Microbial contribution estimated by clumped isotopologues (¹³CH₃D and ¹²CH₂D₂) characteristics in a CO₂ enhanced coal bed methane reservoir

Carbon capture and storage (CCS) of CO₂ is a key technology for substantially mitigating global greenhouse gas emissions. Determining the biogeochemical processes in host rocks after CO₂ injection informs the viability of carbon storage as a long-term sink for CO₂, the complexity of reservoir CH₄ cycling, as well as the direct and indirect environmental impacts of this strategy. The doubly substituted (‘clumped’) isotopologues of methane (¹³CH₃D and ¹²CH₂D₂) provide novel insights into methane origins and post-generation processing. Here, we report the chemical compositions of hydrocarbons (C₁/C₂₊ molecular ratios), and methane bulk and clumped isotopes (δ¹³C, δD, Δ¹³CH₃D and Δ¹²CH₂D₂) of a CO₂ enhanced coal bed methane recovery (CO₂-ECBM) area in Qinshui basin, China and is an analogue for carbon capture and storage. The clumped isotopologue compositions observed in the study area are generally consistent with a range of temperatures spanning 73 to 193 °C. The range in apparent temperature and correlations among clumped and bulk isotopic indices are best explained by mixing between a high maturity thermogenic methane (high in δ¹³C and δD, with a clumped isotope composition equilibrated near ∼249 °C) and biogenic methane formed or processed in the reservoir (low in δ¹³C and δD, with a clumped isotope composition equilibrated near 16–27 °C). We hypothesize that the biogenic endmember may result from slow methanogenesis and/or anaerobic oxidation of methane (AOM). This study demonstrates that the potential of methane clumped isotope approach to identify in situ microbial metabolic processes and their association with carbon cycling in CO₂-ECBM area, improving our understanding of biogeochemical mechanisms in analogous geological reservoirs.