Gas Generation and Retention in the Bakken Shale, Williston Basin

Brian Horsfield¹, Gary P. Muscio², Kliti Grice³, Rolando di Primio¹, Philipp Kuhn¹, and Ercin Maslen^3
1GFZ-Potsdam, Potsdam, Germany
²Chevron, Houston, TX
³Curtin University, Perth, WA, Australia

The Bakken Shale is charged with high concentrations of indigenous gas at low levels of maturity (Ro = 0.3 - 0.7%). A good correlation with TOC demonstrates that the gas is adsorbed to indigenous bitumen and kerogen. During the succeeding levels of early catagenesis the kerogen structure loses diaromatic components by generation and migration and/or cross-linking and condensation, and the gas is no longer retained. The origin and fate of the gas seems intrinsically linked to that of the diaromatic structural units in the organic matter which we know are inherited from green photosynthetic sulphur bacteria living in the water column under photic zone euxinia above the site of source rock deposition. Here we employ organic geochemistry and basin modelling to evaluate the shale gas potential of the Bakken Shale and to place findings within a petroleum systems context. Compositional kinetics have been measured to predict GOR development as a function of maturity, and to establish the phase behaviour of the generated fluids. The Bakken Shale is indeed inherently more gas-prone than many marine source rocks worldwide, exhibiting a lower saturation pressure. Carbon and hydrogen isotopes have been utilised to trace the origins and fate of the light hydrocarbons occurring in a free form, the diaromatic units and also the latter’s thermal degradation products. To facilitate comparisons with known shale gas provinces, star diagrams based on TOC, Tmax, vitrinite reflectance, Transformation Ratio and light hydrocarbon concentrations have been constructed. Potential sweet spot definition will presented.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas