Microbial coalbed gas and its geological formation process in the Jimsar region of the southern Junggar Basin, NW China

Abstract

The southern Junggar Basin is abundant in coalbed gas (CBG) resources, with the Jimsar region in the southern part of the basin emerging as a potential CBG development demonstration area. However, further development of the CBG in this region is restricted by the lack of understanding of the genesis and gas reservoir formation mechanisms of the CBG. This study systematically collected CBG samples and associated water samples from the Jimsar region. Stable isotope, hydrogeochemical, and 16S ribosomal ribonucleic acid (rRNA) sequencing studies were carried out. The results demonstrate that the CBG is the primary microbial gas, and CO2 reduction is its formation pathway. Hydrogeochemical analysis shows that the coalbed water receives recharge from meteoric or surface water. The water environment is relatively active (open system). The total dissolved organic carbon (TDOC) in the coalbed water is primarily from coal, and the supply and consumption of the total dissolved nitrogen (TDN) may be relatively balanced. The dissolution of CO2 is the main source of the dissolved inorganic carbon (DIC). Furthermore, the positive δ13CDIC values and high alkalinity indicate microbial methanogenesis. Microbial community structure analysis revealed that the archaea are hydrogenotrophic methanogens in the Jimsar region. The microbial basis for the production of microbial gas is provided by the existence of hydrolytic, acidogenic, and hydrogen-producing acetogenic bacteria as well as methanogenic archaea. Natural gas genetic identification diagrams and detection of methane-oxidizing bacteria (Methylomicrobium) reveal that the CBG may be affected by slight oxidation. In addition, the methanogenic effect is much greater than the methane oxidation. Finally, based on these results and the regional tectonic background, the CBG enrichment and accumulation mode in the Jimsar region was established. This mode can be divided into three geological stages: burial and coalification, surface water recharge and microbial gas generation after uplift and denudation, and CBG accumulation controlled by reverse faults and hydrodynamic sealing. This study enriches the geological theory of CBG accumulation in the Jimsar region and provides guidance for the subsequent exploration and development of the CBG.