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1972 The aN ture and Origin of Caprock Overlying Gulf Coast Salt Domes. Charles William Walker Louisiana State University and Agricultural & Mechanical College
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Recommended Citation Walker, Charles William, "The aN ture and Origin of Caprock Overlying Gulf Coast Salt Domes." (1972). LSU Historical Dissertations and Theses. 2255. https://digitalcommons.lsu.edu/gradschool_disstheses/2255
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WALKER, Charles William, 1940- THE NATURE AND ORIGIN OF CAPROCK OVERLYING GULF COAST SALT DOMES.
The Louisiana State University and Agricultural and Mechanical College, Ph.D., 1972 Geology
University Microfilms, A XEROX Company, Ann Arbor, Michigan THE NATURE AND ORIGIN OF CAPROCK
OVERLYING GULF COAST SALT DOMES
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
in
The Department of Geology
by Charles William Walker B. A., Southern Illinois University, 1966 M, S,, University of Mississippi, 1968 May, 1972 PLEASE NOTE:
Some pages may have
indistinct print.
Filmed as received.
University Microfilms, A Xerox Education Company ACKNOWLEDGMENTS
The writer wishes to express his sincere thanks to Dr.
Joseph D. Martinez, under whose direction the investigation has been
made. Thanks are due to Dr. Gale K. Billings, Dr. Ray E. Ferrell,
Jr. , Dr. Donald H. Kupfer and Dr. Clyde H. Moore, jr. for their
assistance and encouragement during the preparation of this manu
script. Others of the faculty and many of the students of the
Department of Geology, Louisiana State University, Baton Rouge, have aided in many ways. Special thanks go to Dr. Gary G. Paulson,
Department of Engineering Science, for his assistance in the electron microscopy phase of the study.
This work was financed by the Institute of Saline Studies,
Division of Engineering Research, Louisiana State University and by the New Mexico State Bureau of Mines and Mineral Resources, Socorro,
New Mexico. Thanks are due to the typists and draftsmen of the Bureau staff for their aid in the preparation of this manuscript. Very special thanks go to Mr. Don H. Baker, Jr., Director, New Mexico State
Bureau of Mines and Mineral Resources for his generous support and encouragement. Many companies and individuals as well as two state geological surveys have aided by contributing sample material and data. Thanks go to ..he Mississippi Geological, Economical and
Topographical Survey and to the Louisiana Geological Survey for providing sample material. Among the companies whose assistance the writer wishes to acknowledge are: Freeport Sulphur Company,
Shell Petroleum Corporation and Atlantic Richfield Corporation. To the following individuals who contributed samples the writer extends his thanks: Dr. Joseph D. Martinez, Dr. Donald H. Kupfer and
Dr. Alfred I. Weidie. TABLE OF CONTENTS
P a g e
ACKNOWLEDGMENTS...... i
TABLE OF CONTENTS ...... iii
LIST OF TABLES ...... vii
LIST OF FIGURES ...... x
LIST OF PLATES ...... xiii
ABSTRACT ...... xiv
INTRODUCTION ...... 1
General Statement ...... 1
Purpose of Investigation ...... 2
Salt Domes Studied ...... 3
Sam ples ...... 7
Methods of Investigation ...... 8
Explanation of Statistical T erm s ...... 8
EARLY LITERATURE ...... 11
GENERAL GEOLOGY OF GULF COAST SALT DOMES ...... 12
General Statement ...... 12
Salt Dome M inerals ...... 13
Sum m ary ...... 22
THE CAPROCK ...... 24
General Statement ...... 24
General Geology ...... 24
Caprock Configuration ...... 25
iii iv
P a g e
L ith o lo g y ...... 26
Caprock-Sediment Interface ...... 53
Caprock Deformation ...... 53
Caprock-Salt Interface ...... 56
Circumjacent Sediments ...... 58
Sum m ary ...... 6l
CAPROCK MINERALOGY AND GEOCHEMISTRY ...... 63
General Statement ...... 63
Calcite ...... 66
M i n e r a l o g y ...... 66
Geochemistry ...... 73
B a n d in g ...... 77
Calcite-sulphate interface ...... 79
Dolom ite ...... 80
M in e ra lo g y ...... 80
C elestite ...... 82
Mineralogy ...... 82
Geochemistry ...... 83
Banding ...... 86
Celestite-anhydrite interface ...... 87
B arite ...... 88
M in e r a l o g y ...... 88
Sulphur ...... 88 V
P ag e
M in e ralo g y ...... 88
Geochemistry ...... 91
G y p s u m ...... 92
M in e ralo g y ...... 92
Geochemistry ...... 93
A n h y d r i t e ...... 94
M i n e r a l o g y ...... 94
Geochemistry ...... 98
Banding ...... 108
Anhydrite dissolution and precipitation features ...... 110
Anhydrite-salt interface ...... 110
Comparison between salt residue and caprock anhydrites ...... 113
Anhydrite volume relations ...... 117
Less Common Caprock M inerals ...... 118
Sedim ents Within C a p r o c k ...... 120
Sum m ary ...... 121
THE GEOCHEMICAL ENVIRONMENT...... 125
General Statem ent ...... 125
Geochemistry of Sediment Interstitial W a te rs ...... 125
Geochemistry of Caprock Interstitial Waters ...... 136
Anhydrite-Brine Equilibria ...... 140
Sum m ary ...... 146 P a g e FALSE CAPROCK ...... 148 General Statem ent ...... 148
Authigenic M inerals ...... 150
Form and Size of False Caprock ...... 154
Sum m ary ...... 156
ORIGIN OF CAPROCK ...... I "8 General Statem ent ...... 158
Precipitation In Place T heory ...... 158
Residual Accumulation and Secondary Alteration Theory . . 161
Analysis of Previous Theories ...... 163
Features Compatible with Either T h eo ry ...... 165
Modified Precipitation in Place T heory ...... 168 DEVELOPMENTAL SEQUENCE OF CAPROCK ...... 170
REFERENCES ...... 174
APPENDIX A. Analytical Procedures ...... 182
Thin-Sections ...... 182
Transmit ion Electron M icroscopy ...... 182
Scanning Electron M icroscopy ...... 183
X-Ray D iffraction ...... 184
X-Ray Fluorescence ...... 184
Atomic Absorption ...... 190
Solution-Precipitation Experim ents ...... 191
APPENDIX B. Lithological, Mineralogical, and Chemical Data 193
PLA TES ...... 217
VITA 269 LIST OF TABLES
T ab le P a g e
1 Map Index of Salt Domes for which Information was A v a ila b le ...... 5
2 Water-Insoluble Residue: Size and Mineralogical A n a l y s i s ...... 15
3 Chemical Composition of Residue Anhydrite ...... 17
4 Average Lithology Data from all Wells on Each Dome . . 27
5 Statistical Analysis of Average Lithology D ata ...... 28
6 Statistical Analysis of Data on Texas Salt Domes .... 38
7 Statistical Analysis of Data on Louisiana Salt Domes . . . 39
8 Statistical Analysis of Data on Mississippi Salt Domes . . 40
9 Caprock Minerals ...... 64
10 Dalcite Crystal Properties ...... 67
11 Statistic.'.l Analysis of CaCO^ Caprock Chemical Data, Minden Dome, La ...... 76
12 Statistical Analysis of Salt Dome Sulphur Production . . 90
13 Summary of Features Attributed by Goldman (1952) to Result from Recrystallization or Pressure Solutions in Anhydrite ...... 96
14 Statistical Analysis of CaSO^ Caprock Chemical Data, Minden Dome, La ...... 105
15 Statistical Analysis of CaSO^ Caprock and Salt Residue Chemical Data, Tatum Dome, Miss ...... 106
16 Statistical Analysis of CaSO^ Caprock and Salt Residue Chemical Data, Allen Dome, Tex...... 107
17 Summary of Important Caprock and Associated F e a tu re s ...... 159
vji • • • V l l l
T a b le P a g e
18 Arguments For and Against "Precipitation in Place" Theory ...... 163
19 Arguments For and Against "Residual Ac cumulation" T h e o r y ...... 165
20 X-Ray Fluorescence Instrumental Parameters ...... 185
21 Atomic Absorption Instrumental Param eters ...... 185
22 X-Ray Fluorescence Standard Calibrations ...... 187
23 X-Ray Fluorescence Instrumental Precision ...... 188
24 Accuracy of X-Ray Fluorescence Analysis ...... 189
25 Lithology Data From All Wells on Each Dom e ...... 194
26 Tatum Dome, Miss., AEC Tatum #1 Well. Mineral- ogical and Chemical D ata ...... 203
27 Minden Dome, La. , Jones #1 Well. Mineralogical and Chemical Data ...... 204
28 Kings Dome, Miss. , Hall #1 Well. Mineralogical and Chemical Data ...... 206
29 Utica Dome, Miss., Lee Little #1 Well. Mineralogical and Chemical D ata ...... 206
30 Allen Dome, Tex., Allen #7 Well. Mineralogical and Chemical Data ...... 207
31 Hull Dome, Tex., F. S. Co. Well #6. Mineralogical and Chemical Data ...... 208
32 Jennings Dome, La. , Pan Am #146 Well. Mineralogical and Chemical Data ...... 208
33 Hockley Dcme, Tex. , Salt Mine. Mineralogical and Chemical D ata ...... 208
34 Choctaw Dome, La. Brine Well. Mineralogical and Chemical D ata ...... 209 ix
Table Page
35 Averages of Analyzed Constituents in CaSO^ Caprock and Salt Residue ...... 210
36 Chemical Analyses of Water Taken from Wells Drilled on Clay Creek Dome, T e x a s ...... 213
37 Analyses of Some Louisiana Salt Dome Associated Oilfield Brines ...... 214
38 Statistical Analysis of Some Louisiana Oilfield Brines Not Associated with SaltDomes ...... 216 LIST OF FIGURES
Figure Page
1 Location of salt domes for which information was a v a ila b le ...... 4
2 Analytical procedures flow diagram ...... 9
3 Solubility of halite in the presence of some other electrolytes at 25 C ...... 16
4 Solubility of CaSO^ as a function of temperature and NaCl Concentrations ...... 19
5 Rate of CaSO^ solubility as a function of time and concentration of NaCl in solution ...... 20
6 Generalized lithology with d ep th ...... 32
7 Columnar sections showing lithologic variability of caprock between dom es ...... 34
8 Columnar sections showing lithologic variability of caprock on a single Dome. Allen Dome, Tex ...... 35
9 Map showing the locations of Freeport Sulphur Co. Wells on Allen Dome, Tex ...... 37
10 Triangular diagram illustrating calcite-gypsum- anhydrite thickness for each Dome ...... 42
11 Geographical distribution-Top of salt, feet below surface 44
12 Geographical distribution-Top of caprock, feet below surface ...... 45
13 Geographical distribution-Total caprock thickness, in feet ...... 46
14 Geographical distribution-Calcite thickness, in feet . . 47
15 Geographical distribution-Gypsum thickness, in feet . . 48
16 Geographical distribution-Anhydrite thickness, in feet . 49
x xi
Fig vi Page
17 Geographical distribution-Total sulphate, in feet . . . . 50
18 Geographical distribution-Anhydrite: gypsum ratio . . 51
19 Geographical distribution-Total sulphate: calcite thickness ratio ...... 52
20 Idealized section of a salt dome and circumjacent sedim ents ...... 59
21 Minden Dome, Jones no. 1 well. Depth vs. S i-K „ 0 -F e -Z n in CaCCT caprock z o n e ...... 74 2 3 22 Minden Dome, Jones no. 1 well. Depth vs. MgO-Na^O-Sr-Ti in CaCO^ caprock zone ...... 75
23 Kings Dome, Hall no. 1 well. Depth vs. Si-K-Fe-Sr-Ti in sediments, caprock and salt residue 84
24 Utica Dome, Lee Little no. 1 well. Depth vs. Si-K-Fe-Sr-Ti in caprock and salt residue ...... 85
25 Minden Dome, Jones no. 1 well. Depth vs. Si-K-Zn-Sr-Fe-S in CaSO. z o n e ...... 100 4 26 Tatum Dome, AEC Tatum no. 1 well. Depth vs. Si-K-Zn-Sr-Fe in CaSO^ caprock and salt residue . . . 101
27 Tatum Dome, AEC Tatum no. 1 well. Expanded depth scale from Fig. 3 4 ...... 102
28 Allen Dome, F. S. well no. 7. Depth vs. Si-K-Zn-Sr-Fe-S in CaSO^ caprock and salt residue. . 103
29 Hull Dome, F. S. well no. 6. Depth vs. Si-K-Fe-Zn in CaSO^ caprock and salt resid u e ...... 104
30 Triangular diagram illustrating average Si-K-Fe in entire CaSO^ caprock and salt residue for each dome analyzed ...... 114
31 Triangular diagram illustration Si-K-Fe in CaSO^ caprock immediately overlying salt and in average salt residue ...... 114 x ii
F ig ’ e P a g e
32 Si-K-Fe-Zn-Sr average concentrations in caprock and salt residue CaSO...... 116 4 33 Calcium and magnesium vs. total dissolved solids in subsurface b rin es ...... 127
34 Sulphate and bicarbonate vs. total dissolved solids in subsurface brines ...... 128
35 Pressure vs. depth, Jeanerette Dome, La ...... 131
36 Map showing by contours the altitude of the base of fresh groundwater in the vicinity of Rayburns Salt Dome, Bienville Parish, La ...... 134
37 Geologic section of Rayburns Salt Dome, Bienville P a rish , L a ...... 134
38 Stratton Ridge Dome, Tex.: Shallow water anomaly . . 135
39 Equilibrium distribution of sulphur species ...... 137
40 Stability relations of iron oxides and sulfides in water . 138
41 Stability of some iron oxides and sulfides as functions of PO z and PS2 ...... 139
42 Molar quotient Ca++/(SO^ + HCO^) in salt dome associated subsurface brines ...... 142 LIST OF PLATES
Plates Pages
1-4 The Salt ...... 217-224
5-19 The Caprock ...... 225-254
20 - 22 The Caprock-SaltInterface ...... 255-260
23 - 24 The False Caprock ...... 261-264
25 Dissolution Experim ent ...... 265-266
26 Precipitation Experiment ...... 267-268 ABSTRACT
Caprock overlying Gulf Coast salt domes is an irregular complex of rocks; often composed of anhydrite, calcite, gypsum and occasionally celestite. Over half of the proved domes in the Gulf
Coast area are known to have caprock, with the mantle more likely to occur and to be thickest over the shallow structures.
A study of caprock from 39 salt domes indicates large lithologic variations as well as variances in its chemical composition. Previous theories of origin cannot satisfactorily account for these variations in caprock and its relationship to the surrounding environment. The previous theories are: (1) residual accumulation and secondary alteration and (2) precipitation in place.
Neither of the two theories can account for all the known features of caprock: Great thicknesses of anhydrite caprock means that a tremendous thickness of salt would have had to be dissolved in order to accumulate thick residues. Minerals not found in the salt's less soluble residue are contained in caprock, and their presence cannot always be explained by secondary alteration. It is difficult to account for the large chemical differences between the salt anhydrite and caprock anhydrite by the residual accumulation theory. Precipitation alone of anhydrite can result in a thick caprock, but this process cannot account for the minor minerals included in the caprock which are so similar to those in the salt. XV
Since neith< of the previous theories alone can satisfactorily account for all the known caprock features, it is proposed here that its origin may be explained by a combination of the two theories, with direct precipitation accounting for the bulk of the caprock overlying the shallow salt domes of the Gulf Coast region. The variability of caprock between domes and on any one individual dome would result from local controls and conditions: Structure would partially control fluid movement, along with permeability and osmotically derived pressures in the surrounding sediments. The composition of the associated brines would limit the type of minerals precipitated, and the geochemical environment would dictate the types of minerals which precipitate. INTRODUCTION
General Statement
Caprock is an irregular complex of rock, predominantly
anhydrite, which is present on the top, and occasionally on the sides,
of many salt domes in the Gulf Coast area. Of the 329 proved domes,
181 are known to have caprock (Hawkins and Jirik, 1966). Both the
shallow and deep seated domes are known to have this mantle, but it
is more likely to occur and to be the thickest over the shallow structure
Thickness of caprock may vary from less than 10 to more than 1, 500
feet.
A regional study of borehole data from 39 salt domes has
indicated large variations in the character of caprock from dome to
dome as well as on any single dome. These data were used to define
lithologic variances of caprock and to determine if any qualitative
and/or quantitative relationships exist between caprock and its
surrounding environment.
Geochemical analytical methods were devised and employed to aid in the understanding of caprock genesis. These include major and trace element analyses of caprock and the salts' less soluble
residue material, simulation of caprock growth and diagenesis in controlled laboratory experiments and solubility experiments on the salt-residue minerals. Caprock and salt-residue minerals have been characterized by use of thin sections, the transmission electron microscope and a scanning electron microscope. 2
Irregular zones of indurated sediments, commonly referred to
as "false caprock'' occur erratically above and adjacent to the true
caprock of salt domes. These zones are cemented by various authigenic
minerals which were derived from the true caprock and salt and then
transported tc surrounding sedimentary environments which favored
their precipitarion. Ar. investigation of the false caprock has been
most helpful in understanding the genesis of the true caprock.
Due tc the large amount of data and to avoid constant repetition,
the paper is divided into two sections. The first section presents the
data and is largely descriptive of the nature of caprock and its associated
environment. Interpretations and postulated theories of origin based
on these data are then stated clearly in the second section.
Purpose of Investigation
According to current concepts, caprock is believed to have
formed from the accumulation of anhydrite as halite is dissolved from
the uppermost part of the dome by circulating waters. Some anhydrite
may subsequently alter to gypsum, calcite, sulphur and other minerals.
This postulated sequence of events considered essential in caprock
development is based or. the rather limited information that was available during the early investigations and has not been experimentally tested except for the sulphur-isotope investigation by Feely and Kulp
(1957).
The gcal of this investigation was to develop a more thorough 3
understanding of the origin of caprock and its relationship to the domal
salt, the surrounding sediments and their contained fluids. In order
to attain this goal a comprehensive study of all aspects of the nature of
caprock and its surrounding environment had to be considered. In this
thesis the mineralogy of the salt dome is discussed first. The general
features of the caprock, both internal and external, are then described
in detail. This description is followed by a mineralogy and geo
chemistry section in which each major caprock mineral is considered by itself as well as the interrelationships between the minerals. Next, the geochemical environment, including that of the caprock and circum jacent sediments, is discussed in considerable detail. A description of false caprock is then presented to show its relationship to the true caprock.
Previous theories of caprock origin are analyzed to ascertain whether or not any of them, based on these above data, can satis factorily account for the character of caprock and its relationship to the surrounding environment. This is followed by a modified theory of origin, which is postulated to account for all known facts. Finally, the sequence of events which may have taken place during the develop ment of caprock is presented.
Salt Domes Studied
Figure 1 shows the location of salt domes for which information was available to the writer. Table 1 gives the name and location 20
/ *6 39 •22 MISSISSIPPI
•26 *29 •30 •34
TEXAS .2 7 LOUISIANA
•7 '32 / *37 24 18* *17 ^ .* 3 8
O L84
I0*#| 20 MILES
Figure I. Location of salt domes for which information was available. See Table I for mop index TABLE l Map Index of Salt Domes for which Information was Available Location Type cf information obtained County or Survey or Lithology Thin Chem. Dome State Parish Sec. T, R. Data Samples Section TEM SEM Analy. Other* 1 Allen Tex. Brazoria Alley XXX X 2 Avery Island La. Iberia 39-13S-5E X X 3 Belle Isle L a. St. Mary 28-17S-10E X XX 4 Black Bayou La. Cameron 7-12S-12W X X X 5 Brenham Tex. Austin W illiams, X A -110 6 Bruinsburg Miss. Claiborne 13-11N-1E XX 7 Choctaw La. Iberville 52-9S-11L X XXX 8 Challenger Abyssal Gulf of 23° 27. 3'N, X Plain Mex. 92°35. 2'W 9 Clay Creek Tex. Washington P e rry X 10 Clemens Tex. Brazoria McNeel, X X A-92 11 Cote Blanch La. St. Mary 23-15S-7E X X 12 Damon Tex. B razoria Mills X Mound 13 Danbury Tex. Brazoria Austin, X X A-14 14 Hockley Tex. H arris Coghill XXXX 15 Hull Tex. Liberty Devore XXXX 16 Humble Tex. Harris Adams X 17 Jeanerette La. St. Mary 37-13S-9E X X 18 Jefferson Is. La. Iberia 59-12S-5E X X 19 Jennings L a. Acadia 47-9S-2W X XX 20 Kings Miss. Warren 39-1 7N-4E XXXXX Table 1 cont.
Location Type of information obtained Map County or Survey or Lithology Thin Chem. Index Dome State P arish Sec. T. R. Data 21 Lake Wash. La. Plaquemines 24-20S-26E X X 22 McBride Miss. Jefferson 19-8N-4E X 23 Minden La. Webster 20-19N-8W X X XXX 24 NapoleonvilleLa. Assumption 41-12S-13E X 25 Oakley Miss. Hinds 27-5N-3W X X 26 Oakvale Miss. Jeff. Davis 32-6N-19W X 27 Pine Prairie La. Evangeline 35-3S-1W X 28 Raleigh Miss. Smith 17-2N-8E X X 29 Richmond Miss. Covington 20-6N-15W X X X 30 Ruth Miss. Lincoln 15-5N-9E X 31 Sardis Miss. Copiah 29-10N-9E X Church 32 Section 28 La. St. M artin 33-9S-7E X 33 Stratton Tex. Brazoria Groce X Ridge 34 Tatum Miss. L am ar 14-2N-16W X X X XX X 35 Utica Miss. Copiah 8-2N-4W X X X X X 36 Venice La. Plaquemines 26-21S-30E X 37 Vinton La. Calcasieu 33-10S-12W X V 38 Weeks Is. La. Iberia 38-14S-6E . k X 39 Winnfield La. Winn 19 -11N-3W X Non-Gulf Coast Salt Hutchinson bedded salt, Kansas X X Pug Wash Salt Dome, Nova Scotia, Canada X X Whisky Island Mine, bedded salt, Michigan Basin X X Zechstein 2, Germany X X TEM = Transmission electron microscopy, SEM = Scanning electron microscopy, *Includes water analyses, pressure and temperature data, etc. corresponding to the map numbers as well as the type of information obtained during the study.
Samples
Core samples were generously supplied by the Mississippi
Geological, Economical and Topographical Survey, Freeport Sulphur
Company, Shell Petroleum Corporation, Sinclair Oil Company and the
Louisiana Geological Survey.
Lithology data was contributed largely from the files of Freeport
Sulphur Company. These files consisted of detailed lithology logs of exploratory bore holes on 20 salt domes.
Other samples were given to the writer by Dr. Joseph Martinez,
Dr. Donald Kupfer (both of Louisiana State University, Baton Rouge) and Dr. Alfred Weidie (Louisiana State University, New Orleans).
The selection of specimens which were studied was predom inantly dictated by their availability. From these specimens, the interval examined was chosen on the basis that it appeared repre sentative of that interval. The vertical distance between samples was determined by availability, practical limitations, lithology and proximity to important interfaces.
Many of the cores were drilled 25 to 45 years ago, thus having been subjected to considerable handling and shipping. How ever, most of the cores had been well labeled and were in place.
Missing intervals did not appear to be selectively removed. Methods of Investigation
All samples were treated in a similar man.ier where applica
ble. Minerals were identified by either optical microscopy or x-ray
diffraction analysis. Both the transmission and scanning electron
microscopes were employed to define crystal morphology. Chemical
compositions and quantities were largely determined by x-ray
fluorescence analysis. Atomic absorption was used to chemically
analyze solutions.
Figure 2 indicates the various procedures performed on the
samples by means of a flow diagram. Each sample has not necessarily been subjected to all the entries, and the order of the procedures may have varied slightly for a few specimens. A more detailed description of each procedure is listed in Appendix A. Prior to cutting the core, the megascopic properties; color, hardness, fracturing, and other outstanding characteristics were observed and recorded.
Explanation of Statistical Terms
Statistical methods were used in this study to summarize and present the masses of data and to draw conclusions from these data. The methods allow generalizations from a limited number of observations by statistical inference. The following is a brief explanation of the statistical terms presented in this report: Linear regression analysis-^a method of determining the equation of a straight line which best represents the trend of scattered data plotted on a Co CO3 Salt Sediments
Dissolve salt
TEMSEM Thin Section Slice 8 core Residue SEM
Grind to pass 200 mesh
Wash w/ distil led HgO
Pelletize Dissolve XRF sample XRD sample AA
SEM = Scanning electron microscopy (Jeolco Model JSM-2) TEM = Transmission electron microscopy (Phil lips Model EM 100) XRF = X-ray fluorescence (Norelco 8 -position vacuum spectrograph ) XRD = X-roy diffraction ( Norelco diffractometer) AA= Atomic absorption ( Perki n-Elmer Model 303) Figure 2 Analytical procedures flow diagram graph. Correlation coefficient—used to measure the relationship between two variables. A coefficient equal to plus or minus one means that there is a perfect correlation, whereas, a coefficient equal to zero means that there is no relationship whatsoever between the two variables. A positive correlation coefficient means that there is a direct relationship and a negative coefficient means that there is an inverse correlation. Correlation matrix—a matrix of correlation coefficients which presents summary statistics for a set of data and provides information on linear associations among the variables.
Stepwise multiple regression analysis—a method which permits the study of interrelations among the independent variables and a means of ranking variables by their relative importance. Coefficient of variation—used to quantify differences within each set of data. EARLY LITERATURE
Caprock overlying Gulf Coast salt domes has been of interest
to geologists for over a century. At first, it was only a feature
considered unique to the Gulf Coastal Plain. Interest in caprock
rapidly increased following the discovery of oil at Spindletop Dome,
Texas, in 1901 and economic sulphur deposits at Sulphur, Louisiana,
at about the same time. It was at Spindletop where the term caprock
was first used to describe the upper part of the caprock and the false
caprock, which were thought to function as capping for the cavernous
oil-bearing strata (Taylor, 1938).
One of the first reports (Hopkins, 1870) concerning caprock
described outcropping Cretaceous limestones at Winnfield and Pine
Prairie Domes, Louisiana as erosion remnants of a "Cretaceous
ridge. " Between this early date and about 1920, most authors argued
that the caprock was a sedimentary deposit uplifted by intrusion of the
salt. Rogers (1918) outlined four principal theories of origin with one
of them being the accumulation of residual material after solution of the salt. This residual accumulation theory gained in favor after the classical investigations by Goldman (1929; 1931; 1933; 1952) and
Taylor (1937; 1938).
Most authors at the present time agree with the residual accumulation of halite-water insoluble minerals theory. Feely and
Kulp (1957) believe their data support the accumulation theory.
11 GENERAL GEOLOGY OF GULF COAST SALT DOMES
General Statement
The Gulf Coastal region is underlain by great thicknesses of sediments and sedimentary rocks composed chiefly of sand, clay, marl, limestone, chalk and evaporites ranging in age from at least late Trias sic to Recent. Semicylindrical masses of salt have thrust their way upward through these sediments, and have resulted in the well-known salt domes or salt plugs which are characteristic of the region. As of 1966, three hundred and twenty nine of these structures had been proven to exist in the states of Texas, Louisiana, Mississippi and Alabama (Hawkins and Jirik, 1966). The presence of salt domes has since been established in the Sigsbee Deep area of the Gulf of
Mexico (Leg 1, Glomar Challenger Project, 1968).
The salt domes are usually considered as occurring in one of three general locations: in the interior of the above mentioned states, in the coastal areas or in the offshore area. They are broadly classi fied according to the depth to salt: Shallow, 0-4, 000 feet; inter mediate, 4,000-10,000 feet; and deep, below 10,000 feet (Hawkins and
Jirik, 1966). The domes are individually classified according to shape as mapped (circular, elongate, multiple, anticlinal, tabular), to cross-section (parallel, expanding, teardrop, doublet) or according to origin (Kupfer, 1963).
The economic importance of the Louann salt (believed to be the
12 . source of the domal salt; Andrews, I960) to the Gulf Coastal region is
highly significant. Hawkins and Jirik (1966) report that in 1964, production associated with salt domes amounted to about 74 percent of the elemental sulphur (this value has decreased sharply since the dis
covery of sulphur in the West Texas Permian Basin), about 41 percent
*v of the salt, and about 12 percent of the total crude oil produced in the
United States. They further noted that as of January 1965, about 54 percent of the underground storage capacity in the Nation for liquefied petroleum gas (LPG) was in Gulf Coast salt domes. The environmental significance of salt has been investigated (Martinez, 1971).
Salt Dome Minerals
Halite and the water-insoluble residue minerals which were identified during this investigation will now be examined in the order of their abundance.
Halite (NaCl): This mineral was found to constitute about 89 to 92 percent of the salt samples studied, and has been reported by various authors to range from 80 to 98 percent. The halite which is found in the domes is generally euhedral, and possesses perfect cubic cleavage (plate 1, figs. 1 and 2). A petrofabric study (Muehlberger and Clabaugh, 1968) indicates that a significant number of salt crystals are preferentially oriented, implying that the salt movement occurred along crystal glide planes. Individual crystals range in size from near submircroscopic to as large as 5 centimeters. The color of the 14
mineral, dependent upon the abundance of inclusions, ranges from almost colorless to dark brown.
Several thin-sections of salt were prepared by the method of
Walker (1969) to determine the manner in which the inclusions occur in the salt. Inclusions occur within the individual salt crystals
(plate 1, Fig. 2), apparently in much the same relationship as when originally precipitated (Taylor, 1938). They are scattered throughout all of the salt crystals with greater concentrations along dark bands.
The inclusions were removed from the salt of several domes by dissolving the salt and recovering the water-insoluble residue. A size analysis of the residue from one dome, plus the major mineral com ponents in each rock size class is shown in Table 2. Scanning electron micrographs of each size class are shown in Plate 21, Figures 5 and 6 and Plate 22. Chemical analyses of the water-insoluble residue ob tained from seven salt domes are displayed in Tables 26 through 35, pages 196 through 205. These analyses indicate small chemical variations in the insoluble residue between domes.
Photomicrographs of the insoluble residue from eight salt domes and three bedded salt deposits are shown in Plates 2, 3 and 4.
The solubility of halite is primarily a function of the concen tration and type of other ionic species present. Figure 3 illustrates the effects on the solubility due to other dissolved species found in natural brines associated with salt domes (Table 37, page 207). The 15
T A B L E 2
Water-Insoluble Residue: Size and Mineralogical Analysis Tatum Dome, M iss. , AEC Well, 1947 feet
Sieve Size (mm) Wt. retained on(g) % retained on Mineralogy
I. 0 3.77 1.43 Anhydrite 0. 5 9?. 06 36. 15 Anhydrite 0.25 127. 81 48. 60 Anhydrite some dolomite 0. 12? 33.42 12. 71 Anhydrite, dolom ite 0. 063 2.66 1.01 Anhydrite, dolom ite Pan 0.27 0. 10 Anhydrite, dolomite, quartz, plagioclase solubility of halite varies slightly at temperatures below 100°C
(Lerman, 1970), therefore, the curves of Figure 3 can be considered reliable for the depths involved during caprock formation. Since the solubility of salt in the presence of either univalent or divalent metal chlorides decreases, these ionic solutes must have some effect on the rate and extent that "water-insoluble" minerals (i.e., CaSO^) can accumulate to form a residue.
Anhydrite (CaSO^): Anhydrite constitutes approximately 99 percent of the water-insoluble residue. It ranges in size from minute cleavage fragments to about 5 mm. (table 2), and has been reported to be as large as 14 mm. (Taylor, 1938). The mineral occurs in two major morphologies; tabular crystals and "stem-shaped" (Taylor,
1938; used to designate a morphology having striations similar to those of a plant stem) crystals. The stem-shaped variety, apparently the Mololity of other solute Solubility of halite in the presence of some other electrolytes at 25°C. (from Lerman, 1970) Lerman, (from 25°C. at electrolytes other some of presence the in halite of Solubility iue 3 Figure mNoCl 25°C O' 17
most common type, is occasionally long and slender, but is generally
stubby and often complexly twinned (plate 3, figs. 1 and 2). The
mineral may contain inclusions of smaller anhydrite crystals, pyrite,
dolomite, sulphur, fluid and possibly carbonaceous matter.
Photomicrographs of anhydrite residue can be found in Plate 2;
Plate 3, Figures 1 and 2; Plate 4; Plate 21, Figures 5 and 6; and
Plate 22. Table 3 lists the average chemical composition of anhydrite from four salt dome residues as determined by x-ray fluorescence.
Additional chemical data of residue anhydrite are presented in Tables 26 through 35, pages 196 through 205.
TABLE 3
Chemical Composition of Residue Anhydrite*
CaO(%) S(%) Si(ppm) K(ppm) Fe(ppm) Zn(ppm) Sr(ppm)
Min. 37. 62 23. 21 0 5 57 445 357 Max. 39. 54 23. 55 35 65 125 475 390 Mean 39. 32 23.41 7 21 83 463 371 S. D. 0. 26 0. 03 1. 77 10. 50 14. 70 2. 57 6 C V(%) 0. 66 1. 13 5. 89 48. 92 17. 87 0. 57 1
^Composition of four salt domes residues, ten samples.
Salt domes are: Tatum, Kings, Allen, Hull
The solubility of anhydrite in a natural water system is largely dependent upon ionic strength, temperature and the amount and type of complexes in the solution. Marshall and Slusher (1968) and Lerman
(1970) have indicated that the solubility of CaSO^ can be satisfactorily predicted by a model which takes into account the above mentioned 18 three variables. This model will be discussed in detail in a following section.
Figure 4 displays the solubility of anhydrite as a function of temperature and NaCl concentrations. From this figure it can be noted that there must be some relationship between CaSO^ solubility and caprock formation since anhydrite is the major constituent of most caprocks. More specifically, a portion of the anhydrite residue could be dissolved along with the salt. It is interesting to note in
Figure 4 that the solubility curves for different concentrations cross when the temperature is increased. Figure 5 indicates the rate of
CaSO^ solubility as a function of time and concentration of NaCl in solution. Figure 4 will be referred to again in forthcoming sections.
Dolomite (Ca,Mg(CO ) ): Dolomite is the next most abundant residue mineral found, and has been reported to be in all of the domes in quantities up to several percent (Taylor, 1938). It occurs as well developed rhombohedral crystals, often twinned, which range in size from 0.05 mm. to as large as 1. 3 mm. (plate 2, fig. 2 and plate 3, fig. 4). Many of the rhombohedrons contain a nucleus of a white or dark substance, probably carbonaceous material. The crystals are usually pale brown, but a few are colorless.
