Depletion of 13C in residual ethane and propane during thermal decomposition in sedimentary basins

Explanations for the thermal decomposition of ethane and propane in sedimentary basins have long been conceptual without quantitative evidence. This work bridges the gap between 13C depletion and the conversion of ethane and propane, and confirms their thermal decomposition during the late catagenesis stage. Isotopic fractionation was rigorously investigated in a partly reversible reaction scheme for ethane and propane decompositions. Analytical solutions under isothermal conditions were derived. Non-monotonic isotopic fractionation with respect to conversion is demonstrated, which may be ignored if a steady-state approximation is applied. The results imply that depletion of 13C in ethane and propane as their molar fractions decrease (isotopic inverse with respect to thermal maturity) results from the reversible conversion from alkane to alkyl groups. Kinetic parameters were obtained for ethane and propane decomposition in sedimentary basins, and the chemical kinetic method applies to different isotopic fractionation systems. The results help to better understand the chemical form of deep carbon and the thermal conversion of gaseous hydrocarbons in the geochemical carbon cycle.