Hydrogen isotopic responses to thermochemical oxidation of light hydrocarbon gases in deep clastic reservoirs of the Junggar Basin, China

Abstract The occurrence of high-valence Mn/Fe oxides in deeply buried oxidized reservoir rocks promotes thermochemical oxidation of hydrocarbons (TOH). This study aimed to better understand this process and its effects on hydrogen isotopes via sampling natural gases from the Lower Triassic red beds with depths of >3.5 km in the Junggar Basin, NW China. The gas compositions, hydrogen and carbon isotopes of C1–C4 alkanes, and carbon isotopes of carbon dioxide were analyzed. The gases are almost mature thermogenic gases generated by sapropelic kerogen from the deeper Lower Permian source rock. The hydrocarbon gases display a unique reversed trend of δ2HCH4 > δ2HC2H6   δ2Hn-C4H10, whereas the carbon isotopic compositions (δ13C) of C1–C4 alkanes mostly follow the normal trend of δ13CCH4