NEOFORMATION OF INERT CARBON DURING THE NATURAL MATURATION OF A MARINE SOURCE-ROCK - BAKKEN SHALE, WILLISTON BASIN

The Bakken Shale (Mississippian/Devonian) of the Williston Basin was u sed as a natural laboratory for comparing the disproportionation react ions 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 kero gen formation is enhanced under both natural and closed system simulat ion conditions, ostensibly because of aromatization and condensation r eactions involving primary aromatic structures and possibly cross-link ed moieties. The outcome is that mass balance models normalized to ine rt carbon give overestimates of petroleum generated in nature. This is the second case where we have been able to substantiate that the phen omenon occurs in nature. The presence of high concentrations of aromat ic moieties in the most immature equivalents is common to both the Alu m Shale and Bakken Shale. A maturity zonation based on the residues of simulated and natural maturation allowed the quantitative and qualita tive evolution of liquid natural petroleums to be predicted. The occur rence of enhanced concentrations of low molecular weight hydrocarbons in the immature zone nevertheless remains enigmatic.