## 5 Discussion
### The Impact of Sedimentation on Reservoir Pore Structure in Two Sedimentary Systems
#### 5.1.1 The Impact of Sedimentary Facies on Pore Structure  
Sedimentary facies have a significant effect on the formation of pore structures. Variations in the sediment composition, grain size, and sorting within different sedimentary facies lead to substantial differences in the pore structure. Even within the same sedimentary facies, variations in the grain size and clay content at different locations can cause noticeable differences in the reservoir pore structure (Figures 3 and 6). Turbidity currents are gravity-driven sediment flows with Newtonian rheological properties and turbulent conditions, where sediments are transported in suspension and supported by turbulence. The overall process of turbidity current deposition is characterized by a continuous decrease in energy and the flow velocity, resulting from the gradual settling of suspended sediments. During turbidity current sedimentation, coarse and fine particles settle at different rates, leading to a normally graded sequence with a gradational upper contact [61; 62]. Thin sections of sandstones show that turbidite reservoirs in the area have moderate-poor sorting, with subangular grains predominating. The amount of the matrix is slightly greater, mostly consisting of matrix-supported and pore-filling cementation. The maturity of turbidity current sediments and structures is relatively low, with varying clast sizes. The grain size probability curves typically exhibit a low-slope single-segment feature with a wide range of grain sizes (Figure 5a-h). In the delta front subfacies, the sedimentary microfacies primarily include distributary channels, delta front sandbars, distal sandbars, sheet sands, and interdistributary bars. The overall sorting of the reservoirs is moderate and the roundness is mainly subangular. Owing to significant sedimentary facies control, the compositional and structural maturities of the delta front subfacies reservoirs are intermediate to low (Figure 5i-t). Among the different types of sand bodies within the delta front sedimentary system, distributary channel sands have the best physical properties, whereas delta front sandbars and distal sandbars have intermediate properties. However, sheet sands and interdistributary bay sands have the worst physical properties. The reservoir lithofacies in different sedimentary microfacies of the delta front subfacies are complex, with varying degrees of heterogeneity and relatively noticeable differences in physical properties [63; 64; 65]. Overall, turbidite reservoirs have good physical properties, with lower complexity in terms of their pore structure and weaker heterogeneity, making them favorable reservoirs. In contrast, delta front reservoirs have poorer properties, with relatively complex pore structures and greater heterogeneity (Figures 4 and 6). The composition and structure of tight sandstones affect the reservoir physics and pore structure [66; 67; 68; 69]. The reason lies in the fact that although the matrix content in turbidite reservoirs is slightly higher and sorting is moderately deviated, the rocks have coarser grains, higher quartz and debris contents, and greater sandbody thicknesses, resulting in good reservoir quality. On the other hand, while delta front reservoirs have relatively lower matrix contents, their carbonate cement contents are significantly higher, and both their compositional and structural maturities are intermediate to low. Moreover, delta front reservoirs have thinner single-sand bodies, finer sandstone grain sizes, and better sorting and rounding, but an increased mud content in facies such as sheet sands degrades their physical properties, leading to poorer pore structures and greater heterogeneity, which further deteriorates their physical quality [70]. As the pore structure improves, the proportion of movable fluids in tight reservoirs increases accordingly. Therefore, the proportion of movable fluids and the oil content in gravity flow sedimentary reservoirs are greater than those in delta front reservoirs.