Hydrogeological Control on Coalbed Methane Geochemistry in CO 2 /N 2 -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 CO 2 /N 2 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 CO 2 reduction. CO 2 is biogenic and N 2 is of atmospheric and thermal genesis. CO 2 concentration mainly depends on microbial production and consumption. The higher alkane-degrading bacterial abundance in As/V-enriched environments promotes CO 2 generation, while high HCO 3 − contents inhibit CO 2 dissolution in water and consumption by methanogens. Alkane-degrading bacteria enrichment and discontinuous CO 2 consumption lead to the significantly high CO 2 concentration. Atmospheric N 2 concentration was negatively correlated with groundwater runoff intensity, while thermogenic N 2 concentration showed a weak correlation with total dissolved solids (TDS). High atmospheric infiltration and thermogenic N 2 replenishment lead to a N 2 concentration exceeding 10%. Stable carbon isotopes are related to methanogens, and more methanogens in Li/Zn/Cu/Rb/Sn-enriched environments lead to decreasing δ 13 C–CH 4 . However, when TDS exceeds 15,000 mg/l, the inhibited methanogen production results in δ 13 C–CH 4 higher than −55‰. Three CBM geochemistry distribution models were ultimately established: atmospheric mixed in the runoff–weak runoff area, reformation by shallow microbial gas migration in the weak runoff–stagnant area, and damping microbial gas generation with abundant alkane-degrading bacteria in the weak runoff–stagnant area.