Effects of simulated surface freshwater environment on in situ microorganisms and their methanogenesis after tectonic uplift of a deep coal seam

Secondary biogenic coalbed gas (CBG) is the gas generated by the microbial degradation of coal or wet gas components after freshwater recharge events. However, in situ microorganisms in deep coalbed water with high total dissolved solids (TDS) content may be overlooked. The evolution of the community structure and gene function of in situ microorganisms and their contributions to secondary biogenic CBG are poorly understood. The coalbed water TDS content in the Miquan region of southern Junggar Basin is unusually high (up to 45,000 mg/L), CBG is biogenic, and microorganisms have been detected in coalbed water. This study collected coal and high-TDS coalbed water samples in the Miquan region. Gas production experiments in a simulated freshwater environment (carrying nutrients) were conducted. Microbial community structure and gene function were analyzed using metagenomic sequencing. The gas production capacity of microorganisms in high-TDS coalbed water was significantly enhanced (up to 220.81%) under subculture with deionized water and nutrients. Microbial community analysis indicated that the hydrolytic bacterial group changed from a single genus to a state of synergistic degradation by multiple genera. The abundances of acidogenic bacteria, hydrogen-producing acetogenic bacteria, and methanogens gradually increased. Overall, the relationships between microorganisms were significantly optimized. Gene function analysis showed that most of the functional types were related to metabolism. Low-TDS culture enhanced the metabolic capacity of the microorganisms, thereby improving the fermentation performance. Finally, through combining the succession of the microbial communities, gene function changes, and gas production results, the geological methane-producing process of the in situ microorganisms in deep high-TDS coalbed water against the background of tectonic uplift was recreated.