C, 50 MPa with streamwater, seawater, and 0.008 M and 0.028 M NaCl solutions. The seawater experiments produced fluids of low pH

(approximately pH 3.5), and high dissolved Si02 concentrations, along with the loss of sulfate, Ca, and Mg. Heavy metals such as Fe were also mobilized into the fluid phase.

Anhydrite and mixed-layer chlorite-smectite were precipitated as solid phases. The

NaCl experiments generated near-neutral pH values and dissolved Si02 concentrations

6 7 apparently controlled by quartz solubility. Illite was the only phase that precipitated in the NaCl experiments. Hiltabrand et al, (1973) reacted unconsolidated argillaceous sediments obtained fromth e Gulf Coast with artificial seawater at 100°C and 200°C.

They reported increases in Na, K, and Ca, and decreases in Mg and sulfate relative to seawater. Dissolved silica stabilized near values for quartz saturation. Chamosite and illite were precipitated, and feldspar, kaolinite, and smectite were destroyed. Lentini and

Shanks (1984) reacted two artificial NaCl brines which also contained K, Mg, Ca, Fe, and sulfate with arkosic sediments at 200°C and 500 bar. The artificial brines contained approximately 21 weight percent salts. They reported decreases in K, Mg, and pH, and increases in Na, Ca, Fe, Mn, Zn, and Ba. Dissolved silica concentrations stabilized at values near quartz saturation. Other experiments by Bischoff et a/. (1981), Thornton and

Seyfried (1985), and Hajash and Bloom (1991) have followed similar themes in which artificial NaCl brines or seawater were reacted with natural sediments at elevated temperatures and pressures. Initial lowering of pH, depletion of Mg and sulfate from the starting fluids, and increases in dissolved Ca, Si, and heavy metals are reported, especially when high salinity fluids have been employed in the experiments (Bischoff et al, 1981). In all cases, substantial fluid-rock interaction is observed in the experiments, and the nature of the newly precipitating phases is largely dependent on the composition of the starting fluid. None of these previous experimental studies, however, have systematically approached the problem of sediment diagenesis with CI or NaCl as a widely ranging variable even though it is clear from the earlier results that high-salinity fluids apparently promote more extensive reaction than do low salinity fluids. 8

The purpose of the experimental study is to investigate one facet of fluid-rock interaction: the effects of the introduction of varying amounts of dissolved NaCl into siliciclastic and mixed siliciclastic-carbonate sediments. This work was motivated in part by the observation that saline fluidsi n the south Louisiana Gulf Coast have originated fromth e dissolution of salt diapirs. Although these diapirs are primarily composed of halite (NaCl) with 5 to 10 weight percent anhydrite (CaS04) (Murray,

1966), the fluids which eventually evolve as a result of dissolution are typically enriched not only in Na and Ca, but in such elements as K, Mg, Sr, and Ba. These changes in fluid composition apparently result fromfluid-roc k interactions. Field studies on diagenesis around the margins of salt domes (Leger, 1988; McManus and

Hanor, 1988; Chapters 3 and 4 of this study) have revealed that a number of important diagenetic mineral phases and textures occur which are presumably related to proximity with the diapir and the dissolution of salt. The conversion of calcium and sulfate derived from anhydrite dissolution into calcite and pyrite cements in nearby flank sands has been documented at several salt domes (McManus and Hanor, 1988; Chapters 3 and 4 of this study). What is less well known, however, are the effects of halite dissolution on the diagenesis of silicate minerals, the principal focus of this work.

EXPERIMENTAL TECHNIQUES

Sediments The basic experimental techniques employed in this study consisted of reacting aqueous NaCl solutions of various concentrations with silty clays collected from surficial natural levee deposits along the Mississippi River immediately south of Baton 9

Rouge, Louisiana. Similar sediments constitute a common and volumetrically-important

portion of the Miocene to Recent fluvial and fluvial-deltaic sequences into which salt

has intruded in south Louisiana.

The starting sediment contains quartz, kaolinite, illite, and smectite as major

mineral phases, and detrital muscovite, K-feldspar, and plagioclase as minor mineral

phases. These minerals were qualitatively identified by X-ray diffraction. Semi

quantitative treatment of the X-ray diffraction data using an approach modified from

that of Cook et al. (1975) shows that the bulk sediments contain 61 percent quartz, 10

percent smectite, 14 percent illite (detrital muscovite plus illite), 4 percent kaolinite, 5

percent K-feldspar, and 6 percent plagioclase. The less than two micron size fraction

contains 16 percent quartz, 22 percent smectite, 41 percent illite (detrital muscovite plus

illite), 14 percent kaolinite, 3 percent K-feldspar, and 4 percent plagioclase. Cook et al.

(1975) caution that major errors may occur when smectite is present in such samples

because it is difficult to apply appropriate intensity factors. They report that smectite

intensity factors for their method vary from 3 to 10. An intensity factor value of 3 for

smectites was chosen for the analyses in this study. The choice of 3 was based on

comparison of calculated compositions for smectite, illite, and kaolinite in this study to published analyses of silt-rich sediments that were obtained from the banks of the

Mississippi River for an unrelated study (Hanor, 1980). Normalized values for illite, smectite, and kaolinite in the less than 2 micron size fraction fromth e earlier study are

40 percent, 30 percent and 30 percent respectively. Calculated values for the less than 2 micron size fraction in this study with values normalized to 100 percent for illite, 10

smectite, and kaolinite show that the current samples have 53 percent illite, 29 percent

smectite, and 18 percent kaolinite. These values suggest that the choice of three for the

smectite intensity factor is reasonable.

Because diagenetic and biogenic carbonates occur in some Tertiary Gulf Coast sequences (e.g., McManus, 1991), some of the experiments were conducted with the above sediments amended with a carbonate phase. The carbonates used in these mixed siliciclastic-carbonate experiments were Recent aragonite ooids from Andros Island,

Bahamas. Aragonite was chosen rather than calcite because it is less stable than calcite at earth-surface P-T conditions and is, hence, a more reactive phase, especially in the lower temperature experiments conducted in this study. Dolostone from a sidewall core in Jurassic sediments in Wayne County, Mississippi was also used. Chemical analyses of acid-soluble carbonates in similar dolostones from samples collected approximately

1.5 m above and 1.7 m below this sample are (Ca, 02 Mg09g)CO3, and

(Ca, 0,Mg09g)CO3 respectively (Ezat Heydari, personal communication).

The Mississippi River sediments were homogenized in a dry state by disaggregation and thorough mixing, but were otherwise left untreated. The aragonite ooids were prepared by rinsing 5 times with distilled-deionized water on a vacuum filtration apparatus to remove any residual NaCl from evaporated seawater. They were subsequently used in the experiments without crushing. The dolostone was first crushed to pass a 140 mesh screen (0.106 mm), and rinsed 5 times with distilled-deionized water on a vacuum filtration apparatus. The carbonate phases were mixed 1:1 by weight with the siliciclastic sediments. 11

NaCl Solutions Ten grams total of carbonate-free siliciclastic sediment were reacted with 50 mL

of aqueous NaCl solutions for periods of 30, 60, and 90 days at temperatures of 25°C

and 90°C, and pressures of approximately 1 to 2 bar. In a similar set of experiments

with NaCl solutions, five grams of carbonate sediments were mixed with five grams of

siliciclastic sediments and were reacted for 30, 60, 90, and 270 days at the same P-T

conditions as used in the carbonate-free experimental runs. Pressures slightly exceeded

1 bar in the 90°C experiments as indicated by the escape of gas when opening the

reaction vessels. A wide range of NaCl concentrations was chosen for investigation

because: 1) there are typically wide variations in salinity in pore waters near salt domes;

and 2) it has recently been hypothesized that variations in salinity may be an important

impetus for siliciclastic diagenesis (Hanor, 1994a; 1994b; 1995). The concentrations of

NaCl used in the series of experiments involving carbonate-free sediments were 1,10,

100, 1000,10,000, 35,000, 70,000, 150,000, and 250,000 ppm (corresponding to 1, 10,

100, 1000,10,050, 35,800, 73,400,166,300, and 297,300 mg/L), and concentrations corresponding to halite saturation (ca. 350,000 mg/L). The artificial NaCl brines used in the experiments were made from reagent grade NaCl and distilled-deionized water. The halite-saturated experiments were held at halite saturation by adding an excess of solid

NaCl to the reaction vessel. In subsequent experiments involving sediments containing aragonite or dolomite, NaCl concentrations of 10,1000, 70,000, and 250,000 ppm (10,

1000, 73,400,297,300 mg/L) were used, in addition to solutions at halite saturation. 12

The reaction vessels used in the initial experiments were 60 mL high-density

polyethylene (HDPE) bottles with polypropylene (PP) screw caps, both manufactured

by Nalgene. These vessels performed well in the 25°C experiments, but showed marked

degradation after 90 days in the initial series of 90°C experiments. Subsequent 90°C

experiments were performed in 60 mL Teflon (PFA) bottles with Teflon (PFA) screw

caps.

AH of the 25 °C experiments were conducted in a cabinet within the laboratory

where temperatures ranged between 20°C and 26°C as extremes, with 25°C as an

average. The initial NaCl-siliciclastic experiments conducted at 90°C were held at that

temperature in a HAAKE R-20 hot water circulating bath. The temperature control in the bath was better than +/- 1°C. Subsequent 90°C NaCl-carbonate-siliciclastic experiments were conducted in a THELCO Model 70M Laboratory Oven with temperature control of+/- 3°C. No attempt was made to control redox conditions or

PCQ2. Evaporative losses were monitored in the initial carbonate free experiments by CI" concentration and were not detected in the 25°C or 90°C experiments that ran for 90 days. The experiments that ran to 270 days at 90°C did experience evaporative loss in one vessel. The aragonite/siliciclastic experiment at 250,000 ppm NaCl, displayed a small drop in water level and precipitation of halite. The reaction vessels were gently agitated every day during the 30, 60, and 90 day runs, and ever other day after 90 days during the 270 day runs. 13

ANALYTICAL TECHNIQUES

ICP Analyses Samples of the fluid and solid phases were obtained at 30, 60, 90, and 270 days.

The 30, 60, and 90 day aliquots were sampled fromth e same reaction vessels, and the

270 day runs were performed and sampled in separate reaction vessels. The sampling

procedure caused the fluid-solid mass ratio to vary from 10 during the 0 to 30 day time

period, to 8.9 between 30 and 60 days, and then to approximately 8.6 from6 0 to 90

days. The 270 day experiments had a fluid-solidmas s ratio of 10 throughout their

duration. Reduction in the fluid-solidmas s ratio primarily affects reaction rates (Savage

et al, 1995), and, hence, is not a problem because measurement of reaction rates was

not a goal of the current experiments.

The major, minor, and trace element compositions of the fluid phase were

determined by ICP-AES (Inductively Coupled Plasma Atomic Emission Spectrometry) techniques on a Perkin-Elmer ICP 6500. Aliquots for ICP analysis were withdrawn from the reaction vessels by pipette and immediately filtered, acidified with hydrochloric acid, and diluted to appropriate concentration ranges for ICP analysis. All treated samples had pHs of less than two to prevent precipitation of solutes. The following elements were analyzed for: Al, B, Ba, Ca, Fe, K, Mg, Mn, Na, Pb, Si, Sr, and

Zn. Multi-element standards were prepared from commercial single element standards and used to calibrate the ICP for quantitative multi-element analysis. The standards employed are listed in Table 2.1. 14

Table 2.1. Standards for ICP-AES analysis.

Standard Element(s) Concentration (mg/L) #1 Na 1300 K 100 #2 Na 130.0 K 10.0 #3 Al 1.00 Ba 75.0 Ca 10.0 Fe 100 Mg 100 Mn 10.0 Pb 10.0 Si 10.0 Sr 10.0 Zn iao_ #4 Al 0.1 Ba 7.50 Ca 1.00 Fe 10.0 Mg 10.0 Mn 1.00 Pb 1.00 Si 1.00 Sr 1.00 Zn 1.00 #5 B 10.0 Cu 10.0 Li 10.0 blank #1 (4,600 mg/L HC1) blank #2 (3,600 mg/L NaCl + 1000 mg/L HC1)

To avoid serious plugging problems in the nebulizer and spray chamber assembly of the ICP-AES, it was necessary to dilute the most saline sample aliquots up to 125x (Table 2.2). Dilution raises ideal detection limits relative to undiluted samples by amounts up to slightly more than two orders of magnitude as shown in Table 2.3. Table 2.2. Dilution schedule for ICP-AES analysis.

Starting concentration of NaCl Dilution factor ppm mg/L 1 1 20x 10 10 20x 100 100 20x 1000 1000 20x 10,000 10,050 20x 35,000 35,800 27.8x 70,000 73,400 40x 150,000 166,300 62.5x 250,000 297,300 lOOx ca. 270,000 ca. 350,000 125x

Table 2.3. Spectral lines, limits of detection, and limits of detection with dilution.

* = data from Perkin-Elmer Element A. (nm) Estimated ideal Minimum reportable concentrations at dilutions detection limits used in ICP-AES analyses (img/L ) (mg/L). Winge et al, (1979). lOx 20x 27.8x 40x 62.5x lOOx 125x Al 308.215 0.045 0.45 0.90 1.3 1.8 2.8 4.5 5.6 B 249.773 0.0048 0.048 0.096 0.13 0.19 0.30 0.48 0.60 Ba 455.403 0.0013 0.013 0.026 .036 0.052 .081 0.13 0.16 Ca 422.673 0.010 0.10 0.20 0.28 0.4 0.63 1.0 1.3 Cu 327.396 0.0097 0.097 0.19 0.27 0.39 0.61 0.97 1.2 Fe 259.940 0.0062 0.062 0.12 0.17 0.25 0.39 0.62 0.78 K 766.491 *0.01 0.1 0.2 0.3 0.4 0.6 1 1 Mg 285.213 0.0016 0.016 0.032 0.044 0.064 0.10 0.16 0.20 Mn 257.610 0.0014 0.014 0.028 0.039 0.056 0.088 0.14 0.18 Na 589.592 0.069 0.69 1.4 1.9 2.8 4.3 6.9 8.6 Pb 220.353 0.042 0.42 0.84 1.2 1.7 2.6 4.2 5.3 Pb 216.999 0.090 0.9 1.8 2.5 3.6 5.6 9.0 11 Si 251.611 0.026 (as Si02) 0.26 0.52 0.72 1.0 1.6 2.6 3.3 Sr 407.771 .00042 .0042 .0084 0.012 0.017 0.026 0.042 0.053 Zn 213.856 0.0018 0.018 0.036 0.050 0.072 0.11 0.18 0.23 16

The emission lines used for ICP-AES analysis are also presented in Table 2.3.

The lines were selected from tables published by Winge et al. (1979), and where possible, the most intense lines were used in order to facilitate detection of dissolved species at low concentrations. The selected emission lines were also evaluated for potential spectral interferences from other elements and for background matrix effects in the samples using the graphics mode on the Perkin-Elmer ICP 6500. The graphics mode allows for visual evaluation of emission spectra for 0.5 nm of bandwidth above and below any spectral line of interest between 170 nm and 800 nm wavelength. Winge et al. (1979) state that the detection limit is conventionally defined as the analyte concentration required to yield a net signal equivalent that is three times the standard deviation of background signal beneath the spectral line. Literature supplied by Perkin-

Elmer with the ICP 6500 states that the detection limit is twice the standard deviation of ten replicate measurements of low concentration of the analyte for the wavelength of interest. Random checks performed during analytical runs with the ICP 6500 show that both definitions return comparable detection limit values.

pH, Alkalinity, and Chloride

The pH and alkalinity of the fluids were determined immediately following sampling. The pH was measured at room temperature with a miniature pH combination electrode calibrated against commercial pH buffer solutions. The total alkalinity of the samples was determined by electrometric titration with a standardized H2S04 solution following the standard method of Brown et al. (1970). Chloride titrations were also performed for the firstexperiment s using the Mohr method (Brown et al, 1970). It was 17 soon realized, however, that the Mohr method was not sensitive enough to evaluate any potential uptake or release of CI by minerals besides halite, and was not performed on later samples. The small size of the sample aliquots also prevented the use of the Mohr method on the lower-salinity fluidsbecaus e the total chloride in those samples was below minimum levels required for reliable titration endpoints.

Cation Exchange Characteristics Two short-term experiments were conducted to determine the exchangeable cations present on the siliciclastic sediments. The firstse t of experiments were performed with 10 grams of the bulk siliciclastic sediment in 60 mL HDPE bottles with

50 mL of solution. The sediments were placed in contact with halite-saturated solutions for four hours at 25°C and 90°C. In the second experiment, 5 grams of sediment were reacted with 100 mL of a IN ammonium acetate solution for 24 hours. The IN ammonium acetate solution is utilized in standard ammonium saturation procedures for clays to quantitatively displace exchangeable cations for the measurement of cation exchange capacity (CEC) of clay minerals (e.g. Busenberg and Clemency, 1973). The

CEC for the unreacted bulk sediments also was quantitatively measured by the standard ammonia electrode method of Busenberg and Clemency (1973).

Thermodynamic Modeling

In an attempt to evaluate the equilibrium state of the various systems, selected compositional data from the 90 day fluid analyses were entered into SOLMINEQ.88

(Kharaka et al, 1988) in order to calculate activities for major cations and dissolved silica. In the high salinity experiments, where the ionic strength of the fluidphas e 18 exceeded approximately 1, the Pitzer option in SOLMINEQ.88 was chosen for calculation of activity coefficients. The activity data were then plotted on conventional activity diagrams calculated from data in Bowers et al. (1984).

XRD and SEM Analysis

X-ray diffractometry was used to qualitatively identify the mineral phases that were present in the bulk siliciclastic sediments and bulk reacted sediments. Semi quantitative determinations of mineral abundances were also made for the bulk initial sediments and for selected minerals in the less than 2 um size fractiono f the reacted sediments. The XRD equipment consisted of a Phillips XRG-3000 Generator, a Phillips

9425 Goniometer with theta compensating slit and an APD 3520 operating system. The

X-ray tube had a Cu anode and was operated at 40 kV and 21 mA. Normal scans ran from 2°28 to 50°28 with incremental steps of .040°20. The counting times were 1 second per step. Bulk analyses for the unreacted siliciclastic sediments were performed on random powder mounts, and oriented slides were prepared for the less than 2 um size fraction in reacted and unreacted sediments. The less than 2 um size fraction was gravitationally separated in a tri-sodium phosphate solution which was used to de- flocculate the clays. The 270 day samples have not been examined by XRD because of equipment failure.

Scanning electron microscope secondary electron image analysis (SEM-SEI) was also employed to detect textural changes in the sediments after reaction. The SEM used for most of this study was a JEOL JSM-300 scanning microscope, but later images were collected on a much superior JEOL JSM 840-A scanning microscope. Both were 19

operated at 20 kV accelerating potential and approximately 1 nanoamp beam current.

Samples were prepared for analysis by drying at 60°C in a laboratory oven. Splits of the

dried solid were mounted with conductive glue onto appropriate mounting stubs, and

the samples were subsequently gold coated and analyzed. Only samples of the

carbonate-free experiments have been examined by this approach.

RESULTS Exchangeable Cations The CEC as measured by the standard method of Busenberg and Clemency

(1973) was 68 meq/ lOOg of dried sediment. The exchange experiments reveal that with

the IN ammonium acetate solution in which the sediments were saturated for 24 hours,

similar proportions of cations are produced in comparison to the NaCl exchange

procedure along with small amounts of Na, K, and Si. Analyses fromth e exchange

experiments are presented in Table 2.4. The concentration data are not directly comparable in relation to CEC in the upper half of Table 2.4 because different amounts of fluid and sediment were involved in the two different experiments. The data in the

Table 2.4. Results of cation exchange experiments.

bdl = below detection limit; na = not analyzed Experiment T°C Ba Ca K Mg Na SiO,° Sr mg/L NaCl 90 9.49 800 bdl 60 na bdl 3.05 NaCl 25 4.71 738 bdl 65 na bdl 3.05 NH4C2H302 25 3.00 227 19 43 10 0.4 0.90

meq/100g(bulksedim(:nt) NaCl 90 .0691 20.0 bdl 2.47 na .0348 NaCl 25 .0343 18.4 bdl 2.67 na .0348 NH4C2H302 25 .0874 22.7 0.977 7.09 0.87 .0411 20

lower part of the table have been normalized to meq/100g(sediment) and thus are directly

comparable. Ca and Mg are the predominant cations that were displaced from the

mineral surfaces in the exchange experiments. Summation of CEC values for the

analyzed species in the experiments can only account for about one-half to one-third of

the 68 meq/ lOOg exchange capacity measured by ammonia electrode.

The NaCl-exchange experiments were conducted with identical fluid volume

and sediment mass as used in the primary experiments. This allows direct comparison of

concentration data between the NaCl-exchange experiments and the NaCl-sediment

experiments. When the results for the NaCl-sediment experiments are presented, the

concentration data in Table 4.2 reveal that simple cation exchange can account for a

substantial portion of the initial change in fluid chemistry during the experiments.

Variations in Fluid Composition, Carbonate-Free Systems Charge balance problems

Based on the sum of the cation and anion charges, substantial errors in charge balance are present in the analyses of the highest and lowest salinity fluids.Althoug h most of the species that were analyzed for had good analytical precision and accuracy, with plus or minus one to two percent error, Na in particular had significant accuracy problems in the three highest salinity experiments. The source of the error apparently relates to the precipitation of NaCl near the nebulizer tip, which causes a reduction in the flowo f the nebulizer gas and removal of Na and CI fromth e nebulized sample. The precipitation problem was also exacerbated by the use of HC1 in the diluting solutions rather than nitric acid. Adjustment of the nebulizer gas flow rate to maintain the starting 21

flow rate returned good analyses for all other species analyzed, but did not compensate

for the loss of Na. The effects of Na removal can be seen in Appendix A.7 for the Na

analyses where 10 percent to 15 percent losses of dissolved Na are observed relative to the starting Na concentrations in the three highest-salinity experiments. Such losses are far in excess of anything that chemical reactions with sediments can account for.

Perkin-Elmer states that aspirated solutions may contain up to 20 weight percent total dissolved solids without plugging of the nebulizer assembly on the ICP 6500. In practice, however, analytical accuracy and precision for the Perkin-Elmer ICP 6500 falls off long before such high concentrations are reached with NaCl fluids.Th e highest salinity solutions that were analyzed after dilution contained no more than 0.4 percent

TDS salts. The restriction problem was not apparent when TDS levels in the analyzed fluids were maintained at or below approximately 0.15 percent.

The charge balance problem at low salinities is in the form of excess positive charge. Measured alkalinity as bicarbonate plus measured or starting chloride concentrations account for less charge than is required to balance the reported cations.

This probably is the result of partial analysis of the total alkalinity of the reacted fluids.

The standard method for alkalinity titrations uses a pH value of 4.5 as an arbitrary endpoint for the potentiometric titration and uses the assumption that total alkalinity is primarily due to carbonate. Titration endpoints for common organic acids such as acetic acid are below pH 4.5 and require specialized titrations to detect. The natural sediments used in these experiments contain organic material, and as other reactions proceed, organic acid anions may have been introduced into solution imparting additional 22

alkalinity. Breakdown of the high-density polyethylene bottles may also contribute

some alkalinity in the form of organic acid anions. Thus, the charge balance problems in

the low salinity experiments probably are due to incomplete analysis of dissolved

anions.

There is a systematic drop in pH with increasing chloride concentration at both

25°C and 90°C (Fig. 2.1 and Fig. 2.2). Low salinity waters at 25°C have pHs near 7.0,

--o--25°si30d —e—25°si60d —e—25° si 90d --*--25°asi30d —A— 25°asi60d —*— 25° asi90d - A - 25°asi270d ••o--25°dsi30d —B—25°dsi60d •10 12 3 4 5 6 —B— 25° dsi90d log CI (mg/L) - a - 25°dsi270d

and high salinity waters have pHs of between 5.0 and 5.5. Low salinity waters at 90°C have pHs of 5.5 to 6.0, while high salinity waters have pHs near 5.0. In the lower 23

temperature experiments, there was a slight increase in pH over the time interval of 30

to 90 days. In contrast, there was a slight decrease in pH with time at 90°C.

—e—90°si60d —•—90° si90d --A---90°asi30d —A— 90°asi60d a 90° asi90d - A - 90°asi270d --o--90°dsi30d —a— 90°dsi60d -10 12 3 4 5 6 —e—90°dsi90d log CI (mg/L) - a - 90°dsi270d

Figure 2.2. Plot of pH versus CI at 90°C.

Anions There are pronounced decreases in titration alkalinity with increasing chloride

concentration at both temperatures (Fig. 2.3 and Fig. 2.4). Alkalinities, expressed as

mg(HC03*)/L, in low salinity waters are near 60 to 70 mg/L at 25°C and near 30 mg/L

at 90°C. Alkalinities fall below 10 mg/L in the most saline waters. There are variations

in alkalinity with time at a given chlorinity, but the variations do not appear to be

systematic. pH and alkalinity were not measured at 30 days for the 90°C carbonate-free experiments because the equipment was not available at that time. 24

Titration alkalinity (meq/L) is a measure of the total concentration of weak acid anions in solution minus the concentration of hydrogen ion. Given the bulk composition and pH of the fluids, it is likely that the principal contributor to alkalinity is bicarbonate.

Bicarbonate is the dominant anion in terms of mass abundance in the solutions having chlorinities of less than approximately 60 mg/L in the carbonate-free experiments. At higher chlorinities, chloride is the dominant anion.

250 --o--25°si30d £ 200 —e—25°si60d —®— 25° si90d o 150 --*--25°asi30d u a 25° asi60d ffi 100 £ —•A— 25° asi90d ft 50 -A - 25°asi270d • cI-H --o--25°dsi30d C$ 0 ^ -B—25°dsi60d -10 12 3 4 5 6 -s—25° dsi 90d log CI (mg/L) a - 25°dsi270d

Figure 2.3. Alkalinity versus CI at 25°C. 25

—e—90°si60d —«— 90° si90d --A---90°asi30d —A—90°asi60d —A—90° asi90d - A - 90°asi270d --o--90°dsi30d —B— 90°dsi60d 12 3 4 5 6 • 90° dsi90d log CI (mg/L) - a - 90°dsi270d

Figure 2.4. Alkalinity versus CI at 90°C.

Dissolved Silica

Dissolved silica attained values of between 40 and 80 mg/L after 30 days at

25°C (Fig. 2.5). With increasing time at fixed chlorinity, silica was removed from solution, and concentrations dropped below 20 mg/L. There is a pronounced decrease in dissolved silica with increasing chlorinity at 60 and 90 days, and silica concentrations in the most saline samples dropped below its detection limit of 0.72 to 3.3 mg/L with increasing dilution. Dissolved silica in the 30-day 90°C runs (Fig. 2.6) attained a high of

310 mg/L at halite saturation. Silica was removed from solution with time, but in contrast to the 25°C runs remained in concentrations in excess of 100 mg/L. 100 --©-•25°si30d —e—25°si60d —e—25° si90d ••A---25°asi30d 10 —A—25°asi60d —A—25° asi90d -A - 25°asi270d --o--25°dsi30d —a—25°dsi60d •10 12 3 4 5 6 —B— 25° dsi90d log CI (mg/L) a - 25°dsi270d

Figure 2.5. Dissolved silica versus CI at 25°C.

•o--90°si30d

: -e—90°si60d

8 -e—90°si90d

9 •A---90°asi30d 100 , -A—90°asi60d

-t>—90°asi90d

o--90°dsi30d 10- 1 1 1 1 1 h-^H -B—90°dsi60d •10 12 3 4 5 •a— 90°dsi90d log CI (mg/L)

Figure 2.6. Dissolved silica versus CI at 90°C. 27

Major Cations The principal cations in terms of mass abundance are Na, K, Mg, and Ca. All

show a positive correlation in concentration with increasing chloride at 25°C and 90°C

(Figs. 2.7 through 2.14). There is an excess in dissolved Na in low salinity waters above

concentrations which can be explained by the simple addition of dissolved NaCl. In

high salinity waters, there is a pronounced sodium deficit, i.e., Na concentrations are

lower than in the unreacted fluids. The Na deficit at high concentrations is largely due to

the analytical difficulties previously discussed.

On a log-log plot, the largest rate in the increase in K with increasing chloride

occurs above chlorinities of 45,000 mg/L, while the largest rates of increase in Mg and

Ca occur between chlorinities of 600 and 45,000 mg/L. The concentrations of Mg and

Ca actually level off at higher salinities. K occurs in slightly higher concentrations at

90°C than at 25°C in the most saline waters, while the concentrations of Mg and Ca are

slightly lower.

Minor Species

Strontium and barium both increase in concentration with increasing chlorinity

(Appendix A. 12 and Appendix A.9 respectively). The highest value for strontium was 4 mg/L in both the 25°C and 90°C experiments, and the highest value for barium was 23 mg/L in the 90°C experiment. Iron was detected in a few samples in concentrations below 3 mg/L. Manganese rarely exceeded 1 mg/L in the 25°C runs, but increased significantly to values of 50 to 60 mg/L with increasing chlorinity in the 90°C runs. 28

.-a- •25°si30d / —&- -25°si60d 5 - - --o- -25° si90d ^—^ ^,4- - -- £•- •25°asi30d a . —A—-25°asi60 d w-4 _ ^F a -25° asi90d

*••> W)2 - o *^\sfy%y - A - 25°asi270d o 1—1 —^ft*5W 0JT 1 - .-o- •25°dsi30d —B—-25°dsi60 d 0 - h^-H 1 1 1 1 -25° dsi90d 0 12 3 4 • B « 25°dsi270d log CI (mg/L) - NaCl line

Figure 2.7. Na versus CI at 25°C. NaCl line represents line along which Na versus CI compositions plot from halite dissolution.

..<>- •90°si30d 5 - j{ o -90°si60d 4B^ -90° si90d JT ^> 4- - " •£? *•90°asi30 d Jr w6 , —A—-90°asi60 d 3 - _^^r^^p a K^zSir -90°asi90d ?tofl 2^ - \^mtm^mm^g-^^^n 2S%^Y ^ - A •• 90°asi270d o --Q- •90°dsi30d - 1 - ••-*;••' 0 1 h^H 1 1 1 1 —B—-90°dsi60 d -90°dsi90d -1 0 1 - a • 90°dsi270d CI (mg/L) - NaCl line

Figure 2.8. Na versus CI at 90°C. 29

; --o--25°si30d

s 2 100 , -e—25°si60d

& Ui 10, •25° si90d

0 A - 25°asi270d 1 - 1 1 1 , 1 •10 12 3 4 5 1 a - 25°dsi270d log CI (mg/L)

Figure 2.9. K versus CI at 25°C.

1000 •-o--90°si30d

—&— 90°si60d

£ 100 r —»—90° si90d £ a - A - 90°asi270d

10 - a - 90°dsi270d -10 12 3 4 5 6 log CI (mg/L)

Figure 2.10. K versus CI at 90°C. 30

1000 --o--25°si30d —o—25°si60d 100 —e—25° si90d --A---25°asi30d 10 —A—25°asi60d —A—25° asi90d - A - 25°asi270d 0.1 --o--25°dsi30d —B—25°dsi60d -10 12 3 4 5 6 —a— 25° dsi90d log CI (mg/L) -a - 25°dsi270d

Figure 2.11. Mg versus CI at 25°C.

•-o--90°si30d

—«— 90°si60d

—©—90° si90d

•A---90°asi30d

—A—90°asi60d

—A—90° asi90d

-o--90°dsi30d

-B—90°dsi60d

-10 12 3 4 5 6 —a—90° dsi90d

log CI (mg/L) • a - 90°dsi270d

Figure 2.12. Mg versus CI at 90°C. 1000 ••o--25°si30d o 25° si60d

—e—25° si90d 100 --A---25°asi30d

—A—25°asi60d 10 —A— 25° asi90d - A - 25°asi270d

•-o--25°dsi30d

—a—25°dsi60d

0 12 3 4 5 6 —B—25° dsi90d log CI (mg/L) -a - 25°dsi270d

Figure 2.13. Ca versus CI at 25°C.

1000 --o--90°si30d o 90° si60d —«—90°si90d --A--90°asi30d 100 —A—90°asi60d —A—90° asi90d - A - 90°asi270d --••-• 90° dsi30d 10 —B—90°dsi60d -10 12 3 4 5 6 —e—90°dsi90d log CI (mg/L) - a - 90°dsi270d

Figure 2.14. Ca versus CI at 90°C. 32

The following additional elements were analyzed for, but were below effective

detection limits as presented in Table 2.3: Al, B, Pb, Cu, Zn. Spectral analysis of these

minor species using the graphics mode on the Perkin-Elmer ICP 6500 in a few cases did

detect peaks that could be attributed to Pb or Zn, however the heavily structured

background in the spectral region of interest precluded quantitative analysis during

multi-element analytical runs.

Variations in Fluid Composition, Systems With Carbonate pH and alkalinity

The addition of carbonate mineral phases to the siliciclastic sediments caused some marked changes in the solution chemistry compared to the carbonate-free experiments (Appendix A.l through Appendix A. 12). pH and total alkalinity values still decrease with increasing salinity, but the pHs and alkalinities are significantly higher in the runs containing carbonate than in the carbonate-free systems at comparable times and temperatures (Figs. 2.1,2.2,2.3, and 2.4). For example, pHs in the aragonite- siliciclastic runs at 90°C vary from 8.6 to 7.1, in contrast to the range of 4.95 to 6.1 for the carbonate-free systems.

Dissolved silica

Dissolved silica was substantially lower in the runs containing aragonite or dolomite. Dissolved silica decreases with increasing salinity, in fact, to levels below an approximate reportable detection level of 2.6 to 3.3 mg/L in waters having chlorinities of greater than 73,400 mg/L at 25°C and 180,000 mg/L at 90°C (Figs 2.5 and 2.6). 33

Major species Na, Mg, and Ca (Figs. 2.7,2.8, 2.11, 2.12,2.13, and 2.14) show roughly similar

increases in concentration with increasing chlorinity as they did in the carbonate-free

runs. There are higher concentrations of Na, Mg, and Ca at low salinities relative to

their concentrations under carbonate-free conditions, and lower concentrations of Mg

and Ca at high salinities. Most remarkable is the reduction in K concentrations (Figs 2.9

and 2.10) below approximate detection limits of 0.2 to 1 mg/L for most of the carbonate

runs made at 90°C. K concentrations at 25°C also remain below detection limits during

the first9 0 days, but by 270 days are roughly the same as in the carbonate-free runs.

Minor species

The concentrations of Sr are substantially higher and the concentrations of Ba

and Mn substantially lower in the carbonate runs than in the carbonate-free runs. The elements Al, B, Cu, Pb, and Zn were below detection limits, and Fe was below detection limits except in two cases.

XRD Results

The results of the XRD analysis of sediments fromth e 25 °C experiments with the siliciclastics, and all of the mixed carbonate-siliciclastic experiments analyzed to date show no major changes in bulk mineralogy. The most marked changes in mineralogy were observed in the 90°C carbonate-free siliciclastic experiments at 90 days, where the appearance of a small peak at a low 20 angle, corresponding to a d- spacing of approximately 28 A, appears in the three highest salinity experiments (Fig.

2.15). This d-spacing corresponds to that of rectorite, a mica/smectite mixed layer clay 34

600

t/J

O o

0 5 10 15 20 25 30 35 40 45 50 °29

600 : Na-rectorite, 28 A 500 II/S 400 I,S,Q •\ | 300 o : ' ; K j ° 200 ll 100 1 1 1 1 1 1 1 1 1 1 1 1 |TT 1 1 1 1 I 1 ! 1 1 1 1 ! 1 J -J -1 - I 1 I 1 1 1-1—1 |_L.J..I. 1 1 I—1—1—I 0 0 5 10 15 20 25 30 35 40 45 50 °20

Figure 2.15. X-ray diffractograms for unreacted sediments (upper diagram) and for sediments from90-day , 90°C carbonate-free experiments at high salinities (lower diagram). The 28 A peak likely corresponds to the development of a Na-rectorite phase in the sediments. Abbreviations on plot: I = illite plus detrital muscovite; K = kaolinite; S = smectite; and Q = quartz. 35 mineral. Given the high Na concentrations in these experiments, it is probable that the mineral responsible for the peak is Na-rectorite, which is a 1:1, regularly-stacked paragonite/smectite mixed layer phase. The paragonite layer would have an ideal formula of NaAl2(AlSi3O10)(OH)2, and the smectite layer would have the general formula of Ro 33 (Al( 67Mg0 33)Si4O10(OH)2 for a montmorillonite, or

+ + 3+ Ro.33 Al2(Si3.67Alo.33)010(OH)2 for a beidellite, or R0.33 Fe2 (Si3.67Alo.33)Olo(OH)2 if it

+ + + 2+ were a nontronite (Moore and Reynolds, 1989), where R is K or Na or (Ca )05. It is likely that the interlayer cation, R, in the smectite is Na+. The 28 A peak for rectorite was not observed in the carbonate-bearing experiments through 90 days.

Visual inspection of X-ray diffractograms for the 90°C carbonate-free experiments showed obvious changes in relative peak heights for illite, kaolinite, quartz, and smectite (Fig. 2.15) which become more pronounced with increasing salinity.

Significant changes in peak-height ratios were not observed in the carbonate-bearing experiments. There are general decreases in the XRD peak-height ratios for quartz/illite and quartz/kaolinite as a function of chlorinity in ethylene glycolated siliciclastic sediments reacted for 90 days at 90°C. (Fig. 2.16 and Appendix A. 13). Figure 2.16 shows that air dried samples also follow the decreasing trend up to approximately

50,000 mg/1 Cf, where the ratios then reverse and increase with increasing chlorinity.

The trend at lower chlorinities suggests that either quartz is being destroyed, and/or that kaolinite and illite are simultaneously being precipitated. Peak-height ratios between quartz and smectite produced random variations and were not systematic. This may 36

result from difficulty in attaining consistent expansion of smectite with ethylene glycol

solvation.

Samples from the 270 day mixed carbonate-siliciclastic experiments have not

yet been examined because the Phillips XRD system used for analysis of the prior

experiments currently is out of operation. Given the small changes observed in the fluid

composition for the 270 day experiments relative to those at 90 days, it is unlikely that

any changes detectable by XRD are present.

-B-qtz/kaolEG °3 P4 -»-qtz/ilEG

o -s-qtz/kaol AD

—•—qtz/il AD f-H

0.1 -10 12 3 4 5 6 log CI (mg/1)

SEM Results

During separation of the < 2 um size fraction for preparation of oriented clay smears for XRD, it was observed that the sediments fromth e three highest salinity 37

experiments in all cases had less material present in the < 2 um size fraction than in the

sediments fromlowe r salinity experiments. Subsequent SEM work verifies this

Figure 2.17. SEI images of reacted and unreacted sediments: (a) unreacted sediments, (b) sediments reacted for 90 days in 100 mg/L NaCl solution, (c) sediments reacted for 90 days in 297,300 mg/L NaCl solution. 38

observation (Figs. 2.17a through 2.17c). The SEM work shows that materials are

removed fromth e < 2 um size fraction in the sediments. Removal of kaolinite and illite

from this size fraction at high chlorinities may also explain the reversal of the trend at

high salinities seen in Figure 2.16 for the quartz-illite-kaolinite peak height ratios.

SEM work has also revealed corrosion textures on quartz grains in the high

salinity experiments (Fig. 2.18). SEM analysis reveals that there are at least two

populations of quartz in the starting sediments, and one of these populations apparently

is more susceptible to dissolution in high salinity fluidstha n the other. Surface textures

such as those shown in Figure 2.18 confirm that quartz is being dissolved in the

sediments and eliminates the unlikely possibility fromth e XRD work that kaolinite and

smectite were being simultaneously precipitated.

Figure 2.18. SEI image of corroded surface on quartz grain after reaction with high- salinity fluids. 39

Thermodynamic Modeling Figures 2.19 through 2.24 show activity relations for fluids from the 25°C and

90°C experiments at 90 days. A complete listing of the calculated activity ratios and the

activity of H4Si04 is presented in Appendix A. 14a and A. 14b. The lack of Al data in the

analyses requires the assumption that the solutions are saturated with respect to the Al-

bearing minerals being considered when using the selected stability diagrams.

All of the lower temperature fluids plot within the stability field for kaolinite

(Figs. 2.19,2.20, and 2.21). Each group of analyses straddles the quartz saturation line.

The fluids which reacted with aragonite or dolomite plot near a line representing fluid

-3.5

compositions in equilibrium with aragonite at a PC02 of 10 . The stability lines for

calcite nearly coincide with the aragonite lines on this plot for the same Pco,.

-e-25°si90d -A-25°asi90d -B-25°dsi90d

qtz sat' \ chalcedony \ am silic -5.5 -4.5 -3.5 -2.5

log(flH4Si04)

+ + Figure 2.19. Activity diagram for log (aNa /atH ) versus log (aH4Si04) at 25°C (modified from Drever, 1988). The three vertical dashed lines represent saturation values at 25°C, Psat for quartz (qtz sat), chalcedony, and amorphous silica (am silic). 40

•25°si90d

qtz sat/ \chalcedony Nam silic -5.5 -4.5 -3.5 -2.5

log(tfH4Si04)

+ Figure 2.20. Activity diagram for log (oK7oH ) versus log (aH4Si04) at 25°C (modified from Drever, 1988).

aragonite £ at. PCO2=10A-3.5 at. PCO2=10A-2.0

-o-25° si90d

-A-25° asi90d qtzsat^^ \chalcedony \ am silic -5.5 -4.5 -3.5 -2.5 -B-250 dsi90d log(aH4Si04)

2+ + Figure 2.21. Activity diagram for log (aCa /(aK f) versus log (aH4Si04) at 25°C (modified from Drever, 1988). 41

Fluids fromth e 90°C runs show distinctly different activity relations from the

lower temperature experiments. As shown in the phase diagram for log (aNa+/aH+) vs.

log (aH4Si04) (Fig. 2.22) the lower salinity fluids for the carbonate-free, aragonite, and

8 7 + Paragonite -o-90° si90d 6 5 -A-90° asi90d 4 Gibbsite "Ho -a-90° dsi90d o 3

qtz sat chalcedony\am silic -5 -4 -3 -2 -1 log(aH4Si04)

+ + Figure 2.22. Activity diagram for log (aNa /aH ) versus log (aH4Si04) at 100°C (thermodynamic data from Bowers et al, 1984).

dolomite runs all plot within or near the kaolinite stability field.Wit h increasing

salinity, however, there are marked increases in the activity ratio of oNa+/aH+ which

take the fluids out of the stability field for kaolinite and into the stability fields for albite

and/or paragonite. Fluids moving towards compositions that favor paragonite would

favor the production of Na-rectorite because paragonite is a discrete layer-component of

Na-rectorite. The fluids fromth e carbonate-free runs are supersaturated with respect to quartz, and the degree of supersaturation increases at extreme salinities as the fluids 42

enter the albite stability field.Fluid s fromth e aragonite and dolomite-bearing runs, in

contrast, are slightly supersaturated with respect to quartz at lower salinities and become

undersaturated with respect to quartz at higher salinities. Both sets of carbonate

experiments are trending toward the analcime stability field with increasing salinity.

The most saline fluids in these experiments contained high dissolved Na and silica in

concentrations below detection limits which would most probably drive the activity

trends into the analcime stability field.

The data in Figure 2.23 for log (oNa+/aH+) vs. log (crK+/aH+) at 75°C suggest

that paragonite-muscovite (or Na-illite-K-illite) equilibria control fluid compositions for

Na and K at higher salinities. Figure 2.23 has been constructed with data from Bowers

8 Paragonite Albite 7 .+ .—-*^-"'"^ . -"• * 6 •«*"""""- t 5 Muscovite 4 —e— 90°si90d O Kaolinite \^ 3 parag-musc 100°C

1 1 1 1 1 1 1 l_^ ^ iii i i i i 1.5 2 2.5 3 3.5 log(aK+)/(aH+)

Figure 2.23. Activity diagram for log (oNa+/oH+) versus log (aK+/aH+) at 75°C. The parag-musc xxx°C lines are phase boundaries between paragonite and muscovite at 75°C and at 100°C. 43

et al. (1984) for P-T conditions Psat, 75°C. At higher salinities the plotted data parallel

the metastable extensions of the stability boundaries for paragonite and muscovite

between 75°C and 100°C. 90 day data fromth e carbonate-bearing experiments could

not be plotted because K concentrations were below detection limit. This may indicate

K incorporation into a new mineral phase.

Figure 2.24 is a plot of log (aCa2+/(aH+)2) versus log (oMg2+/(aH+)2) that

—o—90°si90d

•-o--25°si90d 1J - Calcite or Aragonite - /?''' —A—90°asi90d c/-** 14 a 13 - - --A---25°asi90d X 12 - JA + r^ ~cd 11 . - p —B—90°dsi90d V s?t# - & 1° Dolomite --o--25°dsi90d £ 9 /£ * o' 8 ' fa —1—1— H 1 1 — - calc/dolo25°

7 8 9 10 11 12 13 14 calc/dolo 100°

2+ + 2 log(aMg )/(aH ) - - arag/dolo25°

arag/dolo 100°

Figure 2.24. Activity diagram for log («Ca2+/(oH+)2) versus log (aMg2+/(oH+)2). The four lines marked calcite 25°, calcite 100°, aragonite 25°, and aragonite 100° represent the stability boundaries between calcite and dolomite, and aragonite and dolomite at the given temperature. 44

includes the stability boundaries for aragonite/dolomite and calcite/dolomite at 25°C

and 100°C. Nearly all of the fluids plot within the dolomite stability fieldwit h respect to

aragonite and calcite at the respective temperatures. The exception is the experiment

that contained aragonite and siliciclastics at 90°C in which three of the analyses plot within the aragonite stability fielda t 100°C.

The log of the saturation indices (log SI) for aragonite, calcite, and dolomite are less than one for all of the carbonate-free experiments (Appendix A. 15), and thus the fluids apparently are undersaturated with respect to any of the three carbonate phases. In the aragonite plus siliciclastics experiments, all of the fluids except those at the highest salinity step in the 90°C experiments and the two highest salinity steps in the 25°C experiments are saturated with respect to aragonite. In the 90°C aragonite plus siliciclastics experiments the fluids are saturated with respect to calcite and dolomite in the three lower salinity experiments, and in the 25°C aragonite plus siliciclastics experiments the fluids apparently are saturated with respect to calcite and dolomite in all cases. The dolomite plus siliciclastics experiments develop similar trends (see

Appendix A. 15).

DISCUSSION The results show that the salinity of the fluids in the experiments exerts a strong influence over changes in the composition of the fluid phase. The reacted fluids also develop some compositional trends that are similar to natural NaCl-brines from sedimentary basins (see Hanor, 1994a). Ca, Ba, K, Mg, Mn, and Sr increase in concentration with increasing salinity when initial TDS in the experiments exceeds 45 approximately 10,000 mg/L NaCl. pH and total alkalinity drop with increasing salinity, which also is consistent with the behavior of natural brines. However, significant differences exist between the experimental fluids and natural formation waters of comparable salinity. The monovalent and divalent cations in the experiments do not develop the 1:1 and 2:1 increasing trends respectively observed on plots of log cation versus log CI for many natural formation waters with salinities between 10,000 mg/L

TDS and halite saturation (ca. 350,000 mg/L NaCl) (Hanor, 1994a and 1994b). Calcium concentrations never exceed 1000 mg/L, which is a clear deviation because many saline waters in sedimentary basins have Ca concentrations that exceed 4000 to 8000 mg/L Ca

(Hanor, 1994a and 1994b; Land, 1995). K, which normally is present in waters from sedimentary basins, also remains below detection limits of 0.2 to 1 mg/L in many of the reacted fluids.

Equilibrium conditions were not established in the experimental runs because plots of calculated activity data show that minerals such as kaolinite, which persist in the solids of the high salinity experiments, are not in equilibrium with ambient fluid phase. Instead, the experiments appear to be slowly reacting and slowly moving in a direction towards equilibrium. The major compositional changes in the fluidsar e observed within the firstthirt y days, and after thirty days of reaction, the compositional changes generally are much smaller. The large changes in fluid composition in the first thirty days and can be attributed primarily to fast exchange reactions which likely are completed within the first few hours of an experimental run. After the first few hours, compositional changes in the fluids can be attributed to much slower dissolution- 46

precipitation reactions. The occurrence of dissolution-precipitation reactions is

supported by substantial increases in solution of carbonate alkalinity, and the

appearance of dissolved silica and K in the carbonate-free experiments. Furthermore,

increases of Mg, Sr, and Ba in some experiments cannot be entirely accounted for by

exchange.

Exchange reactions versus mineral hydrolysis reactions

The potential effect that mineral-surface exchange reactions have on the fluid

composition will first be examined. Ba, Ca, Mg, and Sr contents were determined by

ICP analysis of the fluidsfro m the short-term NaCl exchange experiments. Ca

concentrations in the NaCl exchange experiments are 800 mg/1 at 90°C and 740 mg/1 at

25°C. Maximum concentrations of Ca occur in the carbonate-free experiments, and they

are 826 mg/1 in the 90° C experiments and 967 mg/1 in the 25° C experiments. This

result reveals that a large portion of the dissolved Ca is derived from exchange,

although some additional Ca may be introduced to solution by mineral dissolution.

In the siliciclastic sediments, the anorthite component of detrital plagioclase in

the sediment probably contributes most of the additional Ca by dissolution.

Hypothetically, if 5% of the bulk siliciclastic sediments are composed of plagioclase

with a composition of 80 weight percent albite and 20 weight percent anorthite, 10

grams of bulk sediments will contain 0.5 grams of plagioclase, or 0.125 grams of the

anorthite component. Dissolution of 0.125 g of anorthite in the experiments would

2+ introduce 0.018 grams of Ca into solution and increase the Ca concentration of 50 mL of brine solution by approximately 360 mg/1. This value in combination with the 47

maximum exchange value for Ca would yield a solution concentration of 1160 mg/L

Ca which is in line with the overall maximum Ca concentrations observed in the

experiments.

Most of the strontium concentration values in the high salinity siliciclastic

experiments are very close to those observed in the NaCl-exchange experiments, and

thus, most of the Sr in the fluids probably is attributable to exchange. However, one

major exception does occur. The mixed carbonate-siliciclastic sediments that included

aragonite ooids have Sr concentration values that exceed the exchange strontium

concentration values by a factor of approximately 7 in the 90°C experiments and

approximately 2 in the 25°C experiments. This is attributable to dissolution of the Sr-

bearing aragonite in the ooids.

Ba and Mg concentrations in the 90°C, carbonate-free experiments at high

salinities exceed exchange concentrations by a factor of approximately 2. This reveals

that dissolution of other minerals in the solids is occurring which contributes the excess

of Ba, which likely is produced from the destruction of K-feldspar (Macpherson, 1989),

and Mg. In some cases Mg falls below effective detection limits which suggests that it

is being reincorporated into the sediments. Reduction in Mg and K at high salinities with time also suggests that mineral precipitation is occurring.

Although pH was not measured in the finalexchang e experiment fluids, the initial drop of measured pH in the main experiments may be in part due to exchange of

+ + H from exchange sites by Na . The pH of the artificial brines at halite saturation is very close to 7 in the unreacted stock solutions, and even at 30 days in the 25°C siliciclastic- 48 only experiments, the pH has dropped to approximately 5. This would seem to be one likely mechanism that could partially drive the initial pH drop.

Dissolution-Precipitation Reactions

Although the variations in cation composition of the reacted fluids with increasing chloride concentration can be explained by cation exchange, the presence of substantial concentrations of silica in solution and changes in XRD peak-height ratios verifies that either congruent dissolution of quartz, or incongruent dissolution of aluminosilicate mineral phases is occurring. Incongruent dissolution of aluminosilicates is indicated because of the lack of dissolved Al. The artificial brines used in the experiments initially contained only Na+ and CI", hence they are initially undersaturated with respect to all silicate and aluminosilicate minerals. The initial dissolution of silicate and aluminosilicate minerals will be congruent followed by incongruent dissolution when saturation is reached for some given mineral phase. In most of the experiments there is evidence of silica incorporation into either amorphous phases, or mineral phases as revealed the lowering of silica concentration with time as observed in the 90°C siliciclastic-only experiment, or by the reduction of dissolved Si02 below detection limits at higher salinities in many of the carbonate-bearing experiments. The loss of silica from solution is especially profound in the 25°C carbonate-free experiments where silica concentrations in the more saline experiments drop from around 50 mg/1

Si02 at 30 days to below ICP-AES detection limits at 60 and 90 days.

No K was detected in the NaCl-exchange experiments possibly as a result of the

125x dilution and a high detection limit of 1 mg/L, but 19.1 mg/L K were detected in 49

the ammonium acetate exchange experiments. 19 mg/L K would be detectable at 125x

dilution and so puts an upper limit on exchanged K in the NaCl solutions. The highest K

concentrations were observed at 30 days in the carbonate-free experiments with 134

mg/L K in the 25° C experiments, and 268 mg/L K in the 90° C experiments. Thus,

most of potassium released into solution was derived fromth e destruction of K-bearing

minerals such as K-feldspars, detrital muscovite, and illite.

Alkalinity is also produced in the experiments through hydrolysis reactions. The

simplest case is the acid hydrolysis of calcite, which is very apparent in the 25 °C

carbonate-bearing experiments. The following reaction describes the acid hydrolysis of

calcite and introduction of alkalinity

+ 2+ CaC03 + H -> Ca + HC03" (2.1)

Alkalinity may also be produced by incongruent acid hydrolysis of

aluminosilicate minerals by carbonic acid

2+ CaAl2Si2Og + 2H2C03 + H20 -> Ca + Al2Si205(OH)4 + 2HC03" (2.2)

Reaction (2.2) is one that potentially may occur in the lower salinity experiments and reflects the incongruent hydrolysis of the anorthite component of plagioclase to form kaolinite by carbonic acid hydrolysis. Carbonic acid is present in the experiments from dissolved atmospheric C02. The reaction potentially generates carbonate alkalinity in 50

the lower salinity carbonate-free experiments in the form of bicarbonate, given the

measured pH range of those experiments.

Precipitation of amorphous silica probably controls the maximum concentration

of dissolved silica in the experiments. One of the more interesting aspects of the

thermodynamic modeling with regard to diagenesis is the trend for data points to track

+ + toward the analcime stability fieldo n plots of log (crNa /aH ) versus log («H4Si04) for

the carbonate-bearing experiments. Analcime is known to precipitate in sediments

around salt domes, and in two reported cases (McManus, 1991; and Esch, Chapter 3,

this dissertation) appears to preferentially precipitate in association with fossil carbonate

microfossil tests. The lack of aluminum data unfortunately prohibits direct calculation

of the saturation index for analcime in the experiments. The apparent tracking of

activity ratios along the paragonite/albite stability boundary in Figure 2.22 also suggests

that paragonite, or possibly Na-illite may be a stable phase around salt domes. At

temperatures approaching 100°C near the flanks of salt domes, analogous reactions to

K-illitization of smectites as observed in non-salt dome settings in the Gulf Coast (Perry

and Hower, 1970) may occur, except that Na fillsth e role that potassium does in the

other settings where K-illitization occurs.

The SEM and XRD work also allow some constraint on potential hydrolysis

reactions. Corrosion of detrital quartz in the high-salinity carbonate-free experiments and the observed reduction in peak ratios for quartz/illite and quartz/kaolinite are

additional evidence that silica dissolution has occurred during the experiments. The

XRD work also suggests that a non-expandable Na-phyllosilicate phase develops in the 51

carbonate-free, high-salinity experiments. Aluminum concentrations remain below

detection limit for ICP-AES in the experiments which also suggests that precipitation of

paragonite or a paragonite-like phase is accomplished through incongruent hydrolysis

reactions with smectite (in the following hypothetical reaction, Na-beidellite)

+ 3Nao.33Al2(Si3.67Al0.33)010(OH)2 + 1.34Na -> + 2.33NaAl2(AlSi3O10)(OH)2 + 4.02SiO2° + 1.34H (2.3)

This reaction also releases silica and produces acid hydrogen which can promote further

diagenesis.

CONCLUSIONS

The addition of dissolved NaCl to the fluids in contact with the clay-rich silts and silt mixtures with carbonates drove a combination of exchange reactions and dissolution-precipitation reactions. These reactions subsequently generated fluidswhic h contained a variety of dissolved species, except for K, that systematically increase in concentration with increasing salinity. Comparable to natural waters from sedimentary basins, definite increases in dissolved species start at about 10,000 mg/L NaCl, but fail to develop the 1:1 and 2:1 trends found in many natural waters fromsedimentar y basins for monovalent and divalent cations.

The exchange experiments show that Ba, Mg, Sr, and especially Ca appear in solution within hours and account for a large portion of the observed increase in those species in the experiments. In the case of Ba and Mg in the 90°C carbonate-free experiments, subsequent dissolution reactions double their concentrations. Ca concentrations were increased by approximately 25 percent by subsequent hydrolysis

reactions. K, however, was not detected as a product of exchange in halite-saturated

NaCl solutions.

By thirty days, dissolved silica appears in solution, which is evidence of silicate

and/or aluminosilicate dissolution. Dissolved silica appears in all of the carbonate-free

experiments as well as in the lower salinity carbonate-bearing experiments. Reduction

in XRD peak-height ratios with increasing salinity for quartz/illite and quartz/kaolinite

and heavily etched quartz grains in the 90°C carbonate-free experiments suggest that

quartz is less stable in the high-salinity fluids. K also appears in the carbonate-free

experiments and along with the Ba mentioned above suggests that K-feldspar is being

destroyed. The Ba and K trends in the carbonate-free experiments also suggest that K-

feldspar is less stable in the high-salinity fluids.

2+ + As early as 60 days, there is evidence for reincorporation of Si02, Mg , and K in precipitating phases, although their abundances in the solids generally were too low to be detected by X-ray diffraction analysis, or readily recognized by SEM image analysis. XRD work does reveal that a Na-rectorite phase develops in the high-salinity carbonate-free experiments. The hypothetical reaction for smectite to paragonite conversion also generates additional dissolved silica which may help explain the very high activities for dissolved silica in the carbonate-free experiments. Thermodynamic modeling of the carbonate-bearing experiments suggests that the fluid compositions at high salinities may favor the precipitation of analcime. This would account for the association of analcime and fossil tests found in flank sediments around salt domes. 53

It is clear that by simply introducing progressively larger amounts of NaCl into a

sedimentary geochemical setting that diagenetic reactions will be driven which can

profoundly alter the fluid composition of many solutions species besides Na+ and CI".

Recommendations for future experiments

A variety of problems in the experiments were encountered regarding the

ambiguities in the contribution to changes in fluid composition by exchange reactions

and dissolution-precipitation reactions. A second problem area is connected with the

analysis of the solid phases by XRD and SEM. The following suggestions are

recommended for future experiments.

To counter the problem of exchange cations masking hydrolysis reactions, two

approaches could be taken: 1) Carefully characterize and quantify all exchangeable

species in NaCl fluids at all salinities of interest. This in itself would be a substantial undertaking, but would lead to a much better understanding of how exchange reactions and salinity affect the fluid composition and also allow for quantitative correction of data. Changes in pH as a result of simple exchange should also be evaluated. These steps are important if mass balance computations are being considered. 2) Pre-saturate the exchange sites in the sediments with some cation that is not an important species in evaluating carbonate, silicate, and aluminosilicate equilibria, such as Li+ from a LiCl solution. Li analyses of reacted solutions would reveal the degree of exchange that occurred during reaction.

The analyses of the solids were confounded by several problems. The natural siliciclastic sediments used in the experiments contained a variety of mineral phases that 54 are difficult to adequately quantify for bulk mineral composition. True quantitative analyses, however, could be accomplished by means of random powder X-ray diffraction for the bulk sample by using a reference material spike (Snyder, 1992).

Although this method would be time consuming, much more useful information could be gathered and applied to explaining reactions and fluid compositions. Bulk compositions of the natural sediments and chemical composition of the individual minerals would also be useful for mass balance calculations. Analytical transmission electron microscopy is another analytical method that would help identify mineralogical changes, especially in the layer-silicates.

Clay coats were also observed on many of the natural detrital quartz grains which serve to partially isolate the coated phase from reactive fluids. The clay coats and the very-fine size of the sediments also made meaningful SEM observations very difficult. In order to avoid these problems in future experiments, and to allow true quantitative XRD analysis and well defined SEM-EDS observations on the morphology of reaction-related textures and qualitative EDS analyses, artificial sediments fabricated from well known standard minerals could also be used as proxies for the real sediments.

By this approach mineral percentages in the bulk artificial sediment package could be accurately known and manipulated, and the composition of the individual mineral phases would also be well known. The individual minerals could also be crushed and screened to different size fractionsi n order to evaluate grain-size effects on the reactions. 55

Future experiments should also be performed at higher temperatures in order to promote the dissolution-precipitation reactions in order to produce more product. This would facilitate the identification of new phases. CHAPTER 3) SEDIMENT DIAGENESIS NEAR THE SALT-SEDIMENT INTERFACE AT THE EUGENE ISLAND 128 SALT DOME

INTRODUCTION

Recent investigations of sediments that flank piercement-type salt domes have produced evidence for pervasive sediment alteration through chemical diagenesis. In a study of the flank sediments at the Black Bayou salt dome, Cameron Parish, Louisiana,

Leger (1988) found massive diagenetic calcite cements, pyrite, and kaolinite. He also observed that diagenetic pyrite and kaolinite increased in abundance with proximity to the salt. At the West Hackberry salt dome in Cameron Parish, Louisiana, McManus and

Hanor (1988 and 1993) and McManus (1991) found calcite cements similar to those reported by Leger (1988). In addition, they also observed iron sulfide cementation of

Miocene sands and analcime cements in Oligocene sands. Calcite cements at the West

Hackberry salt dome also increase in volume with proximity to the salt (McManus and

Hanor, 1988). Substantial secondary porosity in the flank sediments at the West

Hackberry and Black Bayou salt domes was developed from dissolution of calcite cement and framework feldspar grains (McManus, 1991; Leger, 1988).

The observation that diagenetic minerals increase in abundance with proximity to salt suggests that the salt may influence diagenesis in ambient sediments. Insight into this phenomenon can be gained by examining recent work on sediment diagenesis in the presence of high-salinity fluids. Hanor (1994a and 1994b) presented a synthesis of compositional data obtained from oil-field brines fromsedimentar y basins around the

56 57

world and demonstrated that the concentrations of many major and minor dissolved

species in natural formation waters with salinities greater than 10,000 mg/1 and less than

halite saturation (ca. 350,000 mg/L) covary with CI' concentration. Hanor (in press)

attributes the compositional dependence of major and minor cations on CI" in formation

waters to buffering of dissolved species by reactions with multi-phase silicate-

aluminosilicate-carbonate mineral assemblages in a coupled response to changes in total

anionic charge. Because of this, one of the potential controls on diagenesis near salt

domes is halite dissolution at the margin and the subsequent transport of fluids enriched

in CI" into the surrounding sedimentary package.

Coupled dissolution/precipitation reactions in response to halite dissolution can potentially alter the permeability and porosity of sedimentary formations, and knowledge of the diagenetic controls on these two parameters is essential for predicting the quality of hydrocarbon reservoirs and formation water movement around salt domes.

