In situ reservoir biogeochemical evaluation and its indicative significance for coalbed methane extraction: Taking the Shizhuangnan Block in the southern Qinshui Basin as an example

Coal reservoirs are habitats for microorganisms, with some widespread communities linked to carbon cycling. The geographic distribution and metabolic expression of these microorganisms remain largely decoupled from the reservoir environment. Although microbial metabolism under natural conditions is insufficient for gas accumulation, the community influence from organic matter supply and environmental conditions can stimulate a positive response in the in-situ reservoir environment and coalbed methane storage. The southern Qinshui Basin is rich in coalbed methane resources and is one of the first areas in China to realize a commercial exploitation. Taking the Shizhuangnan block in the southern Qinshui Basin as an example, the geochemical characteristics of reservoir water and the biogeochemical sequencing of microbial genes are assessed by assessing ion concentrations, dissolved inorganic carbon isotopes, sulfate isotopes, and microbial abundance and diversity. The results show that the reservoir geochemistry is influenced by hydration, ion exchange, and microbial metabolism. High sodium and bicarbonate ion content indicate relatively reduced or stagnant conditions in the reservoir. Sulfate reducing bacteria and methanogens display synergy during coal degradation. However, sulfate reducing bacteria can outcompete methanogens for substrate when sulfate is sufficient for their metabolism, inhibiting methanogenesis. The sulfate isotopes, dissolved inorganic carbon isotopes, and microbial abundance and diversity may reflect the symbiotic relationship between these microorganisms and the reservoir environment. Regions with active sulfate reduction but weak methanogenesis generally do not provide suitable conditions for effective coalbed methane storage. Relatively reduced or stagnant reservoir environments favor methanogenesis and are beneficial for coalbed methane storage. This research enriches our methods for evaluating reservoir biogeochemistry, guides us in selecting favorable areas for coalbed methane exploration and development, and provides a theoretical basis and guidance for the practical implementation of coalbed methane bioengineering.