Geochemical Constraints on the Generation of Coalbed Methane in the Weizhou Area, South-Central Ningxia, NW China: Insights From Stable Isotopes and Organic Petrology

Amid growing global clean energy demand, coalbed methane (CBM) in the coal-rich Weizhou area holds great development value. This study explores Weizhou CBM's geochemical traits and genesis via analyzing gas compositions (Shanxi and Taiyuan Formations), stable isotopic distributions (δ13C1, δD, δ13CO2, and δ15N), and their burial depth variations. Results show that CBM is hydrocarbon-dominated: Shanxi and Taiyuan Formations have average methane contents of 89.89% and 88.24%, respectively. Coal metamorphism is medium-to-high (Ro: 1.76%–2.52%, subbituminous to anthracite). Isotopic averages were as follows: Shanxi versus Taiyuan (δ13C1: −36.91‰ vs. −36.68‰; δ13C2: −19.46‰ vs. −24.2‰; and δ13CO2: −17.56‰ vs. −15.29‰) and regional δD (−188.7‰) and δ15N (−1.27‰). Further, the results identify gas source differentiation between the Shanxi Formation and Taiyuan Formation, which is controlled by burial depth and coal metamorphism degree. A strong δ13C1–δ13C2 correlation (δ13C1=0.5044 δ13C2−26.188, R2=0.616) confirms the dominant status of thermogenic gas. δ13C1−C1/C2+C3 data fall in the secondary thermogenic zone, indicating modification by diffusion, migration, and fractionation. Gas source compositions differ between the upper Shanxi (0–4 seams) and the lower Taiyuan (5–20 seams) Formations. These findings support optimized Weizhou CBM exploration and utilization.