Formation mechanism of organic-rich source rocks in Bozhong sub-basin, Bohai Bay basin, China

Abstract

As the main source rocks, the third member of Shahejie Formation (Es3), the first member of Shahejie Formation (Es1) and the third member of Dongying Formation (Ed3) have abundant oil reserves. However, the formation mechanism is unclear, especially the paleoproductivity. This paper discussed the paleoproductivity and paleoenvironment such as terrigenous organic matter (TOM) input, sedimentation rate, and redox conditions. Previous studies considered that all the source rocks have high paleoproductivity during deposition, but they ignored the time attribute. Based on the definition of total organic carbon (TOC), we deduced a new formula and calculated the paleoproductivity of Ed3, Ed1, and Es3. The results suggested that the paleoproductivity was high during Ed3 and Es3 deposition, and low during Es1 deposition. The TOM input was moderate during Es3 deposition, high during Ed3 deposition, and low during Es1 deposition. The lake was saline during Es1 deposition, and fresh to brackish during Ed3 and Es3 deposition. The salinization of the lake water in the study area formed good redox conditions, in which the accumulation of organic matter at a low sedimentation rate was the main controlling factor for the formation of organic-rich source rocks in Es1. In contrast, the high paleoproductivity at a high sedimentation rate was the main controlling factor in Ed3 and Es3.