Crude oil stains occur geographically and stratigraphically widely in Bowser and Sustut basins in the Intermontane Belt, British Columbia. Bitumens extracted from Jurassic and Cretaceous Bowser Lake and Sustut groups have compositions indicating at least three petroleum systems. Many of the samples are from outcrops or shallow depths and some are lightly to moderately biodegraded. In some samples the biodegradation affects the normal alkanes generally and isoprenoid compounds partly, such that the polycyclic biomarker compounds are inferred to be unaltered. This is confirmed by systematic variations within and among compositional fractions and compounds. Compositional differences among the samples are best illustrated by variations of steranes and terpanes, but are also observed in gross compositional traits and other saturate and aromatic fraction compounds. One oil family is inferred to be derived from source rock kerogens in Lower Jurassic and older Stikine Assemblage. This petroleum has characteristics of pre-Jurassic, marine, carbonate source rocks deposited in hypersaline to mesohaline environments. A second oil family is derived from Mesozoic open marine source rock kerogens that are inferred to lie within the organic-rich lithologies found in distal fine clastic deposits of the upper Hazelton and lower Bowser Lake groups deposited during progradation of the Bowser clastic wedge. A third oil family that occurs only in the northern Bowser and Sustut basins originates from non-marine Mesozoic source rock kerogens, probably within thick, often coaly, non-marine upper Bowser Lake assemblages. Many of the extracted crude oils occur in rocks where the host rock vitrinite reflectance is significantly higher than the thermal maturity inferred from the crude oil geochemistry. The occurrence and composition of these crude oils expand the petroleum prospectivity of Bowser and Sustut basins by reducing petroleum system risks generally, and by indicating a possible petroleum system for Hazelton Group reservoirs, which now should be attributed a petroleum potential, where before Hazelton Group was considered commonly to be economic “basement”.

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