REPLACEMENT DOLOMITIZATION IN THE UPPER DEVONIAN LEDUC AND SWAN BILLS FORMATIONS, CAROLINE ARE A, ALBERTA, CANADA
By Andre K. Laflamme
Department of Geological Sciences McGill University, Montreal
A Thesis Submitted to the Faculty of Graduate Studies and Research in Partial Fulfillment of the Requirements for the Degree of
Master of Science
September, 1990 © Andre K. Laflamme
( ( ABSTRACT
Replacement matrix dolomite (RMD) is present throughout the Leduc/Cooking Lake Formation, but is restricted to the bank margin in the Swan Hills Formation. RMD has an average crystal size of 140J,Lm and ftuoresces yellow in the Leduc/Cooking Lake Formation but has an average crystal size of 250jlm and fluoresccs green in the Swan Hills Formation. RMD formed before the onset of stylolitization. It has abundant inclusions, a homogeneous red cathodoluminescent response, and simiJar trace element concentrations in both formations. Ô 18 0 values overlap for the most part, but sorne Swan
Hills dolomites are 1 to 2 %0 lighter in ~P80. The Leduc/Cooking Lake dolomite has a slightly greater range in Ô I3C, with several samples lighter than +1.5 % 0. Diagenetic fluids derived from seawater are supported by average ô13 C, ô 18 Q (1.77%0, -4.19%0; 2.810/00, -4.95 % 0), and 87S r/86Sr (.7082; .7086) values in the Leduc/Cooking ,f Lake and Swan Hills dolomite respectively. Yellow fluorescence and '\ pyrolysis parameters in the Leduc/Cooking Lake dolomite could be caused by the presence of mature organic compounds.
III .t RESUME
La dolomie de remplacement est présente dans tous les faciès récifaux de la formation du Leduc/Cooking Lake mais restreinte aux marges récifales dans la formation du Swan Hills. Dans la formation du Leduc/Cooking Lake les cristaux de ce type de dolomie ont une grosseur moyenne de 140~m et sont jaunes lorsqu'observés sous fluorescence alors que dans la formation du Swan Hills ils ont une grosseur moyenne de 250~ m et sont de couleur vert sous fluorescence. La dolomie de remplacement s'est développée avant la stylolitisation, possède de nombreuses inclusions, une couleur rouge homogène sous cathodoluminescence ainsi que des concentrations d'éléments en trace similaires dans les deux formations. Les valeurs de ~ 18 0 sont, pour la plupart, semblables pour les deux formations, , cependant certains cristaux de dolomie sont appauvris de 1 à 2°/0 ° 1 ! li en ~180 dans la formation du Swan Hills. Dans la formation du 1
1 Leduc/Cooking Lake, les valeurs de la dolomie de remplacement sont '1 un peu moins groupées, plusieurs échantillons étant plus légers que +1.5% 0. Les données de ~13C. BlsQ (1.77% 0, -4.19% 0; 2.81 % 0, -4.95 % 0) ainsi que de 87Sr/86Sr (.7082; .7086), dans les formations du Leduc/Cooking Lake et du Swan Hills respectivement, indiquent que les fluides diagénétiques sont derivés de l'eau de mer. La couleur jaune observée en fluorescence ainsi que les paramètres de pyrolyse pourraient indiquer la presence d'hydrocarbures dans la formation du Leduc/Cooking Lake.
IV TABLE OF CONTENTS
INTRODUCTION • • • • • • • • • • • • • • • p.l GEOLOGiCAL SETTING " • • • • • • • • • • • • p.4 PREVIOUS RESEARCH • • • • • • • • • • • • • P.7 RESEARCH METHODS • • • • • • • • • • • • • p.9
PETROGRAPHY LEDUC/COOKING LAKE FORMATION • • • • • • • • p.12 -PETROGRAPHY • • • • • • • • • • • • • • • • p.15 Replacement matrix dolomite • • • e • • • • • • • p.15 cathodoluminescence • • • • • • • • • • p.18 fluorescence • • • • • • • • • • • • • p.18 Saddle dolomite • • • • • • • • • • • • • • • • p.25 Anhydrite· • • • • • • • • • • • • • • • • • p.25 Calcite cements • • • • • • • • • • • • • • • • p.28 Pyri te ••••••••••• • • • • • • • p.28 Sphalerite' • • • • • • • • • • • • • • • • • p.33 Sulfur • • • • • • • • • • • • • • • • • • p.33 Paragenetic sequence • • • • • • • • • • • • • • p.33
SWA~ HILLS FORMATION • • • • • • • • • • • p.35 -PETROGRAPHY • • • • • • • • • • • • • • • • p.38 Replacement matrix dolomite • • • • • • • • • • p.38 cathodoluminescence • • • • • • • • • • pAl fi uorescence • • • • • • • • • • • • • pA4 Saddle dolomite • • • • • • • • • • • • • • • • p.51 Calcite •••••••• • • • • • • • • • • p.54 Anhydrite' • • • • • • • • • • • • • • • • • p.57 Sulfur • • • • • • • • • • • • • • • • • • p.57 Botryoidal dolomite • • • • • • • • • • • • • • p.60 Celestite • • • • • • • • • • • • • • • • • • p.60 Paragenetic sequence • • • • • • • • • • • • • • p.63
v -~ GEOCUEMISIBî -TRACE ELEMENT ANALYSES Replacement matrix dolomite • • • • • • • • • p.69 Variations • • • • • • • • • • • ~ • • • p.74 Interpretations • • • • • • • • • • • • • • p.74 -ISOTOPIC ANAL YSES Carbon and oxygen isotopes Replacement matrix dolomite • • • • • • • • • p.79 Interpretations • • • • • • • • • • • • p.84 Saddle dolomite • • • • • • • • • • • • • • p.86 Strontium isotopes Replacement matrix and saddle dolomite • • • • • p.88 Limestone • • • • • • • • • • • • • • • • p.92 COMPARISON OF LEDUC/COOKING LAKE AND SWAN HILLS REPLACEMENT MATRIX DOLOMITE-SUMMARY • p.92
..... DISCllSSUllS ,. DISTRIBUTION OF DOLOMITE • • • • • • • • • • • P.94 NATURE AND SOURCES OF THE DIAGENETIC FLUIDS • • • • p.98 RECRYSTALLIZA TION • • • • • • • • • • • • • • p.99 MECHANISMS OF PALEOFLUID FLOW • • • • • • • • • p.100 I-Burial Compaction • • • • • • • • • • • • p.100 2-Thermal Convection • • • • • • • • • • • • p.104 3-Topography Driven • • • • • • • • • • • • p.105 4-Reflux • • • • • • • • • • • • • • • • p.105 5-Seismic Pumping • • • • • • • • • • • • p.l05 FLUORESCENCE-FLAME IONIZATlON DETECfION • • • • • P.I06 SUMMARY • • • • • • • • • • • • • • • • • p.112
HYPOTHETICAL DOLOMITIZATION MODELS. • • • • • • p.1 13 TIlERMOCHEMICAL SULPHA TE REDUCTION (TSR)· • • • • p.121
CONCLUSIONS • • • • • • • • • • • • • • • • p.123 REFERENCES· • • • • • • • • • • • • • • • • p.125
VI · t LIST OF FIGURES
Fig. 1 Study area and weil locations of cores. • • • • p.2 Fig. 2 Distribution of Leduc and Swan Hills buildups. • p.3 Fig. 3 Devonian stratigraphy in the subsurface of western Canada. • • • • • • • • • • • p.5 Fig. 4 Core photographs of replacement matrix dolomite in the Leduc/Cooking Lake Formation. p.14 Fig. 5 Petrographic characteristics of replacement matrix dolomite in the Leduc/Cooking Lake Formation. • • • • • • • • • • • • • p. 1 7 Fig. 6 Cathodoluminescence of replacement matrix dolomite in the Leduc/Cooking Lake Formation. p.20 Fig. 7 Fluorescence of replacement matrix dolomite in the Leduc/Cooking Lake Formation. -1- •••• p.22 Fig. 8 Fluorescence of replacement matrix dolomite in the Leduc/Cooking Lake Formation. -2-· • • • • p.24 Fig. 9 Cathodoluminescence and fluorescence of saddle dolomite and anhydrite in the Leduc/Cooking Lake Formation. ••••••••••• p.27 Fig. 10 Petrographic evidence of thermochemical sulphatf: reduction (TSR) in the Leduc/Cooking Lake Formation. • • • • • • • • • • • • • p.30 Fig. Il Sphalerüe, pyrite, and dolomitized submarine cements(?) in the Leduc/Cooking Lake Formation. • • • • • • • • • • • • • p.32 Fig. 12 Paragenetic sequence of the Leduc/Cooking Lake Formation. •• • • • • • • • • • • p.34 Fig. 13 Core photographs of replacement matrix dolomite in the Swan Hills Formation. • • • • • • • p.37 Fig. 14 Petrographic characteristics of replacement matrix dolomite in the Swan Hills Formation. • • • • p.40 Fig. 15 Cathodoluminescence of replacement matrix r dolomite in the Swan Hills Formation. • • • • p.43
VII t Fig. 16 Fluorescence of replacement matrix dolomite ln the Swan Hills Formation. -1- • • • • • • • p.46 Fig. 17 Fluorescence of replacement matrix dolomite 10 the Swan Hills Formation. -2-· • • • • • • p.48 Fig. 18 Fluorescence of replacement matrix dolomite 10 the Swan Hills Formation. -3-· • • • • • • p.50 Fig. 19 Petrographie characteristics of saddle dolomite in the Swan Hills Formation. • • • • • • • • p. 53 Fig. 20 Petrographie characteristics of calcite 10 the Swan Hills Formation. • • • • • • • • • p.56 Fig. 21 Petrographie evidenee of Thermochemical Sulphate Reduction (TSR), in the Swan Hills Formation. • p. 5 9 Fig. 22 Petrographie characteristics of neomorphosed, botryoidal dolomite and celestite in the Swan Hills Formation. • • • • • • • • • p.62 Fig. 23 Stylolitization in the Swan Hills Formation.· • • p.65
~-, Fig. 24 Petrographie relationships between saddle dolomite and blocky calcite in the Swan Hills Formation.·· • • • • • • • • • • • • p. 6 7 Fig. 25 Paragenetic sequence of the Swan Hills Formation. p.68 Fig. 26 Sr concentrations vs stoichiometry of replacement matrix dolomite. • • • • • • • • • • • p. 77 Fig. 27 Trace element concentrations in Cooking Lake Formation weil 7 -34-33-6W5· • • • • • • p. 78 Fig. 28 ô18Q and ô13 C of replacement matrix dolomite, and sadd le dolomite. • • • • • • • • • • • p. 8 1 Fig. 29 ô18 Q and ô 13C for dolomite, calcite, and limestone. •• • • • • • • • • • • • p. 8 2 Fig. 30 Replacement matrix dolomite C and 0 values showing the lack of trends in different wells.· • p. 8 3 Fig. 31 87Sr/86Sr values for dolomite, calcite, and limestone. •• • • • • • • • • • • • p. 8 9 Fig. 32 87S r/86Sr vs Sr concentrations (ppm) in replacement matrix and saddle dolomite.· • • p. 90
VIII r
Fig. 33 BIi~O and BI3C vs 87S r/S6Sr in replacement matrix dolomite .• • • • • • • • • • • • • • p.91 Fig. 34 Comparison of characteristics of Swan Hills and Leduc/Ctwking Lake replacement matrix dolomite. p.93 Fig. 35 Swan Hills southwest-northeast cross-section.· • p.95 Fig. 36 Leduc east-west cross-section. • • • • • • • p.96 Fig. 37 Leduc north-south cross-section. • • • • • • p.97 Fig. 38 Buriai hislory curve for the Swan Hills Formation. p.l Dl Fig. 39 T(OC) vs BISO of replacement matrix dolomite.. • p.103 Fig. 40 Diagram showing the relative proportions of yellow and green fluorescence in the Leduc/Cooking Lake and Swan Hills repbcement matrix dolomite. • p.l07 Fig. 41 Change in fluorescence response with increasing maturity of hydrocarbons in replacement matrix dolomite. • • • • • • • • • • • p.l09 Fig. 42 Pyrolysis parameters in replacement matrix dolomite.· • • • • • • • • • • • • • p.l1l Fig. 43 Interpreted fluid circulation in the Swan Hills Formation. • • • • • • • • • • • • • p.114 Fig. 44 Seawater fluid circulation patterns in the Swan Hills Formation. • • • • • • • • • p.l1S Fig. 45 Interpreted fluid circulation In the Leduc/ Cooking Lake Formation. • • • • • • • • p.116 Fig. 46 Distribution of the Leduc/Cooking Lake and the Swan Hills Formations at Caroline and Leduc- Rimbey in the subsurface. • • • • • • • • p.119 Fig. 47 Flow pathways of the dolomitizing tluids in the Swan Hills and Leduc/Cooking Lake Formations. p.120
IX .' LIST OF TABLES
Table 1 List of wells. • • • • • • • • • • • • • p.IO Table 2 Major/trace element and isotope summary for replacement matrix dolomite. • • • • • p.70 Table 3 Major/trace element and isotope summary for calcite and limestone. • • • • • • • • • p.72 Table 4 Major/trace element and isotope summary for saddle dolomite. • • • • • • • • • • • p.73 Table 5 Geochemical averages for replacement matrix dolomite. • • • • • • • • • • • • • • p.80 Table 6 Geochemical averages for saddle dolomite. • • • p.87
,'.
x ACKNOWLEDGMENTS
1 am very grateful to Dr. E.W. Mountjoy. It has been a privilege to have worked with him and to have been able to discuss questions, observations, and problems relevant to carbonate diagenesis. His knowledge of the regional geology of the Devonian was especially helpful. 1 am also greatly indebted to Shell Canada Ltd, especially the geological research group, for providing financial, technical. and logistical support for this research project. Analytical services provided by Shell and Dr. R. Krouse's laboratory at the University of Calgary are greatly appreciated. It is impossible to name everyone who has contributed to my understanding of carbonate diagenesis, but enlightening discussions with Christian Viau, Michael Fawcett, Kathy Aulstead. Johannes Thiessen, Jan Hutcheon, Paul Fejer, and Dan Potocki were more th an helpful. 1 would especially like to thank : ... Christian Viau who provided guidance and insight throughout the study and who put up with me for so long white in Calgary! 1 enjoyed geological and less geological chats with Elizabeth, Geoff. Kevin, and Mike. My stay at McGill was made more enjoyable by having made new friends such as Hairuo, Chao, Roland, Steve, Alphonso, Dan, and Dave just to name a few. 1 would finally like to thank my family for their continued support. Additional financial assistance has been provided by the Carbonate Research Fund and NSERC to Dr. E.W. Mountjoy, and the American Association of Petroleum Geologists.
" -li\.
XI INTRODUCTION
Pervasive dolomitization in the Caroline and Bearberry/Leduc Rimbey/Harmattan reservoirs is the focus of this study. The objective of this research project is to 1) document, petrographically and geochemically, the dolomite in the Swan Hills Formation at Caroline and in the stratigraphically higher Leduc Formation al Bearberry, Leduc-Rimbey, and Harmattan, and 2) to dctermine whether or not tüe dolomite present in both formations is the result of a siuglp, event, or several related or unrelated diagenetic events. Addressing these problems will also help to put constraints on l) the timing, 2) the fluid source(s), and 3) the fluid flow pathway(s) responsible for dolomitization. A systematic comparatIve sLUdy of dolomitization in two overlying stratigraphie units, in one specifie study area, is rarely docurnented. Comparing dolomites frorn different formations and frorn different areas often several hundreds of kilometers apart is often problematic. Studying dolomites, as done in this research project, should minirnize sorne of the uncertainties involved in the dolomitization problem. The study area (Fig. 1) was selected because of relatively good core control in both the Swan Hills and Leduc Formations. Pervasive dolornitization has erased much of the primary depositional textures making facies analysis difficult, especially in the Leduc/Cooking Lake Formation. For this reason the depositional environments will not be addressed. This study, when cornbined with similar studies, should provide useful information enabling us to better understand and predict when and where dolomite, hence potential hydrocarbon reservoirs, rnight be present.
1 (
;l, 3~ 1 7W5 8W5 ~5!~ 4WS35 5 S .. + (JEIJ" .. ~.ŒE 16-10 "-'0 • I·~ r l J~~ ~ ~3,!F • 11-33 ~ C~.. ~ (,9~ 09-22109-24 34 J • 34 34 34 ';1 34 sw-; CO§;!! 7WS Sw.ï 4WS- ~1~• sr ~07;18J ._- ~1~.. 1~1D 11-35 r,T:33 ~ ~ • ~v LL~ l t=-;:...,Oi-2S " .J.~. O~ 33 33 33 33 33 __ 8~ 1'fS 8~ 6~ ,2Z;t ~ 4WS • 1
( 10Km ~---~,~
• LeduclCooking Lake 1 Alberla cored intervals 1 " SwanHills cored intervals t:. Leduc/Cooking Lake and Swan Hills cored interval
Fig. 1 Study area and weil locations of cores.
1( ...
2 .. I\. 3t 3l ~ SW5 4:i-r- ~!,WS.... '" "SW5 .. .. Ï'. •.. I...... r-. Il J .... ~EDUC/RIMBEY .. ~ -...... / • "- 1" 34 Il ~ 34 34 "" ~ 34 • Il / 34 ~W5 8W5 .... SW5 4WS- • ,~ • 1\. J • "'-. .. '\ V
['.. If .. BEARBERRY ~ _t / ~I-- • • i'- ll. r--- '/ • r-....~ ..
:Il 33 • 33' • 33._ 7WS 4WS ~~ 1 ST Hi\R~~-rxA!1, -(
• LeduclCooklng Lake Leduc Buildups cored Int8lVais
If Swan Hllis cored InteNals
ll. LeduclCooklllg Lake and Swan Hilis cored IntaNai
3~ i ~\ 3! 7'N5 "SWS .. 4WS- sr " • 1 • \ , • 1 .. .. • .. \ • 34 li 34 34 34 • ..r\. 34 aW5 7W5 6W5 4WS- • • sr ..\ .. CAROLINE li \ • • ta .. •
33 • w 33 33 33 4~- a~ ~ 6~ S~ • i
Swan Hills Buildups 10Km
Fig. 2 Distribution of Swan Hills and Leduc buildups (shaded areas).
