## 5 Discussion
### Recognition of Pore in the Es4
#### 5.2.2 Secondary Pore Recognition and Characteristics  
Secondary porosity plays an important role in reservoir quality because it can improve the reservoir quality and modify the original composition by grain dissolution [10; 26; 27; 28].  
Figure 8: Primary pore in the Es4. **(A)** Triangular primary pore, **(B–D)** Irregular primary pores.  
The secondary porosity identification in the present work is based on the formation mechanism and characteristics of the pore type. The development of the pore system in both degradation and retrogradation sequences is similar. However, the percentage of dissolution in the progradational sequences is moderately higher than in the retrogradation sequences. The secondary pore consists of several pore systems grouped into five main pore types: interparticle pores (Figure 6A), intraparticle pores (Figure 6G-D), fractures (Figure 6D,E), moldic pores, and intracrystal pore (Figure 6I). Each pore type can be subdivided into several pore types according to their occurrence and characteristic. The intergranular secondary pores are created by the leaching of cement and depositional matrix [26,28]. However, in this study, the intergranular pore is often formed by the partial dissolution of the framework grain, leading to the widening of the existing intergranular pore network (Figure 10A). This type of pore can be a pure secondary pore or an enlarged mixed pore. An enlarged mixed pore in this study resolves from the widening of the primary pore due to the partial dissolution of the adjacent grain edges (Figure 10C-F). This pore type is more common in the study area, particularly in the distributary (Fan delta) and braided channels (Nearshore subaqueous fan).  
According to their genetic mechanism, the fracture pore consists of the enlarged fracture pore (Figure 6E), rock fracture pore, and grain fracture (Figure 6D,E). A rock fracture is a fracture pore that cuts through several particles or the whole rock. This type of fracture is probably caused by a tectonic event. Grain fracture is a pore that occurs in individual particles. An enlarged fracture pore is formed by overburden pressure in conjunction with dissolution (Figure 6E). This fracture occurs even in individuals or several grains. These types of fracture pores are important pathways for fluid circulation and can be responsible for creating secondary dissolution porosity. The intragranular pores are frequent in the study interval and sometimes are connected to intergranular pores (Figure 6B). Moldic pores are made by the total dissolution of grain. They are common in the degradation and can be recognized by the remains of clay rims (Figure 10A). The intercrystalline pore in the Lijin Sag mainly occurs in kaolinite cement (Figure 6I). However, in the Yanjia area, the carbonate cement also presents some micropores. The secondary porosity in the progradation is dominated by the enlarged intergranular pore. The thin-section observation shows that the enlarged pores are mostly interconnected with each other in the degradation (Figure 10A). In addition, the intragranular pores are also connected to the intergranular pores (Figure 10B). The conjunction of both pores positively affects the reservoir quality in the progradation sequence, mainly in the braided channel facies.  
Figure 9: Range of primary pore in the Es4 in the northern Dongying depression.  
Furthermore, the degradation contains more grain fracture and enlarged pores, which is important for fluid circulation in sediment. Therefore, the progradation presents a good reservoir. On the contrary, the enlarged pore and interconnected intragranular-intergranular pores are less common in retrogradation. The enlarged pore size is not greater than the progradation sequence. Consequently, the reservoir is less good than the progradation sequence.