Carbon Isotopic Characteristics and Evolution of Coal-Measure Gas in the Yushe-Wuxiang Block, Qinshui Basin, China

Abstract

This study experimentally analyzed gas composition and methane carbon isotopes obtained from coal, shale, and silty mudstone. Subsequently, by integrating data on thermal maturity, burial depth, gas content, and geological structural characteristics, the geochemical characteristics and genesis mechanisms of coal-measure gas were systematically investigated. The results show that coal-measure gas in the study area is dominated by CH4, with an average concentration of 90.34% and extremely high drying coefficients. A notable negative correlation between CH4 and N2 concentrations indicates atmospheric intrusion during reservoir formation. δ13C1 values (the carbon isotope ratio of methane) range from −40.412‰ to −16.524‰ (average −32.941‰), generally lower than theoretical thermogenic baselines, suggesting that the early generated thermogenic gas underwent late modifications. Within coal seams, δ13C1 values trend negatively with both gas content and CO2 concentration but decouple from burial depth. In contrast to coalbed methane, shale gas shows greater heterogeneity in its evolution, with δ13C1 values strongly and positively correlated with CO2 concentration. Tectonic faults and folds influence gas preservation and migration, while intense gas diffusion obscures vertical δ13C1 trends. Continuous tectonic uplift and meteoric water infiltration facilitate late-stage microbial colonization, promoting CH4 dissolution and the generation of secondary biogenic gas, which further lowers δ13C1 values. In conclusion, coal-measure gas in the Yushe-Wuxiang block is a composite gas reservoir modified by combined desorption-diffusion–migration effects and secondary biological effects, governed by tectonic movements, atmospheric flushing, and microbial activities.