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Formation Mechanism of High Quality Lacustrine Source Rocks in the Bozhong Depression

Abstract

The Bozhong depression, located in the central part of the Bohai Bay basin, which is a Cenozoic lacustrine rift basin. The source rocks of the first and third members of Shahejie Formation (referred to as Es1 and Es3) and the third member of Dongying Formation (referred to as Ed3) are mainly developed in the Bozhong depression. The source rocks of Es3 and Ed3 are mainly semi-deep to deep lacustrine sediments, while the source rocks of Es1 are mainly sediments of braided river delta, fan delta and shore-shallow lakes. Paleoproductivity was characterized by the parameters of organic carbon, genetic potential, hydrogen index, and algae abundance. The parameters of paleoproductivity show that the productivity of Ed3 and Es3 were relatively high, while that of Es1 was relatively low, suggesting that the stable semi-deep to deep lake environment would increase productivity, while the productivity of the shore-shallow lake environment with hydrodynamic turbulence is lower. Studies of paleoproductivity distribution in different sedimentary facies show that the areas of higher paleoproductivity in Ed3 and Es3 were located in the semi-deep to deep lake areas near the fan delta or braided river delta, and the corresponding Fe/Al and P/Al values were relatively high, while the corresponding Fe/Al and P/Al values of Es1 with lower productivity were relatively low, indicating that the input of terrigenous sediments was an important source of nutrients for the lacustrine paleoproductivity. Preservation conditions study shows that the Pr/Ph ratios of the bearing strata in large-scale mudstones were relatively low in Ed3 and Es3, while the Pr/Ph ratios of the large-scale mudstones intercalated with thin-layered turbidite deposit were relatively large and unstable, indicating that stable deep water was constructive for the preservation of organic matter, while the environmental fluctuation caused by turbidity current deposition had a destructive effect on the preservation of organic matter. Because the water body in Ed3 and Es3 were mainly fresh-brackish lake water with large water depth, the main preservation mechanism should be related to the warm climate and water temperature stratification formed with greater water depth, and the reduction conditions were favourable below the thermocline. The preservation mechanism of Es1 was mainly concerned with the chemical stratification of high saline components, and the reduction conditions were favourable below the halocline.