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

Abstract

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 δ2Hcenter and δ2Hterminal increased by 110 and 131‰. An activation energy (Ea) 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 Ro 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 δ13Ccenter and δ13Cterminal, and 0.815 and 0.838 for δ2Hcenter and δ2Hterminal, respectively. PS isotopic compositions of natural propane in the ΔCc-t - ΔHc-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.