Geochemical evidence for biodegradation in high-rank coals from Qinshui Basin, North China

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 (δ¹³C 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 C₁/C₂₊ ratios and δ¹³C-CH₄ values, and positive δ¹³C-CO₂ values (−9.2 to +24.4 ‰) suggest biodegradation of light wet gases (C₂₊ components) and CO₂ reduction. Negative trends between δ¹³C-CO₂ and CH₄/CO₂ in shallow burial coal seams reveal the mutual conversion of CH₄ and CO₂ and carbon isotope exchange, driven by microorganisms. The calculated isotopic temperatures (33–328 °C) based on the carbon isotope fractionation factors between CH₄ and CO₂ (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.