Three compositionally distinctive, stratigraphically and geographically restricted oil families occur in western Saskatchewan south of the Bakken Formation subcrop. Two oil families occur in Upper Devonian-Mississippian Bakken to Lower Cretaceous Mannville Group reservoirs, each in a different geographic part of western Saskatchewan. Biomarker compositional traits of these two families suggest both have Palaeozoic marine source rocks. Family C(SW) oil pools, in Mississippian, Jurassic and Lower Cretaceous strata of southwest Saskatchewan, are compositionally identical with oil pools entrapped in the Mississippian subcrop plays of eastern Williston Basin. Family C oils have sources in Mississippian Lodgepole Formation. Family E, the other oil pools with a Palaeozoic marine source, occurs primarily in the Middle Bakken sandstone subcrop play and in nearby Mannville Formation in west-central Saskatchewan. Family E oil pools also have slight, but persistent, biomarker compositional differences from both Family C (Lodgepole) and Family B (Bakken) oils that originate in Williston Basin. Family E oil composition and stratigraphic occurrence suggests it has a source in the Bakken Formation. Recent work by others indicates that Exshaw/Bakken Formation sources for Family E oils occur in the Alberta/Montana Trough, west of 113-degrees west longitude (Range 23W4). Family F oil pools in Lower 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 oil family suggests each family has a limited thermal maturity range. Significant maturity differences among families are indicated by gross compositional and physical property variations that follow biomarker maturity 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 biodegradation. Water-washing indicators show that process accompanies biodegradation, although biodegradation does not always affect water-washed oils. Knowledge of source, maturity and alteration provides a framework for understanding oil quality variations within this part of Williston Basin and suggests potential for further discoveries in the Swift Current Platform.