Geochemistry of coal seam gas in the Ordos Basin: Implications for basin-scale carbon cycling

Coal seam gas (CSG) represents an important organic carbon reservoir in sedimentary basins and plays a significant role in the global carbon cycle. Based on 408 geochemical datasets (181 original measurements and 227 compiled datasets) from eight representative subregions of the Ordos Basin (Yulin, Yan’an, Baode, Linxing-Shenfu, Liulin, Sanjiao, Shilou, and Daning-Jixian), this study systematically investigates the compositional, isotopic, and genetic characteristics of CSG and their implications for reservoir-scale carbon cycling. CSG in the Benxi Formation is dominated by CH₄ (mean 89.09%), with N₂ (mean 5.44%) and CO₂ (mean 4.52%) as the principal non-hydrocarbon components. Gas composition and stable isotopic signatures indicate that thermogenic gas predominates in the Ordos Basin, whereas mixed thermogenic and biogenic gases occur along the eastern margin, primarily controlled by differences in burial depth, burial history, and thermal evolution. Systematic spatial variations in gas composition and δ¹³C-CH₄ suggest basin-scale lateral migration of CSG from the central basin toward the eastern margin, while migration is relatively restricted in the deeply buried, low-permeability central basin. Sealing coefficients derived from N₂ concentrations, together with CH₄-N₂ correlations, further suggest stronger CH₄ leakage along the eastern basin margin. These results demonstrate that CH₄ generation, migration, and leakage collectively connect subsurface carbon storage with atmospheric carbon exchange. This study highlights the multiple origins, migration, and leakage of CSG in the Ordos Basin and provides new insights into the role of CSG in the carbon cycling.