In a less restricted sense, understanding the geochemistry of diagenesis influenced by salt dissolution is important for predicting the net effect on sediments from diagenetic reactions in other settings where salty or high ionic strength fluids interact with detrital mineral packages. Deep-well disposal of strong mineral acids and bases, and the current subsalt exploration activities for petroleum in the Gulf of Mexico are prime examples of settings where such knowledge is potentially valuable.

PURPOSE OF STUDY This chapter is an investigation on the physical and chemical diagenetic processes that affect sediments and evaporites at the salt-sediment interface of the 58

Eugene Island Block 128 salt dome, and is a portion of a larger field and experimental

study involving the investigation of water-rock interaction where high salinity, high

ionic strength fluids interact with natural sediments in sedimentary basins. Furthermore,

materials fromth e salt-sediment interface on the flanks of salt domes have not been

closely examined in previously published studies by modern electron beam techniques,

and this investigation makes use of these techniques in order to identify both obvious

and subtle chemical and physical processes that operate in the salt dome flank

environment near and at the salt-sediment interface.

EUGENE ISLAND 128A FIELD

The Eugene Island Block 128A oil and gas field is situated on the southeast

flank of an offshore salt dome which is located 93 km (58 miles) south-southwest of

Morgan City, Louisiana in the Gulf of Mexico (Fig. 3.1). The site lies in approximately

SOmiles SO kilometers

1 Baton Rouge

Morgan City

Eugene Island Block 128 Field

Figure 3.1. Map of Louisiana and offshore Louisiana showing the location of the Eugene Island Block 128 Field. 59

18 m (60 ft)o f water and was discovered by means of reflection seismography (Stipe,

1960). The dome is a shallow, piercement-type structure which penetrates Tertiary and

older sediments and produces hydrocarbons from flank sediments (Stipe, 1960). It is not

known if the EI 128 dome has caprock, or how closely the top approaches the seafloor.

MATERIALS AND METHODS Sidewall cores Petrologic information for this study derives fromborehol e sidewall cores provided by Shell Offshore, Inc. The sidewall cores were taken from two wells located on the southeast quadrant of the EI 128 salt dome in the EI 128 A field (Fig. 3.2), well

Figure 3.2. Map of the southeast quadrant of the Eugene Island Block 128 salt dome showing approximate depth to top of salt, cross-section lines A-A', A'-A", and the location of wells 1, 5,12ST and 23 in the 128A field. 60

12ST and well 23. The two wells are deviated holes drilled immediately adjacent to the

salt in the flank sediments. Well 12ST pierces multiple salt overhangs (Fig. 3.3) which

have developed on the side of the salt dome fromdepth s of approximately 1400 m to

2710 m (4600 ft to 8900 ft).Wel l 23 pierces a salt overhang from approximately 2134

m to 2256 m (7000 ft to 7400 ft).

A total of 175 sidewall cores were taken by Shell from these two boreholes at

irregular intervals. 57 samples are from well 23, and 118 are from well 12ST. The depth

intervals sampled are shown in Figure 3.3. Many of the sidewall cores were taken from formations that appear to be sands having deflections indicating high resistivity on the electric logs. In addition to the sidewall cores, Shell Offshore, Inc. also supplied data from commercial analyses of the sidewall cores by Core Petrophysics, Inc. Those data describe the general lithology, grain-size distribution, porosity, and permeability of the sidewall cores (Appendices B.l, B.2, B.3, and B.4). A representative group of 70 sidewall cores was selected fromth e larger group for further analysis based on the general lithologic descriptions and grain-size distributions determined by Core

Petrophysics, Inc.

Electric wireline logs from 15 wells in the EI 128A field,includin g well 23 and well 12ST, were used to develop cross sections A-A' and A'-A" in Figure 3.3. The correlation of units between the wells had already been performed by Shell Offshore,

Inc. 61

A' A"

2000-

4000 —

6000 — .

8000-

10000-

v 12000— fault LEGEND .1000ft 1000 ft | + salt Y-top of structure | A -sampled interval well

Figure 3.3. Cross sections A-A' and A"-A' for the EI 128 salt dome and southeast flank sediments. Well penetrations of salt overhangs and sampled intervals are shown for well 12 ST and well 23, and structural features are shown for section A-A'. No vertical exaggeration. Lines K, Kl, P2, etc., represent the top of sands taken from structure maps supplied by Shell Offshore, Inc. 62

Electron microscopy The 70 sidewall cores selected as representative samples were prepared for examination by binocular and polarizing light microscopy, and by scanning electron microscopy (SEM). The SEM used in this study was a JEOL 840A SEM equipped with

X-ray energy dispersive spectromety (EDS). Operating conditions were 20 kV accelerating potential and a 0.6 nanoamp beam current. EDS was used to obtain qualitative compositional information from samples in order to positively identify mineralogy. X-ray wavelength dispersive spectrometry (WDS) was performed with a

JEOL 733 Superprobe which was operated at a 15 kV accelerating potential and a 2.0 nanoamp beam current. The WDS technique was used to obtain quantitative compositions for Na-aluminosilicate mineral fillings in microfossil tests. Backscatter electron (BSE) and secondary electron (SEI) images obtained with the SEM were taken from polished thin sections and grain mounts, and were subsequently processed with the

National Institutes of Health Image program, versions 1.51 and 1.55 (Rasband, 1994).

The NIH program was used for labeling, image enhancement, and conversion of images into formats compatible with Macintosh and Windows image files for printing.

Thin-section and grain-mount preparation

Well 12 ST was drilled with halite-saturated drilling muds allowing for the recovery of samples of diapiric salt fromth e salt-sediment interface. Water-soluble evaporite minerals in some samples and the extremely friable nature of some siliciclastic samples required special handling procedures for thin-section preparation and polishing. 63 samples were vacuum impregnated with blue-dyed epoxy to control 63 disaggregation and to preserve primary porosity. Oil or acetone were used to replace water during cutting, grinding, and polishing to prevent the loss of water-soluble minerals. After polishing, the thin sections were carbon coated for SEM-EDS and WDS analysis. Seven grain mounts were prepared and gold coated for analysis by SEM-EDS.

RESULTS Diagenetic minerals and textures Analysis of the polished thin sections and grain mounts showed that a variety of chemical and physical diagenetic features exist in the sediments around the salt- sediment interface of the EI 128 salt dome. Tables 3.2 and 3.3 summarize this information., and the key to abbreviations is given in Table 3.1.

Table 3.1. Key to Table 3.2 and Table 3.3.

mg medium-grained og overgrowths fg fine-grained shr shear textures vfg very fine-grained acr accretion textures shy shaley ba barium-rich vshy very shaley sr strontium-rich ss sandstone fe iron-rich sltst siltstone Anh anhydrite sh shale Anl analcime Is limestone Brt barite cmt cement Cal calcite pcmt pervasive cement Cls celestite * data from Shell Offshore, Inc. HI halite corr corroded Kin kaolinite fcor framework grain corrosion Py pyrite ffrc framework grain fracturing Sp sphalerite frc extended fractures Qtz quartz fs-gtx floating sand-grain texture ps pseudomorph oct octahedral Ccp chalcopyrite lam laminated Po pyrrhotite salt diapiric halite Gn galena spor secondary porosity Chal chalcedony bx breccia 64

Table 3.2. Sidewall core summary for well 12ST.

Depth* lithology* diagenetic; minerals and textures m ft Anl Brt Cal Cls Py Sp misc. 1871 6139 1874 6149 sh X shr, acr 1875 6153 salt spor, corr Anh 1876 6155 vfg ss - vshy sltst sr X shr, acr 1878 6162 vfg ss, vshy sr X shr, acr 1878 6163 salt ba spor 1879 6166 salt & mud X acr, shr 1881 6171 salt X sr, ps spor 1882 6174 vfg ss, vshy sr, ps X acr, shr 1882 6176 vfg ss - vshy sltst X X frc 1905 6249 vshy sltst, X corr X 70%salt 2177 7143 vshy sltst X shr 2177 7144 vfg ss - vshy sltst X acic x acr 2228 7310 vshy sltst, X 20%salt 2232 7324 salt Anh 2697 8850 salt sr spor 2707 8882 sh & salt shr, acr, frc HI veins 2708 8885 vfg ss, vshy X X 2731 8959 vfg ss - vshy sltst X X X acr 2749 9018 sh X X X X acr, shr 2770 9089 sh X X Ccp 2792 9160 sh X 2803 9195 sh X X acr 2832 9290 vfg ss - vshy sltst fe oct Qtzog 2869 9413 salt s por, Anh & HI corr 2871 9420 vfg ss - vshy sltst X cmt 2874 9430 sh X X 3044 9987 vfgss cmt X cmt Gn, ffrc 3045 9989 vfg ss, shy lam X Chal 3048 9999 vfg ss, sshy cmt X cmt ffrc, fcor 3050 10008 vfg ss, sshy X ffrc 3100 10171 vfg ss, shy-sshy cmt X 3103 10182 vfg ss, shy cmt X Ti02, fcorr, ffrc 65

Table 3.3. Sidewall core summary for well 23.

Depth* lithology diagenetic minerals and textures m ft Anl Brt Cal Cls Py Sp misc. 1851 6072 vfg ss, shy - sshy cmt ffrc 1926 6320 fg ss, shy x 1941 6369 fg-mg ss, sshy cmt cmt spor, Ti02 FeS2 intergrowth, ffrc 2002 6568 vfg ss, shy - sshy pcmt X 2015 6611 vfg ss, sshy pcmt X minor s por 2030 6659 vfg-fg ss, sshy pcmt X fs-gtx 2046 6713 sltst, vshy Po.frc 2093 6866 sh X 2096 6876 shy Is Qtzog 2103 6898 fg ss, vshy sr X accr, frc 2266 7436 fg-mg ss, shy- pcmt X cal cmt corr, sshy spor 2272 7454 fg-mg ss, shy cmt ffrc 2284 7494 fg ss, shy-sshy X ffrc 2298 7538 fg-mg ss, sshy cmt cmt bx 2310 7580 fg-mg ss, sshy X ffrc 2371 7780 shy Is stylolites 2378 7802 fg Ss, shy-sshy pcmt X Mg-rich Cal cmt 2404 7886 fg ss, shy-sshy pcmt X

2493 8179 vfg-fg ss, shy- X Ti02 vshy 2503 8213 vfg-fg ss, shy- cmt ffrc vshy 2622 8603 sh x X Ba-rich K- spar 2744 9001 sh X

Identification of drilling mud contamination

Drilling mud contamination was discovered in some of the samples during backscatter-electron imaging (BSE) with the SEM. It was initially recognized by the texture of barite fragments floating in muds which occupy fractures and pores. This 66

texture is clearly recognized during backscatter-electron imaging because barite has a

bright contrast relative to most other minerals contained in the samples (Fig. 3.4). The

presence of this contamination became an important issue when it was realized that

samples from well 12ST and well 23 also contained significant amounts of diagenetic

barite that could be mistaken for the crushed barite that was used to weight drilling-

muds. Diagenetic barite was initially recognized in barite pseudomorphs after anhydrite

in salt samples, and subsequently as disaggregated pseudomorphs. Dismissal of all silt-

sized barite fragments in clay-rich matrices as contamination fromdrillin g muds would

Figure 3.4. BSE image of drill-mud contamination from well 12ST, 2697 m (8850 ft). The image shows bright grains in the upper fracture which are crushed barite in drilling mud. The intermediate gray zone in the lower fracture is a halite fracturefilling . 67 have mistakenly dismissed evidence of natural shearing, mixing, and accumulation of insoluble residues fromhalit e dissolution in the silt-rich and clay-rich sediments that are present at the margin of the salt. Hence, it was necessary to develop criteria to be able to distinguish natural barite and barite contaminants.

The larger barite fragmentsi n the drilling-mud are angular, equant, and range in size from approximately 10 um to 100 um in diameter. The drilling-mud contamination generally occupies clearly identifiable pre-existing fractures or pores, or fractures that may have been induced during the sidewall coring process (Fig. 3.4 and Fig. 3.5).

Figure 3.5. BSE image of drilling-mud contamination occupying secondary porosity in diapiric halite. Well 12ST, 1874 m (6149 ft). 68

In some cases, drilling-mud contamination was observed at the external boundary of the

sidewall cores. The barite used to weight the drill muds is compositionally distinct from

the diagenetic barite at EI 128. The barite used to weight the drilling muds at EI 128 is

relatively pure BaS04 and does not contain Sr in concentrations that can be detected by

X-ray EDS. In contrast, the diagenetic barite contains significant levels of Sr that are

evident in EDS spectra. Thus, the following three criteria can be used to identify barite

in drilling muds at EI 128:1) Sr concentrations below EDS detectability limits in the

barite, 2) mud contamination which occupies clearly identifiable fractures or pores, and

3) barite fragments that are homogeneously dispersed throughout a mud-rich matrix.

Clastic sediments

The samples fromwel l 12ST are either from within the salt or fromwithi n 20 m

(70 ft) of the salt-sediment interface (c.f. Fig. 3.3). In contrast, well 23 is situated further from the salt-sediment interface and the samples are fromwithi n a few meters of salt to

270 m (890 ft) fromsalt . Overall, the siliciclastic samples from well 12ST are finer grained than those from well 23.

Grain size, distribution, shape, and related modification of detrital grains by diagenesis Grain-size distributions in samples from well 12ST show that the modal size ranges from very-fine sands at 100 urn to clays at 2 um (Appendix B.l and B.3). The samples are poorly to very poorly sorted, and a few are polymodal. Most of the very- fine sands have grain-size distributions that are strongly skewed toward finer particle sizes, and most of the clay-rich samples have distributions that are skewed toward coarser particle sizes. The roundness and sphericity of grains within the samples are also 69

quite variable. Optical and SEM inspections reveal that the coarser samples contain

detrital grains that range from being well-rounded with a high degree of sphericity to

angular with a low degree of sphericity. In some cases, detrital grains that were

originally rounded to sub-rounded have been modified to smaller, very angular grains

by crushing (Fig. 3.6). Mineral particles produced fromcrushin g range in size from

Figure 3.6. BSE image of crushed-grain texture, average grain size reduction, and modification of detrital grain roundness to very angular. Well 12ST, 3048 m (9999 ft). The light area in the center is a mass of sphalerite that occupies an area of secondary porosity where detrital framework grains have been removed by dissolution. Secondary porosity is indicated because detrital sphalerite is not reported in Gulf Coast Tertiary sediments and because the area occupied by the sphalerite is grossly oversized for primary porosity. Note that unfractured sphalerite cement also embays fractured framework grains indicating a diagenetic origin for the crushed frameworkgrain s in this sample. 70

grains with maximum dimensions approaching the maximum detrital grain size to

particles in the tenths to hundredths of microns size range.

Most of the samples from well 23 have modal grain sizes that range from

medium sand at 0.25 mm, to medium silt at 15 um (Appendix B.2 and B.4). The well

23 samples vary from moderately to poorly sorted and have trends in skewness similar

to the well 12ST samples. The detrital grains range from well rounded to angular and

display an increase in angularity where framework grains have been crushed. In

sandstone samples with massive calcite cements, floating sand-grain textures and

oversized pores reveal that frameworkgrain s have been dissolved and aggressively

replaced by the calcite cements. Dissolution textures observed in the calcite cements of

some samples indicate that a later stage of partial dissolution has occurred, thus opening

cement-occluded primary and secondary porosity (Fig. 3.7).

The grain-size distributions in the samples do not simply reflect transport and syndepositional processes, but in many cases, also reflect the effects of chemical and physical diagenesis after burial. Alteration of grain-size distributions in the EI 128 samples results fromth e precipitation of large diagenetic minerals in fine-grained sediments and fromcrushin g of detrital frameworkgrain s in sands. Some textures attributable to crushing, however, may have occurred as a result of percussive sidewall coring. Ghost textures in some samples of crushed framework grains in barite and pyrite cements and the presence of unfractured cements embaying fractured frameworkgrain s in others (Fig. 3.6) indicates a diagenetic origin for at least part of the observed fracturing and crushing. Diagenetic fracturing and crushing of frameworkgrain s 71

apparently occurs as a result of stresses induced by salt diapirism and has been observed

around other salt domes (McManus, 1991).

Figure 3.7. BSE image from well 23, 2266 m (7436 ft) showing dissolution of massive calcite cements that have precipitated in both primary and secondary porosity. Secondary porosity is suggested here because of the oversized nature of the pore. The calcite cements apparently are synchronous with the framework grain dissolution event because they have arrested compaction and preserved the oversized pores in this sample and floating sand-grain textures in others. The presence of the calcite cements has prevented further crushing of the quartz grains seen here.

Those sandstone samples fromwel l 12ST and well 23 that were never cemented, or possibly have had massive calcite cements removed, display fracture textures attributable to intense crushing (Fig. 3.8). In the absence of cements, however, it is 72

difficult to ascertain if crushing was naturally induced, or if it is an artifact of percussive

sidewall coring. Crushing in the absence of cementation is also accompanied by the

compactive rearrangement of grains (Fig. 3.8). In cemented samples where normal grain

contacts are observed between detrital grains, massive carbonate cements probably pre

date the grain-crushing event. However, in the sands with massive calcite cements and

floating sand-grain textures, preferential dissolution of finely-crushed material may

Figure 3.8. BSE image of crushed sandstone from well 23, 2284m (7494 ft). This sample does not contain any evidence of calcite cementation, and according to the porosity data supplied by Shell, crushing has caused only a minor loss in porosity if the maximum porosity of this sample never exceeded the maximum of 31.2 percent reported for all samples. Porosity for this sample is 28 percent which represents a potential compaction loss of around 10 percent based on 31.2 percent primary porosity. 73

have occurred along with synchronous cementation by calcite cements. Where the

apparent dissolution of crushed materials has occurred, the absence of finemateria l

suggests that crushed detrital grains have been dissolved regardless of mineralogy.

Synchronous cementation during dissolution prevents further compaction and preserves

floating sand-grain textures (Fig. 3.9).

Figure 3.9. Image of floating sand-grain texture from well 23, 2030 m (6659 ft). The BSE image of the floating sand-grain texture and angular nature of many of the detrital grains suggests that crushing and subsequent dissolution of crushed fines is accompanied by synchronous cementation with calcite. The calcite cements are represented by the light background.

Mineralogy A detailed petrographic study of the detrital materials was not performed because the focus of the present research is to analyze for diagenetic minerals and textures in order to identify the physical and chemical diagenetic processes that operate 74 at the salt-sediment interface. The major detrital components are comprised primarily by quartz along with lesser quantities of K-feldspar, plagioclase feldspar, and lithic fragments. Ilmenite, magnetite, and monazite were detected by SEM-EDS as trace mineral phases. Point counts were not performed, but by visual estimation the sands appear to be similar in composition to the Miocene sands at the West Hackberry salt dome, Cameron Parish, Louisiana (McManus, 1991), and at the Black Bayou salt dome,

Cameron Parish, Louisiana (Leger, 1988). The sands at those fieldswer e described as subarkoses and sublitharenites at West Hackberry (McManus and Hanor, 1988), and as lithic arkoses and feldspathic litharenites at Black Bayou (Leger, 1988).

K-feldspar and plagioclase feldspar grains in most sand samples display various degrees of alteration textures. The alteration textures are from chemical corrosion and can be produced during sediment transport as well as during diagenesis after burial

(Scholle, 1979). In some cases calcite cements replace portions of detrital feldspars leaving partial ghosts of replaced regions giving direct evidence of diagenetic alteration.

In most cases, however, corrosion during transport could not be distinguished from diagenetic corrosion.

Porosity and permeability

The permeability of the samples as measured in air ranges from 0.3 mD to 990 mD for samples from well 12ST (Appendix B.l) and from0. 5 mD to 1150 mD for samples from well 23 (Appendix B.2). Porosity values range from 13.3 percent to 31.5 percent in samples from well 12ST and from 13.3 percent to 30.3 percent in samples 75

from well 23. Inspection of the porosity and permeability data reveals that the larger

permeabilities are associated with coarser samples.

Secondary porosity

Secondary porosity in the clastic sediments has developed primarily by

framework grain dissolution, cement dissolution, and fracturing(c.f . Fig. 3.7; Fig. 3.10).

Figure 3.10. BSE image of linear fractures and irregular fissures. The latter may represent accretion surfaces in the clay rich sediments. Well 23,2103 m (6898 ft). The linear fractures may have either a diagenetic origin from sediment deformation near the salt-sediment interface or a sampling origin from percussive sidewall coring. Dilation of fractures and opening of fissures along potential accretion surfaces may result from sample dessication and do not necessarily represent secondary porosity that continuously existed in the subsurface. 76

Secondary porosity from all three processes can be found in samples from all depths in both wells.

Framework grain dissolution is apparent from the oversized pores and floating sand grain textures that are present in calcite-cemented sands (c.f. Fig. 3.7 and 3.9).

These textures have been preserved by the calcite cements which indicates that the framework grains were aggressively replaced by the calcite cements. Thus, the secondary porosity developed on frameworkgrain s may not have initially contributed to an effective increase in the porosity of the bulk samples. The pervasiveness of the attendant calcite cements would cause a substantial decrease in porosity after the framework grain dissolution/ calcite cementation event occurred.

During later stages of diagenesis, some porosity was restored. The sample from well 23, 2030 m (6659 ft) is a fine-grained sand that is pervasively cemented with calcite producing substantially reduced permeability (c.f. Fig. 3.9). The permeability for this sample is reported to be 1.7 mD with 14 percent porosity. SEM observations of other similarly cemented samples with large porosities and permeabilities reveals that calcite cement dissolution has resulted in the opening of cement-occluded primary and secondary porosity. The sample from well 23,2266 m (7436 ft) (Fig. 3.7) is a good example of this. Dissolution of calcite cement is clearly present in this sample and apparently is responsible for its enhanced permeability (900.0 mD) and porosity (29.5 percent) compared with pervasively cemented samples that have not had their cements corroded. 77

Secondary porosity from fracturingi s present in many of the coarser, poorly cemented sandstone samples. However, it is not clear if all of the observed fracturing is attributable to natural crushing. In those samples that contain evidence for natural crushing (Fig. 3.6), reduction in intergranular volume and porosity can occur if compaction is not arrested (Houseknecht, 1987). The crushed sandstone sample from well 23, 74941 (Fig. 3.8) has a measured permeability of 330 mD and porosity of 28.0 percent, which suggests that the crushing and apparent compaction have not seriously degraded the porosity. This conclusion assumes that the porosity of the sample in Figure

3.8 did not exceed 31.2 percent before being crushed.

The abundance of clay minerals in most of the clastic samples from near the salt- sediment interface effectively occludes porosity in well 12ST. However, in some of the clay-rich samples, there is evidence of porosity along dilated fissures that is due either to fracturing or parting at accretion surfaces (Fig. 3.10). The dilation of fractures or parting surfaces in the sediments may be due in part to dessication and shrinkage of the samples after collection. In some cases diagenetic minerals such as barite and pyrite are found adjacent to the fissures (Fig. 3.11). Because of their proximity to the fissures,i t appears likely that they have precipitated from dissolved components in the fluidstha t moved through secondary porosity that existed in the subsurface along the fissures.

Small isolated pores can also be observed in the clay-rich samples (Fig. 3.12), but it is unclear whether or not these pores are interconnected and contribute to fluid transmission. In some cases, the presence of corroded diagenetic minerals in the clay- 78

rich sediments suggest that fluidsinfiltrat e the clay-rich sediments. However, the spatial

extent of such fluid migration through microporosity is unknown.

Figure 3.11. BSE image of diagenetic barite and pyrite with euhedral habit adjacent to a potential accretion or fracture boundary in a sample from well 23,2103 m (6898 ft). The pyrite and barite have precipitated in clay-rich sediments next to the fissure.Th e occurrence and size of the pyrite suggests that fluids have migrated along secondary porosity that existed along the fracture, transporting the components of the barite and pyrite. This image is an enlargement of the boxed area in Figure 3.10. 79

Figure 3.12. SEI image of micro-pores and barite, well 12 ST, 1882 m (6176 ft). The small pores (<2 um) can be seen to the left of the large, corroded diagenetic barite in the clay-rich sediments. It is unclear if this porosity supports fluid transmission, but the dissolution of the barite crystal suggests that fluids corrosive to barite have moved through the sediments. The dark, homogeneous areas in the barite are epoxy.

Mechanical deformation Inspection of the thin-sections by SEM revealed rotational textures which suggest that shearing of the clay-rich sediments has occurred (Figs. 3.13 and 3.14).

Diapiric salt moves upward relative to flank sediments (Murray, 1966), and it is likely that the rotational textures result from strain in the sediments at the margin of the dome. 80

mmssMwz Figure 3.13. SEI image of chloritized detrital biotite with bent ends that are consistent with post-depositional shearing of sediments. Well 12ST, 1905 m (6249 ft).

Figure 3.14. BSE image of barite fragments outlining accretionary snowball texture. Rotation is likely due to shearing of sediments. Well 12ST, 1876 m (6155 ft). The cleavage fragments of barite pseudomorphs after anhydrite are derived from the margin of the salt and have been released as an insoluble residue fromth e dissolution of diapiric salt. The dissolution residue apparently plates onto finegraine d siliciclastic sediments at the margin of the diapir in a zone of accretion and shear. 81

Halite Crystal size

A meaningful upper size limit on halite crystals within diapiric salt at EI 128 cannot be made because of the small size of the samples available. The largest halite crystals in the sidewall cores greatly exceed 1 cm in diameter, extending beyond the edge of the prepared samples. Isolated remnant grains of halite mixed with siliciclastic sediments are occasionally encountered, but they were derived from larger crystals of halite by dissolution (c.f. Fig. 3.16, well 12ST, 1881 m (6171 ft)).

Composition

The halite in the sidewall cores varied from colorless and clear, to cloudy white and nearly opaque. Random portions of halite were analyzed by EDS and found to be pure NaCl. EDS spectra did not reveal compositional evidence for potash facies salts such as sylvite, or for bromide or iodide substitution for chloride.

Bromide, the most significant anion that substitutes for chloride in halite, is useful as an indicator of halite recrystallization because bromide is strongly partitioned into the aqueous phase in such reactions (Carpenter, 1974). Because of the petrographic evidence of fluid interaction with the halite at EI 128, bromide is of particular interest.

EDS analysis, however, is not an adequately sensitive technique for determining bromide concentrations in halite. Goldstein et al. (1992) state that typical elemental detection limits for EDS analysis are in the range of 0.1 wt % (1000 ppm). Bromide concentrations in a core of the Louann Salt from southwestern Alabama range from a low of 48 ppm Br to a high 812 ppm (Eustice, 1990). Land et al. (1988b) report Br 82

concentrations for the Louann salt throughout the Gulf Coast sedimentary basin above a

depth of 3 km to range from 10 ppm to 290 ppm and less than 60 ppm in diapirs. Hence,

evaluation of bromide concentrations will need to be approached by other means of analysis.

Porosity in halite

Textural evidence in most halite samples fromth e margin of the salt dome reveals that dissolution of halite has formed secondary porosity in the margin of the diapir (Fig. 3.15). Because the samples were prepared under oil, it is unlikely that the secondary porosity is an artifact of sample preparation.

Figure 3.15. BSE image of well-developed secondary porosity from halite dissolution in a sample from well 12ST, 1874 m (6149 ft). 83

The degree of dissolution that the diapiric halite has undergone can also be

deduced fromth e samples. A complete spectrum of increasing secondary porosity is

found in the halite samples which ranges fromn o observable secondary porosity to

nearly complete removal of halite. Figures 3.15 and 3.16 respectively represent portions

of the sequence of increasing dissolution. Figure 3.15 shows an early stage of

dissolution where halite has been dissolved leaving secondary porosity, and Figure 3.16

shows a later stage of dissolution in which halite has been dissolved to the degree that it

has been left as isolated remnant grains.

Figure 3.16. BSE image of halite and barite residua from salt dissolution mixed with fine-grained siliciclastics at the diapir margin. Well 12ST, 1878 m (6163 ft). 84

The halite in Figure 3.17, which contains barite pseudomorphs after anhydrite, has undergone substantial dissolution by formation waters. The corroded halite crystals are residua fromdiapi r dissolution and are apparently being mechanically mixed with fine-grained siliciclastic sediments at the salt-sediment interface. The persistence of corroded halite in the siliciclastic sediments suggests that the formation waters at the salt-sediment interface are close to saturation with respect to halite.

Figure 3.17. SEI image of barite pseudomorphs after anhydrite in halite. The halite is being mechanically mixed with siliciclastic sediments. Well 12ST, 1881 m (6171 ft). EDS spectra fromth e small labeled portion of the large barite pseudomorph indicate that the barite is compositionally zoned and contains substantial amounts of Sr (see Fig. 3.22 for spectrum 1,2, and 3). The rectangular patterns from charging effects on the halite highlight accidentally-induced features frompreferentia l dissolution of halite along strained lattice planes. See text. 85

The rectangular features on the surface of the halite in Figure 3.17 have been

caused by preferential etching and subsequent surface charging effects which were

accidentally induced during the removal of dust fromth e surface of the polished thin

section with a freon propellant. It is likely that these features developed as a result of

strain in the halite lattice which left these regions less stable and more subject to

dissolution than other regions within the halite crystal.

Anhydrite, celestite, and barite

Anhydrite, celestite, barite, and barite-celestite solid solutions comprise the

sulfate minerals found in well 12ST and well 23 (Table 3.2 and 3.3). In the samples

examined, anhydrite, celestite, and Ba-rich celestite are found only in well 12ST,

whereas barite and Sr-rich barite are found in both wells.

Euhedral and subhedral crystals of anhydrite and celestite, and celestite-barite pseudomorphs after anhydrite are present in the bulk evaporites forming the salt dome.

The sulfate minerals are randomly distributed, and by visual estimation comprise less than one percent to approximately five percent of the bulk salt by volume in the sidewall cores.

Morphology

Euhedral lath-shaped crystals of anhydrite were found in halite samples that have undergone little apparent interaction with formation waters. Under crossed polars, unaltered anhydrite typically displays high second-order colors in randomly oriented crystals in the thin sections. Barite-celestite pseudomorphs of the lath-shaped anhydrite crystals were also found in the samples of diapiric halite, but these occur in halite that 86 has evidence of increased interaction with formation waters in the form of secondary porosity caused by the dissolution of halite. The pseudomorphs of barite after anhydrite range in size from 30 um to over 1 mm in length (Fig. 3.17 and Fig. 3.18) and display first-order whites and grays in randomly cut thin sections under crossed polars.

Figure 3.18. BSE image of barite pseudomorphs after anhydrite accreted to fine-grained sediments at the margin of salt. The barite is left as an insoluble residue fromevaporit e dissolution. Sample 12ST, 1876 m (6155 ft).

Diagenetic barite not formed by the pseudomorphic replacement of anhydrite was also found in both wells. Well-formed euhedral crystals of Sr-rich barite were found in the silt and clay-rich sediments of wells 23 and 12ST (c.f. Fig. 3.10 and Fig. 87

3.12). Barite is also found as vein and porosity filling cements in the coarser siliciclastics from well 12ST (Fig. 3.19), and in well 23. The textures along the barite- mineralized fracture shown in Figure 3.19 suggest that frameworkgrain s have been aggressively replaced by barite because there appears to be substantial void space filled

Figure 3.19. BSE image of barite and sphalerite vein-filling and grain-replacing cements in a sandstone from well 12ST, 3048 m (9999 ft). by the barite that cannot be attributed to simple dilation of a fracture.Simila r replacement textures are seen with barite cements in well 23 at 1941 m (6369 ft). Ghosts of fractured and replaced grains are common in the barite cements. 88

The occurrence of celestite is rare in the samples. Diapiric halite from well

12ST, 1878 m (6162 ft) contains celestite as diagenetically formed crystals precipitating

in close spatial association with anhydrite (Fig. 3.20). Anhydrite laths in this same

sample contain replacement zones of celestite. In one unusual instance, acicular celestite

crystals were found within fine-grainedsiliciclastic s near the salt-sediment interface

(Fig. 3.21). The acicular crystals are approximately 5 um thick, and 25 um to 100 um in

length.

