# Diagenesis of Deep Low Permeability Reservoir in Huizhou Sag and Its Influence on Reservoirs
## 5 Discussion
### Impact of Dissolution on Reservoir Quality  
Acidic and alkaline corrosion is crucial in regulating reservoir space within deep sandstone reservoirs [60; 61; 62; 63]. The dissolution of acidic fluids affects the deep, low-permeability reservoir of the Enping Formation in the study area. This leads to the dissolution of soluble components such as clastic particles, calcareous materials, and tufaceous interstitials under the influence of organic acids [38; 54]. As a result, many secondary pore spaces are formed,  
Figure 8: Relationship between intergranular volume and cement content in Enping Formation, Huizhou Depression.  
significantly improving the physical properties of the deep, low-permeability reservoir. This process causes an increase in secondary porosity, which is equivalent to the reservoir's remaining secondary porosity. We express the increased porosity (\(V_{4}\)) resulting from the dissolution of the Enping Formation as follows:  
\[V_{4}=\frac{P_{2}+\ P_{3}}{P_{t}}\times Q_{m} \tag{7}\]  
In the formula above, \(P_{2}\) represents the face rate of dissolved pores within the grain calculated using thin sections observed under a microscope, expressed in %; \(P_{3}\) represents the intergranular dissolved pore face rate, expressed in %; and \(V_{4}\) represents the porosity increase caused by dissolution, expressed in %. The results show an average increase in porosity of 1.2-26.4%, with an average of 8.9%, due to dissolution in deep, low-permeability reservoirs in the Enping Formation. The calculation of the porosity of Enping Formation sandstone reveals that the loss in porosity due to calcareous, argillaceous, siliceous, and ferric cementation accounts for 12.3% of the reservoir volume. An analysis of dissolution illustrated an increase in porosity ranging from 1.4 to 25.6%, with an average value of 10.1%; compaction results in a reduction in reservoir porosity ranging from 11.8% to 27.2%, with an average value of 18.9%. We observe that mechanical compaction is the primary factor influencing the reservoir's physical properties, causing significantly more damage to the original porosity than cementation processes. According to existing research, there were two stages of oil and gas filling in the Enping Formation reservoir in the study area: 12.0-7.5 Ma and 6.5-0 Ma, with the second stage being the primary phase. The first stage, hydrocarbon charging, occurred during the A stage of the middle diagenetic phase. The reservoir was buried at a depth of over 3300 m, resulting in strong acid dissolution and increasing the porosity to 18.6%, and the layout of oil and gas accumulated. Around 6.5 Ma, the reservoir underwent a second oil and gas filling stage. Due to a combination of chemical compaction, calcareous cementation, and clay mineral cementation, the porosity of the reservoir decreased to about 6.3%, rendering deep reservoirs relatively dense. Industrial oil and gas reservoirs can form under large-scale oil and gas accumulation conditions. A comprehensive analysis of the matching relationship between oil and gas charging and the evolution of reservoir pores shows that the deep reservoir type in this area follows a \"tight first, then accumulation\" pattern.