Thermo-compression simulation of hydrocarbon generation and expulsion of inter-salt dolomitic shale, Qianjiang Sag, Jianghan Basin

The inter-salt dolomitic shales in the Qianjiang Sag of Jianghan Basin have a potential to form considerable amounts of shale oil resources. The study on the thermal evolution of inter-salt dolomitic shales plays an important guiding role on shale oil resource evaluation as well as oil exploration and development. A thermo-compression simulation of hydrocarbon generation and expulsion was performed with an immature dolomitic shale source rock from the Qianjiang Formation. The quantitative yields of the products generated from source rocks with an increasing thermal maturity were calculated to explore the hydrocarbon generation and expulsion of inter-salt dolomitic shales. The results showed that there is a precursor-product relationship between residual oil and expelled oil, suggesting that oil generation is a simultaneous two-step process, namely kerogen→bitumen→oil. A strict Boltzmann distribution was observed between TR and EasyR_o value in this study, which can be used as a method to describe hydrocarbon expulsion during maturation within the oil window. Within a TR range of 0-25%, the contents of group fractions of expelled oil show slight changes. However, within a TR range of 25%-100%, the content of asphaltene fraction rapidly decreases while the contents of both saturated and aromatic fractions increase significantly. This indicated that bitumen is the main source of saturated and aromatic hydrocarbons that are gradually enriched in expelled oil. Meanwhile, the generated light hydrocarbons improve the liquidity of hydrocarbon fluid and thus enhance hydrocarbon expulsion, which results in a rapid increase in the product yield of expelled oil in this stage.