Formation of Coarsely Crystalline, Hydrothermal Dolomite Reservoirs in the Presqu'ile Barrier, Western Canada Sedimentary Basin

Abstract

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 ^dgr18O values of both replacement and saddle dolomites increase eastward updip along the Presqu'ile barrier from -16^pmil PDB in the deeper subsurface of northeastern British Columbia to -7^pmil PDB at Pine Point, whereas the corresponding homogenization temperatures of saddle dolomite fluid inclusions decrease from 178 to 92°C. The 87Sr/86Sr ratios of coarsely crystalline and saddle dolomites decrease eastward along the Pres u'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 basin-scale 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.