## 5 Discussion
### The Impact of Sedimentation on Reservoir Pore Structure in Two Sedimentary Systems
#### 5.2.1 Impact of Diagenesis on Pore Structure  
The examination of core-cost thin sections, ordinary thin sections, and SEM data determined that the diagenetic processes significantly affecting the tight sandstone reservoirs in Es\({}_{3}\)[3] in the Linnan Subsag include compaction, cementation, and dissolution. Among these, compaction and cementation reduce porosity, whereas dissolution increases porosity. (1) Compaction  
In the study area, the compaction in Es\({}_{3}\)[3] is primarily mechanical, with chemical compaction (pressure solution) being less common. According to the rock component statistics from core-cost thin sections, the overall clastic components in the area have a relatively high proportion of rigid components (quartz, feldspar, and metamorphic rock debris). However, the tight sandstone particles are generally relatively fine, with moderate sorting and a relatively high content of the clay matrix. As a result, mechanical compaction significantly reduces the original intergranular pores. The compaction phenomena observed under the microscope include the fracturing of rigid particles and deformation and compaction of plastic particles. In addition, the contact relationships of clastic particles are mainly line contacts and point-line contacts, with some particles undergoing compaction (Figure 5i,j). Compaction is one of the important diagenetic processes responsible for reducing primary porosity in the study area, with the primary influencing factor being the burial depth. As the burial depth increases, the compaction intensity also increases, leading to a significant reduction in the primary porosity. However, tight sandstone reservoirs controlled by two different sedimentary systems exhibit significant differences in the burial depth and corresponding degree of compaction. The reservoirs controlled by the gravity flow sedimentary system correspond to the 2 nd sand group, with relatively shallow burial depths ranging from 3200 to 4000 m. The contact types between clastic particles include point-line contacts and line-line contacts, with relatively low degrees of compaction and well-preserved primary intergranular pores (Figure 5b,c). In contrast, the tight reservoirs controlled by the delta front sedimentary system correspond to the 4-6 sand groups in the study area. Owing to a deeper burial (3800-4400 m), the degree of compaction is high, leading to a significant reduction in primary intergranular pores. The particle contacts are predominantly line contacts, with noticeable irregular contacts and the fracturing of rigid particles. These phenomena result in a reduced throat width, with most throat types being sheet-like and sheet-bend-shaped, making the pore structure more complex due to compaction. Mechanical compaction, which significantly reduces the primary porosity, has a weaker effect on turbidites than on delta front sandstone reservoirs. There is a small amount of pressure dissolution in the area, which primarily involves quartz pressure dissolution. This phenomenon manifests as irregular or sutured contactsbetween particles, which reduces the porosity (Figure 5i). Quartz pressure dissolution causes throat narrowing and flattening between elastic particles, increasing the complexity and heterogeneity of the pore structure, which ultimately affects reservoir permeability. After quartz pressure dissolution, the dissolved SiO\({}_{2}\) becomes saturated in the pore water, leading to the precipitation of quartz secondary enlarged edges, which reduces the porosity and permeability of the reservoir. Quartz pressure dissolution seams have developed, with some of these seams containing black carbonaceous films. As the burial depth of the reservoir increases and the geothermal gradient increases, the intensity of quartz pressure dissolution gradually increases. Consequently, the effect of quartz pressure dissolution is stronger in delta front facies reservoirs than in turbidite reservoirs, resulting in a more complex pore structure in the former.