Important oil pools have been discovered on the northeastern and northwestern flanks of the Canadian portion of the Williston basin. These flanks are stable shelf areas of thin sediments, facies changes, and gentle sedimentary truncation. It is in these stable, almost structureless shelf areas that the bulk of the oil is pooled. Although the present major reserves are located on these shelf areas and restricted to Mississippian, Jurassic, and Lower Cretaceous rocks, the basin is complicated by earlier configurations, in particular, the transverse Middle Devonian salt basin.

Oil accumulations in the Canadian portion of the Williston basin apparently have not been affected by salt tectonics or recent uplifts such as the Bowdoin dome. The pools are essentially stratigraphic traps closely associated to the post-Mississippian and post-Jurassic unconformities. Minor tectonics in late Mississippian, Jurassic, and early Lower Cretaceous time have localized these accumulations. This is a sharp contrast to the United States portion of the Williston basin where the major pools are restricted to Paleozoic rocks, and a number of tectonic structures are readily recognized and well known.

This paper discusses some of the features controlling Mississippian and Jurassic oil accumulations.