Highlights:-We present a review and an expanded dataset of methane clumped isotope measurements.-Methane clumped isotope values often indicate equilibrium formation temperature.-Kinetic effects during or after methane production can affect clumped isotope values.-The wide variability in clumped isotope values suggests it will be a useful tracer. AbstractThe isotopic composition of methane is of longstanding geochemical interest, with important implications for understanding petroleum systems, atmospheric 3 greenhouse gas concentrations, the global carbon cycle, and life in extreme environments.Recent analytical developments focusing on multiply substituted isotopologues ('clumped isotopes') are opening a valuable new window into methane geochemistry.When methane forms in internal isotopic equilibrium, clumped isotopes can provide a direct record of formation temperature, making this property particularly valuable for identifying different methane origins. However, it has also become clear that in certain settings methane clumped isotope measurements record kinetic rather than equilibrium isotope effects. Here we present a substantially expanded dataset of methane clumped isotope analyses, and provide a synthesis of the current interpretive framework for this parameter. In general, clumped isotope measurements indicate plausible formation temperatures for abiotic, thermogenic, and microbial methane in many geological environments, which is encouraging for the further development of this measurement as a geothermometer, and as a tracer for the source of natural gas reservoirs and emissions.We also highlight, however, instances where clumped isotope derived temperatures are higher than expected, and discuss possible factors that could distort equilibrium formation temperature signals. In microbial methane from freshwater ecosystems, in particular, clumped isotope values appear to be controlled by kinetic effects, and may ultimately be useful to study methanogen metabolism.

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The stable carbon isotope ratios of coalbed methane (CBM) demonstrate diagnostic changes that systematically vary with production and desorption times. These shifts can provide decisive, predictive information on the behaviour and potential performance of CBM operations. Samples from producing CBM wells show a general depletion in 13 C-methane with increasing production times and corresponding shifts in δ 13 C-CH 4 up to 35.8. Samples from canister desorption experiments show mostly enrichment in 13 C for methane with increasing desorption time and isotope shifts of up to 43.4. Also, 13 C-depletion was observed in some samples with isotope shifts of up to 32.1. Overall, the magnitudes of the observed isotope shifts vary considerably between different sample sets, but also within samples from the same source. The δ 13 C-CH 4 values do not have the anticipated signature of methane generated from coal. This indicates that secondary processes, including desorption and diffusion, can influence the values. It is also challenging to deconvolute these various secondary processes because their molecular and isotope effects can have similar directions and/or magnitudes. In some instances, significant alteration of CBM gases has to be considered as a combination of secondary alteration effects.

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