Isotope Rollover of Gaseous Hydrocarbons Induced by Water Pressure in Laboratory Pyrolysis Experiments: Insights into the Influence of Pressure on Carbon Kinetic Isotope Effects During Methane Generation

作者：Wu, Yuandong; Li, Yuanju^*; Wang, Changfeng; Sun, Lina; Zhang, Zhongning; Ji, Liming; Zhang, Mingzhen; Su, Long

来源：Natural Resources Research, 2022, 31(3): 1523-1537.

DOI：10.1007/s11053-022-10052-9

• Gaseous hydrocarbon
• Isotope rollover
• Kinetic isotope effect
• Carbon isotope fractionation
• Activation volume
• (CH4)-C-12 and (CH4)-C-13

Stable carbon isotope (delta C-13) rollover of natural gas has attracted recent attention due to its association with highly productive shale gas. However, the mechanistic causes of delta C-13 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 x 10(6)-1.2 x 10(8) Pa). It was found that WP induced the isotope rollover of gaseous hydrocarbons. For both sapropelic and humic organic matter, the delta C-13 rollover of CH4 (methane), C2H6 (ethane), and C3H8 (propane) occurred when the WP ranged from 3.25 x 10(7) to 1.2 x 10(8) 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 delta C-13(1) values with increase in WP. Good functions were found between water pressure and the calculated carbon kinetic isotope effect (KIE) for (CH4)-C-12 and (CH4)-C-13 produced from sapropelic and humic organic matter. Further calculations showed that the increments of activation volume (Delta V-12CH2(double dagger) - Delta V-13CH4(double dagger)) were linearly corre- lated with the kinetic isotope effect of methane (Delta KIE) produced from sapropelic and humic organic matter, indicating that WP may affect the KIE of (CH4)-C-12 and (CH4)-C-13 by changing the Delta V-double dagger of (CH4)-C-12 and (CH4)-C-13. Overall, these findings suggest that WP affects the carbon isotope fractionation of gaseous hydrocarbons due to the different thermodynamic properties of (CH4)-C-12 and (CH4)-C-13.

• 单位
  东莞理工学院; 中国科学院