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

Abstract

An intramolecular isotopic study was conducted on natural gases from the Santanghu Basin in China. Propane samples spanned a wide range of ΔC-T (δ13Ccentral-δ13Cterminal) values from –8.4 to 8.5 ‰, δ13Ccentral and δ13Cterminal 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 δ13Ccentral < δ13Cterminal to δ13Ccentral > δ13Cterminal, and the δ13Cterminal decreased by 6.8 ‰ within a Ro range of 0.46–0.50 %. The intramolecular carbon isotopic rollover and a decrease in δ13Cterminal 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 δ13Cterminal values and shifting δ13Ccentral < δ13Cterminal to δ13Ccentral > δ13Cterminal 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.