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 < δ2HC3H8 > δ2Hn-C4H10, whereas the carbon isotopic compositions (δ13C) of C1–C4 alkanes mostly follow the normal trend of δ13CCH4 < δ13CC2H6 < δ13CC3H8 < δ13CC4H10. The average δ13C values of C1–C4 increased by 10.0‰, 6.0‰, 4.4‰, and 5.1‰, respectively, compared with the initial gases from source rocks, and exhibit a gradually declining trend. In contrast, the δ2H values of C1–C4 increased more from those of normal thermogenic gas, and the size of the 2H-enrichment varies, eventually resulting in the unique reversals. The δ2H and δ13C increments of methane are much higher than those of C2–C4. The reversals cannot be explained by current models for hydrocarbon gas generation, such as mixing of gases from different sources and thermal maturities. Instead, the results are consistent with a model where TOH caused a systematic 13C- and 2H-enrichment of C1–C4 after hydrocarbons charged the reservoir beds enriched in high-valence Mn/Fe oxides. Increase of C1 content accompanied by decreasing of C2–C4, heavy isotopic enrichment of C1–C4, and differential hydrogen isotopic fractionation of C1 and C2–C4 relative to theoretical calculation, suggest C2–C4 were preferentially oxidized during TOH, and C1 had been consumed in reaction. As TOH proceeded the 13C- and 2H-enrichment of C1–C4 was mainly caused by the Rayleigh fractionation. Involvement of formation water in the reaction intermediates of TOH is likely another factor causing the abnormal 2H-enrichment of C1 and the reversals. This study demonstrates that in some circumstances the TOH reactions induced by high-valence metal oxides cause large differences in the hydrogen isotopic compositions of natural hydrocarbons, and the process can be identified via their molecular and hydrogen isotopic signatures.