Insights on Using Solid Bitumen Reflectance as a Thermal Maturity Proxy in the Bakken Formation, Williston Basin, USA

To further refine the use of solid bitumen reflectance (BR_o in %) as a measurement of thermal maturity in source-rock reservoirs, we examined its relationship to other thermal proxies in the Bakken Formation. Comparisons included criteria from programmed temperature pyrolysis, gas chromatography (GC), and Fourier transform infrared (FTIR) spectroscopy. Thirty-two organic-rich samples from the lower and upper shale members of the Devonian-Lower Carboniferous Bakken Formation were collected from eight cores across the Williston Basin, USA, at depths (∼7575-11,330 ft) representing immature through post peak oil/early condensate thermal maturity conditions based on proximity to current hydrocarbon production. Unmodified BR_o values were correlated to programmed temperature pyrolysis parameters (hydrogen index, production index, and T _max), normal hydrocarbon and isoprenoid analysis of extractable organic matter (pristane/n-C₁₇ and phytane/n-C₁₈) from GC analysis, and peak ratios from FTIR spectroscopy (branching ratio and A-factor). Strong correlations between unmodified BR_o values (not corrected to a vitrinite reflectance equivalent, VR_e) and other thermal proxies suggest that BR_o can be used as a direct thermal proxy in marine Paleozoic source-rock reservoirs where vitrinite is rare or absent. Moreover, an apparent overestimation of VR_e at the lowest thermal maturity investigated herein may argue against the application of BR_o conversion to VR_e in the Bakken Formation. Solvent extraction caused a consistent decrease in BR_o when average post-extraction values from a given well were compared to BR_o prior to extraction, although the decrease in mean value was not statistically significant. These results are discussed in the context of advocating for the use of unmodified BR_o values as a best practice for thermal maturity determination in Paleozoic marine source-rock reservoirs.