3 GEOLOGIe SETTING
Beaverhill Lake Group
The Caroline Field, located 100 km northwest of Calgary (Fig. 2), contains gas in pervasiveiy dolomitized carbonate sequences of the Swan Hills Formation. The Swan Hills reefs in the Caroline area lie at depths ranging between 3613 and 3770 metres. The Swan Hills Formation is part of the Beaverhill Lake Group (Upper Middle to Lower Upper Devonian; Fig. 3) which also incIudes the carbonaceous basin filling sediments of the Waterways and the Fort Vermilion Formations. Isolated reefs in Swan HiIls, Judy Creek, Carson Creek, Virginia Hills, Ante Creek, Goose River, and Snipe Lake areas are oil producing fields in undolomitized Swan Hills intervals. Carbonate bank-type reefs, in the areas of Kaybob South, Rosevear, Erith, Hanlan, and Caroline are pervasively dolomitized and aligned in a N.W.- S.E. direction (Leavitt, 1968; Fischbuch, 1968; Hemphill et al., 1970; Viau, 1986, 1987; Walls, 1988; Kaufman, 1989). Sedimentation began when a southward transgression of Late Devonian seas onlapped the Peace River Arch. U nder the resulting shallow water conditions, a carbonate platform developed (Murray, 1966; Sheasby, 1971) and reefs started to grow over preexisting topographical highs (Sheasby, 1971; Wendte and Stoakes, 1982; Wendte, 1987; Stoakes, 1988; Kaufman, 1989). The subsiding Western Canada basin was progressively filled by fine sediments of the Waterways during much of the Lower Upper Devonian and this influx of carbonaceous mud continued until the the end of Beaverhill Lake sedimentation (Sheasby, 1971).
Woodbend Group
The Upper Devonian Woodbend Group comprises the carbonate sediments of the Leduc/Cooking Lake reefs and platform and the ( carbonaceous sediments of the Ireton Formation (Stoakes and
4 360 Ma EXSHAW z < Z WABAMUN BIG VALLEY w :E CROSSFIElD """ STEnLER ~ z GRAMINIA·BLUERIDGE 1 z~ CI) CALMAR 1 W C .... Z IRETON c( w () 1- Z -J CO :J LEDUC C C « 0 W -Z 0 ..J 0 ~ DUVERNAY ,\:DUVERNAY~ > w ....~.~~5.~~ .. 0..~5.Z: ...... COOKING LAKE L Cl BASAL COOKING LAKE W ~ WATERWAYS 374 5 SWAN HILLS Ma - J: al SLAVE POINT l W FORT VERMillON ..J * c GILWOOD ...... WAnMOUNTAIN 1 C PRAIRIE· MUSKEG ~ t- l Z KEG RIVER (WINNIPEGOSIS) 1 387 ~ 0 Ma Cl. ------~Q~~~l~~:~------rJ ~ > ...J ..J W ERNESTINA l a: c( LOTSBERG w ------BASAL RED BEDS ~ ------408 Ma *BEAVERHILL Fig.3 Devonian stratigraphy in the subsurface of western Canada (modified from Viau, 1986). 5 ( Wendte, 1987; Fig. 3). The Bearberry, Leduc-Rimbey, and Harmattan Fields (Fig. 2) consist of up to 277m of carbonate sediments of the Leduc/Cooking Lake Formation. These three fields are close to the stratigraphically lower Swan Hills Formation in the Caroline Field. The Woodbend Group was characterized by the development of an extensive carbonate shelf (Klovan, 1964), the Cooking Lake platform. The Cooking Lake platform, subdivided into cycles of deposition by Wendte (1974), was subsequently drowned by a rapid rise in sea level (Stoakes and Wendte, 1987). Presumably depositional highs on the existing Cooking Lake platform provided the initial s;tes for Leduc buildup development. Buildup growth was promoted by episodic sea level rises in the Upper Devonian together with subsidence of the basin. The end of the Woodbend depositional event coincided with an accumulation of fine-grained argillaceous basin filling sediments of the Irelon Formation. ( « 6 r- PREVIOUS RESEARCH Leduc\Cooking Lake Formation Pioneering work on the Leduc/Cooking Lake Formation in Alberta was documented by Layer (1949) just two years after the initial discovery of the Leduc Oil Field. Later publications on the Leduc Formation include Waring t'.nd Layer (1950), Andrichuk (1958), and Illing (1959). Research on the Leduc/Cooking Lake Formation was followed by the work of Klovan (1964), Mossop (1972) and Wendte (~974) on the Redwater reef comp]ex. O'Connor and Gretener (1974) published on differential compaction within the Woodbend Group, while McGillivray and Mountjoy (1975) and McGillivray (1977) published on the Golden Spike reef comple){. Additional work on the Golden Spike reef complex include Walls (1978), Walls, Mountjoy, and Fritz (1979), and Walls (1988). Both Mountjoy (1980) and Stoakes and Wendte (1987) publislled revlew papers. Besides Illing (1959), Wendte (1974), Walls (1978, 1988), Walls et al. (1979), Mattes and Mountjoy (1980), Machel and Mountjoy (1987), and Carpenter and Lohmann (1989) little has been published to help understand the diagenetic events affecting the Leduc/Cooking Lake reef complex. Pervasive dolomitization in the Bearberry, Harmattan and the southern lip of the Leduc/Rimbey trend in the area of Township 33 to 35, Range 4 to 8W5 has not been studied. Swan Bills Formation i ! Several researchers have discussed the sedimentology of the Swan Hills Formation (Murray, 1966; Fischbuch, 1968; Jenik and 1 Lerbekmo, 1968; Leavitt, 1968; Hemphi]J, Smith and Szabo, 1970; Havard and Oldershaw, 1976; Wong and Oldershaw, 1980; Wendte 1 and Stoakes, 1982; Viau, 1986; Kaufman and Meyers, 1988; Kaufman, 1, 1 7 1 1989) but only a handful tried to tackle diagenesis (Wong and Oldershaw, 1981; Walls and Burrowes, 1985; Viau, 1986, 1987; Walls, 1988; Kaufman, Hanson and Meyers, 1988; Kaufman, 1989). Fischbuch (1968), Leavitt (1968), and Wendte and Stoakes (1982) suggested that Swan Hills reef buildups were characterized by distinctive growth stage packages, promoted by eustatic sea level rises, and each growth stage was ended by a sea level drop promoting subaerial exposure and erosion. Rowever Viau (1986) proposed that deep seated faults, not sea level Tises or drops, were mainly responsible for the Swan Hills buildup sedimentation in the Swan Hilis area. Hydrothermal (Viau, 1986, 1987), mixing zone (Walls, 1988; Walls and Burrowes, 1985), and burial (Kaufman and Meyers, 1988; Kaufman, 1989, Machel, 1989) dolomitization models have aIl been evoked to explain localized and pervasive dolomitization and/or dolomite cementation of oil and gas dominated Swan Hills buildups. These sedimentologic and diagenetic models have been 1.. advanced for oil and gas fields in West Central Alberta, north of Township 45. The sedimentology or the diagenesis has not been documented for the Swan Rills Formation south of Township 39 where the newly discovered Caroline gas field is located. ,. 8 t RESEARCH METHODS A study area was selected based on good core control of Ledur/Cooking Lake and Swan Hills intervals (Figs. 1,2). Swan Hills and Leduc/Cooking Lake cores (31)~ in the area of Townships 33 to 35 Range 4 to 8 W5M, were studied in detail (Table 1). Limestone and pervasively dolomitized cores were described, photographed and sampled for retrographic and geochemical analysis. Samples (285) from 31 wells were submitted for thin sections. AIl thin sections were stained using Dickson's (1965) procedure. Thin sections were observed under cathodoluminescence using a Technosyn model 8200 MK II cold cathode luminescence stage mounted on a Leitz Laborlux microscope and under fluorescence using a Zeiss photomicroscope III with a fluorescence attachment. Fluorescence microscope observations were made using a blue-violet filter (400-440, FT 460, LP 470). Selected samples (165) were , ... ,~ analyzed by X-ray diffraction for major elements and clay. Limestone, dolomite, and calcite (77) were submitted for trace element analysis. The ground rock sample was mixed with lithium metaborate in a graphite crucible with a thin wire. The ratio of sample to lithium metaborate was approximately one to five. The graphite crucibles were placed in a cold muffle furnace, heated to 900 oC and fused for approximately half an houT. The molten pellet was poured into a beaker containing 10 ml of 10 % nitric acid. One ml of hydrochloric acid and 10 ml of distilled water were added and the sample stirred magnetically until completely dissolved. The sam pIe was th en diluted to 50 ml and analyzed for elements of interest by inductively coupled plasma spectometry. A National Bureau of Standard sample was routinely run with ail samples. Limestone and dolomite (10) were analyzed for 87Sr/86Sr at the University of Alberta. Aliquots of sample powder weighting 50 to 200 mg were decomposed in a mixture of HF and HN03. Sr was separated by coprecipitation with Ba(N03h and purified by passage through a cation exchange column. Rb was separated and purified by 9 ( TABLE 1 LIST OF WELLS LOCATION WELL NAME FORMATION CORID Swan Oills Leducl INTERVALS Cooking Lake (m) (m) (m) 06-20-33-4WS SIIELL CAROLINE 36368-3712.8 362S.0-3676 4 OS -32-33 -4W5 SIIELL CAROLINE 3687.0-3771.0 3691.0-3741.0 07-13-33-5WS HB ET AL WEST HO 3411.3-3439.4 3398.S-3439.4 12-21-33-6W5 SHEll IDN SUP llEARBERRY 37576-4024 fi 3768.9-3818.2 07 -34-33 -6W5 OOME ET AL LOBLEY 3985.9-4082.2 3825.2-3977 6- 3956.3-4025.8 11-33-33-6W5 MOBIL ET AL LOBLEY 38923-4029.S· 3960.6-3969.7 11-3S-33-7W5 MOBIL GPO BANNER JAMES 3922.8-4200 2 3920.6-3931.9 07-18-34-4W5 SnELL CAROLINE 3645.0-372S.0 36SS.0-3708.0 03-30-34-4WS GULF OOME CAROLINE 3649.0-3699.7 3675.0-3692.8 11-33-34-4W5 SUP ET AL HB GARR 3173.0- 3180.9-320S.3 08-01-34-5W5 SHELL SUNDRE 3612.S-3698.0 3602.0-3666 0 01-04-34-5W5 HUDSON BAY SUNDRE 3444.5- 34S2. 2-3457.3 09-22-34-5W5 SHEll CAROLINE 3699.5-378S.5 3702.0-37590 ( 09-24-34-SWS SHELL CAROLINE 3618 S-3722 0 3641.0-3686.6 06-29-34-5W5 SIIELL CAROLINE 3770.0-3864 S 377S.0-3873.4 10-33-34-SWS SHELL CAROLINE 3722.S-3816.0 372S.0-3824.0 07-12-34-6WS SHEU IDN SUP LOB LEY 3874.6-3979.S 3849 6-3904.S 08-14-34-7WS MOBIL ET AL JAMES 4090.4-4134.6* 41060-4012.7 08-19-34-7WS CANTERRA RICINtJS 4036.0-4384.0 4099.0·4147.0 06-14-34-8W5 ALBANY AMOCO RICINUS 42840- 42306-423S.8 4281.8-43367 06-24-34-8W5 PAN AM A-t RICINUS 4455.6·4331.2 43693-4387.6 02-04-35-4W5 HOrm: ET AL GARRINGTON 3158.5-3401.0 3165.0-3178.8 16-10-35-4W5 OOME ET AL GARRINGTON 3149 0- 3149.7-3176.6 06-02-35-5W5 CANTERRA ET AL CAROLINE 3690.0-3766.0 3698.7-3737.0 03·10-35-5W5 SHEU ET AL CAROLINE 3705.0·3793.0 3710.0-3766.0 1 t-tO-35-5W5 TRIAD BPX CAROLINE 3356.5- 3352.5-3370.S 06-14-35-5W5 CANTIlRRA GARRINGTON 3768.0-3768 0 3720.0-3770.0 10-IS-3S-5W5 SIIELL CHAMPLIN CAROLINE 3721.0-379S.0 3723.0-3757.7 ll-16-35-5W5 SHELL ET AL CAROLINE 3726.7-3812.0 3718.6·3791.0 ll-17-3S-5W5 CHAMPLIN HUSKY CAROLINE 3750.0-3844.0 3810.0-382S 0 • Cooking Lake 10 t evaporating the supernatant solution from the Ba(N03h precipitation and passing the residue through a cation exchange column. Concentrations of Sr and Rb, and hence the 81R b/86 Sr ratios were determined by addition of a mixed 84Sr + 87Rb spike solution to the samples prior to decomposition. For measurement of isotopic ratios the purified Sr or Rb was loaded as a chloride onto the side filament of a double (re) filament assembly. Ratios were measured on a VG MM30 mass spectrometer. Calcite and dolomite (92) were analyzed at the University of Calgary for carbon and oxygen isotopie ratios using the acid digestion method of McCrea (1950). FID (Flame Ionization Detection), TOC (Total Organic Content), and Vitrinite RefJectance were done at Shell's Calgary Research Centre. Finally, 8 samples were studied under Scanning Electron Microscope (SEM) in Backscatter mode for identification of minerai phases and texturaI relationships. 1 1 PETROGRAPHY THE LEDUC/COOKING LAKE FORMATION Introduction Replacement matrix dolomite in the Cooking Lake platform and the Leduc reefs is simiIar. Most of the 12 cores studied are pervasively dolomitized (Table 1). Except for limestones in wells 11- 33-33-6W5 and 8-14-34-7W5, most of the primary textures have been obliterated or greatly modified by dolomitization. These two slightly dolomitiz~d wells are located in the lower forereef and offreef portions of the buildups (Fig. 2). Contacts between dolomitized and undolomitized portions of wells are mostly gradational but can be sharp (Figs. 4DE) when fossil fragments are selectively dolomitized. Replacement matrix dolomite is similar in the three 'lelds studied (Bearberry, Harmattan, and Leduc-Rimbey). In hand ( specimen the dolomite has a sandy beige color possibly attributed to organic matter impurities or hydrocarbon entrapment between crystal boundaries. Anhydrite is common in ail the wells (Figs. 4BCEF) and fills vuggy/moldic or fracture porosity. Anhydrite also occludes porosity created by fracturing and collapse of brecciated fragments of replacement dclomite (Fig. 4B). In sorne of the wells textures resembling geopetal fabries are also infilled by anhydrite. A few wells have micro-fracturing associated with dark organic residues but is more aJundant in weIl 12-21-33-6W5 (Fig. 4A). Stylolites are relatively uncommon (Fig. 4F) but when pres~nt, stylolites can be associated with solid bitumen and may cross-eut both replacement matrix dolomite and anhydrite. Submarine cements are rare; only one example of neomorphosed dolomitized submarine cement is clearly visible in hand specimen (8-19-34-7W5; Figs. IIAB). 12 Fig. 4 CORE PHOTOGRAPHS O~ REPLACEMENT MATRIX J 1. DOLOMITES IN THE LEDUC/COOKING LAKE FORMATION A) Pervasive replacement méltrix dolomite (RMD) displaying a high density of micro-fractures filled by organic residues (Bitumen?). 12-21-33-6W5, 3773.5 m. B) Fragments of replacement matrix dolomite (RMD) and anhydrite cement (AN). Organic residues (Bitumen?) is present on the upper portion of sorne dolomite fragments (arrow). 06-14-34-8W5, 4285.5 m. C) Replacement matrix dolomite (RMD) with good moldic porosity. 16-10-35-4W5, 3156.0 m. D) Pervasive replacement matrix dolomite (RMD) fragments in a dark argillaceous matrix. Anhydrite partially fills sorne intercrystalline porosity dolomite (arrow). 08-14-34-7W5, 4109.3 m. E) Bulbous stromatoporoids partially replaced by replacement matrix dolomite (RMD) and anhydrite (AN). 06-14-34-8W5, 4301.0 m. F) Stylolite truncating replacement matrix dolomite (RMD) and anhydrite (AN). 06-14-34-8W5, 4327.2 m. 13 ( ( 14 t Interpretation of the original Iithofacies prior to dolomitization is very difficult. Nevertheless, sorne partially dolomitized fragments of bulbous stromatoporoids (Fig. 4E), Amphipora, rugose corals, and megalodon bivalve can be recognized as a result of selective dolomitization and cementation processes. PETROGRAPHY REPLACEMENT MATRIX DOLOMITE Matrix dolomite in the Bearberry, Harmattan and Leduc Rimbey Fields is the only type of pervasive dolomite. This replacement dolomite (Figs. 5ABCF), is characterized by 1) anhedral to subhedral textures, 2) crystal size ranging between 60 and 2501l m (average 140llm; Figs. SABC), 3) abundance of solid inclusions (Fig. S), 4) homogeneous red cathodoluminescence (Figs. 6AB), and 5) yellow to greenish yellow fluorescence (Fig. 7). Leduc/Cooking Lake dolomite is non-ferroan, as shown by staining~ and is mostly anhedral to subhedral (nonplanar to planar-S, Sibley and Gregg, 1987; Figs. 5BC). Euhedral rhombs (Planar-E, Sibley and Gregg, 1987) are restricted to pore areas and are relatively minor (Fig. 5E) compared to the volumetrically more important anhedral and subhedral textures. Dolomite crystal sizes can be divided into three determined populations of rhombs: 1) 3S-60~m, 2) 100-140llm, and 3) 200- 250Jlm. The most volumetrically important population of rhombs range between lOO-140llm, while the least important populations vary between 3S-60llm and 200-250llm. Inclusions trapped in the dolomite crystals gives a cloudy du st Y appearance (Fig. SE). Inclusions are evenly distributed in anhedral to subhedral rhombs (Fig. 5C). In contrast, euhedral rhombs often have a higher concentration of inclusions in the centre of the rhombs than in the outer rim (Fig. 5E). Euhedral rhombs are most often present near vugs, molds, fractures or any kind of pore spaces 1 5 ..... Fig. 5 PETROGRAPHIC CHARACTERISTICS OF REPLACEMENT >IJ< t MATRIX DOLOMITE IN THE LEDUC/COOKING LAKE FORMATION A) Very fine-crystalline replacement matrix dolomite (RMD). 11-10-35-5W5, 3358.0 m. B) Fine-crystalline replacement matrix dolomite (RMD). 11-35-33-7W5, 3928.1 m. C) Medium-crystalline replacement matrix dolomite (RMD). 02-04-35-4W5, 3172.4 m. D) Photomicrograph showing replacement matrix dolomite (RMD) rhombs and patch es supported by bitumen (BT). 11-35-33-7W5, 3925.1 m. E) Replacement matrix dolomite (RMD) displaying intercrystalline porosity partially occluded by bitumen (Bl ). 16-10-35-4W5, 3159.5 m. F) Replacement matrix dolomite (RMD) showing good vuggy porosity. 01-04-34-5W5. 3456.7 m. '" v 1 6 , .. - 17 ( (Figs. 5EF) which may or may not be associated with cements such as anhydrite or calcite (Figs. SEF, 9EF). Vuggy and moldic porosity predominate, but intercrystalline porosity is also present (Fig. 4AC, 5E). -Cathodoluminescence- Evidence of primary limestone textures can be detected in replacement matrix dolomite of the Leduc/Cooking Lake Formation. Anhedral to subhedral dolomite rhombs, coalesced in a tight mosaic, display a homogeneous red (Figs. l 6AB) to slightly blotchy red (Figs. 6CD) cathodoluminescence. These signatures ar~ characteristic of wells from the three areas studied. Faint to clear zonations are sometimes visible on rare euhedral rhombs (Figs. 6CDEF) which have brighter centres/dull rims or vice versa (Figs. 6EF) possibly indicating variations in the iron/manganese ratio. Some zonation patterns can be correlated with greater densities of inclusions often present in the centres of euhedral rhombs (Figs. ( 6EF). This relationship is less apparent in anhedral to subhedral rhombs. These minor occurrences of zonation cannot however be locally correlated. -Fluorescence- In the Leduc/Cooking Lake replacement matrix dolomite primary fabries such as fossils can rarely be distinguished (Figs. 7EF). However euhedral rhombs often exhibit zonations characterized by alternating fluorescent and nonfluorescent banding (Fig. 8). These zones cannot be correlated within one weil or locally. Higher/lower densities of inclusions within the crystals can be matched with fluorescing/nonfluorescing responses in certain sampI es (Figs. 8CD). Dolomite fluoresces yellow, white nonfluorescing dolomite is dark green. Replacement dolomite (anhedral to subhedral rhombs) mostly f1uoresces yellow (Figs. 7 AB), but slightly less fluorescing dolomite (yellowish green) is also present (Figs. 7CD). The intensity of the color (yellow) cannot be directly linked to higher densities of ( inclusions visible in euhedral rhombs (Fig. 7). Both fluorescing 18 Fig. 6 CATHODOLUMINESCENCE OF REPLACEMENT MATRIX DOLOMITE IN THE LEDUC/COOKING LAKE FORMATION l A,B) Paired plane light and cathodoluminescence views of replacement matrix dolomite (RMD). The dull homogeneous red signature is characteristic of replacement matrix dolomite. 02-04-35-4W5, 3172.4 m. C,D) Paired plane light and cathodoluminescence views of anhedral to subhedral rhombs (RMO) displaying faint bl~tchy textures and zonations. Anhydrite (AN) is nonluminescent. 11-10-35-5W5, 3360.8 m. E,F) Paired plane light and cathodoluminescence views of zoned replacement matrix dolomite rhombs (RMO) and dull luminescing blocky calcite (BC). 07-34-33-6W5, 3970.8 m. 19 , Il' 1... 20 '" Fig. 7 FLUORESCENCE OF REPLACEMENT MATRIX DOLOMITE IN ) THE LEDUC/COOKING LAKE FORMATION: 1 A,S) Paired plane light and fluorescence views of replacement matrix dolomite (RMD) displaying yellow fluorescence. 07-13-33-5W5, 3438.5 m. C,D) Paired plane light and fluorescence views of replacement matrix dolomite (RMD) displaying an uncommon slightly greenish signature. 08-14-34-7W5, 4112.6 m. E,F) Paired plane light and fluorescence views of replacement matrix dolomite revealing the presence of a fossil (coral?). 07-13-33-5W5, 3399.3 m. 21 ~ ..1 22 Fig. 8 FLUORESCENCE OF REPLACEMENT MATRIX DOLOMITE IN 1 THE LEDUC/COOKING LAKE FORMATION: Il A,B) Paired plane light and fluorescence views of replacement matrix dolomite (RMD) displaying weil developed zones within the more euhedral rhombs. 16-10-35-4W5, 3159.7 m. C,D) Paired plane light and fluorescence views of replacement matrix dolomite (RMD) displaying weil developed zones within on a euhedral rhomb. The pattern of zonation visible under fluorescence correlates weil with zonations visible under plane light. 08-14-34-7W5, 4123.5 m. E,F) Paired plane light and ~Iuorescence views of replacement matrix dolomite (RMD) and blocky calcite (BC) showing yellow and dull fluorescence respectively. 08-14-34-7W5, 4118.4 m. ) 1 li 23 (. 24 (yellow)/Iess fluorescing (yellowish green) signatures are present in matrix dolomite enclosing similar densities of inclusions (Fig. 7). The type of inclusions rather than the density of inclusions is more likely responsible for the variations in fluorescence (see fluorescence discussion). SADDLE DOLOMITE Saddle dolomite is a minor constituent in the Leduc/Cooking Lake Formation. Saddle dolomite is white and coarsely crystalline with a slightly curved to a typical saddle shape (Figs. 9ABCD). Crystals are euhedral to subhedral and relatively devoid of solid inclusions. Crystal sizes vary between a few hundred microns to several mm. Saddle dolomite is only present in one weil as a vug filling cement (Figs. 9ABCD). Cathodoluminescence does not reveal any distinctive zonations and the rhombs are dark, homogeneous red to slightly purplish (Figs. 9CD). Saddle dolomite is relatively nonfluorescent (green, see Figs. 6AB) and does not show any zonation (Figs. 9AB). ANHYDRITE CEMENTS Anhydrite is the most pervasive and volumetrically important cement occluding vuggy, moldic, and/or fracture related porosity (Fig. 4). Present throughout the study area, anhydrite laths greatly vary in size (Jlm to mm; Fig. 11), but the size of the Iaths is not restricted to spcdfic types of porosities and/or dolomite. Anhydrite postdates matrix and saddle dolomite (Figs. 9EF), predates blocky calcite and appears to prcdate bitumen. Anhydrite can partially enclose matrix dolomite but is always truncated by stylolites (Fig. 4F). Anhydrite is present in both pervasively dolomitized and partially dolomitized wells and does not luminesce 25 Fig. 9 CATHOOOLUMINESCENCE AND FLUORESCENCE IN SADOLE DOLOMITE AND ANHYDRITE IN THE LEDUC/COOKING LAKE FORMATION A,S) Paired plane light and fluorescence views of saddle dolomite (SO), blocky calcite (SC), and replacement matrix dolomite (RMO). The slightly greenish fluorescence in the replacement matrix dolomite is uncommon. 08-14-34-7W5, 4109.8 m. C,D) Paired plane light and cathodoluminescence views of saddle dolomite (SO), blocky calcite (BC), and replacement matrix dolomite (RMO). 08-14-34-7W5. 4131.1 m. E,F) Paired plane light and fluorescence views of anhydrite (AN), and replacement matrix dolomite (RMO). Note the faint zones at the anhydrite/dolomite contact. 02-04-35-5W5, 3174.2 m. 26 r f 27 under either cathodoluminescence (Figs. 9EF) or fluorescence (Figs. Il EF). CALCITE CEMENTS The only type of calcite recognized is blocky calcite (Figs. 9ABCD) and it is easily identified by staining. Blocky calcite cement is common in most welh: and is nvt restricted to specifie types of textures or porosity. Blocky calcite is a pore filling cement with coarse anhedral crystals sometimes reaching several mm in size (Fig. 10). The crystals are generally inclusio;. poor but sorne have fJuid inclusions. Sorne crystals are also slightly twinned. Blocky calcite postdates replacement rnatrix dolomite (Figs. 4AB), saddle dolomite (Fig. 5A; Figs. 9ABCD) and postdates or is coeval with bitumen. As fracture filling cement, blocky calcite can be found near native sulfur (Figs. 10CD) and/or pyrite. Blocky calcite does not luminesce under cathodoluminescence (Fil~s. 9CD) and crystals are nonfluorescing green (Figs. 9AB). Blocky calcite is present throughout the study area and is truncated by stylolitization. PYRITE Pyrite is common throughout the Leduc/Cooking Lake Formation. The most common occurrences of pyrite are 1) in association with stylolites, 2) in fractures and/or intercrystalline porosity 7 and 3) in framboidal form. AlI thr~e occurrences of pyrite are present throughout the wells and sorne samples may show more than one occurrences and/or form of pyrite per sample. Pyrite is a relatively late phase and postdates replacement matrix dolomite, anhydrite, and blocky calcite. Two samples from one weIl (11-10-35- 5W5; 11050.9' & 11053.6') show fracture occluding pyrite closely associated with sphalerite (Figs. Il CD). 28 A) Core photograph of replacement matrix dolomite (RMD) displaying moldic porosity occluded by native sulfur (8). 07-13-33-5W5, 3434.2 m. B) Photomicrograph of native suiful (8) occluding vuggy porosity in a tight mosaic of replacement rnatrix dolomite rhombs (RMO). 08-19-34-7W5, 4136.2 m. C) Photomicrograph of anhydrite (AN) partially replaced by calcite (CA). Note the presence of sulfur (8) close to calcite and anhydrite. 07-12-34-6W5, 3890.2 m. D) Photomicrograph showing native sulfur (8) and blocky calcite (LBC). 08-19-34-7W5, 4136.2 m. - 29 ( ( 30 ~ Fig. 11 SPHALERITE, PYRITE, AND DOLOMITIZED } ·It· SUBMARINE CEMENTS(?) IN THE LEDUC/OOOKING LAKE FORMATION A) Core photograph showing a large cavity partially filled by dolomitized submarine cements (?) and pyrite (arrow). This example of weil preserved primary textures is uncommor. in the Leduc/Cooking Lake Formation in the study area. 08-19-34-7W5, 4132.7 m. B) Photomicrograph of dolomitized submarine cements (?) as viewed under cross-nicols. Pyrite (PY) is present at the right hand side. 08-19-34-7W5, 4132.7 m. C,D) Paired plane light and cathodoluminescence views of sphalerite "rosettes" (SPH) in replacement matrix dolomite (RMD), anhydrite (AN), and pyrite (PY). 11-1 0-35-5W5, 3368.3 m. E) Fluorescence view of one sphalerite (SPH) "rosette" \Jlsplaying a series of distinct zones. 11-10-35-5W5, 3368.3 m. F) SEM (Backscatter) view of E. The different diagenetic phases are easily recognized by the different shades of grey. 11-10-35-5W5, 3368.3 m. }<'" 3 1 32 SPHALERITE Only two samples from a single well (l1-10-35-5W5; 11050.9' and 11053.6') have sphalerite (Figs. lIeD). The characteristic "rosette" shaped sphalerite was precipitated near pyrite and anhydrite (Figs. Il CD). Thin, bright and dark micro-bands are visible in plane light and under cathodoluminescence. The bright yellow luminescent color (Figs. 11CD) visible under cathodoluminescence may be indicative of high manganese concentrations as indicated by SEM data. This zonation pattern observed under fluorescence has a dark, rusty-orange coloring (Fig. Il E). Zonations seen under fluorescence and cathodoluminescence can be correlated. SULFUR Native sulfur occludes vuggy and intercrystalline porosity ln a few wells and is more common in dolostones. Native sulfur IS a diagenetic phase postdating replacement matrix dolomite and blocky calcite (Fig. 10). Timing relationships between sulfur and bitumen cannot be adequately assessed. PARAGENETIC SEQUI',NCE The paragenetic sequence illustrated on Fig. 12 is based on texturai relationships and includes 1) replacement matrix dolomitization, 2) two phases of stylolitization, 3) one period of fracturing, 4) solution of calcite and/or limestone, 5) saddle dolomite, 6) cementation by anhydrite and blocky calcite, 7) pyrite and sphalerite deposition, 8) sulfur precipitation, and 9) bitumen. Minor occurrences of stylolitization are present ln undolomitized portions of the reefs. Dolomitization postdates this type of stylolitization since floating patches/rhombs in the Iimestone fabric sometimes crosscut and obscure these stylolites. Fracturing 33 i OLDER YOUNGER TIME Stylolitization - Replacement Matrix Dolomite Fracturing/solution - Saddle Dolomite 1111111 111111111111111111 Anhydrite 1111111111111111111111111111111111111 111111111111 Blocky Calcile 1111111111111111111111111_11111111111111111 Stylolitization 111'111111111111111111111111111111111111111111111111111111111111111 __1111111111111111IIIIIIIIIIIIIIIIIIIIIIItlllllllllili Solid Bitumen , ______.11 .. '.,1111.11 Pyrite ...... 1111 ...... _".,,, ...... " ...... 1111_ .... . "- Sphalerite Sulfur .... '11 ...... , ______11111 ..''' ... 1 ...... '" .... ,' Fig. 12 Paragenetic sequence for the Leduc/Cooking Lake Formation. The thickness of the lines indicates the relative abundance while the dashed lines show the possible ranges. 34 and solution are coeval with or postdate replacement matrix dolomite since those features are commonly healed by saddle dolomite 7 anhydrite, bitumen or blocky calcite. Pyrite and sphalerite were probably coeval and precipitated after blocky calcite. Anhydrite is the predominant type of cement occluding porosity but native sulfur (Fig. 10) and bitumen also occlude residuaJ intercrystalline porosity. The timing of sulfur formation is difficult to constrain but appears to be relatively late. Most stylolitization probably began sometime after replacement matrix dolomite and could have continued under deep burial conditions since stylolites cross-eut ail diagenetic phases. THE SWAN BILLS FORMATION In trod uction The twenty cores examined in the study area are pervasively dolomitized and the dolomite obscures the original lime stone textures. In hand specimen, pervasive replacement matrix dolomitization is either light to dark grey and/or beige (Figs. 13ABCD). The greyish and beige color could be attributed to clay impurities and/or bitumen within or between the dolomite crystals. The good moldic and vuggy porosity (Amphipora and/or bulbous stromatoporoids; Figs. 13BO) is in the beige matrix dolomite, white grey matrix is substantially less porous in most weils (Fig. 13A). Dolomitization is common near the bank edge white limestone is more abundant in the bank interior. Allochems such as Amph ipora and bulbous 7 hemispherical or tabular stromatoporoids are weil preserved in limestone wells (Fig. 13C) and when dolomitization of the matrix is selective (Fig. 13A). Selective dolomitization is present in most lithofacies but is more commonly encountered in association with stromatoporoid rudstones or floatstones (Figs. 13AC). Good fabric retention of selectively dolomitized bulbous stromatoporoids commonly displays partial replacement by grey matrix dolomite and 35 .,,~ '1 Fig. 13 CORE PHOTOGRAPHS OF REPLACEMENT MATRIX ~ 'l DOLOMITE IN THE SWAN HILLS FORMATION A) Core photograph of replacement matrix dolomite selectively replacing bulbous stromatoporoids. The calcareous shale matrix is slightly dolomitized. Weil developed sadd le dolomite (SD) rhombs fills solution vugs in the centre portions of the stromatoporoids. 06-24-34-8W5, 4380.9 m. B) Core photcgraph of replacement matrix dolomite displaying porosity partially occluded by calcite (CA). 03-10-35-5W5, 3721.0 m. C) Core photograph showing selective dolomitization of matrix around bulbous stromatoporoids. 03-10-35-5W5, 3714.3 m. 0) Core photograph showing partially dissolved Amphipora partly filled by bitumen. 05-32-33-4W5, 3728.0 m. E) Green shales (arrow) in a limestone portion of core. 10-33-34-5W5, 3750.0 m. F) Core photograph showing sharp contact between dolomite (light lower half) and limestone (dark). The contact is bounded ~ by a stylolite (right hand side arrow). 10-33-34-5W5, 3761.4 m. ( 36 - 37 precipitation of saddle dolomite and/or sulfur (in that order) towards the centre of the stromatoporoids (Fig. 13A). Saddle dolomite crystals can attain centimetre sizes and are extremely weIl developed when present as mold fiJling cement. Saddle dolomite is also present as a fracture filling cernent or as tens of cms blobs/patches in limestone cores (Fig. 19A). In addition to saddle dolomite, calcite, anhydrite, and celestite also occlude sorne fracture, vuggy or moldic porosity. Two stylolitization events were observed. ID partially dolomitized cores, low amplitude stylolites are often slightly obliterated by pervasjve matrix dolomite (Fig. 23A) suggesting that these stylolites appear to have formed before replacement matrix dolomitization. High amplitude stylolitization is also present and truncates, thus postdates, pervasive matrix dolomite (Figs. 23DE). In addition, high amplitude stylolitization can separate replacement matrix dolomite and limestone (Fig. 23D). Stylolites are commonly associated with thin green shale layers (Fig. 23B) usually present in ~ " cryptai gal laminite lithofacies. 1 PETROGRAPHY REPLACEMENT MATRIX DOLOMITE Replacement matrix dolomite in the Swan Hills Formation is the volumetrically most abundant dolomite and is light to dark grey or beige. The petrographie and geochemical evidence suggest that the grey and the beige dolomite is similar. Replacement matrix dolomite has the following characteristics: 1) variable densities of inclusions (Fig. 14), 2) anhedral to subhedral textures (Fig. 14), 3) crystal sizes ranging between 100 and 300J.1m with modes near 140J.1m and 250J.1m (Fig. 14), 4) a green fluorescence signature (Figs. 16AB), and 5) homogeneous red cathodoluminescence (Figs. 15AB). Replacement matrix dolomite forms a tight mosaic of non ferroan dolomite rhombs (based on staining techniques) with liule 38 Fig. 14 PETROGRAPHIC CHARACTERISTICS OF REPlI'.CEMENT t MATRIX DOLOMITE IN THE SWAN HILLS FORMATION J' A) Photomicrograph of medium-crystalline replacement matrix dolomite (RMD) displaying vuggy porosity. 09-24-34-5W5, 3672.2 m. B) Photomicrograph of fine-crystalline replacement matrix dolomite (RM) showing portions of the original limestone (arrow). 11-16-35-5W5, 3776.8 m. C) Photomicrograph of replacement matrix dolomite (RMO) partially replacing a stromatoporoid stained pink. 05-32-33-4W5, 3707.8 m. D) Photomicrograph of coarse-crystalline replacement matrix dolomite (RMD) displaying clear rims and intercrystalline porosity. 06-02-35-5W5, 3704.1 m. E) Photomicrograph of euhedral replacement matrix dolomite rhombs in Iimestone. 07-18-34-4W5, 3685.1 m. F) Photomicrograph of coarse crystalline euhedral replacement matrix dolomite rhombs (RMD) in Iimestone. Note the absence of weil developed zonation. 03-30-34-4W5, 3677.5 m. ) 39 ( ( ( 40 1 intercrystalline porosity. Cloud y centre/clear rim textures are common in the more euhedral rhombs (Figs. 14CE) at dolomite/lime stone contacts, and in fracture, moldic or vuggy porosity. In the tight matrix dolomite, rhombs are anhedral to subhedral (non planar to planar-S from Sibley and Gregg, 1987; Fig. 14) and their crystal sizes have a bimodal distribution. Finely crystalline rhombs (Fig. 14B) range between 100 and 16011 ru and average 140ll m while the predominant medium crystalline population of rhombs range between 200 and 3001! m and average 250llm (Fig. 14A). On a microscopic scale, matrix dolomite is non fabric selective and obliterates primary textures (Fig. 14C). Kinks are uncommon but are present at the terminations of sorne dolomite crystals. Replacement matrix dolomite enclosed by meshworks of small laths of anhydrite is uncommon and only found in one well (6-29-34-5W5; Figs. 12EF). Anhydrite replacement obliterates most of the dolomite .... textures except for faint outlines of the previous euhedral dolomite rhombs (Figs. 21EF). Stylolites can be in contact with crystal edges (Fig. 23C). Stylolites, sometimes associated with bitumen and/or clay minerai insolubles, truncate matrix dolomite (Fig. 23E). Three types of porosity are present in replacement matrix dolomite: 1) vuggy or moldic (Fig. 14A), 2) intercrystalline (Fig. 14D), and 3) fracture. Vuggy and moldic porosity volumetrically predominates in the Swan Hills Formation and permeability could be controlled by interconnecting vugs and sorne intercrystalline porosity. -Cathodoluminescence (CL) - The overall CL signature of replacement matrix dolomite is duB homogeneous red to slightly blotchy red (Figs. 15ABCD). Primary limestone textures such as fossil remnants or fossit ghosts were not observed. The blotchy CL of replacement matrix dolomite has been attributed by sorne authors to recrystallization processes in other areas and formations (Kaufman, 1989; Machel, 1989). 