Most Williston basin oils belong to one of two basic types: (1) a lower Paleozoic type believed to have originated in Winnipeg shales and found predominantly in Ordovician and Silurian reservoirs, and (2) a Mississippian type expelled primarily from Bakken shales and produced mostly from Madison reservoirs. The two types are isolated vertically by evaporites but commonly are mixed beyond the evaporite limits in basin margin areas.

Both oil types can be related to their source facies. Lithofacies maps of these source sequences, when combined with diagenesis determinations, provide source-area definition. The time of expulsion and volume of expelled oil from each source can be calculated. Paleostructure maps and carrier-bed isopachs indicate the direction and extent of secondary migration, both horizontally and vertically from the source area. Ultimately, the subsurface distribution of each oil type is predicted, to define high-grade areas in which to concentrate exploration activity.

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