Mississippian Oil Reservoirs in Williston Basin1
In the Williston basin, oil occurs principally in carbonate reservoirs of the Mississippian which exhibit structural folding of varying intensity, but facies changes are the basic controlling factor of accumulation. Unconformities also constitute an essential part of the flank habitats. Maps and cross sections of many fields in North Dakota, Saskatchewan, and Manitoba illustrate the controlling factors of accumulation. A lithologic cross section of the Madison group from the outcrop in the Little Rocky Mountains of Montana through the depths of the basin to the eastern wedge edge in North Dakota shows the facies changes in this sequence of rocks.