Development Mechanism and Pattern of the Microscopic Pore Structure in Deep Tight Sandstone Reservoirs: Xihu Depression, East China Sea Basin

Deep tight sandstone reservoirs are characterized by strong microscopic pore structure heterogeneity and commonly exhibit a high-porosity, low-permeability profile, posing significant challenges for effective reservoir evaluation and “sweet spot” prediction. The microscopic pore structure of 209 tight sandstone samples from the deeply buried Huagang Formation in the Xihu Depression, East China Sea Basin, was systematically characterized by integrating multiple analytical techniques, including casting thin sections, scanning electron microscopy (SEM), X-ray diffraction (XRD), nuclear magnetic resonance (NMR), and high-pressure mercury injection (HPMI). The results indicate that the reservoir space is dominated by mesopores (55.48%) and transition pores (32.39%), with macropores (2.09%) and micropores (10.04%) being relatively underdeveloped. A significant vertical heterogeneity in reservoir quality is observed. The H4 member exhibits superior properties, characterized by a higher average movable fluid saturation (averaging 46%) and better pore connectivity. In contrast, the H5 member is more compact, with a notably higher proportion of bound fluid (averaging 47%). The differences in reservoir quality are controlled by a sedimentary–diagenetic coupling mechanism. High-energy, coarse-grained facies underwent a constructive pathway involving chlorite coating protection and dissolution enhancement, forming high-quality pore networks. In contrast, low-energy, fine-grained facies experienced a destructive pathway dominated by intense compaction and cementation, leading to the deterioration of pore structure. The petrophysical properties of the deep reservoirs are primarily governed by the three-dimensional connectivity and spatial distribution of effective “pore-throat assemblages” composed of dominant throats. Accordingly, a “binary” pore structure development pattern is established for the deep tight sandstone reservoirs in the study area. This pattern posits that the reservoir space is heterogeneously composed of a minority of connected “effective percolation assemblages” and a majority of isolated “ineffective assemblages”.