No abstract

In order to study the dominant seepage channel of the third member of Weizhou formation (Ew3) in Weizhou 12-X oilfield, Weixinan Sag, Beibu Gulf Basin, and tap the potential of remaining oil. The distribution and causes of the anomalously high permeability zones in Weizhou Formation were studied by using conventional core physical property analysis, scanning electron microscope, laser particle size analysis, X-ray diffraction and thin section microscopic identification. As the results show, vertically,there are three anomalously high permeability zones in the A11, A12 and A21 micro-stage of the middle diagenetic stage, with the depth range of 2300 m ~ 2400 m, 2400 m ~ 2600 m, 2600 m ~ 2900 m respectively. Grain size, sorting, dissolution and early emplacement of hydrocarbons are the main causes of anomalously high permeability zones. Although both grain size and sorting affect porosity and permeability, the effect of grain size on permeability is stronger than sorting, and sorting has a stronger effect on porosity than grain size. Magmatic hydrothermal and organic acid promote dissolution and concomitant porosity and permeability increase by dissolving unstable minerals. The early emplacement of hydrocarbons retard the cementation and accompanying porosity and permeability reduction by reducing the water-rock ratio. Finally, sandstone reservoirs in the Ew3 are characterized by anomalously high permeability zones.

This study focuses on the diagenetic evolution of the reservoir in the third member of the Oligocene Weizhou Formation (E3w3) 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 E3w3 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 E3w3 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.