# Diagenesis of Deep Low Permeability Reservoir in Huizhou Sag and Its Influence on Reservoirs
## 4 Results
### Diagenetic Processes
#### 4.2.2.2 Cementation  
The cementation of low-permeability sandstone reservoirs in the Huizhou Sag is commonly observed, with cementation occurring in various structural well areas and at different burial depths. Siliceous cementation, carbonate cementation, and a small amount of clay mineral cement (such as some chlorite grain coating clay minerals filled amidst clastic particles) (Figure 6D) are the main types of cement. Specifically, siliceous cementation zones are mainly concentrated in the diagenetic stage between burial depths in the range of 1550-3200 m, while carbonate cementation primarily occurs in the medium diagenetic stage between burial depths in the range of 2920-4280 m.  
Silica Cementation  
The siliceous cementation of the Enping Formation is mainly characterized by quartz secondary overgrowths, while authigenic quartz minerals in primary intergranular or dissolution pores are rare. Under scanning electron microscopy, it can be observed that the quartz secondary enlargement is arranged in an embedded manner (Figure 6D). It is evident that quartz crystals grow towards the pore spaces, and the width of the enlarged edge of quartz (mainly grade II) ranges from 4 to 38 \(\upmu\)m. Furthermore, it was noted that  
Figure 6: Photomicrographs and SEM images of the Enping Formation in Huizhou Sag sandstone: (**A**) Well Y8, 3392 m, with a high argillaceous matrix content and a dense structure; (**B**) Well Y5, 3546.3 m, with a high argillaceous matrix content contact between calcite particles; (**C**) Well Y1, 3601 m, where intense compaction causes the rock to form fractures; (**D**) Well Y4, 3541.3 m, which has microcrystalline quartz and lamellar chlorite aggregate; (**E**) Well Y8, 3313 m, which has intergranular pores filled with cement ferridolomite grains; (**F**) Well Y6, 3507 m, where the elastic particles are in point contact, and carbonate cement is developed; (**G**) Well Y9, 3374.7 m, where the formed intergranular pores are filled with carbonate cement; (**H**) Well Y4, 3684.5 m, with a feldspar solution hole; and (**I**) Well Y5, 3550.1 m, with pores formed by rock debris dissolution. the thickness increased with the burial depth, indicating its influence on multi-stage acidic fluids during burial. Previous studies have shown that siliceous material in clastic rock reservoirs is mainly derived from the pressure dissolution of quartz particles and feldspar mineral dissolution in an acidic environment [52; 53]. Specifically, unstable potassium feldspar and albite dissolved in an acidic environment to form SiO\({}_{2}\)[52]. The formation formula is as follows. \[2\text{KAlSi}_{3}\text{O}_{8}+2\text{H}^{+}+\text{H}_{2}\text{O} \rightarrow\text{AL}_{2}\text{Si}_{3}\text{O}_{5}(\text{O}\,\text{H})_{4}+4 \text{SiO}_{2}+\text{H}_{2}\text{O}+2\text{K}^{+} \tag{1}\] \[2\text{NaAlSi}_{3}\text{O}_{8}+2\text{H}^{+}+\text{H}_{2}\text{O} \rightarrow\text{AL}_{2}\text{Si}_{3}\text{O}_{5}(\text{O}\,\text{H})_{4}+4 \text{SiO}_{2}+\text{H}_{2}\text{O}+2\text{Na}^{+} \tag{2}\]  
The evidence from the thin section and SEM indicates the presence of different types of carbonate cement in the dense, low-permeability sandstone reservoirs of the Enping Formation in the Huizhou Sag. For example, the early formation of some brown iron dolomite cement and red carbonate cement between clastic particles can be seen (Figure 6E-G). A microscopic examination revealed that the total proportion of calcite cementation is pervasively developed in the Enping Formation as an important feature of carbonate cementation, showing different formation levels depending on the depth at which they are buried. The microscopic results indicate a lack of primary pore development, with detrital particles mainly in point contact and relatively weak mechanical compaction (compaction amount < 27.8%), which suggests that calcite cement inhibits mechanical compaction when the content of calcite cement exceeds 9.8% in the study area.