A comparative study of source rocks and soluble organic matter of four sags in the Jiyang Depression, Bohai Bay Basin, NE China

Abstract The Jiyang Depression is a petroleum-rich basin in eastern China and has four petroleum sags: the Dongying, Zhanhua, Huimin and Chezhen Sags. However, the petroleum resources in the four sags differ greatly. We collected Rock-Eval pyrolysis and vitrinite reflectance (Ro) data for source rocks and the contents, group components and GC-MS data for soluble organic matter (SOM) to investigate the reasons for the hydrocarbon differences among the four sags. The results show that the source rock characteristics in the Jiyang Depression can be divided into two types: the type in the Dongying and Zhanhua Sags has greater abundances and better kinds of organic matter than the other type in the Chezhen and Huimin Sags. The Dongying and Zhanhua Sags correspondingly host higher SOM contents and more hydrocarbon components than the Chezhen and Huimin Sags. The evolutions of Ro in the Dongying Sag and the Zhanhua Sag are similar. However, the evolutions of SOM differ: the depth at which the SOM content and the relative content of saturates and resins substantially change in the Dongying Sag is shallower than that in the Zhanhua Sag. Additionally, the evolution of clay minerals corresponds well with the evolution of SOM, which indicates that the differences in clay minerals may cause the differences in the evolution of SOM. Therefore, the distribution in petroleum resource is jointly controlled by the source rock characteristics, thermal evolution and mineral evolution. Thus, organic–inorganic interaction should be a focus of the future research on hydrocarbon generation and petroleum resource evaluation.