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.

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 ( Δ V 12 CH 4 ‡ – Δ V 13 CH 4 ‡ ) were linearly correlated with the kinetic isotope effect of methane ( Δ KIE ) produced from sapropelic and humic organic matter, indicating that WP may affect the KIE of ¹²CH₄ and ¹³CH₄ by changing the Δ V ‡ 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. [ABSTRACT FROM AUTHOR]