Neoformation of Inert Carbon during the Natural Maturation of a Marine Source Rock: Bakken Shale, Williston Basin

Abstract

The Bakken Shale (Mississippian/Devonian) of the Williston Basin was used as a natural laboratory for comparing the disproportionation reactions occurring in nature with those occurring in pyrolysis experiments. A uniform kerogen type and a broad maturity range made this possible. Mass balance calculations, analytical pyrolysis, and MSSV simulation experiments together provided strong evidence that generative yields from open system pyrolysis are not equal to potential petroleum yields in nature for this particular source rock. This is because inert kerogen formation is enhanced under both natural and closed system simulation conditions, ostensibly because of aromatization and condensation reactions involving primary aromatic structures and possibly cross-linked moieties. The outcome is that mass balance models normalized to inert carbon give overestimates of petroleum generated in nature. This is the second case where we have been able to substantiate that the phenomenon occurs in nature. The presence of high concentrations of aromatic moieties in the most immature equivalents is common to both the Alum Shale and Bakken Shale. A maturity zonation based on the residues of simulated and natural maturation allowed the quantitative and qualitative evolution of liquid natural petroleums to be predicted. The occurrence of enhanced concentrations of low molecular weight hydrocarbons in the immature zone nevertheless remains enigmatic.