Quartz (SiO^): This mineral occurs as well-developed individual crystals and as rosettes. The rosettes are very distinctive in appearance, containing nuclei of a white opaque substance, similar g g Ca S04 ^00g H20 .500- .600- .700- .800- 420 Solubility of CaSo of Solubility 5 07 090 80 70 30 25 05 60 50 40 4 a a ucin fTmeaue n NC Concentrations NaCl and Temperature of Function a as .2 m Im 3nrv 4m n MrhlI lse n Jones.1964) and I, Slusher Marshal and (Oafa obtained from Bock,1961 Bock,1961 from obtained (Oafa eprtr C° Temperature iue 4 Figure 100 19 Ca*ppm in solution 0 0 0 2 1000 1400 - 1400 1600- 1200 1800- 400- 200 - 600 800- - - - 0 RafeofCaSo NaCl in solution.Temperature held constant at 25*2° C- 25*2° at constant held solution.Temperature in NaCl 2 4 ouiiy sa ucino tm n Cnetain of Concentration and time of function a as solubility 3 4 C 0 5 aOi I0 Na / wafer l/l aC N I50g CaSO^in SQ 4 n itle water distilled in Figure 6 Oays 7 5 8 1 I 1 13 12 II 10 9 0 2 21 to that found in some of the dolomite rhombs. The rosettes usually have a few anhydrite crystals attached to them. The individual crystals are generally about 0. 02 mm. in length and very rarely as large as
1 mm. (plate 3, figs. 5 and 6). Most of the rosettes are about 0. 1 mm. in size.
Sulphur (S): This mineral occurs as irregular anhedral crystals.
They are usually quite small in size, and never larger than 0.4 mm.
The crystals vary in color from yellow to greenish-yellow. Taylor
(1938) noted that the sulphur distribution is very irregular, as the crystals may be comparatively abundant in salt from one part of a dome, and absent or rare in the salt from other parts of the same dome.
Barite (BaSO^): Barite was found as thin, tabular crystals and very rarely as small rosettes. The tabular crystals range in size from
0. 1 mm. to as large as 1.8 mm. , averaging about 0. 2 mm. Many of the crystals contain small inclusions of anhydrite. The mineral varies from colorless and transparent to brownish and translucent.
Plagioclase: This mineral occurs as a minor phase in the less than
0. 063 mm. fraction of the Tatum Dome insoluble residue. It appears to be nearer in composition to bytownite than any other member of the plagioclase group. The mineral was identified by x-ray diffraction.
Magnesite (MgCO^): Only one crystal of this mineral was noted
(Tatum Dome residue), and it was identified on the basis of its optical properties. The crystal is about 0. 5 mm. in length and colorless.
It has been reported (Taylor, 1938) that the presence of this rare
mineral is one of the distinctive features of the caprock on the
Choctaw Dome, Louisiana.
Unknown material (?): This material was noted during an electron
microscope study of the insoluble residue of the Tatum Dome. It
was actually quite plentiful and was distinguished by its rather odd
shape (plate 3, fig. 3). Electron diffraction was attempted in its
identification, but no pattern could be obtained. The material may be
organic or inorganic in origin, or there is a slight possibility that it
is an artifact from sample procedure. Its small size (about 2 }JL ) prohibited conventional methods of identification.
The reader is referred to Taylor (1938) for a comprehensive list and description of the rare minerals obtained from Louisiana and
Texas salt residues.
Sum m ary
Salt domes contain from 80 to 98 percent halite. The less- soluble constituents of the salt include anhydrite, dolomite, quartz, sulphur, barite and other less commonly occurring minerals. Anhy drite constitutes about 99 percent of the less-soluble fraction. The mineral averages in size 0.25 mm. and is found in either tabular or
•'stem-shaped'’ crystals. Chemical analyses of residue anhydrite from four salt domes indicate that low concentrations of Si, K and Fe are present and Zn and Sr are present in moderate amounts. The trace
element content of residue anhydrite varies slightly from dome to dome.
The solubility of halite is markedly affected by other electrolytes in solution which could control, to some degree, the rate and extent that anhydrite can accumulate to form a residue. Anhydrite solubility is a function of ionic strength, temperature and the amount and type of complexes in solution. Thus, anhydrite could be dissolved or precipitated depending on the above three variables. THE CAPROCK
General Statement
This section is devoted to the general geology, physical character and lithology of caprock. It also includes discussions on the upper and lower boundaries of caprock and of the surrounding sediments. A knowledge of these features is essential in under standing the genesis of caprock because these features are the end product.
General Geology
Caprock is an irregular complex of rocks which is present on the top, and commonly the sides, of approximately half the known salt domes in the Gulf Coast region as well as on the Sigsbee domes in the Gulf of Mexico. Both the shallow and deep seated domes have recognizable caprock, but it is more likely to be present and to be thickest over the shallower domes. The average depth to the salt on those domes having caprock is about 2, 325 feet ranging from near surface to greater than 10, 000 feet deep. The depth to the top of the caprock averages approximately 1,970 feet. Thickness of caprock is very variable, but averages about 420 feet thick.
The caprock commonly consists of three or four different types of lithologies. These lithologies are predominantly mono- mineralic, and contain as their major minerals; anhydrite, gypsum, calcite and less commonly celestite. The term "limestone" is not
24 25
used because oil its genetic implications., Minor minerals include
sulphur, pyrite, dolomite, barite and others. The anhydrite portion
of the caprock is generally the thickest; followed in thickness by
calcite, gypsum and celestite. Interbedded layers of salt and/or
sediments are noted in caprock.
The upper portions of the caprock on many of the domes have
been highly fractured and brecciated, and these features have been
attributed to the pressure exerted on the caprock during upward
movement. The calcite cap is commonly very cavernous with the
cavities ranging in size from minute voids to over a quarter of a
mile in length (Taylor, 1938). Hanna and Wolf (1934) noted that cavities
can comprise up to 50 percent of some caprock. Most of the hydro
carbons and sulphur found in caprock are contained in these cavities.
Capr ock Configuration
The shape of caprock is highly variable. Generally, it
overlies the upper surface and extends down the flanks of the salt dome, with the top of the cap nearly paralleling the salt surface.
Numerous exceptions to this generalization can be found. For
example, caprock extends beyond the edge of the salt at Barbers Hill
Dome, Texas, Allen Dome, Texas and Tatum Dome, Mississippi.
Caprock may be thickest over the central portion of the dome or it may be well developed over only a portion of the dome such as at Belle
Isle Dome, Louisiana, Winnfield Dome, Louisiana and Brenham Dome, 2 6
Texas. Cap’-ock may » --tend down the flanks of the dome for several thousand feet such as at Winnfield and Lake Washington Domes,
Louisiana.
Lithology
The basis of the following discussion is derived from lithology data of all wells on each dome for which information was available to the writer. Most of this information is borehole data obtained from the files of Freeport Sulphur Company. Since all of the domes studied are considered shallow (generally less than 4, 000 feet to the top of the salt), this description of caprock is only pertinent to the shallow domes, as the deep-seated dome caprock may be considerably different.
However, general statements can be made about caprock of the shallow domes with the assumption that the domes studied are representative of all shallow salt domes. The lithology data from all w-ells on each dome is listed in Table 4.
The lithology data (table 4) were averaged for each dome in order to characterize the caprock on each dome and to determine if any similarities or variations exist between domes (table 5). From this table it can be seen that the lithology of the c aprock of the 24 domes listed is quite variable. Table 5 displays information obtained from a statistical analysis of the average lithology data. The coefficient of variation (standard deviation/mean x 100) was used to quantify differences in each variable between the 24 salt domes. The coefficient TA BLE 4 Average Lithology Data from all Wells on Each Dome
Dome TC CT GT AT TT TS t c o 3 TSC> Anhy TSed #Wells 4 Gyp Allen, Tex. 995 51 246. 7 166. 3 420 1394 51 376. 7 7. 39 0. 7 18 15 Bellelsle, La. 859 283 120. 3 10* 339. 8 1199 283 130. 3 0. 46 0. 1 3 7 Black Bayou, La. 1054 76. 3 66 800* 942* 2000* 76. 3 866 11. 35 12. 1 — 7 Brenham, Tex. 1523 74. 8 — 363 459 2253 74. 8 363 4. 85 5 7 Damon Mound, Tex. 220 131. 9 278. 9 — 377 562 131. 9 278. 9 2. 11 — 28 Hockley, Tex. 452 128. 2 110. 7 419 671 1124 128. 2 502 3. 91 3. 8 54 4 Hull, Tex. 418 72. 5 225. 2 86. 8 323. 1 787 72. 5 255 3. 50 0. 4 42 7 Humble, Tex. 1181 60. 2 — 66. 2 126. 4 1307 60. 2 66. 2 1. 10 — 5 Kings, Miss. 3593 155 22 75 252 3845 155 97 0. 62 3. 4 — 1 McBride, Miss. 2095 95. 5 —— 95. 5 2190 95. 5 —— 4 Minden, La. 1175 144 — 586 730 1905* 144 586 4. 07 — 2 Napoleonville, La. 580 104. 7 113. 5 27. 5 151. 2 773 104. 7 112. 6 1. 07 0. 2 8 10 Oakvale, Miss. 1648 94. 6 — 702 826 2396 94. 6 702 7. 42 — 7 Pine Prairie, La. 173 169. 3 157. 8 18. 8 356 530 169. 3 168. 1 0. 99 0. 1 91. 8 12 Richmond, M iss. 1717 34 20 196. 5 228 1950 34 216. 5 6. 37 9.8 — 3 Ruth, Miss. 2331 402. 2 — 37* 439 2700* 402. 2 37 0. 09 — 4 Sardis Church, Mlss.1210 667 23* 400* 1110 2300* 667 423 0. 63 17. 4 30 8 Section 28, La. 1753 20 —— 31. 5 1785 20 — 10 2 Stratton Ridge, Tex. 1391 133. 7 108. 6 131. 7 374 1633 133. 7 240. 3 1. 80 1.2 — 8 Tatum, Miss. 1134 108. 2 35 194. 2 348 1520 108. 2 223 2. 06 5. 5 — 9 Utica, Miss. 2641 494 —— 494 3135 494 —— 1 Venice, La. 1875 381. 5 — 744* 1125 3000* 381. 5 744 1.95 — 6 Vinton, La. 528 104 231. 7 83 419 968 104 315 3. 03 0.4 4 5 Winnfield, La. 100 160 60 140 360 460 160 200 1. 25 2. 3 — 4 Depth to tops is feet below sur. Thickness given in feet. — indicates none recovered, ^indicates value from published data. TC=top of caprock, CT = Calcite thickness, GT=gypsum thick. , AT=anhydrite thick. , TT^total thickness, TS=top of salt, TCOj=total carbonate, TSC> 4=total sulphate, TSed = total sed in cap. TA BLE 5 Statistical Analysis of Table 4, Average Lithology Data. Std. Coeff of Var. *Mean Dev. Var. (%) Step-wise multiple regression analysis© Depd. TC 1276. 92 847.72 66. 39 Var. Variables Deleted Ind. variables in order of im portance CT 172. 73 159. 75 92. 48 TCO TS,TS04, CT, AT, GT, TT, S/C, A/G GT 75. 81 91. 37 120. 05 CTTC S/C, AT, A/G, TS, TSO., TT, GT 4 AT 218. 62 256. 89 117. 50 GT TC, CT TS, A/G, TT, AT, TSO., S/C 4 TT 416. 56 272.59 65. 44 AT TC, CT, GT TS04TS, A/G, TT, S/C TS 1738. 17 889.68 51. 18 TT TC, CT, GT, AT TSO , A/G, TS, S/C 4 TSO. 287.61 243.93 84. 81 TS TC, CT, GT, AT, TT A/G, TS04,S/C 4 S/C 2. 75 2. 91 105. 82 TSO . TC, CT, GT, AT, TT, TS S/C, A/G 4 A/G 10. 63 23. 35 219. 58 S/C TC, CT, GT, AT, TT, TS, TSO . A/G 4
* TC = Top of caprock; CT = Calcite thickness; GT = Gypsum thickness; AT = Anhydrite thickness; TT = Total thickness; TS = Top of salt; TSO^ - Total sulphate; S/C = SO^/CO^; A/G = Anhy/Gyp. © The order of the independent variables is based on the reduction of sums of squares, and the independent variable most important in this reduction is entered in the regression. Table 5 cont. Correlation Matrix
Variable TC CTGTAT TT TS t s o 4 S/CA/G TC 1. 000 0. 254 -0. 643 0. 014 -0.006 0. 942 -0.202 -0. 177 0. 169 CT 0. 254 1. 000 -0.254 0. 072 0. 091 0. 406 -0. 005 -0. 421 0. 064 GT -0. 643 -0. 254 1. 000 -0.310 -0.073 -0. 670 0. 015 0. 148 -0. 389 AT 0. 014 0. 072 -0.310 1. 000 0. 811 0. 300 0. 945 0.659 0. 718 TT -0. 006 0. 091 -0. 073 0. 811 1. 000 0. 241 0. 832 0. 546 0. 679 TS 0. 942 0.406 -0.670 0. 300 0. 241 1. 000 0. 089 -0. 028 0. 357 TSO4 . -0. 202 -0. 005 0. 015 0. 945 0. 832 0. 089 1. 000 0. 739 0. 623 s/c -0.177 -0.421 0. 148 0. 659 0. 546 -0.028 0. 739 1. 000 0. 261 A/G 0. 169 0. 064 -0.389 0. 718 0. 679 0. 357 0. 623 0. 261 1. 000
tv) vO 30
ranges from 51. 18 percent for the depth to the top of salt to as high
as 219. 58 percent for the anhydrite to gypsum ratio.
A stepwise multiple regression computer program was
employed to determine if any relationships exist between the variables
of Table 4. Briefly, this program is a statistical technique for
analyzing a relationship between a dependent variable (y) and a set of independent variables (X^, X^, an ent variables in the order of their importance. The criterion'of importance is based on the reduction of sums of squares, aod the independent variable most important in this reduction in a given step is entered into the regression (i. e. the variable with the highest partial correlation with the dependent variable). Any variable in the original set can be designated as the dependent variable. Table 5 shows the dependent variable, the variables deleted and the independent variables in order of importance for each selection. The correlation coefficient matrix of average lithology data is also shown in Table 5. The top of caprock best correlates with the top of salt, as one would expect, and there is a fair negative relationship between it and the gypsum thickness. Calcite thickness does not strongly correlate with any of the variables, but does show a slight positive relationship with the top of salt. Gypsum thickness is negatively related to the top of salt and somewhat to the anhydrite and calcite thickness. Anhydrite best compares to the total caprock thickness which indicates this is the most abundant 31 lithology. There is a very poor relationship between caprock total thickness and the top of salt, indicating that caprock thickness is not a function of depth for this particular set of shallow domes. The total sulphate (anhydrite + gypsum) to carbonate ratio relates slightly to the total thickness and exhibits a weak negative trend when com pared to the top of salt. The anhydrite to gypsum ratio also shows a slight relationship with total thickness. The sequence of lithology from the top of the caprock to the base in each well examined is illustrated in Figure 6. Flank wells and centrally located wells in the caprock have also been differentiated in the figure. As summarized by Murray (1966). a mature caprock would consist of a calcite-gypsum-anhydrite-salt, top to bottom, sequence of lithology. This complete sequence is found in only 22 percent of the wells examined with a greater part of these wells being centrally located. Six percent of the wells have lithologies which are not in this sequential order: calcite-anhydrite-gypsum-salt, in which gypsum is overlying anhydrite. Anhydrite-gypsum-salt sequence and the gypsum-calcite-salt distribution. The remaining wells have one or two rock types missing, but the sequence remains valid. A calcite- gypsum-salt sequence is the most common distribution present. It is interesting to note the large percentage of wells having only calcite caprock with most of these wells located on the flanks. From the foregoing discussion it can be seen that caprock is quite variable in lithology and lithologic sequence. Furthermore, Lihologic sequence from top % of total number (94) of wells examined of coprock to salt o 10 12 16 18 20 22 24 26 28 30 32 34 Col-Gyp-Anhy Col-Anhy-Gyp Col-Anhy Col- Gyp w /t/w /m Anhy-Gyp Gyp-Col Gyp-Anhy Anhy Gyp Col Cal= calc i te Gyp=gypsum Anhy = onhydrite Flank wells Clear space identifies total percentage Central wells Figure 6 Generalized I ithology with Depth Sediments omitted for simplification variability of lithologies are even much greater when the caprock is studied in more detail. Figure 7 displays columnar sections of boreholes drilled through the caprock of ten salt domes. Note the celestite zones in wells 9 and 10 as this mineral has not been previously noted in this great abundance. The sequence of lithology types is not as simple as most of the literature suggests (ex. , Murray, 1966; Halbouty, 1967). Columnar sections showing the lithologic variability of caprock on a single dome is illustrated in Figure 8. Table 25, page 187 underscores the fact that the caprock on each dome is not uniform in either a vertical or horizontal direction. Tables 6, 7 and 8 list the statistical analysis of a number of salt dome associated variables for the states of Texas, Louisiana and Mississippi, respectively. The data used in the analysis were primar ily obtained from Hawkins and Jirik (1966). Only those domes with information on all variables in each table were analyzed. Counties in Mississippi were given an arbitrary number which increased in value from south to north (landward) and were compared with the other variables to determine if any geographical relationships exist. It was found that increasing caprock thickness only slightly relates, in versely, to a landward direction. In comparing the three tables (6, 7 and 8) together, certain trends can be noted. The top of caprock and top of salt increase in depth from west to east. Total thickness of caprock increases both 34 650 750 850 950 1050 I 150 1250 1350 1- Tatum Dome, F.S.Co. Wei I No.9 2- Hull Dome, F.S.Co. Wei I No.4 3- Bell Isle Dome,F.S.Co.Wei I No.2 4- Hockley Dome,FS.Co.Wei I No .6 5- Stratton Ridge Dome,Tolar Wei I No. I 1450 1550*— Figure 7 Columnar sections showing lithologic variability of caprock between domes. (Fig. continued on next page ) 35 150 2300 2600 3550 9 10 250 2400 2700 3650 2450 350 2800 3750 450 2900 3850 -t 550 3 0 0 0 3950 650 3100 4050 3200 6-Pine Prairie Dome, F.S.Co. Wei I No 6 7 - Napoleonville Dome,F.S.Co.DSL Wei I No.I 8- Minden Dome,Hudson Well No.2 9-Utica Dome,L.Little Well No I 10-Kings Dome, Hal I Well No. I Calcite Anhydrite Shale or gumbo Gypsum Celestite Salt Figure 7 continued 36 750 850 950 1050 1150 1250 1350 1450 1500 Calcite K&l Anhydrite Sand °ypsu EHrj Shale |+t+j Salt Figure 8 Columnar sections showing lithologic variability of caprock on a single lome. Alien Dome,Texas.See figure 9 for location of wells. Ck. AD£ Ag8 AD6 ADI3 ADI Q ADIO AD4 -3 AD 3 WOX I ADI I AD 15 AD5 'AD 14 AD7 ADI2 Figure 9 Mop showing the locations of Freeport Sulphur Co. wells on Alien Dome,Texas. 0 200 600 i_ fe'et 1 TABLE 6 Statistical Analyasis of Data on Texas Salt Domes. (Data obtained from Hawkins and Jirik, 1966)* Standard Coefficient of Variable* Ab Mean Deviation Variation (°, Top of caprock TC 1081.02 1437.30 132.95 Top of salt TS 1609.18 1553.92 96. 56 Caprock thickness^ TT 528.16 456. 33 86. 39 Date dome was discovered Date 1912.42 14. 60 0. 76 Volumn of salt^ Vol 3. 71 4.71 126.95 Oil production^(x 10"^) OP 2543.51 4238.94 166.65 1. 50 observations 2. TS minus TC 3. Estimated from TS to a depth of 10, 560ft. 4. Cumulative prod, to 1964. Correlation Matrix ariable TC TS TT Date Vol OP TC 1. 000 0. 956 0. 107 0. 536 -0. 184 -0. 109 ID o t''- i TS 0. 956 1. 000 0. 393 0. 560 • -0. 109 o o r- 1 TT 0. 107 0. 393 1. 000 0. 220 • -0. 026 Date 0. 536 0. 560 0. 220 1. 000 0. 030 -0. 320 Vol -0. 184 -0. 175 -0. 017 0. 030 1. 000 0. 050 OP -0. 109 -0. 109 -0. 016 -0. 320 0. 050 1. 000 ♦Only those domes with information on all variables were analyzed. u> oo TABLE 7 Statistical Analysis of Data on Louisiana Salt Domes (onshore) (Data obtained from Hawkins and Jirik, 1966 and Halbouty, 1967)* Standard Coefficient of 1 Variable Abb Mean Deviation Variation (%) Top of caprock TC 1866.09 1799.20 96.41 Top of salt TS 2050.76 1716.42 83. 70 2 Caprock thickness TT 335.63 275.03 81.94 Date dome was discovered Date 1920. 71 14. 19 0. 74 Volumn of Salt^ Vol 3. 81 3. 32 86. 91 Oil production^(x 10"^) OP 3420. 08 3370. 74 98. 56 1. 35 observations 2. TS minus TC 3. Estimated from TS to a depth of 10, 560 ft. 4. Cumulative prod, to 1-1-65 Correlation Matrix iriable TC TS TT Date Vol OP TC 1. 000 0.896 -0. 321 0. 364 -0. 301 -0. 121 TS 0.896 1. 000 -0. 226 0. 306 -0. 288 -0. 045 TT -0. 321 -0. 226 1. 000 -0. 253 -0. 068 -0. 104 Date 0. 364 0. 306 -0.253 1. 000 -0. 004 -0. 193 Vol -0.301 -0.288 -0. 068 -0. 004 1.000 0. 344 OP -0. 121 -0. 045 -0. 104 -0. 193 0. 344 1. 000 ♦Only those domes with information on all variables were analyzed. u> nO TABLE 8 Statistical Analysis of Data on M ississippi Salt Domes (Data obtained from Hawkins and Jirik, 1966)* Standard Coefficient of Variable 1______Abbr. Mean ______Deviation ______Variation (%) Top of caprock TC 2972.14 1483. 58 83. 56 Top of salt TS 3313. 39 2362.81 71. 31 Caprock thickness^ TT 402.47 306.60 76.17 Date dome was discovered Date 1945. 12 5. 53 0.28 County Co ------1. 49 observations 2. TS minus TC 3. Counties were given an arbitrary number which increased in value from south to north. Correlation Matrix Variable TC TS TT Date Co TC 1. 000 0. 975 -0.460 0. 360 0. 204 TS 0.975 1. 000 -0.335 0. 356 0. 143 TT -0.460 -0. 335 1. 000 -0. 110 -0. 331 Date 0. 360 0.356 -0. 110 1. 000 0. 052 Co 0.204 0. 143 -0. 331 0.052 1. 000 *Only those domes with information on all variables were analyzed. 41 east and West from .Louisiana. The volume of salt shows a slight increase going eastward and oil production is more prolific su r rounding Louisiana salt domes than in Texas or Mississippi. The thickness of caprock in Louisiana and Mississippi is inversely related to both the top of cap and top of salt, whereas Texas caprock thickness shows a positive relationship to both of these variables. The triangular diagram of Figure 10 which illustrates calcite- gypsum-anhydrite thickness relations for each dome listed in Table 4 is plotted in a manner to differentiate the domes of each state. The three components of the diagram are in feet units. Measures of the three components were added, converted to percentages, and plotted as points within the triangle. The diagram (fig. 10) shows quite a scatter of points, indicating thickness variability of the three com ponents. Mississippi caprock points do not scatter as much as the other two states since gypsum thickness in this state is always less than 20 percent of the total thickness. The geographical distribution of most of the variables listed in Table 4 are plotted on maps of the coastal states in Figures 11 through 19. Depth to the top of salt (fig. 11) does not show any distinctive pattern except the general trend of deepening from west to east. This trend is also apparent in Figure 12 which shows the top of caprock. The geographical distribution of caprock thickness (fig. 13) shows a general trend of thickening from west to east and 42 CT o=Texos Dome ■^Louisiana Dome o=Mississippi Dome GTFigure 10 AT Triangular diagram illustrating calcite-gypsum-anhydrite thickness for each dome. Data from Table 4 possibly the same trend from south to north. Figure 14 shows that the calcite caprock is somewhat better developed in Mississippi and south Louisiana and poorly developed in Texas. The distribution of gypsum thickness clearly indicates an east to west thickening trend (fig. 15), while the anhydrite thickness diagram (fig. 16) shows no apparent trends at alL Figure 17, illustrating total sulphate (anhydrite + gypsum), does not display any apparent trends either. The geographical distribution of the anhydrite to gypsum ratio diagram (fig. 18) displays a low ratio for most of the domes in the states of Texas and Louisiana, and a fairly high ratio for the Mississippi domes. The total sulphate to calcite thickness ratio, shown in Figure 19, displays no discernable geographical trends, indicating the variability of caprock on domes located in similar regions. The character of caprock is very complex on a megascopic examination. The complexity increases as caprock is studied in greater detail: The variety of lithologic banding and variable nature of the boundaries between the bands, the presence of relatively large concentrations of celestite in Mississippi caprock, variable thick nesses of sediments and salt in caprock, crystal morphology, the chemistry of the minerals, stress effects and a host of other features all add to this complexity. These features, among others, will now be discussed in further detail. * i A A Shallow i I i i i i \ ! Figurell Geographical distribution- Top of salt, feet below surface. Data from Table 4 Symbols: 0 = 0 -5 0 0 ,C = 501-1000, A = 1001- 1500, A = 1501-2000 ,□ = 2001-2500,D=>250l i 4- Shcil low A A Figurel2 Geographical distribution-Top of caprock.feet below surface. Data from Table 4 Symbols:O-0-500, U= 501- 1000, A r 1001 -1500, A : 1501-2000,□ = 2001-2500, U =>2501 □ o A A □ O A □ A A O , A O /□ A A □ A A' N Figurel3 Geographical distribution-Totol caprock thickness.in feet. Data from Table 4 Symbols : O :0-100,0-101-300, A : 301 - 500, A= 501-700, □ =>701 Figure 14 Geographical distri but ion- Cnlcite thickness , in feet .Data from Table 4 Sy mbols: 0 = 0 - 5 0 , 0 - 51-100, A ; I0I-I50, A - 151-200, D = >20l Figurel5 Geographical distn bution-Gypsum thickness, in feet. Data from Table 4 Symbols:0=0-50, , - 51-100, A : 101-150, £ : 151-200,□ = >201 o Figure 16 Geographical disf n !vjtion- Anhydrite thickness.in feet.Dnta from Table 4 Symbols:0=0-50, .. - 51-100, A= 101- 200, £=> 201 □ A i ! ' i 1 A i ^ □ O v A A - f' Figure 17 Geographical distribution-Totol sulphate, in feet. (anhydrite*gypsum ).Data from Table 4 / / Symbols: 0 : 0-IOO, 101-200, A-201-300, A- 30l-400,D:40l-600, D = >60l I Figure 18 Geographical distribution-Anhydrite :qypsum ratio Doto from Table 4 Symbols :O=0-l.0,O= 1.1-3.0, A = >3.1 f I I f y Figure 19 Geographical distri' > n - Total sulphate : colcite thickness ratio Data from Table 4 ; \ Symbols:0=0-I.O,C -2.0 , A=2.l-5.0,£=>5.l ' } U1 53 Caprock-Sediment Interface Taylor (1938) states "that the top of the caprock often must be estimated because of the false caprock which grades into it, and because of the broken or brecciated condition of the caprock itself. " The term "false caprock" may be applied to those sediments, above or around the true caprock, which have been indurated and often replaced by an authigenic cement derived from true caprock (Walker, 1968). The relationship between false caprock and true caprock will be examined in a later section. This interface is illustrated in Plate 17, Figure 2. Caprock Deformation The upper portions of the caprock on most of the domes have been highly brecciated, with brecciation generally decreasing down ward toward the salt. The breccia clasts in the calcite cap are usually dark in color and range in size from minute particles to 20 centimeters or greater in their long dimension. The clasts can com monly be fitted together, show little contact with each other, and appear to "float" in a matrix of a white to yellow secondary calcite. Goldman (1952) lists four major types of stress effects in the gypsum anhydrite caprock: (1) mylonization and "flowage". (2) faults, (3) stress effects along surfaces of weakness and (4) steep and irregular gypsified fissures. The more common deformation features noted during this investigation are described below. These 54 features are more likely to occur toward the upper portion of caprock, but they are not restricted to that portion as they are noted in all parts of the consolidated cap. "Rock flowage": This term does not mean flowage in a plastic state, but is used (Goldman, 1952) to describe a process that involves fracture and deformation of the constituents of a caprock in which the particles which are displaced with respect to each other are discern ible under the microscope. This feature is illustrated in Plate 15, Figure 1. Mylonization: This was probably the most striking feature noted. Mylonization is crushing that produces constituents of a rock into a fine gouge and it is usually accompanied by bending, fracturing and fragmentation of the crystals not completely pulverized. An example of mylonization is shown in Plate 10, Figure 3. Faulting: This stress effect was found to range with displacements of at least several feet to as small as one m illimeter or less. The microfaults were the most common feature and were noted in many of the caprock specimens. Accompanying the microfaulting there is usually mylonization and fracturing. One thin-section of caprock from the Allen Dome, Texas, contains a minimum of eight micro faults (plate 10, figs. 3 and 4). Folding: Accompanying the deformation of caprock many bands are folded, usually on a semi-micro scale. Microfolds are generally 55 very asymmetrical on a fairly large scale, but a close view revealed many types of folds such as recumbent, isoclinal and even drag folds. Some of the folds may actually be depositional features and not due to deformation. One of the microfolds is illustrated in Plate 11, Figure 4. Fractures: A very common feature in caprock. A large percentage of the fractures have been healed, or are in the process of being healed, by a lalter generation of coarse calcite (plate 12, fig. 2) or gypsum. Shear zones: These zones comprise an area adjacent to a shear plane, and are restricted to the extent of the affected rock on either side of the plane. The rocks near this zone are often highly mylonized and consist of a fine mylonite meal, and occasionally contain a high concentration of other caprock minerals (plate 11, fig. 3). Goldman (1952), in studying the caprock of Sulphur Dome, Louisiana, concludes that it is not possible to determine the depth at which the deformation took place. He also indicates that the known episodic movement of salt domes suggests that there probably were periods of deformation at different depths. Perhaps a paleontologic study of the sediments included in the fractures would aid in determin ing deformation depth(s). The principal cause of stresses which have resulted in caprock deformation is not clearly understood. Goldman (1952) 56 and Bodenlos (1970) believe that the principal cause is probably due to the upward movement of the dome pushing the caprock ahead of it against the resistance of the overlying and surrounding beds. It has been suggested (Hanna, 1953) that the removal of salt by solution formed insufficiently or unsupported roofs over these areas causing collapse of the materials above, resulting in a high degree of deformation in the caprock. Taylor (1938) believes that the stresses that have deformed the caprock result from both collapse and upthrust. Caprock-Salt Interface This interface may be the most important key to aid in the understanding of caprock genesis. The processes which take place at the interface have not been fully understood. A fairly complete in vestigation of the caprock-salt relationship has indicated large morphological and chemical changes occur within a short distance of the interface. These changes will be discussed in detail in the caprock mineralogy and geochemistry section. The upper surface of the salt, upon which caprock usually forms, is quite variable from dome to dome. It ranges from a near flat surface to a highly rounded surface, or may be very irregular and asymmetrical. Since caprock can form on any of the above surfaces, a "solution table" or flat surface is not a requirement for its formation. Generally, the interface is somewhat gradational; character ized by undissolved salt at the bottom, grading upward into a zone of 57 friable anhydrite sand, and finally grading into the hard anhydrite caprock. It has been reported (Taylor, 1938) that when the interface is encountered in drilling at Jefferson Island and Lake Washington Domes, Louisiana, a cavity of from a few inches to several feet is found. The caprock may also rest directly on the salt with no intervening anhydrite sand, such as at Hockley Dome, Texas (Teas, 1931). Some domes have either calcite or gypsum in contact with the salt, for example; Damon Mound, Texas and McBride, Mississippi. Taylor (1938) remarked that the leached condition of the unconsolidated sediments close to the flanks of salt domes is probably due to the upward movement of ground waters, and that the sediments frequently contain water-insoluble residue minerals from the salt. He further indicates that the shale from near the flank of Darrow Dome, Louisiana, contains solution-worn anhydrite grains at a depth of nearly 8, 000 feet. A discussion by Levi S. Brown on his investigation of caprock (1931) follows the report. A portion of this discussion, which was directed to Dr. Goldman, is as follows: I am familiar with the Hockley structure, and have studied the salt-anhydrite contact in detail as seen in the mine shaft. The horizontal banding and narrow salt-bearing transition zone parallel with the contact are well exemplified. To me the contact is an excellent illustration of a change of facies in a conformable series, and the presence of small quantities of the opposite mineral through a short distance on each side 58 of the contact is to be expected. To assign this horizontal banding to solution, and to regard the transition border as one in which the "removal of the salt. ... is not yet complete. " to my mind introduces an unnecessary and very complicating factor, and Dr. Goldman desired to leave "out of the discussion the difficult question of the origin and circulation of waters" capable of producing such solution. Thus, the existence of the caprock-salt interface has been recognized for years as a clue to understanding the origin of caprock. Many theories of origin have been primarily based on this interface as it was observed and interpreted by the various authors. F. H. Lahee, in a discussion of the same paper (Brown, 1931) makes a statement which is, in the w riter's opinion, just as valid today as it was then. Lahee states: I should like to make the suggestion that investigators of cap-rock material should also study and analyze the underlying salt. There may very well be a genetic relation between the two. For this reason, inferences and theories based on an examina tion of the caprock, unless supported by evidence from the salt, may perhaps be in error. Circumjacent Sediments Figure 20 illustrates an idealized salt dome and its surrounding sediments. Kerr and Kopp (1958) observed brecciation in deep (?) shale sections surrounding salt domes. They report that the brecciation extends laterally at least for several thousand feet, with an apparent roughly circular but somewhat asymmetric ground plan which may expand in depth. The clasts and matrix are mineralogically similar, and some fragments can be pieced together. 59 Surface Normal Shale Caprock Breccia Shale see below Salt Dome Gouge Shale Expanded view of above circle Figure 20 Idealized section of a salt dome and circumjacent sediments.