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

Area of Science:

• Organic geochemistry
• Petroleum geology
• Source rock analysis

Background:

• Vitrinite reflectance (VR e) is a standard thermal maturity proxy, but vitrinite is often absent in Paleozoic marine source rocks.
• Solid bitumen reflectance (BR o) offers a potential alternative for thermal maturity assessment in these formations.
• Understanding thermal maturity is crucial for evaluating hydrocarbon potential in source-rock reservoirs.

Purpose of the Study:

• To refine the application of solid bitumen reflectance (BR o) as a thermal maturity measurement in source-rock reservoirs.
• To examine the relationship between BR o and other established thermal maturity proxies in the Bakken Formation.
• To advocate for the use of unmodified BR o values as a best practice for thermal maturity determination.

Main Methods:

• Collected 32 organic-rich samples from the Devonian-Lower Carboniferous Bakken Formation across the Williston Basin, USA.
• Analyzed samples for thermal maturity using programmed temperature pyrolysis, gas chromatography (GC), and Fourier transform infrared (FTIR) spectroscopy.
• Correlated unmodified BR o values with pyrolysis parameters (HI, PI, T max), GC-derived organic matter analysis (pristane/n-C17, phytane/n-C18), and FTIR peak ratios (branching ratio, A-factor).

Main Results:

• Strong correlations were observed between unmodified BR o values and multiple thermal proxies, validating BR o as a direct thermal indicator.
• The study suggests that BR o can be reliably used in marine Paleozoic source rocks where vitrinite is scarce or absent.
• Solvent extraction showed a consistent but not statistically significant decrease in BR o values.

Conclusions:

• Unmodified solid bitumen reflectance (BR o) is a robust and direct proxy for thermal maturity in Paleozoic marine source-rock reservoirs.
• BR o measurements may be more accurate than vitrinite reflectance equivalent (VR e) conversions, especially at lower maturity levels in the Bakken Formation.
• The direct application of BR o is recommended for improved thermal maturity assessments in similar geological settings.