## 5 Discussion
### The Impact of Sedimentation on Reservoir Pore Structure in Two Sedimentary Systems
#### 5.2.1 Impact of Diagenesis on Pore Structure
clay minerals and their occurrence in the reservoirs. In the northern source area of the Linnan Subsag, the tight sandstone reservoirs widely contain illite, kaolinite, chlorite, and illite/smectite mixed layers. Among these, kaolinite primarily forms book-like or worm-like aggregates by sporadically filling the pores (Figure 5m). It can also create some secondary porosity during the alteration of feldspar to kaolinite (Figure 5g). Illite bridges and partitions the intergranular pores in a flaky manner (Figure 5s). Chlorite acts as a pore lining and filling material within the intergranular pores. Moreover, the illite/smectite mixed layers appear in fibrous and flaky forms, often as pore linings and fillings. Clay minerals significantly reduce pore and throat sizes, affect porosity and permeability, and increase the heterogeneity of the reservoir pore structure. According to the vertical distribution map of clay mineral relative percentages, the kaolinite cementation in the delta front reservoirs is stronger than that in the turbidite sand reservoirs. In delta front reservoirs, the relative content of kaolinite can reach 67% (Figure 11). SEM reveals that kaolinite forms book-like or worm-like aggregates, with kaolinite crystals present to varying degrees in most intergranular pores and some kaolinite filling the throats, reducing their width. Additionally, there are often significant residual pores, dissolution pores, and intercrystalline micropores between kaolinite crystals, which increases the types of pore-throat structures and the heterogeneity of the pore space. However, turbidite sandstone reservoirs are relatively rich in illite and illite/smectite mixed layers. These clay minerals fill the pores in flaky and fibrous forms, with occasional networked and curved flaky oriented illite, reducing the pore and throat sizes. Chlorite is an authigenic mineral in the area and its content is low, ranging from 0.5% to 2.0%. According to X-ray diffraction analysis of clay minerals, the delta front reservoirs are relatively richer in chlorite, accounting for 4% to 23% of the total clay mineral content, with an average of 10.75%, and exhibit chlorite rims. The development of chlorite rims is beneficial for preserving primary intergranular pores. However, in the study area, the chlorite rims in the tight sandstone reservoirs are relatively thin and rarely completely envelop the particles. Therefore, the relatively high content of chlorite cementation does not play a key role in preserving the primary intergranular pores of the delta front reservoirs. In addition, it does not significantly improve the heterogeneity of the reservoir pore structure. Owing to the rich presence of illite and illite/smectite mixed layers in turbidite reservoirs and the abundant kaolinite cementation in delta front reservoirs, the impact of clay mineral cementation on the pore structure of tight sandstones varies. Additionally, the degree of clay filling in the delta front reservoirs is greater than that in the turbidite  
Figure 11: Vertical distribution of relative clay mineral content in Es\({}_{3}\)\({}^{3}\) tight sandstone reservoirs. reservoirs, resulting in different levels of heterogeneity in the pore structures of the two reservoir types (Figure 12). (5) Siliconeous cementation  
In the elastic rock tight reservoirs in the Linnan Subsag, siliceous cementation is characterized by the secondary enlargement of clastic quartz and the formation of new intergranular microcrystalline quartz (Figure 12). Microscopic observations reveal that both the turbidite and the delta front sandstone reservoirs exhibit the secondary enlargement of quartz, with similar amounts of new intergranular microcrystalline quartz. Thus, the impact of siliceous cementation on the heterogeneity of pore structures in the two different reservoirs is not significant (Figure 5e,j).