(Modified from Kerr and Kopp,l958 and Hanna, 1953) 6 0 They concluded that the mechanism of formation for these breccias may be a result of domal movement exerting an upward and outward pressure which exceeds the pressure of overburden. An alternate idea is the possibility of osmotically derived fluid pressure, which could exceed the pressure due to the weight of the overburden, could result in brecciation of the shale. Osmotic pressure is generated when two aquifers of different salinity are separated by a semipermeable membrane, which would be geologically, a fine-grained sediment (White, 1965). As a result, gradients in hydraulic head develop, in the direction of the more saline aquifer. The term "geopressure" is applied to abnormally high subsurface fluid pressure, and has been defined as "any pressure which exceeds the hydrostatic pressure of a column of water containing 80,000 mg/1 (milligrams per liter) total solids" (Dickinson, 1953). Adjacent to the flanks of many salt domes are found shale sheaths composed of fine-grained clayey material. The gouge shale, or sheath, (Fig. 20) is quite variable in age and thickness (Murray, 1966), probably resulting from the material being dragged high above the immediate position of the undisturbed equivalent bed during domal movement (Hanna, 1953). This shale sheath could behave as a semi permeable membrane due to its fine-grainess, permitting selective movement of water toward the salt and the development of osmotic pressure. 6 1 Diapiric shale in contact with salt, both above and adjacent to, has been noted at almost all domes of Louisiana, for example at the Valentine Dome (Atwater and Forman, 1959). This shale may have been derived from osmotic pressure developed in the salt dome area. Su m m a ry Caprock is an irregular complex of rocks which is present on the top. and commonly on the sides, of salt domes. It is composed of predominantly monomeralic zones in which anhydrite, calcite or gypsum is the major mineral. Zones of celestite (Fig. 7) have also been found. Caprock shape is highly variable and can be present on the top. sides or beyond the edge of the salt dome. Lithology and lithologic sequence is quite complex. Caprock on one dome may be almost entirely calcite, whereas a neighboring dome has anhydrite and gypsum as the only minerals, and another dome close by may not have any caprock. There are large lithologic variations of caprock on a single dome such as only calcite in one part of the caprock and only anhydrite in another part or the alternation of one lithology with another type for large vertical intervals. The caprock-sediment interface is gradational due to the presence of false caprock. Deformation of caprock is most pro nounced in the upper portions but is not restricted to these areas. The caprock-salt interface is a most important feature to aid in the understanding of caprock genesis. This interface is generally gradational in which it grades upward from a friable anhydrite sand into the hard anhydrite caprock. Either calcite or gypsum can also be in contact with the salt. A complete discussion of this interface is presented in the next section. The sediments circumjacent to salt domes are generally somewhat brecciated for at least several thousand feet away from the dome. Adjacent to the flanks of many domes is a shale sheath which could behave as a semipermeable membrane and permit selective movement of water toward the salt and the development of osmotic pressures. These pressures may be responsible for the diapiric shales found in contact with many of the salt domes, and the shale itself could serve as a membrane. CAPROCK MINERALOGY AND GEOCHEMISTRY General Statement The major minerals of the caprock framework are sulphates and carbonates. Other associated minerals include sulphides, arsenides, halides, silicates, oxides and a number of pure elements. A list of minerals which have been identified by various students of caprock is presented in Table 9. . This table also tabulates the commonly noted crystal habits and a rough estimate of relative occurrence for each mineral. Of the thirty minerals listed, only nine or ten are found in significant quantities. A somewhat detailed discussion of the more common minerals will follow. This discussion will include such aspects as spatial distribution, abundance, age relations, crystal morphology, stress effects, alteration and replacement features, geochemistry, banding and the association with other minerals and sediments. The order in which the minerals are discussed is based on a typical sequence through the caprock from top to bottom. A few of the rarer minerals are briefly discussed at the end of the section, as well as the sediments found within the caprock. A total of 118 samples from 9 salt domes were analyzed by x-ray fluorescence to determine major and trace element concen trations of Ca, S, Si, K, Fe, Zn, Sr, Ti, Mg and Na. The data obtained from the analysis is listed in Tables 26 through 35, Appendix B. 63 TABLE 9 Caprock Minerals Mineral ______Composition ______Habit ______Occurrence* Anhydrite CaSO 4 Prismatic, radiating, fibrous, "stem shaped" XXXXXX Barite BaSO.4 Radiating, tabular, rosettes, prismatic XXX Celestite SrSO.4 Radiating, fibrous, massive, tabular XXXX Gypsum CaSO4 • 2H.O 2 Prismatic, tabular, fibrous, massive xxxxx Calcite CaC03 Rhombohedral, scalenohedral, equant to bladed XXXXXX Aragonite CaCOs Tabular, prismatic XX Dolomite (Ca,Mg(C03)2 Rhombohedral XXXX Strontianite SrC03 Acicular XX Siderite FeC03 Not reported X Smith sonite ZnC03 Not reported X Pyrite Cubes, pyritohedral, octahedral FeS2 xxxxx Marcasite FeSz Not reported X Galena PbS Cubes XX Sphalerite ZnS Earthy XX Hauerite m„s2 Octahedral X Alabandite MnS Tetrahedral X Realgar Prismatic X A82S2 Chalcopyrite CuFeS2 Massive X Chalcocite Cu£S Massive X Table 9 Continued Mineral Composition Habit Occurrence* Enargite Cu AsS Prismatic X 3 4 Halite NaCl Cubes XXXX X Fluorite CaF2 Cubes Hematite Earthy XX FC2°3 Quartz S i° 2 Terminated xls. , rosettes, chalcedonic XXX Kaolinite Tabular, fibrous XX A12°3' 2SiCY 2H2 ° Tourmaline Mg rich Rhombohedral, prismatic X Sulphur S Orthorhombic xls, massive XXXXX Arsenic As Colloform X Gold Au As inclusions (?) X Silver Ag As inclusions (?) X ♦XXXXXX very common; XXXXX common; XXXX fairly common; XXX fairly rare; XX rare; X very rare O' cn 6 6 Calcite Mineralogy This m ineral is second in abundance to anhydrite. .All of the caprock studied contain large amounts of calcite. and it is the major cementing mineral of the false caprock. It is most abundant in the upper zones of caprock and is occasionally (16% of wells examined) the only major mineral constituent of caprock, such as at McBride and Utica Domes, Mississippi. Calcite may be found as a minor component in any of the lithologies, and constituents from less than 1 percent to somewhat more than 98 percent by weight of the indi vidual specimens as determined by acid-insoluble tests. The crystals vary in size from near submicroscopic to over 12 mm. in the max imum dimension. The size of the individual crystals is apparently indicative of relative age relationships: The finely crystalline variety is generally the oldest generation calcite because it is never found to be cutting other minerals and is generally the most deformed; the coarsely crystalline calcite is associated with the recementation of fractures which may cut either of the other two types; an inter mediate to coarsely crystalline calcite may be either older or younger than the recementing calcite because of recrystallization at the ex pense of either of the above mentioned varieties (plate 11, fig. 1). Crystal morphology may aid in understanding the mode of formation of calcite (Walker and Paulson, 1970). Morphological 67 features include crystal size and shape, macrostructure and micro structure. To define these features, a combination of optical, and transmission and scanning electron microscopes were employed. Samples were obtained from the calcite zone in the caprock of Minden Dome, Louisiana. The Minden calcite caprock zone is composed of essentially two phases as determined by x-ray diffraction and optical micros copy; the major phase CaCO^ and the minor phase, sulphur. There is an apparent relationship between the concentration of sulphur and calcite crystal size, shape, surface microstructure and often color. Five types of crystals, each differing in size and color and easily recognized, were chosen for study. The amount of sulphur ranges from negligible to one percent by weight in the calcite as determined by x-ray fluorescence. Crystal size and macrostructure were defined with the aid of an optical polarizing microscope. Table 10 tabulates the properties. TABLE 10 Calcite Crystal Properties (from Walker and Paulson, 1970) Size (mm) % S J>y wt. Shape Color 9 Neg. Bladed White to It. brown 7 0. 08 Bladed to equant Yellow 2 0. 18 Equant Brown 0. 3 0.40 Equant Gray to black 0. 06 1. 00 Equant Lt. yellow 68 An inverse relationship between increasing sulphur content with increasing crystal size is apparent. There also appears to be some correlation between crystal color and the amount of sulphur. All of the crystal varieties, regardless of their individual properties, appear to have been precipitated in situ. The two smaller crystal line types are usually found in alternating, fairly undisturbed horizontal bands (plate 9. fig. 4). The larger crystals are most often associated with fracture filling in the banded caprock and must be of a late generation (plate 5, fig. 2). Since sulphur, in any ionic or elemental form, cannot be accommodated into the calcite lattice, it must act as an external impurity. This impurity increases the energy necessary for the movement of a grain boundary and inhibits crystal growth. The boundary energy is decreased when it reaches an impurity propor tional to the cross-sectional area of the impurity (Kingery, I960). The boundary energy must be increased again to pull it away from the impurity. Thus, when a number of impurities are present on a grain boundary, its normal curvature becomes insufficient for continued crystal growth after some limiting size is reached. It has been noted that this size is given by D ^ — ~ fd (1) where O is the limiting grain size, d is the particle size of the impurity, and f is the volume fraction of impurities (Kingery, I960). 69 Although this relationship is only approximate, it indicates that the effectiveness of impurities increases as their particle size is decreased and the volume fraction increases. In this type of heterogeneous crystallization there is a reaction interface between the different phases. In order for the reaction to proceed, three steps must take place in series; material transport to the interface, reaction at the interface, and movement of im purities as growth proceeds. A zonation in the crystals occurs because the supply of sulphur is probably sporadic during crystal growth. This zonation can be seen in all of the five sizes of calcite crystals, but becomes more obvious as the amount of impurity in creases (plate 7, figs. 1 and 3; plate 8, figs. 1 and 5; plate 9, figs. 1 and 4). Carbon double replicas of polished and etched surfaces, as well as fractured surfaces, were examined with the transmission electron microscope. It was found that the zones are actually growth lines of three orders of magnitude. First order growth lines represent the initial nucleation of individual calcite scalenohedrons which share common crystallographic planes, resulting in optical continuity (plate 7, fig. 5; plate 9. fig. 6). The small amount of impurity present during crystallization resulted in only partial coalescence of the crystallites (plate 7, fig. 6; plate 9, fig. 7). Second order growth lines could easily be seen with either an optical or scanning electron 70 microscope (plate 7, figs. 3 and 4; plate 8, figs. 5 and 6; plate 9, figs. 1 through 4). These second order growth lines probably represent periods of noncrystallization resulting from temporary undersaturation of the mother solution or a possible influx of im purity (sulphur) resulting in ephemeral disequilibrium. Third order growth lines are the outer limits of optical continuity, in other words, the crystal boundary (plate 7, figs. 1 and 7; plate 9, figs. 1 and 4). Fractured samples of the five types of calcite were examined in the scanning electron microscope to determine grain shape and its relation to the growth lines and sulphur content. The dominant shape is rhombohedral in the smaller crystals (plate 9, fig. 5), while the larger crystals appear to be a combination of rhombohedral and scalenohedral forms (plate 8, fig. 4). Impurities, other than sulphur, will most likely influence calcite crystal morphology because of relationship (1). Fine grained particulate material, such as clay, or even hydrocarbons, present during crystallization (plate 23, fig. 5) must control, to some degree, the morphology. Individual calcite crystals subjected to stresses in the caprock generally exhibit.one or the other of two different deformation features; (1) fracture and (2) induced twinning. Fracturing of the calcite is fairly complex. The main fractures appear to be cleavage related (plate 8, fig. 1), but minor 71 fractures do not show this relationship and are often found trending in directions normal to crystal growth (plate 8, fig. 5). Many of the larger calcite crystals are twinned to varying degrees (plate 10, fig. 6). The twinning appears to be associated with some type of stress feature such as shear zones, and it is probable that the degree of twinning was directly related to the amount of stress. Other minerals are generally associated, in varying amounts, with calcite. The acid-insoluble residue of the calcite caprock from two salt domes illustrates some of the more common associated minerals (plate 14). The relationship between calcite and dolomite is of some interest because the presence of this mineral in the calcite cap has been used to support the residual accumulation origin theory. Taylor (1938) states that the dolomite rhombs have a nucleus of a white or pale-yellow substance, much the same as the rhombs in the salt and, thus, a residual derived from the salt. Less than 5 percent of the dolomite rhombs examined in this study display any nucleus, although those that do, may indeed be of residual origin. The largest percentage of the dolomite associated with calcite appears to be epigenetic and is replacing the calcite as shown by the progressive destruction of the matrix calcite (plate 10. fig. 5). Pyrite is authigenic with calcite. but may show either a •yngenetic or epigenetic relationship. Syngenetic pyrite occurs as finely-disseminated crystals in the oldest generation calcite. which 72 is commonly the darker, finer-grained variety (plate 5, fig. 1). Further evidence of this syngenetic relationship is that pyrite neither is replaced by nor replaces calcite. Epigenetic pyrite is found as sociated with the secondary fracture fill calcite (plate 11, fig. 51 or lining cavities in the calcite caprock (plate 11, fig. 6). Barite, like pyrite, can be either syngenetically or epigenet- ically related to calcite. When the mineral cuts across growth lines and crystal boundaries of the calcite, it is probably a replacement feature (plate 13, -figs. 1 and 2). Contemporaneous deposition is indicated when the barite is found in the growth lines (plate \3, figs. 3 and 4). Celestite occurs predominantly as a syngenetic mineral with calcite as shown by their textural relationship (plate 6, fig. 3), and only occasionally is a replacement feature noted. It is found as discreet crystals, or groups of crystals, in the calcite matrix (Plate 6, figs. 2, 3, and 4), or can alternate in thin bands with the calcite (plate 12, fig. 2\. Both anhydrite and gypsum are found associated with calcite caprock, usually in small amounts. The minerals can be older, younger or the same age as the calcite matrix, but generally show features such as irregular borders and inclusions, which indicate that they are being presently replaced by calcite. Calcite was noted ir. most of the anhydrite and gypsum samples studied, but only as a minor constituent. It occurs as small crystals 73 along cleavage planes (plate 15, fig. 4), as an envelope surrounding dolomite rhcmbs (plate 16, fig. 1) and as single crystals m the sulphate matrix (plate 16, figs. 2 and 3). The single crystals commonly contain a dark nucleus which has the appearance of organic matter. A fairly complete discussion of calcite in sulphate caprock has been presented by Goldman (1952). The association between calcite and sulphur has been previ ously discussed. To summarize, calcite replaces all of the caprock minerals except the sulfides to varying degrees, and is occasionally replaced by sulphur. Geochemistry The variability of trace elements in calcite is well illustrated in Figures 21 and 22. As the calcite zone is not completely mono- mlneralic, the trace element distribution is a function of both solid solution and other minor phases present. It is believed that most of the silica is in the form of quartz, iron in pyrite, sulphur as an external impurity and in pyrite and magnesium in dolomite. Other trace elements occur predominantly as interstitial impurities in calcite as in trace minerals or in solid solution. The averages of the analyzed constituents is shown in Table 11. The coefficients of variation indicate that calcium, iron and sodium remain fairly constant, whereas the other elements are quite variable, with silica and titanium being the most variable. Interrelationships between Oepth 1250- 1230 - 1230 - I190 1290 - 1270 120 - 20 0 70 00 0 1000 750 500 250 0 idn oe,Jns No. Dome , I Jones Minden Wei Si-K vs. I.Depth Sippm 0.05 K,0% iue 21 Figure zone 2 0-Fe-Zn in C in 0-Fe-Zn Feppm 0 CO 3 5 500 250 Znppm CoprocK 74 170- 1210- 1230 - 1250- 1270- 1290 .20 .40 .60 .80 100 .60 .70 .80 .90 1.00 0 100 200 0 100 200 300 MgO % No20 % Sr ppm Ti ppm Figure 22 Minden Oome, Jones No. I Well. Depth vs MgO-No20-Sr-Ti in CaCOj Cap rock zone. TABLE 11 Statistical Analysis of CaCO^ Caprock Chemical Data, Minden Dome, La. Standard Coefficient of Variable Mean Deviation Variation (%) N= 19 CaO 52. 95* 3.63 6. 85 S 0. 02* 0. 02 100 Si 720.47 1767.27 245. 29 K 137.05 91. 72 66. 92 *% value Fe 170. 00 195. 06 11.47 Other values in ppm Zn 257.26 174.92 67. 99 Sr 100. 95 79. 02 78. 27 Ti 491. 85 1058.70 215. 24 MgO 0. 54* 0. 36 66.66 NazO 0.82* 0. 22 26.82 Correlation Matrix Variable CaO S Si K Fe Zn Sr Ti MgO NazO CaO 1.000 -0. 774 -0. 157 -0. 026 0. 033 -0. 675 -0. 085 -0. 543 0.369 0. 022 S -0. 774 1. 000 0. 034 -0. 304 -0. 064 0. 661 -0. 035 0.404 -0. 292 -0. 220 Si -0. 157 0. 034 1. 000 0. 096 0. 753 -0. 092 0. 082 -0. 089 -0. 099 0. 062 K -0. 026 -0. 304 0. 096 1. 000 0. 219 -0. 101 -0. 266 -0. 253 -0. 175 0.249 Fe 0. 033 -0. 064 0. 753 0. 219 1. 000 -0. 317 -0. 124 -0. 178 0. 014 -0. 062 Zn -0.675 0. 661 -0. 092 -0. 101 -0. 317 1. 000 0. 246 0. 360 -0. 184 0. 095 Sr -0. 085 -0. 035 0. 082 -0. 266 -0.124 0. 246 1. 000 0. 070 0. 587 -0. 020 Ti -0. 543 0.404 -0. 089 -0. 253 -0. 178 0. 360 0. 070 1. 000 -0. 095 -0. 071 MgO 0. 369 -0. 292 -0. 099 -0. 175 0. 014 -0. 184 0. 587 -0. 095 1. 000 -0. 373 Na2° 0. 022 -0. 220 0. 062 0. 249 -0. 062 0. 095 -0. 020 -0. 071 -0. 373 1. 000 O' elements is displayed in the correlation matrix (table 11): Calcium is inversely related to most of the other elements as would be expected; sulphur shows a fair positive correlation with zinc; silica relates fairly well with iron; strontium relates slightly with magne sium. The silica and iron relationship is probably due to the in corporation of these elements from interstitial waters at the time of precipitation. The water quality would change with time and depth, but the Fe/Si ratio should remain about the same. The strontium- magnesium relationship may indicate a source for these elements from preexisting limestones or dolomites. The Mg and Sr would be in corporated in a similar manner as described for the Si-Fe relation ship. The relationship between sulphur and zinc is not understood, and it is possible that no true relationship does exist. 13 12 C / C ratios in calcite caprock from a number of domes have been found to be considerably lower than the ratios for normal carbonate rocks (Felly and Kulp, 1957; Downey, 1968). This low ratio is attributed to the incorporation of carbon derived from the oxidation of organic matter. However, different generations of calcite have different carbon isotope ratios within a single sample. Banding Banding in the calcite caprock is a very common feature. One of the most distinctive properties of this banding is the rhythmic alternation of colors (plate 5). The thickness of any one band may range from about 0.25 mm. to 40 mm. or thicker. In the lesser disturbed portions of the cap the banding is generally horizontal or at a small angle from the horizontal. The bands are limited in horizontal extent as they terminate either by pinching out or by truncation (plate 5, fig. 2). When viewed closely, the bands show many irregularities and contortions. Under the microscope the bands appear to grade into one another and a sharp contact cannot be found (plate 9, fig. 4). Cavities lined with drusy calcite are characteristic of the light bands while the dark bands are usually more dense and may contain con siderable pyrite. The dark bands are somewhat harder (a greater resistance to abrasion) than the white bands as indicated by the fact that when the banded rock is polished, the light bands are undercut leaving the dark bands protruding. White bands have an anomalously high concentration of titanium (up to 3800 ppm) but are generally low in sulphur and iron. Darker bands are characterized by high iron, sulphur and often silica but are low in titanium. Calcite is usually the only mineral in the white bands. Sulphides (predominantly pyrite) and sometimes organic matter generally constitute minor phases in the dark banded calcite. Brown in 1931 attributed this banding to diffusional movement. Since that time, little attention has been given to the subject. A possible mechanism of diffusion during early diagenesis would be that dissolved sulphide is capable of diffusing out of organic rich material. Diagenetic models, supported by experimentation, display factors 79 affecting the migration of iron and sulphur within anaerobic sediments (Berner, 1969). The model which appears to best explain calcite caprock banding features contains from four-tenths to ten times the reactive iron concentration over the initial concentration of reducible sulphur. In this situation the is able to diffuse out of the organic rich layer only after solubilizing species have begun to build up fairly high dissolved iron concentrations. Under these conditions there may be steep and approximately equal gradients of + + both Fe and H^S perpendicular to the organic rich layer resulting in the iron diffusing upward and the sulphide downward. Berner adds that due to the fairly high insolubility of iron sulphide, the con centration produce of the interdiffusir.g iron and sulphide will exceed the solubility product to a sufficient degree so that iron sulphide will precipitate at some point below the organic rich layer, giving rise to a dark liesegang band. Alternating bands may then result from repeated increases of interdiffusing iron and sulphide concentrations succeeded by relief of super saturation by precipitation. Berner (1971) lists three principal factors that limit the amount of pyrite which may form; (1) concentration and reactivity of iron compounds, (2) the availability of dissolved sulphate and (J' the concentration of organic material which can be utilized by sulphate-reducing bacteria to produce H^S. Calcite-sulphate interface The interface between calcite and gypsum or calcite and 8 0 anhydrite is somewhat gradational. Taylor (1938) states that "in most cases the anhydrite is not in sharp contact with the calcite zone, but grades into it through a transition zone of variable thickness. " McLeod (I960) believes that there is a "rather complete separation of the calcite layer from the anhydrite zone. " Unfortunately, only seven samples of this interface from six domes were available to the writer, resulting in that a generalization may not be statistically reliable. All of the samples did show some gradation, but the transition zone never exceeded 12 centimeters in vertical distance and averaged approximately less than 2 cm. The sequence of alter ation observed is the anhydrite grading upward into a zone where calcite appears along cleavage traces in the anhydrite. Farther up ward the amount of calcite increases to a degree at which about 40 percent of the original anhydrite remains. At this point there is a fairly abrupt change in which anhydrite almost completely disappears and only calcite is present. Upward from this point there is no evi dence of any anhydrite. Alternating sequences of anhydrite-calcite caprock in one bore hole (Hull Dome, Tex.) showed a similar phenomena. Dolomite M ineralogy Dolomite was noted in each caprock investigated. It com prises up to about 60 percent of some zones of caprock and is only a 8 1 minor constituent in others. It ranges in size fromless than 0.1 mm. up to occasionally as large as 1. 0 m m ., and always displays its characteristic rhombohedral form. The mineral is one of the major constituents in the caprock in a 200 foot section of the Utica Dome, Mississippi and in a 160 foot section of the Kings Dome, Mississippi. Here dolomite is associated with calcite and/or celestite. Plate 12, Figure 3 illustrates dolomite in a celestite matrix, less than one percent calcite is present. Calcite and dolomite of approximately the same quantity is shown in Figure 4 of the same plate. Dolomite as a minor constituent in caprock is very common. It is seen in association with calcite as a contemporaneous precipitate (no replacement or alteration features noted) (Plate 10, fig. 2), a replacement mineral of calcite (plate 10, fig. 5) or a concentration of rhombs along shear planes (plate 11, fig. 3). Dolomite appears in the anhydrite-gypsum caprock as disseminated individual crystals. It is only in this zone where it is common for the rhombs to have a nucleus of light colored material (plate 16, fig. 1). Textural evidence indicates that dolomite replaces calcite to varying degrees and possibly celestite and barite to a very small degree. It is replaced by calcite and seldom by gypsum and barite. 82 C elestite M ineralogy It is believed by the writer that this is the first time that celestite has been recognized as a major caprock mineral. Previous literature (Taylor, 1938; Brown, 1931; Hanna and Wolf. 1934; et al. ) indicates the presence of this mineral., but only in minor quantities. The large concentrations of celestite are found in the caprock overlying Mississippi domes, where it constitutes over 90 percent in some portions of the caprock. Celestite was identified in this study both optically and by x-ray diffraction analysis. The mineral is usually associated with either caicite or dolomite or both. When it is one of the major minerals, it is generally fibrous (plate 12, fig. 3) or in radiating blades (plate 12, figs. 5 and 6). Disseminated throughout any of the lithologi.es, celestite appears as radiating blades or as tabular crystals. Celestite is generally thought to occur as a coprecipitate of the associated minerals (see plate 6, figs. 2, 3 and 4 and plate 12, fig. 2). This relationship is based on such textural features as; commonly curved boundaries between celestite and other minerals, the tendency for the angles of intersection between three ciystals to be 120° and the alternation of bands of celestite with the associated mineral. Celestite is infrequently replaced by caicite, dolomite and sulphur and it slightly replaces anhydrite, gypsum and caicite. 83 Geochemistry The distribution of this mineral, as previously stated, is largely restricted to caprock of Mississippi domes. Figures 23 and 24 illustrate the concentrations of five elements associated with celestite in two of these domes. The mineral is generally character ized by fairly low Si and K concentrations and is commonly associated with dolomite. Two questions can be asked concerning the presence of this mineral in fairly large quantities: (1) Why are large amounts of celestite restricted to the caprock of Mississippi domes? and (2) WTiat is the source of strontium? The following explanation is offered as an answer to both of these questions: Mississippi domes are unique because the mother salt is overlain by a series of carbonate and evaporite facies which range in age from Upper Jurassic to Lower Cretaceous. The carbon ates are composed of materials which range from oolites to quiet water dolomitic beds. Recent carbonate analogues of the above facies contain large concentrations of strontium: Bahama oolites contain about 10, 000 ppm; reef coral aragonites contain around 8, 000 ppm; algal aragonites contain about 9,000 ppm Sr++ (Kinsman, 1969). During diagenesis of these recent aragonites, the Sr++ content decreases with increasing age of the rocks. W'aters moving through these carbonate rocks progressively dissolve the aragonite and precipitate caicite, which would result in the ratio (Sr++/Ca+*) Depth - 0 0 6 3 - 0 0 9 3 - 0 5 4 3 Kings Dome, Hal I No. I Welt. Depth vs. Si-K-Fe-Sr-Ti in sediments .caprock sediments Dome, Hal in No. IKings I Welt. Si-K-Fe-Sr-Ti vs. Depth 00 5000 3000 Kpp n sl residue. salt and Figure 23 400 Sr ppm Ti ppm Ti ppmSr lOOOt 50 5000 2500 o « /> < 84 Depth - 0 5 7 2 - 0 0 7 2 - 0 5 6 2 - 0 5 0 3 - 0 0 0 3 - 0 5 9 2 - 0 5 8 2 - 0 0 8 2 - 0 0 1 3 - 0 0 9 2 3150 tc Dm,e Ltl N I elDph s iKF-rT i cpok and caprock in vs. Si-K-Fe-Sr-Ti I Well.Depth No Little Dome,Lee utico SiO, % SiO, 350 350 ppm K at residue. salt 0 1000+ 500 Sr ppm Sr C 2500 5000+ 5000+ 2500 T ppm i O O . Q 85 86 in solution progressively increasing. When the solubility product of celestite is reached, in the caprock forming environment, it will begin precipitation. The strontium to calcium ratio will decrease during precipitation of celestite to a point where the ratio will reach the equilibrium value, and then either CaCO^ or CaSO^ will begin to precipitate. Thus, the probable major source of strontium in Mississippi caprock originates from the Sr+* rejected from the crystal lattice of aragonite during conversion to caicite at the time, or shortly before the time, of caprock formation. A much more minor source of strontium would be the Sr*"*" contained in the ground waters, which may be the primary source for the rare occurring celestite found in Texas and Louisiana caprock. Banding Celestite and caicite are quite commonly noted in alternating bands in the caprock of Mississippi domes. The bands alternate from a dark gray to white, each averaging approximately 1. 5 mm. in width (plate 12, fig. 2). T hese bands w ere observed in the caprock of Oakley, Clemons and Utica Domes, Mississippi, with the banded intervals sometimes exceeding 100 feet. The darker bands consist mainly of finely-crystalline caicite; the dark color resulting from organic staining and/or a greater abundance of sulphides. White bands are composed mainly of 87 bladed and somewhat radiating celestite crystals. The boundaries between these bands are r.ot very distinct as the celestite and caicite gradually grade into one another. The accessory minerals associated with the banded caprock include quartz in about equal amounts 01 euhedral crystals anc rosettes, dolomite thornbs which have been partially replaced by caicite, pyrite, gypsum and anhydrite. The alternating bands of celestite and caicite are probably due to differences in their solubility, diffusion of strontium out of caicite in the presence of dissolved sulphate and the periodic build up of strontium by this diffusion alternating with a lowering of the strontium concentration by precipitation. Celestite-anhydrite interface The interface between celestite and anhydrite is gradational. This contact was examined in detail in the caprock of Utica Dome. Mississippi. The features, going down section, is as follows: At 2795 feet only celestite was observed; at 2805 feet celestite is still the major mineral and caicite, dolomite and quartz appear in minor quantities; at 2812 feet anhydrite was first noted and continued to increase in abundance downward to a depth of 2825 feet where no celestite could be four.d; at 2825 feet and downward, the anhydrite contains an anomalous amount of strontium when compared to the other analyses of caprock anhydrite (see table 29 and page 85). 88 Ba rite M ineralogy Barite was noted in small amounts in many of the caprock samples. It occurs predominantly as tabular to prismatic crystals (place 13; plate 14, fig. 2), sometimes as radiating blades and rarely as rosettes (plate 14. fig. 3). The crystals vary in size from 0. 1 mm. to large blades over 1. 5 mm. in their long dimension. Barite is generally associated with the carbonate minerals and has been found to be associated with celestite in gypsum and anhydrite rocks (Goldman, 1952). Taylor (1938) states that tabular crystals of barite in anhydrite caprock are of the same age as the anhydrite, dolomite, quartz and celestite, apparently indicating the mineral is of residual origin. Barite was identified during this study in both syngenetic and epigenetic relationships with the carbonate minerals. The mineral can replace or be replaced by caicite, dolomite, sulphur or gypsum. Sulphur M ineralogy This mineral, in economic quantities, is largely restricted to local areas in the caprock. Small amounts of sulphur are dis seminated throughout the carbonate and gypsum zones and occasionally in the anhydrite caprock. The sulphur ore zones are invariably associated with either caicite or gypsum. A gypsum sample from the 89 Lake Washington Dome, Louisiana, contains about 20 percent of the mineral (plate 11, fig. 1). The ore zone sulphur deposits consist of orthorhombic crystals which range in size from very small to over 18 mm. in length, and have been reported to be as large as 3 cm. in length (Taylor, 1938), Sulphur crystals are generally found in cavities or along grain boundaries in the host rock. Disseminated sulphur occurs as very minute particles (plate 7, fig. 6) or occasion ally as massive bundles oi crystals (plate 14, fig. 7). The mineral varies in color from yellow tc greenish-yellow, and commonly con tains inclusions of pyrite, organic matter or anhydrite. Sulphur is mined by the Frasch process from six salt domes in Texas and four in Louisiana. Average yearly production amounts to about 335 long tons of sulphur per dome (table 12). The sulphur producing domes are relatively shallow, caprock thickness is some what greater when compared to the nonproducing domes and the domes are usually located near the coastline. Table 12 displays a cor relation matrix of six variables associated with salt dome sulphur production. The data from this table indicate that sulphur production does net strongly correlate with any of the other variables and only slightly correlates with decreasing degrees of latitude. The top of salt compares more favorably with total caprock thickness on sulphur producing domes than on the nonproducing domes. TABLE 12 Statistical Analysis of Salt Dome Sulphur Production (Data obtained from Hawkins and Jirik (1966)) Standard Coefficient of Variable* ______Abbr. _____ Mean _____ Deviation ______Variation (%) Top of caprock TC 704.43 478.69 67.95 Top of salt TS 1238.00 569. 59 46. 00 Caprock thickness^ TT 533. 56 345.24 64. 70 Oil production^ (x 10"^) OP 2940. 