4 1 Fig. 15 CATHODOLUMINESCENCE OF REPLACEMENT MATRIX DOLOMITE IN THE SWAN HILlS FORMATION A,B) Paired plane light and cathodoluminescence views of replacement matrix dolomite (RMD) displaying a homogeneous dull red signature. 03-10-35-5W5, 3756.3 m. C,D) Paired plane light and cathodoluminescence views of an irregular contact between replacement matrix dolomite (RMO) and limestone (lS). Euhedral rhombs have clear rims with banded luminescence. 07-18-34-4W5, 3685.1 m. E,F) Plane light and cathodoluminescence views of a euhedral rhomb of replacement matrix dolomite (RMD) showing clear rim with banded luminescence. Vuggy porosity is fil/ed by blocky calcite (BC). 07-18-34-4W5, 3661.4 m. ( 1 42 43 Zones visible under ordinary or polarized light are often indicative of weil developed CL zonation (Figs. 15EF). Alternating onght and darker CL bands are common in Swan Hills replacement matrix dolomite (Figs. 15CDEF) with intercrystalline porosity. Slightly euhedral dolomite rhombs (Figs. 15CDEF) at dolomite/limestone contacts (Figs. 15CD), in fractures and/or moldic porosity arp. also zoned. Compositionally different fluids flowing through these high porosity zones or different rates of precipitation could prnmote zonations on certain rhombs. Sorne rhombs may have several alternating bands (Figs. 15CDEF), white others have only two but the banding col our is only bright or dark red. Zones are not restricted to any specific crystal texture and the zones cannot be correlated between samples. -Fluorescence - In most cases replacement matrix dolomite does not fluoresce and is dull green (Figs. 16AB). Sorne minor exceptions include 1) small, euhedral, fluorescent orange limestone replacive rhombs (6-29-34-5W5; Figs. 17EF), and 2) slightly yellowish fluorescent rnatrix (7 -12-34-6W5; Figs. 17CDEF). Crystal size, one of the differences between the Swan Hills and Leduc/Cooking Lake dolomite, does not appear to influence fluorescence of the dolomite rhombs. Although uncommon, primary lime stone textures, almost invisible under ordinary or polarized light, can be clearly identified using fluorescence (7-12-34-5W5 & 6-24-34-8W5; Fig. 18). Gastropods and stromatoporoid fragments were observed. Fluorescent zonations are uncommon (Figs. 16AB) but euhedral rhombs associated with lrimary or secondary pore systems often display alternating fluorescent and nonfluorescent banding (Figs. 16CD). Sorne of these bands correlate with inclusion rich areas or with banding visible under plain light (Figs. 16C,D) and cathodoluminescence. Outlines of dolomite rhombs encJosed in anhydrite (Figs. 21EF) were also observed under fluorescence. 44 ..,. Fig. 16 FLUORESCENCE OF REPLACEMENT MATRIX DOLOMITE IN .. THE SWAN HILLS FORMATION: 1 A,S) Paired plane light and fluorescence views of replacement matrix dolomite (RMD) showing dark green fluorescence. 05-32-33-4W5, 3699.7 m. C,D) Paired plane Iight and fluorescence views of euhedral rhombs of replacement matrix dolomite (RMD) with intercrystalline porosity occluded by blocky calcite (SC) and bitumen (BT). 10-15-35-5WS, 3746.0 m. E,F) Paired plane Iight and fluorescence views of euhedral rhombs of replacement matrix dolomite (RMD) showing clear rims. The slightly yellowish fluorescence is uncommon in Swan Hilis replacement matrix dolomite. OS-02-3S-SWS, 3713.1 m. - " 45 ( ( 46 Fig. 17 FLUORESCENCE OF REPLACEMENT MATRIX DOLOMITE IN l THE SWAN HILLS FORMATION: Il A,S) Paired plane light and fluorescence views of dolomite rhombs with clear rims partially replacing limestone (LS). 05-32-33-4W5, 3739.6 m. C,D) Paired plane light and fluorescence views of euhedral rhombs displaying slightly fluorescent centres but non fluorescent outer rims (1 to 2). 11-17-35-5W5, 3812.0 m. E,F) Paired plane light and fluorescence views of the single example observed of strongly fluorescing fine-crystalline replacement matrix dolomite (RMD) in a micritic limestone (LS). The source of fluorescence is uncertain (see text). 06-29-34-5W5, 3847.2 m. ..., - t 47 ( 48 Fig. 18 FLUORESCENCE OF REPLACEMENT MA TRIX DOLOMITE IN THE SWAN HILLS FORMATION: III A,S) Paired plane light and fluorescence views of replacement matrix dolomite displaying ghosts of primary textures (gastropod ?). 07-12-34-6W5, 3899.2 m. C,D) Paired plane light and fluorescence views of replacement matrix dolomite displaying the outlines of a stromatoporoid fragment. 07-12-34-6W5, 3900.5 m. E,F) Paired plane light and fluorescence views of replacement matrix dolomite display: •• g burrows(?). 06-24-34-8W5, 4375.1 m. ) 't \ • 49 ( ( 50 SADDLE DOLOMITE Saddle dolomite is a pore filling cement (vuggy, moldic or fracture porosity, Figs. 19BEF). Saddle dolo;.1ite can be slightly replacive at dolomite/limestone contacts or near the terminations of rhombs (Fig. 19B). The white color, curved crystal habits, and sweeping extinction of the crystals (Fig. 19A) have been observed in the Cooking Lake and Swan Hills Formations. Dolomite crystals are euhedral to subhedral and sizes vary between a few hundred microns to several mm. Saddle dolomite is usually reJatively free of solid inclusions but has cloudy centres and clear rims (Fig. 19B). Step growth features and kinks are present on sorne rhombs especially in small restricted pore spaces (Figs. 19BEF). Partial replacement of calcite cement by saddle dolomite, observed in weIl 3-10-35-5W5 (Fig. 24B), may indicate that sorne calcite precipitated prior to saddle dolomite. Geographic variations are not observed except that saddle dolomite is more common in the Swan Hills Formation than in the Leduc/Cooking Lake Formation. Under cathodoluminescence saddle dolomite has three distinctive features: 1) homogeneous dull red luminescence (similar to replacement matrix dolomite), 2) alternating bright and dark red zones (Figs. 19EF), and 3) zonations showing purplish centres with dark/bright red rims. Zonation is not distinctive and pervasive enough to allow any kind of cement stratigraphy. Purple luminescing centres on euhedral rhombs have only been observed on crystals growing in fractures that crosscut a micritic Jimestone substrate. Saddle dolomite does not fluoresce and is homogeneous dark green (Figs. 19CD), except for rare fluorescent zones which match with inclusion-rich areas visible under plain Iight. The association of saddle dolomite with bitume n, calcite or anhydrite cements does not appear to influence cathodoluminescence or fluorescence. Saddle dolomite is truncated by stylolites (Fig. 24C) and by fractures filled by blocky calcite cement (Figs. 24CD). 5 1 Fig. 19 PETROGRAPHIC CHARACTERI5TIC5 OF 5ADDLE • DOLOMITE IN THE SWAN HILL5 FORMATION 1 A) Core photograph showing saddle dolomite (SO) in limestone. 1 1 06-29-34-5W5, 3808.6 m. B) Photomicrograph of sadd le dolomite (SD) with clear rims filling 1 vuggy porosity in a limestone matrix (dark). Blocky calcite (BC) also partially occludes sorne of the remaining porosity. Note the .~l step growth features on the dolomite rhombs. j 03-10-35-5W5, 3714.8 m. <;, C,D) Paired plane light and fluorescence views of saddle dolomite (SD) with nonfluorescent green signature and the presence of bitumen (arrow). 06-20-33-4W5, 3644.3 rn. E,F) Paired plane light and cathodoluminescence views of saddle dolomite (50) and blocky calcite (BC) occluding vuggy po rosit y in a limestone matrix (lS). Note the weil developed zonations and step growth features on some dolomite rhombs (1 to 7). 03-30-34-4W5, 3644.3 m. ( 1 52 t ~ ~ 1 l ....".. 53 , . CALCITE At least two clearly distinct types of calcite can be recognized: 1) bladed calcite (Figs. 20ACD), and 2) blocky calcite (Figs. 20ABEF). The elongated crystals of bladed calcite grow normal to the cavity waJls of primary pores (Fig. 20A) but the unclear subcrystals and confusing extinction make a submarine cement interpretation equivocal. Bladed calcite is restricted to a few examples from two undolomitized portions of wells in micritic to slightly peloidal limestone. The rare occurrences of this type of cement may be attributed to obliteration by pervasive dolomitization. Blocky calcite (Fig. 20) forms a tight mosaic of crystals. The crystals range from 200~m to mm in size, have twinning (Fig. 20B), and are commonly found in vuggy, moldic, intercrystalline or fracture porosity. Blocky calcite is present throughout the Swan Hills Formation but is more abundant in dolomitized rocks than in lime stone. Calcite can replace anhydrite as indicated by anhydrite inclusions in calcite (the anhydrite inclusions are in optical conti nuit y with the adjacent anhydrite; Fig. 21 D). Blocky calcite is often associated with bitumen (Fig. 20A), pyrite, sulfur (Fig. 21 D) and in one case celestite (Figs. 22EF). Blocky calcite postdates, but in one example predates(?), saddle dolomite (Figs. 24AB). Blocky calcite predating saddle dolomite can be recognized by inclusions of calcite in saddle dolomite which are optically continuous with surrounding calcite cement (Fig. 24B). Bladed and blocky calcite often display thinly layered black and dark red zones under cathodoluminescence. Zonations are often concentrated on specifie nuc1eation sites (Figs. 20CD). Single or multiple nucleation sites are present at pore margins and are easily recognized by half or fully concentric overgrowth bands (Figs. 20CD). Similar crystals viewed 111 fluorescence can display dogtooth zonations (Figs. 20EF). l', '. 54 FIg.' ,_"JO PETROGRAPHIC CHARACTERISTICS OF CALCITE IN THE 1 SWAN HILLS FORMATION A) Photomicrograph showing sequential cementation of primary po rosit y by 1) bladed calcite (BLC) 2) saddle dolomite with clear rims (SO) 3) bitumen (ST), and 4) blocky calcite (BC). 09-24-34-5W5, 3659.8 m. B) Photomicrograph of weil developed twinning (under crossed nicols) in blocky calcite (BC). 03-10-35-5W5, 3756.3 m. C,O) Paired plane light and cathodoluminescence views of bladed calcite (BLC) displaying nucleation sites with several growth zones. 09-22-34-5W5, 3758.6 m. E,F) Paired plane light and fluorescence views of blocky calcite (BC) displaying dogtooth pattern. 07-18-34-4W5, 3706.5 m. 55 ( r, 56 Blocky calcite cement is dark red (Figs. 15CDEF, 20CD) to almost black under cathodoluminescence, and is dark green under fluorescence (Fig. 16F). Stylolites truncate pore filling blocky calcite. ANHYDRITE Anhydrite is common throughout the Swan Hills Formation. The size of individual laths varies from a few tens of ~ m to several hundreds of !lm (Figs. 21DEF). Anhydrite is a pore filling cement commonly associated with saddle dolomite, blocky calci te and bitumen. Anhydrite can be calcitized (Fig. 21 D) as demonstrated by solid inclusions of anhydrite within calcite in optical continui ty wi th surrounding anhydrite. When present, partially replaced anhydrite is often near native sulfur (Fig. 21D). These observations suggest that native sulfur is related to thermochemical sulphate reduction (TSR) 1 as described by Eliuk (1984), Machel (1987a,b), and Krouse et al. (1987 Anhydrite postdates saddle dolomite but could be coeval with blocky calcite and bitumen. Anhydrite does not fluoresce (Figs. 21EF) or luminesce (Figs. 21 ;E,F) but original oudines of dolomite rhombs can sometime be observed un der fluorescence (Figs. 21 EF). Stylolites truncate anhydrite. SULFUR Traces of native sulfur are found throughout dolomitized and undolomitized wells (Figs. 21ABCD). Native sulfur can occlude interlaminae of large, undolomitized, hemispherical stromatoporoids (Figs. 21AB) but is most often present in dolomitized rocks. Traces of native sulfur are also present in vugs partially filled by saddlc dolomite or intercrystalline porosity. Native sulfur postdates replacement matrix dolomite. saddle dolomite, and blocky calcite but its paragenetic relationship with bitumen cannot be adequately ,~, constrained. 57 Fig. 21 PETROGRAPHIC EVIDENCE OF THERMOCHEMICAL SULPHATE REDUCTION (TSR) IN THE SWAN HILLS FORMATION A) Core photograph of an hemispherical stromatoporoid fragment showing sulfur between latilaminae and in a boring (arrow). 06-29-34-SW5, 3821.7 m. B) Photomicrograph of A) showing sulfur (S) between latilaminae. 06-29-34-SWS, 3821.7 m. C) Core photograph showing sulfur (S) in a vug fi/led by calcite in limestone. 07-34-33-6WS, 4016.3 m. D) Photomicrograph showing partially calcitized anhydrite adjacent to native sulfur (S). 07-12-34-6W5, 3890.2 m. E,F) Paired plane light and fluorescence views of smalliaths of anhydrite (AN) partially replacing replacement matrix dolomite (RMD) (arrow). 06-29-34-5W5, 3848.3 m. .... 58 , , ~ r " 59 BOTRYOIDAL DOLOMITE Weil 6-29-34-5W5 shows an accumulation of saddle dolomite (3cm thick; Fig. 19A) underlain by a vertical fracture (800}1m wide). The fracture, perhaps acting as conduit for the upward migrating f1uids, is filled by two diagenetically different cements: 1) saddle dolomite (Figs. 22AB), and 2) botryoidal dolomite (Figs. 22AB). Each phase fills one half of the fracture along its length. Botryoidal dolomite could have precipitated as aragonite and be later replaced by dolomite while preserving the original fabric. The enlarged fracture could have subsequently allowed saddle dolomite precipitating fluids to move up and precipitate dolomite on top of and along one side of the fracture. Botryoidal, neomorphosed dolomite fluoresces dark green (Figs. 22CD) with the exception of a 140 um band at the outer edge which f1uoresces ye))ow. The cause for this luminescence could be attributed to organic material and/or hydrocarbons trapped in the crystal structure. A luminescing band IS also visible under cathodol uminescence. CELESTITE Celestite (7 -18-34-4W5; Figs. 22EF) was identified by SEM and its royal blue luminescence when viewed under cathodoluminescence (Fig. 22F). Celestite and anhydrite occlude vuggy porosity. This strontium sulphate cement contains sorne solid inclusions but does not reveal any zones under cathodoluminescence. f{ ~ 60 .J\ Fig. 22 PETROGRAPHie CHARACTERISTICS OF NEOMORPHOSED ~' ,. BOTRYOIDAL DOLOMITE AND CELESTITE IN THE SWAN HILLS FORMATION A) Core photograph showing a fracture truncating limestone (Amphipora floatstone). The fracture is filled by saddle dolomite (SD) and neomorphosed botryoidal dolomite (NBD). 06-29-34-5W5, 3803.6 m. B) Photomicrograph of the fracture displayed in A). 06-29-34-5W5, 3803.6 m. C,D) Paired plane light and fluorescence views of both saddle dolomite (SD) and neomorphosed botryoidal dolomite (NBD). Note the slight fluorescence at the crystal termination of the botryoids near the top of the photograph. 06-29-34-5W5, 3803.6 m. E,F) Paired plane light and cathodoluminescence views of blocky calcite (LBC), anhydrite (AN), and celestite (CE) filling primary porosity. 07-18-34-4W5, 3706.5 m. 61 62 t PARAGENETIC SEQUENCE The paragenetic sequence iIlustrated in Fig. 25 is based on texturai relationships. The sequence includes 1) pervasive replacement matrix dolomitization, 2) two phases of calcite cementation; bladed calcite (Fig. 20A) and blocky calcite (Figs. 20AB), 3) at least one fracturing/solution (e.g. calcite and limestone) episode, 4) saddle dolomite cementation, 5) anhydrite and celestite cementation, 6) at least one bitumen porosity occluding event, 7) pyrite precipitation, 8) traces of native sulfur deposition, and 9) styloJitization. Minor occurrences of stylolitization, similar to stylolitization dîscussed in the Leduc paragenetic sequence, IS present in undolomîtized limestone wells and is partially obliterated by pervasive matrix dolomitization. Bladed calcite cementation predates or is coeval with this stylolitization and infills primary porosity (Fig. 20A). The porosity is commonly subsequently filled by saddle dolomite (Fig. 20A). Pervasive replacement matrix dolomite replaces lime stone fabrics and saddle dolomite infills intercrystalline and vuggy porosity. In addition, at least one fracturing and solution event took place prior to saddle dolomite cementation since saddle dolomite often heals fractures and fills solution cavities (Figs. 24EF). Anhydrite and celestite postdate saddle dolomite. Bitumen commonly coats saddle dolomite rhombs in vuggy or mol die porosity (Fig. 20A) and the remaining pore space is often filled by blocky calcite (Fig. 20A). Traces of sulfur are sometimes present between blocky calcite crystals or in vuggy/intercrystalline porosity partially filled by saddle dolomite. Pyrite and sulfur are difficuIt to constrain since theîr relationships with other diagenetic phases are equivocal. Pyrite and bitumen are sometime present along stylolites that cross~cut most previous diagenetic phases (Fig. 23). Most stylolitization began shortly after replacement matrix dolomite (Fig. 23) and probably continued during burial. 63 1 1 l Fig. 23 STYLOLITIZATION IN THE SWAN HILLS FORMATION A) Core photograph showing partial dolomitization of limestone and stylo lite (arrow). 03-10-35-5W5, 3715.3 m. B) Core photograph of a stylolite, fil/ed with insoluble material, truncating a partially dolomitized limestone (arrow). 11-16-35-5W5, 3741.6 m. C) Photomicrograph showing incipient stylolitization in replacement matrix dolomite (arrow). 07-18-34-4W5, 3659.5 m. 0) Photomicrograph of a stylolite, fil/ed with insoluble material, truncating replacement matrix dolomite (RMD), and limestone (LS). 06-02-35-5W5, 3731.8 m. E) Photomicrograph of a stylolite (Ieft is top) truncating replacement matrix dolomite (RMD) and blocky calcite (BC). 07-12-34-6W5, 3900.5 m. F) Photomicrograph of a stylolite (Ieft is top) cross-cutting dolomite rhombs present in limestone. 