Abstract

The thermal structure, hydraulic heads and chemical compositions of water and gas were examined and mapped in the Triassic Montney, Halfway and Doig formations of the Western Canada Sedimentary Basin (WCSB) in British Columbia and Alberta. The thermal data show temperature increasing to the southwest, paralleled by decreasing geothermal gradient magnitudes. The distribution of freshwater hydraulic heads suggests potential for fluid flow is from the south and west to the north and east. The distribution of fluid compositions indicate minimal flow. High hydraulic heads and higher than hydrostatic pressures in the west probably reflect low permeability. Water compositional variations in these areas suggest barriers to flow. Uniform fluid compositions in the central region of the Triassic suggest this area has higher permeability and improved fluid communication. Variations in Ca (super 2+) , HCO 3 (super -) and SO (super 2-) 4 are generally consistent with this interpretation but also reflect the control of lithology, particularly mineral species such as calcite, dolomite and anhydrite. High sulfate and TDS values tend to be associated with the Charlie Lake Formation evaporites. The nonhydrocarbon gases. N 2 and He, seem to reflect crustal radiogenic sources. Hydrocarbon gases in the deepest, hottest parts of the Triassic have high DGI values, suggesting either generation from thermally mature sources or thermal alteration of existing wet gas. An area of high gas pressure coincides with low DGI (wet gas), possibly suggesting active gas generation. The distribution of the reactive nonhydrocarbon gases H 2 S and CO 2 suggests that they result from bacterial sulfate reduction, with the Charlie Lake Formation as the sulfate source. Detailed isotopic and chemical analyses are required to confirm these suggestions about the controls on the distribution and origin of the water and its dissolved constituents and the hydrocarbon and nonhydrocarbon gas.

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