OIL FAMILIES IN CANADIAN WILLISTON BASIN (SOUTHWESTERN SASKATCHEWAN)

Three compositionally distinctive, stratigraphically and geographicall y restricted oil families occur in western Saskatchewan south of the B akken Formation subcrop. Two oil families occur in Upper Devonian-Miss issippian Bakken to Lower Cretaceous Mannville Group reservoirs, each in a different geographic part of western Saskatchewan. Biomarker comp ositional traits of these two families suggest both have Palaeozoic ma rine source rocks. Family C(SW) oil pools, in Mississippian, Jurassic and Lower Cretaceous strata of southwest Saskatchewan, are composition ally identical with oil pools entrapped in the Mississippian subcrop p lays of eastern Williston Basin. Family C oils have sources in Mississ ippian Lodgepole Formation. Family E, the other oil pools with a Palae ozoic marine source, occurs primarily in the Middle Bakken sandstone s ubcrop play and in nearby Mannville Formation in west-central Saskatch ewan. Family E oil pools also have slight, but persistent, biomarker c ompositional differences from both Family C (Lodgepole) and Family B ( Bakken) oils that originate in Williston Basin. Family E oil compositi on and stratigraphic occurrence suggests it has a source in the Bakken Formation. Recent work by others indicates that Exshaw/Bakken Formati on sources for Family E oils occur in the Alberta/Montana Trough, west of 113-degrees west longitude (Range 23W4). Family F oil pools in Low er Cretaceous Viking Formation reservoirs of west-central Saskatchewan have Cretaceous Colorado Group sources. Previous work has shown that Family F oils were expelled from parts of Alberta/Montana Trough lying west of Calgary. Lack of variation in biomarker ratios within each oi l family suggests each family has a limited thermal maturity range. Si gnificant maturity differences among families are indicated by gross c ompositional and physical property variations that follow biomarker ma turity ratios among nonbiodegraded oils. Within each family there is a wide range of n-alkane and acyclic isoprenoid compositions that shows all three families are differentially altered by water washing or bio degradation. Water-washing indicators show that process accompanies bi odegradation, although biodegradation does not always affect water-was hed oils. Knowledge of source, maturity and alteration provides a fram ework for understanding oil quality variations within this part of Wil liston Basin and suggests potential for further discoveries in the Swi ft Current Platform.