Figure 3.20. BSE image of celestite precipitating next to anhydrite and secondary porosity fromhalit e dissolution (black areas). Well 12ST, 1878 m (6162 ft). 89

2 ST. 2178m (7143 ft) j, .. 50 um

<*.^*zr ^>" \.***#»**"

Acicular "celcsiiii «ws»»**

Figure 3.21. BSE image of diagenetic celestite crystals with acicular habit in fine grained siliciclastic sediments. Sample 12ST, 2177 m (7143 ft).

Composition

Pure, end-member diagenetic barite, diagenetic celestite, and evaporitic anhydrite occur, but pure phases appear to be the exception. This is especially true of the pseudomorphic replacements of diapiric anhydrite. Most of the pseudomorphs are barite that contain substantial amounts of Sr. Based on halite dissolution textures, the barium content in the barite pseudomorphs increases at the expense of strontium with increasing degrees of halite dissolution. Anhydrite, anhydrite with celestite replacement zones, or celestite and anhydrite are found where minimal dissolution of halite (< two 90 percent by volume by visual estimation) is evident in the form of sparsely scattered solution channels in the salt (Fig. 3.20), or none at all. With increasing degrees of halite dissolution, substantial solid solution develops between celestite and barite in the pseudomorphs, and anhydrite is no longer found. Evidence for a broad range of solid solutions is found in the gradational compositional zoning that is present in some barite samples. Variations in peak-heights for Ba and Sr on EDS spectra suggest that nearly subequal amounts of Ba and Sr apparently are present (Fig. 3.22, Spectrum 3), as well as spectra which suggest the presence of barite or celestite with compositions that are closer to one or the other end-members (e.g. Fig. 3.22. Spectrum 1). In the final

Figure 3.22. EDS spectra of zoning in barite, Well 12ST, 1881 m (6171 ft)(Fig.3.17) . The variation in the peak heights suggest that Ba and Sr are present in nearly subequal amounts in portions of the zoned areas. These spectra were taken fromth e ends and middle of a six um transect on a zoned portion of the large barite pseudomorph in Figure 3.17. See Figure 3.17 for location. 91 stages of dissolution, where the majority of the halite has been removed, barite with minor amounts of Sr is typically found.

Not all diagenetic barite at EI 128 contains detectable Sr by EDS. For example, the barite vein-filling cement shown in Figure 3.19 (well 12ST, 3048 m (9999 ft)) is pure BaS04 within EDS detection limits. The bulk sample is a very fine sand that probably is located outside of the shale sheath and salt-sediment interface (see cross section, Fig. 3.3).

Textural relations of sulfate minerals with surrounding sediments

Barite, celestite, and anhydrite are found in a variety of textural relations with the evaporites and siliciclastic sediments in the samples. Evaporitic anhydrite is almost exclusively found within the diapiric salt where there is minimal secondary porosity.

With increasing development of secondary porosity in halite, anhydrite is pseudomorphically replaced by barite-celestite solid solutions. Continued development of secondary porosity in halite at the salt margin eventually produces a residue of halite and barite fragments mixed with siliciclastic sediments (Fig. 3.16). With further development of secondary porosity in halite, all halite is removed, resulting in a mechanical mixture of insoluble barite and siliciclastic sediments in accretionary textures.

Accretionary textures in the samples are manifested by irregular layering which in some samples has subsequently been modified almost beyond recognition by mechanical processes. The accretionary textures can be attributed to the accumulation of insoluble residues from the halite at the margin of the diapir. The insoluble residues are 92

comprised primarily by intact or fragmentedbarit e pseudomorphs after anhydrite. The

accretion of barite does not build a continually thickening zone of barite, as anhydrite

does at the base of salt dome caprock. The accretionary textures are highlighted during

backscatter electron imaging in the SEM by the high contrast of the barite

pseudomorphs that have plated onto linear or curvilinear boundaries in the siliciclastic

sediments, or as plating textures of pseudomorph fragmentso n clasts of siliciclastic

sediments that have been rotated (Fig. 3.14). Intact pseudomorphs in the fine-grained siliciclastics typically have their long axes aligned parallel with the accretion surfaces

(Fig. 3.18), and where pseudomorphs have been fragmented, they outline (Fig. 3.23)

Figure 3.23. BSE image of regular accretion boundary containing fragmentedbarit e and no halite. Well 12ST, 2802 m (9195 ft). 93

former accretion surfaces and are heterogeneously distributed through the depth of the

accretion zone. Accretion zones containing fragmentedbarit e pseudomorphs after anhydrite can be confused with drilling-mud contamination. Accretion zones up to 5 mm thick are found in some samples. The layering textures in samples that have not been strongly affected by shear are suggestive of the accretionary textures and relict layering textures inherited from the underplating process observed in samples fromsal t dome caprocks (e.g. Fig. 59 in Murray, 1966; various figuresHann a and Wolf, 1938; and Fig. 5, Hallager et al., 1990).

The accretion residues apparently form from the dissolution of halite by formation waters. Barite, which is relatively insoluble compared to halite, is left as an insoluble residue. The mechanical mixing of the accreted residues with siliciclastic sediments may be attributable to shear.

Anhydrite dissolution

Figure 3.24 shows secondary porosity developed on anhydrite in halite, and remnants of the anhydrite. The remnants do not contain Ba or Sr detectable within the limits of EDS, suggesting that the anhydrite was simply dissolved without an intermediate stage of pseudomorphic replacement by barite or celestite.

Barite dissolution

Some euhedral barite crystals of diagenetic origin (Fig. 3.12) show evidence of corrosion. This indicates that pore fluid compositions vary with time and that they may alternate between undersaturation and oversaturation with respect to barite. 94

Figure 3.24. BSE image of corroded anhydrite. Well 12ST, 1874 m (6149 ft).

Sulfides Pyrite

Pyrite is present in all samples from well 12ST and well 23 except in those samples consisting exclusively of diapiric evaporites. The pyrite typically has framboidal morphology, but small (< 1 um), isolated cubic forms are also encountered.

The small cubic forms also coalesce to form larger, irregular masses of pyrite. The framboidal and cubic forms are present in the silty clays and within the tests of microfossils (Fig. 3.25). The framboids attain maximum diameters exceeding 20 um 95

and occasionally coalesce to form large, irregular clusters in the clay-rich matrix. Large

euhedral forms of pyrite greater than 100 um in diameter are found in silty clays from

well 23,2103 m (6898 ft) and are shown in Figure 3.11. Small pyrite octahedra

Figure 3.25. BSE image of pyrite framboids and analcime in microfossil test, sample 23, 2622 m (8603 ft). approximately 8 um to 10 um in maximum dimension are found in the pore spaces of a sandstone sample from well 12ST, 9290' (2832 m) in association with iron-rich carbonate cements and overgrowths (Fig. 3.26).

Diagenetic pyrite in the sands from both wells has been found as localized cements contained within intergranular porosity and as cements occluding secondary 96 porosity. In cases, pyrite cements in secondary porosity have ghost replacement textures which indicate that the fluids which were corrosive to framework grains were also responsible for precipitating the pyrite. In well 23, intimate anhedral intergrowths of pyrite with an iron-rich Ti02 phase were found (Fig. 3.27).

Figure 3.26. BSE image of octahedral pyrite and Ca-Fe carbonate overgrowth (probably ankerite) on dolomite in a sandstone from well 12ST, 2832 m (9290 ft).

Sphalerite Sphalerite was found only in well 12ST in three samples below a depth of approximately 2743 m (9000 ft). It is present as a fracture and void filling cement and 97

has a granular texture. Figures 3.6 and 3.19 are from well 12ST, 3048 m (9999 ft) and

show sphalerite occupying secondary porosity formed by dissolution of framework

Figure 3.27. BSE image of complexly intergrown diagenetic pyrite and Ti02 in a sandstone sample from well 23,1941 m (6369 ft). silicates and aluminosilicates. The embayment of silicate grains observed in Figure 3.6 also indicates that the frameworkgrain s were aggressively replaced by the sphalerite.

Galena, chalcopyrite, and pyrrhotite

Galena, chalcopyrite, and pyrrhotite are trace mineral phases and were rarely encountered. Galena was found only in well 12ST in a sample from304 4 m (9987 ft), which also contains sphalerite as isolated pore-fillings. Chalcopyrite was found in well 98

12ST at 2770 m (9089 ft) as an isolated mineral in a mudstone. Pyrrhotite was found in

a sample from well 23 at 2046 m (6713 ft) in a mudstone in which no other sulfide

minerals were present.

Carbonates

Carbonate diagenetic minerals are present in samples from both wells as massive, porosity-occluding cements (Fig. 3.9) that are similar to the late stage calcite cements reported by McManus and Hanor (1988). Carbonate also occurs as overgrowths

(Fig. 3.26) and as grain replacements in secondary porosity (see Fig. 3.7 and Fig. 3.9).

EDS analyses of the massive carbonate cements show that they primarily are CaC03 , but samples from well 12ST, 2832 m (9290 ft) contain Fe-rich CaC03 overgrowths

(probably ankerite) and rim cements. Carbonates from well 23, 2378 m (7802 ft) contain massive calcite cements with zones of Mg-rich calcite and evidence of aggressive replacement indicated by extreme floating sand-grain textures. Typical calcite cements in the samples are sparry and have textures that range from very-finely crystalline with equant habit to poikilotopic habit.

Analcime

Diagenetic analcime is found in EI 128 samples as precipitates filling intragranular primary porosity within microfossil tests. Pyrite is commonly found in association with the analcime. Figure 3.25 is a backscatter electron image of a typical microfossil containing both analcime and pyrite. It is not clear if there is a genetic relationship between the two phases. It is likely, however, that the pyrite in the fossil tests formed soon after burial (McBride et al, 1988). In contrast, the analcime probably 99

formed much later in response to the destruction of unstable aluminosilicates and high

Na activities in pore waters close to the salt dome. Thus it is likely that the analcime is

not genetically related to the pyrite in the tests, but simply precipitated in the available

primary intragranular porosity that was partially occupied by early pyrite. The analcime

at EI 128 is Si-rich and has the following average composition by electron-microprobe

analysis: Na13 gAli4 |Si33 g096»nH20. For comparison, ideal analcime has the

composition NaI6Al16Si32096.nH20. Si-rich analcime has also been reported at the West

Hackberry salt dome (McManus, 1991).

Other diagenetic phases

Quartz overgrowths, chalcedony, and Ti02-bearing phases constitute the other

minor diagenetic minerals found in the samples. Both diagenetic and detrital minerals

containing Ti02 are encountered in most samples, and are present only in trace

quantities. Diagenetic quartz and chalcedony are present in insignificant quantities. At

EI 128, the primary diagenetic Ti phase is Ti02, but the specific Ti02 polymorph is

uncertain (D. Henry, person, coram.). In some cases it occurs as doubly terminated

euhedral crystals and in other cases as complex anhedral intergrowths with pyrite (see

Fig. 3.27). Fe-oxides or Fe-oxy-hydroxides are also present in some samples.

Formation water compositions

Formation water analyses fromE I 128 of produced waters (Appendix B.5) had been analyzed primarily to evaluate the potential for mineral-scale formation and corrosion in production equipment. As such, the analyses do not cover a comprehensive suite of dissolved species and are not consistent from analysis to analysis in terms of species reported. However, useful observations on compositional relations can be

derived from the data.

Dissolved calcium versus depth reveals that there is an increasing trend with

depth for Ca (Fig. 3.28). Ca (mg/L) 3000 5000 7000 9000

0 1— 1

1000 -*- EI 128 £ 2000 - t>> OH . *9^ —6 ° 3000 & K^—^ 4000

Figure 3.28. Ca versus depth for wells on the southeast flank of the EI 128 dome. Ca appears to increase with depth.

For Ca versus CI (Fig. 3.29), Ca covaries with chloride. Hanor (1994a) has shown that the concentrations of dissolved Ca, Mg, and Sr in brines from many sedimentary basins increase with increasing concentrations of chloride. When such data are plotted on log- log plots of Me versus CI, where Me represents divalent cations, a 2:1 slope commonly develops relative to chloride. Figure 3.29 contains an arbitrarily placed 2:1 101 reference line on the log-log plot which reveals that dissolved Ca in the formation waters at EI 128 follows the general 2:1 trend shown by Hanor (1994a) for other formation waters. Hanor (1994a) relates such compositional trends to buffering of formation water compositions by multi-phase silicate-aluminosilicate-carbonate mineral assemblages. /4 A / J; IOOO A EI 128

U — 2:1 reference line

100 - h V 1000 10000 100000 1000000 CI (mg/L)

Figure 3.29. Ca versus CI for formation waters on the southeast flank of EI 128. Note that Ca increases with a 2:1 slope relative to CI on the log-log plot. This behavior for major divalent cations in formation waters has been attributed to multi-phase silicate-aluminosilicate-carbonate buffering of pore fluids (Hanor, 1994a).

Mg In contrast to Ca, Mg shows a decreasing trend with depth (Fig. 3.30), and no clear trends with CI (Fig. 3.31). Mg (mg/L) 400 600 800 1000 1200 1400 0

1000

-B 2000

° 3000

4000 Figure 3.30. Mg versus depth showing decrease of dissolved Mg with depth.

L._. A EI 128

— 2:1 reference B, 1000 i // ft line

/ A A

100 £ 1 10000 100000 1000000 CI (mg/L) Figure 3.31. Mg versus CI for formation waters on the southeast flank of EI 128. The Mg concentrations do not follow the 2:1 trend as Ca does and instead disperse vertically. This suggests that Mg concentrations relative to chloride are controlled by halite dissolution rather than by coupled reactions. This may be related to slower kinetics for reactions coupled to halite dissolution which control Mg concentrations. 103

This observation suggests that the control on Mg concentrations is not through buffering by a multiphase mineral assemblage, but that its concentration is controlled by other processes.

Alkalinity

Alkalinity appears to increase with increasing depth (Fig. 3.32), and based on the vertical distribution of data, alkalinity relative to CI (Fig. 3.33) appears to be independent of CI if the two data points for low salinity waters are not considered.

Total Alkalinity as mg (HC03")/L 0 200 400 600 800

0 1 h- 1 -*-EI128 I 1000 £ 2000

Q ^— 3000 A^S^A-A

4000

Figure 3.32. Alkalinity versus depth for formation waters in flank sediments at EI 128. Although the trend is somewhat obscured by the scatter of the data, alkalinity appears to increase with depth. 104

If the two data points are included in the interpretation, alkalinity would appear to

decrease with increasing concentrations of CI. Additional data is needed. The trend of

decreasing alkalinity with increasing CI would be consistent with the observations of

Hanor (1994a) who pointed out that total alkalinity decreases with increasing total dissolved solids (TDS) in formation waters from sedimentary basins. Dissolved Na+ and

CI' are the major species responsible for the increases in TDS in chloride dominated formation waters where chloride concentrations range between 10,000 mg/L CI" and halite saturation (ca. 350,000 mg/L CI") (Hanor, 1994a). Thus, plots of alkalinity versus

CI should show parallel trends to those of total alkalinity versus total dissolved solids as used by Hanor (1994a) for similar formation waters.

10000

CO /-_N fr • 1-H d i a CO *c3 O M U < £ 50 •a H 10000 100000 1000000 CI (mg/L)

Figure 3.33. Alkalinity versus CI for formation waters in flank sediments at EI 128. The overall trend for alkalinity versus CI reveals that alkalinity decreases with increasing salinity. This trend is consistent with those presented by Hanor (1994a) for pore fluids that are being thermodynamically buffered by multi-phase mineral assemblages. Ba and sulfate The barium and sulfate data are presented together in this section because they

are the primary components of barite, and barite is a diagenetically important phase at

the salt-sediment interface. Both the Ba data and sulfate data have numerous analyses

that were below detection limits and are denoted "bdl" in Appendix B.5. Only three

analyses have both Ba and sulfate reported.

-3 A DI uo uctia. \A barite stability + -4 %. A line 75° C, Psat

W) Mo -5

1 I X -6 2- logtfS04

Figure 3.34. Log activity of Ba versus log activity of sulfate calculated in SOLMINEQ.88 (Kharaka et al, 1988) for formation waters in three wells in the flank sediments at EI 128. The barite stability line has been calculated fromdat a presented in Bowers et al. (1984) for 75° C, Psat.

The data for the three wells (Appendix 3.5) were entered into SOLMINEQ.88 (Kharaka

24* 2 et al, 1988) in order to calculate activity values for dissolved Ba and dissolved S04 ".

The plotted activity data suggest that the formation waters in these three wells are saturated with respect to barite. The presence of stable diagenetic barite in many of the

samples is consistent with this result. DISCUSSION The salt-sediment interface on the southeast flank of the EI 128 salt dome is a

complex zone where both physical and chemical diagenetic processes operate. The

physical processes include: crushing and fracturingo f framework grains, fracturing and

brecciation of lithified sediments, and mechanical mixing of poorly-consolidated

siliciclastic sediments with insoluble residues derived from dissolution of evaporites in

the diapir. The chemical processes include: halite dissolution, anhydrite dissolution, pseudomorphic replacement of anhydrite by Ba-Sr sulfates, precipitation of vein-filling barite cements, precipitation of analcime in carbonate fossil tests, generation of secondary porosity in the diapir by dissolution of halite, generation of secondary porosity in the clastic sediments by dissolution of framework grains, precipitation of pervasive calcite cements, and precipitation of pyrite. The following sections will discuss the various physical and chemical processes as they are related to diagenesis at

EI 128.

The cement textures in some samples reveal that crushing and shearing were produced during diagenesis. Grain crushing has important implications on chemical diagenesis around the EI 128 salt dome and will be discussed first.Th e section on mechanical diagenesis will be followed by sections on chemical diagenesis in the evaporites and chemical diagenesis in the siliciclastic flank sediments, both of which are based on the textural and compositional relations in the samples presented in the results. 107

Mechanical controls on diagenesis Crushing of frameworkgrain s is important because there are important geochemical aspects to crushing that serve to enhance reaction rates in the diagenetic environment. The presence of floating sand-grain textures with very angular grains in calcite-cemented sandstones, which apparently have developed at the expense of finely- crushed minerals, suggests that crushing-fines from quartz, feldspar, and other detrital grains are dissolved more rapidly than larger grains of identical mineralogy.

Effect of crushing on reaction rates

The effect of grain crushing on reaction rates can be considered by inspecting the rate equation for silicate hydrolysis formulated by Aagard and Helgeson (1982) and

Helgeson et al. (1984) (Equation 3.1)

nH+ r = (d$)/(dt) = *k(aH+y (3.1) where

r = rate at which reactant mineral is destroyed 4 = reaction progress variable t = time s = effective surface area k = rate constant aH+ = activity of hydrogen ion nH+ = exponent of hydrogen ion activity in solution

The rate equation is highly dependent on the effective surface area s, and increasing the effective surface area as a result of crushing will increase the rate of reaction when all else is held equal. Effect of crushing on stability of geochemical system The effects of grain crushing on the stability of the reacting system can be

considered fromth e standpoint that grain crushing alters the Gibbs Free Energy of the

system, Gsys. At equilibrium, Gsys attains a minimum value and

1988). In a closed system that is capable of reaction which contains a single mineral

phase in contact with an aqueous phase, free energy is associated with the bulk mineral

phase, the bulk aqueous phase, and the solid-liquid interface. (Equation 3.2)

'-'mineral "•" '-Jaq "•" '-'surface — ^sys (V'-v

The energy that is associated with the solid-liquid interface in the system at

equilibrium is proportional to the surface area of the solid phase. Thus in order to achieve thermodynamic equilibrium in the system, the surface area of the solid phase must be minimized relative to the volume of the solid phase that is present in the system.

When mineral grains less than 1 mm in size are crushed and finelydivide d into numerous smaller particles, the numerical value of the ratio, R, of the mineral surface area, S, to the mineral volume, V, increases substantially for each individual particle derived fromcrushin g (Stumm and Morgan, 1981). This effect can be demonstrated for a hypothetical cubic solid where S = 6d2, V = d3, and where d is the length of a cube edge. The effect of crushing is to decrease d and increase R for each individual particle derived from the original grain by crushing (Fig. 3.35). If solid mass is conserved during crushing and no reactions occur, the increase in the surface area relative to the

2 2 original particle is 6nd /6d =n, where n is the number of times the original cube

dimension, d, has been subdivided. Because the surface energy of a mineral grain is proportional to its surface area, it is clear from Figure 3.35 that particles less than 1 um

in size will have large components of surface energy, Gsurface, relative to the free energy associated with the bulk solid phase in the grain, Gmincra|. Crushing will result in an increase in the overall Gibbs Free Energy for the system when summed over all particles in the system that were derived fromth e original particle. Hence, when crushing occurs the system will no longer be at equilibrium, and it should react in order to restore equilibrium by reducing the value of R back to the equilibrium value. Mass I »—i O >

3 CO

& 0 12 3 cube dimension, d (um)

Figure 3.35. Graph of the surface area to volume ratio for a cube as a function of d. Note d is the length of a cube edge, R = 6/d and that the surface area to volume ratio increases dramatically as the particle dimensions drop below 1 um. 110

redistribution in a closed, reacting system with a single mineral phase in contact with an

aqueous phase, is accomplished by preferential dissolution of the smallest mineral

fragments because they react quickly, and subsequent precipitation causes fracture healing and grain enlargement.

Natural systems in which crushing occurs, such as in the salt dome flank environment, are complex and may behave as open systems, closed systems, or either at different times. If the system is open to the transport of dissolved mineral components into and out of the system in solution, the effects of crushing will vary depending on the saturation state of the fluid with respect to a given mineral phase. If the fluidmovin g into the open system is supersaturated with respect to a given mineral phase, precipitation of new mineral will occur to minimize the mineral surface area to volume ratio in the system, thus reducing the Gibbs Free Energy of the overall system. Because solid mass is being added in the open system, porosity will be lost. At EI 128, euhedral quartz overgrowths are observed in samples from well 12ST at 2832 m (9290 ft) and well 23 at 2096 m (6876 ft). Euhedral carbonate overgrowths are observed in well 12

ST at 2832 m (9290 ft). At the Iberia field (Chapter 4, this dissertation) euhedral quartz and analcime overgrowths are observed on detrital grains. Porosity loss occurs where massive calcite cements have precipitated in EI 128 samples, although these are not monomineralic grain enlargements.

If the fluid passing through the open diagenetic system remains at saturation for a given mineral phase, local dissolution/reprecipitation reactions may occur similar to Ill

the closed system in a thermodynamically-driven attempt to redistribute mass to

minimize the surface area to volume ratio which also serves to minimize Gsys.

When the solution moving through the open system remains undersaturated with

respect to the mineral phase, dissolution will preferentially take place at the expense of

the smaller particles at enhanced rates because of their higher effective surface area for

reaction. This can be observed at EI 128 in the calcite cemented sandstone samples (Fig.

3.7, sample 23,2266 m (7436 ft); Fig. 3.9, sample 23,2030 m (6659 ft)) where floating

sand-grain textures occur and where little apparent chemical attack has affected the

larger grains. The barite and pyrite cements with ghost textures of fractured grains also provide evidence for this enhanced dissolution process.

Further aspects of mechanical diagenesis

Sample 12 ST, 3048 m (9999 ft) (Fig. 3.19) is a crushed and fractured sandstone in which barite and sphalerite have subsequently precipitated as localized cements. A large fracture in this sample apparently has conducted mineralizing fluids, and those fluids have dissolved frameworkgrain s and simultaneously replaced them with vein- filling barite cements as evidenced by ghost grain textures in the cement. Sample 12ST,

2697 m (8850 ft) (Fig. 3.4) contains a dilated fracture filled with halite cement. This type of petrographic evidence suggests that fractures play an important role in diagenesis around salt domes both from the standpoint of fluid transmission and from mass balance considerations because dissolution-precipitation reactions occur along the fractures with mass being transported in the fluids. Mass transport in fluids can be significant around salt domes. For instance, McManus and Hanor (1993) estimate that 112

0.8 km of the Louann evaporites have been dissolved in order to supply enough Ca

from the dissolution of anhydrite in order to account for the volumes of calcite cement

encountered in the flank sediments around the West Hackberry salt dome. By their

estimates this requires a fluid volume / pore volume ratio of at least 250:1. Fracture sets

thus offer fluid transmission pathways in addition to intergranular porosity.

Samples fromnea r the salt-sediment interface show textures fromsedimen t

shearing and physical mixing of halite and siliciclastics (Fig. 3.14 and Fig. 3.16).

Kinematic analysis of salt dome evaporites has shown that mechanical distortion of salt

occurs by differential upward plastic flowo f salt within salt domes (e.g. Kupfer, 1962).

The upward flowo f salt relative to the surrounding sedimentary formations also induces

shear stresses at the margin of the diapir. Because of frictional coupling between the salt and the enveloping sediments, shear stresses that act upon the materials in the salt- sediment interface must affect the halite in the margin of the diapir as well. However, the effects of this stress are not as apparent in small thin sections of diapiric halite as they are in the thin sections of sandstones and mudstones fromth e flank sediments near the margin of the salt.

Diagenesis within the evaporites and salt-sediment interface Chemical diagenesis

Affects of chemical diagenesis within the evaporite samples are more apparent than from physical diagenesis. Halite develops secondary porosity, and laths of diapiric anhydrite are pseudomorphically replaced by Ba-Sr sulfates. The Ba-Sr sulfate pseudomorphs after anhydrite are variable in composition and become increasingly Ba- 113

rich with increasing amounts of secondary porosity in the halite. Anhydrite is first

replaced by celestite, then by Ba-rich celestites, then by Sr-rich barites, and finally by

low Sr-barite. This sequence of minerals in relation to secondary porosity in the halite

implies that the Ba and Sr responsible for forming the pseudomorphs are carried as

dissolved components in the pore-fluids which are dissolving the halite. Sr

concentrations in the Louann Salt have been reported to be in the 40 ppm to 100 ppm

range for bulk salt (Eustice, 1991), thus part of the strontium in the sulfates may

actually be derived fromwithi n the diapir. The sequence of Ba-rich celestites altering to

Sr-rich barites suggests that the Sr/Ba ratio in the formation waters decreases with time.

Zoned portions in the Ba-Sr sulfates supply further evidence of a temporal evolution of pore fluid Sr/Ba ratios. The following mass action equations can be written to describe these diagenetic reactions

Halite dissolution, formation of secondary porosity is given by

NaCl->Na+ + Cr (3.3)

Celestite replacing anhydrite is given by

2+ 2+ CaS04 + Sr -> Ca + SrS04 (3.4)

Barite replacing celestite pseudomorphs is given by

2+ 2+ SrS04 + Ba -> Sr + BaS04 (3.5)

Adding equations 3.4 and 3.5 describes the overall net reaction for anhydrite replacement that is observed in the samples fromE I 128 114

2+ 2+ CaS04 + Ba ->BaS04 + Ca (3.6)

Equilibrium constants were calculated with data from Bowers et al. (1984) for

1-76 2 83 4,59 reactions 3.4, 3.5, and 3.6 at 75° C, Psa„. These are 10 ,10 ' , and 10 , respectively.

The equilibrium constants show that the products in each of the reactions are strongly favored, especially for Equation 3.6. Ba and Ca data are available which allows calculation of the molar ratio for Ca/Ba in the formation waters at EI 128. Calculated

Ca/Ba molar ratios for EI 128 range from approximately 110 to 6060. Molar ratios for

Ca/Ba at the Iberia field (Chapter 4, this dissertation) and for seven oil and gas fields from offshore Louisiana (Land et al, 1988a) range from 104 to 3450 for the Iberia field

(Chapter 4, this dissertation), and from 33 to 230 for the seven other fields. If the activities for Ca and Ba are assumed to equal concentrations for Equation 3.6, the molar ratio of Ca/Ba at equilibrium will be 38900. Given that the maximum Ca/Ba ratios are no larger than 6060, the reaction given in Equation 3.6 should have strong thermodynamic driving force toward products when any of these formation waters interact with evaporitic anhydrite in the salt.

Molar ratios for Ca/Sr range from 24 to 65 and 12 to 182 for the Iberia data

(Chapter 4, this dissertation) and Land et al. (1988a) data, respectively. Molar ratios for

Sr/Ba range from 1.84 to 144 and 0.7 to 11 for the Iberia data and Land et al. (1988a) data, respectively. Assuming activity equals concentration, equilibrium ratios for Ca/Sr are 57.5, and for Sr/Ba are 676. Given the equilibrium value for Sr/Ba (Equation 3.5), 115

and the molar ratios of Sr/Ba for the formation waters in the two data sets, it is clear that

the reaction would proceed to form products if the formation waters were reacting with

the diapir. The equilibrium value of 57.5 is exceeded by some formation waters at the

Iberia field (Equation 3.4) and by some waters in the Land et al. (1988a) data set. This

suggests that anhydrite and celestite can coexist during interaction if Ba has been

removed by prior reaction with sulfates along a flowpath (c.f. Fig. 3.20).

Reactions 3.4,3.5, and 3.6 imply that the sulfate in anhydrite is immobilized by

2+ 2+ dissolved Ba and Sr from the formation waters. However, this depends on the

amount of barium and strontium that are available in the pore-fluids to titrate sulfate. If

barium and strontium are depleted below saturation levels for barite and celestite,

dissolved sulfate and Ca can be transported away from the salt dome in solution.

Partially dissolved anhydrite is observed in halite with no attendant replacement by

celestite or barite (c.f. Fig 3.24). In this case, both the calcium and sulfate have been

mobilized and subsequently removed in solution. Water analyses and the observation

that barite and celestite cements and free-growing crystals form in the siliciclastic

sediments external to the salt demonstrates that dissolved sulfate is present in the

diagenetic fluids away from the salt. Diagenesis within the flanksediment s Calcite cements

McManus (1991) presented evidence fromth e Sr isotopic composition of massive calcite cements at the West Hackberry salt dome to suggest that the calcium in the calcite cements had been derived from the dissolution of anhydrite in the salt diapir. 116

Because there is petrographic evidence of anhydrite dissolution (Fig. 3.24) and

pseudomorphic replacement of anhydrite by barite and celestite at EI 128 (Fig 3.17), it

2+ 2 is likely that anhydrite destruction is also a local source for pore fluidCa and S04 ' in

the flank sediments adjacent to the EI 128 salt dome.

McManus (1991) further presented evidence from carbon isotopes that the

source of the carbon in the calcite cements at the West Hackberry salt dome was

methane. Given that EI 128 is in a similar geologic setting with similar clastic and

diagenetic minerals, and equivalent calcite and pyrite cement textures, it is likely that

similar types of complexly coupled redox reactions occur at EI 128. Hydrocarbons at EI

128 are produced from numerous sands between approximately 1.5 km and 4 km, thus hydrocarbons appear to be available at all depths at EI 128 near the salt-sediment

interface to participate in similar redox reactions.

Redox reactions

Based on the sulfur and carbon isotopic composition of sulfide minerals and carbonate cements in flank sediments at the West Hackberry salt dome, McManus

(1991) proposed that the precipitation of calcite was driven by thermochemical sulfate reduction with the oxidation of methane to form carbonate. McManus (1991) proposed the following net reaction to explain the pervasive carbonate cements

CaS04 + CH4 + O.SFeOOH -+ CaC03 + 0.5FeS2 + 2H20 + 0.25H2 (3.7) 117

The highest bottom hole temperatures at West Hackberry were in the range of

90° C to 100° C, which is thought to be the lower temperature limit for thermochemical

sulfate reduction to proceed at significant rates (Orr, 1982). At EI 128 the corrected

bottom hole temperatures range from71° C to 110°C (see Appendix B.6). Some of the

EI 128 temperatures are lower than the apparent minimum temperature necessary for

significant thermochemical sulfate reduction to proceed. Given the cooler temperatures,

microbial sulfate reduction may be locally favored rather than thermochemical.

