Effect of Overpressure Formation on Reservoir Diagenesis and Its Geological Significance to LD Block of Yinggehai Basin

As to the Yinggehai basin, there is much room for further studies on the following issues including the distribution of abnormal pressure, the interaction of fluid and rock under the condition of abnormal pressure and the effect on diagenetic evolution. A systematic analysis on the distribution of overpressure, the effect on geological fluid activities and diagenetic evolution in Yinggehai basin is carried out by means of microscopy, scanning electron microscopy, isotope analysis and fluid inclusion homogenization temperature measurement based on the previous studies.The results show follows: (1) Overpressure drives the deep thermal fluid to release up. The thermal fluid which contains carbonate minerals is driven to the top interface of the overpressure and then deposits again with the changing pressure and temperature. The thermal fluid forms tight plugged zone of carbonate cementation with high content. (2) The overpressure reduces the material sources of carbonate cements and quartz secondary enlargement by inhibiting the transformation of clay minerals, so that the primary pores can be effectively preserved. (3) The reservoirs in LD block generally contain high content of CO₂. In condition of overpressure, the solubility of CO₂ in the fluid increases, which generates a large number of H⁺. On the other hand, overpressure increases the space and time for the release of organic acids in LD block and promotes the dissolution. It is concluded that the main reason for the high porosity in middle-deep overpressure reservoir of study area is that the overpressure constrains the forming of carbonate cement, and the other reason is that the decrease of compaction and the promotion of secondary pores.