Abstract: Bakken and Other Devonian-Mississippian Petroleum Source Rocks, Northern Rocky Mountains-Williston Basin: Depositional and Burial History and Maturity Estimations

James A. Peterson

The three-member Devonian-Mississippian Bakken-Exshaw organic-rich shaly facies is widely distributed in the northern U.S. and southern Canadian Cordillera. Equivalent facies are also present as far south as Utah and Nevada. Paleogeographically, these rocks thin markedly or pinchout to the west approximately along the Devonian-Mississippian carbonate reef-mound belt of the Cordilleran shelf margin. Although these rocks reach maximum organic richness approximately at the Devonian-Carboniferous transition, similar but somewhat less organic-rich Bakken-like beds are also present in underlying Upper Devonian and overlying Lower Carboniferous carbonate depositional cycles. At least ten cycles are identified in the underlying Duperow and Jefferson Formations, characterized by basal organic-rich Bakken-like shale or shaly carbonate that grades upward into carbonate mound or reefal beds, overlain by evaporite or solution breccia. Cycles in the overlying Lodgepole and Mission Canyon Formations, as many as 10-12 in number, are similar except that the carbonates are composed of algal-oolith, crinoid, or mixed skeletal beds, and end-cycle evaporitic units are less prevalent in the lower cycles. These dark shaly beds are the most important source of hydrocarbon reserves in Montana and the Williston basin.

Maximum net thickness of the Devonian-Mississippian organic-rich facies is in the Williston basin. However, variable thicknesses of these potential source rocks is present in parts of Montana as far west as the thrust belt. Burial history studies suggest that in some areas these rocks are probably thermally immature. However, in much of the area original burial depths are sufficient for them to reach the thermally mature stage, and therefore are of importance to further exploration efforts in the Devonian-Mississippian Madison-Duperow-Jefferson Formations.

AAPG Search and Discovery Article #90952©1996 AAPG Rocky Mountain Section Meeting, Billings, Montana