The characteristic of fluid activities and diagenetic responses of the pressure discharging zone in Yacheng area, Qiongdongnan Basin, South China Sea

Based on the multiple tests and analysis results of more than ten wells in Yacheng, West Qiongdongnan Basin, including drilling and logging data, Laser Raman microprobe, thermodynamic modeling of paleopressures, organic geochemistry, cathodoluminescence, casting thin-section, the carbon-oxygen isotope composition of carbonate cements, etc., this paper analyzes in detail the distribution of the top of overpressure system and characteristic of fluid activity in the nearby discharging zone, and also reveals the diagenetic responses of pressure discharging zone caused by fluid activity. In the present study area, the pressure discharging zone is possibly located in formations around the top of overpressure system. Using the data of formation testing, mud density, well logging and velocity spectrum, it has been confirmed that the top of overpressure system is mainly located within the depth of 3 000-4 000 m beneath the earth and its depth increases from structural high to low position; the influential range of pressure discharging in each well around the top of overpressure zone is different, which can be obtained using vitrinite reflectance (R_o). It is indicated that the current top of overpressure system (in the pressure discharging and fluid draining period) has little difference in depth as compared with the top of paleo-overpressure system. The fluid in the discharging zone is characterized by relatively high temperature and pressure as well as the accumulation of acidic materials and hydrocarbons. The fluid activities cause changes in the temperature, pressure and pore fluid medium of diagenetic field in pressure discharging zone, thus exerting influences on water rock interaction. This phenomenon is specifically described as below:(1) The R_o and clay mineral show a transformation in advance; (2) The organic matter T_max is extremely small and S₁/(S₁+S₂)is extremely large; (3) The development of secondary porosity is resulted from hot fluid acidic dissolution in reservoirs of the discharging zone; (4) The albitization of feldspar particles as well as the abnormal development of authigenic quartz and calcite cement are caused by the discharging of deep overpressure organic fluid. In conclusion, the pressure discharging zone around the top of overpressure system may be favorable for gas accumulation, the formation of high-quality reservoirs and the coupling of cap rocks, which is the target zone for natural gas exploration in Qiongdongnan Basin from now on.