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 ((CH3D)-C-13 and (CH2D2)-C-12) provide novel insights into methane origins and post -generation processing. Here, we report the chemical compositions of hydrocarbons (C-1/C2+ molecular ratios), and methane bulk and clumped isotopes (delta C-13, delta D, Delta(CH3D)-C-13 and Delta(CH2D2)-C-12) 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-degrees. 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 delta C-13 and delta D, with a clumped isotope composition equilibrated near similar to 249 C-degrees) and biogenic methane formed or processed in the reservoir (low in delta C-13 and delta D, with a clumped isotope composition equilibrated near 16-27 C-degrees). 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.