REASSESSING THE ORIGIN OF THE HEAVY OILS AND BITUMENS OF THE WESTERN CANADA BASIN, THE WORLD'S LARGEST BIODEGRADED HYDROCARBON DEPOSIT

The Western Canada Sedimentary Basin (WCSB) contains enormous reserves of bitumen and heavy oil, 269.8 billion cubic metres (1.7 trillion barrels) in Alberta alone, accounting for 40% of the world's resources of bitumen. The oil sands and the heavy oil deposits of the WCSB are found in Lower Cretaceous and Jurassic reservoirs in Alberta and Saskatchewan, and in Paleozoic carbonates subcropping beneath the Cretaceous deposits. The origin and timing of emplacement of these hydrocarbons is still controversial. The object of this presentation is to examine both the old and the considerable amount of newer information that has a bearing on the origin of the WCSB tar sands and heavy oils.

Saturate and aromatic hydrocarbon biomarker distributions favour a single major source rock for all the bitumen deposits that was additionally the source of many Mississippian, Jurassic and Cretaceous conventional oils in the WCSB. These biomarker characteristics are also shown by the Late Devonian-Early Mississippian Exshaw Formation which is the only major source rock with the areal extent to source oils with similar biomarker characteristics ranging from northeastern British Columbia to northern Montana. Contributions from other source rocks appear to be minor.

The level of biodegradation of these deposits is highly variable, both regionally and locally although there is a general trend of increasing biodegradation from west to east for the Cretaceous deposits. Bitumens in the Paleozoic reservoirs of northern Alberta tend to be more biodegraded than those in Cretaceous reservoirs. The timing of hydrocarbon generation and migration has commonly been thought to be during Late Cretaceous-Tertiary as result of Laramide burial. We suggest that this could have occurred significantly earlier in northern areas because of a thicker Late Paleozoic to Mesozoic succession. This provides a partial explanation for the observed biodegradation trend.