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

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 delta H-2(CH4) > delta H-2(C2H6) < delta H-2(C3H8) > delta H-2(n-C4H10), whereas the carbon isotopic compositions (delta C-13) of C-1-C-4 alkanes mostly follow the normal trend of delta C-13(CH4) < delta C-13(C2H6) < delta C-13(C3H8) < delta C-13(C4H10). The average delta C-13 values of C-1-C-4 increased by 10.0 parts per thousand, 6.0 parts per thousand, 4.4 parts per thousand, and 5.1 parts per thousand, respectively, compared with the initial gases from source rocks, and exhibit a gradually declining trend. In contrast, the delta H-2 values of C-1-C-4 increased more from those of normal thermogenic gas, and the size of the H-2-enrichment varies, eventually resulting in the unique reversals. The delta H-2 and delta C-13 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 C-13- and H-2-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. As TOH proceeded the C-13- and H-2-enrichment of C-1-C-4 was mainly caused by the Rayleigh fractionation. Involvement of formation water in the reaction intermediates of TOH is likely another factor causing the abnormal H-2-enrichment of C-1 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.

Automated summary

This study analyzed natural gases from deep buried oxidized reservoir rocks, showing a unique trend of hydrogen and carbon isotopes in hydrocarbons. The results suggest that thermochemical oxidation of hydrocarbons induced by high-valence metal oxides can cause significant differences in hydrogen isotopic compositions and carbon isotopic enrichments in natural hydrocarbons.