Hydrogeological Control on Coalbed Methane Geochemistry in CO2/N2-Enriched Secondary Microbial Gas Areas: A Case Study in the Eastern Section of the Southern Junggar Basin, NW China

Secondary microbial gas accompanied by abnormally high CO2/N2 concentrations is widely developed in the southern Junggar Basin (SJB). However, the mechanism of nonhydrocarbon-enriched coalbed methane (CBM) remains unclear. Due to the close correlation between microbial gas and hydrogeology, the geochemical and microbial sequencing experiments were conducted to reveal hydrogeological controls on CBM geochemistry in this study. The results showed that CBM derives from thermogenic and biogenic genesis in SJB, and the biogenic genesis is mainly generated through CO2 reduction. CO2 is biogenic and N2 is of atmospheric and thermal genesis. CO2 concentration mainly depends on microbial production and consumption. The higher alkane-degrading bacterial abundance in As/V-enriched environments promotes CO2 generation, while high HCO3− contents inhibit CO2 dissolution in water and consumption by methanogens. Alkane-degrading bacteria enrichment and discontinuous CO2 consumption lead to the significantly high CO2 concentration. Atmospheric N2 concentration was negatively correlated with groundwater runoff intensity, while thermogenic N2 concentration showed a weak correlation with total dissolved solids (TDS). High atmospheric infiltration and thermogenic N2 replenishment lead to a N2 concentration exceeding 10