Paleoenvironment and Controlling Factors of Oligocene Source Rock in the Eastern Deep-Water Area of the Qiongdongnan Basin: Evidences from Organic Geochemistry and Palynology

摘要

Little research regarding the organic geochemistry of source rock deposited in marine or transitional environment in the eastern deep-water area of the Qiongdongnan Basin (QDNB) has been performed so far. Based on the organic geochemical and petrological data of mudstone samples collected from the eastern deep-water area in the QDNB, combined with palynological data of rock samples in the shallow-water area, this paper studies the paleoenvironment, hydrocarbon potential, and controlling factors of Oligocene source rock in different sedimentary facies in the eastern QDNB. Two distinct models of delta front source rock and neritic source rock are proposedaccording to organic matter sources and geochemical characteristics. Source rock in the delta front subface is of medium to high organic abundance and gas-prone-type kerogen, the geochemical biomarkers are characterized by high ratios of pristane/phytane, oleanane/C-30 hopane, T-bicadinane/C-30 hopane, and low ratios of C-23 tricyclic terpane/C-30 hopane, C-27 sterane/C-29 sterane, (nC(21) + nC(22))/(nC(28) + nC(29)). However, source rock in the neritic face has medium organic richness and kerogen type prone to generate both oil and gas; the biomarkers are characterized by low ratios of pristane/phytane, oleanane/C-30 hopane, T-bicadinane/C-30 hopane, and high ratios of C-23 tricyclic terpane/C-30 hopane, C-27 sterane/C-29 sterane, (nC(21) + nC(22))/(nC(28) + nC(29)). During the Oligocene epoch, the paleoclimate in the QDNB was subtropical warm-humid, and paleovegetation around and within the sags was dominated by pteridophyte and angiosperm plants, causing terrestrial higher plants to be the main organic matter source. Source rock in the delta front subface is formed in oxidation fresh or brackish water environment, whereas source rock in the neritic face is formed in weak oxidization brackish or saline water conditions. Organic matter abundance in the delta front subface is mainly controlled by the terrigenous organic matter input, whereas organic matter abundance in the neritic face is controlled by aquatic organic matter input and influenced by the water redox environment.