Geochemical Characteristics and Sedimentary Control of Pinghu Formation (Eocene) Coal-bearing Source Rocks in Xihu Depression, East China Sea Basin

Coal-bearing source rocks of the Pinghu Formation in the Xihu Depression comprise an important material basis of oil and gas resources in the East China Sea Basin. Based on drilling core observation results combined with the analysis and test results of macerals, trace/rare earth elements, and rock pyrolysis, the geochemical characteristics and sedimentary control of coal-bearing source rocks formation are discussed in a high-frequency sequence framework. The results indicate that the macerals composition of the coal-bearing source rocks of the Eocene Pinghu Formation in the Xihu Depression is dominated by vitrinite, with low-medium abundance of exinite and almost no inertinite. The coals and carbonaceous mudstones display higher amounts of total organic carbon (TOC) (14.90%-65.10%), S1 + S2 (39.24-136.52 mg/g), and I-H (191-310 HC/g TOC) respectively, as compared to the mudstones. Organic matter is plotted in type III kerogens and partially in type II; it is mainly in the low maturity stage. The trace elements results imply that the samples were deposited in a weakly reducing to weakly oxidizing environment and were occasionally affected by seawater. The coal-bearing source rocks were deposited in a relatively oxygen-containing environment. The coal-bearing source rocks development is jointly controlled by the coal accumulation environment, the water conditions affected by ocean currents in offshore basins in China, oxidation-reduction cycles of aqueous media and paleoclimate evolution in a high-frequency sequence framework.