Biogenic methane clumped isotope signatures: Insights from microbially enhanced coal bed methane

[Display omitted] •Coal and amendments significantly increase methane production in incubation.•Machine learning help predict missing biogenic Δ12CH2D2 data.•Clumped isotopes indicate differential reversibility and combinatorial effects.•Coal bioavailability rather than coal thermal maturity determine CH4 production. Biogenic (microbial) methane is an imperative natural gas resource and plays an important role in the global carbon cycle. Determining isotope systematics in microbially enhanced coal bed methane (MECBM) informs methane biogeochemical processes and environmental impacts. Here, we conduct laboratory MECBM experiments using coal samples of differing maturities (from lignite to anthracite) and report analytical results of methane production, bulk and clumped isotopes (δ13C, δD, Δ13CH3D and Δ12CH2D2). We reproduce the differential reversibility in methanogenic reactions, which exhibits a correlation with kinetic clumped isotope fractionation (Δ13CH3D from +0.17 ‰ to +4.04 ‰). The exogenous combinatorial effects play a greater role in determining the negative Δ12CH2D2 (