## 3 Results and Discussion
### Evaluation of Basic Characteristics of Tight Reservoir
#### 3.2.4. Grain Size Characteristics of Reservoir Sand Bodies  
The grain size parameters and their spatial distribution are useful indicators for distinguishing the paleogeographic environment and sedimentary hydrodynamic conditions during the process of sand body deposition. Additionally, they are crucial for determining the physical properties of sand body reservoirs, which are of great significance for reservoir evaluation. As illustrated in Figure 7, the underwater distributary channel sand body of fan-delta facies is characterized by a well-sorted particle size distribution, exhibiting a positive skewness. This indicates that the sediments in the sand body are predominantly coarse-grained components. Additionally, the kurtosis of the particle size-frequency curve is  
Figure 5: Composition triangle of Chang 9 sandstone. Figure 6: Optical microscope images of sand bodies from the Chang 9 oil layer formation. (**a**) Coarse- to medium-grained debris feldspar sandstone with grain support (C19, 840.47 m); (**b**) Fine-grained feldspar sandstone with grain support (C88, 864.77 m). flat and very sharp, suggesting a uniform distribution of particle sizes. The particle size probability curve is primarily shaped by channelized traction flow, which is exemplified by the classical two-stage formula comprising a jump component and a suspension component. Additionally, the three-stage formula of a double jump component reconstructed by lake waves also plays a role. The braided river delta sedimentary system is characterized by the development of onshore and underwater braided distributary channel sand bodies. According to Figure 8a, the sand particle size distribution in the braided distributary channel of the mainland is positively skewed, indicating that the sediments in the sand body are mainly coarse-grained components. The kurtosis of the particle size-frequency curve is from flat to very sharp. The particle size probability curve is primarily shaped by channelized traction flow, which is encapsulated in the classical two-stage formula comprising a jump component and a suspension component. As illustrated in Figure 8b, the particle size distribution of the underwater distributary channel sand body exhibits a positive skewness, indicating that the sediments in the sand body are predominantly coarse-grained components, and the kurtosis of the particle size-frequency curve is medium to very sharp. The particle size probability curve is primarily shaped by channelized traction flow, which is represented by the classical two-stage formula comprising a jump component and a suspension component. Additionally, the three-stage formula of a double jump component reconstructed by lake waves also plays a role. Figure 8: Grain size probability diagram of braided river delta sandstone. (**a**) for the sand particle size distribution in the braided distributary channel; (**b**) for the particle size distribution of the underwater distributary channel. Figure 7: Grain size probability diagram of channel sandstone in fan delta. The meandering river delta sedimentary system comprises two main types of sand bodies: onshore sand bodies and underwater distributary channel sand bodies. The constituent particles of the former are obviously finer than those of the latter. Figure 9a illustrates that the grain size distribution of the sand body in the land distributary channel is positively skewed, indicating that the sediments in the sand body are mainly coarse-grained components. The kurtosis of the grain size-frequency curve is from flat to sharp. The particle size probability curve is mainly formed by the channelized traction flow, which is represented by the classical two-stage formula composed of the jump component and the suspension component. Figure 9b illustrates that the particle size distribution of the underwater distributary channel sand body is positively skewed, indicating that the sediments in the sand body are mainly coarse-grained components. The kurtosis of the particle size-frequency curve is medium to very sharp.