Integrated Hydrocarbon Geochemical Characterization and Pore Size Distribution Analysis for Bakken Shales, Williston Basin, USA

Zhang, Tongwei; Wiggins-Camacho, Jaclyn; Ruppel, Stephen C.; Sun, Xun

The conventional model for determining petroleum expulsion efficiency is based solely on petroleum generation and needs to be revised by combining hydrocarbon generation, pore characterization and mineralogical/lithological characterization of organic-rich shales. In this study, 24 core samples from a single well which penetrated the intervals of Upper and Lower Bakken organic-rich shales and Middle Bakken dolomite/carbonate were used to characterize residual oil and gas geochemistry and to determine the porosity and pore size distribution (PSD).

Gas chemical compositions from the released gas of crushed rock samples provide important clues to fluid phase behavior in organic-rich source rocks and tight carbonate rocks. Gases in the Upper and Lower Bakken shales contain low levels of methane (0.34%) and ethane (6.4%), but high volumes of propane (47.5%), butane (33.7%) and pentane (12.1%). In contrast, gases in the Middle Bakken lower permeability dolomite-rich rocks contain high amounts of methane (53.8%) and ethane (16.4%), and relatively low levels of propane (12.6%), butane (10.4%) and pentane (6.5%). These differences in gas composition suggest that pores in the Upper and Lower Bakken are mainly filled with oil, and that gas is dissolved in oil rather than in a free gas phase.

Variations in oil saturation in the Upper and Lower Bakken shale intervals provide a basis for determining petroleum generation index (PGI). The middle sections of each of these shale intervals exhibit relatively constant residual oil saturation (12.8mg oil/g rock). By contrast, saturation values display a gradual decrease through a 3-5 ft interval near the contacts of these organic-rich shale source rocks and underlying and overlying carbonates, indicating that lithological stacking patterns greatly affect petroleum expulsion efficiency (PEE). More detailed study of SARA fractions of the residual oils is underway to refine our understanding of oil expulsion at the molecular compound level.

N2 adsorption/desorption studies of PSD shows that both the Upper and Lower Bakken are dominated by mesopores (2-32nm) and macropores (32nm - 270nm). About 60% of the total porosity (8%) in these rocks comes from mesopores. Middle Bakken carbonates, by contrast, have higher total porosity and greater pore volume of mesopores and macropores. Based on mass balance calculation, the residual oil in Middle Bakken carbonates only takes about 20% of total pore volumes.

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