The variation of polycyclic aromatic hydrocarbon compound-specific carbon isotopic composition in crude oil during thermal evolution

Compound-specific carbon isotopic compositions (δ13C) of aromatic hydrocarbons offer a promising solution to the long-standing challenge of correlating ultra-deep oils with their source rocks. However, systematic studies on the evolution of these isotopic signatures during thermal maturation remain scarce. In this study, we conducted closed-system anhydrous gold-tube pyrolysis experiments using a representative marine crude oil from the Tarim Basin to systematically investigate the evolution of polycyclic aromatic hydrocarbon (PAH) compositions and their compound-specific δ13C values during thermal maturation. The results show that the abundance and relative distribution of the naphthalene, phenanthrene, fluorene, and dibenzothiophene series vary significantly with increasing maturity. Based on the variation patterns of δ13C values, the aromatic hydrocarbons can be divided into two categories. The first category includes parent PAHs such as naphthalene, phenanthrene, fluorene, and dibenzothiophene, along with some alkylated dibenzothiophenes, whose δ13C values remain essentially invariant during thermal evolution. The second category comprises other alkylated aromatic hydrocarbons, whose δ13C values remain stable at lower temperatures but become progressively enriched in δ13C at higher temperatures due to demethylation. Considering the diverse origins of PAH precursors and the thermal invariance of δ13C in certain aromatic hydrocarbons, compound-specific carbon isotope analysis represents a powerful tool for identifying source rocks in ultra-deep petroleum systems, thereby advancing our understanding of ultra-deep hydrocarbon accumulation.