Diagenetic Evolution Control on Reservoir Quality of the Oligocene Weizhou Formation in Weixinan Sag of Beibuwan Basin, Northern South China Sea

This study focuses on the diagenetic evolution of the reservoir in the third member of the Oligocene Weizhou Formation (E(3)w(3)) in Weinan Sag of Beibuwan Basin, northern South China Sea. Based on the contents and occurrence states of chlorite, illite, kaolinite and quartz cement were described by scanning electron microscopy (SEM), thin section and X-ray diffraction. In the E(3)w(3) reservoir, three different occurrences of chlorite are recognized. These include chlorite coating, leaf-shaped chlorite and tongue-shaped chlorite. Furthermore, three major types of diagenetic kaolinite are observed. These include kaolin crystallization promoted by fluids of meteoric origin, kaolin formed by organic acids and blocky kaolin. In addition, feldspar, by organic acid dissolution, can mainly form kaolinite but also form illite or chlorite in the E(3)w(3) reservoir. Different clay minerals have different effects on reservoir quality, which are controlled by occurrence and diagenetic environment. Compared to the other minerals, chlorite coating has a greater effect on quartz overgrowth. However, If minerals are packed in intergranular pores, they have a negative effect on reservoir quality.