Geochemical characteristics and oil–source rock correlation in the Paleogene source rocks of the Qinnan Sag, Bohai Bay basin

The Paleogene Shahejie Formation, including the first (Es₁) and third (Es₃) members of the Shahejie Formation, and the third member of Dongying Formation (Ed₃) are important source rocks in the Qinnan Sag, Bohai Bay Basin. In order to uncover the source of oil and gas in this sag, basic geochemical, molecular geochemical, organic petrological, and n-alkane monomer carbon isotopic characteristics of source rocks and oils were analyzed in detail. In combination with the geological settings, the percentage contribution of these three sets of source rocks was assessed in the Qinnan Sag. Results obtained from the source rocks offered the following insights: (1) total organic carbon (TOC) ranged from 1.0% to 3.0%, and the S₂-TOC indicated that the qualities of source rocks followed the order of Es₃ > Es₁ > Ed₃. (2) Organic matter (OM) of the Ed₃ source rock belonged to Type II₂, while those of Es₁ and Es₃ belonged to Type II₁. The molecular geochemical parameters (CPI, C₂₄TeT/C₂₆TT, C₂₇–C₂₉ regular steranes, and 4-MSI) also revealed that the organic matter in the source rocks mainly originated from mixtures of algae and terrestrial higher plants. (3) The % R_o values of Ed₃, Es₁, and Es₃ were 0.61%, 0.80%, and 0.88%, respectively. These results were consistent with the maturity molecular parameters (including T-max, Ts/Tm, C₂₉-ααα20S/(20S + 20R) and C₂₉-ββ/(ββ + αα)). (4) Pr/Ph, Pr/nC₁₇, and Ph/nC₁₈ values indicated that the three sets of source rocks were deposited in a reduced lacustrine environment with different salinities (analyzed with G/C₃₀H), and the salinities of Ed₃ and Es₃ indicated a freshwater brackish water environment, while that of Es₁ suggested a brackish semi-saline water environment. (5) combining the geological settings with the geochemical results of oil–source correlations, it was found that the contribution of Es₁ source rock dominated (>50%) in the sag. Overall, this study provides more accurate data of oil source formation and is significant for oil exploration in the Qinan Sag, Bohai Bay Basin.