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

Abstract

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