Stabilization of early-formed dolomite: a tale of divergence from two Mississippian dolomites (original) (raw)
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Ichnos, 2014
Formation of northwestern Alberta, Canada form the primary reservoir intervals in the Dunvegan gas field. Sedimentological and ichnological analyses suggest a carbonate ramp setting that includes subenvironments such as sabkhas, hypersaline lagoons, restricted subtidal lagoons, intertidal mud flats, and peloidal shoals. Dolomitization occurs primarily within oxidized muds and highly bioturbated sediments, with the primary mode being sabkha-associated precipitation. In this context, dolomitization within the burrows also appears to be mediated by sulfatereducing bacteria. d 18 O values for dolomite within burrows (mean 2.4%) are enriched by 1.3% relative to calcite values (mean 1.1%) within the burrows. This degree of fractionation is similar for dolomite and calcite that have precipitated from the same solution. It is therefore suggested that the protodolomite precipitated in equilibrium with calcite rather than by replacement of preexisting calcite. Isotopic values of d 13 C measured for dolomite associated with burrows (mean 3.4%) and matrix (mean 3.5%) is slightly enriched relative to measured calcite values (mean 3.2% for matrix; mean 3.1% for burrows). These isotopic trends are common for modern dolomite that has precipitated in equilibrium with seawater where concomitant sulfate reduction and organic carbon-oxidation is inferred to occur near the surface.
Origin and characterization of hydrothermal dolomite in the Western Canada Sedimentary Basin
Journal of Geochemical Exploration, 2003
Regional and localized extensive fluid flow events may have occurred during tectonic thrusting, sediment loading, uplift and compression in the Western Canada Sedimentary Basin (WCSB). These fluids were responsible for the formation of sedimenthosted ore deposits; petroleum migration and dolomitization affecting the majority of Devonian and Mississippian carbonate reservoirs. The timing and origin of these fluid flow events remain a controversial issue. Pre-, syn-and post-Laramide fluid flow events have been invoked in the literature based on a multitude of paleomagnetic, geochemical and other evidence. The composition and evolution of ancient sedimentary fluids have been successfully reconstructed using the techniques of stable and radiogenic isotopes and fluid inclusion analyses.
Aquatic Geochemistry, 2000
Mississippian shoal carbonates of Western Canada Sedimentary Basin are important hydrocarbon hosts. Dolomitization plays a major role in the evolution of reservoir porosity in these carbonates. This process varies across the basin and reflects, in part, divergent sources and chemistry of pore fluids. Dolomites from several petroleum reservoirs were analyzed for mineralogical, geochemical and isotopic variation. The data clearly demonstrate the progressive and complex recrystallization of dolomite during shallow and deep burial in modified marine, meteoric and burial fluids. These data include: change in crystal size, stoichiometry, cathodoluminscence characteristics, stable oxygen and carbon isotopic shifts and changes in radiogenic Sr isotopic composition. However, regional geology, tectonic history and fluid flow evolution play important roles in the diagenetic imprints and the degree of recrystallization.
Bulletin of Canadian Petroleum Geology, 2017
Carbonate rocks of the Pekisko Formation make up an important reservoir in west-central Alberta, especially in fields along the Pekisko subcrop edge. They represent a transgressive-regressive carbonate platform sequence comprised of upward shallowing facies, which subsequently underwent extreme erosion leading to the development of karst topography. As a result, diagenetic alteration, mainly through dolomitization and karstification, has affected reservoir characterization for most of the carbonate facies. Several generations of calcite cementation and dolomite are the result of complex diagenetic changes. Calcite cements include isopachous fibrous, equant drusy mosaic, pendant/meniscus, blocky spar, syntaxial, fibrous, and bladed. These cements formed during early and late diagenetic events; pre-syn-and post exposure in shallow and deeper burial realms. There are five types of dolomite identified in the Pekisko Formation, based on petrographic and geochemical analyses: 1) pervasive, fine to coarse crystalline, subhedral to anhedral replacive dolomite; 2) void-filling, coarse crystalline, euhedral dolomite cement; 3) selective, fine to coarse crystalline, euhedral to anhedral dolomite; 4) dissolution seamassociated, fine crystalline, euhedral dolomite; and 5) saddle dolomite. Dolomite types 1), 3) and 4) are interpreted to have formed early in the diagenetic history and subsequently recrystallized, whereas void-filling, coarse crystalline, euhedral dolomite and saddle dolomite formed later in deeper burial setting. Petrographic evidence for recrystallization of dolomite types, excluding void-filling and saddle dolomite, includes: etching, displayed mainly on euhedral crystals; dissolved cores on many crystals of varying shapes; non-planar crystal boundaries, exclusively in pervasive dolomites; and coarsening crystal size, evident in both pervasive and