Tracing the Contribution of Abiotic Methane in Deep Natural Gases From the Songliao Basin, China Using Bulk Isotopes and Methane Clumped Isotopologue 12CH2D2

Abstract

The identification and evaluation of abiotic methane remain an active research area due to uncertainties in traditional indicators that may lead to “false-positive” detections. As an emerging isotopic tool, methane clumped isotope can provide novel information about the generation and post-generation processes of methane gases. Using six deep natural gas samples from the Songliao Basin, we explored the potential of clumped isotopes in identifying abiotic methane. The results indicate that the Δ¹³CH₃D values of all samples are consistent with the thermodynamic equilibrium values at the inferred formation temperatures, whereas the Δ¹²CH₂D₂ values show small but detectable deficits (1∼6‰) relative to equilibrium. This particular non-equilibrium clumped isotopic signature of natural gas from the Songliao Basin is similar to the clumped isotopic signature of abiotic methane predicted by an isotopologue-specific kinetic model, suggesting a possible contribution of abiotic methane in the studied natural gases. However, we found that the mixing of abiotic and thermogenic gases could not fully explain the isotope data of all the samples. The proportion of abiotic gas calculated based on clumped isotopes would be underestimated because methane isotopic bond re-ordering at the late burial temperatures partially erases the disequilibrium signatures inherited from the original mixtures. The carbon isotopic reversals in C₁–C₃ of the samples provide additional constraint for evaluating the contribution of abiotic gas. Therefore, the coupling of intra-molecular clumped isotope and inter–molecular carbon isotope signatures may be a more robust approach for identifying abiotic methane, which can help us to quantitatively evaluate abiotic methane in petroleum systems.