Understanding the generation of secondary microbial methane (SMM) is important for the evaluation of natural gas resources and instructive for the stimulation of methane production. Coal seams are popular targets for extracting in situ preserved methane and studying microbe-stimulated methane yield. However, few studies have been done on overmature coals. Here we collected gas samples from coals varying from bituminous to meta-anthracite in the Qinshui Basin, North China, and analysed the molecular and stable isotopic compositions to systematically evaluate the influence of biodegradation in high-rank coals in geological settings. The stable isotope signatures (δC and δD) of methane are dominated by the thermal decomposition of organic matter in deep coals but inconsistent with the maturity rank of shallow burial coals. The decoupling of coal maturity with C/C ratios and δC-CH values, and positive δC-CO values (−9.2 to +24.4 ‰) suggest biodegradation of light wet gases (C components) and CO reduction. Negative trends between δC-CO and CH/CO in shallow burial coal seams reveal the mutual conversion of CH and CO and carbon isotope exchange, driven by microorganisms. The calculated isotopic temperatures (33–328 °C) based on the carbon isotope fractionation factors between CH and CO (1.024–1.069) demonstrate that carbon isotope exchange is prevalent in high-rank coals. It also reveals that the burial depth is an imperative factor in controlling microbial environments and thus the biodegradation process. This study implicates the potential of high-rank coals as the target for microbial-enhanced methane recovery and also implies that microorganisms are widely involved in reservoir carbon cycling.

中文翻译：

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华北沁水盆地高阶煤生物降解的地球化学证据

了解次生微生物甲烷（SMM）的生成对于天然气资源评价具有重要意义，对甲烷增产具有指导意义。煤层是提取原位保存的甲烷和研究微生物刺激的甲烷产量的热门目标。然而，对过熟煤的研究却很少。在这里，我们从华北沁水盆地的烟煤到变无烟煤等不同煤种中采集了气体样品，并分析了分子和稳定同位素组成，以系统评估地质环境中高阶煤生物降解的影响。甲烷的稳定同位素特征（δ13C和δD）主要由深部煤中有机质的热分解决定，但与浅埋藏煤的成熟度等级不一致。煤成熟度与 C/C 比和 δ13C-CH 值以及正 δ13C-CO 值（−9.2 至 +24.4 ‰）的脱钩表明轻湿气体（C 成分）的生物降解和 CO 还原。浅埋煤层δ13C-CO和CH/CO呈负趋势揭示了微生物驱动下CH和CO的相互转化和碳同位素交换。根据 CH 和 CO 之间的碳同位素分馏因子 (1.024–1.069) 计算的同位素温度 (33–328 °C) 表明碳同位素交换在高阶煤中普遍存在。它还表明埋藏深度是控制微生物环境以及生物降解过程的重要因素。这项研究表明高阶煤作为微生物增强甲烷回收的目标的潜力，也意味着微生物广泛参与储层碳循环。