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

Abstract

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 (<–30 ‰). In the microcosm of MECBM, coal and amendments are suggested to significantly increase methane production (from 82.4 ± 17.1 to 344.6 ± 22.8 μmol CH4/g coal), with effects controlled by substrate coal bioavailability rather than coal thermal maturity. This suggests the possibility of MECBM attempts to utilize high thermal maturity coal. We perform machine learning models and improve the accuracy of biogenic 12CH2D2 predictions, which help understand the mechanism of methane formation, and further provide new insights into clumped isotope effects among different biogenic methane sources. Our incubation experimental and compiled dataset (n = 275) better constrain biological participation in the methane formation process and potentially be used for methane sources tracing.