17 3656.10 124.34 Sulphur production"* SP 334.66 370.48 110.70 Lattitude (°N) Lat 29.46 0. 39 1. 32 1. 23 observations 1. TS minus TC 3. Cumulative prod, to 1964 4. Average yearly prod, to 1964 (long tons) Correlation Matrix triable TC TS TTOP SP Lat TC 1. 000 0. 797 -0. 072 0. 150 0. 171 -0.227 TS 0.797 1. 000 0. 545 0.248 0. 128 -0.054 TT -0.072 0. 545 1. 000 0. 202 -0. 026 0.225 OP 0. 150 0. 248 0. 202 1. 000 0. 197 0. 120 SP 0. 171 0. 128 -0. 026 0. 197 1. 000 -0.230 Lat -0.221 -0. 054 0.225 0. 120 -0.230 1.000 vOo Geochemistry Sulphur, which is ccmmonly present in caprock. is thought to be an alteration product from the reduction of anhydrite (Feely and Kulp, 1957). The reducing ager.t in the alteration process may be hydrocarbons or de*d organic matter, aided by sulphate-reducing anaerobic bacteria. One type of sulphate reduction reaction may be CH + CaSO aC eria •- H S + CaCO + H O (2) 4 4 Z 3 Z in which methane is used only for simplicity. Elemental sulphur may later be formed by the produced in equation (2) being reoxidized by reaction with more sulphate ion (Feely and Kulp, 1957). They show that the reaction in this case is: S04= + 3H2S ~--N 4S° + 2HzO + 20H" (3) Davis and others (1^70) indicate that hydrogen sulphide is not oxidized by sulphate ions under the conditions proposed by Feely and Kulp (1957). They believe that acid conditions would be more favorable thermodynamically for the sulphate to be reduced to sulphur in this type reaction, SG4= + 3H2'i + 2H+ ~~~ N 4S° + 4H20 (4) but acid conditions do not exist in caprock. When the CaS04 is reduced by bacteria to H2S, the pH increases, rather than becoming acidic, due to hydrolysis of caicite formed by the reaction of calcium ion with aqueous carbon dioxide (Davis, et al. , 1970). They suggest 92 that a deep, anaerobic oxidation of H S by sulphate iar.s is not an I ^ acceptable explanation for th ^ or*' T;n of the sulphur, but that the sulphur is formed within 1, 000 feet of the surface due to contact of hydrogen sulphide with oxygenated groundwater in this type of reaction: 2H25 + °2 v 2S° + 2H2° (5) Gyp sum M ineralogy The lithology data of Tables 4 and 5 show that gypsum is found on only a little over half of the domes studied. The thickness of gypsum is quite variable, having a coefficient of variation of 120 percent, considerably greater than the coefficient for caicite thickness and somewhat greater than the coefficient for anhydrite thickness. Gypsum thickness generally increases from an east to west direction with Texas domes having the greatest amount of the mineral. From Table 4, page 27 and Figure 10. page 42 it can be seen that gypsum never exceeds 75 percent of the total caprock on any of the domes. The mineral comprises less than 15 percent of the total caprock thickness on any Mississippi dome, while Louisiana and Texas domes may have gypsum thicknesses comprising from 0 to 75 percent of the total thickness. Gypsum occurs as prismatic, tabular and fibrous crystals. The tabular selenite variety is the most common form with the 93 crystals ranging ir: size from less than 1 mm. to 8 mm. in their long dimension. Minerals associated •with gypsum include caicite, which is usually noted to be replacing the gypsum (plate 15, figs. 3 and 4) and sometimes in an apparent syngenetic relationship with gypsum (plate 16, figs. 2 and 3); dolomite (plate 16, fig. 1), aragonite (plate 16, fig. 4), barite (plate 16, fig. 5), sulphur (plate 17, fig. 1) plus a few rarer minerals. Anhydrite crystals brought up to the surface by a brine well on the Choctaw Dome, Louisiana are displayed in Plate 18, Figures 4, 5 and 6. The crystals were left on the surface for an undetermined amount of time, and have since been cemented together by gypsum. This rock was examined to determine if any similarities exist between the rock formed on the surface and the caprock anhydrite. The crystals show some solution features and are cemented only where they are in contact with each other. Given some amount of time, they may completely coalesce and form a solid gypsum rock; perhaps similar in morphology to the gypsum caprock. Since temperature, pressure and chemical conditions are so different in the two rock forming environments, any further comparisons would probably be invalid. Geochemistry The largest percentage cf gypsum resulted from the direct alteration from anhydrite with a smaller amount of the mineral resulting from direct precipitation. The hydration of anhydrite to form gypsum rarely proceeds at depths greater than 1820 feet (MacDonald, 1953). Gypsum thickness shows a fairly good inverse relationship with both the depth to the top of caprock and top of salt (table 5) which indicates that the process of hydration with depth is increasingly less effective. The reader is referred to Goldman (1952) for further information regarding the process and features of gypsification. A nhydrite M ineralogy Anhydrite is the most abundant mineral with respect to the total number of domes which have caprock, however, a few domes with caprock do not contain this mineral in any of the wells examined (table 4). Anhydrite thickness averages approximately 220 feet thick on the domes listed in Table 4, but the thickness is quite variable from dome to dome as well as on any one single dome. Anhydrite is most commonly found in the lower portions of caprock. although it is not restricted to this area, and is occasionally found in relatively smaller quantities associated with other major minerals. Anhydrite occurs in a number of forms; prismatic, radiating, fibrous and "stem shaped". The prismatic variety is characteristic of most of the massive anhydrite caprock, often referred to as •' sac char cidal" (Taylor, 1938) or "pile-of-brick" structure (Brown, 95 1931). The ''ftfcm shaped" anhydrite was noted by the writer to be only present in the caprock-salt interface, both in the salt and directly above the salt, and a search of the literature did not reveal any mention of this form in the massive anhydrite. The massive anhydrite caprock is characterised by prismatic crystals which vary in size from about 0. 05 mm. up to 2. 1 mm. in their long dimension. The color of the mineral in thin-section varies from nearly colorless to a light brown. The anhydrite crystals form an interlocking mosaic, resulting in low porosity, and exhibit a slight preferred orientation trending in a horizontal direction (plate 18, figs. 1, 2 and 3). Coalescence of crystals has resulted in fairly indistinct boundaries when a fractured surface of the rock is ex amined (plate 19, figs. 1 through 5). Cleavage fragments, and pos sibly individual crystals, from a crushed core of massive anhydrite a re shown in Plate 19. Figure 6. Goldman (1952) discussed recrystallization and pressure solutions in anhydrite caprock. These features, along with his comments, which he attributed to one or the other of the above processes are briefly listed in Table 13. It is the writer's con tention that many of these features can be interpreted as precipi tation features: Recrystallization features 2 and 5, inclusions of other crystals--this could simply indicate that these impurities were present during the precipitation of anhydrite, and therefore included in the 96 TA BLE 13 Summary of Features Attributed by Goldman (1952) to Result from Recrystallization or Pressure Solutions in Anhydrite A. Features attributed to recrystallization. F eature Comment s 1. Elongate crystals Distinct from the more equant or stocky crystals. Usually occur in zones. 2. Inclusion of fine Include carbonates, sulphides, quartz, grained impurities probably sulphur, etc. and vesicles. 3. Coalescence Boundaries become vague. 4. Compound grains Crystals in close contact with closely related extinction, birefringence and cleavage. 5. Intergrowth andmutual A larger crystal includes a smaller one. inclusion of crystals 6. Interstitial voids Voids between grains. B. Features attributed to pressure solutions. 1. Deformation of indi Found in nonmylonized areas. vidual crystals 2. Trapped grains Narrow remnants between larger grains. 3. Serrate and irregular b o rd ers 4. Interpenetration of grains 5. Elongation of grains Have very irregular external form. 6. Closing of in te r stitial voids 7. Parallel elongation Not accompanied by parallel orientation 8. A ssoc, with other Faulting, shearing, "flowage", stress effects increase with depth. 97 crystals. An example of this process would be sulphur included in caicite (see page 67). Recrystallization features 3 and 4, coalescence and compound grains--crystals growing from many centers in a super saturated brine would eventually coalesce to form an interlocking mosaic. Compound grains would also result from this coalescence which is the common case for many precipitates, such as halite. Interstitial voids, feature 6, could result from incomplete coa lescence. Pressure solutions, feature 2, trapped grains--this may be due to precipitation in which the smaller crystal is enclosed by two larger coalescing crystals. Interpenetration of grains, feature 4, m a.y be due to protruding intergrowth. Feature 6, closing of inter stitial voids, this could simply indicate precipitation. There appears to be no diagnostic textural criteria to positively identify a recrystallization feature from a pressure solution feature or from a precipitation feature. The writer does not conclude that Goldman was wrong in his interpretation, only that there may be other explanations for the various features. Thus, due to the lack of cer tainty in recognizing what textures are a result of what process, other criteria must be employed. Anhydrite is associated with a host of rare occurring minerals. These minerals can be dispersed throughout the anhydrite cap or can be found in concentrated zones which normally parallel the banding. 98 A list of the rarer minerals, accompanied by a brief description follows: Caicite, usually finely-crystalline and found along cleavage traces and grain boundaries, coarsely-crystalline in veins; dolomite, ranges in size from minute inclusions up to 0. 8 mm. , generally contain, in this zone, small nuclei; quartz, occurs as small ter minated crystals and rosettes as inclusions within anhydrite grams; pyrite, minute inclusions; sulphur, minute inclusions in both anhydrite and dolomite; barite, small bladed crystals; celestite, occurs as small radiating clusters and rarely as rosettes; and a number of very rare minerals. Barnes (1933) studied samples col lected from the mine dump at Winnfield Dome, Louisiana. He indi cates that the samples are probably from the basal portion of the anhydrite caprock. A portion of his report which describes the relationship between the anhydrite and the metallic minerals he noted is quoted below. The metallic minerals have formed along parallel planes producing a distinct banding in the anhydrite. One mineralized layer is composed of native arsenic spheroids as much as a quarter of an inch in diameter. Other bands are composed predominantly of individual minerals (metallic sulphides), suggesting that selective replace ment has taken place along lines of banding in the anhydrite, which in the mineralized zone shows no pronounced transverse breaks. Geochemistry The chemical analyses of anhydrite caprock from six domes is listed in Tables 26 through 31, pages 196 through 201. Some of the 99 analyzed constituents are plotted against the depth at which they occur for four of these domes in Figures 25 through 29. The elements listed are not identifiable in separate phases in the anhydrite as determined by x-ray diffraction and microscopy. It can be seen from these figures that the trace element concentrations vary considerably with depth in each dome analyzed, and probably would show an even greater variation if analyzed in shorter intervals. The relationships among the constituents, plus the average values for three of the domes is listed in Tables 14, 15 and 16. Considering the three tables together, certain correlation coef ficients share common trends: Si relates to K. indicating that both are incorporated at the same time; in the Minden Dome anhydrite, Fe correlates with Zn, and in the Allen Dome, Fe is inversely related to Zn, indicating differing conditions at each dome. Coefficients for the Allen Dome show better correlations between the variables than the other two domes, which may mean that conditions were more stable during its formation. The abnormally high strontium content in the anhydrite of Utica Dome, Mississippi (table 29, pagel99), appears to be related to the celestite caprock which overlies it. This probably indicates that Sr++ was present during anhydrite formation. The strontium is diadochically included in the anhydrite. The greatest amount of Sr included is 0. 125 percent, while the average strontium content Oepth 1500- 1400 - 1350 40 - 1450 1300 Minden Dome , No.Jones IWei I. Depthvs. in CaS Si-K-Zn-Sr-Fe-S i ppm Si Kppm 4 n ppm Zn Figure25 e ppmSrFe ppm 04 zone. % S 0 0 1 Depth 1100 - 1050 150- 0 5 11 1300- 1250- 1200 1350- 1450- 1500 - 1500 - 5 50 5 10 0 1000 750 500 250 0 Totum Dome, A EC Tatum No. I Wei I. Depth vs. CaSO*Si-K-Zn-Sr-Fe cap in ok n sl eiu . ahd ie idct no indicate linessample.Top of residuesalt . Dashed salt at rock and i ppm Si 0 20 0 4020 5 40 5 30 5 0 10 200 100 0 450 350 250 450 350 250 400 300 200 100 p Z pm r p F ppm Fe ppm Sr ppm Zn Kppm Figure 26 FeppmSr ppm 1510. 1 0 1 Depth 1450 - 0 8 4 1 - 0 7 4 1 - 0 0 5 1 - 0 9 4 1 1460 - 1460 0 250 500 750 1000 0 100 200 300 400 250 350 450 250 350 450 0 100 200 200 100 0 450 350 250 450 350 250 400 300 200 100 0 1000 750 500 250 0 au Dome, Fig.ATatum from26 No.EC Tatum I Wet scale depth I.Exponded i p K p Z pm r p Feppm ppm Sr ppm Zn ppm K ppm Si o o Salt of Top Figure 2 7 2 0 1 1280 1290 1300 1310 1320 1330 .c J-1340 o 1350 1360 1370 1380 1390 TS 1400 250 500 750 1000 0 100 200 300 350 450 250 350 0 100 22.89.0 I .2 .3 4 Si ppm K ppm Zn ppm Sr ppm Fe ppm S % Figure 28 len Dome, FS Well No.7.Depth vs. Si-K-Zn-Sr-Fe-S in C 0 SO4 caprock and salt residue. Depth 695 670- • 0 6 6 680- u l Ootne,Hull F.S.Co.Wei I No. Si ppm Si 500 500 CoS 1000 04 arc ad at residue salt and caprock 5 300 150 Figure 29 6 . in Depth vs.Si-K-Fe-Zn 90 Feppm Kppm 1 10 40 500 400 130] _ 110 300 Zn ppm Zn 104 TABLE 14 Statistical Analysis of CaSO^ Caprock Chemical Data, Minden Dome, La. Standard Coefficient of Variable ______Mean ______Deviation Variation (%) N= 13 CaO 40. 10 * 0.93 2.31 S 23.43 * 0.40 1.70 Si 70. 85 61.57 86.90 *% value K 76. 77 74.45 96.97 Other values in ppm Fe 66.85 67. 53 101. 01 Zn 334. 38 32.24 9.64 Sr 326.00 46.77 14.34 Correlation Matrix Variable CaO S Si K Fe Zn Sr___ CaO 1.000 0.779 -0.214 -0.318 -0. 078 -0. 067 0.218 S 0. 779 1. 000 -0. 573 -0. 567 -0. 213 -0.037 0. 017 Si -0. 214 -0. 573 1. 000 0. 653 0. 332 0. 070 0.391 K -0. 318 -0. 567 0. 653 1. 000 0. 748 0.469 0. 087 Fe -0.078 -0.213 0. 332 0.748 1. 000 0. 852 0. 108 Zn -0. 067 -0. 037 0. 070 0.469 0. 852 1. 000 0. 007 Sr 0.218 0. 017 0. 391 0. 087 0. 108 0..007 1. 000 105 TABLE 15 Statistical Analysis of CaSO^ Caprock and Salt Residue Chemical Data, Tatum Dome, Miss. Standard Coefficient of Variable Mean Deviation Variation (%) N= 38 CaO 40.25* 0. 65 1.61 S 23.49* 0. 22 0. 94 Si 373.68 307.41 82.26 *% value K 201.68 115. 81 57.42 Other values in ppm Fe 61.92 31.43 50. 76 Zn 311.45 53. 84 17.29 Sr 353.39 39. 56 11. 19 Correlation Matrix ariable CaO S Si K Fe Zn Sr CaO 1.000 0.400 0.298 0.414 -0.461 -0.719 -0. 122 S 0.400 1. 000 0.077 0. 004 -0. 316 0.024 0.001 Si 0.298 0. 077 1. 000 0. 662 -0. 083 -0. 298 -0. 060 K 0.414 0. 004 0. 662 1. 000 -0.324 -0.392 -0. 130 Fe -0.461 -0.316 -0. 083 -0. 324 1. 000 0.255 0.034 Zn -0.719 0. 024 -0.298 -0. 392 0. 255 1. 000 0.228 Sr -0. 122 0. 001 -0. 060 -0. 130 0. 034 0.228 1.000 o O' TABLE 16 Statistical Analysis of CaSO^ Caprock and Salt Residue Chemical Data, Allen Dome, Tex. Standard Coefficient of Variable Mean Deviation N= 12 CaO 39.47* 0.46 1. 16 S 23. 15* 0. 16 0. 69 Si 458. 75 273.87 59.69 *% value K 143.50 110.67 77. 12 Other value in ppm Fe 110. 33 37. 11 33. 63 Zn 392.17 39. 04 9.95 Sr 325. 67 38.61 11. 85 Correlation Matrix Variable CaO S Si K Fe Zn Sr CaO 1. 000 0.858 -0.463 -0.364 0.215 -0. 191 0.512 S 0. 858 1.000 -0. 675 -0. 510 -0. 001 0.089 0. 392 Si -0.463 -0.675 1.000 0.817 0. 074 -0.613 -0.708 K -0. 364 -0.510 0.817 1.000 -0. 226 -0. 345 -0. 579 Fe 0. 215 -0. 001 0.074 -0.226 1. 000 -0.625 0.271 Zn -0. 191 0. 089 -0.613 -0.345 -0. 625 1. 000 0. 264 Sr 0. 512 0.392 -0. 708 -0. 579 0. 271 0.264 1. 000 107 1 0 8 of worldwide anhydrite deposits, in the absence of celestite, is about 0.2 percent (Braitch, 1971). The German Zechstein anhydrites range in strontium content from 0. 11 to 0.43 percent, whereas a recent gypsum deposit in Trapani, Sicily contains 0.28% strontium. Braitch (1971) states that nothing positive can be concluded of the strontium content upon distance from the former coastline but there ++ appears to be less in the interior of the basin. The Sr content barely changes during gypsification. Banding Generally two somewhat different types of banding are com mon to the massive anhydrite caprock. One type is composed of irregular and usually horizontal dark bands, about a millimeter in width, alternating with thicker, lighter colored bands (Taylor, 1938). The second type of banding, referred to by Goldman (1933; 1952) as "katatectic" banding, is characterized by near parallel layers, gen erally horizontal or dipping at low angles, which alternate between a band of a centimeter or less of fine-grained anhydrite and a thicker band of coarse-grained anhydrite. The first type of banding has been attributed (Taylor, 1938) to secondary pyrite and carbonaceous material deposited along horizontal joint planes and in narrow shear zones. Katatectic banding has been stated (Goldman, 1952) to result from intermittent compaction of anhydrite accumulating on the top of the salt dome by solution of the salt. It is difficult for the writer to 109 understand the alternation of coarse and fine-grained anhydrite by simple accumulation and compaction, even when recrystallization is considered. The top of katatectic bands directly below the katatectic surface often display a lighter color which grades downward to a darker anhydrite. Goldman (1952) indicates that this is due mainly to mylonization and the accumulation of carbonates. The following is his discussion on the concentration of secondary minerals at the top of katatectic bands: I believe this concentration of secondary minerals is correlated with the upward progress of recrystallization that I assume to explain the upward grading from dark to light in the polished faces of so many katatectic layers. I have pointed out that mineralization generally tends to be concentrated in more porous parts of the anhydrite caprock. Its concentration at the top of the katatectic layers may therefore be due either to m iner alization at a late stage when upward progress of recrystal lization in the katatectic layers left only or mainly this upper part of the layer in a porous condition; or some of the secondary minerals may be forced out and driven ahead, as recrystallization advances upward. The density of the recrystallized base of the overlying katatectic layer may arrest them at the katatectic surface. Katatectic banding in the anhydrite caprock may result from either diffusion or an episodix influx of other elements, or even the two processes combined. The darker bands generally contain less silica, potassium and iron, and these elements gradually increase as the band becomes lighter. The very light bands may also contain small amounts of calcite and rarely celestite or barite. The darker bands which occur at high angles with respect to the nearly horizontal 1 1 0 bands are probably developed by diffusional processes. Anhydrite dissolution and precipitation features Anhydrite displays characteristic features which indicate whether the mineral is undergoing dissolution or precipitation. The solubility of anhydrite is slightly different for each of the three crystal faces (Brown, 1931), resulting in the peculiar, but diagnostic, shapes formed during either of the two processes. Diagnostic dissolution features are (1) protruding plates and needles which represent the less soluble faces and, (2) a very irregular surface on all crystal faces. Photomicrographs of these features are displayed on Plate 25. Gypsum was precipitated in the experimental phase of this investigation, but the characteristic features of precipitation are thought to be similar to those of anhydrite because gypsum also has different solubilities for each crystal face. The features most diagnostic of this process are (1) pronounced growth lines as shown in Plate 26, Figure 1 and, (2) differential growth rates resulting in platy type structures as shown in Figure 2 of the same plate. This platy structure is probably due to the crystallographically controlted solubility. Anhydrite-salt interface The morphologic changes in the anhydrite caprock-salt interface cone of the Tatum Dome, Mississippi, were examined in detail. The massive anhydrite grades downward into increasingly unconcolidated anhydrite at about a depth of 1505 feet, and finally into the salt at 1510 feet. The following are those features observed both optically and with a scanning electron microscope: At 1506 feet, the rock is slightly friable, crystals show a prismatic haoit and only precipitation features are present (plate 20. figs. 1 and 2). One foot below this, the anhydrite is friable, the "stem shaped" form predominates and dissolution features are found on most of the crystals (plate 20, figs. 3 through 6). At a depth of 1509 feet the anhydrite is almost completely unconsolidated, representing the "anhydrite sand" which is characteristic of this interface. The crystals show a fairly high degree of dissolution and they are all of the "stem shaped" variety (plate 21. figs. 1 through 4). The top of the solid salt is at a depth of 1510 feet. The water insoluble residue from this depth, primarily consisting of anhydrite, is illustrated in Plate 21, Figures 5 and 6 and Plate 22. The only anhydrite habit noted was the "stem shaped" crystals and no dissolution features were found. Thus, within four feet from the top of the salt, the caprock anhydrite completely changes morphology and dissolution features are re placed by diagnostic features of precipitation. It has just been stated that the anhydrite in the salt is of different morphology than the anhydrite in the massive caprock. Likewise, trace element concentrations differ significantly between the two anhydrites. Referring again to Figures 26 through 29, pages 101 through 104^abrupt changes in trace element content at the caprock-salt interface can be noted. Silica, iron and potassium increase in concentration immediately above the interface, while zinc and strontium decrease in each of the three domes illustrated. A comparison of morphological features with trace element content, with decreasing depth, in the Tatum Dome is as follows: At a depth 1509, just above top of salt, the anhydrite sand is "stem shaped" and shows dissolution features. The concentration of Si. K and Fe in the anhydrite caprock has increased while Zn and Sr decreased with respect to the anhydrite in the salt. The "stem shaped" variety and dissolution features still predominate at 1507 feet. At this point, no silica was detected and both K and Fe content decrease by about a factor of two, with zinc and strontium remaining almost unchanged. At a depth of 1506 feet, crystals abruptly display a prismatic habit and only precipitation features are noted. The crystals at this depth again increase in Si and K; while Fe, Zn and Sr remain fairly constant. Above 1506 feet, smaller variations in concentration occur except at four different depths, each separated by approximately 100 foot intervals, where both silica and potassium decrease and iron slightly decreases. The dissolution features of the anhydrite sand just above the salt indicates that the interstitial water in this zone is under sat urated with respect to anhydrite. The sudden increase in Si, K and Fe above the salt is not fully understood, but it may be due to a type of interstitial solid solution in which foreign ions enter into the open places of the anhydrite crystal structure, even though the m ineral is undergoing dissolution. The decreased concentrations of Zn and Sr probably mean that they are being lost to the water during the dis solution of anhydrite. Above the depth 1506, at which only precipi tation features are found, smaller trace element concentration variations exist. This may indicate that fairly uniform water quality conditions were present during the formation of the anhydrite section. The four abrupt changes in concentration at about 100 foot intervals result from environmental variations at the time of anhydrite forma tion. One possible explanation of this variation would be the upward movement of the salt dome. Comparison between salt residue and caprock anhydrites Figure 30 illustrates Si-K-Fe concentrations in the entire CaSO^ caprock and in the salt residue anhydrite for each dome analyzed. It can be noted from the figure that all but one salt residue points plot toward the bottom of the diagram, and caprock points plot toward the silica apex. The residue point which groups with the caprock is a sample brought up in a brine well on the Choctaw Dome, Louisiana, and lefr on the surface for an undetermined amount of time. The anomalously high silica was probably introduced at the 114 Si o = cap rock • = salt residue K Figure 30 Triangular diagram illustrating average Si-K-Fe in entire Ca S 0 4 Caprock and salt residue for each dome analyzed Si 0 = cap rock • = salt residue K Figure 31 Triangular diagram illustrating Si-K-Fe inCaSo 4 Caprock immediately over- lying salt and in average salt residue 115 surface. The caprock point isolated in the center of the diagram represents the Minden Dome, Louisiana. The triangular diagram (fig. 31) illustrating Si-K-Fe in anhydrite caprock immediately overlying the salt and in the average salt residue also indicates fairly large differences between the two anhydrites. This difference is largely due to the enrichment of silica with respect to the other two elements in the caprock CaSCL. Figure 32 displays Si-K-Fe-Zn-Sr average concentrations in both caprock and salt residue anhydrite for five domes. Silica, potassium and strontium show the largest variations in caprock CaSO . between domes, while concentrations of iron show less 4 variation and zinc remains fairly constant. The most striking feature of this figure is the uniformity of the trace elements in the anhydrite salt residue, since the domes listed are located in two different states. This could possibly mean that the salt and included anhydrite were precipitated contemporaneously in one large basin or that conditions were very similar in several smaller basins. Further evidence is the work done by Feely and Kulp (1957) in which they show 32 34 that the S /S isotope ratios of anhydrite salt residues from a number of Gulf Coast domes vary just slightly, even less than the ratios of modern sea water sulphate. 32 34 Feely and Kulp (1957) indicate that the S /S ratios of anhydrite caprock vary much more widely than do the ratios cf >1600 1400- 1200- 1 0 0 0 - £ 8 0 0 - 600- 4 0 0 - □ Cap rock 200 - Salt residua JluU n I Fa I Zn Zn Sr Fa Zn Zn Dome TATUM KINGS UTICA ALLEN HULL Figure 32 Si-K-Fe-Zn-Sr average concentrations in cap and salt residue CaSO> 117 anhydrite salt residues from all domes. They further state that 32 enrichment of S in caprock may indicate original variations in the isotopic composition of anhydrite salt residue, based on inclusions of bedded anhydrite rock in the salt. This does not appear likely, since the "bedded anhydrite" is probably a locally high concentration zone of the mineral and should have the same isotopic composition as the more dispersed anhydrite. Isotopic analysis of Spindletop Dome 32, 34 sulphate compounds by Feely and Kulp show that the S /S ratio of anhydrite caprock is different from that of the salt residue, but is very similar to the ratio of bleedwater sulphate. Anhydrite volume relations Great thicknesses of anhydrite caprock means that a tremendous amount of salt would have had to be dissolved off the top of the domes to accumulate thick residues. Accompanying this dis solution of salt should be tremendous collapse areas over the top of the domes if the salt did not move up as fast as the dissolution rate. Collapse features, however, are not found above all domes that have caprock. The surface area available to dissolution at the top of the dome is quite small in relation to its entire surface area. Waters moving past the top would soon approach saturation which would result in a reduced rate or possibly even halting of the dissolution process. Anhydrite is not insoluble, therefore, a portion of the residue anhydrite may go into solution. Two models are now presented to 118 illustrate two different processes of obtaining anhydrite for caprock formation. The first model only considers residual accumulation of anhydrite and the second model illustrates how another process can result in a source of the anhydrite. This second model assumes that both halite and anhydrite are dissolved from the flanks of the dome and that anhydrite can later be reprecipitated. Model I: A hypothetical salt dome; one mile in diameter, with the shape of a cylinder, and containing 5 percent of evenly dispersed anhydrite impurity. 20, 000 feet of this salt would have to be dissolved off the top of the dome in order to accumulate 1, 000 feet of anhydrite (assuming that none of the anhydrite is dissolved). Model II: The same hypothetical dome as above. For a caprock 7 1000 feet thick and one mile in diameter its volume would be 21.9 x 10 cubic feet of anhydrite. If only 40 feet of salt were dissolved by upward moving waters on the flanks of the dome for a distance of 10, 000 feet, 7 32.9 x 10 cubic feet of anhydrite would be available, or approximately 33 percent more anhydrite than necessary for a 1000 foot thickness. Less Common Caprock Minerals Pyrite (FeS^): This mineral occurs in varying amounts in all lithology types, but its largest concentrations are in the calcite zone. Pyrite ranges in size from nearly submicroscopic to as large as 2.4 m m ., with many of these crystals being euhedral and having the form of cubes, pyritohedrons or octahedrons. The mineral generally 119 occurs in the darker banded calcile (plate 5, fig. ]) and less com monly is associated with the recementing calcite (plate 11, fig. 5). Pyrite was also found filling voids, and was occasionally noted to form on one side of a cavity or fracture with calcite forming on the opposite side (plate il, fig. 6). Pyrite replaces sulphur and gypsum to a small degree; it is replaced by none of the other minerals. Quartz (SiO^): This mineral is usually dispersed in small amounts throughout the caprock, although quartz is occasionally found in higher concentrations in shear zones (plate 11, fig. 3). Quartz is commonly noted as terminated crystals (plate 14, fig. 5) or rosettes (plate 14, fig. I), with both forms being generally small in size. The rosettes often contain dark nuclei similar to those in the dolomite crystals. Quartz is replaced only by calcite, and it replaces none of the other minerals. Aragonite (CaCO^): Almost all of the aragonite observed had been partially to completely replaced by calcite. It is predominantly associated within the gypsum caprock (plate 16, fig. 4', but is not restricted to this zone. The crystals average appi oxirnately 0, 3 mm. in size, with the largest observed crystals never exceeding 0. 6 mm. Ar-.g-. nite does not replace any* of the other caprock minerals. Halite (NaCl): This m ineral is probably disseminated throughout many of the interstitial voids and cavities of the caprock, but due to its high solubility it is probably dissolved in the drilling process. 1 2 0 Halite also forms as discontinuous beds, or lenses, in any part of the caprock and can separate other lithology types. One well drilled into the caprock of the Hull Dome, Texas, encountered salt 40 feet into the cap and had to drill 100 more feet to reach its base. The salt hi this well is both overlain and underlain by anhydrite. The presence of a small lens of pure salt (no water-insoluble residue was re covered) in the anhydrite caprock at Hockley Dome, Texas h s beer, reported (Teas, 193i). Sediments Within Caprock Detrital minerals, either disseminated or in bedded deposits, within the caprock is much more common rhan the literature would lea.d one to believe. Sediments have been observed in all lithology types and are chiefly composed of quart/, clay and feldspar detrital minerals. Disseminated detritus may be quite abundant (plate 15, *ig- 1) or found in zones of higher concentration (plate 10, fig. 3; pla te 15, fig. 2). Sediments were often noted filling fractures ir. the caprock (plate 11, fig. 2). Napolenoville Dome, Louisiana has considerable clay dispersed through gypsum; Hull Dome, Texas contains both sard and clay dispersed in anhydrite; and Vinton Dome, Louisiana contains quite an abundant amount of disseminated sand in the calcite caprock. 