Stable carbon isotope (δ¹³C) rollover of natural gas has attracted recent attention due to its association with highly productive shale gas. However, the mechanistic causes of δ¹³C rollover are not fully understood. In this investigation, pyrolysis was carried out using calcareous shale and carbonaceous mudstone under high water pressure (WP) (i.e., 5 × 10⁶–1.2 × 10⁸ Pa). It was found that WP induced the isotope rollover of gaseous hydrocarbons. For both sapropelic and humic organic matter, the δ¹³C rollover of CH₄ (methane), C₂H₆ (ethane), and C₃H₈ (propane) occurred when the WP ranged from 3.25 × 10⁷ to 1.2 × 10⁸ Pa. This result can be explained by high WP conditions retarding oil cracking, and enhancing hydrocarbon expulsion and oil generation. The promotion of oil generation resulted in increasing trends of vitrinite reflectance, and inhibition of gaseous hydrocarbons generation resulted in decrease in δ¹³C₁ values with increase in WP. Good functions were found between water pressure and the calculated carbon kinetic isotope effect (KIE) for ¹²CH₄ and ¹³CH₄ produced from sapropelic and humic organic matter. Further calculations showed that the increments of activation volume (\({\Delta V}_{{12}_{{\mathrm{CH}}_{4}}}^{\ddagger }\)–\({\Delta V}_{{13}_{{\mathrm{CH}}_{4}}}^{\ddagger }\)) were linearly correlated with the kinetic isotope effect of methane (\(\Delta \mathrm{KIE}\)) produced from sapropelic and humic organic matter, indicating that WP may affect the KIE of ¹²CH₄ and ¹³CH₄ by changing the \(\Delta {V}^{\ddagger }\) of ¹²CH₄ and ¹³CH₄. Overall, these findings suggest that WP affects the carbon isotope fractionation of gaseous hydrocarbons due to the different thermodynamic properties of ¹²CH₄ and ¹³CH₄.

天然气的稳定碳同位素（δ ¹³ C）翻转由于与高产页岩气相关而引起了最近的关注。然而，δ ¹³ C 翻车的机理原因尚不完全清楚。在本次调查中，利用钙质页岩和碳质泥岩在高水压（WP）（即5 × 10 ⁶ –1.2 × 10 ⁸  Pa）下进行热解。发现WP引起了气态碳氢化合物的同位素翻转。对于腐泥质和腐殖质有机质， CH ₄（甲烷）、C ₂ H ₆（乙烷）和 C ₃ H _{8的 δ} ¹³ C 翻转（丙烷）发生在 WP 从 3.25 × 10 ⁷到 1.2 × 10 ⁸  Pa 的范围内。这一结果可以用高 WP 条件延缓石油裂解，促进烃类排出和生油来解释。促进生油导致镜质体反射率增加趋势，抑制气态烃生成导致δ ¹³ C ₁值随着WP的增加而降低。¹² CH ₄和¹³ CH ₄的水压与计算的碳动力学同位素效应 (KIE) 之间存在良好的函数关系由腐泥和腐殖质有机物产生。进一步的计算表明激活量的增量 ( \({\Delta V}_{{12}_{{\mathrm{CH}}_{4}}}^{\ddagger }\) – \({\Delta V}_{{13}_{{\mathrm{CH}}_{4}}}^{\ddagger }\)）与甲烷的动力学同位素效应（\(\Delta \mathrm{KIE} \) ) 由腐泥质和腐殖质有机质产生，表明 WP 可能通过改变 12 CH 4 和 13 CH 的^\ ( _\ Delta ^{ V }^{\ddagger }\)影响¹² CH ₄和¹³ CH _{4的 KIE 4}. 总之，这些发现表明，由于¹² CH ₄和¹³ CH ₄的不同热力学性质，WP会影响气态烃的碳同位素分馏。