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1 Shell International Exploration and Production Inc., 3737 Bellaire Boulevard, Houston, Texas 77025; Jeff.Lonnee{at}shell.com
2 Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada, T6G 2E3
Jeff Lonnee is a carbonate reservoir geologist with Shell International Exploration and Production. He received a Ph.D. from the University of Alberta under the supervision of Hans G. Machel. He previously obtained an M.Sc. degree (1999) and an B.Sc. degree (honors) (1997) from the University of Windsor, Ontario. His research interests include carbonate diagenesis and the evolution of subsurface fluids.
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, low-temperature 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 190°C (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 60–80°C, 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 invasion of a gypsum-saturated brine during periods of extremely high heat flow and regional plate-margin tectonics in the Late Devonian to Mississippian. Fluid-inclusion homogenization temperatures suggest that hydrothermal alteration occurred between 230 (uncorrected) and 267°C (corrected), which is significantly higher than the maximum temperature of about 190°C attained by the Slave Point Formation during burial. The sources of the halite- and gypsum-saturated brines are Middle Devonian evaporite depositional environments roughly 200 km (124 mi) south and/or east of Clarke Lake, near the Peace River arch.
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