About 750 billion bbl of viscous heavy oil is in place in the southern interior plains of western Canada. Most of this oil is in sandstone beds of the Lower Cretaceous Mannville Group along a 600-mi belt that extends from Peace River, Alberta, through the "Athabasca tar sands" area to Lloydminster, Saskatchewan. Regional study of the belt provides information on geologic controls of the accumulations and on genesis of the oil that is not obvious in separate examination of individual accumulations.

The accumulation at Peace River (50 billion bbl) is in a regional updip sandstone pinchout. Accumulations in the Athabasca-Lloydminster region (700 billion bbl) lie across the crest or on the southwest flank of a regional features that appear to control the accumulations already had formed at the end of Early Cretaceous time.

The oils in Lower Cretaceous rocks belong to a single oil system and all appear to be young, immature, and unaltered. The heavy oils appear to be at or very near the site where a discrete oil phase was formed. The most acceptable hypothesis of origin is that hydrocarbons which moved out of the deep basin in micellar or colloidal solution in compaction waters were "precipitated" on anticlinal structures or in sandstone pinchouts, possibly as a result of a change of salinity of the formation water.