Crude oil type classification and source rock identification in Miaoxi area of Bohai Sea area based on stoichiometric method

The crude oil types in the Miaoxi area of the Bohai Sea area are highly complex, and the source rocks and genetic types remain unclear. Using multivariate statistical analysis, researchers can comprehensively examine the interrelationships among multiple correlated variables, which is particularly suitable for large-scale data mining and regional oil-oil and oil-source analysis. In this study, based on the biomarker parameter index system, hierarchical cluster analysis (HCA) and principal component analysis (PCA), common methods in multivariate statistical analysis, were applied for oil-oil and oil-source correlation of crude oil from multiple layers in the Miaoxi area. Three types of crude oil were detected. Type Ⅰ crude oil are characterized by a low C₂₃TT/C₃₀H ratio with relatively low maturity. It may be derived from a freshwater lacustrine reducing environment with abundant input of terrigenous organic matter. This type of crude oil shows a strong correlation with the source rocks in the first and second members of the Shahejie Formation in the eastern sag of the Huanghekou Depression. Type Ⅱ crude oil is at a mature stage and features lower C₂₃TT/C₃₀H and G/C₃₀H ratios compared to Type Ⅰ, but with slightly higher Pr/Ph, sterane/hopane, and C₁₉TT/C₂₃TT ratios. These features also indicate a freshwater lacustrine environment with terrestrial organic matter input. It is inferred that Type Ⅱ oil is mainly sourced from the third member of the Shahejie Formation, with contributions from source rocks in both the eastern sag of the Huanghekou Depression and the southern sag of the Miaoxi Depression. Type Ⅲ crude oil is at a mature stage, exhibiting a wide distribution range in multiple biomarker parameters, including ETR(C₂₈TT+C₂₉TT)/(C₂₈TT+C₂₉TT+ Ts), G/C₃₀H, C₂₃TT/C₂₁TT, Pr/Ph, C₂₃TT/C₃₀H C₂₄Te/C₂₆TT, C₂₇/C₂₉ regular sterane, and 4-methyl sterane/ C₂₉ regular sterane. These variations reflect heterogeneity in the depositional environment of oil source rocks and organic matter types. It can be concluded that type Ⅲ crude oil is mixed-source oil, likely derived from the third and fourth members of the Shahejie Formation. Alternating least squares analysis results indicated that Type Ⅲ crude oil is mainly derived from the source rocks in the fourth member of the Shahejie Formation, with a contribution rate of 85% to 93%, while the contribution from the third member is only 7% to 15%.