Fracture development characteristics and its formation mechanism in Lower Member 1 Shahejie biolithite reservoir, Z oilfield, Bohai Bay
Yan Ruitao1, Zeng Lianbo1, Zhao Xiangyuan2, Liao Zonghu1, Chen Minzheng3
1. College of Geoscience, China University of Petroleum, Beijing 102249;
2. Exploration & Production Research Institute, SINOPEC, Beijing 100083;
3. Research Institute of Shenzhen Branch Company, CNOOC, Guangzhou 510240
Based on cores, thin sections and FMI data, the characteristics and controlling factors of natural fractures in biolimestone reservoir in Lower Member 1 of Shahejie Formation in Bohai Bay area were studied. The formation mechanism was then analyzed. The results showed that fractures were mostly generated by structure movement and diagenesis. The fractures caused by structure movement could be further divided into shear fractures and extension fractures. Most of the structural fractures were high-angle fractures and the strike were mainly NE-SW, NW-SE and near EW. The height of the fractures was always less than 170 cm, the length was less than 35 m and the aperture was less than 100 μm. The distribution of the fractures was controlled by lithology and thickness of the reservoir and faults. The fractures were mainly formed in NW-SE extensional stress field in the mid-late rift period of Eocene-Oligocene and in NEE-SWW compressive stress field of Late Dongying period. The fractures formed in early stage were largely NE-SW normal fault style fractures and the fractures in late stage were mainly NE-SW, NW-SE shear fractures and near EW extensional fractures.