10-15-35-5W5, 3749.0 m. ( 64 65 ( Fig. 24 PETROGRAPHIC RELATIONSHIPS BETWEEN SADDLE ;1 DOLOMITE AND BLOCKY CALCITE IN THE SWAN HILLS 'w FORMATION 1~ ., j A) Photomicrograph showing sadd le dolomite rhombs (50) with / clear rims and blocky calcite (BC) filling vuggy porosity. 11-16-35-5W5, 3789.8 m. 1 B) Photomicrograph showing saddle dolomite (SO) partially replacing (?) calcite (CA). 03-10-35-5W5, 3716.0 m. 1~ 1 C,O) Photomicrographs showing saddle dolomite (SO), and fracture l filling blocky calcite (BC), truncated by a stylo lite in C) (arrows). ~ 03-10-35-5W5, 3718.2 m. c. 66 67 l" OLDER YOUNGER TIME Stylolltization - Bladed Calcite Replacement Matrix Dolomite ___1111111111 Fracturing/Solution - Saddle Dolomite 11111111_111111111111111 Anhydnte 11111111111111111111111"'111111'-111111'111" Celesbte BIocky Calcite 11/1111/1111111111 ___111/11111111/1111111 Stylolitization 11111111111111111111111111111111111111111111111111111111111111111111 ___11111111111111111111111 ______... 1..... 111 .. 4 i Soli Sulfur 1lIIIIIIIIIIIIIIIIIIIIIIIIIItulllllllllllllllili 1111111111 Fig. 25 Paragenetic sequence for the Swan Hills Formation. The thickness of the lines indicates the relative abundance while the dashed lines show the possible ranges. 68 GEOCUEMISTRY TRACE ELEMENTS Replacement Matrix Dolomite. Replacement matrix dolomite in the Leduc/Cooking Lake (50.2% mol MgC03) and the Swan Hills Formations (49.8% mol MgC03) is nearly stoichiometric (Table 2). Elemental composition averages from both formations are 1623 ppm Fe, 104 ppm Mn, 418 ppm Na, and 81 ppm Sr (Table 5). Strontium averages could only be calculated from samples with concentrations greater than the detection limit of 50 ppm. In the Leduc/Cooking Lake Formation Fe averages 1531 ppm. This value is comparable to Fe concentrations found in Swan Hills replacement matrix dolomite (1741 ppm). Mn concentrations are similar in both Formations: 104 ppm in Leduc/Cooking Lake and 100 ppm in Swan Hills. Sodium and strontium concentrations are also very similar in both formations: 441 ppm Na and 84 ppm Sr in Leduc/Cooking Lake and 383 ppm Na and 78 ppm Sr in Swan Hills. These averages reflect replacement matrix dolomite values from the four areas studied. No geographic trends occur in the geochemistry of the replacement matrix dolomite. Limestone. Four lime mud samples from A mp h i po ra and stromatoporoid floatstones were analyzed for comparison with dolomite (Table 3). Fe concentrations range between 90 ppm and 270 ppm and average 188 ppm. Mn is relatively constant and ranges between 30 ppm and 50 ppm averaging 35 ppm. Na varies between 120 ppm and 300 ppm averaging 220 ppm. Sr is also extremely constant at 140 ppm for three out of four samples and averages 143 ppm. Fe(188 ppm), Mn(35 ppm), and Na(220 ppm) average concentrations are substantiaJly lower in Iimestone than ln replacement matrix dolomite (1623 ppm Fe, 104 ppm Mn, and 418 ppm Na). Strontium(Sr) concentrations are higher in limestone (143 .,.-, ppm) than in replacement matrix dolomite (81 ppm) . 69 TABLE 2 SUMMARY OF TRACE/MAJOR ELEMENTS AND STABLE/RADIOGENIC ISOTOPES IN REPLACEMENT MATRIX DOLOMITE Weil Depth Fe Mn Na K Ca Mg Sr I)l3C 1)1110 87Sr/86 S r (ppm) (ppm) (ppm) (ppm) (Ij!, mol) (Ij!, mol) (ppm) (PDD) (PDB) 06-20-33-4W5 3652.3 16000 90 380 80 55.6 44.4 70 2.94 -3.63 0.70832 3661.8 17000 100 480 90 49.9 50.1 50 3.03 -2.94 36640 34000 130 480 540 53.3 46.7 130 2.43 -7.35 36703 23000 110 450 210 50.8 492 80 3.77 -5.99 OS-32-33-4WS 3703.3 1600 130 410 120 50.9 49.1 50 2.65 -3.80 0.70861 3719.8 800 70 500 190 51.0 49.0 80 3.02 -4.54 07-13-33-7WS 34034 3600 130 590 390 53.7 46.3 120 0.4 -3.97 3408.9 500 90 300 120 50.4 49.6 50 1.44 -3.88 34361 830 210 390 100 50.4 49.6 12-21-33-6WS 3772.4 550 100 370 70 50.7 493 80 1.28 -3.35 37918 800 100 470 170 507 49.3 100 2.26 -3.67 3803.1 790 90 310 80 50.5 49.5 07-34-33-6WS 3961.3 18000 2000 360 5700 49.7 50.3 11-3S-33-7W5 39273 880 110 540 190 47.5 52.5 100 o 15 -3.39 3931 1 420 100 320 80 47.0 53.0 90 047 -5.03 07-18-34-4WS 36587 2000 120 360 130 46.4 53.6 50 2.75 -5.60 3685 1 3100 130 460 310 47.0 53.0 11·33·34·4W5 31942 580 90 520 120 52.4 47.6 150 1.26 -3.91 3194.7 440 80 500 100 50.4 49.6 70 1.17 -4.29 3197.9 570 90 560 300 50.6 494 08-01-34-5W5 36403 53000 180 450 270 51.8 48.2 01·04-34-5W5 3453.3 400 80 460 240 504 49.6 09-22-34·SWS 37322 750 60 220 80 44.8 55.2 10·33-34·5W5 37630 1500 90 290 480 51.6 48.4 60 2.76 -4.12 07-12-34-6W5 3900 S 2.32 -3.82 3900.S 2.12 -3.99 3903.9 850 160 310 190 51.3 48.7 50 1.83 -3.61 08·14-34-7W5 4106.0 3900 160 370 700 50.0 50.0 50 2.45 -4.87 41084 3.32 -4.15 41194 3.76 -4.13 41309 1100 160 350 390 51.S 48.5 60 4.08 -5.10 0.70810 ( 70 ...". ~, TABLE 2 CON'T Weil Depth Fe Mn Na K Ca Mg Sr SI3C SilO 87S r /86S r (ppm) (ppm) (ppm) (ppm) (~ mol) (~ mol) (ppm) (POO) (POB) 4131.1 2100 130 380 500 51.2 48.8 80 499 -4.77 08-19-34-7W5 4138.0 500 100 430 160 46.5 53.5 70 1.88 -4.22 4141.2 2.12 -2.85 06-14-34-8W5 4292.5 370 90 340 130 50.1 49.9 50 2.91 -3.59 070823 4309.4 580 100 400 180 49_8 50.2 90 2.99 -392 4315.8 257 -4.94 4315.8 1000 110 430 470 52_6 47.4 100 3.?'; -2.88 4322.5 400 100 370 190 524 47.6 50 276 -4.12 06-24-34-8W5 43740 800 100 350 190 528 47.2 260 2.14 -678 4384.9 490 70 360 270 5U 48.5 60 1.93 -4.86 02-04-35-4W5 3169.7 290 80 450 100 48.9 51.1 60 1.63 -4.00 3171.2 640 90 610 310 52.3 47.7 130 2.14 -4 10 16-10-35-4W5 3159.7 640 90 470 100 47.5 52.5 90 1 38 -5 09 06-02-35-5W5 37131 13000 90 370 110 477 523 50 2.72 -3.98 3730.1 3.49 -5.14 03-10-35-5W5 37168 26000 100 420 350 47.0 53.0 <50 2.\3 -568 3725.4 2.18 -458 375'1.1 13000 100 320 560 46.9 53.1 60 3.45 -5.08 ,,",'- 1I-10-35-5W5 3357.3 34000 80 600 5000 496 504 120 -0.54 -328 3358.0 63000 90 660 10000 47.2 52.8 90 0.12 -3 10 3368.3 1300 80 610 430 49.6 50.4 80 052 -3.74 70829 10-15-35-5W5 3737.7 2.94 -445 37465 1500 100 410 500 54.0 460 <50 351 -4.96 11-16-35-5W5 3753.4 850 70 370 240 51.7 48.3 80 3773.4 23000 170 390 220 51 8 48.2 70 3.55 -7.42 7 1 TARLE 3 SUMMARY OF TRACE/MAJOR ELEMENTS AND STABLE/RADIOGENIC ISOTOPES IN CALCITE AND LIMESTONE Weil Depth Fe Mn Na K Ca Mg Sr Sl3C SIlO 87Sr/US r (m) (ppm) (ppm) (ppm) (ppm) ('II> mol) ('II> mol) (ppm) (POO) (POB) Llmestone 07-18-34-4W5 1685 1 070828 1)3-30-34-4W5 3681 7 270 50 140 50 100 0 140 Û6-29-34-5W5 3808.3 90 30 220 270 99 150 2.93 -372 3821.7 130 30 120 50 99 140 232 -5.68 10-33-34-5W5 37608 260 30 300 90 694 30.6 140 321 -5.70 08-19-34-7W5 40995 350 130 233 -361 Calcite i 12-21-33-6>\15 37891 -23.1 -9.28 .~ 08-14-34-7W5 41326 -16.23 -9.17 08-19-34-7W5 41036 -2656 -8.74 11-16-35-5WS 3775.0 -9.01 -774 72 TABLE 4 SUMMARY OF TRACEIMAJOR ELEMENTS AND STABLEfRADIOGENIC ISOTOPES IN SADDLE DOLOMITE Weil Depth Fe Mn Na K Ca Mg Sr BI3C SUO 87S r /BIIS r (ppm) (ppm) (ppm) (pp 'Tl) (' mol) (' mol) (ppm) (POB) (POO) 5-32-33-4WS 37056 2500 170 360 130 489 41.1 60 -2.63 -806 J707.2 195 -8 53 7-34-33-6WS 40046 -007 -S 15 6-29-34-5WS 38083 <10 70 380 70 525 47.5 70 167 -671 070880 7-12-34-6WS 3899.3 liOO 160 260 110 568 432 110 -1075 -630 8-14-34-7WS 4108.4 1200 190 240 60 520 480 50 -1.76 -643 071055 41194 290 140 230 6-14-34-8W5 42970 -666 -503 3-10-35-5W5 3757 1 -3.29 -468 ~ 73 1 Sadd le Dolomite. Replacement matrix dolomite (see above) and saddle dolomite (1273 ppm Fe, 146 ppm Mn, 294 ppm Na, and 70 ppm Sr) have similar trace element concentrations but saddle dolomite is not as stoichiometric (Table 4). TRACE ELEMENT VARIATIONS Trace element concentrations in the Leduc/Cooking Lake and Swan Hills replacement matrix dolomite lack any significant trends with depth except for the Cooking Lake platform weil 7-34-33-6W5. Mn and Na follow an exponentially decreasing and increasing curve respectively over 20 metres of increasing burial depth (Fig. 27). This relationship was not observed in other Cooking Lake, Leduc or Swan Hills wells. No covariance is observed between Fe, Mn, Na, and Sr concentration~ from Leduc/Cooking Lake and Swan Hills replacement matrix dolomite, saddle dolomite, and calcite. INTERPRETATIONS Trace element concentrations 10 Leduc/Cooking Lake and Swan Hills Formations are similar. Replacement matrix dolomite and limestone precursors have different trace element concentrations. Geochemical differences observed between limestone and replacive dolomite can be attributed to f1uid composition, original limestone composition, or other diagenetic or kinetic factors present during dolomitization, or a combination of sorne or ail of the above. According to Veizer (1983), calcite and dolomite, if in equilibrium with seawater, shouJd precipitate with 2 to 39 ppm and 3 to 50 ppm Fe respectively. These concentrations are mu ch lower than 188 ppm Fe and 1623 ppm Fe concentrations for calcite and dolomite of this stud}. The differences between seawater 74 precipitated calcite-dolomite and calcite-dolomite of this study can be attributed to the replacement nature of the dolomite and the associated distribution coefficients (Veizer, 1983; Land, 1985; Machel, 1988), local reducing/oxidizing conditions, rate of dolomite precipitation, and/or fluid composition. Mn concentrations in limestone (35 ppm) and in replacement matrix dolomite (1(' l ppm) are much higher than predicted for limestone and dolomite precipitated in equilibrium with sea water (1 ppm). The reasons for the se differences are similar to those responsible for Fe variations. Fe and Mn concentrations reported are comparable to results reporled by Kaufman (1989) for a similar dolomite from the Swan Hills Formation in the Rosevear area. In contrast, strontium concentrations in limestone (140 ppm) and replacement matTÏx dolomite (81 ppm) are substantially lower than the 1000 ppm, and 470-550 ppm concentrations predicted by Veizer (1983) for limestone and dolomi te preci pItati n g 10 equilibrium with seawater. Unlike Fe and Mn the distribution coefficient for Sr is lower th an 1, hence it IS preferentially excluded from carbonates during diagenesis. Alternatively, Machel (1988) argues that dolomite preclpllating in equilibrium with seawater (between 25 -50°C) should have 128- 155 ppm Sr rather than 470-550 ppm as proposed by Veizer (1983). Machel further advocates that seawater/meteoric water mixtures with more than 5% seawater has a mSr/rnCa ratio close to seawater and if such a mixture contains a dissolved precursor carbonate with elevated concentrations of Sr (more than 2000 ppm), prccipitated dolomite should have less than 43-150 ppm Sr. Replacement matrix dolomite, with an average S: concentration of 81 ppm rather than the proposed 128-155 ppm for seawater precipitated dolomite, could have precipited in the presence of slightly modified seawater using Machel's values (Machel and Anderson, 1989). On the other hand, Vahrenkamp and Swart (1990) suggest that Sr concentrations and % mol MgC03 are covariant. Based on Vahrenkarnp and Swart's new evidence, low Sr concentrations (60 ppm) are prcdicted for seawater 75 , precipitated stoichiometric dolomite from the Bahamas (Late Tertiary), but such covariance is not observed in this study (Fig. 26). Sodium (Na) concentrations in lime stone (220 ppm) are lowe; than those ln replacement matrix dolomite (418 ppm). Na concentrations in limestone fall within the range of seawater precipitated calcite estimated at 200-300 ppm (Veizer, 1983). In contrast, replacement matrix dolomite reported in this study (418 ppm) has a much higher sodium concentration than seawater precipitated dolomite (110-160 ppm). This has also been observed in dolomite from the Nisku in north central Alberta (Machel, 1988), Keg River in the Rainbow area (Qing and Mountjoy, 1989), and the Upper Oevonian Miette Buildup (Mattes and Mountjoy, 1980) for which a high concentration of sc!id or fluid inclusions rich in Na have been proposed as a possible explanation. Another alternative is the tendency for Na to be substituted into the carbonate lattice of dolomite (Land, 1980). Trace element concentrations encountered in saddle dolomite (1273 ppm Fe, 146 ppm Mn, 294 ppm Na, and 70 ppm Sr) are comparable to concentrations found in replacement matrix dolomite (1623 ppm Fe, 104 ppm Mn, 418 ppm Na, and 70 ppm Sr). Although both dolomite types are similar in terms of trace element concentrations, their stable and radiogenic isotopes indicate that they precipitated from very different fluids (see stable and radiogenic isotope discussions). Lateral or vert 1 - 11 trace element distribution patterns are not presen t In replacemen t matrix dol omi te. Trace element concentrations plotted against burial depth or cross-ploted against other elements are scattered and overlap the distribution patterns of other minerais. Increasing or decreasing trends in trace element concentrations with respect to f1uid f10w direction, advanced by Machel (1988), were not observed except in one weil (7-34-33-6W5) from the Cooking Lake platform (Fig. 27). Three samples taken over a 20 metre interval show respectively decreasing and increasing Mn and 76 l 280 260 - • 240 220 200 E 180. -a. a.... 160 - -(J) c 140 Cl 120 Cl • 0 100 [J [J [] [J cm 80 • CKl [J .[] [J 60 • •CJ C cc Cl [0 Cl "!' 40 •• • • 44 46 48 50 52 54 56 Stoichiometry (°10 mol MgC03) • Swan Hills Replacement Matrix Dolomite [] Leduc/Cooking Lake Replacement Matrix Dolomite Fig. 26 Sr concentrations vs stoichiometry of replacement matrix dolomite. 77 A B 2200 620 2000 580 1800 1800 540 Ê 1400 500 Q. -E .s 1200 S: c: 460 1000 -l'II ~ Z 800 420 600 380 400 / 200 340 3960 3964 3968 3972 3976 3980 3960 39601 3968 3972 3976 3980 3984 Depth (m) Depth (m) C 2200 2000 D 1800 1600 Ê 1400 ft 1200 c: ::!: 1000 a 800 600 400 a 200 340 380 420 460 500 540 580 620 Na (ppm) Fig. 27 Trace element concentrations in Cooking Lake Formation weil 7-34-33-6W5. A) Mn concentration decreases with increasing bunal depth B) Na concentration increases with Increasing burial depth C) Mn and Na appear to covary and may indlcate a downward fluid flow direction (see text). 78 1 Na concentrations with increasing burial depth (Fig. 27). Machel's qualitative model suggests that trace elements with coefficients of distribution higher than 1 should be decreasing in concentration in the downflow direction of the fluid, while elements with coefficients of distribution higher than 1 should be increasing. Hence, Mn (coefficient of distribution > 1) should decrease while Na (coefficient of distribution <1 ) should increase in concentration in the downftow direction of the diagenetic fluid. If such a mechanism operated during replacement matrix dolomitization, the fluid flow wou Id be downward. This trend is opposite to the flow direction interpreted by Machel for sorne Nisku wells. The relatively small interval (20 m), the few data points, the fluid composition, and f10w rales (Machel, 1988) with respect to the carbonate precursor composition, ma)' cause local variations and/or reversais from theoretically predicted trends. ISOTOPIe ANALYSES CARBON AND OXYGEN ISOTOPES Replacement Matrix Dolomite. The average isotopie composition for combined Leduc/Cooking Lake and Swan Hills replacement % % matrix dolomite is 2.23 0 0 13C and -4.53 0 0 18 0 based on 68 samples (Table 5). 38 Leduc/Cooking Lake replacement matrix % 0 dolomites have an average of 1.77 0 0 13C and -4.19 /00 018 0 while 30 replacement matrix dolomites from the Swan Hills have an % % average of 2.81 00 13 C and -4.95 0 0 18 0. 0 18 0 of the two formations overlap but sorne Swan Hills samples are 1 to 2 () /00 more negative in Ô 18 0. The Leduc/Cooking Lake dolomite has a slightly greater range in Ô 13C, with sevt!ral samples lighter than + 1.5 () /0 () (Figs. 28, 29). No systematic trends occur between B 18 0 and ô 13C , relative to burial depths or relative to location (Fig. 30). Trace 79 TABLES COMPUTED AVERAGES OF TRACE ELEMENTS AND STABLE ISOTOPES IN REPLACEMENT MATRIX DOLOMITE Fe Mn Na Sr ~13C ~180 (ppm) (ppm) (ppm) (ppm) (PDB) (PDB) Leduc/Cooklng Lake n=33 n=33 n=36 n=25 0=38 n=38 x=2030 x=188 x=441 x=84 x=1.77 x=-4.19 x1=1531 x2=104 Swan "Ills n=25 n=25 0=23 n=17 n=30 n=30 x=1741 x=100 x=383 x=78 x=2.81 x=-4.95 x1=1623 x2=104 80th formations n=58 n=58 n=59 n=42 n=68 n=68 x=1906 x=155 x=418 x=81 x=2.23 x=-4.53 x1=1623 x2= 1 04 n= number of samplcs x= computcd avcragcs :x 1= corrcclcd avcragcs dlscardmg one extreme value of 18000 ppm Fe x 2= correctcd avcragcs dlscardmg three extreme values of 2000, 870, and 370 ppm Mn 80 1S a o (PD8) -7 -5 -3 -2 ~--~~--~--~~--~~----~-----+6 4 13 A 2 a C (POB) o -2 1S a o (PDB) -9 -s -7 -6 -5 -4 -3 -2 B c ~ •c , 4 ltJ C el tQJ~. " + C.,., 6J ••• . + .' ..• 0 13 8 B C (PDB) -4 -s ~------~~------~-12 • LedudCooklOg Lake Replacement Matnx Dolomite c Swan Hilis Replacement Matnx Dolomite A LedudCooklng Lake Saddle Dolomite + Swan Hlils Saddle Dolomite Fig. 28 Leduc/Cooking Lake and Swan Hills replacement matrix dolomites (A) and saddle dolomite (8). 8 l 'f '" 18 Ô 0 (PDB) -10 -8 -6 -4 -2 . L a 4 Il q. D~l)~IfI.·" • il ... Q A + + A • -4 A -8 + -12 13 Ô C (PDB) -16 -20 ~ ..,. -24 ~... • -28 • Leduc/Cooking Lake Replacement Matrix Dolomite Il Swan Hills Replacement Matrix Dolomite Il Leduc/Cooking Lake Sadd le Dolomite + Swan Hills Saddle Dolomite • Limestone  Blocky Calcite Fig. 29 C and 0 isotopes for ail respective diagenetic phases. Carbon and oxygen is depleted in blocky calcite. 82 .. • 18 Ô 0 (POB) -8 -7 -6 -5 -4 -3 -2 -1 0 • • .- 5 C 4 • C A C • • • 3 • A Â. •• 2 +• • • + + • 13 • +0 Ô C (POB) 0 0 0 -1 -2 ,,"" -3 • • -4 • 7 -34-33-6WS • 6-20-33-4WS o 11-1 0-35-5WS + 7-13-33-7WS • 0 I-04-34-5W5 C 8-14-34-7WS • 8-19-34-7WS  3-1 0-35-5WS Fig. 30 Replacement matrix dolomite C and 0 values showing the lack of trends in different wells. 