However, periodic expulsion of hot, geopressured fluids from deeper within the section

(Evans, 1989) may occasionally drive episodes of thermochemical sulfate reduction.

Here, net reactions are written based on petrographic observations. The similarity of the diagenetic mineral suite at EI 128 to that at West Hackberry with the presence of pervasive calcite cements, detrital magnetite, and diagenetic pyrite cements, suggests the following general reaction

7CaS04 + Fe304 + 7CH4 -> 7CaC03 + 3FeS2 + 11H20 + 2H2 + H2S (3.8)

Reaction (3.8) utilizes magnetite as a source for reducible ferric (Fe3+) iron in a manner similar to McManus's proposed reaction in which goethite was used. However, reactions may also be written in which detrital minerals containing ferrous (Fe +) iron and titanium, such as ilmenite, are destroyed. Complex diagenetic intergrowths of pyrite and

Ti02 (Fig. 3.27) are observed in sample 23,1941 m (6369 ft) and suggest the following reaction (3.9) in which ferrous iron is not further reduced: 118

2CaS04 + FeTi03 + 2CH4 -> 2CaC03 + Ti02 + FeS2 + 3H20 + H2 (3.9)

In addition to methane, other hydrocarbons could also be partially oxidized to form bicarbonate.

When considering net reactions that involve sulfate reduction driven by microbial or thermochemical means, important intermediate reactions may occur. The sulfate which is being reduced must be present in aqueous solution, and as such, the minerals barite, celestite, and anhydrite must first undergo dissolution in order to make sulfate available for reduction. Orr (1982) reports that the rate of thermochemical sulfate reduction appears to be first order with respect to sulfate concentration in solution. This implies that the concentration of barium in the formation waters can slow the thermochemical sulfate reduction reaction by controlling the available dissolved sulfate, or, conversely, that sulfate reduction may drive barite dissolution.

Reactions (3.7), (3.8), and (3.9) generate approximately 1.5 to 2.0 moles of water as a product of the net reaction for every mole of S04 " reduced. At very high salinities, the activity of water is substantially reduced because significant numbers of water molecules are retained in the hydration spheres around charged solution species.

By applying Le Chatlier's Principle of Equilibrium, the effect of lowering the activity of a product in a reaction will cause the reaction to produce more of the product in order to restore equilibrium. Consequently, halite dissolution may serve to shift the reaction to 119

the right to some degree, thus favoring complexly coupled sulfate reduction reactions

near dissolving salt domes where fluids may be near saturation with respect to halite.

Sulfates

Precipitation of celestite and barite in flank sediments may also occur in

reactions unrelated to local anhydrite destruction in the diapir

2+ 2 Sr + S04 "->SrS04 (3.10)

2+ 2 Ba + S04 ' ->BaS04 (3.11)

Reactions (3.10) and (3.11) can proceed anytime that the value of Q/K, the saturation

index for celestite or barite, exceeds 1.

Silicates and aluminosilicates

Well-defined compositional trends exist for formation waters from sedimentary

basins (Hanor, 1994a and 1994b). Ca and Mg were the only dissolved species that had analyses available in the commercial data set for the formation waters from EI 128, and when they are plotted against chloride on log-log plots conflicting results develop. Ca versus CI (Fig. 3.29) shows the expected 2:1 trend that should be present if the hypothesized coupled reactions are proceeding in response to halite dissolution, but Mg versus CI (Fig. 3.31) does not develop the expected 2:1 trend. For Mg versus CI the observed pattern of the data is consistent with that observed by Hanor (1994b) for waters in southwest Louisiana near salt domes. There, rapid addition of Na and CI by halite dissolution, with little further coupled reaction because of kinetic limitations of

the silicate reactions (Hanor, personal communication), best explains the vertical trend

of the data. Further evidence in the EI 128 samples does exist, however, for the

operation of reactions that are hypothesized to be coupled to halite dissolution

Sodium that is introduced into formation waters by halite dissolution should be

partially reincorporated into new sodium-rich mineral phases in order to maintain

overall charge balance in the reacting fluid-mineralsyste m (Hanor, 1994b). Thus, it

should be possible to identify diagenetic sodium-rich phases in the vicinity of salt

domes where halite is being dissolved. Silica-rich diagenetic analcime is found in the

flank sediments around the EI 128 salt dome in microfossil tests, as was observed by

McManus at West Hackberry (1991). The presence of diagenetic analcime in an

environment where halite is being dissolved and its components introduced into the

ambient formation waters is consistent with Hanor's (1994b) requirement for the

precipitation of authigenic Na-bearing minerals in response to reactions that are coupled

to halite dissolution.

One unusual aspect to the analcime found at EI 128 and West Hackberry is that it preferentially precipitated in fossil tests. This implies that aluminum has been mobilized and transported in solution. At EI 128 the detrital sediments are Late Miocene or younger, and they contain relatively few volcanic lithic fragments, a potential source for Al and Si proposed by McManus at West Hackberry (1991). In addition, the fossil tests in the EI 128 samples are almost exclusively found in the clay-rich sediments from the sheared zone next to the salt. If it is assumed that aluminum is relatively immobile 121

because of its demonstrated very low solubility in typical basinal waters, aluminum

derived from nearby unstable aluminosilicates within silty clays seems to be the most

reasonable source. Detrital feldspars are present within the silty clays at EI 128 as well

as other detrital aluminosilicates. Many of these aluminosilicates have textures which

indicate corrosion during diagenesis. It is likely that their dissolved components became

available for reincorporation into analcime within the silty mudstones.

Sulfides

The pyrite observed in the microfossil tests at EI 128 is interpreted to be an early

sulfide resulting from reducing conditions caused by the decomposition of organic

material in the fossil tests shortly after burial. Early pyrite in foraminifera tests has also

been reported in samples fromVermilio n Block 31, offshore Louisiana (McBride et al,

1988), and fromth e West Hackberry salt dome (McManus, 1991) and Black Bayou salt dome (Leger, 1988).

Late stage diagenetic pyrite is observed at EI 128 ( e.g. Fig. 3.26). Leger (1988),

McBride et al. (1988), and McManus (1991), all report a late stage of pyrite precipitation in their respective study areas which occurs concurrently with or postdates a cement and frameworkgrai n dissolution event. McManus (1991) reports that the late stage of pyrite precipitation at West Hackberry occurs in several events and that some of the events coincide with the precipitation of the pervasive calcite cements.

Diagenetic sequence

The apparent diagenetic sequence at the Eugene Island 128 salt dome was deduced based on 113 SEM images and optical microscopy. The diagenetic sequence is 122

remarkably similar to those at Vermilion Block 31 McBride et al. (1988), West

Hackberry (McManus, 1991), and Black Bayou (Leger, 1988). Similarities in the

diagenetic sequences between Vermilion Block 31, which is not directly associated with

Mineral Early- -Late

pyrite pyrrhotite sphalerite chalcopyrite barite celestite quartz ? ? analcime Ti-oxides calcite Fe-calcite compaction and crushing NaCl dissolution framework grain dissolution

Figure 3.36. Sequence of diagenetic events at the Eugene Island Block 128 salt dome, offshore Louisiana. a salt dome, EI 128,and the other salt domes suggest that the diagenetic influence of dissolving halite and anhydrite, and the crushing of mineral grains near the salt- sediment interface may extend well beyond the immediate vicinity of the dome.

Diagenetic zones around the EI 128 salt-sediment interface

Figure 3.37 is a schematic summary based on the apparent spatial relations between the degree of halite dissolution, predominant sulfate minerals, and the accumulation and accretion zone at the margin of the EI 128 diapir for insoluble mineral residues left from halite dissolution. There is no requirement for all zones to be

universally present. Figure 3.37 defines five zones around the salt-sediment interface at

EI 128: 1) the flank sediment zone; 2) the shale sheath zone; 3) the accretion zone; 4)

the halite dissolution zone; and 5) the unaltered evaporite zone. The halite dissolution

zone has two sub-zones, the Sr-rich barite zone and the celestite-anhydrite zone.

The flank sediment zone contains mildly deformed strata that becomes

increasingly deformed with proximity to salt. The outer limits have not been defined.

The sediments in proximity to the salt in the flank sediment zone may or may not

contain massive calcite cements, and in closer proximity to the dome framework grain

crushing is observed in sands.

The shale sheath zone corresponds to that identified by Hanna (1953) and is a

zone that accommodates shear as a result of differential vertical movement between the

salt and the flank strata. Hanna (1953) also identified an anhydrite accumulation zone.

At EI 128 the corresponding zone to the anhydrite accumulation zone is the accretion

zone where barite pseudomorphs after anhydrite are found. The mineralogy of insoluble materials derived from dissolving evaporites in the margins of diapirs apparently is a function of the chemistry of the formations waters that are dissolving the evaporites.

Hence, a more general term such as "accretion zone" may be preferred for the flanks of 124

shale, unaltered evaporite flank sediment zone sheath zonj zone halite dissolution accretion zone zone u no scale V Sr-rich barite zone salt-sediment interface'

% porosity in halite formation waters 100- -0

Figure 3.37. Schematic diagram for EI 128 defining and describing the spatial relations between the flank sediment zone, shale sheath zone, the accretion zone, the salt- sediment interface, the halite dissolution zone, and the unaltered evaporite zone. Not to scale. salt domes to "anhydrite accretion zone" because it is clear from the EI-128 study that

anhydrite is not the only mineral that can remain as an insoluble residue. Residual

crystals of corroded halite may also be mixed with the barite and siliciclastics. Textures

observed in the EI 128 samples also reveal that sediment shearing occurs in the

accretion zone which tends to mix the various materials and destroy relict layering

inherited from accretion.

The salt sediment interface can be defined as the surface that separates

individually disaggregated halite clasts, and in situ halite with well-developed

secondary porosity. The in situ salt with secondary porosity defines the halite

dissolution zone and itself may be subdivided into two sub-zones at EI 128 based on the

composition of the sulfate minerals. Beyond the halite dissolution zone is normal

diapiric salt that does not contain evidence of significant formation water interaction. CONCLUSIONS

The petrographic study of samples from around the salt-sediment interface at the

Eugene Island 128 salt dome revealed that a variety of processes in addition to halite dissolution influence diagenesis in the vicinity of the salt-sediment interface. Textural and compositional information suggest five zones at the salt-sediment interface on the flanks of the EI 128 salt dome: 1) the flank sediment zone, 2) the shale sheath zone, 3) the accretion zone, 4) the halite dissolution zone, and 5) the unaltered evaporite zone.

The halite dissolution zone has two sub-zones, the Sr-rich barite sub-zone and the celestite-anhydrite sub-zone. Kinetic and thermodynamic considerations based on the surface area of a

mineral and the relation of a mineral's surface area to its volume, respectively, show

that reaction rates are increased with crushing of silicate minerals (Aagard and

Helgeson, 1982; Helgeson et al. 1984), and that stability of the overall diagenetic

system decreases as the overall size of a mineral particle is diminished to micron and

submicron sizes (Stumm and Morgan, 1981). At EI 128 the presence of crushed grains

in some samples and floating sand-grain textures in others apparently results from the

preferential dissolution of fines derived fromcrushin g of detrital grains with

synchronous cementation by calcite and suggests that the theoretical rate and stability

considerations apply at EI 128.

Petrographic evidence in this study also reveals that substantial secondary

porosity has developed in halite at the margin of the salt diapir. This observation

confirms that salt is actively removed by dissolution from the flankso f the EI 128 salt

dome. In addition, Posey and Kyle (1988) hypothesized that vertical fluid movement

around salt domes may in part rely on fluidtransmissio n through "conduits" developed by dissolution of halite at the margin of the salt dome. This work confirms that dissolution porosity exists in the halite at the margin of the salt dome. However, the extent of interconnection in this porosity is presently unknown.

The EI 128 study shows that Ca is liberated from anhydrite both by dissolution and during the pseudomorphic replacement of anhydrite by Ba-Sr sulfates. The sequence of compositions of the Ba-Sr sulfate pseudomorphs and accretion residues of complete and fractured pseudomorphs in the silt and clay rich sediments at the margin of the salt reveal that the formation waters moving through the salt are first strontium

rich and then become barium rich. The persistence of barite in samples from the salt-

sediment interface also reveals that Ba in the mobile formation waters is in part

responsible for controlling dissolved sulfate derived fromdestructio n of anhydrite in the

margins of the EI 128 salt dome.

At West Hackberry, reactions coupled to thermochemical sulfate reduction and

oxidation of methane to carbonate were proposed to explain the presence of associated

pyrite and calcite cements. Late diagenetic pyrite and calcite cements found in the EI

128 sediments are similar to those found at the West Hackberry salt dome. Based on the

similarity of the geologic setting of the two salt domes and the similarity in the

diagenetic minerals and apparent timing of events, it is reasonable to conclude that the

late pyrite and calcite cements at EI 128 are also the result of sulfate reduction reactions

and oxidation of hydrocarbons. The presence of hydrocarbons may be essential for

formation of the massive carbonate cements, and the cements may subsequently form

up-dip seals near the salt and thus form traps. In addition, diagenetic Ti02 and Ti02

intergrowths with pyrite suggest that the redox reactions associated with sulfate

reduction may also be complexly coupled to titanium and iron-bearing minerals such as

ilmenite.

This study thus finds petrographic evidence for halite dissolution on the flank of the EI 128 salt dome, a series of pseudomorphic replacements for anhydrite first by celestite and then by barite in the dissolving salt, fivezone s around the salt-sediment 128 interface, crushing of framework grains, and enlianced dissolution rates for crushed sediments. CHAPTER 4) FAULT AND FRACTURE CONTROL OF FLUID FLOW AND DIAGENESIS AROUND THE IBERIA SALT DOME, IBERIA PARISH, LOUISIANA

INTRODUCTION

In the Gulf Coast sedimentary basin, fluid flow from deep, overpressured formations up into shallower sediments around salt domes requires pathways which can transmit fluid flow across kilometers of section. The vertical component to fluid migration around salt domes may seem remarkable because the sediments that are penetrated by many salt domes are composed of alternating sequences of sands and mudstones, and mudstones are commonly thought to act as baniers to fluid flow.Hence , fluid flowmus t be accommodated either through features in the mudstone that act as fluid conduits, such as dilated fractures, or around the mudstones where mudstone strata buttress the salt. From a broader perspective, not limited to the salt dome environment, the pathways that accommodate vertical fluidmovemen t through mudstones or shales are important because they have implications for the vertical migration of petroleum and on the fate of toxic and hazardous wastes disposed of by deep-well injection.

Posey and Kyle (1988) have summarized current hypotheses on the nature of vertical flow paths around salt domes. They state that fluid-flow around salt domes occurs within fractures, through interconnected porosity developed in the margin of the salt, and imply that faults may act as vertical fluid conduits in salt dome flank sediments. Complexly interconnected sands have also been proposed as another potential pathway for vertical fluidmigratio n around salt domes (Hanor, 1987).

129 Of the vanous pathways listed above, fluid migration through interconnected

porosity in the margin of the salt, in particular, has additional influence on diagenesis in

flank sediments around salt domes. Upward migrating fluids fromdee p sedimentary

formations beneath the surface of geopressure in south Louisiana typically have

salinities less than 35 g/L (Hanor, 1994b), which is a value far below that for halite

saturation (ca. 350 g/L). Hence, when upward migrating fluidsar e transmitted through

pathways contained within the margins of salt domes, they are capable of dissolving

large quantities of halite. For example, McManus and Hanor (1993) calculate that 0.8

km of evaporites have been dissolved from the southwest flanko f the West Hackberry

salt dome in Cameron Parish, Louisiana.

Introduction of large masses of Na+ and CI' into formation waters substantially

alters their composition and can subsequently drive diagenetic reactions when they

emerge fromth e salt back into siliciclastic flank sediments (Hanor, 1994a, 1994b, 1995

in press). Petrographic evidence fromth e salt-sediment interface also shows that

anhydrite, which comprises approximately 5 to 10 weight percent of a typical diapir

(Murray, 1966), is dissolved or pseudomorphically replaced by barite, thus liberating

2+ 2 + Ca and S04 *, or in the case of pseudomorphic replacement, Ca only (see Chapter 3,

this dissertation; and McManus and Hanor, 1988 and 1993). McManus (1991) presents

isotopic evidence that Ca liberated from anhydrite is partitioned into massive calcite

2 cements in flank sediments, and that S04 ' from anhydrite dissolution is rapidly reduced and incorporated into pyrite and other diagenetic sulfides in the flank sediments. 131

The research described here is part of a study designed to identify potential fluid migration pathways that accommodate vertical fluid transport on the southwest flank of the Iberia salt dome in Iberia Parish, Louisiana. This study is also intended to identify diagenetic minerals and reactions that result from fluid migration and salt dissolution along the flowpaths. The Iberia salt dome was chosen because of earlier evidence developed there for vertical fluid flow within the flank sediments (Workman and Hanor,

1985; Hanor, 1987) and because of an extensive set of fluidsamples , drill cuttings, and electric logs that were available fromwell s in the Iberia oil field.Th e distribution of the wells will ultimately allow a three dimensional approach to the identification of flow paths.

IBERIA FIELD

The Iberia salt dome is located in Iberia Parish, Louisiana, approximately 8 km

(5 miles) east of the town of New Iberia (Fig. 4.1). It is a shallow piercement-type salt dome with the top of salt at -245 m (-805 ft) (Spillers, 1961). A topographic high, surface gas seeps, and paraffin dirt led to its discovery. The Iberia dome has a nearly circular cross section with no apparent overhangs, and it has an associated radial fault system that offsets Miocene and Pliocene sands and mudstones (United States Strategic

Petroleum Reserve Office, 1977). The top of geopressure in the field is deeper than

2866 m (9402 ft) (Hanor and Workman, 1986). Hydrocarbons have been produced along the flank from the Miocene and upper Oligocene (Spillers, 1961). Hydrocarbons are also produced from above the dome in the Pliocene (Workman, 1985). The study area is located on the southwest quadrant of the Iberia salt dome (Fig. 4.2). Figure 4.1. Map of Louisiana showing the location of the Iberia field.

1000 2000 ft

B'tlOoo ft 0 250 500 m

Figure 4.2. Map of study area on the southwest flanko f the Iberia dome. The locations of the eighteen selected oil wells are shown along with their projections onto cross section lines A-A'and B-B'. The contours show the approximate depth in feet from the ground surface to the top of the salt (modified from Workman and Hanor, 1985). Well 19 is the source of the drill cuttings used in this study. METHODS Flow path identification Two approaches have been taken to identify specific fluid flow-paths around the

Iberia dome. In the firstapproach , formation water samples from oil producing sands in the Iberia Field have been analyzed for their major, minor, and trace cations. Selected data were subsequently plotted and contoured on a detailed cross section in order to identify any correlation between spatial variations in pore fluid chemistry and large- scale structural or stratigraphic features.

The second approach involved the analysis of drill-cuttings with the scanning electron microscope (SEM) and X-ray energy dispersive spectrometry (EDS). This approach was taken recognizing the potential for diagenetic interaction between migrating fluids and the ambient detrital minerals and pore fluids they encounter during their ascent. Migrating fluids potentially record their passage through preferential flow paths in the form of diagenetic cements, overgrowths, pore-fillings, or grain replacements, and they may be potentially enriched in elements such as Ag, Ba, Fe, Mg,

Mn, Pb, Sr, and Zn. Thus, drill-cutting analysis in this study primarily focuses on diagenetic minerals. No drill cuttings or core samples were available fromth e salt- sediment interface in the study area, and the electric logs did not reach the salt. Thus, the hypothesis that fluidflo w occurs in porosity developed along the margin of the salt could not be directly verified through petrography or well log analysis, however, the fluid analyses were graphically analyzed and applied to this question. 134

Formation water analysis Fluid samples that were archived from eighteen wells (see Fig. 4.2 and Table

4.1) in the Iberia fieldb y Workman and Hanor (1985) were analyzed as a part of this study by ICP-AES on a Perkin-Elmer ICP 6500 for dissolved Na, K, Mg, Ca, Sr, Ba,

Mn, Fe, Cu, Pb, Zn, Al, Si, and B. The original samples had been preserved by dilution and acidification with nitric acid and were stored in high-density polyethylene bottles.

Upon initial inspection, the archived samples did not contain observable precipitates and still retained pH values of approximately one. Sample aliquots were extracted and diluted to lOx total dilution for analysis of K, Ba, Mn, Fe, Cu, Pb, Zn, Al, Si, and B,

Table 4.1. List of wells and fluid sample depth at the Iberia field.

Well Well Name Depth of sample(m / ft) Type of sample m ft 1 Duhe-58 660 2165 fluid 2 Duhe-18 1222 4010 fluid 3 Bryant-4 1267 4158 fluid 4 Duhe-41 1283 4209 fluid 5 Duhe-30 1406 4612 fluid 6 Duhe-60 1474 4837 fluid 7 Duhe-14 1537 5043 fluid 8 Duhe-66 1697 5566 fluid 9 Decuir-6 1746 5728 fluid 10 Duhe-7 1776 5827 fluid 11 Duhe-84 1825 5987 fluid 12 Bullock-5 1831 6008 fluid 13 Bullock-2 1850 6070 fluid 14 Bullock-4 1892 6207 fluid 15 Gralino-8 2679 8791 fluid 16 Duhe-79 2703 8869 fluid 17 GulfFee-3 2747 9014 fluid 18 Duhe-73 2866 9402 fluid and aliquots of the samples were diluted to lOOx total dilution for Na, Ca, Mg, and Sr.

The sample dilutions for minor and trace cations were held to a minimum of lOx in order to facilitate their detection. The practical detection limits for ICP-AES analysis of the various elements analyzed for in this study are based on published data of Winge et al.,(1979) for the emission lines used. Winge et al. (1979) state that the detection limit is conventionally defined as the analyte concentration required to yield a net signal equivalent that is three times the standard deviation of the background signal beneath the spectral line. During multi-element analysis of the diluted brines, the effective detection limits approached the published estimated detection limit values. Table 4.2 is a summary of the estimated detection limits published by Winge et al. (1979), and

Table 4.2. Spectral lines and detection for elements in ICP-AES analyses.

Element wavelength, A, Estimated detection limit Estimated minimum reportable from Winge etal, (1979) concentration at lOx dilution nm mg/L mg/L Al 308.215 0.045 0.45 B 249.773 0.0048 0.048 Ba 455.403 0.0013 0.013 Ca 422.673 0.010 0.1 Cu 327.396 0.0097 0.01 Fe 259.940 0.0062 0.062 K 766.491 0.01 (Perkin-Elmer) 0.1 Mg 285.213 0.0016 0.016 Mn 257.610 0.0014 0.014 Na 589.592 0.069 0.7 Pb 220.353 0.042 0.42 Pb 216.999 0.090 0.9

Si 251.611 0.026 (as Si02) 0.26 (as Si02) Sr 407.771 0.00042 0.0042 Zn 213.856 0.0018 0.018 the minimum reportable element concentrations based on the lOx dilutions in this study.

The most sensitive line for Al at 396.152 nm could not be used because it is strongly

interfered with by intense emission wings that are associated with the Ca 396.847 nm

line.

Drill-cutting analysis

The available drill cuttings were from Bryant no. 9, which is well 19 in Figure

4.2 and cover the depth interval between 2896 m and 3203 m (9500 ft and 10510 ft).

The sampled interval covers 307 m (1010 ft)o f section that lies immediately above the

salt-sediment contact on the steeply sloping flank of the salt dome. Thirty-six bags of

drill cuttings were collected and washed with samples taken every 9 m (30 ft) in the

upper 277 m (910 ft) of the sampled interval. Samples were taken at irregular spacings

across the lower 30 m (100 ft). The contents of all sample bags were inspected under the

binocular microscope, and selected sandstones and siltstones were acid tested for the

presence of carbonate cements. A total of sixty-three individual cuttings were hand

picked from thirteen different samples of drill cuttings under the binocular microscope.

This number of samples was deemed adequate to characterize the diagenetic mineralogy

and its gross spatial distribution for the present study. The selected cuttings were mounted with conductive glue on metallic stubs as grain mounts and were gold coated

for SEM-EDS analysis on a JEOL 840A scanning electron microscope (SEM) with a

Tracor Northern X-ray energy dispersive spectrometry (EDS) system. Well logs Electric well logs were used to construct the detailed cross section A-A' shown in Figure 4.3. Work is in progress on cross section B-B'. Cross section A-A' was constructed by correlating individual sands that are greater than 15 m (50 ft) in thickness. Sands thinner than 15 m (50 ft) in mixed sand/mudstone intervals were ignored unless they made up greater than approximately 30 percent of the bulk

+ salt LEGEND • 1000 ft. \fault approximate contact 1300 m ' \ sand s«? mudstone production mid-poinlid-nnint T »

Figure 4.3. Detailed cross section along section line A-A' in Figure 2. thickness of those intervals by visual estimation. The units designated as mudstones on

the detailed cross section closely correspond to the < 25 percent sand regions in an

earlier cross section published by Workman and Hanor (1985).

Faults were identified during the construction of the cross section where there

was signiflcant missing section, or wherever an anomalous steepening of formation dips

occurred between two adjacent wells on the cross section. The dip of the strata between

wells that are not separated by faults is relatively shallow at approximately 10° to 15°.

In the vicinity of probable faults, apparent formation dips exceed approximately 20°

between well control points but return to the shallower dips in laterally adjacent well

control segments along the section line.

Thermodynamic modeling

Activities and activity ratios for the dissolved species in the formation waters were calculated using SOLMINEQ.88 (Kharaka et al, 1988) in order to evaluate potential fluid-mineral equilibria in the formation waters around the Iberia dome. The

Iberia formation waters have ionic strengths greater than two, and hence, the Pitzer option was chosen when calculating activity coefficients. The Pitzer method is reported to more accurately model activity coefficients at high ionic strengths (Drever, 1988).

Direct evaluation of the saturation indices for aluminosilicate minerals with

SOLMINEQ.88 was not possible because aluminum values in the brines were below detection limits for ICP-AES. To circumvent this problem, activity diagrams were constructed fromth e thermodynamic data set presented in Bowers et al. (1984), and various cation/hydrogen ratios were plotted in order to evaluate potential fluid- aluminosilicate equilibria. This approach assumes that the formation waters are

saturated in aluminum with respect to the aluminosilicate mineral phases.

Modeling runs in SOLMINEQ.88 were performed at corrected bottom-hole

temperatures (calculated in Workman, 1985) up to 75° C. When modeling with the

Pitzer option at temperatures above 75° C with version 0.85 of SOLMINEQ.88, the

program would fail to run properly for unknown reasons.

A large data set from 47 producing oil and gas wells in seven different fields

from offshore Louisiana (Land et al., 1988a) was also evaluated in SOLMINEQ.88 for

comparative purposes. These analyses do not include pH data, and the option of

calculating pH by assuming calcite equilibrium in SOLMINEQ.88 was enabled in order

to calculate activity data. RESULTS Formation waters Spatial distribution of dissolved solutes at the Iberia field

The results of the formation water analyses are tabulated in Appendixes C.la,

C.lb, and C.lc. As discussed by Workman and Hanor (1985), the chlorinities of most of the formation waters in the Iberia field study area fall within a very narrow range of 74 to 80 g/L. The two exceptions are the sample from Well 3 near the top of the dome, which has a CI concentration of 151 g/L, and the sample from Well 18, the deepest sample collected, which has a CI concentration of 66 g/L. Because of the generally narrow range of chlorinities, spatial variations in chloride do not provide as sensitive a tool for investigating flowpath s at this field as do boron, iron, and the group IIA elements, Ca, Sr, and Ba, which have among the widest measured ranges in concentration. Figures 4.4 through 4.8 show the spatial variations in each of these elements in cross section A-A'.

There is a marked difference in composition between the deepest waters, which have relatively low concentrations of Ca, Ba, and Fe and high concentrations of B and

Sr, and the shallowest waters, which have significantly higher concentrations of Ca, Ba, and Fe, and lower concentrations of B and Sr. The contours define a tongue of low-Ca-

Ba-Fe, high-B-Sr fluid which extends from depth up in a direction subparallel to the

Ba,mg/L -1

-2

-4

a -5' o 3 -6 2- -7

1-9

Figure 4.4. Spatial variations in dissolved Ba. 141

§ 3

Figure 4.5. Spatial variations in dissolved B.

Ca, mg/L -1

-2

-3

-4

o 3 -6 2-1 -7

Figure 4.6. Spatial variations in dissolved Ca. 142

Figure 4.7. Spatial variations in dissolved Fe.

A A' 0 Sr, mg/L -1

-2 •

-3 1- "< K .•••'

-8 Kh + + -9

Figure 4.8. Spatial variations in dissolved Sr. 143

Figure 4.9. The arrow marks the core of the tongue of low Ca-Ba-Fe and high B-Sr water thus demarcating the core of the apparent flowpath .

side of the dome. The core of this tongue is situated approximately 200 to 400 m

laterally from the salt-sediment interface (see Fig. 4.9).

Figures 4.10 and 4.11 are plots of Ca versus depth and CI versus depth. Two trends can be observed in the Iberia data set relative to the wells that are spatially located along the deep tongue of fluid.Th e trends in each diagram have been marked by a horizontal line and a subvertical line. The marked trends reveal important information about formation water interaction with the diapir. McManus and Hanor (1993) state that

Na , CI", Ca , and S04 ' are introduced into formation waters around salt domes by the dissolution of diapiric halite and anhydrite, hence when mobile fluids are dissolving salt and anhydrite during upward transport, increases in the concentrations of Na+, CI', Ca2+, • Iberia data

a Land et al data

0 2000 4000 6000 Depth (m)

Figure 4.10. Ca versus depth for the Iberia field and data presented in Land et al. (1988a). The trend lines suggest that Ca is introduced into formation waters at depth.

160000 • Iberia data 120000 a Land et al data E 80000

40000

0 0 2000 4000 6000 Depth (m)

Figure 4.11. CI versus depth for the Iberia field and data presented in Land et al. (1988a). The trend lines suggest that CI is introduced into formation waters below 2600 m. A likely source is from halite dissolution. and S04 ", assuming no other reactions, should be observable in the formation waters.

The subvertical trend line with negative slope shown for the Iberia data in Figures 4.10

and 4.11 reveals that Ca2+ and CI" respectively are being introduced into the formation

waters below approximately 2600 m as depth decreases. Above 2600 m, the trend lines

have zero slope for Ca2+ and CI' for the wells along the tongue of low-Ca-Ba-Fe, high-

B-Sr fluid. The horizontal trend above 2600 m reveals that the concentrations of Ca2+

and CI" become independent of depth and suggest that halite and anhydrite are no longer

being dissolved above 2600 m. This also suggests that the low-Ca-Ba-Fe, high-B-Sr

tongue of fluidi s isolated from the salt in the margin of the diapir above 2600 m.

Dissolved solutes versus chloride for the Iberia field and seven offshore Louisiana fields Hanor (1994b) has shown that formation waters from many sedimentary basins with salinities greater than 10,000 mg/1 and less than halite saturation (ca. 350,000 -

400,000 mg/L) have predictable concentration trends for major and some minor dissolved species as a function of chlorinity. Hence, similar plots for Ba, B, Ca, K, Mg,

Na, dissolved silica, Sr, and alkalinity (Figures 4.12 through 4.20) have been made for the Iberia fielda s well as for the seven offshore fields published in Land et al. (1988a).

Hanor (1994a and 1994b) has shown that a 2:1 linear increasing trend on log-log plots exists for the divalent cations such as Ca2+, Mg2+, and Sr2+ in saline formation waters with increasing CI. Similarly, monovalent cations are shown to increase, but with a 1:1 linear trend on similar log-log plots, and total alkalinity was shown to decrease with increasing chloride in a non-linear trend. 1000 ri D Land et al data

• Iberia data 100 a 2:1 reference line ffl* 10

1000 10000 100000 CI (mg/L)

Figure 4.12. Ba versus CI for Iberia data and Land et al. (1988a) data.

