Wettability plays a critical role in dictating the fluid percolation within low-permeability sandstone reservoirs. However, the wettability evolutionary mechanisms remain ambiguous given the frequent changes in the fluid properties and mineral types and compositions in these reservoirs throughout diagenesis. This opacity severely hampers the research on the mechanisms of hydrocarbon accumulation in the reservoirs. This study centers on the upper submember of the 4th member of the Paleogene Shahejie Formation in the Dongying Sag, Bohai Bay Basin. Based on the analysis of geological data, we undertake a systematical analysis of diagenesis’s controlling effects on the wettability of lowpermeability sandstone reservoirs through observation of casting thin sections and the X-ray diffraction analysis of mineral compositions, Amott wettability tests using nuclear magnetic resonance (NMR) equipment, and experiments of contact angles in a solid-oil-water system, both under high temperature and pressure. The results highlight distinct major pore types across various diagenetic states attributable to complex diagenesis. Specifically, the early diagenetic stages A and B evidenced the major pore types of residual pores by compaction and both intragranular pores from feldspar dissolution and intergranular pores from carbonate dissolution, respectively, while middle diagenetic stages A1, A2, and B witnessed the presence of dissolution pores at quartz edge, both intragranular pores from feldspar dissolution and intergranular pores from carbonate dissolution, and fractures, respectively as major pore types. As diagenesis advanced, residual pores by compaction, dissolution pores at quartz edge, and intragranular pores from feldspar dissolution grew increasingly hydrophilic. Furthermore, intergranular pores from calcite dissolution trended toward lipophilicity, while intergranular pores from dolomite dissolution evolved from water wetting to intermediate wetting. The overall wettability of the low-permeability sandstone reservoirs is governed by the major pore types and their surface wettability throughout the diagenetic timeline. The wettability of low-permeability sandstone reservoirs was dominated by water wettability across all the diagenetic stages, showcasing strong hydrophilicity, weak hydrophilicity, hydrophilicity, intermediate wetting, and hydrophilicity sequentially. Finally, a wettability evolution model of sandstone reservoirs under the action of diagenesis is established, which will guide the prediction of sweet spots in low-permeability sandstone reservoirs.

The swift shifts in sedimentary water bodies in continental petroliferous sedimentary basins result in frequent intercalation of sandstone and mudstone layers. Various sandstone-mudstone intercalation patterns (sand-mud assemblages) lead to significant differences in the storage spaces in sandstone reservoirs. Focusing on the lower submember of the 3^rd member of the Paleogene Shahejie Formation in the Linnan sub-sag, Huimin Sag, Bohai Bay Basin, we first analyze the spatial assemblages and single-layer thickness of sandstone and mudstone layers. Using casting thin section observations, physical property tests, and micro-CT scanning, we systematically elucidate the physical properties, pore types, and pore structures of sandstone reservoirs with different sand-mud assemblages. As indicated by the findings, the lower submember contains nine types of sand-mud assemblages, namely thick mudstone interbedded with thin sandstone, medium mudstone interbedded with thin sandstone, thick mudstone interbedded with medium sandstone, intercalated thin sandstone and thin mudstone, intercalated medium sandstone and medium mudstone, intercalated thick sandstone and thick mudstone, medium sandstone interbedded with thin mudstone, thick sandstone interbedded with medium mudstone, and thick sandstone interbedded with thin mudstone. The ionic interactions between sandstones and mudstones lead to strong heterogeneity in the storage capacity of sandstone reservoirs with different sand-mud assemblages. For sand-mud assemblages with low net-to-gross ratios, mudstones supply ample CO₃^2-, Ca²⁺, Fe²⁺, and Mg²⁺ to sandstones, and sandstones are completely filled with cements. Consequently, the sandstone reservoirs become extremely tight. However, when this ratio rises, mudstones cannot provide sandstones with sufficient ions mentioned above. In this case, sandstones near the sand-mud interfaces exhibit strong carbonate cementation, forming extremely tight reservoirs. In contrast, the interior of the sandstones shows weak carbonate cementation, with a small number of primary pores present. Additionally, the sand-mud assemblages with relatively thick sandstones promote organic acid infiltration, enhancing reservoir quality through the formation of numerous intergranular dissolution pores. Based on the differences in sand-mud assemblages, we reveal the influence of sand-mud assemblages on the evolutionary path and model of the storage spaces in sandstone reservoirs. Our insights are pivotal for predicting sweet spots in tight sandstone reservoirs.