## 2 Formation Mechanisms of Effective Reservoir in Ultra-Deep
### Sedimentation Lays the Foundation for Mineral Fabric and Primary Porosity of Reservoirs
#### 2.2.5 Pore-Increasing Effect of Dissolution  
The development of dissolution processes in deep-seated reservoirs significantly enhances the reservoir properties of sand bodies. Qualitative petrological observations have led many geologists to conclude that secondary dissolution pores predominate in these  
Figure 8: Microscopic characteristics of typical diagenesis in deep reservoirs of the Midwest Basin. (**a**) Silurian, Tarim Basin, Shun-901 well, 5298 m, early hydrocarbon charging effectively inhibited cementation; (**b**) Silurian, Tarim Basin, Shun-9 well, 5397.32 m, quartz overgrowths in bitumen-free reservoir; (**c**) Jurassic, Junggar Basin, Yong-1 well, 5878 m, quartz secondary enlargement outside discontinuous oil film; (**d**) Jurassic, Junggar Basin, Dong-7 well, 5224 m, feldspar dissolution; (**e**) Silurian, Tarim Basin, Shun-902 well, 5518.3 m, two stages of hydrocarbon charging occurred after feldspar dissolution; (**f**) Triassic, Ordos Basin, Honghe-12 well, 2091 m, kaolinite dissolution pores Qog: quartz overgrowth.  
deep-seated sandstone reservoirs [23; 63; 64; 65; 66; 67; 68]. Microscopic examinations often reveal the dissolution of silico-aluminate minerals, such as feldspar (Figure 8d), which are effective for the migration and accumulation of hydrocarbons. For instance, in the Silurian reservoirs of the Tarim Basin, feldspar grains exhibit pronounced dissolution along joint fractures, with fluorescence imaging indicating that hydrocarbons from two phases migrated along these dissolution pathways (Figure 8e). Beyond the dissolution of detrital minerals, the interaction between diagenetic fluids and authigenic minerals plays a dual role in both preserving and enhancing porosity. Diagenetic early-stage cementation can protect pores from compaction and subsequent acidic or alkaline fluid invasion can dissolve these cements, thereby increasing porosity. Advancements in reactive transport modeling have improved our understanding of subsurface processes by integrating chemical reactions with fluid flow through porous media. These models simulate the spatial and temporal distribution of chemical reactions along flow paths, helping to predict mineral dissolution and precipitation patterns. However, accurately capturing the complex interplay between geochemical reactions and fluid dynamics remains a significant challenge. Early syntaxial calcite cements, while initially occluding pore spaces during shallow burial, can effectively resist compaction. Subsequent diagenetic processes may partially dissolve these cements. In the Silurian reservoirs of the Tarim Basin, evidence shows that quartz grains remain uncontacted, with calcite dissolution residues present in the pores, followed by hydrocarbon migration (Figure 8e). Similarly, the precipitation and later dissolution of kaolinite minerals contribute to pore preservation by releasing additional pore space upon dissolution (Figure 8f).  
During burial diagenesis, significant mineral dissolution and the removal of dissolution products by pore-water migration determine the degree of improvement in porosity and permeability. [PERSON] [69] argued that in a closed geochemical system unaffected by hydrothermal activity, and constrained by material balance, the development of secondary dissolution pores and a net porosity increase are not feasible. Although secondary pores are observable under thin-section microscopy, quantifying the amount of material dissolved and precipitated specifically during a certain diagenetic stage remains highly challenging.  
During shallow burial, reservoirs act as open geochemical systems due to meteoric water influence, but in deeper, closed environments, existing geochemical models struggle to predict the formation and distribution of secondary pores, limiting our ability to forecast the dissolution porosity distribution of dissolution pores.