Quantitative Evaluation of Lower Cretaceous Mannville Group as Source Rock for Alberta's Oil Sands

Abstract

The Western Canada sedimentary basin hosts about 12 billion bbl of conventional oil in Devonian to Cretaceous reservoirs. Lower Cretaceous oil sands contain an additional 1.3 trillion bbl in place. The oil sands represent the biodegraded remnants of supergiant conventional oil deposits, the source for which has been thought to be mature rocks of the equivalent-age Mannville Group. This work shows, however, that the known Mannville rocks alone are incapable of generating the required volume of hydrocarbons. The volume of hydrocarbons generated in the Mannville beneath central Alberta was calculated by combining measured geochemical and geologic data with Lopatin's method for thermal maturation. Original hydrocarbon-generative capacity of the Mannville rocks was calculated from geochemical analyses of immature samples. Using average values for total organic carbon (TOC) (1.3%) and Rock-Eval hydrogen index (100 mg HC/g TOC), maximum hydrocarbon generation per unit volume of source rock was calculated. The maturation model was then employed to estimate the extent to which maximum yield has actually been achieved. Total volume of source rock in the basin was obtained from isopach maps of Mannville shale. Multiplication of actual oil generation per unit volume by source rock volume gave a generated volume of 450 billion bbl. These calculated values are exceedingly optimistic, however, because they ignore inefficiencies in expulsion and migration. Inclusion of oil generated in Mannville-equivalent source rock in the Foothills belt would less than double this quantity, assuming that source potential there is similar to that of the central Alberta rocks. It is clear that the Mannville Group of the Western Canada sedimentary basin cannot be the major source of Alberta's oil-sand hydrocarbons. Either the hydrocarbons were generated in as yet undocumented, very organic-rich Mannville-equivalent rocks in the Foothills, or they are derived from multiple sources throughout the basin.