Cluster Analysis of Crude Oil Genesis and Oil Source Correlation: A Case Study from the Cangdong Depression, East China

In a petroliferous basin with multiple sets of source rocks, determining the origins of hydrocarbons and their mixed ratios is a crucial aspect of hydrocarbon accumulation mechanisms. Focusing on the Mesozoic-Paleozoic reservoirs within the Cangdong Depression, this study began with source-source comparisons to identify key distinguishing parameters. This was followed by oil-oil correlations using the cluster analysis method. And then, oil source correlations were performed to determine the origins of each type of crude oil. On this basis, a quantitative method for predicting the proportion of mixed crude oil sources was proposed. The results show that the Ek₂ and C-P source rocks are both rich in organic matter. However, the Ek₂ source rocks are predominantly sapropelic and humic-sapropelic and favorable for oil generation. However, the C-P source rocks are primarily humic-sapropelic and more conducive for gas generation. Using cluster analysis, the Meso-Paleozoic crude oils in the Cangdong Depression are classified into two main types. Type I crude oil is primarily derived from C-P coaly mesure source rocks, while Type II crude oil is mainly sourced from the Ek₂ source rocks. Additionally, the oil source index (OSI) was proposed, involving physical property, carbon isotope, Ts/Tm, C₃₀-/C₂₉-Hopane, C₂₇-/C₂₉-regular sterane, and tricyclic terpene/C₃₀-Hopane. An OSI greater than 1.20 indicates a source of Ek₂, while an OSI less than or equal to 0.40 suggests humic oils derived from C-P source rocks. When the OSI falls between 0.40 and 1.20, crude oil is classified as a mixed-source oil. The proportion of mixed sources can be determined by the OSI value's proximity to the two end-point values. This study provides a valuable reference for analyzing the origin of crude oil in similar basins.