# Diagenesis of Deep Low Permeability Reservoir in Huizhou Sag and Its Influence on Reservoirs
## 4 Results
### Diagnetic Stage and Diagnetic Sequence  
According to the diagenetic stage classification criterion and the Chinese standard classification scheme of sandstone diagenesis, \"the division of diagenetic stages of the clastic rocks\" (SY/T5477-2003) [57] based on the current industry standards, combined with the distribution characteristics of cement, the contact relationship of clastic particles (point contact, line contact, and concave-convex contact), the diagenetic sequence of the Enping Formation in Huizhou Sag has been reconstructed (Figure 7). In conclusion, the reservoirs in the study area are in the stage of middle diagenesis A-B.  
In early diagenesis stage A (IA), the reservoir had a shallow burial depth, where a weak alkaline environment gradually transformed montmorillonite into illite, and mechanical compaction significantly transformed the pore space. The formation water, which contains a substantial amount of alkali metal ions (Na\({}^{+}\) and K\({}^{+}\)), will undergo acidic dissolution with pyroclastic materials, resulting in a small amount of chlorite film and secondary quartz enlargement, while the pore water in the sediments will precipitate and form micritic calcite.  
In early diagenesis stage B (IB), a large amount of montmorillonite was transformed to create an illite-montmorillonite mixed layer. In addition to authigenic clay minerals, small amounts of siderite and quartz minerals were also formed. At the same time, the whole sandstone was unconsolidated--and it was mostly weakly consolidated in loose sand--and the elastic particles were mainly in point contact. In middle diagenesis stage A (IIA), with the burial depth, the development degree of the source rocks gradually matures, and the amount of organic acids gradually increases. Pore water discharges a significant amount of organic acids, leading to the corrosion of soluble minerals like potassium feldspar and plagioclase. The relatively enclosed diagenetic environment delays the discharge of the  
Figure 7: Diagenetic sequence of sandstone reservoir of Enping Formation in study area.  
precipitated material resulting from dissolution, leading to the formation of various types of cement, including kaolinite, illite, quartz, and calcite.  
In middle diagenesis stage B (IIB) of the Enping rock formation, the relatively closed diagenetic environment gradually changed, showing a weak alkaline environment and leaf-like chlorite precipitating in intergranular and dissolution pores. Meanwhile, dolomite metasomatism, calcite, and quartz occurred.  
The diagenetic evolution sequence of the Enping Formation reservoir is as follows: it starts with early diagenetic stage A (IA), which involves mechanical compaction and the formation of a chlorite film; then, it moves on to early diagenetic stage B (IB), where mechanical compaction, tufaceous alteration, and the dissolution of feldspar occur; then, it moves on to middle diagenetic stage A (IIA), which involves chemical compaction, dolomite cement, feldspar dissolution, debris dissolution, and kaolinite consolidation; and then it moves on to late diagenetic stage B (IIB).