Differential diagenetic evolution and evaluation of reservoirs in Santan Deep Depression, Xihu Sag, East China Sea Shelf Basin

The Santan Deep Depression in the Xihu Sag of East China Sea Basin has favorable conditions for oil and gas accumulation, and multiple gasfields such as Y, Q, and G have been discovered, indicating abundant oil and gas resources. The key factor for accumulation and enrichment in this area is the reservoir. However, the study area experienced early deep burial, resulting in overall poor reservoir physical properties and unclear distribution of sweet spot reservoirs, which constrains the exploration process of oil and gas in the middle and deep formations. To identify large-scale high-quality reservoir zones, based on data such as thin section observation, X-ray diffraction, and physical properties, two conclusions were drawn through comparison of sedimentation, microscopic pore structures, and differences in diagenetic evolution of reservoirs in the southern, central, and northern parts of the Santan Deep Depression: (1) In terms of reservoir characteristics and diagenesis, the study area mainly consisted of low-porosity and low-permeability, ultra-low-porosity and ultra-low-permeability, and tight reservoirs, with reservoir evolution at the middle diagenetic stage B. Secondary dissolution pores were the main type of reservoir space, and chlorite film and dissolution were constructive diagenesis processes. (2) In terms of differences in reservoir physical properties, influenced by provenance, diagenesis, and geothermal gradient variations, the burial depths of the top boundaries of tight reservoirs between the southern and northern parts of the Santan Deep Depression differed. The top boundary of tight reservoirs in the southern part was buried at 4 000 m, corresponding to a temperature of 140 ℃. In the central and northern parts, the top boundary was at 4 700 m, with a corresponding temperature of 160 ℃. Compared with the Huagang Formation, Pinghu Formation reservoirs experienced stronger carbonate cementation, weaker compaction, and stronger dissolution. More high-quality reservoirs were developed in conventional reservoir units, and more effective reservoirs were developed in tight reservoirs controlled by the overpressure-induced diagenetic inhibition effects within the source. Based on the above understanding, a high-quality reservoir development model controlled by “coarse-grained facies, main channel sand bodies, and internal source overpressure” was proposed, providing important guidance for exploring large-scale oil and gas reservoirs in the middle and deep formations of the Santan Deep Depression in the Xihu Sag.