83 l element concentrations plotted against a13C or a180 do not show any covariance. Carbon isotopic values for replacement matrix dolomite from both formations are similar to values reported for other Western Canada Devonian replacement dolomites (Kaufman, 1989; Qing dnd Mountjoy, 1989; Machel and Anderson, 1989). Oxygen isotopie values are slightly less depleted than those reported by Kaufman (1989) for a similar replacement dolomite. Furthermore, replacement matrix dolomite from the two formations combined has an average carbon (2.23% 0) and oxygen (-4.53%0) isotopic values very close to <.:arbon (2.70%0) and oxygen (-4.70%0) isotopie values present in its carbonate precursor and Upper Devonian marine lime stone values suggested by Carpenter and Lohmann (1989). However sadd le dolomite isotopic averages differ considerably from replacement matrix dolomite averages. Saddle dolomite isotopie v llues for the two formations combined are -2.53%0 a13 c and -6.50%0 B18 a. Average B13C for Leduc/Cooking Lake (-3.05% 0) dolomites are almost 1° /00 more depleted than Swan Hills Saddle dolomites (-2.19%0), but their respective BI8 0 signatures are comparable; -6.41 ° /00 for Leduc/Cooking Lake and -6.57°/00 for Swan Hills. Interpretations. The oxygen isotopic fractionation at equilibrium and low temperatures between dolomite and water IS unknown. Holocene ex amples (Land, 1980) and low temperature experimental data extrapolated from high temperature ~ata (O'Neil and Epstein, 1966; Sheppard and Schwarcz, 1970; Hamza and Broecker, 1974; Land, 1980, 1985) indicate that dolomite should be % enriched in 18 0 by 2-3 0 compared to a coexisting calcite. The B18 0 '"c .... ;,,~ composition of Upper Devonian "calcite cement is close to -4.5% ° (Carpenter and Lohmann, 1989; Hurley and Lohmann, 1989) thus dolomite precipitating from Upper Devonian seawater should range % % between -2.5 0 and -1.5 0. Although replacive dolomite in the Devonian of Western Canada shows a wide range of values, matrix 84 0 1 dolomite is generally depleted by 0.5 /00 or more rather than being enriched relative to its carbonate precursor. B 180 values in precipitating dolomite can be affected by the BI80 of the water, the temperature, the type of carbonate precursor, and other water/rock interactions (Land, 1980). Several researchers studying replacement matrix dolomite from the Devonian of Western Canada (Mattes and Mountjoy, 1980; Qing and Mountjoy, 1989; Kaufman, 1989; Machel and Anderson, 1989) have interpreted the light B 180 isotopes observed resulting from elevated tempe ratures associated with burial. The absence of heavier B 18 0 values in dolomites over a theoretically determined value or a coexisting precursor assumes that the carbonate precursor B18 0 contribution to the precipitating dolomite was minoT. However dolomite precipitating in a locally c10sed system (Sass and Katz, 1982) will be principally isotopically overprinted by the Iimestone precursor. Based strictly on carbon and oxygen isotopes and trace element data, three diagenetic models are not applicable for the origin of replacement matrix dolomite. One is the evaporitic environment of dolomitization. Isotopie signatures of dolomite precipitating in a near surface evaporative environment should have positive Ô 180 (Moore, 1989; Machel and Anderson, 1989; Qing and Mountjoy, 1989; Veizer, 1983; Land, 1985). Clearly replacement matrix dolomite is much too depleted in B18 0 (-4.53 % 0) to have formed under such conditions unless the observed dolomite is the result of neomorphism and the addition of more negative oxygen. In addition, dolomite forming in an evaporative environment should have higher concentrations of Sr (Veizer, 1983; Land, 1985) but replacement matrix dolomite from Leduc/Cooking Lake or Swan Hills has low concentrations of Sr. The low Sr content, light B18 0 isotopie values, replacement natul e of the dolomite, and petrographic textures do not corroborate an evaporative environment of dolomitization. Another diagenetic model is the meteoric environment. Carbon and oxygen isotopie ratios for dolomi te forming in a meteoric environment are variable (Lohmann, 1987). B18 0 values in meteoric 85 waters are usually more depleted in 180 than normal seawater (Land, 1983; Lohmann. 1989) due to fractionation processes during ev aporati on/precipita tion. Rock/water interaction could enrich meteoric water in 18 0 but carbonate solubility in meteoric water is very low (Allan and Mathews, 1982) unless it contains high levels of C Q 2 (which would lower the pH favoring rock/water interaction). Replacement matrix dolomite should be depleted in 18 0 and have lower sodium concentrations, if it precipitated from C02 depleted meteoric water. The geochemistry of replacement matrix dolomite does not support a meteoric dolomitization model in either formation. Mixing zone dolomitization proposed by Badiozamani (1973) is also problematic. Hardie (1987) contends that disordered dolomite precipitating in the mixing zone should form from waters of approximately 30 and 40% seawater rather than 5 to 47% seawater advanced for ordered dolomite (Badiozamani, 1973). Precipitation of disordered dolomite is mort! realistic in most environments of dolomitization hence the theoretical range of salinity favoring dolomitization, in today's mixing environment, is substantially reduced. Lower concentrations of Na (although equivocal) should be observed in fluids diluted by meteoric waters as weil as in the resulting precipitated dolomite. However, replacement matrix dolomite contains higher concentrations of Na (418 ppm). % Saddle Dolomite. Saddle dolomite cements (averaging -2.53 0 S13C and -6.50%0 (518Q); Table 6) ale more depleted in 18 0 than replacement matrix dolomite. Light S 18 0 values for this type of pore filling saddle dolomite (Figs. 28, 29) are attributed to high temperatures or fluids with light S 18 0 values or both (Radke and Mathis, 1980). Fluid inclusion work from Middle Devonian and Upper Devonian dolomite (Aulstead and Spenc~r, 1985; Aulstead et al., 1988; Qing, personn. comm; Kaufman, 1989) suggest that saddle dolomite forms at temperatures of 90°C or more. Two b~:;ic models have been evoked for explaining the presence of this high 86 · ,( TABLE 6 COMPUTED A VERAGES OF TRACE ELEMENTS AND STABLE ISOTOPES IN SADDLE DOLOMITE Fe Mn Na Sr I)13C SIIIO (ppm) (ppm) (ppm) (ppm) (POO) (POB) Leduc/Cooklng Lake n::2 n=2 n=2 n=2 n=4 n=4 x::1490 x=165 x=235 x=55 x=-3.05 x=-6.41 Swan Hills n::2 n=3 n=3 n=3 n=6 n=6 x::1800 x=133 x=333 x=80 x=-2.19 x=-6.57 Both Formations n::4 n=5 n=5 n=5 n=10 n=10 }(::1273 x=146 x=294 x=70 x=-2.53 x=-6.50 n= nurnber of samples x= compuled averages ( ;{ '\ 87 1, 1 temperature saddle dolomite. These model sare burial/press ure solution (Qing, 1986; Qing and Mountjoy, 1989; Mattes and Mountjoy, 1980) and hydrothermal circulation (Morrow et al., 1986; Aulstead and Spencer, 1985; Aulstead et al., 1988, Viau~ 1986, 1987; Viau and Oldershaw, 1984). Both models are based on data indicating that hot fluids are responsible for saddle dolomite precipitation. Sadd le dolomite precipitation is minor in both Swan Hills and Leduc/Cooking Lake Formations in the Caroline area. Pressure solution generated by normal burial could be responsible for saddle dolomite precipitating near stylolites. The required Mg could be provided by pressure dissolved replacement tratrix. dolomite and/or by dolomite related to hydrocarbon oxidation (Machel, 1987). The latter possiblility could explain why sorne saddle dolomite is present in !imestone. STRONTIUM ISOTOPES Replacement Matrix and Saddle Dolomite. Three replacement matrix dolomites in the Leduc/Cooking Lake have values (.7081 to .7083; Fig. 31) close to the expected Frasnian seawater value of .7082-.7083 (Burke et al., 1982; Popp et al., 1986; Fig. 31). The three Swan Hills replacement matrix dolomites (.i083, .7086, and .7089) are more radiogenic than Frasnian seawater values. Leduc/Cooking Lake saddle dolomite values are much more radiogenic (.7102 and .7106) than replacement matrix dolomite. One Swan Hills replacement matrix dolomite (.7089) overlaps with the sadd le dolomite. Radiogenic saddle dolomite is common and similar resuIts have been reported by Machel (1987), Kaufman (1989), Mountjoy and Halim-Djhardja (in press), and Qing (pers. comm.). Sadd le dolomite could have acquired radiogenic strontium from contact of precipitating fluids with shales, the basement or other sources (Machel and Anderson, 1989; Kaufman, 1989). No correlation or trends are observed between Sr content (Fig. 32) 180, 13C (Fig. 33), and 87Sr/86Sr. 88 0.7110 0.7108 0.7106 0.7104 0.7102 -- 0.7100 ... 07098 :st/) 0.7096 ï::; en 0.7094 IX; 0.7092 0.7090 0.7088 0.7086 Frasnian seawater• ...J 0.7084 ~ 0.7082 - ( 07080 w w X X W ....J ..J z 0 0 il: il: 0 0 l- I- < «0 ~ < en CI) U) ~ ~ w Q) ::i!! ~ !II ~ ~ ~ Ï cu ~ Ï tU ....J c ..J C !!. cu Cl tU CIc Ï ~ c ~ CI) :i: (f) :i2 c:: 0 0 co 0 0 ~ () u en Cl Il :J :l "C al Q) ..J ....J Fig. 31 Strontium isotope values for dolomite, calcite, and limestone. 4" 89 0.7110 0.7108 0.7106 L~ 0.7104 0.7102 r'1 -~ 0.7100 0.7098 .. 0.7096 (/) lB 0.7094 1:; Cf) 0.7092 r-. Q) 0.7090 r, 0.7088 1. .... 0.7086 f'\ 0.7084 T 0.7082 A Q Q 0.7080 1 40 50 60 70 80 Strontium (ppm) o Saddle Dolomite o Replacement Matrix Dolomite Fig. 32 Strontium isotope values vs strontium concentrations, in replacement matrix and saddle do!omite. 90 -3~------~ l D + D + -4 -5 c 18 o 0 (POB) -6 -7 .. ~+-----~------~----~------~----~07080 07082 07084 07086 0.7088 07090 87 86 Sri Sr 5~------~ 4 C ~ + ~ " 3 C + + o 13C (POB) 2 D ~------~----~------~----~ 07080o+--- __07082 07084 07086 07088 07090 87 86 Sri Sr C Leduc/Cooking Lake Replacement Matrix Dolomite + Swan Hills Replacement Matrix Dolomite Fig. 33 Carbon and Oxygen isotopes vs Strontium isotopes " 1 in replacement matrix dolomite. 91 Limestone. The single lime stone sample submitted for strontium analysis has a .7083 which is characteristic of Frasnian seawater. A coexisting replacement matrix dolomite (.7089) is more radiogenetic. COMPARISON OF LEDUC AND SWAN HILLS REPLACEMENT MATRIX DOLOMITE-SUMMARY. Replacement matrix dolomite has similar petrographic and geochemical characteristics ln both formations (Fig. 34). Dolomitization is restricted to the bank margin in the Swan Hills Formation but IS pervasive throughout the buildups In the Leduc/Cooking Lake Formation. These differences may be attributed to permeability and hydrologic considerations (see discussion). Aside from the smaller crystal size of the dolomite rhombs in the Leduc/Cooking Lake, the yellow/green fluorescence is the most distinctive difference between the Leduc/Cooking Lake and Swan Hills replacement dolomite. However yellow and green fluorescence is observed in both formations thus it is not a discriminating criteria. The green fluorescence in replacement matrix dolomite could be the result of 1) the nature of primary organic material/hydrocarbon inclusions in the crystal structure, 2) the absence of f1uorescing inclusions in the crystal structure, 3) thermal maturity of the organic material/hydrocarbons, and less likely, 4) trace elements within the crystal structure. Fluorescence differences between Leduc/Swan Bills replacement matrix dolomite cou Id result from diagenetically different f1uids and/or depositional differences (see discussion). Replacement dolomite has comparable trace element concentrations but slightly contrasting stable and radiogenic values. The slightly 13C enriched dolomite in the Swan Bills Formation compared to dolomite in the Leduc/Cooking Lake Formation contrasts with the slightly less 18 0 depleted dolomite in the Leduc/Cooking Lake Formation compared to dolomite in the Swan Hills Formation. There is an insufficient number of Sr isotope analyses in the Swan 92 1 LEDUC/COOKING LAKE SWANHILLS A) DISTRIBUTION Whole Buildup Reef Margin B) PETROGRAPHY C%rs Sandy Belge Sandy Beige Grey Crystal sizes 35-60 um 100-160 um 100-140 um 200-300 um 200-250 um - - X=250 um X= 140 um Texture Anhedral to Anhedral to subhedral - subhedral CL Homogeneous to Slightly Homogeneous to SlIghtly Blotchy Red Blotchy Red Fluorescence Yellow Green Porosity MOldic, Vuggy, and Microfractunng Moldic and Vuggy ( C) GEOCHEMISTRY Trace Elements Fe 1531 ppm 1623 ppm Mn 104 ppm 104 ppm Na 441 ppm 383 ppm Sr 84 ppm 78ppm MgCD3 50.2% mol 49.8% mol Ô'3C 1.77°/00 281%0 ô'8D -4.19°/00 -4.95°/00 87Sr/ 868r .7082 .7086 Fig. 34 Comparison of characteristics of Swan Hills and Leduc/Cooking Lake replacement matrix dolomite. 93 t Hills and Leduc/Cooking Lake Formations to make meaningful comparisons. DISCUSSION DISTRIBUTION OF DOLOMITE Pervasive dolomitization in the Swan Hills For.nation IS preferentially encountered at the margin of the carbonate bank (Fig. 35). Wells located in the interior of the buildups only show minor amounts of dolomite. This suggests that the hydrologie flow regime responsible for dolomitization was present along the reef margin but absent in the reef interior. Permeability barriers sueh as tight mudstones/wackestones and/or better porosity developmen t at the margin cou Id explain the dolomite distribution observed. The more pervasive Leduc/Cooking Lake dolomite distribution in the Leduc-Rimbey trend, although more difficult to explain (Figs. 36, 37), suggests 1) a large seale hydrologie regime affecting the entire reef, 2) better overall fluid flow, porosity, and permeability, or 3) the presence of an extensive conduit system which may have been absent in the Swan Hills Formation. DolomIte could have replaced limestone relatively early during Leduc/Cooking Lake and Swan Hills sedimentation. Early dolomitization is suggested by broken fragments of dolostone embedded ln sligh tly dolomitized carbonaceous basin fi Il i ng sediments present in both Leduc/Cooking Lake and Swan Bills Formations. These fragments could have been derived from a portion of the reef that was already dolomitized. The truneation of the dolomite crystals comprising the fragments supports early dolomitization. Similar observations have been interpreted by Sass and Katz (1982) for the Soreq dolomite (Cretaceous). Although less likely, it is also possible that pressure solution could have truncated ...... the dolomitized fragments. 94 .:. .~ . .1 ." J ~ :'" • 1- t-.... J 1\ b 1 1 J r· .J \ M . .:. ...;. "":' ~ 1. ~ \ JI ~ 1\ . l' l' l~ '':'' ~ ..... U f Swan Hiis Bulldupa 1-._ l·IMMWI INl.M-IIIS .-- ..-. "'~ ...... -DSIl._ I~-. DB Top 01 Beavertull Lake -DE_ DSHL Top of Swan HIUs DE Top of Elk POint Ei:s::9 Llmestone 0 Intervsl Not Cored ~ Dolomite Fig.35 Swan Hills southwest-northeast cross-section. 95 t \. .~ ,.l. .,1 J "-:- "!'" , ~, '!' l, 1 :. 1 " 1 , Il 1 ...... 1 1 ~ N .1 ~ 1.'. .~ -~ r-... .:. ~ .,:. 1. ,,~ -;" ~ .". " l' l\. .. ~v ,/ r---. "- l' ~ \ .L: .' ~- r:" ":" 7' ..:L '- Il Bulldups 1-35-33-7WS 11-33-33-tWS 7·34-33-IM 01-04-34-6WS 11-33·~W5 l' .ft., •• ~ - 1 J J II~ ii""~ ~OR. V DI Top of freton DR Top 01 Leduc DCO Top of Cooklng Lake DB Top of Beaverhlll Lake 1\ ~ Llm&stone Dolomite 0= InlSrval Not Cored ~ ~- Fig. 36 Leduc west-east cross-section. 96 1'\ ,,r . ~ ~. J l'~. 0;- 1 .. ~'':''-; 1. "i 1 1 1 l '" 1 1 I- - f' J " t-... ~ ...,.. r-... / " I~ . 1\. ,. ~ r. . / 14. ~. 1. ~7WI , ...... j . " , 1\. 17 . , i"- l' '-~ !'--" . ~, .; ',3' ' 1- [oM' .:~- 7 --:" s,:" 1 1111\ Leduc Buildups 7-13-33-6W5 01-04-34-5W5 11-1 0-35-5W5 ~ ·ur - " E 1~ DR --on------Ei DI Top ollrelon DR Top 01 Leduc ~ Dolornrte 0 Interval Not Cored Fig. 37 Leduc north-south cross-section. 97 NATURE AND SOURCES OF THE DIAGENETIC FLUIDS Potential sources of Mg incIude 1) high Mg-calcite, 2) Mg absorbed on organic matter, biogenic silica and clay minerais, 3) Mg structurally bo~ded in clay minerai, 4) Mg in the structure of organic matter (chlorophyl), and 5) pressure solution (Land, 1985; Machel and Mountjoy, 1986, 1987; Machel and Anderson, 1989). Petrographie and geochemical 0vidence discussed in the previous sections suggest that pervasive dolomitization fluids were very close to Frasnian seawater composition. Mg contributions from other sources, noted above, may have been present 10cally, but the bulk composition of the dolomitizing fluid must have been principally controlled by the volumetrically overwhelming seawater component based on stable and radiogenic isotopes. Oxygen (-4.530/00) and carbon (-2.23 % 0) isotopes from replacement matrix dolomite in both formations support a seawater precipitated dolomite at normal to slightly higher temperatures (up to 60°C ?). In addition similar carbon and oxygen stable isotopie signatures in the limestone precursor suggest buffering by the carbonate precursor in a locally cIosed system. Buffering from the precursor limestone is also suspected, at least for carbon isotopie values, in replacement matrix dolomite at Rosevear (Kaufman, 1989). 87Sr/86 Sr isotopie ratios fall in the range of Frasnian seawater composition (Burke et al., 1982) supporting a seawater derived diagenetic f1uid. I!1terpretations based on strontium concentrations (81 ppm) are equivocal since the Sr partioning coefficient is not well constrained for seawater prècipitated dolomite (470-550 ppm, Veizer, 1983; 128-155 ppm, Machel, 1989; 80 ppm, Vahrenkamp, 1988). Nevertheless, new evidence (Vahrenkamp and Swan, 1990) suggest that nearly stoichiometric d()loPlite that precipitated from seawater (Bahamas; Late Tertiary) should have low Sr concentrations (60 ppm). Na as trace element cannot be used as an accurate indicator of fluid composition since it is affected by Na-Ca 98 ( substitutions but values observed could be characteristic of a seawater to slightly modified seawater composition. RECRYSTALLIZATION Recrystallization is a process that could take place as dolomite is buried and/or influenced by periodic influx of meteoric water (Machel and Mountjoy, 1987; Kaufman, 1989). Geochemical characteristics of dolomite could be attributed to recrystallization episodes which could have partly or completely obliterated the geochemical properties of the primary dolomite. Kaufman (1989) advocates that multiple recrystallization episodes are responsible for i5oi.