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 Δ_C-T (δ¹³C_central-δ¹³C_terminal) values from –8.4 to 8.5 ‰, δ¹³C_central and δ¹³C_terminal values varied from –37.3 to –25.8 ‰ and –35.7 to –28.9 ‰, respectively. During thermal maturation, the position-specific carbon isotopic composition of propane shifted from δ¹³C_central < δ¹³C_terminal to δ¹³C_central > δ¹³C_terminal, and the δ¹³C_terminal decreased by 6.8 ‰ within a R_o range of 0.46–0.50 %. The intramolecular carbon isotopic rollover and a decrease in δ¹³C_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 δ¹³C_terminal values and shifting δ¹³C_central < δ¹³C_terminal to δ¹³C_central > δ¹³C_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.