100.0 0DDcPaftg-17 D gr£T Q ° ^15+11 |-Mo f 10.0 5 • Iberia data PQ V a Land et al data 1.0 1000 10000 100000 1000000 CI (mg/L)

Figure 4.13. B versus CI for Iberia data and Land et al. (1988a) data. 10000 /fa 11,10,5,4 /—\ • Iberia data J; 1000 D i* 17 a Land et al data

/a 2:1 reference E) a line 100 -•- 1 ' i 10000 100000 1000000 CI (mg/L)

Figure 4.14. Ca versus CI for Iberia data and Land et al. (1988a) data.

1000 11,15 10,17 jf 100 • Iberia data • Landetal 10 data 1:1 reference 1000 10000 100000 line CI (mg/L)

Figure 4.15. K versus CI for Iberia data and Land et al. (1988a) data. 148

1000

100 toD • Iberia data

• Landetal 10 data 1000 10000 100000 — 2:1 reference CI (mg/L) line

Figure 4.16. Mg versus CI for Iberia data and Land et al. (1988a) data.

1000000 a 100000

1000 10000 100000 1000000 CI (mg/L)

Figure 4.17. Na versus CI for Iberia data and Land et al. (1988a) data. 100.0 • Ibena data a 1 a Land et al o Dqb • H 8 V^15 data CZ5

13 xn *§ 10.0 1 1 1000 10000 100000 1000000 CI (mg/L)

Figure 4.18. Dissolved Si02 versus CI for Iberia data and Land et al. (1988a) data.

1000 / •/ • Iberia data I 100 1% a Land et al data (Z) Bgi o 2:1 reference D/ 7 • line 10 rf 1 10000 100000 1000000 CI (mg/L)

Figure 4.19. Sr versus CI for Iberia data and Land et al. (1988a) data. 1000

s • CO

I 100 • GO • Iberia data a

• H a Land et al data 10 , 10000 100000 1000000 CI (mg/L)

Figure 4.20. Total alkalinity versus CI for Iberia data and Land et al. (1988a) data.

For Ba, Ca, Mg, and Sr in the Land et al. (1988a) data set the 2:1 trend clearly exists. The Iberia data, in general, groups close to the Land et al. (1988a) data, however it tends to disperse vertically on the plot down fromth e 2:1 trend for the divalent cations. Hanor (1994b) noted similar deviations for saline waters in Calcasieu Parish, southwest Louisiana that were obtained from close proximity to salt domes. Na and K for the Land et al. (1988a) data and the Iberia data follow the predicted 1:1 trend, and total alkalinity follows a similar non-linear decreasing trend as observed by Hanor

(1994b). Other than the vertical trend for the Iberia data, trends for dissolved Si02 and

B are not clearly defined. 151

The data points for wells 17,15, 11,10, 5, and 4 have been identified on several

of the plots. This sequence of data points represents formation water analyses in wells

from deepest to shallowest along the tongue of low-Ca-Ba-Fe, high-B-Sr fluid.Th e

sequence, in most cases, is fairly well replicated in the string of data points indicating that the composition of the fluids also is a function of depth along the tongue of low-Ca-

Ba-Fe, high-B-Sr fluid.

Diagenetic minerals

The general lithologies of the drill cuttings, as summarized in Tables 4.3 and

4.4, ranged from coarse sandstones to mudstones. The sandstones and siltstones yielded

Table 4.3. Key to Table 4.4

eg coarse grained spor secondary porosity mg medium grained og overgrowths fg fine grained shr shear textures vfg very fine grained acr accretion textures shy shaley mfrac mineralized fracture vshy very shaley bx brecciated ss sandstone fe iron-rich sltst siltstone diss dissolution mdst mudstone Anh anhydrite Is limestone Anl analcime salt diapiric halite Brt barite sps sparse Cal calcite corr corroded Cls celestite fcor framework grain corrosion sem analyses ffrc framework grain fracturing Py pyrite frc extended fractures Sp sphalerite fs-gtx floating sand-grain texture Qtz quartz oct octahedral ps pseudomorph lam laminated vrm vermiform cmt cement Kin kaolinite pcmt pervasive cement Table 4.4. Dnll cutting lithology and diagenetic mineral summary, well 19.

Depth drill-cutting diagenetic; minerals and textures m ft lithologies Anl Brt Cal Py Kin Sp misc. 2896 9500 mg ss+mdst cmt X Anh % - M$ \ mm' eg ssfnwM ' ^ ;x cmt x vna frc , 2914' 9560 mg ss+mdst cmt x vrm frc mdst ^m £590 fps+mdst x cmt X ^ mfrac 2932 9620 sltst+mdst cmt X frc 2941 9650 vfg ss+mdst sps frc mdst 2950 9680 vfg ss+mdst cmt X frc 2960 9710 vfg ss+mdst cmt X frc mdst * 296*9 9740 V% $s+mdst x" cmt X jfrc Mdst 2978 9770 fg ss+sltst cmt X 2987 9800 mgss - sps 2996 9830 vfg ss+sltst cmt X 3005 9860 vfg ss+sltst cmt sps 3014 9890 vfg ss+sltst cmt sps 3021 9910 vfg ss+mdst cmt 3030 9940 vfg ss cmt X 3039 9970 eg ss+fg ss sps 3048 10000 sltst cmt frc mdst 3057 10030 vfgss 3066 10060 vfgss sps 3075 10090 fgss sps 3085 10120 sltst sps sps 3094 10150 ig ss+sltst Og -' .. sps J _ • ' <5fe 3121 10240 fgss-fmdst cmt X - frc mdst* 3130 10270 fg ss+mdst X X < 3139 10300 sltst - X 3149 10330 sltst 3158 10360 fgss , u&r 10390 vfgss - J«c 3176 10420 vfgss cmt - frc(dilated) 3182 10440 fgss rnfrac 3184 10445 fgss 3194 10480 fgss+bxmdst •• mfirac 3203 10510 fgss nearly all of the information on the diagenetic mineralogy. The drill cuttings with

average grain sizes finer than coarse silt will require future X-ray diffraction work to

determine how they may have been modified by diagenesis.

The average grain sizes were visually estimated using the Wentworth size

classification, but the mineralogy of the detrital grains were not investigated because of

the primary interest in stable diagenetic minerals and textures. Calcite cements, quartz

overgrowths, Na-aluminosilicate overgrowths (Fig. 4.21), pyrite (4.22), vermiform

kaolinite (4.23), and sphalerite (Fig. 4.24) were observed as diagenetic phases

associated with the intergranular porosity.

Figure 4.21. SEI image of diagenetic analcime overgrowths on detrital framework grains, 3094 m (10150 ft). 154

"9*'A ,J

>«}•

2933 m(95

W J.

Figure 4.22. SEI image of diagenetic octahedral pyrite, 2923 m (9590 ft).

2905 m(953

#7^

':>%**

Figure 4.23. SEI image of diagenetic vermiform kaolinite, 2905 m (9530 ft). analcime

Au,Zn

Figure 4.24. EDS spectra for (a) analcime at 3094 m (10150 ft); (b) barite at 2923 m (9590 ft); and (c) sphalerite at 3130 m (10270 ft). Au peaks are due to sample coating.

The carbonate cements were present throughout most of the sampled interval above

3149 m (10330 ft), and pyrite was present in most of the sandstones and coarse siltstones. Below 3149 m (10330 ft) pyrite was not observed, and calcite cement is present in only one sample at 3176m (10420 ft). Vermiform kaolinite is present only in sandstones fromth e upper 27 m (90 ft)o f the sampled interval, and the analcime overgrowths were found in sandstones at 3094m (10,150 ft). Bante is present in tabular dnll cuttings that appear to represent fracture fills

which have separated from friable host sediments, and the tabular fragments were

observed randomly throughout the sampled interval. The drill cuttings from 3194m

(10480 ft) were brecciated and contain an unidentified Al-Cu-Zn precipitate that was

present as a cement and fracture filling (Fig. 4.25 and 4.26).

Brecciated and cemented mu'dslone

Dilated I'rueture with AI-C'u-/n precipitate

,' ..A }()94 m(l()l Breeciatccf i 500 um

Figure 4.25. SEI image of Al-Cu-Zn vein-filling in fracture, 3194m (10480 ft). The fracture is dilated nearly 1 mm and is contained in a mudstone. The mudstone along the margins of the fracture has been brecciated and subsequently cemented by the same mineral cements that fill the dilated fracture. Thick fracture mineralizations support the hypothesis that dilated fractures support fluid flow through the mudstones. |Au center of fracture

Zn Au

Au Al

1 UW • «»..WWI/&LA. Al lAu margin of fracture

Au Cu I v Ca .55 10 keV

Figure 4.26. EDS spectra of Al-Cu-Zn mineral shown in Figure 4.23. Spectrum (a) is from the center of the fracturean d spectrum (c) is fromth e margin of the fracture. Spectrum (b) was in the traverse from a to c. Au peaks are due to sample coating.

The Al-Cu-Zn vein-fill was variable in composition and was crudely zoned fromth e center of the fracture out to the margins as shown by the sequence a-b-c in Figure 4.26.

Faults

Four faults, labeled a, b, c, and d on Figure 4.3, were identified during the construction of cross section A-A'. All four dip at high angles and have normal offsets.

Work which is in progress on the B-B' cross section indicates that the strike of fault a is somewhere between NW-SE and NNW-SSE. The exact strike is uncertain because the well control is too widely spaced on cross section B-B' between Well 3 and Well 2.

Faults b, c, and d do not cut cross section B-B', but it is likely that they strike parallel or

subparallel to fault a.

Fractures and fracture dilation

Fractures exist in sandstone drill cuttings from Well 19 in samples scattered

throughout the sampled interval (Table 4.4). The presence of vein fill mineralization,

such as that shown in Figure 4.25, confirms that the fractures are not simply artifacts

induced during the drilling operations, but are fracturestha t existed in the formation

prior to sampling. Dilation of the fractures is also evident because space existed to

accommodate fluids and the precipitation of minerals. The thickness of the fracture fill

in Figure 4.25 indicates the fracture had been dilated in excess of 1.0 mm from wall to

wall, and more importantly, the precipitation of Cu, Fe, Zn, and Al bearing mineral(s) in

the fracture indicates that the fractures transmitted metal-rich fluids.Th e drill cuttings

of the mudstones, in general, were too small in dimension and too few in number in to

display fractures except in those fromth e bottom 20 m (65 ft) of the sampled interval.

The tabular drill-cutting fragments that contain barite, especially in the samples below 3048m (10000 ft), have linear striations on the planar sides that appear to be slickensides. Slickensides on fracturemineralization s indicate that fractures are accommodating strain, as well as fluid transmission and space for mineral precipitation.

Thermodynamic modeling

The activity ratios that were calculated using SOLMINEQ.88 (Kharaka et al,

1988) for the Iberia field and the seven oil and gas fields from Land et al. (1988a) are 159

+ + tabulated in Appendix C.2. Data for log(aNa /oH ) versus log(aH4Si04) and for log(oNa+/aH+) versus log(aK+/aH+) have been plotted on the activity diagrams shown in

Figures 4.27 and 4.28.

• Iberia data

+ • Land et al a data +

log(aH4Si04)

+ + Figure 4.27. Plot of log(oNa /aH ) versus log(oH4Si04) for the Iberia data and the Land et al. (1988a) data. The phase boundaries were calculated for 75° C, Psat with thermodynamic data from Bowers et al. (1984) assuming aH20 = 1, and mineral activities =1. 160

8 Albite—; Analcime- - 7.5 7 - 6.5 analcime-par: gonite 100°C -Paragonite albite-pafa'go iite 100°C 6 5.5 - ^/analcime- K-feldspir 00 Kaolinite 5 kspar o albite-kspai • Iberia data 4.5 Muscovite 4 • Land et al data 1 2 3 4 log(aK+/*H+)

Figure 4.28. Plot of log(aNa+/aH+) versus log(aK+/aH+) for the Iberia data and the Land et al. (1988a) data. The phase boundaries were calculated for 75° C, Psat with thermodynamic data from Bowers et al. (1984) assuming aH20 = 1, quartz saturation, and mineral activities = 1.

Paragonite in the activity diagrams proxies for a Na-illite (Aagaard and Helgeson,

1983). The activity diagram presented in Figure 4.27 for log(aNa+/oH+) versus log(aH4Si04) shows that most of the data points cluster around the kaolinite-paragonite- albite triple point which is also bisected by the quartz stability line. The clustering of data points suggests that these four phases, or thermodynamically similar phases such as brammalite (Na-illite) instead of paragonite, are in equilibrium with the formation waters. Figure 4.28 shows similar results with the exception that dissolved Si02 does not appear on this diagram, but is assumed to be at saturation with respect to quartz. The 161

assumption of silica saturation appears to be acceptable considering the near

coincidence of data points with the quartz stability boundary in Figure 4.27. Figure 4.27

suggests that albite is the stable frameworkphas e rather than analcime. However the

morphology of euhedral grain overgrowths in the SEM images has not yet permitted a

definitive determination. Si-rich analcime has been reported as cements and pore-

fillings in fossil tests in the flank sediments at two other south Louisiana salt domes (see

Chapter 3, this dissertation; and McManus, 1991), and it is possible that the stability

field of analcime may enlarge at the expense of the albite field as analcime becomes

silica-rich. However, appropriate thermodynamic data for Si-rich analcime were not

available in the Bowers et al. (1984) data set. The documented presence of Si-rich

analcime in the flank sediments of other salt domes suggests that an enlarged stability

field for Si-rich analcime is a possibility.

From Figures 4.27 and 4.28, it appears that at least six mineral phases are

buffering the formation water compositions. These include quartz, albite or Si-rich

analcime, Na-illite or paragonite, K-feldspar, muscovite, and kaolinite.

DISCUSSION Fault influence on the concentration of dissolved species The behavior of faults as barriers or conduits for fluid migration continues to be a controversial topic both in the Gulf Coast and elsewhere (Hooper, 1991; Berg and

Avey, 1995). However, a few simple criteria can be developed which utilize relations of spatial variations in the concentrations of dissolved species in formation waters to the presence of faults in order to determine whether a fault may be transmitting or inhibiting fluid flow. These cnteria can also be applied to stratigraphic and diagenetic features that potentially act as barriers to fluid migration, such as mudstones, shales, or pervasively cemented horizons in sands. The tests can also give gross directional information on fluid flow when sufficient numbers of contours describing the concentration gradients are present and can also help constrain the physical location of the flow path. For the purpose of the present study, the tests will be restricted to two dimensions in sand and mudstone sequences that are cut by high-angle faults.

In order to use fluid compositions to identify fluid motion and flowpath s in the subsurface, several conditions must initially exist: first, there must be compositionally distinct fluidspresent . Second, chemical reactions must not occur which significantly add or remove the dissolved species that are being used to identify the different fluid masses. Third, there must be a driving force present that causes fluidst o flow, and fourth, a sufficient number of control points must be present in the subsurface around the fault or feature in question. The problem of diagenetic reactions modifying the concentrations of the dissolved species that are being used to identify the different fluid masses and skewing interpretations can be circumvented by considering a number of different dissolved species that possess differing geochemical behavior in the diagenetic environment.

When the preconditions listed above are met, four tests may be applied to test the seal of faults as will be subsequently explained. The firsttw o tests, illustrated in

Figures 4.29a and 4.29b, are used to determine if the plane of the high-angle fault is Figure 4.29. Hypothetical spatial variations in fluidcomposition s across faults in alternating sand-shale sequences which are either transmissive or barriers to fluid flow. This diagram assumes that the compositional data for the fluidswer e obtained from the sands, and that the spatial distribution and coincidence of the contour lines within the mudstones is an artifact of interpolation between data points rather than actual compositional variations of the pore fluidswithi n the mudstones. See text for discussion. laterally permeable and involve a hypothetical fluid mass which is migrating laterally through a layer of sand towards a second fluidmas s of different composition contained within the same layer. The sand body is bounded above and below by mudstones, and the sand-mudstone sequence is cut by a high angle fault. Lateral fluid flow potentially

may be blocked by the fault. The second set of tests, illustrated in Figures 4.29c and

4.29d, are used to determine if the plane of the fault is acting as a fluid conduit and

applies to sand-mudstone sequences cut by high angle faults. In these two tests, a fluid

mass in one sand has the potential to migrate through a fault which potentially connects

one sand to another.

High angle fault acting as a barrier to lateral fluid migration

If the fault plane behaves as a barrier to lateral fluid flow,larg e concentration differences may develop on opposite sides of the fault plane as a result of compartmentalization of compositionally different fluid masses. In this situation, the contours describing concentration variations for the dissolved species in the formation waters will turn fromparalle l with the mudstone strata to trend parallel with the plane of the fault. A strong compositional disconformity, or very sharp compositional gradient will be present along the plane fault (Fig. 4.29a).

High angle fault open to lateral fluid migration

If the fault plane is laterally permeable, significant concentration differences will not develop across the plane of the fault (Fig. 4.29b). In this case, the concentration contours will cross the fault plane at high angles and remain parallel to the mudstone strata that enclose the sand. Mixing from dispersion between the two different fluid masses may result in a gradual compositional gradient that develops along the flowpat h in the sand. High angle fault acting as a barrier to vertica! fluid migration In the case where the fault is vertically impermeable, a strong compositional

difference may be present between the upper and lower sands (Fig. 4.29c). Under these conditions, the concentration contours will parallel the strata.

High angle fault acting as a conduit for vertical fluid migration

In the case that the fault plane is permeable and acts as a conduit for vertical fluid flow across the mudstone, the contour lines will deflect from parallel with the mudstone to parallel with the fault as the fault is approached (Fig. 4.29d). The contours will remain parallel to the fault plane as long as the sediments through which the fault cuts have low lateral permeability. The contours will deflect away from the fault horizontally when the fault again intersects laterally permeable strata. Fault influence on flow paths and dissolved species in the Iberia field The simple tests developed above can be applied in the Iberia field in order to determine the influence of faults on fluid migration because the preconditions that are required to use the tests are satisfied in portions of the Iberia study area. First, the chemical analyses from this study and those fromWorkma n and Hanor (1985) and

Posey et al. (1985) show that there are significant variations in the compositions of formation waters in this field.Second , the analyses contain a variety of dissolved species with different geochemical behaviors. Third, Hanor (1987) has shown that there are fluiddensit y instabilities that result fromtemperatur e and salinity differences in formation waters present to move the fluids, and fourth, there are a sufficient number of data control points to constrain interpretation of fluid flow directions in some portions

of the field.

Ba, B, Ca, Fe, and Sr were chosen for this portion of the study because they are

present in significantly different concentrations in the deep fluids and shallow fluids.

They also have diverse geochemical behavior during diagenesis. Figures 4.4,4.5,4.6,

4.7, and 4.8, for Ba, B, Ca, Fe, and Sr respectively, show a compositionally distinct

tongue of fluid low in Ba, Ca, and Fe and high in B and Sr which has moved or is

moving up the flank of the salt dome. A shallower body of fluidcoul d be interpreted as

having moved down and away fromth e salt dome. This apparent fluidmotio n agrees with the sense of fluidmotio n proposed by Workman and Hanor (1985), and Hanor

(1987), where they interpreted deep fluidsmovin g upward along the flanks of the dome, and fluids at shallower levels apparently moving downward and away fromth e salt dome. For each of the species considered in the diagrams, the contour lines associated with the deep tongue of fluidcros s faults a and b at high angles. The contour lines, however, do not strictly follow formation dips and cross mudstone strata. By the criteria presented in the first two arguments concerning lateral permeability and faults, it is apparent that faults a and b are laterally permeable and allow mobile fluidst o cross. The fact that the contours describing compositional variations also cross the mudstones at low angles is significant and will be discussed further in a subsequent section. The contours at faults a and b do not become parallel with the faults which suggests that significant vertical fluid flowi s not occurring within the planes of fault a or b through the overlying mudstones. When the fluid tongue reaches fault c it apparently turns parallel to and spatially

coincides with the fault, as is suggested by the contour lines. Fault c cuts across a thick

mudstone interval between 1600 m (5250 ft) and 1722 m (5650 ft), which allows the

tests for vertical permeability to be applied. By the reasoning used to develop Figure

4.29d, the plane of fault c is vertically permeable and allows fluidst o be conducted

upward across the thick mudstone.

The lack of control points and fluid analyses immediately to the east of fault d

preclude making judgments about its control on fluid migration. The volume of

sediments bounded by faults a and c, the bottom of the mudstone at approximately 1280

m (4200 ft), and the top of the mudstone at approximately 1600 m (5250 ft) contains

fluids that appear to be moving down and away from the salt dome. Unfortunately, the

lack of control points outside of faults a and d and above the bottom of the mudstone at

1280 m (4200 ft)prevent s drawing reliable conclusions about fault influence on fluid

flow above 1600 m (5250 ft) for all of the faults except fault c. The contour lines for Ba,

B, Ca, and Fe continue to parallel fault c at depths shallower than 1600 m (5250 ft) which suggests that fault c forms a vertical flow path nearly one kilometer in extent.

Fault a appears to be directing downward flow of the shallow fluids, but it is not clear if this fluid mass is being deflected downward because fault a is acting as a barrier, or if the fluids are preferentially being conducted along fault a in a fracture system.

Fractures and fracture dilation

The contour lines that describe the deep tongue of fluid show that its apparent flow path crosses mudstone strata at low angles. Because mudstones are normally thought to inhibit fluid flow, one would expect to see contour lines parallel the general

dip of mudstone strata in the cross section where obvious vertical conduits are absent

rather than crossing the mudstones at an angle. The low angle relation of the flow path

to the mudstone strata, especially in the absence of apparent faults, suggests that the

mudstones in the lower part of the section, which are within approximately 500 m of the

salt, are not effective barriers to cross-formational fluid migration. Hanna (1953) stated

that fracture porosity in the Gulf Coast is important as a permeability regime through

which fluid migration can occur. From the analyses of core samples obtained from

around salt domes, Hanna (1953) recognized pervasive fracturingwithi n the shale

sheath that mantles the flanks of some salt domes. In a later study of polished sidewall cores, Kerr and Kopp (1958) described a large dome shaped zone in the mildly deformed mudstone strata around salt domes in south Louisiana in which the mudstones are fractured and brecciated. The term "salt-dome breccia" was applied to these mudstones. Salt-dome breccias are not part of the shale sheath as described by Hanna

(1953), but occur throughout a much larger volume of stress-affected sediments that surround the salt dome. Kerr and Kopp (1958) also state that the zone containing the salt dome breccia may extend approximately one kilometer laterally from the salt dome and that the lateral extent of the zone may expand with increasing depth.

The fracturingo f bedded mudstones around salt domes is significant to fluid transmission. Some reasonable mechanism must be found to explain why the apparent flow path for the deep fluids at the Iberia field apparently crosses mudstone strata in the absence of obvious faults. Mudstone fracturingoffer s one reasonable explanation. However, simple fracturing alone is not completely adequate to support fluid flow.

Hooper (1991) points out that fracture dilation in association with growth faults is

required to support significant fluid flow in fractured sediments along the fault. By

following a similar line of reasoning, fluidflo w through fractures in the mudstones

around salt domes would also require some degree of dilation in order to become

significant fluid conduits. For instance, dilation of fractures in salt-dome breccias would

increase the permeability of fracturedmudstone s and offer substantial vertical flow

paths.

The presence of mineral precipitates with significant thickness in fractures

reveals that fractures in the Iberia fieldhav e been dilated and have acted as pathways for fluid migration. Given the pervasiveness of fracturing that is reported around salt domes

(Hanna, 1953; Kerr and Kopp, 1958), and potential fluid flow rates as high as tens of meters per year (Hanor, 1987), dilated fractures may over time accommodate the transport of large volumes of fluid and substantial quantities of dissolved metals.

Hanna (1953) also observed that fracture systems are present in competent beds around salt domes, and that the fracturesapparentl y are associated with faults. The presence of these fractures also raises the possibility that fault associated fracturing forms significant vertical fluid conduits through pervasively cemented sands that occur in proximity to salt domes. Pore-occluding calcite cements that would inhibit vertical fluid flow have been described in flank sands around salt domes (McManus and Hanor,

1993; Chapter 3 of this dissertation). Pervasive fracture textures have also been 170

documented in flank sands around salt domes (McManus and Hanor, 1988; Chapter 3 of

this dissertation).

Accommodation of fluid flow in intergranular porosity

The presence of diagenetic minerals within the intergranular porosity of sands is

physical evidence that the intergranular porosity around the Iberia dome also supports

fluid migration. Significant volumes of vermiform kaolinite, Na-aluminosilicate overgrowths, quartz overgrowths, pyrite cements, and calcite cements present in various sands throughout the sampled interval attests to the fact that fluids saturated in these various phases have moved through the intergranular porosity.

Fluid-mineral equilibria in the presence of dissolved NaCl

The systematic variations in the composition of natural formation waters have been used to support the theory that multi-phase silicate-aluminosilicate-carbonate buffering systems operate to control the composition of ambient formation waters in sedimentary basins with salinities greater than 10,000 mg/1 and less than halite saturation (ca. 350,000 mg/L) (Hanor, 1994a, 1994b, 1995 in press). Hanor (1994a and

1994b) conceptually develops an argument supporting this theory by using activity- activity diagrams. In the argument, the activity ratios of certain dissolved species

(aK+/oH+, aMg2+/ (aH+)2, aNa+/oH+) are arbitrarily fixed at triple points on activity diagrams containing the hypothetical buffering mineral assemblage (smectite, kaolinite, illite, chlorite) which fixes equilibrium composition of the fluid. If the activity of one of the species is increased by an order of magnitude, for instance that of Na+ by the dissolution of halite, the system must respond accordingly by increasing the pH in order 171 to preserve the ratio of aNa+/aH+ for equilibrium. The change in pH will require requisite changes in all other dissolved species that are involved in the coupled equilibria as well, because the change in pH alters all other activity ratios where oH+ appears in the denominator (i.e. oCa2+/ (aH+)2, aK+/aH+, etc.). The theoretical argument for multi-phase buffering based on activity diagrams is an implicit prediction that calculated activity ratio data fromnatura l waters should also group near triple points on activity diagrams developed with appropriate minerals if this type of buffering exists.

The activity ratio data for the Iberia field and the Land et al. (1988a) study that are plotted on the activity diagrams in Figures 4.27 and 4.28 are clearly consistent with the implicit prediction and supports the hypothesis that such coupled reactions occur.

CONCLUSIONS

Spatial variations in the concentrations of Ba, B, Ca, Fe, and Sr in formation waters on the southwest flank of the Iberia salt dome, Louisiana, support the earlier hypothesis of Workman and Hanor (1985) that kilometer-scale vertical migration of brines has occurred in this area, even in the presence of numerous mudstone beds. There is a tongue of low Ca-Ba-Fe and high B-Sr water which extends from a depth of approximately 2.5 km that parallels the salt-sediment interface and then makes a more vertical ascent upward within one km of the surface. These waters apparently are mixing with shallow waters having distinctly different compositions. Waters below

2600 m along the tongue of low Ca-Ba-Fe and high B-Sr water appear to be dissolving halite and anhydrite, but above 2600 m they do not. The deep tongue of water extends obliquely through the planes of two normal

faults, and it is reasonable to conclude that these portions of the faults are not sealing

with respect to lateral flow of aqueous fluid. The pronounced upward deflection of these

waters is approximately coincident with the plane of a third normal fault, and it is

possible that the presence of this fault is serving to channel flow upward.

The compositional trends cut across the dip of the bedding even in areas where

spatial variations are not obviously related to faults, and it is reasonable to conclude that

some other type of cross-formational transport has occurred. Samples of deep sediments

within approximately 300 m of the salt flank contain dilated and mineralized fractures

that attest to the past migration of fluids through fractureporosity . If such fracturing is

pervasive, the vertical permeability of the numerous mudstone beds around the salt

dome should be greatly enhanced. This would facilitate the broader pattern of cross-

formational transport evidenced by the spatial variations in formation water

compositions.

Systematic variations in formation water compositions show that fluids at the

Iberia dome are similar to those identified by Hanor (1994b) as "waters near salt

domes." The Iberia waters do not display the 1:1 and 2:1 trends found in other

formation waters, such as those fromth e seven Land et al. (1988a) fields.Th e calculated activity ratio data supply additional evidence that support the hypothesis of silicate-aluminosilicate-carbonate mineral buffering of formation water compositions.

The grouping of calculated activity ratio data at triple points on appropriate activity diagrams fulfills the implicit prediction of fixed activity ratios. CHAPTER 5) SUMMARY

EXPERIMENTAL WORK

The experimental work performed in this study shows that increases in the starting salinity of the fluids resulted in increases in the concentrations of a variety of dissolved species after reaction. Direct interpretation and correlation of the results from the raw fluidanalyse s to diagenesis around salt domes are difficult to make, however, because the compositional changes were due in part to exchange reactions and were not strictly due to dissolution/precipitation reactions. Nonetheless, important insights into the controls on diagenesis around salt domes can be made by referring to the results from the thermodynamic modeling of the fluid compositions.

Analcime and carbonate minerals

+ + The activity diagram of log(aNa /aH ) versus log(aH4Si04) (Figure 2.22 in

Chapter 2), on which the calculated activity data for the 90°C experiments at 90 days were plotted, displays two trends that are consistent with observed patterns of sediment diagenesis around salt domes. Both sets of data points from the carbonate-bearing experiments at low salinities plot in the kaolinite stability field, and with increasing salinity, the points trend upward toward the analcime stability field. The carbonate-free experiments have data that plot in the kaolinite stability field at low salinities, as above, but trend into the albite stability fielda t high salinities. The two differing trends between carbonate-bearing and carbonate-free experiments suggest that carbonate minerals must be present in the diagenetic system around salt domes in order to evolve fluids during evaporite dissolution that are capable of precipitating analcime at high

173 salinities. It thus appears that the presence of carbonate minerals in the chemical

diagenetic system acts as a diagenetic control. Field evidence bears out the apparent

interdependence between analcime and carbonate minerals. Around the West Hackberry

salt dome (McManus, 1991) and the EI-128 salt dome (Chapter 3 this dissertation),

analcime has been reported to preferentially precipitate in void space within carbonate

fossil tests. Diagenetic analcime at the Iberia salt dome (Chapter 4, this dissertation)

was not observed to precipitate in the direct presence of carbonate minerals, however,

diagenetic calcite cements are present in nearby strata. Given the apparent extent of

vertical fluid movement in flank sediments at the Iberia dome, it is likely that the

carbonate cements are chemically in communication through formation waters in the

geochemical system in a spatially extended sense. What is still unknown is why the

apparent link between analcime precipitation and carbonate minerals exists.

Salinity, kaolinite ,and analcime

A second observation fromFigur e 2.22 is that under low-salinity conditions

around salt domes, with or without carbonates in the system, diagenetic kaolinite may

be expected to form. At higher salinities, as discussed above, albite or analcime would be expected to form. Salinity thus appears to be a controlling factor in the experiments, and we would expect to findkaolinit e around salt domes in formations with low salinity pore fluids. Diagenetic kaolinite has been reported in flank sediments at the Black

Bayou salt dome (Leger, 1988) and at the Iberia salt dome (Chapter 4, this dissertation), but the low-salinity link cannot be established with the information that is presently available. In the case of the Iberia field, there is no direct information regarding 175

salinities in well 19, and thus it is not possible to evaluate if diagenetic kaolinite

precipitates under relatively lower salinity conditions within shallower strata in that well

in which it is found. At the Black Bayou salt dome, kaolinite is reported only within

100m (325 ft) of salt (Leger, 1988), which suggests that higher-salinity conditions may

prevail in those sediments. This observation seems to contradict the proposed salinity

dependency, however there are other considerations.

Spatial distribution of diagenetic minerals

Predicting the salinity structure around salt domes based on proximity to salt is likely to be a highly uncertain process. The Iberia study has shown that faulting and fracturing in part control the distribution of formation waters around the Iberia dome, and thus can influence the spatial variations in formation water compositions as well.

