Abstract: Geochemical Evaluation of the Bakken Shale in the Williston Basin: Implications for Migration in Fractured Shale Plays

S. W. Rushworth

The Bakken shale in the Williston basin is both an extremely rich, regionally effective oil source rock and a locally fractured reservoir. Regional geochemical studies of the Bakken shale provide data necessary to quantitatively predict localized petroleum migration and entrapment within fractured Bakken reservoirs.

Thermal maturity is the key to fractured Bakken oil production, all of which occurs where the vitrinite reflectance exceeds 0.55, which indicates the onset of Bakken oil generation. The zone of Bakken oil generation is also characterized by high formation resistivity, caused by oil saturation of Bakken pore space and high fluid pressures in areas where exceptional source rock quality and thermal maturity overlap. The extremely large volumes of oil generated in these areas may enhance reservoir porosity by generating fractures or by propping open existing tectonic fractures.

A methodology is presented for calculating petroleum migration and trapping efficiencies in the Williston basin by considering the spatial and temporal aspects of Bakken oil generation, secondary migration, and entrapment. This methodology would also be applicable to the prediction of migration and entrapment efficiencies for other fractured source rock reservoir plays such as those in the Monterey, Niobrara, and Woodford shales in North America.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994