## 4 Results  
### Petrography and Lithofacies Assemblage Characteristics  
The reservoir rocks in the Shunan area are rich in quartz and feldspar, forming feldspathic quartz sandstone and lithic quartz sandstone, respectively (Figures 2 and 3a). The average porosity of the Shaximiao Formation in the Shunan area is 7.19%, with an average permeability of 0.63 mD. The effective reservoir (porosity greater than 5%) has an average porosity of 9.1% and an average permeability of 0.72 Md (Figure 4). Overall, the reservoirs from these members are characterized by low porosity and permeability mainly caused by intergranular contact of grains and feldspar dissolution (Figure 3d). The sandstone is dominated by medium- to fine-grained, sub-angular to sub-rounded grains (Figure 3b), with moderate sorting, and predominantly point (tangential) contacts (Figure 3c). Further cementation is primarily porous (Figure 3d). Lithofacies provide important information for a detailed understanding of the diagenetic impacts on reservoirs, as various lithofacies assemblages exhibit distinct reservoir properties, as described by [PERSON] et al. [37]. Core observations from three wells in the study area reveal that effective reservoir lithologies in the Shaximiao Formation lie in medium-grained, medium- to fine-grained, and fine-grained sandstones, which comprise features of massive bedding, cross-bedding, and parallel lamination, respectively (Figure 3e,f). Cross-bedding is classified by scale: small-scale cross-bedding (<3 cm), and medium-scale cross-bedding (3-10 cm). Medium-grained sandstone lithofacies primarily develop massive bedding, while medium- to fine-grained sandstones develop both massive-bedding and cross-bedding. Fine-grained sandstone containing higher plastic minerals and lower quartz and feldspar contents, typically develops parallel lamination. This study focuses on effective reservoirs, excluding siltstone lithofacies from further discussion. Based on vertical stratigraphy, lithofacies types, and bedding characteristics, the Shaximiao Formation has been classified into three primary lithofacies assemblages identified in Figure 5, including: (1) Type I Lithofacies Assemblage (High-Porosity and High-Permeability) (Figure 5a), (2) Type II Lithofacies Assemblage (Medium-Porosity and Medium-Permeability) (Figure 5b), and (3) Type III Lithofacies Assemblage (Low-Porosity and Low-Permeability) (Figure 5c). Type I is associated with two lithofacies i.e., massive medium-grained sandstone (Type I 1) and medium-scale cross-bedded medium-grained sandstone (Type I 2). This assemblage is dominated by medium-grained sandstone with uniform grain size and an absence of mudstone interbeds. The sandstone exhibits massive  
Figure 4: Histogram of porosity and permeability distribution in tight sandstones of the study area. Figure 3: Typical thin section photographs from the study area. Note: (**a–d**) are thin sections. (**e,f**) are core samples. (**a**) Well C18, 1166.11 m, J\({}_{2}\)S\({}_{2}\)\({}^{1}\), quartz and feldspar; (**b**) Well C14, 1495.75 m, J\({}_{2}\)S\({}_{1}\)\({}^{2}\), sub-angular to sub-rounded grains; (**c**) Well C14, 1495.75 m, J\({}_{2}\)S\({}_{1}\)\({}^{2}\), point and line contacts; (**d**) Well C18, 1164.98 m, J\({}_{2}\)S\({}_{2}\)\({}^{1}\), intergranular pores; (**e**) Well C14, 1497.65–1498.00 m, J\({}_{2}\)S\({}_{1}\)\({}^{2}\), parallel and cross-bedding; (**f**) Well C14, 1500.14–1500.31 m, J\({}_{2}\)S\({}_{1}\)\({}^{2}\), massive bedding. and medium-scale cross-bedding, typical of depositional environments in the delta plain distributary channel and delta front subaqueous distributary channel. The Type II lithofacies assemblage comprises three lithofacies: massive medium- to fine-grained sandstone (Type II ), small-scale cross-bedded medium- to fine-grained sandstone (Type II ), and parallel-laminated medium-grained sandstone (Type II ). The overall grain size ranges from medium to fine, displaying normal grading with coarser grains at the base and progressively finer grains toward the top.