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Structural and Sedimentary Characteristics of Hydrocarbon-Rich Sags in Lacustrine Sub-Basin of Half Graben Type, Pearl River Mouth Basin, South China Sea

Pearl River Mouth Basin undergoes complex tectonic evolution processes and forms lacustrine, transitional and marine sediments. Drilling shows that there exist large petroleum reserves in the hydrocarbon-rich sags of Pearl River Mouth Basin, South China Sea. To reduce the risk, the exploration, structural and sedimentary characteristics of the hydrocarbon-rich sags should be identified and described. Drilling, seismic, and microfossil data are integrated to interpret the structural and sedimentary evolution of the hydrocarbon-rich sags in Pearl River Mouth Basin. By analyzing the tectonic and sedimentary evolutionary characteristics, three conclusions may be drawn: 1) The present regional tectonic characteristics of the PRMB are formed by mutual interactions of the Eurasian Plate, Pacific Plate, Philippine Plate and Indian Plate. During the Paleocene to early Eocene and late Eocene and early Oligocene, the PRMB is at the rifting stage. During the late Oligocene, the PRMB was at the rifting-depression transitional stage. After the Oligocene, the PRMB is at the depression stage. 2) Tectonic conditions control the sedimentation process in the hydrocarbon-rich sags. During the lacustrine sedimentation stage, synsedimentary faults and intense faulting control the sedimentation. During the transitional sedimentation stage, weak fault activity influences the deposition process in the hydrocarbon-rich sags. During the marine sedimentation stage, weak fault activity and depression activity control the deposition process in the hydrocarbon-rich sags. 3) Tectonic evolution affects the deposition process. The lacustrine, transitional and marine sedimentation corresponds to different tectonic conditions. The lacustrine sedimentation is formed under fault activity during the rifting stage. The transitional sedimentation is formed under weak fault activity during the late rifting stage. The marine sedimentation is formed under weak fault activity and depression activity during the rifting-depression and depression stages. 4) The half graben is beneficial for the formation of lacustrine source rocks, which is responsible for the hydrocarbon-rich sag. Therefore the half graben mode contributes to the hydrocarbon-rich sags.