Because fluid flow apparently is spatially heterogeneous around salt domes, diagenetic reactions that are salinity dependent will also be spatially controlled by the location of flowpaths. For example, kaolinite precipitation may occur along flowpaths fromdee p formations before they intersect with salt or where low salinity fluidsremai n isolated from salt by low-permeability gouge at the margin of the diapir. More detailed information clearly is needed on the salinity structure around salt domes in order to verify the connection between salinity and the species of diagenetic mineralization observed in flanksediments . EI 128 SALT DOME Crushing, dissolution rates, and stability of geochemical system The petrographic study of samples fromaroun d the salt-sediment interface at the

Eugene Island 128 salt dome revealed that a variety of physical and chemical processes influence diagenesis in the vicinity of the salt-sediment interface. The most obvious physical process is the crushing of framework grains. Kinetic and thermodynamic considerations based on the surface area of a mineral and the relation of a mineral's surface area to its volume predict that reaction rates will be increased when silicate minerals are crushed to micron and submicron sizes. At EI 128, the presence of crushed grains in some samples and floating sand-grain textures in others apparently results from the preferential dissolution of fines.Aroun d salt domes, framework-grain crushing must be considered as an additional potential control on diagenesis.

Salt dome dissolution and sulfate minerals

Petrographic evidence in this study also shows that substantial secondary porosity has developed in the halite at the margin of the salt diapir. This observation confirms that salt is actively removed by dissolution from the flankso f the EI 128 salt dome. Less obvious is the apparent connection of halite dissolution to the physical process of geopressured fluid expulsion. The development of barite pseudomorphs after anhydrite in the margin of the salt shows that the fluids responsible for dissolving the salt are rich in barium. Fluids that are barium-rich are consistent with the destruction of

K-feldspar (Macpherson, 1989), and in the Gulf Coast, massive K-feldspar destruction has been documented to occur deeper within the basin below the surface of geopressure (Land et al, 1987). The formation of barite pseudomorphs after anhydrite thus points

toward geopressured fluidexpulsio n fromdee p formations as being responsible for the

dissolution of halite in the samples from EI-128. The interaction between barium and

sulfate around salt domes may act as an important control on their concentrations in

nearby formation waters.

The sequence of compositions of the Ba-Sr sulfate pseudomorphs and the final composition of the pseudomorphs in the accretion residues at the margin of the salt suggest that the formation waters moving through the salt are initially strontium rich and then become barium rich.Diageneti c barite in the flank strata contain Sr at levels below detection limits by EDS, and this fact suggests that Sr is largely derived from within the diapir rather than being supplied by the formation waters dissolving the salt.

Land (1988b) reports that Sr is present in evaporitic anhydrite in Gulf Coast salt domes in concentrations as high as 1000 ppm. Because of this, anhydrite destruction either by dissolution or replacement would liberate strontium thus raising the Sr/Ba molar ratios as dissolution proceeds along a flow path contained within the margin of the diapir.

Down-path Sr/Ba ratios can thus be increased to the point that Ba-rich celestites precipitate.

Redox reactions

At the EI-128 salt dome, the presence of pore-occluding calcite cements in the flank sediments in conjunction with diagenetic pyrite, in textures and relations that are nearly identical to those at the West Hackberry salt dome, suggest that coupled redox reactions occur at EI-128 that are similar to those proposed to occur at the West 178

Hackberry salt dome by McManus and Hanor (1988). Some of the details of the redox reactions at EI-128, however, appear to differ. For instance, the EI 128 study shows that

Ca is liberated from anhydrite during the pseudomorphic replacement of anhydrite by

Ba-Sr sulfates as well as by direct dissolution as was proposed at West Hackberry by

McManus and Hanor (1988). It is also apparent that iron and titanium-bearing detrital minerals at EI-128 are altered in the redox reactions. Additionally, reaction 3.9 in

Chapter 3 (this dissertation) reveals that iron derived fromdetrita l minerals in the redox reactions does not need to be reducible ferric iron. Thus, the redox reactions appear to be coupled to the destruction of a variety of detrital iron-bearing minerals.

IBERIA SALT DOME

The spatial variations in the concentrations of Ba, B, Ca, Fe, and Sr in formation waters on the southwest flank of the Iberia salt dome, Louisiana, not only support the earlier hypothesis of Workman and Hanor (1985) that kilometer-scale vertical migration of brines has occurred in this area, but also demonstrate that there is a heterogeneous distribution of the various fluidbodie s relative to salt that is in part controlled by local structural features. As mentioned above, this means that diagenetic processes related to halite dissolution around salt domes cannot simply be related to distance fromsal t as we might expect in a diffusion dominated setting, but rather that the spatial distribution and character of diagenetic minerals will be related to the location of flow paths in and around the salt dome. The work of Bennett and Hanor (1987) and Bray and Hanor

(1990) has shown that saline fluids have migrated kilometers from dissolving salt domes. As such, those fluids will exert influences on diagenesis at distance as well. Slopes on log cation-log chloride plots: A measure of equilibrium Variations in formation water compositions when plotted on log-log plots of cation versus chloride show that formation waters on the southwest flanko f the Iberia dome are similar to those identified by Hanor (1994b) as waters near salt domes in southwest Louisiana. These waters do not plot on 1:1 or 2:1 sloping lines which represent equilibrium compositional trends. This feature is consistent with formation waters that are not in equilibrium with the sediment package as a result of the slower kinetics of the silicate-aluminosilicate-carbonate reactions relative to the more rapid halite dissolution reaction (Hanor, personal communication). Such fluidsar e capable of inducing further coupled diagenetic reactions until they once again have compositions which plot along equilibrium concentration trends.

FUTURE RESEARCH

The research in this dissertation raises a number of questions that will require further exploration. Among the potential projects are the following: 1) specific theoretical and experimental investigations should be performed to determine the nature of the link between analcime precipitation and carbonate minerals. 2) More refined experiments should be performed in order to completely characterize the link between stable diagenetic minerals around salt domes, such as analcime and kaolinite, and salinity. 3) A field oriented project should be performed to characterize on a finer scale the spatial variations in salinity around a salt dome. Such a project should include at a minimum drill-cuttings from all wells, a complete set of electric logs fromal l wells, and formation water analyses from all wells. If an appropriate field could be located, it 180

would be desirable to have a portion of the data set that included wells which penetrate

the surface of geopressure, and wells that form a transect from the salt-sediment

interface and outward approximately 1 km. 4) Laboratory experiments should be

performed with common detrital minerals that measure hydrolysis rates both as a

function of grain size and salinity. 5) Information on reaction rates could also be used to

formulate and refine theoretical explanations for the compositional behavior of

formation waters around salt domes on plots of log cation versus log chloride plots. 6)

Investigations should be performed to determine if fluid expulsion around salt domes is

periodic in nature, steady-state, or a combination of the two. 7) Given that fluid

expulsion from beneath the surface of geopressure around salt domes may be periodic,

investigations should be initiated to investigate the amount of time it takes for spatial geochemical anomalies in flank pore waters to decay. BIBLIOGRAPHY

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191 192

A.l. pH ANALYSES bdl=below detection limit; na=not analyzed; si=siliciclastic; asi=aragonite+ siliciclastics; dsi=dolomite+siliciclastics; xx25 or xx90=25°C or 90°C experiment

Exp. mgNaC|/L 30 days 60 days 90 days 270 days si90 1 na 5.90 5.85 na si90 10 na 6.10 5.80 na si90 100 na 5.98 5.65 na si90 1000 na 5.80 5.55 na si90 10050 na 5.65 5.48 na si90 35800 na 5.45 5.41 na si90 73400 na 5.40 5.35 na si90 166300 na 5.20 5.19 na si90 297300 na 5.05 5.01 na si90 ca. 350000 na 4.95 4.98 na si25 1 7.00 6.95 7.20 na si25 10 6.75 6.90 7.05 na si25 100 6.50 7.05 7.03 na si25 1000 6.50 6.95 6.91 na si25 10050 6.35 6.65 6.75 na si25 35800 6.25 6.57 6.75 na si25 73400 6.15 6.35 6.45 na si25 166300 5.7 5.95 6.08 na si25 297300 5.25 5.50 5.58 na si25 ca. 350000 5.00 5.48 5.32 na asi90 10 7.98 8.13 8.55 na asi90 1000 7.75 8.05 8.43 na asi90 73400 7.12 7.72 8.12 na asi90 297300 6.92 7.10 7.30 7.75 asi90 ca. 350000 6.83 6.88 7.12 7.55 asi25 10 7.58 7.68 8.30 7.85 asi25 1000 7.88 7.05 8.07 7.85 asi25 73400 7.50 7.00 7.90 7.55 asi25 297300 6.75 6.62 7.10 6.92 asi25 ca. 350000 6.80 6.38 7.10 6.80 dsi90 10 7.83 7.86 8.20 na dsi90 1000 7.68 7.74 8.12 na dsi90 73400 7.50 7.53 7.88 na dsi90 297300 6.81 6.85 7.01 7.63 dsi90 ca. 350000 6.73 6.73 7.25 7.58 dsi25 10 7.52 7.98 8.50 8.00 dsi25 1000 7.11 7.78 8.00 7.78 dsi25 73400 7.15 7.20 8.02 7.50 dsi25 297300 6.70 6.58 7.28 6.70 dsi25 ca. 350000 6.58 6.55 6.80 6.68 193

A.2. TOTAL ALKALINITY as mg(HC03")/L bdl=below detection limit; na=not analyzed; si=siliciclastic; asi=aragonite+ siliciclastics; dsi^dolomite+siliciclastics; xx25 or xx90=25°C or 90°C experiment

Exp. mgNaC|/L 30 days 60 days 90 days 270 days si90 1 na 30.5 28.7 na si90 10 na 27.5 25.6 na si90 100 na 25.6 23.2 na si90 1000 na 19.0 13.4 na si90 10050 na 18.3 17.1 na si90 35800 na 14.6 16.5 na si90 73400 na 11.0 15.9 na si90 166300 na 2.4 12.8 na si90 297300 na 1.2 1.8 na si90 ca. 350000 na 6.1 38.4 na si25 1 62.2 60.4 73.2 na si25 10 56.0 73.8 75.6 na si25 100 53.7 69.5 53.7 na si25 1000 43.3 54.0 53.1 na si25 10050 34.2 40.3 37.8 na si25 35800 33.0 25.6 29.3 na si25 73400 26.8 17.7 15.3 na si25 166300 20.1 9.2 11.6 na si25 297300 20.7 4.0 1.8 na si25 ca. 350000 10.4 6.7 0.6 na asi90 10 64.7 65.9 75.6 na asi90 1000 54.3 62.2 72.0 na asi90 73400 64.7 59.8 65.9 na asi90 297300 30.5 28.7 25.0 37.8 asi90 ca. 350000 25.0 20.7 22.0 43.9 asi25 10 130 174 189 218 asi25 1000 118 157 174 207 asi25 73400 102 122 133 129 asi25 297300 52.5 53.7 53.7 51.0 asi25 ca. 350000 42.1 42.1 49.4 53.7 dsi90 10 56.1 56.1 61.0 na dsi90 1000 51.2 51.2 56.1 na dsi90 73400 51.2 50.0 52.5 na dsi90 297300 26.2 23.8 20.1 30.5 dsi90 ca. 350000 20.7 20.1 26.2 29.3 dsi25 10 138 160 170 173 dsi25 1000 139 153 166 161 dsi25 73400 111 126 141 128 dsi25 297300 59.8 100 50.0 37.8 dsi25 ca. 350000 55.5 44.5 42.7 30.5 194

A.3. CI" ANALYSES (mg/L)

bdI=below detection limit; na=not analyzed; si=siliciclastic; asi=aragonite+ siliciclastics; dsi=dolomite+siliciclastics; xx25 or xx90=25°C or 90°C experiment

Exp. mgNnn/L Clatt=0 30 days 60 days 90 days 270 days si90 1 0.61 na na na na si90 10 6.06 na na na na si90 100 60.7 na na na na si90 1000 607 na na na na si90 10050 6100 na na na na si90 35800 21700 22000 22100 22800 na si90 73400 44500 45500 45900 47400 na si90 166300 101000 101000 102000 102000 na si90 297300 180000 184000 183000 177000 na si90 ca. 350000 207000 209000 210000 209000 na si25 1 0.61 na na na na si25 10 6.06 na na na na si25 100 60.7 na na na na si25 1000 607 na na na na si25 10050 6100 na na na na si25 35800 21700 22500 22500 22600 na si25 73400 44500 46500 46500 45600 na si25 166300 101000 104000 103000 94000 na si25 297300 180000 186000 187000 187000 na si25 ca. 350000 198000 199000 198000 200000 na asi90 10 6.06 na na na na asi90 1000 607 na na na na asi90 73400 44500 na na na na asi90 297300 101000 na na na na asi90 ca. 350000 207000 na na na 204000 asi25 10 6.06 na na na na asi25 1000 607 na na na na asi25 73400 44500 na na na na asi25 297300 101000 na na na na asi25 ca. 350000 198000 na na na 194000 dsi90 10 6.06 na na na na dsi90 1000 607 na na na na dsi90 73400 44500 na na na na dsi90 297300 101000 na na na na dsi90 ca. 350000 207000 na na na 203000 dsi25 10 6.06 na na na na dsi25 1000 607 na na na na dsi25 73400 44500 na na na na dsi25 297300 101000 na na na na dsi25 ca. 350000 198000 na na na 194000 A.4. Ca ANALYSES (mg/L) bdl=below detection limit; na=not analyzed; si=siliciclastic; asi=aragonite+ siliciclastics; dsi=dolomite+siliciclastics; xx25 or xx90=25°C or 90°C experiment

Exp. mgN«ci/L 30 days 60 days 90 days 270 days si90 1 20 29 38 na si90 10 15 28 39 na si90 100 25 36 49 na si90 1000 70 82 97 na si90 10050 322 342 361 na si90 35800 593 572 610 na si90 73400 687 755 768 na si90 166300 766 789 826 na si90 297300 800 792 777 na si90 ca. 350000 792 763 722 na si25 1 5 10 9 na si25 10 4 6 8 na si25 100 13 14 17 na si25 1000 75 69 75 na si25 10050 373 366 363 na si25 35800 622 671 704 na si25 73400 626 717 882 na si25 166300 717 938 855 na si25 297300 842 967 846 na si25 ca. 350000 947 967 846 na asi90 10 67 95 116 na asi90 1000 106 139 164 na asi90 73400 521 664 698 na asi90 297300 515 678 755 850 asi90 ca. 350000 568 654 743 898 asi25 10 45 55 59 75 asi25 1000 87 100 89 107 asi25 73400 492 513 412 492 asi25 297300 541 526 474 510 asi25 ca. 350000 486 535 448 520 dsi90 10 69 85 133 na dsi90 1000 103 126 154 na dsi90 73400 509 575 622 na dsi90 297300 531 604 618 815 dsi90 ca. 350000 510 546 616 858 dsi25 10 51 57 58 56 dsi25 1000 93 96 92 94 dsi25 73400 453 501 466 482 dsi25 297300 483 579 441 498 dsi25 ca. 350000 509 599 425 489 A.5. K ANALYSES (mg/L) bdl=below detection limit; na=not analyzed; si=siliciclastic; asi=aragonite+ siliciclastics; dsi=dolomite+siliciclastics; xx25 or xx90=25°C or 90°C experiment Exp. mgNnn/L 30 days 60 days 90 days 270 days si90 1 16 11 17 na si90 10 11 15 14 na si90 100 21 17 14 na si90 1000 22 20 21 na si90 10050 28 29 32 na si90 35800 42 33 38 na si90 73400 45 37 52 na si90 166300 88 117 112 na si90 297300 181 205 246 na si90 ca . 350000 268 287 306 na si25 1 bdl 11 4 na si25 10 6 bdl 4 na si25 100 2 bdl 8 na si25 1000 6 14 13 na si25 10050 25 17 17 na si25 35800 36 52 57 na si25 73400 45 85 72 na si25 166300 65 164 47 na si25 297300 131 226 136 na si25 ca., 350000 134 290 261 na asi90 10 bdl bdl bdl bdl asi90 1000 bdl bdl bdl bdl asi90 73400 bdl bdl bdl bdl asi90 297300 bdl bdl bdl 62 asi90 ca,.35000 0 bdl bdl bdl 28 asi25 10 bdl bdl bdl 8 asi25 1000 bdl bdl bdl 11 asi25 73400 bdl bdl bdl 31 asi25 297300 bdl bdl bdl bdl asi25 ca,.35000 0 bdl bdl bdl bdl dsi90 10 bdl bdl bdl bdl dsi90 1000 bdl bdl bdl bdl dsi90 73400 bdl bdl bdl bdl dsi90 297300 bdl bdl bdl bdl dsi90 ca. 350000 bdl bdl bdl 48 dsi25 10 bdl bdl bdl 9 dsi25 1000 bdl bdl bdl 18 dsi25 73400 bdl bdl bdl 52 dsi25 297300 bdl bdl bdl 104 dsi25 ca. 350000 bdl bdl bdl 192 A.6. Mg ANALYSES (mg/L) bdl=below detection limit; na=not analyzed; si=siliciclastic; asi=aragonite+ siliciclastics; dsi=dolomite+siliciclastics; xx25 or xx90=25°C or 90°C experiment Exp. mgNnci/L 30 days 60 days 90 days 270 days si90 1 6 6 6 na si90 10 3 6 8 na si90 100 5 8 11 na si90 1000 12 11 15 na si90 10050 56 59 64 na si90 35800 99 102 109 na si90 73400 106 132 138 na si90 166300 126 135 136 na si90 297300 119 125 128 na si90 ca . 350000 115 113 111 na si25 1 bdl bdl bdl na si25 10 bdl bdl bdl na si25 100 1 bdl 1 na si25 1000 14 11 13 na si25 10050 74 74 72 na si25 35800 123 128 135 na si25 73400 109 126 150 na si25 166300 132 181 149 na si25 297300 154 196 136 na si25 ca,. 350000 176 196 124 na asi90 10 4 1 1 na asi90 1000 6 bdl 2 na asi90 73400 34 7 8 na asi90 297300 48 20 21 bdl asi90 ca,.35000 0 56 30 20 bdl asi25 10 10 10 11 16 asi25 1000 20 18 16 24 asi25 73400 94 86 74 90 asi25 297300 102 78 80 83 asi25 ca. ,350000 94 79 85 84 dsi90 10 8 5 14 na dsi90 1000 9 9 7 na dsi90 73400 58 50 36 na dsi90 297300 60 62 62 27 dsi90 ca. 350000 59 46 65 20 dsi25 10 13 11 14 13 dsi25 1000 24 20 21 23 dsi25 73400 90 85 96 98 dsi25 297300 97 92 87 94 dsi25 ca. 350000 104 110 89 91 198

A.7. Na ANALYSES (mg/L)

bdl=below detection limit; na=not analyzed; si=siliciclastic; asi=aragonite+ siliciclastics; dsi=dolomite+siliciclastics; xx25 or xx90=25°C or 90°C experiment

Exp. mgNaa/L Naatt=0 30 days 60 days 90 days 270 days si90 1 0.39 8 47 32 na si90 10 3.94 21 38 33 na si90 100 39.3 51 79 67 na si90 1000 393 310 322 333 na si90 10050 3950 3480 3600 3680 na si90 35800 14100 12800 13100 13500 na si90 73400 28900 26700 28600 28200 na si90 166300 65300 59000 58600 52500 na si90 297300 117000 98000 98400 99900 na si90 ca. 350000 134000 116000 126000 122000 na si25 1 0.39 bdl 84 bdl na si25 10 3.94 11 101 4 na si25 100 39.3 38 114 23 na si25 1000 393 329 409 309 na si25 10050 3950 3770 3400 3580 na si25 35800 14100 14200 14000 14600 na si25 73400 28900 29500 25400 27500 na si25 166300 65300 63200 58400 61200 na si25 297300 117000 108000 116100 103000 na si25 ca. 350000 128000 110000 109000 113000 na asi90 10 3.94 39 58 135 na asi90 1000 393 308 396 540 na asi90 73400 28900 25100 29400 27800 na asi90 297300 117000 104000 95600 119000 129000 asi90 ca. 350000 134000 110000 123000 131000 126000 asi25 10 3.94 38 125 106 26 asi25 1000 393 396 406 464 366 asi25 73400 28900 31700 32700 25200 27900 asi25 297300 117000 12400 118000 116000 120000 asi25 ca. 350000 128000 132500 135000 126000 148000 dsi90 10 3.94 34 81 17 na dsi90 1000 393 310 532 410 na dsi90 73400 28900 23900 30800 33000 na dsi90 297300 117000 107000 108000 127000 121000 dsi90 ca. 350000 134000 129000 124000 123000 128000 dsi25 10 3.94 bdl 10 38 38 dsi25 1000 393 342 456 400 422 dsi25 73400 28900 27900 38700 31420 29200 dsi25 297300 117000 118000 137000 121000 118000 dsi25 ca. 350000 128000 128000 159000 136000 126000 A.8. DISSOLVED Si02 (mg/L) bdl=below detection limit; na=not analyzed; si=siliciclastic; asi=aragonite+ siliciclastics; dsi=dolomite+siliciclastics; xx25 or xx90=25°C or 90°C experiment Exp. mgNnr,/L 30 days 60 days 90 days 270 days si90 1 107 107 128 na si90 10 94 126 126 na si90 100 107 105 120 na si90 1000 98 94 105 na si90 10050 92 92 105 na si90 35800 98 81 90 na si90 73400 116 83 96 na si90 166300 135 86 90 '' na si90 297300 263 94 94 na si90 ca . 350000 310 107 118 na si25 1 77 11 17 na si25 10 58 17 15 na si25 100 51 11 13 na si25 1000 49 11 13 na si25 10050 43 9 9 na si25 35800 41 2 4 na si25 73400 45 bdl 2 na si25 166300 47 bdl bdl na si25 297300 62 bdl bdl na si25 ca,. 350000 49 bdl bdl na asi90 10 52 65 67 na asi90 1000 45 61 59 na asi90 73400 12 23 21 na asi90 297300 bdl bdl bdl bdl asi90 ca,.35000 0 bdl bdl bdl bdl asi25 10 6 6 7 7 asi25 1000 4 3 4 4 asi25 73400 bdl bdl bdl bdl asi25 297300 bdl bdl bdl bdl asi25 ca..35000 0 bdl bdl bdl bdl dsi90 10 44 60 58 na dsi90 1000 40 50 51 na dsi90 73400 15 24 19 na dsi90 297300 bdl bdl bdl bdl dsi90 ca. 350000 bdl bdl bdl bdl dsi25 10 8 9 8 9 dsi25 1000 5 4 5 5 dsi25 73400 bdl bdl bdl bdl dsi25 297300 bdl bdl bdl bdl dsi25 ca. 350000 bdl bdl bdl bdl A.9. Ba ANALYSES (mg/L) bdl=belpw detection limit; na=not analyzed; si=siliciclastic; asi=aragonite+ siliciclastics; dsi=dolomite+siliciclastics; xx25 or xx90=:25oC or 90°C experiment Exp. mgN„ri/L 30 days 60 days 90 days 270 days si90 1 bdl 0.1 bdl na si90 10 0.1 0.2 0.2 na si90 100 0.1 0.2 0.1 na si90 1000 0.4 0.5 2.2 na si90 10050 2.4 3.0 3.2 na si90 35800 7.5 8.5 9.8 na si90 73400 13 14 16 na si90 166300 18 20 23 na si90 297300 22 21 20 na si90 ca . 350000 21 23 18 na si25 1 bdl 0.5 bdl na si25 10 bdl 0.1 bdl na si25 100 bdl 0.1 bdl na si25 1000 0.1 0.3 0.1 na si25 10050 1.5 1.6 1.3 na si25 35800 4.8 4.9 4.9 na si25 73400 7.4 6.9 8.8 na si25 166300 11 9.0 11 na si25 297300 11 8.0 11 na si25 ca, 350000 12 9.0 11 na asi90 10 bdl bdl bdl na asi90 1000 bdl bdl bdl na asi90 73400 bdl bdl bdl na asi90 297300 bdl bdl bdl 6.0 asi90 ca,.35000 0 bdl bdl bdl 3.8 asi25 10 bdl bdl bdl bdl asi25 1000 bdl bdl bdl bdl asi25 73400 bdl bdl bdl bdl asi25 297300 bdl bdl bdl bdl asi25 ca.. 350000 bdl bdl bdl bdl dsi90 10 bdl bdl bdl na dsi90 1000 bdl bdl bdl na dsi90 73400 4.0 4.0 4.0 na dsi90 297300 bdl bdl bdl 10 dsi90 ca. 350000 bdl bdl bdl 10 dsi25 10 bdl bdl bdl bdl dsi25 1000 bdl bdl bdl bdl dsi25 73400 bdl bdl bdl 1.2 dsi25 297300 bdl bdl bdl 2.0 dsi25 ca. 350000 bdl bdl bdl 1.25 201

A.10. Fe ANALYSES (mg/L) bdl=below detection limit; na=not analyzed; si=siliciclastic; asi=aragonite+ siliciclastics; dsi=dolomite+siliciclastics; xx25 or xx90=25°C or 90°C experiment

Exp. mgNan/L 30 days 60 days 90 days 270 days si90 1 bdl bdl 1 na si90 10 bdl 2 1 na si90 100 bdl bdl bdl na si90 1000 1 bdl bdl na si90 10050 bdl bdl bdl na si90 35800 bdl bdl bdl na si90 73400 bdl bdl bdl na si90 166300 bdl bdl bdl na si90 297300 1 bdl bdl na si90 ca . 350000 bdl bdl 1 na si25 1 3 bdl bdl na si25 10 1 bdl bdl na si25 100 bdl bdl bdl na si25 1000 bdl bdl bdl na si25 10050 bdl bdl bdl na si25 35800 bdl bdl bdl na si25 73400 bdl bdl bdl na si25 166300 2 bdl bdl na si25 297300 1 1 1 na si25 ca,. 350000 bdl 3 3 na asi90 10 bdl bdl bdl na asi90 1000 bdl bdl bdl na asi90 73400 bdl bdl bdl na asi90 297300 bdl bdl bdl bdl asi90 ca,.35000 0 bdl bdl bdl bdl asi25 10 bdl bdl bdl bdl asi25 1000 bdl bdl bdl bdl asi25 73400 bdl bdl bdl bdl asi25 297300 1 bdl bdl bdl asi25 ca.,35000 0 bdl bdl bdl bdl dsi90 10 bdl bdl bdl na dsi90 1000 bdl bdl bdl na dsi90 73400 bdl bdl bdl na dsi90 297300 bdl bdl bdl bdl dsi90 ca. 350000 1 bdl bdl bdl dsi25 10 bdl bdl bdl bdl dsi25 1000 bdl bdl bdl bdl dsi25 73400 bdl bdl bdl bdl dsi25 297300 bdl bdl bdl bdl dsi25 ca. 350000 bdl bdl bdl bdl 202

A.11. Mn ANALYSES (mg/L) bdl=below detection limit; na=not analyzed; si=siliciclastic; asi=aragonite+ siliciclastics; dsi=dolomite+siliciclastics; xx25 or xx90=25°C or 90°C experiment Exp. mgN„rl/L 30 days 60 days 90 days 270 days si90 1 1 1 1 na si90 10 bdl 1 1 na si90 100 1 1 2 na si90 1000 2 3 3 na si90 10050 11 13 15 na si90 35800 22 26 29 na si90 73400 28 37 43 na si90 166300 41 49 54 na si90 297300 55 59 61 na si90 ca. 350000 56 63 63 na si25 1 bdl bdl bdl na si25 10 bdl bdl bdl na si25 100 bdl bdl bdl na si25 1000 bdl bdl bdl na si25 10050 bdl bdl bdl na si25 35800 bdl bdl bdl na si25 73400 bdl bdl bdl na si25 166300 bdl bdl bdl na si25 297300 1 1 bdl na si25 ca,. 350000 bdl 1 bdl na asi90 10 bdl bdl bdl na asi90 1000 bdl bdl bdl na asi90 73400 bdl bdl bdl na asi90 297300 bdl bdl bdl bdl asi90 ca,.35000 0 bdl bdl bdl bdl asi25 10 bdl bdl bdl bdl asi25 1000 bdl bdl bdl bdl asi25 73400 bdl bdl bdl bdl asi25 297300 1.0 bdl bdl bdl asi25 ca,. 350000 bdl bdl bdl bdl dsi90 10 bdl bdl bdl na dsi90 1000 bdl bdl bdl na dsi90 73400 bdl bdl bdl na dsi90 297300 0.1 bdl bdl bdl dsi90 ca., 350000 2.5 bdl bdl bdl dsi25 10 bdl bdl bdl bdl dsi25 1000 bdl bdl bdl bdl dsi25 73400 bdl bdl bdl bdl dsi25 297300 bdl bdl bdl bdl dsi25 ca.,35000 0 bdl bdl bdl bdl 203

A.12. Sr ANALYSES (mg/L)

bdl=below detection limit; na=not analyzed; si=siliciclastic; asi=aragonite+ siliciclastics; dsi=dolomite+siliciclastics; xx25 or xx90=25°C or 90°C experiment

Exp. mgN„n/L 30 days 60 days 90 days 270 days si90 1 bdl bdl bdl na si90 10 bdl bdl bdl na si90 100 0.1 0.1 0.1 na si90 1000 0.2 0.3 0.3 na si90 10050 1.3 1.3 1.6 na si90 35800 2.7 2.9 3.2 na si90 73400 3.4 3.8 4.0 na si90 166300 4.0 4.0 4.0 na si90 297300 4.0 4.0 3.0 na si90 ca. 350000 4.0 4.0 3.0 na si25 1 bdl 0.1 bdl na si25 10 bdl bdl bdl na si25 100 bdl bdl bdl na si25 1000 0.3 0.2 0.2 na si25 10050 1.5 1.4 1.3 na si25 35800 2.7 2.7 2.9 na si25 73400 3.1 2.8 3.7 na si25 166300 3.0 2.0 3.0 na si25 297300 3.0 2.0 3.0 na si25 ca. 350000 4.0 3.0 4.0 na asi90 10 1.3 1.6 2.2 na asi90 1000 2.0 2.4 3.0 na asi90 73400 12.0 15.2 16.4 na asi90 297300 14.0 18.0 20.0 23.0 asi90 ca. 350000 16.3 18.0 21.3 22.5 asi25 10 0.2 0.3 0.5 0.7 asi25 1000 0.4 0.4 0.6 0.8 asi25 73400 5.6 6.0 5.6 7.2 asi25 297300 6.0 5.0 7.0 8.0 asi25 ca. 350000 3.8 5.0 5.0 6.3 dsi90 10 0.2 bdl 0.1 na dsi90 1000 bdl 0.2 0.2 na dsi90 73400 1.6 1.2 1.6 na dsi90 297300 2.0 bdl 1.0 1.0 dsi90 ca. 350000 1.3 bdl bdl bdl dsi25 10 bdl bdl 0.1 bdl dsi25 1000 0.4 bdl bdl bdl dsi25 73400 0.8 0.8 1.6 0.8 dsi25 297300 bdl bdl 1.0 bdl dsi25 ca. 350000 1.25 bdl 2.5 bdl 204

