Diagenetic evolution of the Slave Point Formation, Clarke Lake, British Columbia, Canada

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. Some of the saddle dolomite is replacive, some is cement, and both are associated with dissolution porosity and recrystallized matrix dolomite. A rigorous petrographic and geochemical examination of the reservoir dolomites was conducted, as well as consideration of the size and shape of the dolomitized rock body.; Pervasive matrix dolomitization was accomplished by long-distance migration of halite-saturated brines during the Late Devonian to Mississippian. Fluid inclusion homogenization temperatures suggest 150°C (uncorrected) to 190°C (corrected) at the time of matrix dolomitization. The replacive and cement saddle dolomites, as well as the associated dissolution and recrystallized matrix dolomites, at Clarke Lake are hydrothermal alteration products of the matrix dolomites, and formed from the invasion of a hydrothermal gypsum-saturated brine during periods of extremely high heat flow and regional plate-margin tectonics in the Late Devonian to Mississippian. Fluid inclusion temperatures suggest hydrothermal alteration occurred between 230°C (uncorrected) and 267°C (corrected), which are 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 south and/or east of Clarke Lake, near the PRA.; The present chemistry of the formation waters in the Slave Point Formation is the result of mixing between the previously entrained halite- and gypsum-saturated brines and meteoric water. This meteoric water end-member entered the basin through recharge in the Rocky Mountains to the west and was driven by topography. Isotopic compositions and ionic hydrochemistry suggest the meteoric water end-member was involved in water-rock interaction predominantly with the Proterozoic Muskwa Formation along its migration pathway. The migration pathways of the meteoric water, in combination with the overall geometry of the Slave Point Formation platform margin, are important for both trap development and preventing the previously emplaced hydrocarbons from being flushed through the active migration of formation waters in the Devonian. This observation may prove critical in the exploration for hydrocarbon accumulations in the Middle Devonian across northeastern British Columbia.