Abstract

The post-Mannville sediments of the Western Canada Basin constitute a hydrocarbon system which is isolated hydraulically from the underlying sedimentary column. Thus, oils in Viking, Cardium and Belly River reservoirs are sourced exclusively from the Colorado Shales. The oils are high-quality, low sulphur crudes which, geochemically speaking, show very little variation other than that induced through source maturity differences.

The organic carbon content and the organic matter type of the Colorado Shale are controlled by the overall sedimentation rate. In the condensed sections of the First and Second White Speckled Shales and Fish Scales Zone, the organic matter is prolifically oil-prone. However, commercial oil has also been generated from leaner shales between the condensed intervals. Using geochemical data obtained from core samples, it can be shown that there is a threshold value of organic carbon content below which commercial oil sourcing ceases. Dilution of oil-prone marine organic matter with transported land-derived organics and inorganics controls effective source distribution.

Variable secondary migration pathways have been effective within the section. Mapping of oil, source rock and reservoir rock maturities, coupled with oil geochemical data, allow deductions to be made about migration distances within the post-Mannville hydrocarbon system. Maximum secondary migration distances are seen in the Viking where oil has travelled as far as the Eureka-Dodsland fields of Saskatchewan from mature sources close to the Disturbed Belt.