Intramolecular isotopic distributions of associated propane in shale oil determined by an improved GC-Pyrolysis-GC-IRMS approach: Insights into influence of kerogen structures on enrichment of hydrocarbon deposits

Abstract

The intramolecular isotopic compositions of associated propane in shale oil from the Chang-7 sub-member of the Yanchang Formation in the Ordos Basin were firstly determined by an improved GC-Pyrolysis-GC-IRMS method, which was calibrated by the quantitative isotopic ¹³C NMR method. The results show that the Δ_C-T values (δ¹³C_Central-δ¹³C_terminal) of associated C₃ are ranging from 1.3 ‰ to 2.9 ‰, the δ¹³C_Central values range from −34.2 ‰ to −31.8 ‰, while the δ¹³C_terminal values vary between −35.6 ‰ and −34.9 ‰. The molecular carbon isotopes of propane suggest that the associated gas originates from the sapropelic organic matter (OM) at the early maturation stage, while the intramolecular carbon isotopic compositions of associated C₃ indicate that the associated propane is dominated by n-propyl reaction pathway with contribution more than 75 %. The intramolecular isotopic compositions of C₃ derived from typical humic and sapropelic OM shows that the compositions of normal and isomeric groups within the kerogen structures could influence the chemical compositions in generated hydrocarbons. OM with high proportion of normal groups prefers to generate liquid hydrocarbons, while OM with high proportion of isomeric groups tends to generate gaseous hydrocarbons at the same maturation stage. This study demonstrates that the intramolecular isotopic composition of associated C₃ in shale oil can reflect the chemical structure of kerogen of source rock, and may further assess the shale oil potential.