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Hans G. Machel is a professor at the Department of Earth and Atmospheric Sciences, University of Alberta. His research involves carbonate and evaporite facies and the diagenesis, lowtemperature geochemistry, and petroleum geology of Alberta, particularly dolomitization, cathodoluminescence, and diagenetic redox processes relevant to sour gas, sulfur, and Mississippi Valley -type sulfide deposits, and magnetic exploration for hydrocarbons.The Clarke Lake gas field in British Columbia, Canada, is hosted in pervasively dolomitized Middle Devonian carbonates of the Slave Point Formation. The Clarke Lake field consists mostly of pervasive matrix dolomite and some saddle dolomite, the latter varying in volume from about zero in limestones to normally 20 -40% (locally up to 80%) in dolostones over any given 10-m (33-ft) core interval. Some of the saddle dolomite is replacive, some is cement, and both varieties are associated with dissolution porosity and recrystallized matrix dolomite.The major objective of this study is to identify the causes and timing of matrix and saddle dolomite formation, specifically, whether these dolomites are hydrothermal. A comprehensive petrographic and geochemical examination indicates that pervasive matrix dolomitization was accomplished by long-distance migration of halite-saturated brines during the Late Devonian to Mississippian. Fluid-inclusion homogenization temperatures suggest about 150 (uncorrected) to 190jC (corrected) at the time of matrix dolomitization. These temperatures differ markedly from most published work on the dolomitized Devonian reefs in the Alberta Basin south of the Peace River arch, where pervasive matrix dolomitization was accomplished by advection of slightly modified seawater at temperatures of about 60-80jC, and where no hydrothermal influence was ever present. The saddle dolomites at Clarke Lake are not cogenetic with matrix dolomite and are not the product of hydrothermal dolomitization (sensu stricto). Instead, they formed through the hydrothermal alteration of matrix dolomite by way of

Petrographic and geochemical studies of the Middle Devonian (Givetian) Sulphur Point Formation in the vicinity of the Rainbow South Field, northwestern Alberta, reveal that dolomitization was a direct result of precipitation by chemically distinct fluids, and that recrystallization of these dolomites significantly altered their original chemical signatures. Sulphur Point carbonates were deposited in a restricted peritidal environment. Lithofacies include grainstones, sparsely fossiliferous packstones, mudstones, algal mudstones, and intraclast breccia mudstones. Multiple episodes of calcite cementation and dolomitization have affected these rocks to varying degrees. Five dolomite types were identified: 1) dolomicrite, 2) fine-crystalline matrix dolomite, 3) medium-crystalline matrix dolomite, 4) saddle dolomite and 5) fracture-lining dolomite.Dolomicrite (2-20 gm) replaced both micrite and calcite cement in the mud-supported facies before early compaction. A trend toward more negative 8180 values of-9.22 to -3.10%o Vienna Pee Dee Belemnite (VPDB) with respect to postulated Middle Devonian marine carbonate values suggests that dolomicrite was recrystallized by later fluids. Geochemical modelling of the isotope and trace element trends in the dolomicrite support this interpretation.Both fine-and medium-crystalline matrix dolomites (40-200 gm) are usually fabric destructive. However, some intervals have retained lamination and algal structures. Matrix dolomite was formed during intermediate burial, as suggested by its association with dissolution seams, high Fe and Mn concentrations, and 8180 values of -12.20 to -8.34%0 VPDB. This evidence, in addition to the presence of high salinity fluid inclusions (-18 wt% NaC1 equivalent), indicates that matrix dolomite was precipitated by basinal fluids between the Mississippian and Late Jurassic.The precipitation of saddle dolomite (0.5-2.0 mm) is genetically related to fractures and breccia zones where it partially to completely occludes the fractures, breccias and vugs that were developed through the dissolution of the earlier matrix dolomites. Geochemical and petrographic evidence suggests that saddle dolomite was precipitated from a hot, slightly saline (10.5 to 13.3 wt% NaC1 equivalent), calcium-rich fluid that was funnelled upward along faults and fractures that developed during the Late Cretaceous to early Tertiary Laramide Orogeny. Strontium isotope modelling confirms that saddle dolomite was precipitated from a two-component hydrothermal fluid incorporating a significant quantity of Middle Devonian brines and radiogenic basement fluids.Fracture-lining dolomite (0.2-1.0 mm) was the last dolomite phase to precipitate, and is intimately associated with blocky calcite, quartz, sulphide mineralization and pyrobitumen. Isotopic and fluid inclusion evidence imply precipitation from slightly saline brines (-8 wt% NaC1 equivalent) at elevated temperatures. Extremely low Fe and Mn concentrations, negative 813C values (~ -5%0 VPDB), and significant volumes of HzS gas suggest th...

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