1 2 1 The following is a list of examples of caprock which contain more or less bedded sediments and a short description of their associations: (Data from F. S. Co. lithology logs) Allen Dome. Tex. Lenses of sandy clay, up to 111 feet thick, are found throughout most of the calcite cap. One well displays a sandy shale alternating with anhydrite in beds about 10 feet thick. Hockley Dome, Tex. A shale bed, which may be continuous from one side of the cap to the other side, separates the calcite zone in one well and separates the massive anhydrite at the same depth but on the other side of the dome. Hull Dome, Tex. A lens of shale and gravel about 50 feet thick separates anhydrite. Napoleonville Dome, La. A fairly continuous bed of shale about 25 feet thick is located within the gypsum caprock. Pine Prairie Dome, La. In one well a 450 foot section of shale separates the gypsum cap from the calcite cap, and in another well a 20 foot section of sand separates the same lithoiogies. Caprock is known to contain at least thirty different minerals, of which only nine or ten are found in significant quantities. Calcite, the second most abundant caprock mineral, probably originates from direct precipitation from solution as determined from its morphology and chemistry. Trace element concentrations in the mineral vary widely from sample to sample but do show interrelationships between 1 2 2 the elements. Commonly the calcite zcne is distinctly- banded. These bands are attributed to diffusional processes of iron and H S during very early diagenesis of the rock. The interface between calcite and gypsum or anhydrite is only somewhat gradational. Dolomite is infrequently one of the major constituents of caprock, but it is quite common as a minor constituent. The mineral is found as a presumably direct precipitate, replacement mineral and much less commonly as residue rhombs from the salt. Celestite is frequently a major mineral in the caprock of Mississippi domes and is usually associated with either calcite or dolomite. Celestite and the associated minerals occur as contem poraneous precipitates. The probable major source of strontium in Mississippi caprock originates from the Sr++ rejected from aragonite during conversion to calcite in the surrounding sediments. Celestite is commonly found in alternating bands with calcite. The interface between celestite and anhydrite is gradational as shown by celestite grading downward into an increasing amount of anhydrite and finally into the massive anhydrite which contains an anomalously high concentration of strontium. Barite occurs as a minor constituent of caprock. It is usually associated with the carbonate minerals. Sulphur, like barite, is only a minor m ineral except rarely it is found in economic quantities. The mineral is thought to be an alteration product from the reduction of 123 anhydrite. Gypsum is found in the caprock on a lircle over half of the domes studied and is restricted to depths less than 1820 feet. The largest percentage of gypsum originated from the direct alteration from anhydrite. Anhydrite is the most abundant mineral when all caprock is considered, but not every caprock contains this mineral. Anhydrite commonly occurs in the lower portions of caprock, although it is not restricted to this area. Crystals in the massive anhydrite caprock form an interlocking mosaic which results in low porosity. Many other minerals, mostly in minor quantities, have been noted to be associated with anhydrite. Trace element concentrations in the massive anhydrite vary considerably in any single core, but do show some relationships between elements. The high strontium content in the anhydrite of one Mississippi dome appears to be related to the celestite caprock which overlies it, indicating that strontium was present during the formation of the anhydrite. Color and density banding in the anhydrite caprock may result from either diffusion or an episodic influx of minor elements, or the two processes combined. Experiments indicate that anhydrite displays characteristic features which show whether the mineral is undergoing dissolution or precipitation. These features were used to interpret morphologic changes in the anhydrite caprock-salt interface. It was found that the anhydrite crystals included in the salt are "stem-shaped" and do not 124 display dissolutior features. Directly above the salt is a zone of anhydrite sand in which the crystals are "stem-shaped", but are undergoing dissolution. Within four feet of this zone, going up into the caprock, the anhydrite completely changes morphology to pris matic crystals and dissolution features are replaced by features of precipitation. Trace element concentrations differ significantly between caprock anhydrite and the salt residue anhydrite. Sulphur 32 34 isotope data also indicate that the S /S ratio of anhydrite caprock varies much more widely than do the ratios of salt residue anhydrite from all domes. The volume of anhydrite caprock can be more easily explained in terms of dissolution and later reprecipitation of the mineral than by only simple accumulation on top of the salt. Less common caprock minerals include pyrite, which is most common in the calcite zone; quartz, as terminated crystals or ro settes and appear very similar to those crystals within the salt; aragonite and halite. Sediments occur within all caprock lithology types. The detritus may be finely dispersed, filling fractures or can be a bedded deposit. THE GEOCHEMICAL. ENVIRONMENT General Statement The geochemical environment associated with the caprock m ust be quite complex, as shown by the large number of mineral species and their variable distribution. Trace element analysis of the minerals themselves has also indicated this complexity. A more thorough understanding of caprock genesis requires knowledge of the interaction between the solid-water phases in the environment. This knowledge must include information on the geochemistry of interstitial waters, both in the caprock and in the circumjacent sediments. Water movement in the surrounding sediments is also discussed in this section. Due to the lack of data in caprock-forming environments, primarily water chemistry, the geochemical environment must be approximated from the known mineral assemblages, the chemical constituents of the rock and groundwaters and the organisms which live in the environment. Certain limits can now be set on the caprock geochemical environment, based on the above mentioned natural features. Geochemistry of Sediment Interstitial Waters Oil field brines associated with salt domes contain concen trations of dissolved constituents of from less than 25,000 to more than. 145, 000 milligrams per liter. Table 37, page 207 presents 125 126 the data that were ootained (Collins, 1970) by analyzing the petroleum associated water samples. A statistical analysis of some Louisiana oil field brines (Collins, 1970) not associated with salt domes is shown in Table 38 for comparison with those brines associated with the domes. The associated brines are characterized by the predominance of Cl among the anions, and Na and Ca among the cations. Those brines not associated with salt domes contain less total dissolved solids, but considerably more Ca is present. This greater abundance of Ca is probably due to most of these oil fields being located in the northern portion of Louisiana, where limestone in the subsurface is more abundant than in the southern part. The data of Figure 33 show that at values less than approxi mately 30 grams per liter dissolved solids, the concentrations of Ca and Mg differ but slightly. As the brines become more concent rated, the Ca concentration increases more rapidly than Mg. This process may be due to dolomitization reactions between calcite and magnesium in the brines, resulting in an increase of calcium in the brines as indicated in Figure 33. The SO^ and HCO^ concentrations plotted in Figure 34 scatter greatly, but both show a slight negative relationship to the total dissolved solids. It is interesting to note the large variation of SO^ concentration in caprock brines. There is little evidence from the 127 4000 3000 - O’ A A 1000 50 100 150 Totol Dissolved Solids (g/l) Figure 33 Calcium and Magnesium vs Total Dissolved Solids in Subsurface Brines (Data from Col lins, 1970,Tables 36 ond 37 ). Symbols! A= Co in sediment brines,A = Mg in sediment brines, @ = Co in caprock brines, Q=Mg in caprock brines (Data for salt dome associated brines only ) S04 and HC0 3 (mg/l) I000+-1 200 400 0 - 600 0 - 800 - from Collins , 36 and37 ).Symbols 1970,Tables 1 A = S0 HCO Sulphate and bicarbonate vs.Total Dissolved Solids in Subsurface Brines (Data Brines (Data Dissolved in Subsurface vs.Total Solids bicarbonate and Sulphate 3 in sediment brines,© = brines,© inS0 sediment A ▲ Dt fr at oe soitd rns only) brines dome associated for salt (Data © © 0 100 50 Total Dissolved Solids (g/l) 4 © © © © n arc bie, #=HC brines, in caprock A 2407 2570 Figure34 A i i ii 4 A ▲ n eiet brines, ▲ in sediment= 03 in brines. caprock A. A A M 128 50 1 plotted dat& that there is a reciprocal relationship between the sulphate being reduced by the oxidation of organic matter and bicarbonate being formed. From the correlation matrix of Table 37, it can be seen that the only correlation with the top of the salt dome is a poor one with the sulphate concentration. This correlation may be due to an increasing amount of anhydrite dissolved with depth. The m ag nesium in the interstitial water decreases slightly with depth while the concentrations of potassium, calcium, bicarbonate and sulphate increase. There appears to be a small positive correlation between pH and depth. Abnormal reservoir pressures, defined in page 60, are encountered in the area surrounding salt domes. Jones (1968) states that the entrapment of water in geopressured reservoirs of the northern Gulf Basin would be geologically short lived except for osmotic confinement. He believes that water expelled from the reservoir through clay beds is diluted by hyperfiltration and dis solved solids in remaining water are concentrated until osmotic pressure opposing the flow equals the head differential due to the overburden. The distribution of salinity with depth is extremely variable above depths ranging locally from 8.000 to 12.000 feet in geo pressured areas, but below this depth range there is a general 130 progressive freshening (Jones, 1968). Figure 35 illustrates conditions which exist in the geopressured area surrounding Jeanerette Dome, K Louisiana. A sharp decrease in salinity, accompanied by a fairly abrupt increase in geopressuring can be noted at a depth of approx imately 12,000 feet, or about 4,000 feet below the shallowest salt penetration. Transferring the borehole data of Figure 35 to Fig ure 20, page 59. a well drilled on the flank of the idealized salt dome may encounter geopressuring and a decrease in salinity at the top of the gouge shale serving as a semipermeable membrane. The osmotic efficiency of this shale would depend upon the water-salinity contrast on either side of the membrane (Young and Low, 1965). Temperatures of the caprock fluids at depths of 1,000 to 2,000 feet generally range from about 40 to 95°C (Table 36; Guoyd, 1950). Temperatures as high as 136°C and greater have been en countered in geopressured reservoirs in south Louisiana, with the geothermal gradient being steepest in that part of the clay beds im mediately overlying geopressured reservoirs (Jones, 1968). Nonionized dissolved solids, among them silica and bicar bonate, are able to move upward with water that escapes through clay beds that behave as semipermeable membranes. The upper parts of these clay beds often contain precipitates of carbonate and silica which may be a result of chemical reactions with exchange cations of clay minerals in the zone of steep pressure and temperature Depth 10,000 18,000 16,000- - 14,000 12,000 - 0 0 0 6 - 0 0 0 8 - 0 0 0 4 2000 2000 - - - 0 00 000 200 400 600 I8P00 16,000 14,000 12,000 10,000 8000 0 0 0 6 0 0 0 4 0 0 0 2 ore fdt casfe b otiuo' request) by contributor's classified ( of data Source at Dome La., Salt Flank Well Located Near Jeanerette Jeanerette Near Located WellFlank rsue s et Plot Depth vs Pressure aiiy 1 (ppm) 10 X Salinity ,o Pressure Psi Psi Pressure Figure 35 Esf. Pm Press. Press. Pm Esf. Solinity «.Gro>\ > o r G p«c. S Gradient ^o dome =^-on — Shallowest salt penetration penetration salt Shallowest — 20 131 132 gradients (Jones, ’968), or possibly by the simple mixing of the natural waters with different chemical and/or physical properties. Movement of the interstitial water occurs in response to potential fields (Hubbert, 1953). The potential fields may originate by differential pressure due to the overburden, differences in ele vation, osmcric pressures, differences in temperature, movement of the salt dome or bv a chemical potential gradient. Lerman (1970) suggests several possible models which approximate conditions under which movement of interstitial water is due to the physical characteristics of the environment. A brief outline of his models which may be applicable to the salt dome environment is as follows. His first model consists of an "infinitely high" brine column in a porous medium overlying and "infinitely thick" halite bed. The initial NaCl content of the brine is C - 0. 2mNa Cl, the concentration o at the brine-salt interface is maintained at the saturation value of halite C =6. lmNaCl. At a distance z = 100 meters above the s brine-salt interface, the NaCl concentration would approach C. „ ^ 100 (90% of the saturation vnlue or 5. SmNaCl) as a function of time according to the relationship. C100 = Co + (Cs ' Co) erfc (Z/S Dt* } (6) where erfc is the error function complement (empirically derived), -5 2 D is the diffusion coefficient of NaCl (taken to be 1x10 cm /sec) 133 and t_ is time. Thus, the NaCl concentration at 100 meters above the salt comes to C in approximately 155x10 years. This process is quite slow when molecular diffusion is the only transporting agent. A second model is identical to the first one, except the brine flows upwards in the £ direction normal to the salt at a rate of _5 V=lxl0 cm/sec (approx. 3m/yr). The change in NaCl concen tration at z = 100 meters is described by: z-vt , z+Vt (7) c , _ = c •100 - J o + *(Cs - Co> erfc f o r + 1 V D> erfc 2Dt The NaCl concentration at £ above the salt obtains Cjqq in approxi mately 320 years. 'This is a relatively fast process, and illustrates how a flow rate as low as 1 cm/day can increase the brine concentra tion by a large factor when compared with diffusion without flow. Figures 3b and 37 illustrate that the flow of water moves upward adjacent to and above the salt dome. Fresh water reaches a greater depth toward the flanks of the dome than in the surrounding area, and it is usually absent altogether over the apex of the dome (Rollo, I960). Taylor (1938) notes that the areas in which the more shallow salt domes occur are characterized by' saline and "sour" wafer springs. Figure 38 displays shallow water quality associated with the Stratton Ridge Dome, Texas. On a larger scale, the effect a salt dome has on the altitude of the base of fresh ground water is less apparent. Salt domes of Louisiana were plotted on a map of the state which showed, by 134 EXPLANATION Control Point Approximate outline of salt plug T. Contour interval 100 feet 13 Dotum-mean sea level N. 0 1 2 3 4 5 MILES -700' T. 14 N T. 13 N. R.6W R.5W- Figure36 Mop showing by contours the altitude of the base of fresh ground water in the vicinity of Rayburns salt dome, Bienvil le Parish, (after Rollo,l960) J g Jgjfi»^ FRESH WATER — -= 1 0 0 0 - co 1* 0 0 - MIDW ROU 43 (scc fiouecse son l i n e of scctioni Figure 37 Geologic section of Rayburns salt dome, Bienville Poruh ( after Rollo,l960) 135 Average depth to salt =1,633 feet II Secondary saline waters Primary alkaline waters MILES 2 a Figure 38 Stratton Ridge Dome Shallow water anomaly (From Problems of Pet. Geol., 1942, p. 896 ) 136 contours, tho base of fresh water (Rollo, 1960, plate 3). The top of the salt was then correlated with the depth to salt water and it was found that no correlation exists (coefficient=0. 055). Geochemistry of Caprock Interstitial Waters Interstitial waters in caprock are of moderate salinites and are characterized by an abundance of calcium, sulphate and bicar bonate ions in solution. Also associated with these waters is a fairly high concentration of dissolved H^S. The equilibrium distri bution of sulphur species under standard P-T conditions is illustrated in Figure 39. This figure indicates that nonequilibrium conditions probably exist in the caprock environment due to the expected low Eh and fairly high pH values in the environment with the association of SO^ and H^S. Higher P-T values only slightly effect the equilib rium boundaries, and the ionic species are independent of the total dissolved sulphur since the boundaries are only where the ratios of sulphur species are unity. H S is the stable species under acid reducing conditions, but is the stable sulphur species under basic reducing conditions which are more likely in the caprock. The iron minerals present in caprock are also an indication of reducing conditions. Pyrite is the major iron mineral and only rarely has hematite been reported. The mineral is stable under a wide pH range but is restricted to negative Eh values above a pH of 6 (fig. 40). The occasionally found copper sulphides are also 137 +08- C7 + 0.6- +0.4- E h +0 .2 - * 0.0 v - 0 . 2 - -0.4- - 0.6- CT no (/) - 0.8- PH Figure 39 Equilibrium distribution of sulfur species in water at 25°C and I at mosphere total pressure for activity dissolved sulfur = 10“*.Under these conditions,native sulfur is a stable phase. Oashed line indi cates equal values of dissolved species within sulfur field.(After Garrels and Christ,l965) 138 + IjO -4 +0.8 +06 -4 +0.4- Hematite Fe?0 •+Q 2- E h 0.0 - 4 - 02 - -0.4- - 0 6- -0 8 - - 1.0 PH 8 Figure 40 Stability relations of iron oxides and sulfides in water at 2 5 °C and I atmosphere total pressure at an activity of dissolved sulfur of I0 '1. Boundaries between ions ond solids are ot an activity of lO‘*of dissolved iron species.The numeral -4 is the log of the iron activity used to show the rate of change of "solubility." (After Garrels and Christ, 1965 ) - 0 9 - - 0 8 - log Po2 - 0 6 - 50- 0 -5 - 0 4 - 70- 0 -7 - 0 3 - - - 0 2 10 6 5 -30-40 -50 -60 - - at water are superimposed . (After Garrels and Christ, 1965 ) 1965 Christ, and Garrels . superimposed (After are water tblt f oeio xdsad ufdsa ucin f o n Ps and Po2 Stabilityironof some sulfidesfunctionsofas oxides and Fe + H + Fe antt Fe Magnetite °C 5 2 eoie Fe Hemotite n I topee oa pesr.Saiiy eain for relations Stability pressure. total I atmosphere ond Pyrrhotite Pyrrhotite FeS+ H 3 2 0 Qj+H20 4 +H20 2 Figure 41 o Ps log yie FeS Pyrite 2 -20 -10 2 +H20 + 10 0 2 139 140 restricted to a reducing condition and generally require fairly high pH as shown by Eh -pH diagrams of the system Cu-Fe-S-O-H (Garrels and Christ, 1965), Pyrite also is an indication of the activity of the gas phases, if the assumption is made that activity is approximately equal to its partial pressure at caprock P-T conditions. The Po -Ps_ b Ct diagram (fig. 41) shows that pyrite is the stable mineral when log Po2 ranges between about -30 to -83 and log Ps^ is in the range of O to - 34. The widespread occurrence of the sulphate-reducing bacteria Desulfovibrio in the caprock brines is another indication that the environment is a basic reducing one. The organisms survive best between a pH of 6 to 10 and Eh values of +0. 2 to -0. 4 (Becking, I960). Feely and Kulp (1957) state that acid conditions depress their growth rate greatly, and a high pH increases the growth rate but the stationary population is reduced. Anhydrite-Brine Equilibria The anhydrite-brine equilibrium relationship may be a most important factor in the development of anhydrite caprock. The solubility of anhydrite can be satisfactorily predicted by a model which takes into account ionic strength, temperature and the amount and type of complexes in the solution (Marshall and Slusher, 1968; Lerman, 1970). The following is a discussion of the model applied to those brines in and around caprock: 141 The molar quotient Ca**/(SO * + HCO^ ) was first calculated for each brine analysis. These values were then plotted against total dissolved solids, and are shown in Figure 42. The molar quotient is considered an indicator of whether a brine can become depleted in sulphate (and carbonates) by precipitation of CaSO^ (and CaCO^) mineral phases (lerman, 1970). Precipitation of anhydrite or gypsum would not deplete the brine of its sulphate if the quotient is smaller than one, but would deplete it if the value is greater than one. In order to determine the anhydrite-brine equilibria curve of Figure 42 the composition of the brines was averaged within succes sive intervals of 25 g/l total dissolved solids, and the concentrations of Ca+ + and SO, were calculated for each of the "mean" brines at 4 equilibrium with anhydrite at 75°C. New values of the molar quotient Ca++/ (SO^ + HCO^ ) were computed and the smooth equilib ria curve was drawn between the calculated values. The method and the calculations to obtain this curve were taken from reports by Lerman (J 970), Marshall (1967) and Marshall and Slusher (1968) and are as follows: Using the initial values of the ionic strength I the "practical" ionic solubility product for anhydrite at 75°C is calculated by the equation 2 log Ksp = log K° + -Bi-Cl (8) 142 I00-. •=sediment brines o = coprock brines x = colculated equilibria 10- 110 o Anhydrite-Brines equilibria ♦ at 75°C— r o o 2 1.0 - 50 1000 150 Total Dissolved Solids (g/l) Figure 42 Molar quotient Ca*V(SQ| + HC0 3 ) in salt dome associated sub surface brines. Data from Tables 36 and 37. 143 where K is the thermodynamic dissociation constant at the specified temperature, S is the Debye-Huckel limiting slope and A, B and C are empirical constants dependent on temperature derived by Marshall and Slusher (1968). From the definition of log Ksp in equation (8) it follows that for anhydrite at equilibrium with a brine the activity of water is compensated for in the parameters B and C. It was concluded that MgSO^° is the most important complex which controls the solubility of the calcium sulphate minerals (Marshall and Slusher, 1968), making it necessary to know the amount of complexed in the ion pair. The concentration of MgSO^° is expressed as (MgSO°) = Ksp (Mg + +) / [^CAIq ) Kd + KspJ (9) where the parentheses indicate molar concentrations, Mg++ is the total magnesium in the brine, C a^ is the calcium concentration at eq equilibrium with anhydrite (in the first step Ca++ = Ca++ from chemical eq analysis} and Kd is the dissociation constant of MgSO^° as determined by Marshall (1967). Kd is calculated for anhydrite at 75°C by the following equation: 8 x 0. 5645 x/T log Kd = -2. 846 + ( 10) (1 + n/T) When equilibrium is attained with anhydrite, equivalent ^ — amounts of Ca and SC) are either added to, or subtracted from, 4 the brine; depending on whether it is undersaturated or supersaturated 144 with respect to anhydrite. A more reliable solubility product of anhydrite thar. the ^ jlues from equation (8) may be calculated in the equation X2 + X(Ca++ + SCT - MgSO °) + Ca++ (SCf -MgSO° J-K sp = O (11) or 4 or 4 or 4 or ® 4 where the subscript or indicates the original molar concentration in solution and X is the number of moles of Ca++ and per 1000 g H^O added or subtracted from the solution when it attains equilibrium with anhydrite. A new value of ionic: strength is now calculated by the relation ship: I' = I - 4MgS04° + 4X (12) Using this new value of I' irom equation (12), the entire procedure was repeated three times by recalculating equations (8', (9), (10), (11) and (12). In equation (9) Ca++ eq was used from the proceeding step and in equation (12) I is always the original value of the ionic strength. The values of I1" and Ca + + eq converged within the three steps indicating the procedure was completed. The values of X obtained in the third step were algebraically added to the concentration of Ca++ and gi'rer. by the analysis. New values of the molar quotient Ca++/(SC>4 HCOj ) were then computed and plotted on Figure 42 for each "mean" brine. The curve was calculated at 75°C because this is regarded as 145 reasonable for the depths of caprock formation. The brines super saturated with respect to anhydrite are the points below the curve (fig. 42) and those brines undersaturated fall above the curve. An increase in temperature would only slightly move the curve upward, whereas a decrease in temperature to around 30°C would move the curve considerably downward (see fig. 4, page 19). It can be seen in Figure 42 that the caprock brines are either close to equilibrium or are supersaturated with respect to o anhydrite at 75 C. Most of the sediment brines are fairly well undersaturated with respect to anhydrite, and those supersaturated may indeed be brines located directly on the flanks of the dome. If Ca + + and SO^ were added in equivalent amounts to the under saturated sediment brines, the values of the Ca++/ (SO^ + HCO^ ) quotient would decrease and fall closer to the equilibria curve. Thus, brines migrating toward the salt would be under saturated with respect to anhydrite. Upon contact with the dome and moving upward along the flanks the brines would increase in Ca++ and by dissolving the anhydrite which would result in a decrease of the molar quotient. At s jmft point the brines would become supersaturated with respect to anhydrite, and anhydrite would possibly be precipitated. The addition of HCO^ by the bacterial sulphate reduction process (equation 2, page 91) to the supersaturated brines would increase the molar quotient, and CaCO^ could be precipitated. 146 Summary The caprock geochemical environment consists of basic and reducing conditions with the probable limits of pH of 7 to 10 and Eh values ranging from -t little over O down to as low as -0. 4 volts. Equilibria relationships between the interstitial water and minerals found in caprock, salt and sediments are very complex. The species and reactions in caprock and sediment brines considered important involve both acid and basic aqueous solutions containing fairly high sodium chloride concentrations. The brines associated with the sediments, and believed to migrate upward along the flanks of the domes, are somewhat acidic; whereas the brines in the c~prock are probably basic. Temperatures vary widely in the salt dome environment, but it has been noted that the temperature increases more rapidly above the dome and at the flanks than within the salt (Hawtof, 19 30). Increases in both pH and NaCl concentration effect the stability of the clastic silicates: The amounts of K+, Fe++ and Ca++ introduced into the brines increase with increasing total salt content (Althaus, 1969). The solubility of halite is primarily a function of the concentration and type of other ionic solutes present (iig. page 16). Gypsum-anhydri te equilibrium relations are of interest: MacDonald (1953) indicates that gypsum is not in equilibrium with buried halite except at temperatures lower than 14°C, yet 39 percent of the boreholes studied have this lithologic relationship. Anhydrite-brine equilibria calculations show that sediment interstitial witei-s are ur.dt re..iur HCO.^ to the caprock brines could result in the precipitation of FALSE CAPROCK General Statement An understanding of false caprock (sediments circumjacent to the salt dome which have been indurated by an authigenic cement derived from the true caprock) is essential to an understanding of the true caprock. The origin of false caprock in the Gulf Coast region is directly related, and therefore very similar, to that of the true caprock of the region. The cementing materials of the rock are the same as the major minerals of the true caprock. The form and size of false caprock will be discussed. A large percentage of the information concerning false caprock in this section was obtained from a thesis by Walker (1968'. All of the samples which were studied are cemented by calcite to some degree, and the majority contain only calcite as the cementing agent, with the amount of the mineral, by weight, ranging from 1. 8 to 73. 8 percent of the samples. A smaller percentage of the samples are cemented by other authigenic minerals to varying degrees. In a few of the wells, and very locally, gypsum is the major cementing agent. Anhydrite also locally cements some of the sediments, but never exceeds four percent of the cement in any of the specimens. Pyrite was occasionally noted as a cementing mineral, but it is most commonly found as disseminated crystals cemented in either calcite or gypsum. 148 149 The average fah-e > iprock sample contains about t>0 percent clastic material and 40 percent authigenic material. These authigenic minerals were found as individual crystals, in clusters and as the cementing agent. The samples also contain fragments and individual grains from the true caprock (plate 24, fig. 2), and false caprock fragments from other areas adjacent to the dome (plate 24, fig. 3). When the rocks are highly fractured, mylonized materials are likely to be found (plate 24, fig. 5). It was noted that many of the samples had contained organic matter during the time of precipitation (Plate 23, fig. 5). Thus, the constituents which make up the false caprock are numerous and they vary greatly from one area to another. The primary properties of the indurated sediments were found to be practically absent in most of the examined samples. Original form and sire of grains were often destroyed by various physical and chemical changes. Very few of the samples contain any primary porosity and the largest percentage of pore space was a result of dissolution of the cement. The secondary features alter the sediments to an extent where differentiation between the true caprock and the false caprock becomes impossible. The first major secondary feature to occur was the cementation of the sediments by authigenic minerals derived from migrating fluids. A prerequisite to the induration was environmental factors which were favorable to authigenic mineral precipitation, but were apparently unfavorable to the stability of the 150 major clastic minerals, ,viih the result 01 the leplacement of the elastics by the stable authigenic minerals. Authigenic Minerals Calcite (CaCO^): Calcite is the most abundant authigenic mineral. The size of calcite varies from finely crystalline to crystals as large as 8. 5 mm. Color of the mineral ranges from almost colorless to brown, depending on the amount and type of impurities present. Calcite replaces to varying degrees all of the false cap minerals except pyrite and possibly halite. The mineral totally replaced some of the clastic grains, leaving only their relict boundaries, and partially replaced almost all of the grains (plate 23, figs. 1 and 2). None of the clastic m inerals replace calcite, but calcite is partially replaced by gypsum, sulphur and possibly barite. Calcite, in a few of the samples, is partially dolomitized. Adjacent to shear zones, the calcite crystals are commonly twinned and even slightly mylonized where the shear movement was great enough. Recrystallization of the mineral is usually associated with some type of superimposed structure, but it is not restricted to these areas. Pyrite (FeS,)t This m ineral was found to be second to calcite in abundance. The size of the individual crystals ranges from almost submicroscopic to about 2 mm. , and clusters of the mineral are as large as 15 mm. The euhedral crystals are found as cubes, pyritohedrons and octahedrons, and are usually associated with 151 secondary ca'uue rec.err.en v«ti'-n. The granular pyrite clusters are mostly found with older generation calcite and gypsum (plate 24, fig. 1), and locally the clusters are the cementing agent. Pyrite was observed in all of the false caprock samples, with an average concentration of about 3 percent. A few of the samples have locally high concentrations of the mineral, but most frequently this mineral is disseminated throughout the entire sample. Sulphur, gypsum and possibly chert is replaced by pyrite and it is replaced by none of the minerals. Gypsum (CaSO • ?H O): Gypsum is the third most abundant mineral in the false caprock. Four varieties of the mineral occur in the samples, with each varfety usually associated with certain type features: The granular variety of gypsum, alabaster, is the most common, and is responsible for most gypsum cementation (plate 24, fig. 1); the "chalcedonic" variety (refers only to form and not to the presence of chalcedony, Taylor, 1938) is rather common in recemented fractures and occasionally is found in with the calcite cemented elas tics; satin spar, the fibrous variety, is found mostly in calcite cemen ted areas where the gypsum had partially replaced the calcite (plate 23, fig. 2); the selenite, or tabular variety, is commonly associated with the clastic grains. The size of the gypsum crystals is highly variable, with the selenite variety being the largest, and having crystals as large as 1. 3 mm. A few of the samples contain as much as 20 percent of the mineral, but it is usually found in sm aller amounts and many 152 samples do r.'.: rvr.t.vLr. inv g rf.&um. All of the -uthigenic minerals except anhydrite and halite replace gypsum, and gypsum replaces anhydrite, calcite, sulphur, barite, and dolomite. Anhydrite (CaSO^): Anhydrite was found to be the next most abundant minerals. The greatest percentage of the mineral is contained in the sediments near the true caprock and the amount generally decreases in an upward direction. The form of anhydrite crystals also varies with the distance from the true caprock. The crystals near the cap tend to be euhedral, and with increasing dis tance, the crystals become increasingly fibrous (plate 23, fig. 4), although there are many exceptions. When the mineral was mylonized, other forms were created (plate 24, fig. 5). The size of the anhydrite crystals ranges from nearly submicroscopic up to 2 mm. Anhydrite is easily altered to gypsum in the false caprock environment, which may account for a fairly large percentage of the gypsum found in these areas. Anhydrite does not replace any of the other minerals, and is replaced by all of the authigenic minerals except pyrite and halite. Barite (BaSC) ): No distinction was made between barite and 4 celestite in the f »lse caprock study. It is believed that barite pre dominates in the wells examined, but probably celestite is also present. The mineral occurs mostly in fractures and cavities, but is occasionally associated with the cementing calcite. Barite found in fractures is always of a tabular to bladed form when sufficient space was available 153 for growth (plite 24, fig. 4) and, when spice was not available, the mineral form is anhedral. In most of the samples, barite is younger than the cementing calcite and is often the same age as the recementing calcite. Euhedral crystals range in size from 0. 05 mm. to 1. 2 mm. in length. Barite is replaced by calcite and sulphur, and it replaces most of the other authigenic minerals, but to a very small degree. Dolomite (Ca, M glC O ^): Dolomite was found to be the sixth most abundant mineral. It generally occurs in definite zones which are associated with some structural movement, and occasionally occurs as disseminated crystals in areas with no evidence of struc tural movement. Dolomitization of the cementing calcite was occasion ally noted where the calcite is firely-crystallir.e (plate 24, fig. 6). The rhombohedrons range in size from approximately 0. I mm. to 0. 8 mm. Dolomite is replaced by calcite and gypsum. Sulphur (S); Sulphur contained in the samples was largely found as pseudomorphs after calcite or the sulphate minerals, and occasionally as individual crystals. The mineral is disseminated in small amounts throughout the majority of samples, with the largest concentrations occurring in samples containing a large amount of clay minerals. Individual orthorhombic crystals of sulphur vary in size from microcrystalline to as large as 1. 3 mm. Sulphur replaces barite, anhydrite, gypsum and calcite, and is replaced by calcite, pyrite and gypsum. 