Characterization of Polycyclic Aromatic Hydrocarbons in a Shale Strata Profile from the First Member of the Upper Cretaceous Qingshankou Formation in the Sanzhao Sag, Songliao Basin, NE China

The first member of the Upper Cretaceous Qingshankou Formation (K₂qn¹) stands as the most significant source rock layer in the Songliao Basin, concurrently serving as the principal target for shale oil exploration. Polycyclic aromatic hydrocarbons (PAHs), as one of the main components of soluble organic matter in mudstone and shale, are of significant importance for revealing the hydrocarbon generation mechanisms in source rocks and the formation conditions of shale oil. However, systematic research on PAHs in the K₂qn¹ layer of the Songliao Basin has not yet been conducted. Our study concentrated on a comprehensive set of 34 rock core samples, covering the entire K₂qn¹ layer, retrieved from the SYY3 well in the Sanzhao Sag of the northern Songliao Basin. The geochemical characteristics of a diverse range of PAHs were meticulously assessed through gas chromatography–mass spectrometry (GC–MS). Meanwhile, this study preliminarily discussed possible influential factors on the formation of alkylated PAHs (a-PAHs) and the isomerization of parent PAHs (p-PAHs) in our samples. The results revealed that PAHs predominantly consist of the phenanthrene, naphthalene, and chrysene series, trailed by the pyrene, fluorene, dibenzothiophene, and benzopyrene series. In contrast to the lower unit (>2015.00 m) of the K₂qn¹ layer, the upper unit (<2015.00 m) exhibits generally lower PAH concentrations and reduced levels of dibenzothiophene series, implying lower biological productivity and more oxidized sedimentary waters. The upper unit exhibits a higher content of 1,2,5-trimethylnaphthalene, 1,2,5,6-tetramethylnaphthalene, retene, pyrenes, fluoranthene, benzopyrenes, and benzofluoranthene, suggesting elevated levels of contributions from terrestrial higher plants. Maturity parameters of alkyl naphthalene and methylphenanthrene, along with vitrinite reflectance (R_o), indicate a close maturity in both units. Most of the a-PAHs/p-PAH ratios are higher in the lower unit than in the upper unit, indicating more pronounced alkylation. The ratios of p-PAH isomers, including benzo[b]fluorene/benzo[a]fluorene, benzo[a]pyrene/benzo[e]pyrene, and benzo[a]anthracene/chrysene, exhibit a vertical distribution pattern similar to the a-PAHs/p-PAH ratios, indicating that less stable p-PAH isomers are more prevalent in the lower unit. By comparing the a-PAHs/p-PAH ratios and the ratios of p-PAH isomers with conventional geochemical parameters of saturated hydrocarbons, it was preliminarily revealed that the catalytic effects of clay minerals, along with fluctuating biological inputs, can substantially affect PAH alkylation and p-PAH isomerization. Sediment reductivity slightly enhances PAH alkylation without obviously impacting p-PAH isomerization, and salinity shows no significant effect on these processes. The above insights offer molecular geochemical evidence of PAHs, which aids in understanding the heterogeneity of the K₂qn¹ source rock, facilitates oil source correlation, and optimizes the selection of sweet spots within shale oil formations.