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

摘要

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 C_1-C_4 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 δ~2H_(CH4) > δ~2H_(C2H6) < δ~2H_(C3H8) > δ~2H_(n-C4H10), whereas the carbon isotopic compositions (δ~(13)C) of C_1-C_4 alkanes mostly follow the normal trend of δ~(13)C_(CH4) < δ~(13)C_(C2H6) < δ~(13)C_(C3H8) < δ~(13)C_(C4H10). The average δ~(13)C values of C_1-C_4 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 δ~2H values of C_1-C_4 increased more from those of normal thermogenic gas, and the size of the ~2H-enrichment varies, eventually resulting in the unique reversals. The δ~2H and δ~(13)C increments of methane are much higher than those of C_2-C_4. 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 ~(13)C- and ~2H-enrichment of C_1-C_4 after hydrocarbons charged the reservoir beds enriched in high-valence Mn/Fe oxides. Increase of C_1 content accompanied by decreasing of C_2-C_4, heavy isotopic enrichment of C_1-C_4, and differential hydrogen isotopic fractionation of C_1 and C_2-C_4 relative to theoretical calculation, suggest C_2-C_4 were preferentially oxidized during TOH, and C_1 had been consumed in reaction.