Intramolecular carbon isotopic rollover in propane from natural gas reservoirs of the Santanghu Basin: Insights into chemical structure of kerogen

An intramolecular isotopic study was conducted on natural gases from the Santanghu Basin in China. Propane samples spanned a wide range of Delta(C-T) (delta C-13(central)-delta C-13(terminal)) values from -8.4 to 8.5 parts per thousand, delta C-13(central) and delta C-13(terminal) values varied from -37.3 to -25.8 parts per thousand and -35.7 to -28.9 parts per thousand, respectively. During thermal maturation, the position-specific carbon isotopic composition of propane shifted from delta C-13(central) < delta C-13(terminal) to delta C-13(central) > delta C-13(terminal), and the delta C-13(terminal) decreased by 6.8 parts per thousand within a R-o range of 0.46-0.50 %. The intramolecular carbon isotopic rollover and a decrease in delta C-13(terminal) with a slight increase in maturation could be explained by kinetic effects in thermal cracking of kerogen with a low proportion of branched chains. As isomeric groups were rapidly exhausted, propane generation pathway rapidly changed and primary kinetic isotope effect completely shifted from central carbon position to terminal carbon positions in propane. These changes increased carbon isotope fractionation at terminal carbon positions, thereby reducing delta C-13(terminal) values and shifting delta C-13(central) < delta C-13(terminal) to delta C-13(central) > delta C-13(terminal) within a small maturity range. This work demonstrated that position-specific isotope distribution of propane can be applied to elucidate the chemical structure of kerogen.