Source Rock Potential of the Bakken Shales in the Williston Basin, North Dakota and Montana
Hui Jin and Stephen Sonnenberg
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 in place 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. Anhydrous pyrolysis analysis has been conducted using the Source Rock AnalyzerTM (SRA) to measure organic richness, identify kerogen type, and indicate source rock maturity. For SRA analysis, over 150 core chip samples of Bakken shales from 24 different locations, providing good coverage of the North Dakota and Montana portions of the Williston Basin, have been collected from the US and North Dakota Geological Surveys and industrial donations. Based on the SRA pyrolysis results of Bakken samples, lower and upper Bakken shales exhibit a wide range in Total Organic Carbon (TOC) contents, from 3 wt% at shallower basin margins up to 28 wt% in the deeper central basin. This high variation of TOC content may result from mixed effects of the original depositional environment and progressive post-depositional diagenesis (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. The pyrolysis temperature (Tmax) of 425ºC and production index (PI) of 0.08 correspond to a threshold of intense HC generation from mature source rocks. During maturation and HC generation, TOC contents are diminished by about 5 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 #90156©2012 AAPG Rocky Mountain Section Meeting, Grand Junction, Colorado, 9-12 September 2012