A.13. XRD PEAK-HEIGHT RATIOS nc=not calculated; si=siliciclastic; asi=aragonite+siliciclastics; dsi=dolomite+ siliciclastics; xx25 or xx90=25°C or 90°C experiment Exp. mgNaC|/L qtz/kaol, glyc qtz/kaol, ad qtz/il, glyc qtz/il, ad si90 1 0.72 0.38 0.68 0.72 si90 10 0.41 0.33 0.50 0.52 si90 100 0.51 0.36 0.51 0.52 si90 1000 0.38 0.27 0.35 0.42 si90 10050 0.34 0.17 0.37 0.17 si90 35800 0.51 0.19 0.53 0.21 si90 73400 0.42 0.14 0.38 0.14 si90 166300 0.21 0.39 0.21 0.40 si90 297300 0.58 0.31 0.51 0.42 si90 ca. 350000 0.40 0.43 0.38 0.71 si25 1 nc nc nc nc si25 10 nc nc nc nc si25 100 nc nc nc nc si25 1000 nc nc nc nc si25 10050 nc nc nc nc si25 35800 nc nc nc nc si25 73400 nc nc nc nc si25 166300 nc nc nc nc si25 297300 nc nc nc nc si25 ca. 350000 nc nc nc nc asi90 10 nc nc nc nc asi90 1000 nc nc nc nc asi90 73400 nc nc nc nc asi90 297300 nc nc nc nc asi90 ca. 350000 nc nc nc nc asi25 10 nc nc nc nc asi25 1000 nc nc nc nc asi25 73400 nc nc nc nc asi25 297300 nc nc nc nc asi25 ca. 350000 nc nc nc nc dsi90 10 nc nc nc nc dsi90 1000 nc nc nc nc dsi90 73400 nc nc nc nc dsi90 297300 nc nc nc nc dsi90 ca. 350000 nc nc nc nc dsi25 10 nc nc nc nc dsi25 1000 nc nc nc nc dsi25 73400 nc nc nc nc dsi25 297300 nc nc nc nc dsi25 ca. 350000 nc nc nc nc 205

A.14a. CALCULATED ACTIVITY DATA bdl=below detection limit; nc=not calculated; si=siliciclastic; asi=aragonite+ siliciclastics; dsi=dolomite+siliciclastics; xx25 or xx90=25°C or 90°C experiment T Exp. mgNBC,/L log(aH4Si04) log(aCa"7(H )") log(aMg'7(HY) si90 1 nc nc nc si90 10 -2.6207 8.483 8.019 si90 100 nc nc nc si90 1000 -2.6985 8.218 7.644 si90 10050 nc nc nc si90 35800 nc nc nc si90 73400 -2.6457 8.173 7.675 si90 166300 si90 297300 -2.5170 8.133 7.684 si90 ca. 350000 -2.4101 8.168 7.714 si25 1 nc nc nc si25 10 -3.5185 10.33 bdl si25 100 nc nc nc si25 1000 -3.5786 10.86 10.33 si25 10050 nc nc nc si25 35800 nc nc nc si25 73400 -4.2763 10.66 10.17 si25 166300 nc nc nc si25 297300 bdl 9.651 9.315 si25 ca. 350000 bdl 9.173 8.821 asi90 10 -2.9126 12.83 10.90 asi90 1000 -2.9664 12.61 10.48 asi90 73400 -3.3104 12.06 10.39 asi90 297300 bdl 11.33 10.11 asi90 ca. 350000 bdl 11.29 10.08 asi25 10 -3.8483 13.60 13.10 asi25 1000 -4.0706 13.22 12.72 asi25 73400 bdl 13.27 12.80 asi25 297300 bdl 12.34 12.02 asi25 ca. 350000 bdl 12.45 12.20 dsi90 10 -2.9726 12.43 11.67 dsi90 1000 -3.0251 12.26 11.16 dsi90 73400 -3.3571 11.63 10.64 dsi90 297300 bdl 10.59 10.29 dsi90 ca. 350000 bdl 11.36 10.74 dsi25 10 -3.8092 14.00 13.61 dsi25 1000 -4.0275 13.11 12.70 dsi25 73400 bdl 13.49 13.08 dsi25 297300 bdl 12.69 12.43 dsi25 ca. 350000 bdl 11.80 11.59 206

A.14b. CALCULATED ACTIVITY DATA

bdl=below detection limit; nc=not calculated; si=siliciclastic; asi=aragonite+ siliciclastics; dsi=dolomite+siliciclastics; xx25 or xx90=25°C or 90°C experiment T z iT Exp. mgNaci/L log(aNa7rT) log(oK7H ) log(aCa 7oMg ) si90 1 nc nc nc si90 10 2.938 2.334 0.4638 si90 100 nc nc nc si90 1000 3.639 2.205 0.5734 si90 10050 nc nc nc si90 35800 nc nc nc si90 73400 5.163 2.152 0.4981 si90 166300 nc nc nc si90 297300 5.595 2.590 0.4493 si90 ca . 350000 5.687 2.661 0.4539 si25 1 nc nc nc si25 10 3.273 3.043 bdl si25 100 nc nc nc si25 1000 4.977 3.368 0.5281 si25 10050 nc nc nc si25 35800 nc nc nc si25 73400 6.351 3.490 0.4900 si25 166300 nc nc nc si25 297300 6.307 2.963 0.3354 si25 ca..35000 0 6.147 2.990 0.3524 asi90 10 5.505 bdl 1.927 asi90 1000 5.941 bdl 1.768 asi90 73400 7.130 bdl 1.673 asi90 297300 7.303 bdl 1.216 asi90 ca .350000 7.299 bdl 1.210 asi25 10 5.923 bdl 0.4989 asi25 1000 6.307 bdl 0.5057 asi25 73400 7.772 bdl 0.4727 asi25 297300 7.838 bdl 0.3198 asi25 ca.,35000 0 7.875 bdl 0.2510 dsi90 10 4.372 bdl 0.7569 dsi90 1000 5.659 bdl 1.101 dsi90 73400 7.026 bdl 0.9926 dsi90 297300 7.195 bdl 0.6567 dsi90 ca.,35000 0 7.337 bdl 0.6209 dsi25 10 5.682 bdl 0.3916 dsi25 1000 6.174 bdl 0.4066 dsi25 73400 7.971 bdl 0.4129 dsi25 297300 8.005 bdl 0.2592 dsi25 ca. 350000 7.602 bdl 0.2052 207

A.15. CARBONATE SATURATION INDICES bdl=below detection limit; nc=not calculated; si=siliciclastic; asi=aragonite+ siliciclastics; dsi=dolomite+siliciclastics; xx25 or xx90=25°C or 90°C experiment Exp. mgN„n/L log SI aragonite log SI calcite log SI dolomite si90 1 nc nc nc si90 10 -1.881 -1.772 -2.221 si90 100 nc nc nc si90 1000 -2.217 -2.108 -3.001 si90 10050 nc nc nc si90 35800 nc nc nc si90 73400 -2.347 -2.227 -3.311 si90 166300 nc nc nc si90 297300 -3.113 -2.993 -4.796 si90 ca . 350000 -1.723 -1.603 -2.020 si25 1 nc nc nc si25 10 -1.639 -1.499 bdl si25 100 nc nc nc si25 1000 -1.170 -1.030 -1.488 si25 10050 nc nc nc si25 35800 nc nc nc si25 73400 -1.661 -1.521 -2.432 si25 166300 nc nc nc si25 297300 -2.757 -2.617 -4.470 si25 ca,. 350000 -3.439 -3.299 -5.850 asi90 10 0.824 0.933 1.726 asi90 1000 0.693 0.801 1.624 asi90 73400 0.076 0.196 0.359 asi90 297300 -0.509 -0.389 -0.354 asi90 ca . 350000 -0.502 -0.382 -0.333 asi25 10 0.724 0.864 2.330 asi25 1000 0.524 0.664 1.922 asi25 73400 0.418 0.558 1.744 asi25 297300 -0.111 0.029 0.837 asi25 ca., 350000 -0.028 0.112 1.072 dsi90 10 0.601 0.709 2.450 dsi90 1000 0.440 0.549 1.785 dsi90 73400 -0.285 -0.165 0.318 dsi90 297300 -0.844 -0.724 -0.465 dsi90 ca.,35000 0 -0.418 -0.298 0.422 dsi25 10 0.868 1.008 2.725 dsi25 1000 0.459 0.599 1.891 dsi25 73400 0.493 0.633 1.953 dsi25 297300 0.014 0.154 1.148 dsi25 ca. 350000 -0.438 -0.298 0.298 208

A.16. SILICA SATURATION INDICES bdl=below detection limit; nc=not calculated; si=siliciclastic; asi=aragonite+ siliciclastics; dsi=dolomite+siliciclastics; xx25 or xx90=25°C or 90°C experiment

Exp. mgNaC|/L log SI quartz log SI chalcedony log SI amorph. si90 1 nc nc nc si90 10 0.576 0.342 -0.336 si90 100 nc nc nc si90 1000 0.498 0.265 -0.413 si90 10050 nc nc nc si90 35800 nc nc nc si90 73400 0.743 0.493 -0.237 si90 166300 nc nc nc si90 297300 0.967 0.717 -0.013 si90 ca. 350000 1.095 0.845 0.115 si25 1 nc nc nc si25 10 0.412 0.212 -0.818 si25 100 nc nc nc si25 1000 0.352 0.152 -0.878 si25 10050 nc nc nc si25 35800 nc nc nc si25 73400 -0.309 -0.509 -1.539 si25 166300 nc nc nc si25 297300 bdl bdl bdl si25 ca. 350000 bdl bdl bdl asi90 10 0.284 0.050 -0.C28 asi90 1000 0.231 -0.003 -0.681 asi90 73400 0.076 -0.174 -0.904 asi90 297300 bdl bdl bdl asi90 ca. 350000 bdl bdl bdl asi25 10 0.082 -0.118 -1.148 asi25 1000 -0.140 -0.340 -1.370 asi25 73400 bdl bdl bdl asi25 297300 bdl bdl bdl asi25 ca. 350000 bdl bdl bdl dsi90 10 0.224 -0.010 -0.688 dsi90 1000 0.172 -0.062 -0.740 dsi90 73400 0.033 -0.217 -0.947 dsi90 297300 bdl bdl bdl dsi90 ca. 350000 bdl bdl bdl dsi25 10 0.121 -0.079 -1.109 dsi25 1000 -0.097 -0.297 -1.327 dsi25 73400 bdl bdl bdl dsi25 297300 bdl bdl bdl dsi25 ca. 350000 bdl bdl bdl 209

B.l. EI-128, WELL 12ST GENERAL LITHOLOGY, POROSITY, AND PERMEABILITY ss=sand; shy, sshy=shaley, slightly shaley; mg, fg, vfg =standard size modifiers Is = limestone; lam = laminated; mD = millidarcys (data from Core Petrophysics , Inc.) Depth lithology % porosity permeability (mD) m ft 1871 6139 sh 18.4 1.0 1874 6149 salt not measured not measured 1875 6153 vfg ss - vshy sltst 18.1 0.9 1876 6155 vfg ss, vshy, salty 18.7 1.5 1878 6162 salt not measured not measured 1878 6163 salt & mud not measured not measured 1879 6166 salt not measured not measured 1881 6171 vfg ss, vshy, salty 19.0 2.2 1882 6174 vfg ss - vshy sltst 18.0 0.7 1882 6176 vshy sltst, 70% salt 17.9 0.5 1905 6249 vshy sltst 18.6 1.0 2177 7143 vfg ss - vshy sltst 19.5 1.5 2177 7144 vshy sltst, 20% salt 18.9 1.1 2228 7310 salt not measured not measured 2232 7324 salt not measured not measured 2697 8850 sh & salt not measured not measured 2707 8882 vfg ss, vshy, salty 18.6 1.0 2708 8885 salt & mud not measured not measured 2731 8959 vfg ss - vshy sltst 18.1 0.9 2749 9018 sh 17.6 0.5 2770 9089 sh 18.0 0.9 2792 9160 sh 18.1 0.9 2803 9195 sh 17.7 0.6 2832 9290 vfg ss - vshy sltst 19.0 2.1 2869 9413 sh 17.8 0.5 2871 9420 vfg ss - vshy sltst 18.6 1.3 2874 9430 sh 17.6 0.3 3044 9987 vfgss 30.3 475.0 3045 9989 vfg ss, shy lam 24.8 70.0 3048 9999 vfg ss, sshy 30.1 220.0 3050 10008 vfg ss, sshy 21.5 25.0 3100 10171 vfg ss, shy-sshy 28.7 220.0 3103 10182 vfg ss, shy 24.3 50.0 210

B.2. EI-128, WELL 23 GENERAL LITHOLOGY, POROSITY, AND PERMEABILITY

ss=sand; shy, sshy=shaley, slightly shaley; mg, fg, vfg =standard size modifiers; sdy= sandy; Is = limestone; lam = laminated; mD = millidarcys (data from Core Petrophysics, Inc.) Depth lithology % porosity permeability (mD) m ft 1851 6072 vfg ss, shy - sshy 27.1 150.0 1926 6320 fg ss, shy 22.7 30.0 1941 6369 fg-mg ss, sshy 31.2 1150 2002 6568 vfg ss, shy - sshy 21.7 15.0 2015 6611 vfg ss, sshy 22.5 30.0 2030 6659 vfg-fg ss, sshy 14.0 1.7 2046 6713 sltst, vshy 18.1 0.8 2084 6836 ssdy-sshy Is 14.2 0.5 2093 6866 sh 17.9 0.7 2096 6876 shy Is 19.4 2.1 2103 6898 fg ss, vshy 19.0 1.5 2266 7436 fg-mg ss, shy-sshy 29.5 900.0 2272 7454 fg-mg ss, shy 26.1 70.0 2284 7494 fg ss, shy-sshy 28.0 330.0 2298 7538 fg-mg ss, sshy 30.3 860.0 2310 7580 fg-mg ss, sshy 28.7 280.0 2371 7780 shy Is 13.6 0.9 2378 7802 fg ss, shy-sshy 25.0 110.0 2404 7886 fg ss, shy-sshy 24.5 80.0 2493 8179 vfg-fg ss, shy-vshy 20.1 6.1 2503 8213 vfg-fg ss, shy-vshy 18.6 1.4 2622 8603 sh 18.2 0.9 2744 9001 sh 17.5 0.5 211

B.3. EI-128, WELL 12ST GRAIN-SIZE DISTRIBUTION

(data from Core Petrophysics , Inc.) Depth Sand Silt Clay m ft % % % crs med f vf crs med f vf 1871 6139 0.0 0.0 1.6 9.6 10.3 10.0 13.3 15.1 40.2 1874 6149 1875 6153 0.0 0.0 0.0 3.8 7.6 10.6 16.9 19.8 41.5 1876 6155 0.0 0.0 0.0 4.1 5.4 12.2 17.4 25.0 36.0 1878 6162 1878 6163 1879 6166 1881 6171 0.0 0.0 1.4 5.9 8.2 13.4 16.1 21.8 33.2 1882 6174 0.0 0.0 0.4 9.6 8.1 11.5 16.3 21.2 32.8 1882 6176 0.0 0.0 0.0 0.0 4.3 14.8 20.3 20.7 40.0 1905 6249 0.0 0.0 0.0 2.3 13.2 16.0 16.6 15.2 36.7 2177 7143 0.0 0.0 0.0 3.0 9.4 18.4 20.5 17.9 30.8 2177 7144 0.0 0.0 0.0 0.0 4.7 15.5 20.5 19.9 39.5 2228 7310 2232 7324 2697 8850 2707 8882 0.0 0.0 0.0 6.1 9.2 15.3 15.8 21.3 32.2 2708 8885 2731 8959 0.0 0.0 0.0 1.1 9.2 16.1 17.6 21.6 34.5 2749 9018 0.0 0.0 1.9 4.5 7.1 8.5 11.6 17.9 48.5 2770 9089 0.0 0.0 0.0 0.0 4.1 13.4 17.3 22.8 42.4 2792 9160 0.0 0.0 0.0 0.0 9.6 18.9 19.0 17.7 34.8 2803 9195 0.0 0.3 0.0 0.0 2.8 16.7 19.4 20.4 40.5 2832 9290 0.0 0.0 0.0 1.4 12.0 18.3 17.1 19.4 31.7 2869 9413 0.0 0.0 0.0 0.0 8.7 20.8 20.7 19.0 30.9 2871 9420 0.0 0.0 0.0 2.3 12.9 19.6 17.2 17.8 30.2 2874 9430 0.0 0.0 0.0 1.0 18.5 21.4 16.0 14.9 28.3 3044 9987 0.0 0.5 15.3 48.9 9.3 7.5 5.4 5.6 7.5 3045 9989 0.0 0.0 8.7 26.5 15.1 13.7 13.2 11.8 11.1 3048 9999 0.0 0.4 8.7 20.5 15.9 17.1 13.1 10.5 13.7 3050 10008 0.0 0.0 2.6 19.5 16.5 17.2 16.3 14.5 13.5 3100 10171 0.0 0.9 15.2 31.7 11.3 13.0 10.7 7.9 9.4 3103 10182 0.0 0.0 12.6 30.0 14.7 12.0 10.7 9.4 10.7 212

B.4. EI-128, WELL 23 GRAIN-SIZE DISTRIBUTION (data from Core Petrophysics, Inc.) Depth Sand Silt Clay m ft %> % i % crs med f vf crs med f vf 1851 6072 0.0 0.0 2.3 27.6 33.0 13.1 8.7 6.8 8.5 1926 6320 0.0 3.5 12.6 18.1 17.4 15.7 11.6 8.6 12.5 1941 6369 1.4 15.2 25.0 14.7 12.8 10.7 8.1 6.2 5.9 2002 6568 0.0 1.2 8.8 21.9 19.6 16.7 13.8 8.8 9.1 2015 6611 0.0 0.0 5.7 19.6 18.7 17.1 13.5 10.4 15.0 2030 6659 0.0 3.7 34.6 27.0 11.6 8.4 5.7 4.0 5.1 2046 6713 0.0 0.0 0.0 0.0 4.7 14.0 19.6 23.9 37.8 2084 6836 2093 6866 0.0 0.0 0.0 2.8 9.1 22.2 24.6 20.4 20.9 2096 6876 2103 6898 0.0 0.0 4.3 20.5 15.8 15.0 16.2 14.9 13.4 2266 7436 0.0 24.7 32.4 12.4 10.0 7.5 5.4 3.8 3.8 2272 7454 0.0 5.9 19.1 14.1 15.7 15.6 12.3 8.7 8.5 2284 7494 0.0 16.1 24.5 12.7 13.2 11.4 9.1 6.1 7.1 2298 7538 0.4 21.2 30.7 12.0 10.8 9.2 6.8 4.7 4.2 2310 7580 0.0 11.6 16.6 12.9 14.8 15.0 11.8 8.1 9.1 2371 7780 2378 7802 0.0 2.8 24.2 19.1 14.2 13.0 11.6 7.7 7.4 2404 7886 0.0 8.2 31.7 17.6 13.1 10.4 7.7 5.3 6.0 2493 8179 0.0 4.2 23.3 9.0 8.7 16.7 16.2 11.8 10.1 2503 8213 0.0 0.8 14.0 11.1 15.6 16.0 14.3 13.1 15.1 2622 8603 0.0 0.0 0.0 1.7 10.3 13.1 17.7 17.5 39.7 2744 9001 0.0 0.0 0.0 0.5 15.7 28.4 21.4 19.5 14.6 213

B.5. EI-128 FORMATION WATER ANALYSES bdl = below detection limit (data from Shell Offshore, Inc.) Well Prod.depth pH Ba Ca Fe Mg Total Alkalinity S04 CI mid-point as HC03 m ft mg/L 1 bdl 4588 847 bdl 118215 1 bdl 5346 1137 19 127440 1-D bdl 4014 935 bdl 100820 1-T 2723 8933 6.4 4482 46 1007 483 bdl 91848 2 7.5 1752 157 736 1182 968 10620 2 7 247 190 789 1577 813 15576 3 bdl 3591 605 bdl 105790 3 bdl 3511 774 bdl 94785 3 bdl 3836 668 bdl 110760 3D bdl 4237 881 bdl 102950 4 bdl 5080 1215 bdl 102940 4 6.2 4142 22 1385 306 15 102125 5 3829 12562 6 8940 531 422 308 bdl 133422 6 2774 9102 6.5 146 4701 31 1119 262 bdl 95408 6 2774 9102 6.2 5780 3 1135 385 bdl 99680 6 2822 9260 130 6 2822 9260 450 680 6 2991 9813 5.4 6276 33 697 318 bdl 115456 6D 6.3 4677 84 1214 143 41 92115 6D 5.3 7144 53 1325 408 86 119438 6D 4.9 7531 185 1076 285 5 120400 8-D bdl 4190 968 bdl 102240 8 3069 10069 4956 950 18 124584 9E 2232 7294 6.6 2 3536 34 1190 309 181 110004 9 2335 7661 6.4 bdl 3400 32 1290 102 103 108800 9 2335 7661 6.4 3870 44 1320 154 57 110400 IOC 1853 6079 6.2 49 3702 12 1285 27 34 93628 10 bdl 397 1386 270 20180 11-D 6.4 75 5210 38 625 97 bdl 91610 11 1973 6472 6.7 4080 32 1390 144 bdl 96200 11D 2863 9394 2.2 3780 47 1290 0 416 99800 12A 2618 8588 6.7 126 5034 26 1285 192 4 102172 12-D 3004 9857 6.5 bdl 3380 23 450 365 15 104700 12-D 3004 9857 6.3 bdl 4000 33 675 270 bdl 103600 12 3158 10360 6.4 4720 3 722 303 108 112900 13-D 2967 9734 6.7 bdl 4375 3 850 225 bdl 101000 13 3210 10533 6.4 75 5040 10 730 610 bdl 104385 214

B.6. CALCULATED BOTTOM-HOLE TEMPERATURES (BHT) AND CALCULATED MAXIMUM TEMPERATURES, EI 128

BHT = bottom hole temperature (data from Hanor, personal communication) \ ' / .. = highest temperature if different than BHT Well No. BHT Depth Max.T Depth" °C m ft °C m ft EI 128 #1 86 3463 11360 86 3463 11360 EI 128 #2 86 3505 11497 91 3061 10040^ EI 128 #3 84 3528 11576 84 3528 11576 EI 128 #4 89 3598 11806 89 3598 11806 EI 128 #5 108 4454 14612 108 4454 14612 EI 128 #6 97 3258 10690 97 3258 10690 EI 128 #8 83 3342 10964 83 3342 10964 EI 128 #9 74 2659 8720 82 2182 7158 EI 128 #10 78 3114 10216 78 3114 10216 EI 128 #11 71 2722 8929 71 2722 8929 EI 128 #13 88 3373 11065 88 3373 11065 EI 128 #14 82 3434 11265 82 3434 11265 EI 128 #17 110 4261 13980 110 4261 13980 215

CIA. IBERIA FIELD FORMATION WATER ANALYSES: pH, ACETATE, ALKALINITY * :data fromWorkma n (1985); bdl = below detection limit Well Depth* Depth* BHT* pH* Acetate* Total Alkalinity* m ft °C mg/L as mg/L HC03" 1 660 2165 41 6.21 8.1 193 2 1222 4010 57 6.53 8.5 338 3 1267 4158 58 6.09 1.5 366 4 1283 4209 58 6.27 8.7 271 5 1406 4612 62 6.89 10.3 245 6 1474 4837 64 6.80 9.2 327 7 1537 5043 66 6.75 0.0 448 8 1697 5566 71 6.66 26.6 348 9 1746 5728 72 6.26 9.0 254 10 1776 5827 73 6.42 3.0 310 11 1825 5987 74 6.05 3.9 464 12 1831 6008 74 6.21 7.5 211 13 1850 6070 75 6.43 7.1 272 14 1892 6207 76 6.21 2.7 277 15 2679 8791 89 6.51 8.9 311 16 2703 8869 89 6.46 42.1 466 17 2747 9014 89 6.22 51.7 496 18 2866 9402 90 7.00 66.2 837 216

C.1B. IBERIA FIELD FORMATION WATER ANALYSES: Na, K, 0 Mg, Ca, Sr, Ba, SiO2

* :data fromWorkma n (1985); bdl = below detection limit; Si0=2 °dissolve = d silica u Well Na K Mg Ca Sr Ba Si02 mg/L 1 50000 110 889 2830 109 93 19.5 2 49000 180 845 2770 104 69 17.3 3 95000 200 788 2760 93 bdl 21.4 4 51000 180 840 2560 97 53 24.4 5 46000 200 801 2580 98 66 19.3 6 50000 200 884 2750 116 74 25.5 7 50000 240 825 2640 98 76 19.5 8 51000 270 755 2800 140 78 21.2 9 53000 310 943 2940 119 67 27.6 10 48000 210 768 2590 130 35 22.0 11 58000 270 711 2590 136 7 22.9 12 51000 240 850 2850 109 72 21.4 13 48000 210 825 2660 113 69 19.9 14 52000 210 976 3170 123 69 23.1 15 50000 270 506 2320 128 7 29.1 16 49000 240 308 1650 149 3 32.5 17 53000 210 330 1960 172 2 33.4 18 48000 210 167 1000 92 1 34.9 217

C.IC. IBERIA FIELD FORMATION WATER ANALYSES: B, Cu, Fe, Pb, Zn, Mn, CI * :data fromWorkma n (1985); bdl = below detection limit ~Weli B Cu Fe Pb Zn Mn *CT~ mg/L 1 12.6 bdl 26.4 bdl bdl 2.7 74000 2 10.0 bdl 44.7 bdl 0.2 1.4 78000 3 17.4 1.1 66.5 bdl bdl 1.0 151000 4 11.0 0.1 26.8 bdl bdl 0.9 75000 5 10.0 bdl 22.5 bdl 0.4 1.0 78000 6 12.8 bdl 31.8 bdl 0.1 1.1 79000 7 11.9 0.1 29.5 bdl 0.3 1.1 79000 8 16.9 0.9 16.2 bdl bdl 2.7 76000 9 14.7 0.5 17.6 bdl bdl 0.7 78000 10 16.0 0.3 4.6 bdl 0.1 0.4 75000 11 23.4 0.5 26.9 bdl bdl 1.7 79000 12 13.3 bdl 17.5 bdl bdl 0.8 79000 13 15.1 0.2 14.2 bdl bdl 0.5 80000 14 14.8 0.8 42.2 bdl bdl 0.6 79000 15 23.3 0.6 26.5 bdl bdl 1.3 77000 16 50.7 bdl 7.7 0.8 0.1 1.1 70000 17 48.4 0.7 10.8 bdl bdl 0.4 80000 18 63.7 0.2 6.5 bdl bdl 0.4 66000 218

C.2A. CALCULATED ACTIVITY RATIOS, PCG2, AND SI'S FOR IBERIA FIELD

Iberia log log log log log 2 + 2 + + 2 + 2 + + well (oH4Si04) (aCa 7(aH ) ) (aK /aH ) (aMg 7(aH ) ) (dNa /aH ) 1 -3.22 10.41 3.29 10.20 6.25 2 -3.31 10.94 3.78 10.70 6.51 3 -3.14 10.55 3.48 10.36 6.54 4 -3.15 10.38 3.52 10.17 6.28 5 -3.27 11.45 4.08 11.21 6.75 6 -3.14 11.29 3.99 11.07 6.71 7 -3.26 11.18 4.03 10.95 6.66 8 -3.23 10.98 3.99 10.68 6.58 9 -3.11 10.34 3.72 10.11 6.26 10 -3.22 10.59 3.70 10.32 6.36 11 -3.19 9.88 3.47 9.59 6.10 12 -3.23 10.26 3.56 10.00 6.20 13 -3.27 10.65 3.71 10.40 6.38 14 -3.19 10.28 3.50 10.03 6.20 15 -3.10 10.70 3.88 10.30 6.45 16 -3.07 10.45 3.79 9.98 6.40 17 -3.04 10.15 3.53 9.65 6.23 18 -3.04 10.97 4.10 10.45 6.75 Iberia Pc02 log (SI quartz/ log(SIca,cite) log(SIdo|omitc) well 1 0.06583 0.546 -0.307 0.444 2 0.07449 0.284 0.374 1.966 3 0.1685 0.494 0.353 1.970 4 0.1067 0.418 -0.021 1.211 5 0.02997 0.256 0.518 2.307 6 0.04839 0.365 0.582 2.471 7 0.07806 0.223 0.686 2.695 8 0.07117 0.211 0.478 2.260 9 0.1184 0.306 0.061 1.512 10 0.1121 0.181 0.291 1.945 11 0.3695 0.210 0.103 1.554 12 0.1138 0.184 -0.031 1.320 13 0.09583 0.120 0.287 1.980 14 0.1513 0.193 0.117 1.638 15 0.09819 0.282 0.347 1.947 16 0.01595 0.316 0.309 1.800 17 0.2676 0.349 0.232 1.616 18 0.1252 0.345 0.724 2.581 219

C.2B. CALCULATED ACTIVITY RATIOS FOR LAND et al. (1988a) DATA

Lander log log log log log 2 + 2 + + 2 + 2 + a/.well (aH4Si04) (aCa 7(aH ) ) (aK /(aH ) (aMg 7(aH ) ) (aNa7aH ) A-A4 -3.27 10.58 3.77 10.16 6.21 A-A7 -3.27 11.16 4.09 10.78 6.51 AB111 -3.23 11.19 4.18 10.70 6.61 A-C3 -3.28 11.14 4.11 10.79 6.49 A-l -3.27 11.20 4.06 10.81 6.53 B-4 -3.17 10.63 3.69 10.29 6.22 C-3 -3.35 10.74 3.76 10.39 6.19 C-3D -3.34 11.23 4.13 10.95 6.58 C-12 -3.20 10.89 4.04 10.47 6.37 C-30 -3.16 11.30 4.14 11.04 6.53 C-41 -3.15 10.69 3.75 10.42 6.31 C-54 -3.20 11.11 4.07 10.85 6.42 C-66 -3.35 10.89 3.84 10.54 6.38 C-69 -3.18 10.51 3.84 10.06 6.17 C-80D -3.19 10.50 3.78 9.95 6.16 C-84 -3.18 10.62 3.71 10.32 6.19 C-91 -3.10 11.34 4.23 11.08 6.61 C-98 -3.26 11.68 4.36 11.38 6.77 D-171 -3.18 11.14 3.97 10.76 6.46 D-183 -2.70 11.05 3.82 10.46 6.54 E-Al -3.20 10.46 3.67 10.03 6.32 E-Bl -3.19 10.36 3.66 9.96 6.25 E-B3D -3.30 10.34 3.53 9.99 6.24 E-33 -3.27 10.43 3.58 10.07 6.31 F-2 -3.30 10.79 3.83 10.47 6.25 F-25 -3.47 10.67 3.74 10.32 6.11 F-54 -3.14 10.53 3.95 10.06 6.22 F-60 -3.36 10.76 3.79 10.44 6.24 F-62 -3.16 11.08 3.97 10.77 6.40 F-70 -3.10 10.52 3.89 10.09 6.21 F-78 -3.13 10.49 3.99 9.90 6.24 F-79 3.46 10.55 3.59 10.27 6.18 F-85 -3.19 10.79 3.84 10.3 6.33 F-114 -3.23 11.08 4.13 10.61 6.45 G-4 -2.86 10.11 3.43 9.47 6.11 G-5 -3.01 10.36 3.84 9.83 6.23 G-18 -3.12 10.29 3.66 9.91 6.11 G-1+19 -3.24 10.75 3.70 10.36 6.25 VITA William Lee Esch was born September 2,1956 in Colorado Springs, Colorado.

He is the first of five children born to William F. and Connie J. Esch. He graduated

from William J. Palmer High School in June 1974. In December, 1990 he received a

Bachelor of Arts degree in distributed studies from the University of Colorado in

Colorado Springs. In August, 1991 he entered the Ph.D. program at Louisiana State

University. In September 1995 he was employed by Exxon Production Research,

Houston, Texas.

220 DOCTORAL EXAMINATION AND DISSERTATION REPORT

Candidate: William Lee Esch

Major Field: Geology

Title of Dissertation: Effects of Salt Dome Dissolution on Sediment Diagenesis: An Experimental and Field Study

Approved:

Major Professor and Chairman

EXAMINING COMMITTEE:

^ £• W*^

Date of Examination:

August 28, 1995