Characteristics and origin of the Lower Oligocene marine source rocks controlled by terrigenous organic matter supply in the Baiyun Sag, northern South China Sea

Abstract To analyze the dominant factors of the origin of marine source rocks of the northern South China Sea (SCS), this paper focuses on the Lower Oligocene neritic source rocks in the Baiyun Sag of the Pearl River Mouth Basin. This study assesses the source rocks based on the principles and methods of sedimentology, geochemistry, and palynology and data on pyrolysis parameters, biomarker compounds, major and trace elements, and sedimentary organic debris. The paleoproductivity, input of terrigenous organic matter (TOM), redox conditions and sediment flux are characterized by the above parameters. The dominant effects of organic matter (OM) supply, preservation, and dilution on the origin of the source rocks are studied, and a depositional model of the source rocks is established. Finally, the origin of the Lower Oligocene marine source rocks in the northern SCS is clarified. The results show that the main source of the marine source rocks was terrestrial higher plants, while the contribution of planktonic algae was small. The origin of the marine source rocks was strongly controlled by the input of TOM during the depositional period. The source rocks developed in an oxic environment, and the correlation between the redox indices and OM abundance is poor, which indicates that the redox conditions were not the main factor dominating the origin of the source rocks. With an increase in the sediment flux and the barrier formed by low uplifts, the dilution of the OM decreased, and the contact time between OM and oxygen was markedly reduced; therefore, some OM in the oxic bottom water was not oxidized but instead became enriched, thereby forming the source rocks.