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

Jin, Hui; Sonnenberg, Stephen

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.

The objective of this study is to understand the source rocks' potential and its relationship with Bakken oil present in reservoirs of the BPS across the Williston Basin. Important geochemical characteristic parameters of the Bakken shales, such as organic richness, kerogen type, source rock maturity, and kinetics, are derived from the Total Organic Carbon (TOC) and pyrolysis analysis in Rock-Eval and Source Rock AnalyzerTM (SRA). Over three thousand TOC and pyrolysis results, providing good coverage of the North Dakota and Montana portions of the Williston Basin, have been collected from the Colorado School of Mines SRA Lab and US Geological Survey Energy Geochemistry Database.

Based on the TOC and pyrolysis results of Bakken samples, lower and upper Bakken shales exhibit a wide range in TOC contents, laterally from 1 wt.% at shallower basin margins up to 35 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 I/II, but along the shallow east flank of the basin there is Type III kerogen input. Original hydrogen index and TOC across the basin are empirically and mathematically restored, which are averaged at ~600 mg hydrocarbon (HC)/g Carbon and ~19-20 wt.%, respectively. Kinetic analysis suggests that averaged activation energy for Type II kerogen in Bakken shales is ~52 kcal/mol. The pyrolysis temperature of 425^oC, production index of 0.1, and conversion fraction of 0.1~0.15, correspond to a threshold of incipient HC generation from mature shales. Due to maturation and HC generation, TOC contents are diminished by about ~7-8 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.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013