ABSTRACT: Oil Families and Their Sources in West-Central Williston Basin

OSADETZ, KIRK G., LLOYD R. SNOWDON, and PAUL W. BROOKS, Institute of Sedimentary and Petroleum Geology, Geological Survey of Canada, Calgary, Alberta, Canada

Three stratigraphically restricted, compositionally distinctive oil families dominate west-central Williston basin in southwestern Saskatchewan, south of the Mississippian subcrop. Oils in the stratigraphic interval Mississippian Madison Group to Lower Cretaceous Mannville Group, of the Battle Creek and Rapdan-Battrum area, are variously biodegraded and compositionally identical to low-maturity oils in the northwestern Williston basin Madison subcrop. The oils are attributed to a Mississippian Lodgepole Formation source rock and their low maturities suggest they have migrated from at least where the North Dakota-Montana state line meets the international border. The limited maturity spectrum of these oils suggests that structures induced by the dissolution of Middle Devonian Prairie F rmation over the Swift Current platform altered migration pathways early in the main hydrocarbon generation stage, and that more mature, higher quality crudes may have been trapped elsewhere in southwestern Saskatchewan. Another pervasively biodegraded oil family occurs in pools trapped at, or near, the uppermost Devonian-Mississippian Bakken Formation subcrop edge. These oils share some compositional similarities with Lodgepole-sourced oils, but in detail they are distinguished by both compositional traits, suggesting derivation from a Paleozoic clastic marine source rock, and their restricted stratigraphic occurrence in the Bakken Formation. The Bakken Formation is inferred to be the source of these oils, but the geographic location of their generation remains to be determined. Differe ces in thermal maturity and expulsion threshold of these Bakken-sourced oils compared to Bakken-sourced oils in the central and eastern Williston basin suggests that oils in southwestern Saskatchewan are derived from a distinctive, probably western Bakken organic and lithological facies. The final family is a sometimes biodegraded, very thermally mature, Cretaceous oil trapped in the Lower Cretaceous Viking Formation. Maturity levels suggest that these oils are far migrated. Despite their consistently lower densities (higher API gravities) these oils are also biodegraded, but the effects of biodegradation are mitigated by thermal maturity.

AAPG Search and Discovery Article #91017©1992 AAPG Rocky Mountain Section Meeting, Casper, Wyoming, September 13-16, 1992 (2009)