Characterization for Source Rock Potential of the Bakken Shales in the Williston Basin, North Dakota and Montana

Abstract

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.