üjiic Sr ratios and trace element concentrations encountered in replacement dolomite of the Swan Hills Formation at Rosevear. In addition to geochemical evidence, cathodoluminescence (CL) blotchy features observed in dolomites have also been ascribed to recrystallization. Trace element ratios (Sr/Ca, Fe/Ca, Mn/Ca, Ba/Ca) and strontium isotopie ratios cao also be explained in terms of 1) local reducing/oxidizing conditions, 2) degree of openess of the system, 3) crystal growth rate, 4) fluid flow rates, 5) rock-water interactions, and 6) carbonate precursor. CL blotchy features may not necessarily be the result of recry~tallization but could be caused by inhomogeneities in the original limestone substrate or to different densities of inclusions trapped in the dolomite rhombs. In fact, inclusion rich areas are generally brighter, under CL, than inclusion poor areas (Figs. 6CD). '{ 99 , 1~, J MECHANISMS OF PALEOFLUID FLOW A number of potential mechanisms of paleofluid f10w include 1) compaction f1ow, 2) thermal convection, 3) topography driven flow, 4) reflux, and 5) seismic pumping. 1) Durial setting for compaction derived diagenetic fluids Burial dolomitization (Fig. 38) of carbonate sequences in Western Canada has been extensively discussed by several authors (Illing, 1959; Mattes and Mountjoy, 1980; Machel, 1985, 1987; Machel and Mountjoy, 1986, 1987; Qing and Mountjoy, 1989; Machel and Anderson, 1989; Kaufman, 1989). Burial compaction derived fluids funneIled into porous units is an appealing model if only for explaining the distribution and the Flame Ionization Detection(FID)/Fluûrescence behavior of Leduc/Cooking Lake and Swan Hills replacement matrix dolomite. One of the fundamental premises of this model is that increasing temperature during burial, reduces the kinetic barriers to dolomitization present at surface temperatures (Mattes and Mountjoy, 1980; Machel and Mountjoy, 1986; Hardie, 1987). Mass balance, petrographie and geochemical constraints must also be taken into consideration. The evidence for this model is petrographie .nd geochemical. Stylolitization could begin at shallow depths between a few to several hundred metres of burial. Dolomite rhombs truncating such stylolites are interpreted to be burial derived. Replacement matrix dolomite, from Leduc/Cooking Lake and Swan Hills Formations, is problematic. In aIl cases stylolitization truncates dolomitization hence stylo lites formed after dolomite. This information does not however dispute the applicability of a burial setting. The geochemical line of evidence of burial dolomitization is derived from Land's (1980, 1983, 1985) calcul.ated relationshïp between temperature and ô 18 0 of the precipitated dolomite. Determining diagenetic fluid temperature and burial depth 100 1 (m.y.) 144 66 0 Cret Tert 30 ~O 51 1000 ...... ~ ...... ~ , ... ,...... / ? ? ? 84 2000 (OC) (m) 111 3000 :K 138 " 4000 l \, ? , 165 Fig. 38 Burial history plot for the base of the Swan HUIs Formation al Bearberry (8-14-34-7W5). The curve is representative of the Swan HUIs Formation in the study area. The Leduc/Cooking Lake Formation follows a similar curve 20-40m. above the Swan Hills curve. 101 1", ~ 1 (assuming a geothermal gradient) is done by assuming the S 180 (SMOW) composition of the dolomitizing fluid. One way to constrain the problem is to use fluid inclusion tempe rature data in conjunction with Ô180 of the precipitated dolomite to derive the Ô18 0 of the fluid (Fig. 39). However fluid inclusion temperatures from pervasiv~ matrix dolomite are difficult to obtain. Added to these difficulties is estimating paleogeothermal gradients and surface paleotemperatures. Following Land's ca1culations (1980, 1983, 1985) 35 to 55°C temperatures (Fig. 39) were calculated for Leduc/Cooking Lake replacement matrix dolomite (-2.85°/00 to -6.05°/00) and 35 to 60°C temperatures were ca1culated for Swan Hills replacement matrix 0 0 dolomite (-2.94 /00 to -6.54 /00). These results are based on a presumed -2.5°/00 ô 18 0 (SMOW) value for Frasnian seawatcr (Anderson, 1985; Kaufman, 1989) and the diagenetic fluid being principally derived from Frasnian seawater. Assuming that surface paleotemperatures were close to 30° C and the present day 1 geothermal gradient of 27°C/km was also representative for the Devonian, replacement matrix dolomite can be estimated to have been buried 190-930m for Leduc/Cooking Lake dolomi te and 190- 1110m for Swan Hills dolomite (Fig. 38). These buriai depths are in close agreement with burial depths determined for simiJar dolomites from other formations (Mattes and Mountjoy, 1980; Qing and Mountjoy, 1989; Machel and Anderson, 1989; Kaufman, 1989). Determining what was an adequate source and delivery mechanism for Mg (Machel and Mountjoy, 1987) is aiso problematic. Shale dewatering has been proposed but mass balance calculations in the Rosevear area (Kaufman, 1989) indicate that this kind of Mg contribution cannot account for the overall pervasive dolomitization of reefal units in the Swan Hills Formation. , i i 102 1 ( o-.______~----~~~~--~ 50 -&:>, W 100 OC :J ~ 150 OC W n. :?! 200 w t- 250 -15 -10 -5 o 5 DOLOMITE 5180 (PDB) Fig.39 Graph of temperature vs ~180 %o(PDB) of replacement matrix dolomite for several water compositions (%0, SMOW). Leduc/Cooking Lake 0 isotope compositions vary between -2.85 and -6.1 (0/00) which corresponds to Iines A and C (giving temperatures of approximately 35 to 55°C). Swan HiII~ 0 isotope compositions vary between -2.94 and -6.5 (%0) which corresponds to Unes Band D (giving temperr.tures of approximately 35 to 60 OC). (moditied trom Kaufman, 1989). 103 1 2) Thermal convection Thermal convection cells can become potential f1uid f10w mechanisms if a hydrodynamic system is maintained for a long period of time and if the rock and fluid characteristics are favorable (Kohout et al., 1977; Simms, 1984; Saller, 1984; Assaoui et al., 1986; Aulstead and Spencer, 1985; Viau, 1986; Morrow et al., 1986; Spencer, 1987; Aharon et al., 1987; Machel and Anderson, 1989). Temperature gradients promoted by abnormally high heat f10w (Aulstead and Spencer, 1985) or by geothermally heated fluids (Kohout et al., 1977) can initiate convection cells. One of the fundamental problems associated with this type of convection is driving fluids through semi-permeable lithologies. One solution to the problem is the presence of faults and fractures which enable fluids to freely pass through impermeable units (Aulstead and Spencer, 1985; Viau, 1986; Morrow et al., 1986; Aulstead et al., 1988; Mountjoy and ,"'- Halim-Diharadja, in press). It is unlikely that f1uids circulated without restriction through the tight shaly sediments ùf the Waterways and Ireton Formations and fault systems have not been investigated in the study area. Added lO this difficulty is the absence of a Devonian or Carboniferous heat source. Moreover if large scale convection cells had been active at the time of dolomitization fluorescence and FID differences between the Leduc/Cooking Lake and Swan Hills dolomite wûuld not be expected. Thermal convection in a shallow setting may have occurred at the time of deposition if warm f1tlids circulated in the reef similar to the present day Florida Plateau (Kohout et al., 1977; Saller, 1984; Assaoui et al., 1986). Warm ground waters (heated up by normal geothermal temperatures) circulating towards the bank margin of the carbonate platform could have encountered relatively col der seawater filtering through the porous reef and platform. The resulting circulation system would have to be maintained for hundreds of thousands or even millions of years to bring in the Mg required for dolomitization. This type of fluid flow could explain why 104 sorne dolomite is restricted to the bank margine However, the low relief of the Cooking Lake and Swan Hills basal platforms and the small size of many buildups could prevent active thermal convection (Assaoui et al., 1986). 3) Topography driven flow Topography driven flow has been proposed by Toth (1980), Hitchon (1984), Garven (1985, 1989), and Morrow et al. (1986) as a viable flow mechanism. Topographie recharge from the West Alberta Ridge could be possible during the Swan Hills deposition. However the absence of topographical highs during the Leduc/Cooking Lake deposition hinders its application during the Middle Upper Devonian. 4) Reflux Si mms (1984) and Aharon (1987) argue that slightly evaporated seawater could move downward due to its higher den si ty. Shallow water carbonate platforms could provide environments for seawater evaporation. The reflux model could explain the high sodium content of most dolomite as weB as the downward directed fluid flow implied by trace elements in one Cooking Lake platform weil (7-34-33-6W5). Restricted water bodies and denser fluids would be expected in the back barrier and lagoonal portions of the reef, yet these areas are only slightly dolomitized if at aIl at Caroline. However this mechanism could provide enough fluids to pervasively dolomitize thick carbonate sequences such as those encountered in the Leduc/Cooking Lake and Swan Hills Formations if dolomitization persisted over a long period of time. 5) Seismic Pumping Sei smic pumping of fluids (Sibson et al., 1975) is possible in the 1 Leduc/Cooking Lake and Swan Hills Formations but faulting has not 4. 105 " been investigated in this study area. Hydrothermal f1uids moving along fracture systems have been advocated for Swan Hills saddle dolomite at Swan Hills (Viau, 1984, 1986), for the Middle Devonian Keg River Formation (Aulstead and Spencer, 1985), and for the Wabamun Group (Mountjoy and Halim-Diharadja, in press). Although not directly linked to seismic activity these studies suggest that hydrothermal fluids can migrate upward along faults and fracture systems possibly in pulses (Viau, 1986; Viau and Oldershaw, 1984). FLUORESCENCE-FLAME IONIZATION DETECTION (FID) Utilizing identical analytical conditions, replacement matrix 1 dolomite has two distinctive fluorescence signatures: green and yellow fluorescence. The majority of samples in the Swan Hills Formation display a green fluorescence while most samples in the Leduc/Cooking Lake Formation display a yellow fluorescence (Fig. 40). Fluorescence in carbonates is poorly understood but the type or maturity of organic malter appear to be the main factors that control fluorescence (Bustin, 1989; Dravis and Yurowicz, 1985). Fluorescence has been used for sorne time for recognizing macerals (vitrinite, liptinite, and inertinite) in sediments and coals (Hagemann, 1986; von der Dick, 1986; Bertrand et al. 1986; Lo, 1986; Khorosani, 1987; Otten jan n, 1988). The red/green ratio IS often used to determine organic maturity of sedimentary source rocks (Stach, 1982). This technique is based on the principle that peak intensity of fluorescence is shifted towards longer wavelengths in the visible range of 400-700nm when i ! the maturity of organic matter is increased. A ratio of relative ! intensity in the red (at 650nm) over relative intensity in the green (at 500nm) gives an approximate indication of organic maturity f which can be correlated with a vitrinite reflectance maturity index. Red/green ratios were not calculated but a similar principle was 106 ( FLUORESCENCE IN SWAN HILLS ..• 1- 1 cl li j 80 E :::1 Z 60 40 YELLOW GREEN Color . 1 FLUORESCENCE IN LEDUC/COOKING LAKE 100 90 80 .. 70 "Ii.• 60 E cl '0... 50 40 1:::1 Z 30 20 10 0 YELLOW GREEN Color Fig. 40 Diagrams showing the relative proportions of yellow and green fluorescence in the Leduc/Cooking Lake and Swan Hills replacement f matrix dolomite. 107 1 ~ '1 qualitatively appHed using the green (495-560nm approx.), yellow (560-600nm approx.), and orange (600-625nm approx.) spectral colors. A relative change in fluorescence from green to yellow to orange is presumably indicative of a lower to higher maturity index (Fig. 41). Khorasani (1987) suggests that a shift towards longer wavelengths could also indicate an increasing aromatic concentration and a reduction in concentration of saturates during thermal maturation. Green fluorescence could result from higher concentrations of saturates diluting small quantities of aromatics. Based on fluorescence coloTs alone, organic inclusions ln Leduc/Cooking Lake dolomites are presumably more mature than organic material trapped in Swan Hills dolomites or Leduc/Cooking Lake dolomites are rich in organic inclusions whereas Swan Hills dolomites are poor in organic inclusions. Iî maturity of hydrocarbons is responsible for the green and yellow fluorescence of Swan Hills and Leduc/Cooking Lak~ dolomite respectively. then what l mechanism was responsible for the presence of more mature 1 compounds overlying less mature compounds in these dolomites? In order to better constrain the maturity of organic matter a number of Leduc/Cooking Lake and Swan Hills replacement matrix dolomite samples were analyzed for total organic content (TOC), level of metamorphism (LOM)/vitrinite reflectance, and SI, S2 pyrolysis parameters. TOC was extremely low in ail samples which is expected for most dolomite. LOM or vitrinite reflectance data obtained from these dolomites proved to be unreliable due in part to the low organic content. Sorne FID (Flame Ionization Detection) graphs obtained from samples submitted for pyrolysis could not be used because of low TOC values. SI and S2 pyrolysis peaks shown in Fig. 42 are representative of Leduc/Cooking Lake and Swan Hills replacement matrix dolomite. SI represents the free hydrocarbons released al lempt:'ratures up to 300°C. while S2 represents hydrocarbons generated by thermal cracking at temperatures up to 600°C (Hunt. 1979; Bustin. 1989). A strong SI peak indicates that a sample has reached the oil generating lOS { ( Violet Blue Green Yellow Red 400 430 460 490 520 550 580 610 640 670 700 Wavelengh (nm) Incr6aslng Maturlty Of The Sam pie Fig.41 Diagram showing the relative change in fluorescence with increasing maturity of hydrocarbons (1) in replacement matrix dolomites. The green fluorescence is from the Swan HUIs Formation (5-32-33-4W5), while the yellow fluorescence is trom the Leduc Formation (7-13-33-5W5). 109 , 1 stage while a weIl defined S2 peak indicates that the sample is potentially capable of producing hydrocarbons. With increasing depth of burial and organic maturity the SI peak tends to increase while the S2 peak decreases (Hunt, 1979). In the Leduc/Cooking Lake, SI peaks are !Jetter defined th an S2 peaks (Figs. 42AB) whereas in the Swan Hills Formation, S2 peaks are much more prominent th an S 1 peaks (Figs. 42DE). Assuming that the samples have not been contaminated, hydrocarbons in Leduc/Cooking Lake dolomite are believed to be slightly more mature (weil defined SI) than hydrocarbons trapped in Swan Hills replacement matrix dolomite (weil defined S2), hence FID derived hydrocarbon maturity correlates with the fluorescence maturity index. Organic maturity appears to be the most likely cause of fluorescence differences between the two formations but the presence/absence of orga.