The lower and upper Bakken shales in the Williston Basin are world class source rocks, sourcing reservoirs in the Bakken, upper Three Forks, and lower Lodgepole formations, which comprise the economically significant Bakken Petroleum System (BPS). Based on the Total Organic Carbon (TOC) and pyrolysis results of Bakken samples, lower and upper Bakken shales exhibit a wide range in TOC contents, laterally from 1 wt.% along shallower basin margins up to 20 wt.% in the deeper basin, and vertically with recurrent patterns in each shale section. This high variation of TOC content may result from mixed effects of the original depositional environment and progressive maturation. Based on the modified van-Krevelen diagram, the kerogen type present in Bakken shale is primarily Type II marine oil-prone kerogen, but along the shallow east flank of the basin there is Type III kerogen input. Original hydrogen index (HI) and original TOC across the basin are empirically and mathematically restored, which are averaged at ~580 mg hydrocarbon (HC)/g Carbon and ~19–20 wt.%, respectively. The pyrolysis temperature of 425ºC, production index of 0.08, and conversion fraction of 0.1~0.15, correspond to a threshold of incipient bitumen generation from early mature shales. Due to maturation and HC generation, TOC contents are diminished by about 5~8 wt.% in thermally mature areas of the Williston Basin. Early results indicate that the upper and lower Bakken shales in the central, deeper Williston Basin are organic rich, contain oil-prone kerogen, and are thermally mature and in the oil generation window.
The Williston Basin is an intracratonic sedimentary basin, with the Nesson and Cedar Creek anticlines as major structures in the basin (Figure 1). The Devonian-Mississippian Bakken Formation of the Williston Basin in central North America is recently described as part of a continuous and unconventional petroleum system (Figure 2). The Bakken Formation consists of four members, in ascending order: the Pronghorn calcareous siltstone member, lower Bakken shale member, middle Bakken dolomitic siltstone and sandstone member, and upper Bakken shale member. The contact between Bakken and Three Fork formations is unconformable along the basin margin but conformable in the central part of the basin. The Bakken Formation is conformably overlain by Lodgepole Formation.
The lower and upper Bakken shales are world class source rocks in the Williston Basin, sourcing reservoirs in the Bakken, upper Three Forks, and lower Lodgepole formations, which comprise the economically significant Bakken Petroleum System (BPS). 10 to 400 billion barrels of oil have been estimated to have been generated from the Bakken shales, charging both unconventional and conventional plays in the BPS.