The Western Canadian basin hosts about 12 billion bbl of conventional oil in Devonian to Cretaceous reservoirs. Lower Cretaceous heavy-oil sands contain 1,300 to 2,600 billion bbl in place. They represent the biodegraded remnants of supergiant conventional deposits, the source for which has been thought to be mature rocks of the equivalent-age Mannville Group. This work shows, however, that the Mannville rocks alone are incapable of generating the required volume of hydrocarbons.

Volume of hydrocarbons generated in the Mannville under central Alberta was calculated by combining measured geochemical and geologic data with a model (modified from 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 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 been achieved.

Total volume of source rock in the basin was obtained from isopachs of Mannville shale. Multiplication of actual oil generation per unit volume by source rock volume gave a generated volume of 450 billion bbl. Inclusion of oil generated in the Foothills belt would less than double this number. These calculated values are exceedingly optimistic, however, because they ignore inefficiencies in expulsion and migration. It is therefore clear that the Mannville Group cannot be the major source of the heavy oils. Dominant contributions probably come from Paleozoic and other Mesozoic rocks.

End_of_Article - Last_Page 610------------