154 Halite (N.i.r.1): The actual abundance of this mineral in false caprock was r..ot detei mined because of the mineral's solubility. However, it is believed that halite should be fairly abundant in the rocks surrounding the dome, because the circulating fluids contain large amounts of dissolved NaCl. Halite was preserved in one thin- section of a sample taken In m the Black Bayou Dome, Louisiana (plate 23, fig. b). in this thin-section, halite had crystallized in a void space in the calcite cement. The mineral is probably restricted, in all cases, to void filling since it is thought that halite does not replace any of the other minerals. Quartz (SiO ): This mineral may or may not be authigenic to the false caprock. Rarely, both euhedral crystals (plate 23, fig, 4) and rosettes were found, and these were associated with the cementing calcite. They were of the same size as those crystals and rosettes of the true caprock. An amorphous crust of silica, at or near the surface, was noted above a Louisiana salt dome (oral communication, Clay Durham, 1970). This silica may originate from the material removed from the clastic grains while they were being replaced by the authigenic cement in the false caprock. Form and Size of False Caprock The physical form of false caprock bodies is dependent on the extent of the environment which was favorable for its formation. It 155 is bekeved th»'. these environments are continually changing position, so it is expected that the indurated bodies are likewise continually changing their position as well as their form. In general, favorable environments would probably have their greatest extent parallel to the bedding of the sediments which would result in a tabular - shaped body. If the rock formation was influenced by superimposed physical features, such as a fault, it may take the form of veins or pipes. At any one particular time, the form of the false caprock is the form of the environment which permitted its construction, and this environ ment is thought to be quite restricted. The relict shape and size of a pre-existing false caprock body may be preserved in the form of a solution cavity having a dimension of a few inches to greater than a few feet. While the sediments are undergoing induration, the clastic grains are displaced farther apart by the force of crystallization and result in the grains being tightly squeezed together near the boundary between the indurated and nonindurated zones. The cemented grains are unstable in this environment and are readily replaced by the cementing material. When conditions become unfavorable to the cementing m aterial's stability, the material will be removed in solution and the sediments will again be unconsolidated, but their original volume would be highly decreased. The result of the grain volume decrease and cement dissolution would be a solution cavity with its size and form 156 dependent upon the extent oi the preceding factors. These solution cavities aie encountered frequently while drilling in the vicinity of a salt dome. Less direct indications of the processes which create solution cavities in the false caprock are the cemented clastic fragments which were occasionally found ir.. the samples, These fragments range in size from about 0 5 rr.m. to as large as 10 cm, , and consist of clastic grains cemented by some authigenic mineral. They are only recognized when there is a distinct grain size difference between the fragments or a difference in the types of cement. Recementatior. of the fragments masks their original boundaries and further hinders their identification Intermixing of false caprock fragments could occur vithin the many solution cavities present. It is now believed that these fragments would then simply be the result of constant environmental changes. As these processes take place, portions of rock could possibly be dislocated from their original position and collect in the cavities. The fragments may then be recemented together during a later cementing generation. The boundary between two of these fragments is illustrated in Plate 24. Figure 3. Summer y False caprock is very similar in nature to the true caprock. The cementing minerals are the same as the major minerals of caprock, and thus must originate from the same source since false 157 caprock is only associated with salt domes. No residual components from the salt are present m the false caprock. The processes of its formation are similet to some of the processes involved in true caprock genesis except the distance to the source of the minerals is greater. The form and size of false caprock is dependent on the extent of the environment which was favorable for its formation, as well as the distance to the source minerals and the ground water conditions which transported these minerals m solution. ORIGIN OF CAPROCK General Statement The previous discussions were primarily concerned with the features of caprock and its surrounding environment. Table 17 summarizes these important caprock and associated features. The features listed in this table will now be examined to ascertain if any of the theories of caprock origin can satisfactorily account for the character of caprock and its relationship to the circumjacent environment. Only two previous theories will be considered; (1) precipitation in place and (2) residual accumulation and secondary alteration. Other theories, such as a block of sedimentary material upthrust by the salt, or the alteration of limestone to form caprock, will not be discussed because these theories have already been disposed of and are seldom seriously considered in the present literature (Taylor, 1938; Murray, 1966; et al.). Precipitation In Place Theory Stuart (1931) proposed that precipitation of anhydrite or gypsum would result when meteoric waters containing dissolved calcium carbonate mixed with salt dome waters containing soluble sulphates. Paul Weaver in a written communication with Ralph Taylor during 1937 (in Taylor, 1938) states: "There is also the possibilicy that the CaSO^ may be precipitated from circulating waters originally connate in offside sediments; as these waters. 158 159 TA B LE 17 Summary of Important Caprock and Associated Features I. Physical Features: 1. Caprock shape is highly variable. 2. Caprock may rest directly on the salt with no intervening anhydrite sand. 3. The upper surface of the salt can be very variable. 4. The upper portions of caprock are more highly deformed than portions near the salt. 5. Sediments adjacent to sat domes are generally brecciated. 6. The shale sheath around the salt could behave as a semipermeable membrane and permit selective movement of water toward the salt and the development of osmotic pressures. 7. Caprock is present on the Sigsbee Knolls in the Gulf of Mexico. II. Lithological Features: 1. The lithological sequence in caprock is variable. 2. Caprock lithology is variable from dome to dome. 3. Caprock lithology on a single dome can be very variable. 4. The interface between celestite and anhydrite is gradational. 5. The interface between calcite and anhydrite caprock is somewhat sharp. 6. Anhydrite can be found anywhere in the caprock and may alternate with other lithologies. 7. The volume of anhydrite caprock can be more easily explained in terms of dissolution and later reprecipitation of the mineral than by accumulation alone. 8. Sediments are common in the caprock; as dispersed inclusions or in beds separating any lithology types. III. Mineralogical Features: 1. The top of the salt is commonly undergoing dissolution and the less soluble minerals accumulate in this interface. 2. At least 30 different minerals have been identified in caprock. 3. Only approximately 20 different minerals have been identified in the salt. 4. Dolomite or celestite may be a major mineral in the 160 caprock over a few domes. 5. Anhydrite is the most abundant caprock mineral, but not every caprock contains the mineral. 6. Crystals in the massive anhydrite caprock form an interlocking mosaic. 7. Crystals of the massive anhydrite caprock differ from those in the salt in morphology. 8. Dolomite and quartz crystals, characteristic of the salt residue, are found in the caprock. 9. False caprock contains the same authigenic minerals as the true caprock. and false caprock grades into true caprock. IV. Geochemical Features: 1. Trace element content of residue anhydrite varies but slightly from dome to dome. 2. Halite solubility is markedly influenced by other electrolytes in solution. 3. Anhydrite solubility is a function of ionic strength, temperature and the amount and type of complexes in solution. 4. Trace element concentrations in the massive anhydrite caprock range considerably, but do show interrelationships with each other. 5. Caprock banding is largely due to diffusion or an episodic influx of minor elements, or both. 6. Trace element concentrations differ significantly between caprock anhydrite and the salt residue anhydrite. 7. Waters move upward along the flanks of the domes and only locally affect the quality of the groundwater. 8. The caprock geochemical environment may consist of basic and/or reducing conditions. 9. Most brines are undersaturated with respect to anhydrite -in the surrounding sediments but are at near equilibrium or supersaturated in the caprock environment. 161 already containing some CaSO^ lie along the edge cf the salt dome, they dissolve salt and anhydrite from the salt dome until saturated by the latter, and then as upward circulation brings them to the top of the salt dome, they precipitate the CaSO^. " Residual Accumulation and Secondary Alteration Theory The residual accumulation theory of caprock genesis has gained in favor since the early 1930's, and most students of caprock at the present time agree with this theory. The sequence of events which Taylor (1938) theorized to take place during the formation of caprock is presented verbatim below: 1. Intrusion of the salt plug into a zone of active water circulation, probably up to within a few hundred feet of, but not to, the surface, or if to the surface, in an environment in which the surface of the salt would soon be covered by sediments. Where intrusion did not bring the salt into the zone of active water circulation, caprock would be thin or wanting, as at Henderson, Convent, or Timbalier Bay. Where intrusion has been recent, caprock would also be thin or wanting, as at Avery Island, Cote Blanche, and the spines at New Iberia and Jefferson Island. 2. Rapid solution of the salt, taking place with the greatest rapidity at the apex of the cone-shaped salt plug. Cementation of the surrounding sediments would begin at this stage. Anhydrite sand would begin to accumulate in irregular pockets and solution cavities in the salt surface. Waters moving up the sides of the salt plug would effect seme solution and residue would also accumulate on the flanks; false caprock would also begin to form at this time. Anse La Butte is probably an example of this stage. 3. Gradual truncation of the top of the salt plug, decapitating the folds in the salt and forming a solution table, with the anhydrite sand derived from the salt accumulating upon it, relatively uncontaminated by surrounding sediments due to the covering of false caprock. Bay Marchand salt dome may be at about this stage of development. 4. Beginning of the compaction of the caprock as a result of occasional collapse of the false caprock and up thrust of the salt, accompanied by some precipitation of anhydrite from solution and intergrowth of anhydrite grains. From what is known of Vermilion Bay salt dome, it is an example of this stage. 5. Continued solution of the salt, with the upthrust of the salt plug compensating for the salt removal, with more rapid upthiust than removal, or with upthrust lagging behind solution. The tendency toward deeper burial of the top of the salt plug, due to continued sedimentation, also would have to be overcome by upthrust of the salt. Shearing of the anhydrite as it became consolidated would result from stresses set up by upthrust and collapse. The upper part of the caprock and that along the flanks would be repeatedly broken and recemented. Sorrento, Gueydan, and East Hackberry salt domes apparently are near this stage. 6. Entrance of altering solutions and inauguration of the transition zone in which anhydrite alters to gypsum and both in turn alter to calcite and sulphur, with the relict anhydrite structure retained. In some cases hydration might be the only alteration occurring, with only gypsum being formed. The transition zone might appear early, after the formation of a relatively thin anhydrite caprock, or might be delayed until a fairly thick caprock had been formed. The caprock at Garden Island Bay is at about this latter stage of development. 7. Transgression of the transition zone downward, escape of hydrogen sulphide or its oxidation to sulphur in place or within the overlying caprock, and deposition of calcite. Secondary veins of calcite and sulphur, and of pyrite, other sulphides, barite, and celestite, develop in the upper part of the calcite zone, or replace sulphur in the transition zone. Continued influx of hydrocarbons results in the reduction of sulphur and its redeposition in another part of the caprock, or its escape. Caverns develop in the upper calcite zone, owing to active 163 circulation of ground water, and possibly as a result of removal of sulphur, and collapse of the lower part because of solution of salt. Several of the salt domes that have commercial sulphur deposits, and others that have considerable thicknesses of calcite caprock virtually barren of sulphur, are examples of this stage. Lake Washington is included in the former group and Hockley in the latter. 8. Cessation of caprock growth, due to quiescence of the salt plug and the development of a seal that retards circulation of water. Sulphur salt dome appears to be an example of this stage. 9. Final stages might be the removal of caprock by uplift and subsequent erosion, stranding of relatively thick caprock at a lower level by a sudden upthrust of the salt plug, or shattering of relatively thin caprock owing to a considerable upthrust of the salt plug. Caprock is exposed to surface erosion at Winnfield and Pine Prairie, it has been stranded at a lower level at Jefferson Island, and apparently is shattered by uplift at White Castle. Analysis of Previous Theories A comparison between fact and theory will now be presented. A fact which agrees favorably with one theory may unfavorably agree with the other theory. Table 18 lists arguments for and against the "Precipitation in Place" theory. Arguments for and against the "Residual Accumulation" theory are presented in Table 19. TABLE 18 Arguments For and Against "Precipitation in Place" Theory Compatible Features 1. Variability of caprock lithology in single domes and from one dome to another. 164 2. Most of the calcite is definitely precipitated in place. 3. Dolomite may be a major caprock mineral. 4. Large volumes of celestite in some caprock. 5. Trace element concentrations in anhydrite caprock vary considerably. 6. Trace element concentrations differ significantly between caprock anhydrite and the salt residue anhydrite. 7. The volume of anhydrite caprock requires dissolution of much less salt from the top of the dome than the "residual accumu lation" theory. 8. Sediments are common to caprock. 9. Waters more upward along the flanks of the domes. 10. Most brines are undersaturated with respect to anhydrite in the surrounding sediments but are at equilibrium or supersaturated in the caprock. 11. False caprock contains authegenic minerals the same as the true caprock, and false caprock grades into true caprock. 12. Caprock is present on the Sigsbee Knolls. Incompatible Feature Dolomite and quartz crystals, characteristic of the salt residue, are found in the caprock. 165 T A B L E 19 Arguments For and Against "Residual Accumulation" Theory Compatible Features 1. The top of the salt is commonly undergoing dissolution and the less soluble minerals accumulate in this interface. 2. Dolomite and quartz crystals, characteristic of the salt residue, are found in the caprock. 3. The upper portions of caprock are more highly deformed than portions near the salt. 4. Anhydrite is the most abundant caprock mineral. Incompatible Features 1. Most of the calcite is definitely precipitated in place. 2. Dolomite may be a major caprock mineral. 3. Large volumes of celestite in some caprock. 4. Large amount of salt dissolution required from top of dome. 5. Variability of caprock lithology. 6. Trace element difference in caprock and salt residue anhydrite. 7. Presence of false caprock and its gradational boundary with true caprock. 8. Presence of caprock on the Sigebee Knolls. Features Compatible with Either Theory A number of the features do not give strong preferential arguments, either for or against, one or the other caprock 166 theories. Other less direct features may support the "precipitation in place" theory, but the features are not incompatible with the other theory. A discussion of these features is forthcoming: Many of the features attributed to recrystallization and pressure solutions in Table 13, page 96 (Goldman, 1952), maybe features of precipitation. The "saccharoidal" structure of massive anhydrite caprock which was thought to have resulted from compaction and recrystallization of residue anhydrite could also be due to precipitation in place forming the interlocking mosaic. The deformation at the tops of anhydrite and calcite zones has been attributed to "downbuiiding" with an increase in age from the salt-anhydrite contact upward, but the feature can also be explained by the caprock forming at depth, with subsequent movement upward resulting in deformation. The fact that anhydrite is the most abundant caprock mineral can support either theory since the ultimate source for the mineral must be the residue anhydrite. The morphological differences between caprock anhydrite and salt residue anhydrite would indicate either that the crystals were reprecipitated from dissolved residue anhydrite or that the crystals precipitated from CaSO^ rich solutions migrating in from external sources. The interface between calcite and anhydrite caprock is fairly sharp, which may result from an alteration "front" in which anhydrite alters to calcite in a progressive, but vertically limited, space or the feature may be attributed to changes in the geochemical environment during precipitation. The fact that caprock may rest directly on the salt with no intervening anhydrite sand indicates that dissolution is not now occurring at this interface. The brecciated nature of the sediments adjacent to salt domes would increase porosity and permeability allowing greater fluid movement. The shale sheath adjacent to the salt could serve as a semipermeable membrane and permit selec tive escape of water toward the salt, resulting in osmotic pressures and a more or less enclosed channel (bounded by the salt on one side and shale on the other) in which upward moving water could flow. The fact that partially dissolved anhydrite crystals are found in the shale-salt interface. The interface between celestite and anhydrite is gradational, indicating that strontium was incorporated in the anhydrite during its formation. Direct precipitation is supported by the fact that anhydrite can be found anywhere in the caprock and may alternate with other lithologies or even sediments* The arguments presented in Tables 18 and 19. and the discussion on the less direct features seem to indicate that the caprock is largely formed from direct precipitation. The main feature that the precipitation theory cannot account for is the presence cf dolomite and quartz crystals, characteristic of the salt residue, dispersed throughout the caprock. Because of this fact, it is proposed that the origin of caprock can be satisfactorily explained by a "modified precipitation in place" theory. Modified Precipitation in Place Theory The pa . agenesis of the caprock minerals is postulated as follows: Anhydrite and salt would be dissolved from the flanks of the dome by upward moving waters, and anhydrite would later be re precipitated, a3 the caprock, from these solutions when the geo chemical conditions were favorable to this process. A smaller amount of anhydrite coula be incorporated in the caprock by dis solution of the upper salt surface releasing the less soluble residue. The degree of dissolution at this surface, as well as the amount of included impurities in the salt, would determine what percentage the residue anhydrite contributes to the total bulk. The characteristic dolomite and quartz salt-residue minerals would be included in the caprock in an amount dependent upon the above process. Petrographic recognition of the residue anhydrite in caprock would be most dif ficult, if not impossible, because of such processes as recrystal lization and overgrowth. Gypsum can form by either the hydration and alteration of anhydrite or by direct precipitation. Sulphur is an alteration product from the reduction of anhydrite. Calcite is pre cipitated from solutions containing Ca++ from the upward moving waters and HCO^ from the anhydrite reduction process. Celestite is precipitated v/hen sediment interstitial waters containing fairly large amounts of strontium come in contact with the caprock associated waters. Much cf the dolomite could be formed by a similar process as the celestite but the sediments waters would contain Mg++. The less common caprock minerals would largely be formed by the interaction of sediment interstitial waters with those waters in the caprock. The remaining less common minerals have come from the less-soluble fraction in the salt and by alteration from other minerals. The probable development sequence of caprock, based on this modified precipitation theory, is presented in the following summary. DEVELOPMENTAL SEQUENCE OF CAPROCK The following is the sequence of events which may have taken place during the development of caprock. 1. The salt dome is intruded into the overlying sediments, its movement upward is episodic. 2. Dissolution of the salt along the flanks and the top of the dome will occur when the dome encounters waters which are undersaturated with respect to halite. Since salinity can greatly decrease with depth below 8,000 to 12,000 feet in geopressured areas, the depths at which dissolution first takes place can be very great. 3. Substantial movement of water probably occurs at depths as deep as 15,000 feet, moving in response to potential fields resulting from physical or chemical differentials. The brines migrating toward the dome would be undersaturated with respect to both halite and anhydrite. The waters move upward along the flanks dissolving halite and anhydrite. Some of the overhangs of salt have been attributed to these circulating waters, such as at Belle Isle and Anse la Butte Dome, Louisiana (Judson and Stamey, 1933). 4. At depths of 4, 000 to 10, 000 feet, the flow rate of the hydrologic system would be sufficient enough, as shown by the presence of cap> rock at this depth, to transport the dissolved solids toward the apex of the dome. During periods of quiescence of the episodic growth, the Ca+r and incorporated in the waters would become 170 171 supersaturated near the upper surface of the salt. Since the waters would only be supersaturated with respect to anhydrite and not to halite, anhydrite would be precipitated and the salt would continue to dissolve. Because of the two simultaneous processes, the newly formed anhydrite would contain the less soLuble residue minerals such as quartz and dolomite. 5. The variations in the trace element concentrations would then reflect the concentrations of these elements in the brines from which anhydrite is precipitating. As the dome moves up episodically, it encounters different quality brines and therefore the anhydrite changes in composition. 6. The anhydrite grows both upward and downward in direction, with the amount of growth depending on the rate of anhydrite pre cipitation and the rate of halite dissolution off the apex of the dome. 7. Anhydrite continues to precipitate until the environment changes from acid reducing to basic reducing. This change occurs at a depth where organic material (hydrocarbons), along with other favorable conditions, exist for the growth of the sulphate reducing bacteria. In this environment the pH would increase because of the sulphate re duction process. The bicarbonate ion is a reduction product, therefore due to differences of solubility between calcite and anhydrite, the calcite would precipitate, also increasing the pH or at least buffering above pH7. 172 8. If the supply- of organic matter was temporarily diminished, bacterial growth would cease, pH would probably lower due to the ionization of H S, and anhydrite (or gypsum) would again precipitate. w This could possibly result in the alternation of anhydrite and calcite. 9. If the supply of organic matter was deficient in only parts of the caprock, different minerals would be precipitated in different areas. 10. If other elements were present in fairly large quantities in the associated brines, they could become incorporated in the caprock. This is the case in some of Mississippi caprock where large amounts of strontium were incorporated. 11. The upper deformed surface of anhydrite and calcite zones simply result from the caprock forming at greater depths, followed by later movement upward and resulting in the upper surfaces being deformed. 12. All of the anhydrite dissolved does not necessarily form in the caprock, but some escapes into the surrounding sediments to form the false caprock. 13. The room necessary for caprock growth develops primarily from the dissolution of the top of salt (inother words only 1000 feet of salt wculd have to be dissolved off the top of the salt in order to accommodate a 1000 foot caprock). Other ways of developing room would be the dissolution of the unstable elastics (common to the false caprock environment) or simply by the force of crystallization. (Weyl, 1959). 14. The variability of caprock between domes and on any one individual dome would result from local controls and conditions. Structure would control fluid movement, along with the permeability and osmotically derived pressures in the surrounding sediments. The composition of the associated brines would limit the types of minerals precipitated, and the geochemical environment would dictate the type of minerals which precipitate from these brines. REFERENCES Althaus, E. , and Johannes, W. , 1969, Experimental metamorphism of NaCl-bearing aqueous solutions by reaction with silicates: Am. Jour. Sci. , v> 267, p. 87-98. Andrews, D. I. , I960, The Louann salt and its relationship to Gulf Coast salt domes: Gulf Coast Assoc. Geol. Soc. Trans. , v. X, p. 215-240. Atwater, G. , and Forman, M. J. , 1959, Nature and growth of southern Louisiana salt domes and its effect on petroleum accumulation: Am. Assoc. Petroleum Geologists Bull. , v. 43, p. 2592-2622. Angino, E. E. , and Billings, G. K. , 1967, Atomic absorption spectrometry in geology: Elsevier Pub. Co. , N. Y. , 144 p. Barnes, V. E. , 1933, Metallic minerals in caprock, Winnfield salt I dome, Louisiana: Am. Mineralogist, v. 18, p. 335-340. Baas Becking, L. G. M. , Kaplan, L R. , and Moore, D. , I960, Limits of the natural environment in terms of pH and oxidation-reduction potentials. Jour. Geol. , v. 68, p. 243-284. Berner, R. A. , 1969, Migration of iron and sulphur within anaerobic sediments during early diagenesis: Am. Jour. Sci. , v. 267, p. 19-42. 174 175 Berner, R. A. , 1971, Principles of chemical sedimentology, McGraw-Hill Co. , N. Y. , 240 p. Billings, G. K. , and Ragland, P. C. , 1969, Atomic absorption spectrometry: Geochemical techniques and problems: Canadian Spectroscopy, v. 14, no. 1, p. 8-12. Bock, E. , 1961, On the solubility of anhydrous calcium sulphate and of gypsum in concentrated solutions of sodium chloride at 25°C, 30°C, 40°C, and 50°C: Canadian Jour. Chem. , v. 39. p. 1746-1751. Bodenlos, A. J. , 1970, Cap-rock development and salt-stock movement: in_ Geology and technology of Gulf Coast salt, La. State Univ. , Sch. Geosci. , Baton Rouge, p. 73-86. Braitsch, O. , 1971, Salt deposits, their origin and composition: Springer-Verlag Co. , Berlin and N. Y. , 297 p. Brown, L. S., 1931, Cap-rock petrography: Am. Assoc. Petroleum Geologist Bull., v. 15, p. 509-529. Collins, A. G. , 1970, Geochemistry of some petroleum-associated w aters from Louisiana-; U. S. Bur. Mines Rept. Inv. 7326, 31 p. Davis, J. B. , Stanley, J. P. , and Custard, H. C. , 1970, Evidence, against oxidation of hydrogen sulfide by sulfate ions to produce elemental sulphur in salt domes: Am. Assoc. Petroleum Geologists Bull. . v. 54, p. 2444-2447. 176 Dickinson, G. , 1953, Geological aspects of abnormal reservoir pressures in Gulf Coast Louisiana: Am. Assoc. Petroleum Geologists Bull. , v. 37, p. 420-432. Downey, M. W. , 1969. Rock description, core 5, Challenger Knoll, Gulf of Mexico, in Initial reports of the Deep Sea Drilling Project, v. 1:U. S. Govt. Print. Off., p. 427. Feely, H. W. , and J. L. Kulp, 1957, Origin of Gulf Coast salt dome sulfur deposits: Am. Assoc. Petroleum Geologists Bull., v. 41, p. 1802-1853. Ferguson, W. B. , and Minton, J. W. , 1936, Clay Creek salt dome, Washington County, Texas: Am. Assoc. Petroleum Geologists Bull. , v. 20, p. 68-90. Garrels, R. M. , and Christ, C. L. , 1965, Solutions, Minerals, and equilibria: Harper and Row, N. Y. , 450 p. Goldman, M. I. , 1925, Petrography of salt dome cap rock: Am. Assoc. Petroleum Geologists, v. 9, p. 42-78. ------1929, Features of gypsum-anhydrite salt-dome cap rock (abs. ): Geol. Soc. America Bull. , v. 40. p. 99-100. ------1931, Bearing of cap rock on subsidence on Clay Creek salt dome, Washington County, Texas, and Chestnut dome, Natchitoches Parish, Louisiana: Am. Assoc. Petroleum Geologists Bull., v. 15, p. 1105-1113. -----1933, Origin of the anhydrite cap rock of American salt domes: U. S. Geol. Survey Prof. Paper 175, p. 83-114. 177 Goldman, M. I. ,1952, Deformation, metamorphism, and mineralization in gypsum-anhydrite cap-rock, Sulphur salt dome, Louisiana: Geol. Soc. America Memoir 50, 169 p. Guyod, H. , 1950, Temperature well logging: Well Instrument Developing Co. , Houston, 47 p. Halbouty, M T. , 1967, Salt domes; Gulf region, United States and Mexico: Gulf Pub. Co. , Houston, 425 p. Hanna, M. A., 1953, Fracture porosity in Gulf Coast: Am. As soc. Petroleum Geologists Bull. , v. 37, p. 266-281. Hanna, M. A. , and Wolf, A. G. , 1934, Texas and Louisiana salt- dome cap-rock minerals: Am. Assoc. Petroleum Geologists Bull., v. 18, p. 212-225. ------1941, Gold, Silver, and other elements in salt-dome cap rocks. Am. Assoc. Petroleum Geologists Bull. , v. 25, p. 750-752. Hawkins, M. E. , and Jirik, C. J. , 1966, Salt domes in Texas, Louisiana, Mississippi, Alabama and offshore tidelands; a survey: U. S. Bur. Mines Inf. Circ. 8313, 78 p. Hawtof, E. M. , 1930, Result of deep well temperature measurements in Texas: Am. Petroleum Inst. Production Bull. 205, p. 62-108. Hopkins, F. V. , 1870, First annual report of the Louisiana State Geological Survey: La. St. Univ. Ann. Rept. for 1869, p. 77-109. 178 Hubbert, M. K. , 1953, Entrapment of petroleum under hydrodynamic conditions: Am. Assoc. Petroleum Geologists, v. 37, p. 1954-2026. Jones, P. H. , 1968, Hydrodynamics of geopressure in the northern Gulf of Mexico Basin: Soc. Petroleum Engineers Preprint 2207, 12 p. Judson, S. A., and Stamey, R. A. , 1933. Overhanging salt on domes of Texas and Louisiana: Am. Assoc. Petroleum Geologists Bull., v. 17, p. 1492-1520. Kerr, P. F. , and Kopp, O. C. , 1958, Salt-dome breccia: Am. A s soc. Petroleum Geologist Bull., v. 42, p. 548-560. Kingery, W. D. , I960, Introduction to ceramics: John Wiley and Sons, N. Y. , 781 p. Kinsman, D. J. J. , 1969, Interpretation of 5r++ concentrations in carbonate minerals and rocks: Jour. Sed. Petrology, v. 39, p. 486-508. Kupfer, D. H. , 1963, Structure of salt in Gulf Coast salt domes: in Symposium on Salt, Northern Ohio Geol. Soc. , Inc. , Cleveland, p. 104-123. Leake, B. E. , Hendry, G. L. , Kemp, A. , Plant, A. G. , Harrey, P. K. , Wilson, J. R. , Coats, J. S. , Aucott, J. W. , Lunel, T. , and Howarth, R. J. , 1969/1970, The chemical analysis of rock powders by automatic x-ray fluorescence: Chem. 179 Geol. Special Issue 5, Elsevier Pub. Co. , Amsterdam, p. 7-86. Lerman, A. , 1970 Chemical equilibria and evolution of chloride brines: Mineral. Soc. Amer. Spec. Pap. 3, p. 291-306. MacDonald, G. J. F. , 1953, Anhydrite-gypsum equilibrium relations: Am. Jour. Sci., v. 251, p. 884-898. Marshall, W. L. , 1967, Aqueous systems at high temperature. XX. The dissociation constant and thermodynamic functions for o magnesium sulfate to 200 C: Jour Phys. Chem. , v. 71, p. 3584-3588. - , Slusher, R. , and Jones, E. V. , 1964, aqueous systems at high temperature. XIV. Solubility and thermodynamic o relationships for CaSO^ in NaCl-H^O solutions from 40 to 200°C. , O to 4 molar NaCU Jour. Chem. Eng. Data, v. 9r p. 187-191. - , and Slusher, R. , 1968, Aqueous systems at high o temperature. XIX. Solubility to 200 of calcium sulphate and its hydrates in sea water and saline water concentrates, and temperature-concentration limits: Jour. Chem. Eng. Data, v. 13, p. 83-93. Martinez, J. D., 1971, Environmental significance of salt: Am. Assoc. Petroleum Geologists Bull. , v. 55, p. 810-825. McLeod, R. R ., I960, A theory for the formation of limestone caprock of salt domes: Gulf Coast Assoc. Geol. Soc. Trans. , v. X, p. 151-153. 