Solid-state NMR characterization of Mowry shale from the Powder River Basin

Abstract

Solid-state 13C NMR measurements were carried out on a suite of petroleum source rocks from the Mowry shale of the Powder River Basin in Wyoming. The objectives of this study were to use CP/MAS 13C NMR measurements to monitor changes in the carbon structure of the kerogen that result from depth of burial. NMR measurements were made on a suite of samples covering a present-day depth interval of 3000–11,500 ft. Because total organic carbon values were mainly in the range of 1–2 wt%, a large-volume sample spinner was used for the 13C NMR measurements. Washing the samples with HCl was found to improve the quality of the 13C spectra. The carbon aromaticity of the kerogen increased with depth of burial, and at depths greater than ∼10,000 ft the kerogen showed little capacity to generate additional oil because of the small fraction of residual aliphatic carbon remaining in the kerogen. By combining NMR and Rock-Eval measurements, an estimate of the hydrogen budget was obtained. The calculations indicated that ∼20% of the kerogen was converted to hydrocarbons, and that sufficient hydrogen was liberated from aromatization and condensation reactions to stabilize the generated products.