Early Generation Characteristics of a Sulfur-Rich Monterey Kerogen {1}

Abstract

This paper documents the critical role of organic sulfur in the early formation of bitumen and oil from a sulfur-rich kerogen from the Miocene Monterey Formation in California. Monterey kerogen commonly contains abundant chemically bound sulfur (8-14 wt. %) which appears responsible for early, low-temperature generation of heavy, sulfur-rich petroleum due to the lower energy required to break carbon-sulfur bonds compared to carbon-carbon bonds. Hydrous pyrolysis was used to simulate the natural generation of petroleum from a sulfur-rich, type II kerogen from the Phosphatic member of the Monterey shale. The amount and distribution of sulfur in the kerogen, bitumen, and expelled oil generated at various temperatures indicate that (1) initial bitumen and oil generation is directly related to cleavage of weak sulfur linkages, and (2) most of the expelled oil is formed by decomposition of bitumen and not directly from the kerogen. Finally, comparison of hydrous pyrolysis results with other type II kerogens suggests that the temperature of peak bitumen generation is inversely related to the heteroatom content (primarily sulfur) of the kerogen. Low-temperature bitumen generation is an important exploration concept because kerogen rich in sulfur can generate producible oil at thermal maturities significantly below conventionally accepted values. Furthermore, established relationships between thermal maturity, sulfur content, and other oil properties such as API gravity can be used to rank exploration prospects.