Abstract
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, but an advanced geochemical and geological characterization of source rock property of Bakken shales enables more realistic oil resource estimation.
The technical contribution of this study enhances our understanding of the source rocks' potential and sequence stratigraphy of Bakken shales and the associated relationship with Bakken oil presence in reservoirs of the BPS across the basin. Over three thousand total organic carbon (TOC) content and other geochemical results, such as kerogen type, maturity, and kinetics, have been analyzed at the Source Rock Analysis Lab of Colorado School of Mines. The correlations of wireline logs, geochemical TOC and hydrogen index (HI) logs, XRF mineralogy- associated elemental logs have been integrated to infer depositional paleo-redox conditions and establish sequence stratigraphy for the Bakken shales.
The results indicate that lower and upper Bakken shales exhibit a wide range of TOC content, and the kerogen present in shales is primarily Type II kerogen. Original HI and TOC across the basin are restored and averaged at ~580 mg HC/g C and ~19-20 wt.%, respectively. The pyrolysis Tmax temperature of 425°C, 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. Lower and upper Bakken shales also exhibit vertical recurrent patterns in the TOC, GR and silicon logs, which may result from mixed effects of the original depositional environment and process and the progressive post-depositional diagenesis and catagenesis. The maximum flooding surface was identified near the top contact of lower and upper Bakken shales, indicated by the highest enrichments of Vanadium and Nickel from XRF logs. Overall, the upper and lower Bakken shales in the central basin are organic rich, contain oil-prone kerogen, and are thermally mature and in the oil generation window, with very prolific oil-generation potential.
