The evolution of intra- and inter-molecular isotope equilibria in natural gases with thermal maturation

Naturally occurring hydrocarbon fluids in sedimentary basins have economic, geological and environmental significance. Connecting basin temperature-time evolution with petroleum generation transformation is a long-studied problem. In this study, we investigate the use of novel tool – multiply substituted isotopologues methane, for distinguishing between different chemical mechanisms catagenesis characterizing extent thermal maturation thermogenic natural gases. We analyze stable isotope compositions suite gas samples that are globally distributed cover wide range composition maturation, from dominantly unconventional shale formations few conventional plays. Our data show methane generated at early maturity has governed by kinetics, characterized pronounced deficit Δ12CH2D2; signature can be explained its formation chemistry combines more D-rich methyl radical pool D-poor H pool. Methane higher increases Δ12CH2D2, reaching equilibrium vitrinite reflectance (Ro) approximately 1.5% (equivalent to 170–210 °C peak burial temperature) higher, which interpreted result exchange erasing disequilibrium catagenetic chemistry, mediated CH activation during either chain reactions or organic-inorganic interactions on mineral surfaces. further examined hydrogen fractionations among ethane propane compiled global dataset found intermolecular fractionation exhibits trend similar seen Δ12CH2D2 value departing low maturities moving towards as increases. These findings indicate inter- intra-molecular structures components transition chemical-kinetic control toward thermodynamic maturities, reactions. propose these systematic relationships could used identify exact stages gases their associated fluids, especially oil-associated maturation.