Evaluation of sweet spots for a tight sandstone reservoir: A quantitative study of diagenesis in the fourth member of the Oligocene Huagang Formation, Xihu Depression, East China Sea Shelf Basin

Reservoir sweet spot evaluation is very important for tight sandstone gas exploration and development. Due to the characteristics of large burial depth, strong diagenesis heterogeneity, and prominent importance of diagenetic facies, sweet spot evaluation is the focus and difficulty of unconventional oil and gas exploration and development. By combining with core observation, conventional core analysis, clay x-ray diffraction analysis, bulk rock x-ray diffraction analysis, powder grain size analysis, thin section quantitative statistics, fluid inclusion homogenization temperature, scanning electron microscopy, scanning electron microscopy mineral quantitative evaluation, well logging data, and pre-stack seismic data, integrated simulation approach of seismic diagenetic facies and diagenetic numerical simulation was used to simulate porosity evolution and spatial distribution of tight sandstone of second submember (Smbr 42) of the fourth member (Mbr 4) of the Oligocene Huagang Formation in the Xihu Depression in the East China Sea Shelf Basin, and to evaluate reservoir sweet spot distribution. Based on mineral texture relationship and diagenetic sequence of Smbr 42 sandstone compaction, clay mineral transformation, calcite cementation, quartz cementation, and dissolution, five diagenetic facies were identified. Based on various seismic elastic parameters and core-derived diagenetic facies, the distribution of seismic diagenetic facies was interpreted by seismic diagenetic facies prediction method. Seismic diagenetic facies distribution can provide a "hard-constraint" three-dimensional diagenesis heterogeneity model for diagenetic history reconstruction, and seismic diagenetic facies distribution provides constraints on grain size and quartz grain content distribution in diagenetic numerical model. Based on the detailed consideration of input parameters of burial history, thermal history and diagenetic history, integrated diagenetic numerical simulation was used to reproduce porosity evolution and spatial distribution of Smbr 42 sandstone, and was applied to reservoir sweet spot evaluation of tight sandstone. Smbr 42-1 sublayer developed type III reservoir sweet spot and non-sweet spot. Smbr 42-2 to Smbr 42-6 sublayers mainly developed type II1, type II2, type III, and minor type I reservoir sweet spot. Simulation results were consistent with the locations of multiple sets of drilled gas layers and proven gas reservoirs.