Carbon and hydrogen isotope fractionation for methane from non-isothermal pyrolysis of oil in anhydrous and hydrothermal conditions

In this study, to ascertain the carbon and hydrogen isotope fractionation of oil cracking gas (secondary gas) in hydrothermal conditions, non-isothermal pyrolysis of oil with and without water was carried out by a gold-tube system. By determination of the yields of individual gas products, it is found that the presence of water enhanced the yields of hydrocarbon gases. However, kinetic calculations indicate that E_a for the generation of methane and C_2–5 in pyrolysis in hydrothermal conditions are essentially identical with those in anhydrous pyrolysis. The yields of carbon dioxide (CO₂) and alkene gases in pyrolysis in hydrothermal conditions are evidently higher than those in anhydrous pyrolysis. It is reasonable that water–hydrocarbon reactions occurred and contributed to the generation of secondary gas in hydrothermal conditions. Meanwhile, the presence of water resulted in a slight depletion of ¹³C for methane and an evident depletion of ¹³C for CO₂. Thermodynamic calculations suggest that water–hydrocarbon reactions in non-isothermal pyrolysis are dominated by free radical mechanism rather than ionic mechanism. Moreover, δ²H values of methane are apparently different in pyrolysis involving water with different δ²H. This result demonstrates that water provided hydrogen for hydrocarbon gas generation. Finally, we established mathematical models based on isotope fractionation to quantitatively determine the contribution of water–hydrocarbon reactions for gas generation in both experimental and geological conditions.