Tracing the origin and formation mechanisms of coalbed gas from the Fuxin Basin in China using geochemical and isotopic signatures of the gas and coproduced water

Chen, Xiangrui and Wang, Yunpeng and Tao, Mingxin and Zhou, Zheng and He, Zhihua and Song, Kailin (2023) Tracing the origin and formation mechanisms of coalbed gas from the Fuxin Basin in China using geochemical and isotopic signatures of the gas and coproduced water. International Journal of Coal Geology, 267: 104185. ISSN 0166-5162

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Abstract

Coalbed gas (CBG) is an unconventional natural gas with a large resource potential. In order to determine its origins, formation pathways and mechanisms, studies on the geochemistry of CBG, coalbed water and coal have been carried out for years. However, the relationship between geochemistry of CBG and coalbed water is still not clear, especially with respect to the CO2 dissolution process. Here, a comprehensive study on the geochemistry of CBG and coproduction water in samples from the Fuxin Basin, China, is presented. Twenty-four gas and water samples were collected directly from the CBG producing well heads. C1/(C2 + C3) values are far >1000, δ13C1 values range between −62.8 and −57.6‰, δDCH4 values are from −252 to −225‰, ΔDH2O-CH4 values are from 148 to 178‰, and αCO2-C1 values are from 1.04 to 1.05. The composition characteristics, associated with the genetic diagrams of δ13C1 vs. δDCH4 and δ13C1 vs. δ13CCO2, and low coal rank (0.4–0.6% Ro), suggest that the CBG in the Fuxi Basin is mainly microbial gas. The Na-HCO3-Cl type of water in the coalbed is favorable for methanogenesis. δDH2O and δ18OH2O plot along the global meteoric water line (GMWL) and to the left of the local meteoric water line (LMWL), suggesting that the coalbed water is mainly from meteoric water recharge. However, isotope values in water may have been modified by methanogenesis and water-rock interaction. ΔDH2O-CH4 values of 148 to 178‰ suggest that methanogenic pathway is mainly CO2 reduction. However, αCO2-C1 values of 1.04 to 1.05 suggest that methanogenesis pathway might be acetoclastic or methylotrophic. This inconsistency may be caused by low δ13CCO2 values (−19.2 to −14.2‰) and thus low αCO2-C1 values due to high dissolution effect of CO2 and relative strong hydrodynamic activity in coal aquifers in the Fuxin Basin. Groundwater flow can carry away 13C-enriched CO2 dissolved in water, thus, the residual CO2 and DIC are depleted in 13C. Consequently, CO2 reduction is likely the main methanogenic pathway in the basin. Although CH4 and H2O may be close to isotope equilibrium for some samples, the CH4, CO2 and HCO3− are isotopically in disequilibrium at present reservoir conditions. Overall, kinetic processes largely control isotopic composition and distribution of the CBG and coalbed water in the Fuxin Basin. The CBG in the Fuxin Basin has been continuously generated, probably since the deposition of the coal-bearing formation. Although part of the CBG may have been lost during coalbed uplift stage, uplift to near surface allow re-inoculation of coalbeds with methanogenic microbial consortia via meteoric water recharge, which can accelerate the formation and accumulation of the microbial CBG. Consequently, most CBG in the present coalbed has been generated likely after the coal strata uplift.