## 5 Discussion
### Genetic Mechanisms of Reservoir Pore Structures in Different Sedimentary Systems
Medium pore-fine throat pore structures (Type E) can occur in calcareous siltstone lithofacies, eventually developing under the combined effects of strong compaction, strong cementation, and medium dissolution. Type E pore structures also develop at the top of delta sedimentary reservoirs in the eastern part, but the structural position is lower than that of Type D pore structures. In terms of physical properties, the porosity distribution ranges from 4-8%, and the permeability ranges from 0.02-1 mD (mainly less than 0.3 mD). The pores are mainly residual intergranular pores (70.60%), followed by dissolution pores (approximately 21%). The pore diameter range is the largest among the delta front reservoirs, from 0.45 to 6.52 um, with an average pore diameter of 1.89 um. However, within the relatively impoverished argillaceous layered siltstone lithofacies of the central and western Linnan Subsag, reservoir porosity reduction is most pronounced. Compaction and cementation are intense, whereas dissolution is weak. As a result, the worst pore structure type, fine pore-fine throat (Type F), has developed. This material has the poorest physical properties among all pore structure types, with the greatest complexity and heterogeneity. The porosity varies widely from 0-6%, with a concentration in the range of 0-4%. The permeability is mainly 0.1-0.2 mD. Although Type F pores are still predominantly residual intergranular pores (64.88%), they are characterized by a significant proportion of intercrystalline micropores (22.18%), which mainly consist of kaolinite intercrystalline micropores. The pore diameter distribution ranges from 0.12 to 3.57 um, with an average of only 0.88 um, which is the smallest among all pore structure types. The pore structure of tight sandstone reservoirs significantly affects the oil content. Overall, turbidite reservoirs have notably higher oil contents than delta front reservoirs and the oil content decreases sequentially as the pore structure deteriorates. The fluorescence images reveal that some rock fragments are impregnated with oily bitumen and asphaltene bitumen, resulting in fluorescence (Figure 14a4-f4). The two sedimentary systems exhibit different oil-bearing characteristics, and the quality of the pore structure has an important effect on the oil content. High-quality reservoir pore structures demonstrate good oil-bearing properties, whereas those with relatively poor pore structures present less noticeable oil-bearing characteristics. Coarse pore-coarse throat pore structures show spotted, patchy pale yellow, and brownish-yellow fluorescence, indicating a high bitumen content. The medium-to-coarse-throat sandstones exhibit diffused or localized patchy bright yellow, yellow, off-white yellow, and slightly brownish-yellow fluorescence, with bitumen contents slightly lower than those of Type A pore structures. The medium-pore-medium-throat sandstones mainly display patchy medium-dark, medium-bright yellow, yellow, and brownish-yellow fluorescence, with bitumen contents ranging from medium to high. In the medium-pore to fine-sized-pore-throat and fine-pore to medium-sized-throat types with a moderate-pore structure quality, patchy dull yellow-white and blue-white fluorescence are present in the intergranular spaces, with the bitumen content ranging from medium to low. In contrast, the poorest-quality fine-pore-fine-throat sandstones have only a small amount of bitumen-impregnated fragments, with the intergranular fluorescence being mostly patchy and spotted medium-dark blue-white and blue-white with a low bitumen content. Based on this analysis, a summary of the tight sandstone reservoirs controlled by different sedimentary systems in the study area is provided, clarifying the genetic mechanisms behind the differences in the reservoir heterogeneity. The pore structures are classified into six types, which are formed under the combined control of sedimentary lithofacies and diagenetic facies.