Geochemical Composition and Microstructure of Coal Measure Shale in the Upper Permian, Western Guizhou, China: Implications for Methane Generation and Storage

Substantial coal measure gas is reserved in the Upper Permian shale in western Guizhou, China, but the mechanism of gas generation and enrichment remains unclear. In this study, rock-eval pyrolysis, stable carbon isotope, and low-temperature N-2/CO2 adsorption (LN2/CO(2)A) experiments were conducted on coal measure shale collected from western Guizhou. The geochemical composition and pore morphology were clarified, and the shale gas generation and storage characteristics were ultimately revealed. The results show that the total organic carbon (TOC) contents of most samples exceed 2%, indicating a high organic matter (OM) abundance. The average R-o and T-max are 2.89% and 576 degrees C, respectively, corresponding to the overmature stage, leading to the poor current hydrocarbon generation potential. Unlike marine shale with dense organic pores, these pores are generally poorly developed and heterogeneous in the samples. Densely distributed microfractures are favorable methane migration channels. The pores are dominated by mesopores, with an average pore volume (PV) proportion of 53.55%, while the specific surface area (SSA) is mainly contributed by micropores. The LCO(2)A-SSA and LCO(2)A-PV show a positive correlation with the TOC, T-max, and S-1, but the LN(2)A-SSA and LN(2)A-PV are poorly correlated with these parameters, indicating that micropores are mainly derived from organic pores and that hydrocarbons generated by OM pyrolysis are mainly reserved in micropores. When delta C-13(OM) shows a negative anomaly, R-o, T-max, and S-1 all show a negative drift in the vertical sequence, suggesting that the chemical parameters are closely related during thermal maturation. Accompanied by hydrocarbon generation, (CH4)-C-12 is preferentially separated from the OM, resulting in a higher delta C-13(OM). Meanwhile, the generated methane is mainly reserved in the form of adsorption, and the micropores/mesopores provide a favorable place for shale gas storage.