Abstract

Selected upper Devonian Three Forks and lower Mississippian Bakken shale cores from North Dakota were described. Four lithofacies were identified in the upper Three Forks Formation, four lithofacies in the Grassy Butte Member of the lower Bakken shale, and one in the lower Bakken shale.

The base of the upper Three Forks is red silty shale and siltstone that were deposited in continental sabkha environments (Lithofacies A). Conformably overlying Lithofacies A is dolomitic shale with rip-up clasts that were created during very shallow marine reworking (Lithofacies B). Lithofacies C is highly deformed and brecciated silty and sandy dolomite and gray-green shale that were deposited in tidal flat and sabkha environments. Lithofacies D is silty dolomite, dolomitic siltstone, and gray-green shale that were deposited in tidal flat and sabkha environments. In the area of investigation, Lithofacies C and D are petroleum reservoirs.

Following deposition of the upper Three Forks beds, an erosional surface developed at the top of the formation. Three Forks lithofacies were progressively truncated from the center of the basin to its flanks. This is an angular unconformity (sequence boundary) that was capped within the central Williston Basin by the Grassy Butte Member of the lower Bakken shale. This member has four lithofacies which are genetically related to the transgression of the lower Bakken sea. Lithofacies 1, at the base of the Grassy Butte Member, is burrowed and bioturbated and has abundant siltstone and very fine-grained sandstone beds. This lithofacies is the Sanish sandstone, and it should be included with the Grassy Butte Member and not the upper Three Forks Formation. The other Grassy Butte Member lithofacies (Lithofacies 2, 3, and 4) thicken or are additional sediments that were deposited during the lower Bakken transgression. The Grassy Butte Member is thickest at the center of the basin in Williams and McKenzie counties, North Dakota.

Erosion at the top of the Grassy Butte Member marks the final transgression of the lower Bakken sea. The lower Bakken shale (Lithofacies LBS) is black, organic-rich silty and dolomitic shale that was deposited in an oxygen-stratified marine basin.