180 Muehlberger, W. R, , and Clabaugh, P. S. , 1968, Internal structure and petrofabrics of Gulf Coast salt domes: in_ Am. Assoc. Petroleum Geologists Memoir 8, p. 90-98. Murray, G. E. , 1966, Salt structures of Gulf of Mexico Basin - a review: Am. Assoc. Petroleum Geologists Bull. . v. 50, p. 439-478. Rollo, J. R. , I960, Ground water in Louisiana: Louisiana Dept. Conserv. , Geol. Survey and Dept. Public Works in coop, with the U. S. Geol. Survey, Water Resources Bull. , no. 1, 84 p. Stuart, M. , 1931, A Contribution to "Salt Dome" geochemistry: Inst. Petrol. Tech. Jour., v. 17, p. 338-345. Taylor, R. E. , 1937, Water-insoluble residues in rock salt of Louisiana salt plugs: Am. Assoc. Petroleum Geologists Bull. , v. 21, p. 1268-1310. ------1938, Origin of the cap rock of Louisiana salt domes: Louisiana Geol. Survey Bull. 11, 191 p. Teas, L. P ., 1931, Hockley salt shaft, H arris County, Texas: Am. Assoc. Petroleum Geologists Bull. , v. 15, p. 465-469. Walker, C. W. , 1968, False caprock overlying Gulf Coast salt domes-Analysis and origin: Unpublished M. S. Thesis, Univ. of Mississippi, Oxford, 240 p. --1969. A method for the preparation of halite thin-sections: 181 Jour. Sed. Petrology, v. 39, p. 799. Walker, C. W. , and Paulson, G. G. , 1970, The effect of sulphur impurity on the crystallization of calcite: Electron Microscopy Soc. Am. Proc. , v. 28, p. 498-499. Weyl, P. K. , 1959, Pressure solution and the force of crystal - lization-a phenomenological theory: Jour. Geophys. Research, v. 64, p. 2001-2025. White, D. E. , 1965, Saline waters of sedimentary rocks, jji Fluids in subsurface environments: Am. Assoc. Petroleum Geologists Mem. 4, p. 342-366. Young, A. and Low, P. F. , 1965, Osmosis in argillaceous rocks: Am. Assoc. Petroleum Geologists Bull. , v. 49, p. 1005-1007. APPENDIX A Analytical Procedures Thin-Sections A total of 133 thin-sections were cut perpendicular to the long axis of the cores. The objectives of this examination include the following; mineral identifications and their size and shape, determi nation of the degree and type of replacement and/or alteration of the minerals, the porosity or the percentage of unfilled pore space of the rocks determined by point count or visual estimate, the degree of intragranular fracturing, the degree of recrystallization and the type and percentage of fossils or fossil fragments in the section. Any unusual characteristics of the thin-sections were also recorded. Transm ission Electron Microscopy (TEM) This procedure enabled samples to be observed at magnifi cations up to approximately 250, 000 times with a resolution of less o than 50A . The microscope was used to determine morphology and impurity sites in calcite cap rock as well as to give some information on the minerals genesis. Sample preparation was as follows: The cores were cut in such a manner as to orientate most of the crystals in their maximum dimension. The cut surfaces were then polished with progressively finer grit to remove all scratches. This was followed by submerging the samples in an acid bath, containing 10 percent formic acid, for 182 183 5 seconds. A few drops of a 6 percent collodion in amyl acetate solution was poured over the areas to be studied, and then allowed to dry. The film was then stripped from the specimen and taped at the edges onto a microscope slide, impression up. These peels were examined and photographed with an optical microscope. The slides were then placed into a. vacuum evaporator and shadowed with chromium, followed by a deposition of a thin carbon film. Several layers of filter paper were placed into a shallow dish and 200 mesh grids were arranged over the paper. The composite film was then placed over the grids, carbon film up. Amyl acetate was added slowly to the edges of the filter paper and the dish was covered to allow the collodion to dissolve. After thorough drying, the double replicas were examined in the electron microscope. Scanning Electron Microscopy (SEM) The utility of the SEM stems from a unique combination of features which include magnifications up to 50, 000 times, resolution less than 30oJ?, great depth of focus-nearly 300 times that of the light microscope, realistic "three-dimensional" imaging and direct examination without replication. The samples observed with this microscope were electrically non-conductive which necessitated a vacuum deposit of a thin film of carbon and/ or Au-Pd on the specimen surface, so as to preclude electrical changing of the specimen, with attendant image distortion. 184 The objectives of this procedure were to study crystal morphology, grain to grain relationships, and to identify small inclusions and minor phases with the electron probe attachment. Samples of calcite and anhydrite cap rock and salt residue minerals were observed. Specimens were prepared by polishing and etching the surface, fracturing the surface or making particulate matter grain mounts. X-Ray Diffraction All of the samples studied were subjected to x-ray diffraction to determine mineral phases present. A Norelco diffractometer equipped with a copper target and a nickel filter was used. As a result of this radiation, iron bearing minerals were seldom detected. Samples were prepared first by grinding and then seiving them with a 200 mesh screen. The material which passed through the seive was then packed into a glass sample holder, and x-rayed. X-Ray Fluorescence X-rav fluorescence data were obtained on a Philips 8-position vacuum spectrograph equipped with a simultaneous output printer. Counting was done completely in the fixed-time mode. Operating conditions of the spectrograph are summarized in Table 20. Samples were prepared according to the method outlined by Leake and others (1969). Separate standards were made for CaCO^ and CaSO^ matrix rocks to avoid mass absorption correction problems. 185 TABLE 20 X-Ray Fluorescence Instrumental Parameters Col- Fixed z Peak Tube C rystal Det. Gas limator Path kV mA Time Na Ka Cr Gyp GFPC Nat. Coarse Vac. 45 25 20 Mg Ka Cr Gyp GFPC Nat. C oarse Vac. 45 25 20 A1 Ko Mo Gyp GFPC P-10 Fine Vac. 45 25 50 Si Ka Cr EDDTGFPC P-10 Coarse Vac. 45 25 20 S Ka Cr EDDTGFPC P-10 C oarse Vac. 40 25 10 K Ka Cr EDDTGFPC P-10 C oarse Vac. 45 25 10 Ca Ka Cr EDDTGFPC P-10 Fine Vac. 20 5 10 Ti Ka Cr EDDT GFPC P-10 C oarse Vac. 45 25 10 Fe Ka Mo L iF Scin. — Coarse Vac. 50 38 10 Cu Ka Mo L iF Scin. — Fine Air 30 20 20 Zn Ka Mo L iF Scin. — C oarse Air 45 40 10 Rb Ka Mo L iF Scin. — Coarse Air 45 40 10 Sr Ka Mo L iF Scin. — C oarse Air 50 38 10 Ba L£, W L iF Scin. — C oarse Vac. 50 45 20 GFPC = gas flow proportional counter. Nat = natural. FT = fixed time Scin. - scintillation counter. Vac. - vacuum TABLE21 Atomic Absorption Instrumental Parameters Wave- Cathod Z length(&) current(mA) Range Slit (mm) Fuel Qxid Na 5890 15 Vis 1 A ir C2H2 Mg 2852 10 UV 3 Air C2H2 A1 3093 28 UV 1 C„H n 2o 2 2 Si 2516 40 UV 0. 3 C2«2 N2° K 7665 30 Vis 1 A ir S « 2 Ca 4227 20 Vis 1 A ir C2»2 Fe 2483 35 UV 0. 3 Air C2«2 Cu 3247 20 UV 1 Air C2H2 Zn 2138 15 UV 3 A ir C2»2 Sr 4607 20 Vis 1 A ir Ba 5536 30 Vis 0. 3 n 2o ‘ 2h 2 Vis - visable spectrum. UV = ultraviolet spectrum Boling burner head used throughout 186 These standards were prepared by spiking spectrochemical grade calcite and anhydrite with Spex Mix No. 1000 (a spiking standard containing 1. 27 percent each of 49 elements) in amounts calculated to approach the anticipated concentration levels of the samples. Six standards for each matrix type were made with the following concentrations: (1) Pure matrix material (2) 10 ppm (3) 50 ppm (4) 100 ppm (5) 500 ppm and (6) 1000 ppm. The weight of the Spex Mix added to the matrix sample was equal to the desired concentration level in the standard divided by 1. 27% (concentration of elements in the Spex Mix) multiplied by 6 grams (desired weight of pellet). Other matrix type rocks were analyzed by the method of one standard, an amphibolite, BL 3571 (Leake, et al. , 1969). To obtain the highest possible precision a ratio technique was used exclusively to avoid both long and short term variations in the performance of the spectrometer. Each set of standards was run ten times for each element to be analyzed. The average values were then divided by the standard containing the 500 ppm concentration level. This standard was kept in one position of the spectrograph and counted after running each set of seven unknowns. Therefore, all the calibra tion curves are based on plots of oxide percentage or element ppm against the ratio of counts collected on the standard, which was always the standard containing the 500 ppm concentration level. Equations tor the standard calibrations are listed in Table 22. TABLE 22 X-Ray Fluorescence Standard Calibrations CaCO. Matrix CaSO. Matrix 3 4 Corr. Standard Corr. Standard z Coef. Equation error of est. Z Coef. Equation error of Na 0. 996 Y=0. 472 x+0. 560 0. 083 Si 0.991 Y=0. 001 x+0. 860 0. 041 Mg 0.997 Y=0. 753 x+0. 911 0. 056 S 0.998 Y=0. 072 x -0. 638 0. 001 Si 0. 982 Y=0. 001 x + 0 .485 0. 048 K 0.956 Y=0. 001 x+0. 612 0. 045 S 0.998 Y=0. 002 x + 1. 154 0.019 Ca 0.999 Y=0. 029 x -0. 150 0. 001 K 0.998 Y=3. 013 x +0. 531 0. 038 Ti 0.998 Y=0. 001 x+0. 574 0. 012 Ca 0. 997 Y=0. 015 x+0. 158 0. 006 Fe 0.995 Y=0. 001 x+0. 328 0. 031 Ti 0.982 Y=0. 001 x+0. 454 0. 046 Zn 0. 991 Y=0. 002x-0. 020 0. 064 Fe 0.995 Y=0. 001 x+0. 528 0. 025 Rb 0.991 Y=0. 001 x +0. 233 0. 042 Zn 0.998 Y=0. 001 x+0. 299 0. 021 Sr 0.999 Y=0. 001 x+0. 758 0. 008 Rb 0. 999 Y=. OOOx+O. 775 0. 002 Sr 0.999 Y=0. 000x+0. 633 0. 007 Table 23 X-Ray Fluorescence Instrumental Precision Sample: CaCC>3 matrix. Jones #1 well* 1195 ft., Minden Dome, La. Average of duplicate analysis of 14 pellets. CaO* S MgO* NazO* Si k 2o* Fe Zn Sr Ti Min. 53.03 99. 0.61 0. 89 450. 0. 04 123. 527. 113. 161. Max. 53.39 121. 0. 85 0. 92 480. 0. 06 147. 533. 127. 179. Mean 53.21 110. 0.73 0. 90 465. 0. 05 135. 530. 120. 170. S. D. 0.28 7.08 0. 07 0.01 10. 38 0. 01 7. 54 2. 04 5. 01 4. 90 C V (%) 0. 52 6.43 9.45 0.99 2. 23 10. 91 5. 59 0. 38 4. 17 2. 88 Sample: CaSO^ matrix. Jones #1 well, 1485 ft., Minden Dome, La. Average of duplicate analysis of 14 pellets. CaO* s* Si K Fe Zn Sr Min. 39.80 23.26 80. 190. 250. 432. 335. Max. 40. 22 23.61 115. 220. 280. 438. 352. Mean 39.96 23.41 96. 203. 265. 435. 343. S. D. 0. 13 0. 11 12. 51 9.95 10. 01 1. 96 5. 35 C V (%) 0. 32 0.45 13. 03 4. 90 3.78 0. 45 1. 56 ★ % value, other values in ppm From Table 23 it can be seen that the coefficients of variation are largely dependent upon the element and the concentration. They are well within acceptable limits for this particular study. 189 TABLE 24 Accuracy of X-Ray Fluorescence Analysis Sample Tl-1 Tl-1 Tl-34 Tl-34 Dol-88 Dol-88 la Ta Method XRF AA XRF AA Std. An. XRF Std. An . XRF Na or Na^O 0. 83 0. 89 — 120 0. 08 0. 10 0. 39 0. 41 Mg or MgO 0. 20 0. 18 — 312 21. 48 + 2. 19 2. 16 Al or Al^O^ — 0. 06 — 0. 005 0. 067 — 4. 16 4. 11 Si or SiO 385 378 205 188 0. 31 0. 28 14. 11 13. 90 S% <. 005 * 23. 65 * 0. 027 0. 035 0. 266 0. 273 K or Kz O 1435 1395 195 203 0. 03 0. 02 0. 71 0. 69 CaO 50. 02 * 40. 52 * 30. 49 30. 55 41. 32 41. 60 Ti or TiOz 45 * — * 0. 005 0. 005 0. 16 0. 17 Fe or Fe203 390 395 75 60 0. 084 0. 079 1. 63 i. 65 Cu * 122 * 575 ** ** Zn 490 510 310 385 * 25 * 118 Rb — # — * * U * 310 Sr 1250 1265 332 338 <100 42 2300 2280 Ba — 310 — £5 * — * Tl-1 = Tatum Salt Dome, AEC Tatum #1, 974 ft. , CaCO caprock Tl-34 - Tatum Salt Dome, AEC Tatum #1, 1506 ft. , CaSO caprock Dol-88 = Dolomite, Bureau of Standards, Standard Sample pIo. 88 la = Argillaceous limestone, Bureau of Standards, Standard Sample No.la XRF = X- ray fluorescence analysis AA = Atomic absorption analysis Std. An. = Bureau of Standards analysis * = Not analyzed — = Below detection limit + = Beyond practical range of standard calibration curve Underlined values indicate parts per million of the element. All other values are given in percent as the oxide or percent S. Each sample run in duplicate with mean value shown. 190 The reproducibility of the data obtained from the spectrometer was evaluated by analyzing 14 pellets in duplicate of a calcite and of an anhydrite matrix sample. Table 23 summarizes the obtained resu lts. The accuracy of the x-ray fluorescence analysis was determined by comparing wet chemical and x-ray analyses, and by treating standards prepared and analyzed by the U. S. Bureau of Standards as unknowns. These two analyses were then compared. The results of these tests indicate that the method of analysis is highly accurate for most elements, and adequate for the remaining elements. The data obtained from the accuracy tests are listed in Table 24. Atomic Absorption The atomic absorption method of analysis was employed to determine trace element concentrations in CaCO and CaSO. minerals, 3 4 to test the accuracy of x-ray fluorescence standards and to determine certain elemental concentrations in the solution-precipitation laboratory experiments. The technique used in the analysis was adopted from the Atomic Absorption Newsletter as well as other pertinent information(Angino and Billings, 1967; Billings and Ragland, 1969). Operating conditions of the Perkin-Elmer 303 spectrometer are listed in Table 21. 191 Solution-Precipitat ion Expt^riments The purpose of these experiments was to develop a better understanding of the genesis and diagenesis of CaSO^ caprock. The solubility of the caprock mineral is controlled mainly by natural parameters in the three phase system CaSO^ - NaCl - H^O. The experiments were designed to take into account solute and solution concentrations, temperature and time independent variables. The precipitation experiments were performed by two methods. The first method consisted of evaporating an artificial sea water solution to about 0. 02 percent of its original volume. The precipitate was periodically removed from solution and analyzed for mineralogy and crystal morphology. The second method accomplished precipi tation by additions of small concentrations of sodium sulphate to saturated calcium sulphate solutions with excess solid salt residue anhydrite. The solid material was examined every day for ten days to determine charges in mineralogy and crystal form. Dissolution experiments were conducted on the salt residue anhydrite. The rate of dissolution was determined by placing one gram of CaSO^ in 100 mi of distilled water in one bottle and one gram of CaSO^ in 100 m l of 150 g NaCl p e r lite r of w ater in another bottle. Five ml of solution were removed every 24 hours for the first 10 days, followed by removal of the same amount at ten day intervals for-the next 90 days. Each sample was then analyzed for the amount of Ca++ in solution. 192 The effect of mixing different concentrations of NaCl solutions on the solubility of anhydrite open to the atmosphere was experi mentally investigated. This procedure involved passing a solution of 150 g NaCl per liter through 1. 5 grams of CaSO^ at a flow rate of 1 ml per 63 seconds. Distilled water was added at a rate of 1 ml per 21 seconds to the above passed solution and allowed to pass through a second 1. 5 grams of solid CaSO^, resulting in a dilution factor of three. At intervals of 24 hours for ten days a 5 ml sample was recovered below each one of the two CaSO^ charges and analyzed for Ca+* concentration and crystal morphology changes. APPENDIX 3 Lithological, Mineralogical, and Chemical Data Appendix B is a compilation of data derived from chemical and mineralogical analyses of caprock and associated materials. These data were primarily obtained by the writer, although some of the information originated from the literature. Table 25 lists lithology data from all wells on each dome for which information was ^variable. A large percentage of this informa tion is bore hole data derived from the files of Freeport Sulphur Company. Mineralogical and chemical data are shown in Tables 26 through 34. These data were obtained by x-ray diffraction and x-ray fluores cence analyses. Table 35, which lists the averages of constituents in anhydrite caprock and salt residue must be viewed with some caution. The number of observation are too few to be statistically reliable for some of the domes. They were calculated, however, in hopes that the values might indicate some similarities or differences between domes. Tables 36 and 37 show water chemistry concerning caprock and surrounding sediments. Statistical analyses of some Louisiana oil field brines not associated with salt domes is listed in Table 38 for comparison with those brines associated with domes. It should be noted that most of these brines are from oil fields located in the northern portion of the state. 193 TABLE 25 Lithology Data From All Wells On Each Dome Well TC Thick TG Thick TA Thick TT TS TSC> TSed S° 4 4 CO3 Allen Dome, Tex. FS1 — 916 348 1264 117 465 1381 465 — FS2 1509 — —- NR FS3 — 985 222 1207 172 394 1379 394 — FS4 815 17 832 45 877 503 565 1380 548 — 32. 23 FS5 — — 1363 20 20 1383 20 4 FS6 867 31 898 475 1373 20 526 1393 495 — 15. 97 FS7 942 18 — 960 434 452 1394 434 12 24. 10 FS8 1081 5 1086 278 1364 19 302 1383 297 14 59.43 FS9 —. 1154 101 1255 123 224 1378 224 — FS10 807 16 823 377 1200 192 585 1392 569 — 35. 56 FS11 791 11 802 408 1210 184 603 1394 592 — 53. 82 FS12 • 806 77 883 427 1310 85 589 1395 512 — 6. 65 FS13 1237 245 1482 3 — 248 1485 3 119 0. 01 FS14 896 143 1039 217 1256 127 487 1383 344 128 2.40 FS15 760 100 920 60 860 NR Belle Isle Dome, La. FS1 1247 438 —— 442 1689 — 4 FS2 672 632 — — 632 1304 — — FS3 1672 234 —— 253 1925 — 19 FS4 393 38 405 137 — 189 582 137 11-salt 3.61 FS5 546 163 709 164 — 327 873 164 — 1. 04 FS6 1058 387 — — 387 1445 —— FS7 426 89 503 60 — 149 575 60 — 0. 67 Table 25 cont. s o 4 Well TC Thick TG Thick TA Thick TT TS TSO TSed 4 CU3 Black Bayou Dome, La. FS1 — 954 NR FS2 — 953 NR FS3 1027 46 1086 20 1106 NR FS4 1164 NR FS5 1389 164 1553 NR FS6 — 881 NR FS7 1011 19 1130 112 1242 NR Brenham Dome, Tex. FS1 1383 106 1489 NR FS2 1973 70 — 2043 NR FS3 1694 4 — 1698 NR FS4 1794 96 — 1890 363 459 2253 363 5 3. 78 FS5 1022 93 — 1115 NR FS6 — — 2369 NR FS7 1272 45 — 1317 NR Damon Mound Dome, Tex. FS1 103 181 284 276 — 457 560 276 1. 53 FS2 148 186 334 230 — 416 564 230 — 1. 26 FS3 170 221 391 167 — 388 558 167 — 0. 75 FS4 179 81 260 314 — 395 574 314 — 3. 90 FS5 152 158 310 NR FS6 191 100 291 NR 195 Table 25 cont. so4 Well TC Thick TG Thick TA Thick TT TS tso4 TSed FS7 128 118 246 296 — 414 542 296 — 2. 57 FS8 M issed Cap FS9 157 38 195 342 — 380 537 342 — 9. 03 FS10 227 112 339 193 — 305 532 193 — 1.74 FS11 — 157 380 — 380 537 380 — FS12 233 369 602 40 — 405 642 40 — 0. 12 FS13 108 53 161 399 — 452 560 399 — 7. 52 FS14 — 173 394 — 394 567 394 — FS15 380 159 539 45 — 204 584 45 — 0.28 FS16 105 — NR FS17 1247 158 1405 NR FS18 — 189 369 — 369 558 369 — FS19 — 178 NR FS20 — 180 NR FS21 85 158 243 NR FS22 — 291 242 — 242 533 242 — FS23 108 12 120 410 — 422 530 410 — 34.22 FS24 227 42 269 NR FS25 186 96 282 NR FS26 199 43 242 366 — 409 608 366 — 8. 54 FS27 140 158 298 NR FS28 151 196 347 NR Hockley Dome, Tex. FS1 345 35 380 77 457 720 847 1192 797 15 22. 86 FS2 417 190 607 173 780 286 649 1066 459 — 2.41 FS6 666 201 — 1011 177 522 1188 177 144 0.88 FS7 382 87 428 82 510 493 666 1048 575 4 6. 60 Table 25 cont. so4 Well TC Thick TG Thick TA Thick TT TS TSO. TSed 4 CCh Hull Dome, Tex. FS1 298 7 305 393 — 400 698 393 56. 11 FS2 598 341 939 84 — 425 1023 84 — 0. 25 FS3 309 10 598 86 684 11 107 695 97 — 9. 76 FS4 685 47 — 718 164 253 1033 164 42 3. 50 FS6 — 36 Z 298 660 29 327 689 327 — FS7 333 28 361 237 598 82 347 680 319 — 11.43 FS9 285 2 287 253 540 148 403 688 401 _ 200.51 Humble Dome, Te: La. 4 1098 32 — 1130 84 116 1214 84 — 2. 64 Bender 1 1200 75 — 1275 30 105 1305 30 — 0.40 Bender 3 1322 65 — 1387 93 158 1480 93 — 1.43 La. 1 1140 51 — 1191 67 118 1258 67 — 1. 33 Cortes 1 1144 78 — 1222 57 135 1279 57 — 0. 73 Kings Dome, Mis celestite Hall 1 177 — 3770 247 3840 247 3593 McBride Dome, Miss 22 2201 57 2208 — 145 2185 — 158 2168 _ 197 Table 25 cont. SO4 Well TC Thick TG Thick TA Thick TT TS TSO. TSed 4 T O 3 Minden Dome, La • Jones 1 1160 124 — 1284 616 740 1900 616 4. 97 Bridgeman 1 1190 165 — 1355 557 622 1912 557 ----- 3. 37 Hudson 2 — — 2320 80 80 2400 80 --- Napoleonville Dome, La. Le Blanc 1 — 430 216 670 24 263 693 240 23 La Bar re 1 361 20 350 175 655 NR — La Barre 2 — 414 NR La Barre 3 898 18 891 247 1181 31 296 1187 278 — 15.41 Duval 3 647 59 706 84 — 143 790 84 — 1.46 UniS. 1 423 322 — — 322 686 — Duval 5 — 551 134 — 134 685 134 — Duval 1 — 703 26 — 26 729 26 — TGS2 — 680 23 — 23 703 23 _ TGS5 — 708 3 — 3 711 3 — Oakvale Dome, Miss. Newman 1 1507 150 — 1657 NR Newman 2 1808 111 — 1919 NR Thurm an 1 1819 7 — 1826 NR Taylor 1 1570 124 — 1694 702 826 2396 702 — 5.66 Fortenberg 1 — NR Sun 1 1538 NR Table 25 cont. so4 Well TC Thick TG Thick TA Thick TT TS TSO. TSed 4 CO3 Pine Prairie Dome, La. FS 1 105 7 132 356 115 12 380 492 368 5 52. 62 FS 2 237 49 246 216 — 265 499 216 — 4. 45 FS 3 29 357 415 64 — 466 495 64 45 0. 18 FS 4 160 214 374 101 475 22 337 497 123 — 0. 57 FS 5 345 94 449 78 527 3 185 530 81 10 0.86 FS 6 172 19 191 329 — 354 526 329 — 17. 30 FS 7 83 150 233 274 — 426 509 274 — 1. 85 FS 8 — 465 15 480 9 24 . 489 24 — FS 9 144 84 675 19 694 58 608 752 77 — 0. 92 FS 10 122 334 456 48 504 9 391 513 57 — 0. 17 FS 11 189 49 257 236 — 304 493 236 19 4. 93 FS 12 31 505 — — 530 561 — 25 Richmond Dome, Miss. Beasley 1 1607 43 — 1650 295 338 1945 295 — 6.86 Watts 1 1708 32 1740 5 1745 NR Scarborough 11 1837 27 1940 45 1842 45 117 1954 90 — 3. 32 Ruth Dome, Miss. Clark 1 Missed cap Clark 2 2212 464 — 2676 NR Lee 1 2244 485 — 2729 NR G. Clark 1 2438 322 — 2760 NR B of SI 2432 338 _ 2770 NR Table 25 cont. SP4 Well TC Thick TG Thick TA Thick TT TS TSO . TSei 4 CO3 Sardis Church Dome, Miss. Bell 1 1102 759 — 1861 NR Bell 2 1174 NR Cliburn 1 1498 400 — 1898 NR Allen 1 1067 842 1909 NR Section 28 Dome, La. M artin 1-A 1126 32 _ — 55 1181 23 Martin 2-A 2381 8 -- — 8. 2389 — Stratton Ridge Dome, Tex. Seaburn 1 2030 361 2391 82 — 443 2473 82 — 0. 23 T & D 1 _ 910 398 — 398 1308 398 — T & D 2 1212 42 1254 62 — 104 1316 62 — 1.51 T & D 4 1246 12 1238 81 — 93 1319 81 — 6. 73 Brock 1 1239 61 1300 12 — 73 1312 12 — 0 . 20 Seaburn 4 994 26 1020 17 1003 331 340 1334 348 — 13. 46 Stratton 1 1208 52 — 1260 63 115 1323 63 — 1. 20 Rycads 1 2300 382 — 2682 1 382 2682 1 — Tatum Dome, Miss. FS 1 1370 67 1465 69 1437 28 164 1534 97 — 1.46 FS 2 1784 11 — 1792 NR FS 3 967 104 1071 3 1074 459 566 1533 462 — 4.45 FS 4 872 146 1018 2 1020 NR Table 25 cont. Well TC Thick TG Thick TA Thick TT TS TSO„ TSed £ 2 ? ______4______TXT-3 FS 5 905 124 1029 33 1062 NR FS 6 1184 202 1386 6 1400 112 320 1512 118 — 0. 58 FS 7 M issed cap FS 8 961 68 1029 95 1124 NR FS 9 1161 127 1288 37 1325 178 342 1503 215 — 1.70 FS 10 1002 125 — 1127 NR Utica Dome, Miss, celestite L. Little 1 2641 96 310 494 3135 398 — 4. 14 2737 1 Venice Dome, La. GP 2 2216 180 2396 NR Venice 1 1535 583 — 2118 NR Tidewater 1 2935 Tidewater 2 4735 Vinton Dome, , La. G ray 1 466 1 467 168 561 NR G ray 2 645 123 768 118 945 83 324 968 201 — 1.63 Gray 3 420 159 593 116 835 NR G ray 5 724 159 — 906 NR G ray 6 384 78 465 525 1001 NR 201 Table 25 cont. S0 4 Well* TC Thick TG Thick TA Thick TT TS TSO. TSed 4 “CUT Winnfield Dome, La. 3 325 105 430 30 — 135 460 30 0. 28 18 — 150 125 310 175 300 460 300 — 22 225 125 350 10 360 100 235 460 110 — 0.88 31 250 175 MM 425 35 210 460 35 MM 0. 20 ♦Information from Guidebook, Shreveport Geol. Soc. , I960 TC = top of calcite, TG = top of gypsum, TA = top of anhydrite TT = total thickness of caprock, TS = top of salt, TSO^ = total sulphate TSed = total sediments in caprock, — = absent, NR = none recovered Thick = thickness of preceding lithology 202 TABLE 26 Tatum Dome, Miss. AEC Tatum #1 Well. Mineralogical and Chemical Data Depth M inerals* Color CaO% S% Si K Fe Zn Sr Ti MgOfoNa^C ppm ppm ppm ppm ppm ppm 974 Cal Bk. &Wt. Bds. 50. 02 0.005 385 180 390 490 1250 45 0. 20 0. 83 1028 Cal Gray 54. 10 0.0145 590 ------305 500 1242 0. 17 0. 94 1045 Anhy Gray 40. 83 23. 80 950 245 45 305 380 mm MM 11 II 1050 40. 79 23. 79 470 140 53 295 390 ___ .p . II 11 1054 39.96 23. 57 255 250 30 325 378 m — — II II 1061 39. 58 22. 94 645 244 100 295 330 __ II 11 1101 40. 16 23. 28 ------90 32 317 410 _ ------II 11 1115 40. 44 23. 43 540 155 80 330 408 mm II II 1117 40. 42 23. 73 730 345 52 300 390 — mm ------II 11 1122 40. 43 23. 51 705 270 5 315 350 _ ------II II 1201 40. 34 23. 52 100 175 50 295 378 «_ - n II 1208 40. 26 23. 25 375 320 40 313 355 mm ii 11 1210 40. 20 23. 46 400 125 81 293 357 mm - — ii II 1215 40. 80 23. 58 ------60 294 335 _ MM ii II 1230 39. 54 22. 95 575 195 65 285 331 w— _ n II 1237 40. 73 23. 40 920 310 60 260 380 ------mm MW it II 1239 40. 80 23. 65 945 345 5 270 318 — ii II 1241 40. 80 23. 57 425 195 45 280 355 — mm it II 1312 40. 54 23. 00 270 290 75 283 360 MW -- ii 11 1318 40. 04 23. 35 350 255 90 278 361 mm mm MW ii II 1320 40. 32 23. 80 ------65 255 318 ww MW ii 11 1324 40. 44 23. 55 425 165 73 282 361 _ - — ii II 1440 40. 62 23. 56 345 320 80 280 400 --- - - ii II 1450 40. 20 23. 67 680 300 10 383 350 _ MW ___- ii II 1456 40. 34 23. 37 400 280 60 275 375 mm MM it II 1458 40. 42 23. 48 170 400 30 294 360 _ -r-T - — ii II 1463 mm - — 40. 62 23. 44 275 205 85 281 330 MM ii 11 203 1465 40. 80 23. 46 ------60 260 400 - — Table 26 cont. Depth Minerals* Color CaO% S% Si K Fe Zn Sr Ti MgO%Na 0% ppm ppm ppm ppm ppm ppm w 1470 Anhy Gray 40. 60 23. 60 525 275 65 295 405 ----- — 1477 11 11 40. 68 23. 64 605 375 60 327 330 — — — 1493 11 11 40. 80 23. 69 380 215 25 300 250 — ------1500 11 11 40. 82 23. 70 185 285 45 315 280 — ----- — 1502 It II 40. 67 23. 61 275 225 40 ' 225 277 — ----- — 1506 II II 40. 52 23. 65 205 195 75 310 332 — ------1507 II II 39. 70 23. 10 ------110 75 316 280 — ----- — 1509 II II 39. 74 23. 43 1075 280 160 316 300 — ----- — 1510. 5 Salt res. " 37. 62 23. 33 ------5 110 446 380 — ------11 II 1513 39. 54 23. 55 ------5 125 445 360 — ------1847 II 11 39. 48 23. 54 ------10 85 447 390 — ------2228 II II 38. 82 23. 54 ------65 57 450 385 — — TABLE 27 Minden Dome, La. Jones #1 Well. Mineralogical and Chemical Data Depth Mine rals *Color CaO% S% Si K Fe Zn Sr Ti MgO%Na 0% P P m P P m P P m P P m ppm ppm M 1165 Cal-Q Bk. &Tan Bds. 54. 42 0. 0130 945 207 575 70 40 0. 90 0. 92 1170 Cal-D Tan 56. 04 0. 0120 ----- 83 ----- 155 100 110 0. 98 0. 75 1180 Cal Bk. some Wt. 55. 23 ----- 227 187 85 80 50 45 0. 35 0. 98 1185 Cal Bk. &Wt. Bds. 55. 32 0. 0178 ----- 94 65 528 168 210 0. 88 0. 73 1185 Cal-D Bk. 55. 20 ----- 295 — 175 165 310 510 1. 00 0. 72 1190 Cal Bk. &Wt. Bds. 56. 81 0. 0154 160 137 125 145 125 175 0. 75 0. 84 1195 Cal II 53. 21 0. 0110 465 228 135 530 120 170 0. 73 0. 91 1200 Cal If 55. 34 0.0110 260 166 145 175 95 180 0. 63 0. 92 1220 Cal 11 55. 25 0. 0203 285 104 70 200 70 180 0. 50 0. 95 1250 Cal-Q II 48. 80 0. 0155 870 153 105 360 135 210 0. 63 0. 93 204 Table 27 Depth M inerals* Color CaO% S% Si _ Fe Zn Sr Ti MgO%Na 0% ------PPm ppm ppm ppm ppm ______ppm B 2 1255 Cal Yellow 54. 00 0. 0055 ------116 ----- 175 50 140 0. 10 1. 02 1255 Cal Wt. 52. 06 ------403 104 70 225 180 3120 0. 90 0. 93 1264 Cal-S Bk. StWt. Bds. 45. 20 0.0703 225 75 5 580 215 255 0. 55 0. 76 1274 Cal Yellow 50. 81 0. 0040 77 436 195 245 65 85 0. 11 0. 94 1280 C al-Q -Py Bk. &Wt. Bds. 50. 77 0.0212 7922 174 715 200 135 135 0. 38 0. 86 1284 Cal Bk. some Wt. 55. 20 0.0140 285 120 120 265 5 35 0. 10 0. 96 1286 Cal Wt. 54. 12 0. 0120 185 104 5 245 ----- 20 — 0. 93 1290 Cal-S Wt. 43. 65 0.0750 210 62 100 580 25 3800 _ 0. 65 1290 Cal-Q Bk. 54. 60 0.0230 875 54 395 225 —— --- _ 0. 93 1304 Anhy Gray 40. 38 23. 80 ■ 1 20 73 324 320 — II II 1312 40. 63 23. 80 — — ------15 349 300 --- ... II II 1320 40. 71 23. 80 —— ----- 43 340 275 _ _ 1330 11 11 39. 46 23. 08 85 150 80 325 200 — _ 1350 It II 37. 40 22. 56 70 110 45 325 330 _ 1370 II II 40. 28 23. 56 65 ----- 5 310 328 _ II II 1400 40. 74 23. 62 — ■ i ----- 15 324 332 — 1420 II II 40. 80 23. 80 75 ----- 16 308 360 — _ 1430 II It 40. 22 23. 11 160 120 110 330 375 _ 1450 II II 39. 56 23. 17 195 125 52 332 358 — _ 1470 II II 40. 86 23. 80 70 150 65 315 362 _ II 1485 II 39. 96 23. 41 96 203 265 435 343 _ — - —_ 1500 11 II 40. 85 23. 12 105 120 85 330 355 --- —* 205 TABLE 28 Kings Dome, Miss. Hall #1 Well. Mineralogical and Chemical Data Depth Minerals* Color CaO% s% sio2%K F e2° 3% Zn Sr% Ti MgO%Na C ppm ppm or ppm ppm 3431 K -Q -Pl Gray 1. 78 0. 0352 69. 92 4440 3. 40 770 533 ppm 4796 0.90 — 3451 Q -K -Pl II 1. 81 0. 0332 76. 00 3942 2. 02 790 454 ppm 4496 0. 50 — 3499 K -Q -Pl II 2. 53 0. 0719 62. 41 4482 4. 60 735 505 ppm 5036 0. 90 — 3519 Q -K -Pl II 1. 88 0.0375 77. 71 3735 1. 25 795 715 ppm 3537 0. 20 — 3583 Q -K -Pl II 1.71 0.0475 77. 28 3444 0. 80 825 453 ppm 3117 — — 3619 D-Q-Ce Bk. some Wt. 32. 01 1. 10 1. 70 124 0. 65 520 3. 01% 10 17.33 0.22 3748 Ce-D-Q Gray 4. 31 15. 70 12. 20 77 1. 45 50 36. 32% 10 7. 95 — 3778 Anhy II 40. 56 23. 67 0. 03 125 0. 05 425 1700 ppm — — — 3825 Anhy II 38. 34 21. 80 0. 90 155 0. 15 412 4800 ppm — — — 3845 Salt Res. II 39. 47 23. 54 ----- 10 0. 024 453 380 ppm — — — 4050 It II 39. 51 23. 53 --- 15 0. 017 460 380 ppm — — — TABLE 29 Utica Dome, Miss. LeeLittle #1 Well. Mineralogical and Chemical Data Depth M inerals* Color CaO% S% S io2% K f = 2o 3% Zn Sr% Ti MgO%Na C ppm ppm or ppm ppm w 2646 Cal-D -Q - Gray 31.93 0. 0225 10. 28 668 1. 05 538 270 ppm 2818 11. 45 0. 30 K -Pl 2651 D-Cal-Q II 32. 55 ---- 1. 10 153 0. 49 540 235 ppm — 17. 50 0. 30 2715 Cal-D Gr. &Wt. Bds. 50. 71 ---- 0.0845 178 0. 21 70 65 ppm — 5. 56 0. 84 2725 Cal-D II 47. 23 0.0110 0.0577 166 0. 50 115 27 ppm 65 5. 80 0. 61 2775 Ce-Cal- II 13. 44 10. 00 4. 61 41 0. 53 175 27. 31% 305 2. 60 — D-Q 2785 Ce- Cal Bk. &Wt. Bds. 5.30 14. 31 0.0577 83 0. 02 160 39. 06% 9292 0. 05 -- 2795 Ce Gray < 1. 00 16. 28 ------620 44. 62% 10731 0. 05 — 206 2805 Ce - Cal- II 6. 00 13. 97 2. 85 ---- 0. 35 ---- 38. 15% — ■ ■■ 1. 20 D-Q Table 29 cont. Depth Minerals* Color CaO% S% SiO % K Fe O % Zn Sr% Ti MgO%Na_0% ______2____ ppm ____ 2 3_____ ppm or ppm _____ ppm ______2 2863 Anhy Gray 39. 64 23. 55 ------0. 03 425 820 ppm — — — 2924 Anhy " 39.68 23.49 0.85 0.10 400 760 ppm — — — 2964 Anhy " 39. 56 23. 56 0. 93 ------0. 08 282 880 ppm — — — 3114 Anhy " 39.55 23.55 0.90 0.09 450 1250 ppm — — — 3136 Salt Res. " 39. 54 23. 54 ---- 15 0. 03 463 375 ppm — — — TABLE 30 Allen Dome, Tex. Allen #7 Well. Mineralogical and Chemical Data Depth Minerals* Color CaO% S% Si K Fe Zn Sr Ti MgO%Na ppm ppm ppm ppm PPm ppm w 1285 Anhy Gray 38. 98 23. 02 812 330 26 395 260 ------ii it 1295 39. 12 23.07 600 200 102 386 278 — ------1310 n it 39. 60 23. 21 424 130 134 366 358 — ------1330 39. 99 23. 26 212 ------148 357 376 — ------ m ii 1340 39. 36 23. 19 436 25 140 376 283 — ------n ii 1365 38. 98 23. 09 588 180 134 384 304 — ------— ii n 1373 39. 48 23. 08 580 305 130 380 332 — ------— ii n 1383 39. 84 23. 20 434 150 130 376 353 — — ------1390 ii ti 40. 48 23. 47 100 120 100 372 320 — ------1394 ii ii 38. 97 22. 81 925 210 136 368 316 — ------1395 Salt Res. " 39. 40 23. 21 35 19 66 472 371 — ------1399 Sait Res. " 39. 44 23. 21 25 23 78 474 357 — _ — 7 0 2 TABLE 31 Hull Dome, Tex. F. S. Co. Well #6. Mineralogical and Chemical Data Depth Minerals* Color CaO% S% Si K Fe Zn Sr Ti MgO%Na_0% ppm ppm PPm _. ppm ppm ppm 2 663 Anhy Gray 40. 14 23. 51 515 120 127 315 413 — — — 670 II II 39. 54 23. 09 705 180 103 318 415 — — — 680 II II 40. 46 23. 62 250 10 111 305 399 — — — 689 II II 39. 22 22. 88 970 290 119 302 429 — — — 690 Salt Res. " 39. 50 23. 39 ------35 94 475 363 — — — 692 Salt Res. " 39. 54 23. 41 ------29 96 475 357 — — — TABLE 32 Jennings Dome, La. Pan Am #146 Well. Mineralogical and Chemical Data Depth Minerals* Color CaO% S Si K_0% F e .O % Zn Sr Ti MgO%Na 0% ppm ppm 2 2 3 ppm ppm ppm w 2769 Cal-Py Gr.&Wt. Bds. 54. 01 280 ---- 0. 06 0.25 505 315 73 0.48 0.94 2850 Cal-Py Gr. &Wt. Bds. 50. 62 530 603 1.07 0.45 515 506 — 0.90 0.82 TABLE 33 Hockley Dome, Tex. Salt Mine. Mineralogical and Chemical Data Depth Minerals* Color CaO% S% Si K Fe Zn Sr Ti MgO%Na 0% ppm ppm ppm ppm ppm ppm £ approx.Salt Res. Gray 38. 69 22. 27 ---- 145 132 452 385 — — 1500 208 TABLE 34 Chactaw Dome, La. Brine Well. Mineralogical and Chemical Data Depth Minerals* Color CaO% S% Si K Fe Zn Sr Ti MgO%Na.O% ppm ppm ppm ppm ppm PPm 6 2 ResidudSalt Res. ?Gray 38. 70 22. 12 860 180 220 448 336 on surface Only those minerals identified by x-ray diffraction are noted. Minerals listed in decreasing order of abundance. Cal = calcite, Anhy = anhydrite, Q = quartz, D = dolomite, K = kaolinite, Ce = Celestite, PI = plagioclase, S = sulphur, Py = pyrite. = Below XRF detection limit. TABLE 35 Averages of Analyzed Constituents in CaSO^ Caprock and Salt Residue Dome Zone Value CaO% S% Si K Fe Zn Sr No. of ppm ppm ppm ppm ppm sample! Tatum, Miss. Caprock min. 39. 54 22. 94 —— 5 225 277 34 max. 40. 82 23. 80 1075 400 160 500 410 m ean 40. 49 23. 49 417 223 58 295 350 a 0. 37 0. 23 295.26 102. 51 29. 75 27. 57 40. 64 C(%) 0. 92 0. 97 70. 81 45. 97 51. 30 9. 34 11. 61 Salt residue min. 37. 62 23. 33 — 5 57 445 360 4 max. 39. 54 23. 55 — 65 125 450 390 mean 38. 86 23.49 — 21 94 447 379 <7 0. 89 0. 10 31. 42 29. 84 3. 91 13. 12 C (%) 2. 29 0. 43 149. 86 31. 71 0. 90 3. 54 Minden, La. Caprock min. 37. 40 22. 56 ______- - - 5 308 200 13 max. 40. 86 23 , 80 195 203 265 435 375 mean 40. 23 23. 43 71 77 67 335 326 a 0. 95 0. 39 61. 19 74. 02 67. 53 32. 24 46. 77 C(%) 2. 36 1. 69 86. 18 96. 13 100. 79 9. 62 14. 31 Salt residue* 0 Kings, Miss. Caprock min. 38. 34 21. 80 140 125 300 412 1700 2 max. 40. 56 23. 67 4200 155 400 425 4800 mean 39. 45 22. 73 2800 140 350 418 3250 Dome Zone Value CaO% S% Si K Fe Zn Sr No. of ppm ppm ppm ppm ppm sam pl Kings, Miss. a 0. 03 0. 01 3. 53 19. 09 4. 95 0. 0 C(%) 0. 08 0. 04 29. 46 26. 52 1. 08 0. 0 Utica, Miss. Caprock min. 39. 55 23. 49 ------111 282 760 4 max. 39. 68 23. 56 4334 ------345 450 1250 m ean 39. 61 23. 54 3150 ------264 389 927 Salt residue 39. 54 23. 54 ------15 112 463 375 1 Allen, Tex. Caprock min. 38. 97 22. 81 100 - 26 357 260 10 max. 40. 48 23. 47 925 330 148 395 376 m ean 39. 48 23. 14 512 165 118 376 318 Hull, Tex. Caprock min. 39. 22 22. 88 250 10 103 302 399 4 max. 40. 46 23. 61 970 290 127 318 429 mean 39. 84 23. 25 610 150 115 310 414 (7 0. 56 0. 35 304. 00 116. 90 10. 33 7. 70 12. 27 C(%) 1. 41 1. 50 49. 83 77.94 8.98 2.48 2.96 211 Table 35 cont. Dome Zone Value Ca 0% S% Si K Fe Zn Sr No. qf JPPm _ ppm ppm ppm ppm sam ples Hull, Tex. Salt residue min. 29. 50 23. 39 29 94 475 357 2 max. 39. 54 23. 41 ---- 35 96 475 363 m ean 39. 52 23. 40 ---- 32 95 475 360 cr 0. 04 0. 01 4. 24 1. 41 0. 0 4. 24 C(%) 0. 10 0. 06 13. 26 1. 48 0. 0 1.18 Hockley, Tex. Caprock* 0 . Salt residue 38. 69 22. 27 ------145 132 452 385 Chactaw, La. Caprock* 0 Salt residue 38. 70 22. 12 860 180 220 448 336 1 * = Samples not available — = Below XRF detection limit Table 36 Chemical Analyses o f Water Taken from Wells D rilled on Clay Creek Dome, Texas Farm Depth Farts per Million Temp. and in CO, °F. Cl .and SO, Na Ca Mg Total Well Feet neo, Solids A. Freeman 2,029- No. 1 2,064 34,550 887 2,407 20,473 1,117 60 59,494 195 J. Crote 2,485- No. 2 2,733 33,276 679 2,570 21,487 1,376 Trace 59,383 165 Wm. Schirmer 3,822- 110 No. 2 3,851 22,676 381 787 13,708 1,170 95 38,817 approx. W.F. Schlottman 1,771- 11,196 766 ------7,158 258 51 19,428 95 No. 3 1,809 approx. Table 36.shows chemical analyses of water taken from cavities in some of the cap rock wells. The listing of the wells is arranged in increasing distance from the center of the dome. Note the increase in the temperature of the water from the well nearest the center of the dome as compared with those farther out. (From Ferguson and Minton, 1936.) TABLE 37 Analyses of Some Louisiana Salt Dome Associated Oilfield Brines (Data from Collins, 1970 and Hawkins and Jirik, 1966) Top of Depth of Dome salt analysis pH Na K Ca Mg Sr Ba Cl h c o 3 S° 4 Jennings 2512 1855 6. 90 46937 428 2497 1154 173 7 80807 64 132 do. 2512 6309 6. 90 29768 263 2176 331 375 15 51603 416 86 Sulphur Mines 1460 3728 5. 20 44465 176 2492 949 145 68 76191 134 0 Edgerly 3985 2465 5. 10 17736 79 867 409 92 51 32032 167 0 do. 3985 3538 6. 20 40816 248 2645 840 128 73 66974 134 0 Starks 1925 4192 6. 60 35017 214 2886 844 70 45 61819 195 219 do. 1925 806 6. 20 11791 45 912 375 15 5 20548 171 466 Lockport 7207 3228 6. 30 40756 110 2386 836 93 74 69814 86 0 Vinton 925 4873 6. 50 45000 328 3026 976 180 48 78136 116 100 do. 925 2258 6. 20 35008 218 2187 981 105 6 60933 334 0 E. Hackberry 2950 5978 6. 60 44607 195 2473 747 143 90 75837 157 0 W. Hackberry I960 3297 5. 60 46011 168 2097 646 130 48 77085 153 0 Avery Is. 6 9557 7. 20 49920 310 2920 253 91 70 88329 128 200 Choctaw 629 3322 6.99 46416 216 2796 876 118 55 79513 134 0 do. 629 8132 7. 50 32204 198 1133 336 200 5 52861 701 388 St. Gabriel 11230 8601 6. 30 41453 224 3450 2140 63 30 76023 256 910 White Castle 2313 5635 6. 40 47798 246 2874 883 120 73 81864 88 0 LaFitte 13947 4516 6. 80 31773 190 2531 1172 68 60 57117 163 0 Iowa 7902 8122 6. 20 23261 262 462 120 70 10 37301 800 167 Leeville 3800 8714 6. 70 48077 562 2419 389 208 128 86350 4490 0 Chacahoula 1100 3517 6. 50 54556 348 1307 370 100 88 88076 116 0 Lk. Washington 1500 1135 8. 00 8501 120 195 0 33 8 13584 93 74 Paradis 13538 10066 6. 60 30735 186 803 156 118 5 49328 783 397 Port Barre 3642 2927 7. 