selective dolomite types. Geochemical evidence, such as pronounced negative shift in oxygen isotopes (by up to 8‰ VPDB) and enrichment of radiogenic Sr isotopes further support this interpretation. There is a definite negative trend whereby wells closest to the subcrop edge have the most negative isotopic values and those farthest away show the least depletion. This trend in δ 18 O isotope values points to recrystallization of the earlier formed dolomites. Résumé Les carbonates de la Formation de Pekisko représentent un important gisement dans le Centre-Ouest de l'Alberta, surtout dans les champs le long de la bordure subaffleurante de Pekisko. Ils représentent une séquence de plate-forme carbonatée issue d'événements transgressifs à régressifs comprenant un faciès décroissant vers le haut soumis ultérieurement à une érosion extrême menant à une topographie karstique. Résultat : l'altération diagénétique, causée essentiellement par la dolomitisation et la karstification, a affecté la caractérisation du gisement dans la plus grande partie du faciès carbonaté. Les changements diagénétiques complexes résultent de plusieurs générations successives de cimentation calcitique et de dolomite. Les ciments calcitiques incluent ce qui suit : fibre isopache, mozaïque drusique équidimentionnelle, pendants/
Sedimentary Geology, 2011
The Denault Formation (2.1-1.9 Ga) crops out in the Labrador Trough, northeastern Québec and western Labrador. Rocks surrounding the town of Schefferville, Quebec contain textural characteristics consistent with deposition on the middle and outer portions of a storm-influenced shallow ramp. Mid-ramp facies consist of intraclastic grainstones with hummocky cross-stratification (HCS), swaley cross-stratification (SCS), current ripples, and graded event beds. Further outboard, grainstones grade into deeper-water laminites that are composed of even, mm-scale couplets of flat-lying organic and dolomudstone laminae. Scours within the laminites suggest periodic storm activity. Laminites gradually grade into outer ramp deep-water shales. An isolated eastern stromatolitic buildup is separated from these ramp facies by 50 km (present day). This succession can be interpreted as the remnant of a near-continuous margin or may simply represent an isolated accumulation that developed on a pre-existing topographic high. The presence of gypsum pseudomorphs in all lithofacies indicates that the Denault margin was restricted and evaporitic. Four paragenetic stages are recognized in the diagenetic evolution of the Denault Formation: (1) carbonate deposition, contemporaneous marine cementation, authigenic gypsum growth, and precipitation of authigenic chert; (2) synsedimentary mimetic dolomite precipitation; (3) pore-rimming and pore-occluding shallow burial dolomite cement; and (4) fabric destructive, sutured, anhedral burial dolomite. Gypsum crystals occur in all lithofacies, form the nuclei of interstitial dolomite rhombs, average 10 μm in length, and often display swallowtail twinning. Paleoproterozoic ocean water had very low concentrations of dissolved sulfate and evaporation in restricted settings would have been required to form gypsum. Formation of microcrystalline gypsum across this restricted ramp facilitated dolomite precipitation by increasing pore water Mg/Ca ratios and lowering its dissolved sulfate concentrations. Such an interpretation may explain why there is an abundance of synsedimentary dolostone in the Precambrian and the relative paucity of Phanerozoic analogs.
AAPG Bulletin, 1994
Late-stage, coarsely crystalline replacement dolomite and associated saddle dolomite cement form a widespread diagenetic facies in the Middle Devonian Presqu'ile barrier that extends southwestward from Pine Point to the subsurface of the Foothills in northeastern British Columbia. These dolomites create hydrocarbon reservoirs in otherwise tight limestones in the subsurface of the Northwest Territories and northeastern British Columbia, and host Mississippi Valley-type deposits at Pine Point. The coarsely crystalline replacement dolomite and saddle dolomite cement are interpreted to have formed during burial because they replace blocky sparry calcite cements, occur continuously across the sub-Watt Mountain unconformity, postdate stylolites and earlier replacement dolomites, and overlap sulfide mineralization. The δ 18 O values of both replacement and saddle dolomites increase eastward updip along the Presqu'ile barrier from-16‰ PDB in the deeper subsurface of northeastern British Columbia to-7‰ PDB at Pine Point, whereas the corresponding homogenization temperatures of saddle dolomite fluid inclusions decrease from 178 to 92°C. The 87 Sr/ 86 Sr ratios of coarsely crystalline and saddle dolomites decrease eastward along the Presqu'ile barrier from about 0.7106 in the subsurface of northeastern British Columbia to 0.7081 at Pine Point. These geochemical trends suggest a possible basinscale migration of hot, radiogenic dolomitizing fluids updip eastward along the Presqu'ile barrier. Such large-scale fluid movements probably were related to Western Canada sedimentary basin tectonic compression and sedimentary loading, which occurred at least twice: during early burial between the Late Devonian and Early Carboniferous and during deep burial between the Late Jurassic and early Tertiary.
