Position-Specific Carbon Isotope Constraints on Propane Formation from Kerogen during Open-System Pyrolysis

Position-specific carbon isotopic compositions of propane have been proven to be a powerful tool for identifying the processes involved in natural gas generation. However, current understanding of intramolecular isotope distributions in propane and their geochemical implications is mainly derived from kerogen pyrolysis in closed-system experiments. In this study, open-system pyrolysis experiments were performed to investigate changes in intramolecular carbon isotope distributions of propane with increasing thermal maturation. Given the low abundance of propane in pyrolysis products, an offline enrichment protocol combining alkaline CO₂ scrubbing and cryogenic trapping was developed. Coupled with GC-pyrolysis-GC-IRMS, this experimental setup was applied to collect and measure position-specific carbon isotopes of propane from a gas sample with a dryness coefficient up to 0.99. Open-system pyrolysis of a type II kerogen reveals that the isotopic difference between the central and terminal carbon atoms of propane increases from 2.7‰ to 6.7‰ across the oil window (Easy%R ₀ = 0.6-1.6%), followed by a decrease to 2.8‰ at the overmature stage (Easy%R ₀ = 1.8-2.6%). In contrast to the monotonic increase observed during closed-system pyrolysis, open-system pyrolysis yields a unique intramolecular isotope trajectory. This pattern is further supported by the correlation between δ¹³C_propane and δ¹³C_central. With respect to the secondary cracking of propane at high- to overmature stages, pure-propane cracking experiments constrain the onset of substantial secondary cracking at approximately 520 °C (Easy%R ₀ = 1.9%), which provides a thermal reference for discriminating primary generation signals from secondary overprinting. The findings of this study provide a valuable complement to previous closed-system pyrolysis-based investigations and establish a framework for interpreting intramolecular isotope signatures of propane generated during kerogen thermal degradation in petroliferous basins.