Mechanisms of Shale Gas Generation from Typically Organic-Rich Marine Shales

The evaluation system of shale hydrocarbon generation is mainly addressed in the static condition at present, ignoring the dynamic process of hydrocarbon generation. Consequently, the original hydrocarbon generation potential of the shale cannot be properly evaluated. In this study, a typically marine shale sample with relatively low maturity and its kerogen were artificially matured by a half closed pyrolysis system and a closed pyrolysis system. Samples with different maturity levels obtained from the two systems were then pyrolyzed for gas generation in sealed gold tubes (i. e., pyrolysis experiment in sealed gold tubes). The quantitative analysis based on the products of C₁-C₅ gases, C₆-C₁₂ light hydrocarbons and carbon isotopes of gases from the simulation experiments indicates that the generation process of methane in kerogen can be divided into four stages:oil-generation ( < 1.0% EasyR_o), condensate-generation (1.0%-1.5% EasyR_o), wet-gas-generation (1.5%-2.2% EasyR_o) and dry-gas-stage (>2.2% EasyR_o). Kerogen, expelled bitumen and residue bitumen contributes 22.7%, 57.6% and 19.6% of maximum yield of methane in shale, respectively. Abundant soluble bitumen still exists in the shale matrix after the hydrocarbon expulsion, which becomes the major source of shale gas by interacting with kerogen and insoluble bitumen at high maturity levels.