Kinetic isotope effect and reaction pathways of thermogenic propane generation at early maturation stage: Insights from position-specific carbon isotope distribution

An intramolecular isotopic study was conducted on natural gases from the Turpan-Hami Basin of China. The set of natural gases exhibits larger 813C values of the terminal position compared with those at the central position of propane, with AC-T (813Ccentral-813Cterminal) values ranging from - 3.8 parts per thousand to -0.7 parts per thousand. During the thermal cracking of kerogen, the primary kinetic isotope effect is on the terminal carbon position to generate n-propyl radicals and on the central carbon position to produce i-propyl radicals. Therefore, distinct reaction pathways significantly in-fluence the position-specific carbon isotope distribution in thermogenic propane. In the early stage of matura-tion, propane is preferentially generated from the i-propyl radicals by capturing hydrogen atoms owing to the lower dissociation energies of i-propyl radicals cleavage from kerogen compared with those of n-propyl radicals, resulting in larger 813C values in the terminal position compared with those at the central position of generated propane. The position-specific 13C isotope composition of propane could provide new insights into revealing the generation mechanism of natural gas in the geological period at the atomic level.