00 43853 260 2776 934 180 95 75836 144 0 Constituents given as milligrams per liter Table 37 cont. Standard Coefficient of Variable ______Mean ______Deviation ______Variation(%) ______N=24 TS* 3854.46 3995.71 103.66 DA 4763. 77 2868. 00 60. 20 PH 6. 52 0. 65 9. 97 Na 37352 12177 32. 60 1. Top of salt, ft K 233 110 47. 21 below surface 2096 912 43.51 Ca 2. Depth of anal- Mg 696 461 66. 24 ytraia si3) in it•t f Sr 126 73 57. 94 Ba 48 35 72. 92 Cl 63873 20852 32. 65 HCO 418 894 213. 87 J0 S ° 4 132 219 165. 90 Correlation Matrix K Ca Mg Sr Ba Cl HCO, SO. Var. TS DA PH Na 3 4 TS 1. 000 0. 352 -0.095 -0. 179 -0. 132 -0.061 0. 256 -0. 224 -0. 094 -0. 155 0. 087 0. 360 DA 0. 352 1.000 0. 246 0. 285 0. 401 0. 147 -0. 068 0. 251 0.069 0. 279 0.427 0.419 PH -0. 095 0. 246 1. 000 -0. 063 0. 249 -0.054 -0.219 0. 130 -0.145 -0.075 0.080 0. 105 Na -0. 179 0. 285 -0.063 1. 000 0. 610 0. 723 0. 406 0. 307 0. 623 0. 994 0. 103 -0. 234 K -0. 132 0. 401 0. 249 0. 610 1. 000 0. 357 0. 063 0. 454 0. 386 0. 621 0.610 -0. 148 Ca -0.061 0. 147 -0. 054 0. 723 0. 357 1. 000 0. 747 0. 237 0. 477 0. 776 -0. 047 -0. 039 Mg 0. 256 -0.068 -0.219 0. 406 0. 063 0. 747 1. 000 -0.073 0. 124 0. 473 -0.228 0. 278 Sr -0.224 0. 251 0. 130 0. 307 0. 454 0. 237 -0. 073 1. 000 0. 108 0. 312 0. 273 -0. 237 Ba -0.094 0. 069 -0. 145 0. 623 0. 386 0. 477 0. 124 0. 108 1. 000 0. 636 0. 347 -0. 485 Cl -0.155 0. 279 -0. 075 0. 994 0. 621 0. 776 0. 473 0. 312 0. 636 1. 000 0. 137 -0. 210 HCO3 0.087 0. 427 0.080 0. 103 0. 610 -0.047 -0.228 0. 273 0. 347 0. 137 1. 000 -0. 031 s o 4 0. 360 0.419 0. 105 -0. 234 -0. 148 -0.039 0. 278 -0.237 -0. 485 -0.210 -0.031 1.000 215 TABLE 38 Statistical Analysis of some Louisiana Oilfield Brines not Associated with Salt Domes (Data from Collins, 1970) Standard Coefficient of Variable Mean Deviation Variation(%) N=6l DAa 4934. 44 2925.37 59. 28 PH 6. 64 0. 93 14. 01 Na 29702 14833 49. 94 K 218 171 78. 44 1 . Depth of Ca 3339 5470 163.82 analysis, in ft. Mg 499 563 112.82 2. Total dissolved Sr 283 322 113. 78 solids, in g/1. Ba 72 106 147. 22 Other constituents Cl 53536 30582 57. 12 in mg/1. HCO 270 247 91. 48 s o . 152 252 165.79 TD%2 87.8 49. 8 56. 72 Correlation Matrix Var. DA Na K Ca Mg Sr Ba Cl h c o 3 TDS PH S°4 DA 1.000 -0.493 0. 332 0. 391 0.408 0. 286 0. 400 -0.237 0. 400 -0.294 0. 384 0. 398 PH -0.493 1. 000 -0.387 -0.431 -0.647 -0. 397 -0. 565 0. 151 -0. 524 0. 530 -0.354 -0.520 Na 0. 332 -0.387 1. 000 0. 786 0. 536 0. 471 0. 507 0. 141 0. 955 -0.319 0. 135 0. 957 K 0. 391 -0.431 0. 786 1. 000 0. 642 0. 401 0. 718 0. 136 0. 826 -0. 224 0. 137 0. 827 Ca 0. 408 -0. 647 0. 536 0. 642 1. 000 0. 556 0. 870 -0.150 0. 760 -0.386 0. 456 0. 755 Mg 0. 286 -0.397 0. 471 0. 401 0. 556 1. 000 0. 441 -0. 020 0. 591 -0. 333 0. 105 0. 581 Sr 0.400 -0. 565 0. 507 0. 718 0. 870 0. 441 1. 000 0. 025 0. 693 -0. 324 0. 287 0. 690 Ba -0.237 0. 151 0. 141 0. 136 -0. 150 -0. 020 0. 025 1. 000 0. 060 -0. 045 -0. 351 0. 061 Cl 0.400 -0. 524 0. 955 0. 826 0. 760 0. 591 0. 693 0. 060 1. 000 -0. 388 0. 250 0. 999 HCO3-O. 294 0. 530 -0.319 -0. 224 -0.386 -0.333 -0.324 -0.045 -0.388 1. 000 -0. 081 -0. 380 so 4 0. 384 -0.354 0. 135 0. 137 0. 456 0. 105 0. 287 -0.351 0. 250 -0. 081 1. 000 0. 252 TDS 0. 398 -0. 520 0.957 0. 827 0. 755 0. 581 0. 690 0. 061 0. 999 -0.380 0. 252 1. 000 216 P late 1 The Salt Figure 1. Euhedral halite crystal illustrating perfect cubic cleavage and numerous inclusions. AEC salt core, Tatum Dome, Mississippi, 1847 feet. Optical micrograph, plane polarized light. Figure 2. Thin-section of salt showing that the inclusions occur within the individual crystals, apparently in much the same relationship as when originally precipitated. The included anhydrite crystals are !'stem-shaped". AEC salt core, Tatum Dome, Mississippi, 151.3 feet. Optical micrograph, nicol prisms removed. 217 218 PLATE 1 219 Plate 2 The Salt Figure 1. Water-insoluble residue, Belle Isle Dome, Louisiana. Sample from dark band in Rocm 6. Anhydrite crystals dispHy slight dissolution features. Scanning electron micrograph. Figure 2. Water-insoluble residue, "hydrocarbon zone", Belle Isle Dome, Louisiana. This zone has a high concentration of dolomite (d) and quartz crystals associated with solution-worn anhydrite and solid hydrocarbon material. The minerals were probably once at the salt-sediment interface and became incorporated in the massive salt during later upward move ment of the dome. Scanning electron micrograph. Figure 3. Water-insoluble residue, Cote Blanch Dome, Louisiana. Anhydrite crystals. Scanning electron micrograph. Figure 4. Water-insoluble residue, Weeks Island Dome, Louisiana. "Stem-shaped" anhydrite crystals. Scanning electron micrograph. Figure 5. Water - insoluble residue, Jefferson Island Dome, Louisiana. "Stem-shaped" anhydrite crystal. Scanning electron micrograph. Figure 6. Water-insoiuble residue, Avery Island Dome, Louisiana. Anhydrite crystals appear to have undergone considerable dis solution. Dolomite crystals do not show any dissolution features. Scanning electron micrograph. 220 PLATE 2 P late 3 The Salt Figure 1. Water-insoluble residue, Tatum Dome, Mississippi. "Stem-shaped1' anhydrite crystal showing crystallite terminations (arrow). AEC well, 1510 feet. Scanning electron micrograph. Figure 2. Water-insoluble residue, Tatum Dome, Mississippi. Twinned, "stem-shaped" anhydrite crystals. AEC well, 1513 feet. Optical micrograph, reflected and plane polarized light. Figure 3. Water-insoluble residue, Tatum Dome, Mississippi. Unknown material which is quite plentiful in the residue. AEC well. 1847 feet. Transmission electron micrograph. Figure 4. Water-insoluble residue, Tatum Dome, Mississippi. Twinned dolomite rhomb. AEC well, 1513 feet. Scanning electron micrograph. Figures 5 and 6. Water-insoluble residue, Tatum Dome, Mississippi. Double terminated quartz crystals. AEC well, 1513 feet. Scanning electron micrographs. PLATE 3 223 P late 4 The Salt Figure 1. Water-insoluble residue. Hockley Dome, Texas. Tabular (+) and "stem-shaped" (s) anhydrite crystals. Well and depth unknown. Scanning electron micrograph. Figure 2. Water-insoluble residue. Zechstein 2, Germany. Prismatic anhydrite crystals. Exact location unknown. Scanning electron micrograph. Figure 3. Water-insoluble residue, a Nova Scotia Dome, Canada. "Stem-shaped" anhydrite crystals. Depth unknown. Scanning electron micrograph. Figure 4. Water-insoluble residue, Hutchinson bedded salt, Kansas. Tabular and prismatic anhydrite crystals. Exact location unknown. Scanning electron micrograph. Figure 5. Water-insoluble residue, bedded salt, Michigan Basin. Irregular and possibly somewhat dissolved anhy’drite crystals. Whisky Island Mine, International Salt Corporation. Scanning electron micrograph. 225 Plate 5 The Caprock Figure 1. Core sample, Jennings Dome, Louisiana, Polished surface of calcite caprock illustrating the rhythmic banding which is distinctive of this lithology. The banding is most surely developed by diffusional processes as indicated by their orientation with respect to each other. The bands are limited in horizontal extent as they terminate either by pinching out or by truncation. Cavities lined with drusy calcite are characteristic of the light bands while the dark bands are usually more dense and may contain considerable pyrite (arrows). Figure 2. Core sample, Minden Dome, Louisiana. Polished surface of calcite caprock showing how the original banding has been truncated by fracturing and later filled with a coarsely-crystalline calcite. Note the irregularities in the dark bands. 226 PLATE 5 Fig. 2 , J cm 227 P late 6 The Caprock Figure 1. Core sample, polished and etched surface, Tatum Dome. Mississippi. Cavity in calcite caprock containing twinned calcite crystals. AEC well, 974 feet. Scanning electron micrograph. Figure 2. Core sample, polished and etched surface, Tatum Dome, Mississippi. A group of celestite crystals radiating from calcite. AEC well, 974 feet. Scanning electron micrograph. Figure 3. Core sample, polished and etched surface, Tatum Dome, Mississippi. Radiating blades of celestite which are believed to have been coprecipitated with the calcite. AEC well, 974 feet. Scanning electron micrograph. Figure 4. Core sample, polished and etched surface, Tatum Dome, Mississippi. Prismatic celestite crystal in calcite matrix. AEC well, 1028 feet. Scanning electron micrograph. PLATE 6 229 P late 7 The Caprock Figure 1. Core sample, collodion replica of polished and etched surface, Minden Dome, Louisiana. Calcite caprock; contact between light brown (larger crystals) and dark (smaller crystals) bands. Dark bands contain 0. 40 percent sulphur and light brown bands contain 0. 18 percent sulphur. Third order growth lines (arrow) are the outer lim its of optical continuity. Jones no. 1 well, 1185 feet. Optical micrograph, plane polarized light. Figure 2. Core sample, polished and etched surface, Minden Dome, Louisiana. Calcite caprock; rhombic crystals in cavity illustrating third order growth lines(3). Jones no. 1 well, 1185 feet. Scanning electron micrograph. Figure 3. Core sample, collodion replica of polished and etched surface. Minden Dome, Louisiana. Calcite caprock; brown crystals with 0. 18 percent sulphur and numerous second order growth lines (arrow). Jones no. 1 well. 1202 feet. Optical micrograph, plane polarized light. Figure 4. Core sample, fractured surface, Minden Dome, Louisiana. Calcite caprock with 0. 18 percent sulphur. Second order growth lines are normal to the direction of crystal growth. Variable spacing between growth lines probably indicate differing durations of precipitation. Jones no. 1 well, 1202 feet. Scanning electron micrograph. Figure 5. Core sample, carbon replica of polished and etched surface. Minden Dome, Louisiana. Calcite caprock with 0. 18 percent sulphur. Individual scalenohedrons sharing the same crystal- lographic orientation. First order growth lines are some what irregular but trend in a NE-SW direction. Jones no. 1 well, 1202 feet. Transmission electron micrograph. Figure 6. Core sample, carbon replica of polished and etched surface. Minden Dome, Louisiana. Calcite caprock with 0. 18 percent sulphur. The sulphur(s) is found as an external impurity sur rounding the scalenohedron crystallite. Jones no. 1 well, 1202 feet. Transmission electron micrograph. 230 PLATE 7 \ Fig. 2 3 ^ 231 Plate 8 The Caprock Figure 1. Core sample, colloidion replica of polished and etched surface, Minden Dome, Louisiana. Calcite caprock; no sulphur detected, large white, bladed crystals. The dark lines are fractures with apparent secondary recrystal lization. No growth lines are evident. Jones no. 1 well, 1248 feet. Optical micrograph, plane polarized light. Figure 2. Core sample, carbon replica of polished and etched surface, Minden Dome, Louisiana. Calcite caprock with no sulphur detected. First order growth lines trending NE-SW (arrow) between rhombic crystallites. Jones no.. 1 well, 1248 feet. Transmission electron micrograph. Figure 3. Core sample, carbon replica of polished and etched surface, Minden Dome, Louisiana. Enlarged view of above Figure illustrating a first order growth line. Transmission electron micrograph. Figure 4. Core sample, fractured surface, Minden Dome, Louisiana. Calcite caprock with 0. 08 percent sulphur. The fairly large crystals appear to be a combination of rhombohedral and scalenonedral forms. Jones no. 1 well, 1264 feet. Scanning electron micrograph. Figure 5. Core sample, colloidion replica of polished and etched surface, Minden Dome, Louisiana. Calcite caprock con taining 0. 08 percent sulphur showing second order growth lines in bladed crystals. The decreasing frequency of growth lines toward the right of the photo may indicate a decreasing amount of impurity (sulphur^ present during precipitation. Jones no. 1 well, 1264 feet. Optical micrograph, plane polarized light. Figure 6. Core sample, carbon replica of polished and etched surface, Minden Dome, Louisiana. Calcite caprock containing 0. 08 percent sulphur. Enlarged view of second order growth lines (arrows). Jones no. 1 well, 1264 feet. Transmission electron micrograph. 232 PLATE 8 233 Plate 9 The Caprcck Figure 1. Core sample, colloidion replica of polished and etched surface, Minden Dome, Louisiana. Calcite caprock containing 0. 40 percent sulphur. Numerous second and third order growth lines. Jor.es no. 1 well, 1285 feet. Optical micrograph, plane polarized light. Figure 2. Core sample, fracture surface, Minden Dome, Louisiana. Calcite caprock with 0. 40 percent sulphur. Second order growth lines resulting in a "platy" structure in a single crystal. Jones no. 1 well, 1285 feet. Scanning electron micrograph. Figure 3. Enlarged view of above photo showing how the "plates" are separated by second order growth lines. Scanning electron micrograph. Figure 4. Core sample, colloidion replica of polished and etched surface, Minden Dome, Louisiana. Indistinct boundary between calcite containing 0.40 percent sulphur I. 00 percent sulphur (smaller crystals). Note the high frequency of second order growth lines. Jones no. 1 well, 1290 feet. Optical micrograph, plane polarized light. Figure 5. Core sample, fractured surface, Minden Dome, Louisiana. Calcite caprock containing 1.00 percent sulphur. The dominant habit of the smaller crystals is rhombohedral. Jones no. 1 well, 1290 feet. Scanning electron micrograph. Figure 6. Core sample, carbon replica of polished and etched surface, Minden Dome, Louisiana. Calcite caprock with 1. 00 percent sulphur. Individual crystallites separated by first order growth lines which trend in a NE-SW direction (arrow). Jones no. 1 well, 1290 feet. Transmission electron micrograph. Figure 7. Enlarged view of Figure b showing a crystallite surrounded by sulphur impurity (s). Transmission electron micrograph. 234 PLATE 9 235 Plate 10 The Caprock Figure 1. Core sample, fractured surface, Utica Dome, Mississippi. Calcite caprock; second order growth lines appearing as "steps” in fractured c rystal. Lee Little no. 1 well, 1765 feet. Scanning electron micrograph. Figure 2. Core sample, fractured surface, Utica Dome, Mississippi. Calcite caprock; the dolomite rhomb (d) appears to be of the same age as the calcite since no replacement features can be noted. Lee Little no. 1 well, 2765. Scanning electron micro graph. Figure 3. Core sample, thin-section, Allen Dome, Texas. Calcite caprock which has been highly deformed. A minimum of eight microfaults can be observed in the photo. The very dark material is mylonite which was probably produced during the faulting. Fractures have been recemented by secondary calcite. The white specs dispersed in the fine material are clastic grains. F . S. Co. well no. 7, 952 feet. T ransm itted light. Figure 4. Enlarged view of one of the many microfaults in Figure 3. The faulted fibrous material is mylonite partially replaced by calcite. Some fracturing has occured since the last recementing generation. Optical micrograph, crossed nicols. Figure 5. Core sample, thin-section, Allen Dome, Texas. Dolomite (d) in calcite caprock which appears to be epigenetic and replacing the calcite. F. S. Co. well no. 7, 955 feet. Optical micrograph, plane polarized light. Figure 6. Core sample, thin-section, Allen Dome, Texas. Sheared and partially mylonized calcite caprock. The twinning is com mon to shear zones and may indicate the amount and direction of relative force involved in the shearing. F. S. Co. well no. 7, 956 feet. Optical micrograph, plane polarized light. 236 PLATE 10 237 P late U The Caprock Figure 1. Core sample, thin-section, Allen Dome, Texas. Brecciated sandy calcite caprock with a minimum of three generations of calcite. No. 1 is the oldest generation and is probably the original matrix. No. 2 generation is both older and younger than No. 3 since this is the recrystallized calcite. Generation No. 3, which is the coarsest, is the recementing calcite after fracturing occurred. F. S. Co. well no. 7, 952 feet. Optical micrograph, crossed nicols. Figure 2. Core sample, thin-section, Allen Dome, Texas. Brecciated calcite caprock containing many clastic grains between clasts. Dolomite rhomb (left, d) has a halo of recryttal- lized calcite. F. S. Co. well no. 7, 956 feet. Optical micro graph, crossed nicols. Figure 3. Core sample, thin-section, Bruinsburg Dome, Mississippi. Shear zone in calcite caprock containing many dolomite rhombs and quartz crystals. On either side of this zone the rock has been highly mylonized and consists of a fine mylonite meal. F. S. Co. Hammett no. 2 well, 2000 feet. Optical micrograph, crossed nicols. Figure 4. Core sample, thin-section, Black Bayou Dome, Louisiana. Deformation of calcite caprock resulting in microfolding. F. S. Co. Watkins no. 5 well, 1390 feet. Optical micrograph, crossed nicols. Figure 5. Core sample thin-section, Black Bayou Dome, Louisiana. Euhedral pyrite crystals of the same age as the recementing calcite. F. S. Co. Watkins no. 5 well, 1405 feet. Optical micrograph, crossed nicols and reflected light. Figure 6. Core sample, Black Bayou Dome, Louisiana. Polished surface of calcite caprock. Calcite crystals (upper) lining one side of cavity and pyrite crystals lining the other side. Pyrite is the same age or possibly younger than the calcite. F. S. Co. Watkins no. 5 well, 1405 feet. Optical micrograph, reflected light. 238 PLATE II i# .» S 239 P late 12 The Caprock Figure 1. Core sample, thin-section, Oakley Dome, Mississippi. Organic stained celestite adjacent to calcite recemented fracture indicating hydrocarbon movement prior to recementation. Blanco and Fairchild no. 1 Shuff, 2324 feet. Optical micrograph, crossed nicols. Figure 2. Core sample, thin -section, Clemens Dome, Mississippi. Alternating bands of celestite (upper and lower portions) and calcite (central portion) which have been fractured, re cemented by calcite and fractured again. T. S. S. Co. Proebstle no. 3 well, 828 feet. Optical micrograph, crossed nicols. Figure 3. Core sample, thin-section, Kings Dome, Mississippi. Celestite caprock; the celestite ( •) is fibrous in habit, and forms the matrix of the rock in which dolomite rhombs (d) are dispersed. Hall no. 1 well, 3761 feet. Optical micrograph, crossed nicols. Figure 4. Core sample, fractured surface, Kings Dome, Mississippi. Celestite caprock. Hall no. 1 well, 3691 feet. Scanning electron micrograph. Figure 5. Core sample, thin-section, Utica Dome, Mississippi. Radial clusters of bladed celestite. Some of the celestite has been partially replaced by calcite.. Lee Little no. 1 well, 2795 feet. Optical micrograph, c rossed niuols. Figure 6. Core sample, thin-section, Utica Dome, Mississippi. Banded celestite and calcite caprock which has been slightly fractured and recemented by calcite. The matrix in this portion of the thin - section is dominantly bladed celestite. Lee Little no. 1 well, 280.5 feet. Optical micrograph, crossed nicols. 240 241 P late 13 The Caprock Figure 1. Core sample, thin section, Allen Dome, Texas. Calcite caprock; barite (b) which originally replaced calcite is now partially being replaced by calcite. F. S. Co. no. 7 well, 948 feet. Optical micrograph, crossed nicols. Figure 2. Core sample, polished and etched surface, Minden Dome, Louisiana. Calcite caprock; barite cutting across growth lines which indicates that it has replaced calcite. Jones no. 1 well, 1284 feet. Scanning electron micrograph. Figure 3. Core sample, polished and etched surface, Minden Dome, Louisiana. Calcite caprock; barite contemporaneously precipitated with calcite as indicated by the barite being along growth lines and the lack of replacement features. Jones no. 1 well, 1185 feet. Scanning electron micrograph. Figure 4. Enlarged view of above photo showing the morphology of the barite. Scanning electron micrograph. PLATE 13 243 P late 14 The Caprock Acid-In soluble Residue of Calcite Caprock Figure 1. Lake Washington Dome, Louisiana. Quartz rosette with dark nuclei. F. S. Co. no. 879 well, 1450 feet. Optical micrograph, low index oil, plane polarized light. Figure 2. Lake Washington Dome, Louisiana. Tabular barite crystal with anhydrite inclusions. F. S. Co. no. 879 well, 1450 feet. Optical micrograph, plane polarized light. Figure 3. Black Bayou Dome, Louisiana. Barite rosette. F. S. Co. Watkins no. 5 well, 1400 feet. Optical micrograph, plane polarized light. Figure 4. Black Bayou Dome, Louisiana. Radiating cluster of pyritohedrons. F. S. Co. Watkins no. 5 well, 1400 feet. Optical micrograph, reflected light. Figure 5. Black Bayou Dome, Louisiana. Doubly terminated quartz crystal. F. S. Co. Watkins no. 5 well, 1389 feet. Optical micrograph, low index oil, plane polarized light. Figure 6. Black Bayou Dome,Louisiana. Complexly twinned anhydrite crystal. F. S. Co. Watkins no. 5 well, 1389 feet. Optical micrograph, low index oil, plane polarized light. Figure 7. Black Bayou Dome,Louisiana. Radiating cluster of amorphous sulphur. F. S. Co. Watkins no. 5 well, 1389 feet. Optical micrograph, low index oil, reflected and plane polarized light. 244 PLATE 14 Fig. 7 Fig. 6 (I0 P la te 15 The Caprock Figure 1. Core sample, thin-section, Allen Dome, Texas. Photograph of entire thin-section of impure (many clastic grains) gypsum caprock. An indistinct microfault traverses the entire section vertically in the center of the photo. Some mylonization and a great deal of "rock flowage" can be observed. F. S. Co. no. 1 well, 929 feet. Transmitted light. Figure 2. Core sample, thin-section, Allen Dome, Texas. Slightly mylonized gypsum caprock containing fibrous gypsum cementing clastic grains in fracture. F. S. Co. no. 1 well, 998 feet. Optical micrograph, crossed nicols. Figure 3. Core sample, thin-section, Allen Dome, Texas. Shear zone in gypsum caprock with fracturing through coarse carbonate crystal. Criculating waters in the fracture has probably resulted in its partial dissolution. F. S. Co. no. 1 well, 998 feet. Optical micrograph, crossed nicols. Figure 4. Core sample, thin-section, Allen Dome, Texas. Sheared gypsum which has been partially replaced by calcite. Dark area in carbonate crystal is a void. F. S. Co. no. I well, 1069 feet. Optical micrograph, crossed nicols. 246 PLATE 15 247 P la te 16 The Caprock Figure 1. Core sample, thin-section, Allen Dome, Texas. Dolomite rhomb in gypsum caprock. The rhomb has a calcite envelope surrounding it and a nucleus of a light colored opaque material. This dolomite may be syngenetic with the gypsum. F. S. Co. no. 1 well, 1069 feet. Optical micrograph, crossed nicols. Figure 2. Core sample, thin-section, Allen Dome, Texas. Coarse carbonate crystal in gypsum caprock. The dark nucleus may have been derived from organic matter. F. S. Co. no. 1 well, 921 feet. Optical micrograph, crossed nicols. Figure 3. Sodium thiosulphate-insoluble residue, Allen Dome, Texas. Coarse calcite crystal from the same interval as above Figure showing the dark nucleus. Optical micrograph, low index oil, plane polarized light. Figure 4. Core sample, thin-section, Allen Dome, Texas. Aragonite which has partially altered to calcite in gypsum caprock. F. S. Co. well no. 1, 998 feet. Optical micrograph, plane polarized light. Figure 5. Core sample, thin-section, Allen Dome, Texas. Barite in gypsum caprock. No replacement features were noted. F. S. Co. no. 1 well, 998 feet. Optical micrograph, plane polarized light. 248 P L A T E 16 249 P late 17 The Caprock Figure 1. Core sample, Lake Washington Dome, Louisiana. Gypsum caprock in sulphur ore zone. This sample contains about 20 percent of orthorhomibic sulphur crystals (S). Gypsum crystals (G) average about 6 mm. in their long dimension. The sulphur is largely in cavities although a smaller percentage is along grain boundaries. F. S. Co. , well and depth unknown. Figure 2. Core sample, Minden Dome, Louisiana. Gradational contact between calcite caprock and overlying clayey sediments. The sediment show a high degree of replacement by the primary fine-grained calcite. Secondary calcite (white, coarse crystals) penetrates the sediments for an undetermined distance. Taylor (1938) stated "that the top of the caprock often must be estimated because of the false caprock which grades into it", as is clearly shown in this photo. Jones no. 1 well, 1160 feet. 250 251 P la te 18 The Caprock Figure 1. Core sample, thin-section, Minden Dome, Louisiana. Massive anhydrite caprock in which the crystals form an interlocking mosaic, resulting in low porosity, and exhibiting a slight preferred orientation. Jones no. 1 well, 1304 feet. Optical micrograph, crossed nicols. Figure 2. Core sample, fractured surface, Minden Dome, Louisiana. Anhydrite crystal with precipitated features. Jones no. 1 well, 1485 feet. Scanning electron micrograph. Figure 3. Core sample, thin-section, Kings Dome, Mississippi. Massive anhydrite caprock with no apparent preferred orientation of crystals. Crystal size in this one sample varies from 0. 5 to 1.8 mm in their long dimension. Hall no. 1 well, 3780 feet. Optical micrograph, plane polarized light. Figure 4. Hand specimen, Choctaw Dome, Louisiana. Anhydrite crystals brought up to the surface by a brine well. The crystals were left on the surface for an undetermined amount of time, and have since been cemented together by gypsum. Optical micrograph, plane polarized light. Figures 5 and 6. Fractured surface of anhydrite resid\ie from the Choctaw Dome, Louisiana. The crystals left on the surface show some dissolution features and are cemented together only where they are in contact with each other. Scanning electron micrographs. 252 253 P late 19 The Caprock Micrographs on this Plate represent the massive anhydrite caprock. They are all fractured surfaces of cores from the Tatum Dome, Mississippi, AEC well. Scanning electron micrographs. Figure 1. Contact between two anhydrite crystals. 1050 feet. Figure 2. Precipitation features. 1210 feet. Figure 3. Coalescence of crystals has resulted in fairly indistinct boundaries. 1324 feet. Figure 4. Contact between two anhydrite crystals. 1465 feet. Figure 5. Contact between two anhydrite crystals. 1500 feet. Figure 6. Fractured and crushed core. 1502 feet. 254 PL A T E 19 255 Plate 20 The Caprock-Salt Interface The morphologic changes, with depth, in the anhydrite caprock-salt interface zone of the Tatum Dome, Mississippi, AFC well, were examined in detail with the aid of a scanning electron microscope. The massive anhydrite grades downward into increasingly unconsolidated anhydrite at about a depth of 1505 feet, and finally into the salt at 1510 feet. This progressive sequence is now presented in Plates 20, 21 and 22. Figures 1 and 2. 1506 feet. The rock is only slightly friable, crystals show a prismatic habit and only precipitation features are present. Fractured core, scanning electron micrographs. Figure 3. 1507 feet. Friable anhvdrite caprock. Note large amount, of pore space between grains and the lack of any preferred orientation. Thin-section, optical micrograph, plane polarized light. Figures 4 and 5. 150 7 feet. The friable rock consists of "stem shaped" anhydrite crystals which show dissolution features. Fractured core, scanning electron micrographs. Figure 6. 1507 feet. Diagnostic dissolution feature resulting from crystallographicaliy controlled solubility. Fractured core, scanning electron micrograph. 256 P late 21 The Caprock-Salt Interface Figure 1. 1509 feet. Unconsolidated crystals, representing the "anhydrite sand" which is characteristic of this interface. Thin-section, optical micrograph, plane polarized light. Figures 2, 3 and 4. 1509 feet. The crystals show a fairly high degree of dissolution and they are all of the "stem-shaped" variety. Fractured core, scanning electron microscope. Figure 5. 1510 feet. Insoluble residue from the salt. "Stem shaped" anhydrite crystals only, no dissolution features can be noted. 1.0 mm. and greater size class. Scanning electron micrograph. Figure 6. 1510 feet. Insoluble residue, 0.5 to 1.0 mm. size class. Scanning electron micrograph. 258 PL A T E 21 259 Plate 2? The Caprock-Salt Interface Figure 1. 1510 feet. Insoluble residue. 0. 25 to 0.5 mm. size class. Dolomite rhombs first appear in this size class. Scanning electron micrograph. Figure 2. 1510 feet. Insoluble residue. 0. 125 to 0. 25 mm. size class. Scanning electron micrograph. Figure 3. i510 feet. Insoluble residue. 0.063 to 0. 125 m m . size class. Scanning electron micrograph. F igures 4 and 5. 1510 feet. Insoluble residue. 0.063 to 0. 125 m m . size class. "Stem-shaped" anhydrite crystals with diagnostic crystallite terminations. Scanning electron micrograph. Figure 6. 1510 feet. Insoluble residue. Less than 0.063 mm. This size contains, besides anhydrite, doubly terminated quartz crystals (Q), dolomite and plagioclase feldspar as determined by x-ray diffraction. Scanning electron micrograph. 260 PLATE 22 Fig. 1 Fig. 2 ,100^ JjdS tig. 4 i Fig. 6 ,30^ 2 6 1 P la te 23 The False Caprock Figure 1. Core sample, thin-section, Black Bayou Dome, Louisiana. Clastic grains which have been partially to almost completely replaced by the calcite cement. F. S. Co. Watkins no. 5 well, 1132 feet. Optical micrograph, crossed nicols. Figure 2. Core sample, thin-section. Black Bayou Dome, Louisiana. Fibrous gypsum (C) replacing the calcite cement and calcite replacing the clastic grains. F. S. Co. Watkins no. 5 well, 1212 feet. Optical micrograph, crossed nicols. Figure 3. Core sample, thin-section, Black Bayou Dome, Louisiana. Fractured quartz grain cemented in calcite. The fractures have beer, healed by calcium carbonate rich solutions of a later generation than the cement. F. S. Co. Watkins no. 5 well, 1212 feet. Optical micrograph, crossed nicols. Figure 4. Core sample, thin-section, Black Bayou Dome, Louisiana. Fibrous anhydrite (a) which is probably the same age as the cementing culcite. Euhedral quartz crystal (Q) is very similar to those crystals found in the true caprock. Note the high degree of replacement of the elastics by calcite. F. S. Co. Watkins no. 5 well, 1381 feet. Optical micrograph, crossed nicols. Figure 5. Core sample, thin-section, Black Bayou Dome, Louisiana. Organic matter entrapped during various stages of calcite crystal growth. F. S. Co. Watkins no. 5 well, 1103 feet. Optical micrograph, crossed nicols. Figure 6. Core sample, thin-section. Black Bayou Dome, Louisiana. Authigenic halite (h), having perfect cubic cleavage, which has crystallized in a void in the calcite cement. The mineral is probably restricted to void filling since it is thought that halite does not replace any of the other minerals. Halite is found very rarely in thin- section due to its high solubility. Dark areas are pyrite. F. S. Co. Watkins no. 5 well, 1242 feet. Optical micrograph, plane polarized light. 262 P L A T E 23 263 P la te 24 The False Caprock Figure 1. Core sample, thin-section. Black Bayou Dome, Louisiana. Fibrous and granular gypsum cementing clastic grains. The large amount of pyrite (dark) present is re placing the gypsum to a small degree. F. S. Co. Watkins no. 5 well, 1361 feet. Optical micrograph, crossed nicols and reflected light. Figure 2. Core sample, thin-section. Black Bayou Dome, Louisiana. Photograph of entire thin-section showing highly mylonized fragments of probable true caprock origin. The top fibrous portion of the largest fragment is anhydrite which has been almost completely replaced by calcite. The dark, almost opaque, area is dominantly recrystallized quartz with some calcite. Transmitted light. Figure 3. Core sample, thin-section, Allen Dome, Texas. Boundary between siltstone fragment (right) from some other portion of the false caprock, and the presumably "in situ" larger elastics. Recementation and replacement by calcite has made this boundary indistinct. F. S. Co. no. 1 well, 776 feet. Optical micrograph, crossed nicols. Figure 4. Core sample, thin-section, Allen Dome, Texas. Bladed, authigenic barite which has grown in an open fracture in calcite cemented mud. Roxana Petrol. Corp. Randon no. 2 well, 4790 feet. Optical micrograph, crossed nicols. Figure 5. Core sample, thin-section, Allen Dome, Texas. Bent and fractured authigenic anhydrite crystals accompanied by slight mylonization. These crystals show low interference colors between crossed nicols. F. S. Co. no. 7 well, 920 feet. Optical micrograph, crossed nicols. Figure 6. Core sample, thin-section, Bruinsbury Dome, Mississippi. Authigenic dolomite replacing the finely-crystalline calcite cement. F. S. Co. Hammett no. 2 well, 1285 feet. Optical micrograph, plane polarized light. 264 PL A T E 24 Fig. 6 ,300a , 265 P late 25 Dissolution Experiment The solubility of anhydrite is different for each of the three crystal faces, resulting in the peculiar, but diagnotic, shapes formed during dissolution of the crystals. The starting material for this experiment was obtained from the 0. 5 mm fraction of the Tatum Dome, Mississippi insoluble residue, AEC well, 1510 feet. Scanning electron micro graphs. Figure 1. Crystals removed after three days of dissolution. Figure 2. Crystals removed after seven days of dissolution. Figures 3 and 4. Crystals removed after 10 days of dissolution. Similar features were observed up to the point where ail the anhydrite was dissolved. 266 PL A T E 25 267 P late 26 Precipitation Experiment Certain features have been determined as being diagnotic of precipi tation. The mineral precipitated was gypsum, but similar features are observed in anhydrite crystals. Scanning electron micrographs. Figure 1. Crystals with pronounced growth lines indicate precipitation. Figure 2. Differential growth rates due to crystallographically controlled solubility result in platy type structures which are characteristic of precipitation. Figure 3. Needle-shaped gypsum protruding from massive gypsum. Figure 4. Carbonate crystal precipitated due to the presence of CO^ during the experiment. 268 PL A T E 26 VITA Charles William Walker, the son of Charles Weir Walker and Evelyn Cadieux Walker, was born on April 17, 1940 in Chicago, Illinois. Mr. Walker moved to Highland Park, Illinois at the age of four and g rad u a ted from. H ighland P ark High School in 1958. Mr. Walker attended Southern Illinois University fr 1962 to 1966 and received a B. A. degree in geology. In June, 1966, he m arried Jean Alice Duffy, also from Highland Park, Illinois. Mr. Walker received an assistantship and attended The University of M ississippi in Oxford, where he was awarded the M. S. degree in 1968. After receiving this degree, he taught geology at the University for one semester. Mr. and Mrs. Walker became the parents of a daughter, Christine Leigh, in 1967. In 1968, M r. Walker began working toward the Doctor of Philosophy degree at Louisiana State University, Baton Rouge. He was awarded , -esearch assistantship in the Institute of Saline Studies. In 1970, Mr. Walker and his family moved to Socorro, New Mexico where he began work for the New Mexico State Bureau of Mines and Mineral Resources a* Mineralogist. EXAMINATION AND THESIS R E F °RT Candidate: Charles William Walker Major Field: G eology Title of Thesis: The Nature and Origin of Caprodk Overlying Gulf Coast Salt Domes. Approve^: y M ^i<^rofessor and Cliairman of the Graduate School E xA iyfINING COMMITTEE: _ Qy ^ Qv- )lAjrzn^J\. Date of Examination: January 20. 1972