Kinetics of propane cracking and position-specific isotope fractionation: Insights into the origins of natural gases

Position-specific isotope analysis (PSIA) of propane can provide novel and valuable information to constrain the origin and history of natural gases. We conducted a systematic experimental study to determine the kinetics of propane pyrolysis, including bulk and position-specific carbon and hydrogen isotope fractionation of propane at 415 – 465 °C. As the dominant gaseous products, the yields of CH₄ and C₂H₆ increased with increasing C₃H₈ consumption. A significant amount of aromatic residues (C/H: ∼1) was generated, which was comparable to the yields of CH₄ and C₂H₆. Position-specific δ¹³C values of propane increased by up to 8.3 and 8.8‰ at the center and terminal positions respectively, while δ²H_center and δ²H_terminal increased by 110 and 131‰. An activation energy (E_a) of 69.9 (±29.1) kcal/mol and frequency factor (A) of 3.1 × 10¹⁵ (±7.9 × 10⁸) s⁻¹ were obtained based on a first-order reaction model. Using the average kinetic parameters from this study and previous studies on wet-gas cracking, propane cracking starts above 215 (±10.5) ^oC at a geological heating rate of 5 °C/m.y., corresponding to Easy R_o of 2.31%. An Arrhenius plot of kinetic isotope fractionation indicates both central C and H of propane show larger isotope fractionations compared with those at the terminal position in natural reservoirs. Position-specific (PS) isotopic fractionation factors (k*/k) at 215 °C were estimated as 0.983 and 0.987 for δ¹³C_center and δ¹³C_terminal, and 0.815 and 0.838 for δ²H_center and δ²H_terminal, respectively. PS isotopic compositions of natural propane in the ΔC_c-t - ΔH_c-t diagram may serve as a diagnostic indicator for the origins (kerogen vs. oil) and degradation (thermal cracking vs. biodegradation) of natural gases in sedimentary basins.