• ic matter could also control fluorescence. During subaerial exposure, surface organic material could be incorporated in dolomite and result in FID and fluorescence properties similar to higher levels of maturity. Although p.ot very likely since lime stone does not fluoresce yellow, different organic matter may have been incorporated into the Swan Hills and Leduc/Cooking Lake dolomite because of different depositional and early diagenetic histories. If the original organic matter does not differ between the two stratig~a~hic !evels and maturity is responsible for the FID and fluorescence, the yellow fluorescence and FID character of the LedlJc/Cooking Lake dolomite may be linked to a hydrocarbon event. Hydrocarbons may have been present at the time Leduc/Cooking Lake dolomite formed but were absent when Swan Hills replacement matrix dolomite l~ormed. Organic matter that matured downdip in the basin could have migrated updip into porous reefs located weil above the oil window along faults and fractures and/or a stratigraphic conduit system. Fault control!ed upward migration of hydrocarbons is unlikely sÎnce porous Swan t-Hlls dolomite is barely affected by - yellow fluorescence. Vertical and posslbly downward migration could 1 1 0 t 6-14-34-5W5 Leduc A S 1 =268 (yellow) S2=364 1 1 1 1 1 1 100 200 300 400 500 600 700 11-33-34-4W5 Leduc B S1 =291 (yellow) S2=407 1 1 1 1 1 1 1 100 200 300 400 500 600 700 12-21-33-6W5 Leduc C S1=170 S2=417 (greenish) 1 1 1 1 1 1 1 100 200 300 400 500 600 700 i 5-32-33-4W5 D -- Swan Hills 51=293 S2=420 (green) 1 1 1 1 1 1 1 100 200 300 400 500 600 700 8-1-34-5W5 Swan Hilis E S1=192 S2=419 (green) ±1 1 1 1 1= 100 200 300 400 500 600 700 Fig.42 FID signature of 5 samples from dltferent wells. A, B, C are from the Leduc'Cooking Lake Formation and 0 and E are trom the Swan Hilis Formation The bottom scale IS the temperature (OC). S1 represents peak hydrocarl'X>ns released below 300 (oC) while S2 represents peak hydrocarbons released above 300 ("C). Samples wilh a yellow fluorescence have a weil defined S1 peak (A, B) whereas samples with a green fluorescence have a weil defined 82 peak (C, D, E). The bottom curve shows an exageration of the peaks present. Il 1 have occurred on a local scale (which could explain sorne slightly fluorescing dolomite in the Swan HiIIs) but lateral migration is more likely, based on the distribution of yellow fluorescent dolomite. The Cooking Lake platform may have acted as fluid conduit for updip migration of hydrocarbons. Aqueous fI uids might have migrated prior to or coeval with hydrocarbons using the same conduit system. The Cooking Lake platform has been proposed before as a possible hydrocarbon and diagenetic fluid pathway (Illing, 1959; ~(achel and Mountjoy, 1987). The absence of dolomitization in the Golden Spike and portions of the Redwater reef complex, which are not underlain by the Cooking Lake platform, tends to support this interpretation. The factors that control fluorescence and FID in dolomite are poorly understood. Additional work will be required to better constrain these parameters. SUMMARY Based on petrographie evidence discussed earlier the following has been established: 1) replacement dolomitization is the single most pervasive diagenetic event affecting the buildups following sedimentation and early Iithification, 2) replacement dolomitization could have occurred relatively early as evidenced by dolomitized fragments found in basin filling sediments, and 3) yellow fluorescence and green fluorescence may respectively indicate the presence of mature and less mature hydrocarbons trapped 10 replacement matrix dolomite. Geochemical evidence supports the following: 1) the dolomitizing fluid is most Iikely seawater to slightly modified seawater indicated by carbon, oxygen, and strontium isotopes, and to a lesser degree trace element values, 2) buffering by the Iimestone precursor may indicate loeally closed diagenetic environments, 3) recrystallization is possible but petrographie and geochemical data is ineonclusive, and 4) pyrolysis parameters suggest that fluorescence may be caused by hydrocarbons enclosed in dolomite and that those 1 1 2 ------ t hydrocarbons were not present or were less mature during the Swan Hills pervasive dolomitization event. The following is uncertain: 1) the exact timing of dolomitization, 2) the source of Mg (likely seawater), and 3) the type of fluid flow system. HYPOTHETICAL DOLOMITIZATION MODELS IN THE LEDUC/COOKING LAKE AND SWAN HILLS FORMATIONS A) EARL Y OOLOMITIZATION BY DOWNWARD CIRCULATION OF SEAWATER, INDUCED BY SALINITY GRADIENTS (Late Devonian). Simms (1984) contends that downward fluid flow can be initiated with marginal salinity gradients between underlying and overlying porewaters. Salinity gradients between pore fluids in the Swan Hills and Leduc reefs and pore fluids in their underlying platforms could have resulted in dolomitization. Downward circulation of Leduc or Swan Hills porewaters (seawater) could have been initiated if the Cooking Lake and the Swan Hills basal platfoTms acted as conduit ~ystems and if fluids in the se conduits had a salinity less than the overlying porewaters (Figs. 43, 44, 45). Circulation of seawater in the Leduc and Swan Hills reefs could provide enough magne sium for dolomitization. During Swan Hills deposition, meteoric water recharge from the West Alberta ridge (Fig. 43) could have provided the less saline water circulation required. Since the Swan Rills carbonate platform onlaps the West Alberta ridge, fresh wa.er could have moved eastward in the basal platform. The presence of sali nit y gradients in the Leduc/Cooking Lake Formation could be caused by lower salinities in the less restricted basin associated with the Fairholme Complex (downdip) far to the south west (over 100 km) compared to higher salinities in the 113 -.. ~------2~~kml------~~~ 20mt I~~ Seawaler + Downward Circulation of Seawater ~ Fresh Water Recharge Fig. 43 Interpreted fluid circulation patterns to explain the dolomite distribution in the Swan Hills Formation. Fresh water recharge trom the West Alberta Ridge would place Iow salinity fluids in the basal platform. Downward circulation would result from higher salinity seawater overlying the less saline fluids in the platform (see Fig. 44 for details). 1 1 4 Backreef D lImestone iii Dolomite S Seawater Sahnlty Fluid Flow Carbonate Forereef ~ Platform El] Umestone cS Salinity Lower Than Seawater ~ Fig. 44 Interpreted fluid circLilation patterns to explain the dolomite distribution in the Swan Hills Formation. Fluids with lower sali nit y «S) than normal seawater (S) in the basal platform (wide arrows) could induce downward circulation of seawater (small arrows). More permeable lithologies would allow better circulation of fluids hence promote dolomitization (dark pattern). Areas with Iower permeability would only be slightly dolomitized. t.. , 115 ....41------100 km-----~~. l Mlbarra Basin Seawater A , 1 ~, l'1 B 700m 1 8 Salinity <8 Lower Sahnlty AbouI50X Vertical Exagerallon Fig. 45 Interpreted fluid circulation patterns to explain the dolomite distribution in the LedurlCooking Lake Formation. Less saline waters circulating in the Cooking Lake platform, combined with slightly evaporated seawater to the east in the Alberta Basin, could induce downward circulation of porewaters. Most dolomitization WOJld have occurred during Leduc deposition (A), or before the end of Upper Devonian deposition {B). 1 1 6 somewhat restricted and slightly evaporated seawater associated with the Leduc Rimbey trend (updip; Fig. 45). Differentiai compaction along this SW -NE trend could have forced waters of seawater salinities to be funnelled updip in the Cooking Lake platform during Upper Devonian deposition. Differentiai compaction would be caused by 1) the greater thickness of the Devonian units to the west in what is now the Main Ranges (eg. The Fairholme is nearly three times as thick in the Main Ranges as it is in the Front Ranges; Mountjoy, 1980), and 2) the deposition of the Sassenach and Wabamun Formations which also thicken westward. If the fluid flow models suggested above are realistic, dolomitization may not have been a continuous process but the result diffcrent fluid flow events periodically interrupted by "dolomitization still stands" and controlled by the relative salinities of fluids trapped or moved in different parts of the basal platforms of the Swan Hills and Cooking Lake Formations. Assuming the Cooking Lake platform acted as a fluid conduit system it is possible that hydrocarbons(?) could have used the same pathway and filtered upward into the reef where dolomite was forming. Permeability, as weIl as hydrodynamic barriers would control the amount of hydrocarbons(?) reaching the reef. The resulting fluorescence and pyrolysis maturity parameters would be dependent on the volume of hydrocarbons present at the dolomitization site. Il is difficult to conceive that hydrocarbons could be present this early and at such shallow depths. However this problem can be overcome if the nature of organic matter, rather than the maturity of organic compounds, is responsible for the fluorescence and pyrolysis character of the dolomite (see previous discussion). A better understanding of what causes fluorescence is needed befoTe unequivocal interpretations can be made. The diagenetic fluid flow models proposed above explain 1) petrvgTaphic textures (e.g. dolomitized fragments), 2) carbon, 3) oxygen, and 4) strontium isotopes. However the models should be furtheT tested to determine the following: 1) the minimal salinity 117 1 gradient required to initiate downward fluid circulation, 2) regional salinity gradients, 3) regional trace element variations in platforms and reefs, and 4) what controls fluorescence and FID in dolomite. B) DOLOMITIZATION RESUL TING FROM DIAGENETIC FLUID CIRCULATION IN CARBONATE BASAL PLA TFORMS FOLLOWING TILTING OF THE SEDIMENTARY BASIN Fluids moving at different times in carbonate platforms could have been responsible for dolomitization (Fig. 46), provided these fluids contained sufficient magnesium. Basin sediment dewatering is not sufficient to pervasively dylomi tize the thick carbonate sequences of the Leduc/Cooking Lake and Swan HHls Formations. Dolomite could form if adequate magnesium could be supplied from a combination of sources including density driven fluids, basin sediment dewatering, metamorphic fluids, or other unknown sources (Figs. 46, 47). These fluids could have migrated updip during the '.' westward tilting of the basin, and progressively dolomitized the more permeable recfal units (Fig. 47). As shown on Fig. 45 tilting of the Leduc level was initiated during Upper Devonian sedimentation. The tilting process continued during the Mississippian until relatively abrupt tilting took place from the Permian to early Triassic Progressive tilting from Upper Jurassic to Early Tertiary also continued during deposition of the foreland basin clastic wedge. Diagenetic fluids in the Leduc/Cooking Lake Formation may have flowed updip southwest to northeast along the reef trend (Leduc/Rimbey). In the underlying Swan Hills Formation diagenetic fluids could have circulated updip from the bank interior towards and along the bank margin (Figs. 46, 47). A more extensive recharge area in the south west providing a larger volume of diagenetic fluids in the Leduc/Cooking Lake Formation could explain the pervasive distribution of replacement dolomite. A smaller recharge area and/or the presence of permeability barriers could explain why replacement ..,. 1 1 8 fi , Leduc Reefs ~(L.edUcIRimbeY Trend) COI~klrlg Lake Platform _~~W tL.eCluclflimb~ Trend) (Caroline) ( Basal Platform (Caroline) m Limestone Reefs and Platforms _ DolomHized Reefs and Platforms ~ Direction of DolomHizing Fluids Fig. 46 Distribution of the Leduc/Cooking Lake and the Swan Hills Formations at Caroline and LeduclRimbey in the subsurface. Diagenetic fluids are inferred to be circulating in a SW-NE direction in the Cooking Lake platform but in a E-NE direction in the Swan Hills basal platform. ( 119 J, l 1 r·:::·:~j Backreef/Forereef EIII Dolomitization EEi"7l Carbonate (Basal) ~ Shale (Carbonaceous) ~ Platform ~ Most Important Diagenetic Fluid Contribution and Direction Minor Diagenetic Fluid Contribution Fig.47 Diagram showing the inferred main flow pathways of the dolomitizing fluids in the Swan Hills and the Leduc/Cooking Lake Formations. Dolomitization f1uids could be derived trom a combination of pressure driven tlows (trom the SW in the Leduc/Cooking Lake and trom the W·SW in the Swan Hills Formation), shale dewatering, metamorphic or other sources. Fluids circulating in the Cooking Lake platform and the basal Swan Hills platform would be tunnelled upwards in the Leduc and Swan Hills reefs respectively. 120 1 matrix dolomite is restricted to the Swan Hills bank margin at Caroline. Hydrocarbons that matured downdip (or in deeper stratigraphie levels) and migrated updip could explain the fluorescence and FID character of replacement matrix dolomite in the Leduc/Cooking Lake Formation. Dolomitization in a burial environment rather than in a shallow setting better explains the fluorescence and FID variations observed between Swan Hills and Leduc/Cooking Lake replacement matrix dolomite. However the model is seriously hindered by the fact that the source of the diagenetic fluids is unknown. THERMOCHEMICAL SULPHATE REDUCTION (TSR) Thermochemical sulphate reduction (TSR) is defined as the oxidation of hydrocarbons in the presence of sulphates (Eliuk, 1984; Machel, 1987; Krouse et al., 1988). TSR is believed to have generated the high concentrations of H2S now found in many deep Devonian sour gas wells of Western Canada. For example Bearberry and Caroline H2S concentrations average 90% and 30% respectively. TSR requires elevated temperatures of at least 100°C according to Orr (1974), Machel (1987), Eliuk (1984), Krouse et al. (1988), and Sassen (1988), a temperature much lower than the estimated 250°C contended by Trundinger et al. (1985). One possible explanation of the discrepancy is that H2S is acting as a catalyst and enables the reaction to occur at lower temperatures (lOOOC). Bacterial Sulphate Reduction (BSR) can also produce H2S but this process alone cannot account for the large H2S accumulations at Caroline and Bearberry. Although dolomite precipitation resulting from TSR is possible TSR cannot account for the large volume of pervasive replacement dolomite present. The most common byproducts of TSR are elemental sulfur, { bitumen, and calcite or dolomite cement (Eliuk, 1984; Machel, 1987). 121 l Petrographie..; and geochemical evidence suggests that TSR IS responsible for blocky calcite cementation and ca1citization of anhydrite. One diagnostic characteristic of sulphate reduction is the presence of ca1citized anhydrite (Fig. 21 D). Calcium ions released during the reduction of sulphate (S04-2 to H2S) allow calcite or even dolomite to precipitate (if free Mg ions are present). If calcite is precipitated during sulphate reduction, the anhydrite precursor may be present as inclusions in optical continuity with anhydrite outside the calcite crystal (Fig. 21D). The three TSR byproducts (bitumen, elemental sulfur, and calcite) next to each other in a given sample (Fig. 21 D) strongly support TSR. These associations have been documented in both Leduc/Cooking Lake and Swan Hills dolomitized and undolomitized wells especially in the Bearberry and Caroline Fields. The presence of light l'C isotopes in blocky calcite supports the involvement of TSR (Fig. 29). Four blocky calcite samples from Bearberry and Caroline (Table 3) are very depleted in l3C: -26.56'1 /00, % -23.1%0, -16.23 0, and -9.01%0. This depletion is interpreted to be caused by oxidation of hydrocarbons which releases 13C depleted C02. Carbonates precipitating in the presence of such C02 are 13C depleted (Krouse et al., 1988). 13C depleted cements have also been ob:.erved by Machel (1987), Heydari et al. (1988), and ~(aufman (1988) at intermediate and deeper parts of the Alberta Basin. The presence of C02 and/or H2S dissolved in formation waters can cause carbonate dissolution. H2S or C02 production during TSR may have increased reservoir porosity by enabling H2S or C02 rich formation waters to dissolve earlier precipitated carbonate cements that occlude moldic porosity. TSR produced H2S may have contributed to reservoir creation. TSR byproducts may have also decreased porosity with the precipitation of calcite and dolomite cements, sulfur, and bitumen. - 122 CONCLUSIONS 1) Replacement matrix dolomite is nearly stoichiometric, is distri buted throughout the Leduc/Cooking Lake buildups, but is mostly concentrated at the bank margin in the Swan Hills Formation. 2) Replacement matrix dolomite in the Leduc/Cooking Lake Formation is anhedral to subhedral, has crystal sizes ranging between 60 and 250Jl m and averaging 140J.1 m, has abundant inclusions, a homogeneous red cathodoluminescence, and a yellow to slightly greenish yellow fluorescence. 3) In the Swan Hills Formation, replacement matrix dolomite is anhedral to subhedral, has crystal sizes varying between 100 and 300J.1m with two fine modes near 140Jlm and 300llm but average 250 J.1 m, has abundant inclusions, a homogeneous red cathodoluminescence, and a green fluorescence. 4) Swan Hills and Leduc/Cooking Lake replacement matrix dolomites have relatively similar trace element concentrations. The aI8 0 of the two formations overlap for the most part, but sorne Swan Hills samples are 1 to 2°/00 lighter in BISO. The Leduc/Cooking Lake dolomite has a slightly greater range in ô 13C, with several samples lighter th an +1.5%0. 87S r/86Sr values in the Leduc/Cooking Lake dolomite (.7082) are near the estimated Late Devonian seawater composition (.7082), but 87Sr/86Sr values in the Swan Hills Formation are more variable (.7083-.7089). a13c, aI8 o, and 87Sr/86Sr (although variable in the Swan Hills Formation) support a seawater derived diagenetic fluid in both formations. 5) Based on oxygen isotopie values and on an assumed ô 18 0 water composition, replacement matrix dolomite cou Id have formed at temperatures ranging between 35 and 55°C in the Leduc/Cooking Lake Formation, and between 35 and 60°C in the Swan Hills Formation. 6) Yellow fluorescence, and pyrolysis parameters could indicate the presence of mature hydrocarbons when Leduc/Cooking Lake replacement matrix dolomite forlll.;d. Green fluorescence and 123 1 1 pyrolysis parameters may suggest that hydrol..arbons were not present when Swan Hills dolomite formed, or that hydrocarbons were less mature than those present during dolomitization in the Leduc/Cooking Lake Formation. However what causes fluorescence is uncertain at the present. 7) Recrystallization of replacement matrix dolomite cannot be ruled OUlt but petrographie a.~d geochemical evidence does not unequivoc;ally support 3everal episodes of dolomite recrystallization. 8) Pervasive replacement matrix dolomile could have formed relatively early (Late Devonian) by means of fluids that were possibly driven by regional salinity differences. Fluids relatively less saline than seawater circulating in the Cooking Lake platform and in the Swan Hills basal platform could have induced downward migration of seawater porewater present in the overlying reefs thus causing dolomitization. Alternatively, diagenetic fluids, including, density driven, basin sediment dewatering, metamorphic fluids or other fluids derived from downdip in the basin and migrating updip cou Id have dolomitized reefat units, specifically in the Leduc/Rimbey trend, during and/or following tilting of the basin (Late Mississippian-Tertiary). However, these models are hypothetical and need to be critically tested. 9) Thermochemical Sulphate Reduction (TSR) was active during the burial of Leduc/Cooking Lake and Swan Hills carbonate sequences and was responsible for the precipitation of 13C depleted blocky calcite. TSR could have also caused sorne carbonate dissolution if high levels of H2S were present in formation waters. ,f 1 1 i ! l 124 l REFERENCES AHARON. P., SOC KI, R.A., and CHAN, L.,1987, Dolomitization of Atolls by Sea Water Convection Flow: Test of a Hypothesis at Niue South Pacifie. 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