Gas content, geochemistry and accumulation of coalbed methane in Suzhou mining area of the Huaibei coalfield, China

Abstract Coalbed methane (CBM) resources in the Huaibei coalfield, particularly within the Suzhou mining area, have garnered significant attention due to their substantial energy potential. This study investigated the gas content, geochemical characteristics, and accumulation mechanisms of CBM from Nos. 7 and 8 coal seams in the Suzhou mining area through a comprehensive analysis of 26 CBM and 23 groundwater samples. Results indicate that gas content ranges from 0.70 to 26.97 m 3 /t, with average values of 10.67 m 3 /t for No. 7 coal seam and 11.63 m 3 /t for No. 8 coal seam, respectively. The methane concentration varies between 0.72% and 96.20% with an average of 76.48%. The variation of δ 13 C CH4 (− 68.20‰ to − 43.90‰) and δD CH4 (− 229‰ to − 204‰) reflects complex secondary transformation of primary thermogenic gas, and quantitative end-member calculation demonstrates that secondary biogenic gas contributes an average proportion of 57.21%. Hydrogeochemical data show that the weakly alkaline groundwater with total dissolved solids (TDS) of 145.57–3146.85 mg/L is dominated by Na⁺ and HCO 3 ⁻. Integrated analysis of hydrogeochemical characteristics and δ 13 C CH4 data indicate that thermogenic CH 4 with heavier δ 13 C CH4 value is well preserved within the core of the Su’nan syncline, while shallow fractured recharge-runoff zones host mixed gas characterized by lighter δ 13 C CH4 value. Yanshanian-Himalayan tectonic activities together with thermal evolution and burial depth dominate the generation of CBM, and fracture networks coupled with hydrodynamic conditions reshaped the final spatial differentiation of CBM. This study clarified the coupled controls of tectonic evolution, subsurface hydrodynamic conditions and dual gas genetic processes on CBM enrichment within medium-rank coal in the Suzhou mining area.