Sedimentology, 1998
The petrography and geochemistry of ®ne-and medium-crystalline dolomites of the Middle Devonian Presqu'ile barrier at Pine Point (Western Canada Sedimentary Basin) are different from those of previously published coarse-crystalline and saddle dolomites that are associated with late-stage hydrothermal¯uids. Fine-crystalline dolomite consists of subhedral to euhedral crystals, ranging from 5 to 25 lm (mean 8 lm). The dolomite interbedded with evaporitic anhydrites that occur in the backbarrier facies in the Elk Point Basin. Fine-crystalline dolomite has d 18 O values between A1á6 to ±3á8& PDB and 87 Sr/ 86 Sr ratios from 0á7079±0á7081, consistent with derivation from Middle Devonian seawater. Its Sr concentrations (55±225 p.p.m., mean 105 p.p.m.) follow a similar trend to modern Little Bahama seawater dolomites. Its rare earth element (REE) patterns are similar to those of the limestone precursors. These data suggest that this ®ne-crystalline dolomite formed from Middle Devonian seawater at or just below the sea¯oor. Medium-crystalline dolomite in the Presqu'ile barrier is composed of anhedral to subhedral crystals (150±250 lm, mean 200 lm), some of which have clear rims toward the pore centres. This dolomite occurs mostly in the southern lower part of the barrier. Medium-crystalline dolomite has d 18 O values between A3á7 to A9á4& PDB (mean A5á9& PDB) and 87 Sr/ 86 Sr ratios from 0á7081±0á7087 (mean 0á7084); Sr concentrations from 30 to 79 p.p.m. (mean 50 p.p.m.) and Mn content from 50 to 253 p.p.m. (mean 161 p.p.m.); and negative Ce anomalies compared with those of marine limestones. The medium-crystalline dolomite may have formed either (1) during shallow burial at slightly elevated temperatures (35±40°C) from¯uids derived from burial compaction, or, more likely (2) soon after deposition of the precursor sediments by Middle Devonian seawater derived from the Elk Point Basin. These results indicate that dolomitization in the Middle Devonian Presqu'ile barrier occurred in at least two stages during evolution of the Western Canada Sedimentary Basin. The geochemistry of earlier formed dolomites may have been modi®ed if the earlier formed dolomites were porous and permeable and water/rock ratios were large during neomorphism.
Journal of Sedimentary Research, 2012
The Lonely Bay Formation, located to the west of Great Slave Lake, Northwest Territories, Canada, is a thickbedded limestone succession that includes four facies that are characteristic of a Devonian middle-ramp depositional setting. One facies in the Lonely Bay Formation is intensely bioturbated with some burrows filled with calcite and others with dolomite. The calcite-filled burrows are found close to the paleo-shoreline of the Canadian Shield, whereas the dolomite-filled burrows are found in deeper ramp deposits. In the calcite-filled burrows the parent burrows, each surrounded by a diagenetic halo, are readily apparent, whereas the dolomite-filled burrows are largely devoid of original structures. Each burrow type has its own distinctive geochemical suites of rare-earth-elements (REE), trace-elements, and d 18 O (PDB) and d 13 C (PDB) isotopes. These data indicate that sulfate-reducing bacteria, reducing conditions, and marine organic matter were present in the dolomite-filled burrows. Conversely, geochemical data from the calcite-filled burrows indicate that they remained in suboxic conditions and contained little to no marine organic matter that would have contributed to the formation of early dolomite. For these burrows, continent-derived organic matter may have hindered dolomite formation. The contrast between the two types of burrows clearly shows how the different diagenetic environments influenced the evolution of the carbonate. This study, based on interpretations of various geochemical signatures, highlights the roles that oxygen concentrations and types of organic matter (continental versus marine derived) played in dolomite precipitation.
Dolomitization of the Middle Devonian Winnipegosis carbonates in south-central Saskatchewan, Canada
Sedimentology, 2006
The Middle Devonian Winnipegosis carbonate unit in south-central Saskatchewan is partially to completely dolomitized. Two major types of replacive dolomite are distinguished. Microcrystalline to finely crystalline dolomite (type 1) displays nonplanar-a to planar-s textures, mimetically replaces the precursor limestone, accounts for about four-fifths of dolomite phases volumetrically, and mainly occurs in the Winnipegosis mounds and the Lower Winnipegosis Member directly underlying the mounds. Medium crystalline dolomite (type 2) shows planar-s to planar-e textures, commonly occurs in the Lower Winnipegosis and Brightholme members, and decreases upward in abundance. The 87 Sr/ 86 Sr ratios of type 1 dolomite (0AE70795 to 0AE70807) fall within the estimated Sr-isotopic range for Middle Devonian marine carbonates. Stratigraphic, petrographic and geochemical data constrain the formation of type 1 dolomite to hypersaline sea water in a near-surface environment, after marine cementation and sub-aerial diagenesis and prior to precipitation of the Middle Devonian Leofnard salts. Movement of dolomitizing fluids could be driven by density differences and elevation head. The shift to lower d 18 O values of type 1 dolomite [)7AE4 to )5AE1& Vienna Pee Dee Belemnite (VPDB)] is interpreted as the result of recrystallization at elevated temperatures during burial. Type 2 dolomite has higher 87 Sr/ 86 Sr ratios (0AE70809-0AE70928), suggesting that the dolomite probably formed from basinal fluids with an increased richness in the radiogenic Sr isotope. In type 2 dolomite, Sr 2+ concentrations are lower, and Fe 2+ and Mn 2+ concentrations are higher, compared with the associated limestone and type 1 dolomite. Type 2 dolomite is interpreted as having been formed from upward-migrating basinal fluids during latest Devonian and Carboniferous period.