## 4 Results and Discussion
### Characteristics of the Shales' Pore Structures
Pore volume increase and cumulative pore volume distribution plots (Figure 11c,d, respectively) reveal that the Dalong Formation shales contain a significant amount of mesopores in the size range of 2-50 nm. The cumulative pore volume curves increased slowly at pore sizes >50 nm, indicating a low proportion of macropores in the shales. Based on the results of our CO\({}_{2}\) and N\({}_{2}\) adsorption tests (Table 2), we conclude that the pore volume of the Dalong Formation shales is dominated by mesopores (68.9% of the total pore volume), followed by micropores (23.5%) and macropores (7.6%). In contrast, micropores dominate the specific surface area of pores (61.2% of the total specific surface area), followed by mesopores (38.6%) and macropores (0.3%). A shale's pore structure can be influenced by the development of different interparticle and intraparticle pore types. Micropore volumes and specific surface areas in our shale samples (Figures 12 and 13, respectively) are positively correlated with clay mineral content, negatively correlated with quartz content, and weakly negatively correlated with carbonate content. Similarly, mesopore volumes and specific surface areas are positively correlated with clay mineral content and weakly negatively correlated with quartz and carbonate content. In contrast, the volumes and specific surface areas of large pores were low and were not correlated with any mineral content. These results indicate that clay minerals favor the development of pore volume and specific surface area, whereas quartz and carbonate minerals resist pore development. Figure 12: Correlations between pore volumes and mineral contents in the Dalong Formation. Correlations are shown between pore volume, separated by pore size class, and (**a**) quartz, (**b**) clay mineral, (**c**) carbonate mineral contents, as well as (**d**) between total pore volume and individual mineral contents. Many nanoscale organic pores were formed during the thermal evolution of organic matter and hydrocarbon generation in the Dalong shales (Figure 8). The development of organic pores generates significant pore volumes and specific surface areas that can provide storage space and adsorption sites for shale gas enrichment. Organic pore development in shales is thus closely related to organic matter content, type, and maturity. Most organic matter in the Dalong Formation shales in well ED-2 were type I and overturned (\(R_{\text{o}}\) = 2.56-2.79%) (Table 1). Thus, the organic matter content in the Dalong Formation shales had the greatest influence on organic pore development. The volumes (Figure 14a) and specific surface areas (Figure 14b) of both micropores and mesopores were highly correlated with organic matter content. Therefore, organic pores mainly contributed to the micro- and mesoscale porosity in the shales, and the organic matter content was the main factor controlling the pore structures. Indeed, the average pore size of the shales decreased with increasing organic matter content (Figure 14c). Our SEM analysis of pore types (Section 4.3.2) indicated that high organic matter contents might favor compaction (e.g., Figure 8l), thus reducing the size of many organic pores. The correlations between the mineral and organic matter contents of the shales can be summarized as follows. Clay mineral contents were highly correlated with organic matter content but weakly negatively correlated with quartz and carbonate contents. This suggests that a large amount of organic matter might have been adsorbed by clay minerals during the deposition of the Dalong Formation shales, resulting in the observed positive correlations between clay mineral content and shale pore volume and specific surface area. In general, the pore structure was influenced by the organic matter content. Figure 13: Correlations between pore-specific surface areas and mineral contents in the Dalong Formation.