## 5 Discussion
### Multi-Scale Coupling Development Model of Pores and Fractures  
In this study, macroscopic pores and fractures were studied at the centimeter scale through drilling core research, and then microscopic pores and fractures were studied at the micro and nanoscale through SEM (Figure 9). The results show that the reservoir space of Fengcheng Formation shale has multi-scale and multi-type characteristics. Then, through the quantitative calculation of the pores and fractures of the samples of four lithologies, the respective porosity of the fractures and the pores of each lithology were calculated, the contribution rate of the fractures was calculated according to the results, and the pore and fracture development characteristics of the four lithologic reservoirs in the Fengcheng Formation were obtained. The results are shown in Table 2.  
Figure 8: NMR curves of each lithology. It can be clearly seen from Table 2 that the overall total porosity of siltstone interlayers is the highest, and its pore and fracture development is obviously better than that of the shale lithology. The development of pores and fractures is relatively balanced, and the porosity degree of the two is basically the same. Among the three shale types, the porosity of dolomitic felsic shale is the best, and its pore development is at the medium level among the three lithologies, only 2.57%. However, its fracture development is particularly good, and the fracture porosity is the highest, even exceeding the sandstone fracture porosity, reaching 10.37%. The porosity development of felsic shale is superior to that of the other three lithologies, reaching 3.85%, showing that the porosity development is superior to the fracture development in the pore structure. Although the fracture development of clay-bearing felsic shale is very close to that of felsic shale, the pore development is very poor, and the porosity is only 1.12%. From the perspective of pore structure, the development of pores and fractures is relatively balanced. The shale of the Fengcheng Formation in Mahu is dominated by felsic shale. Therefore, in this study, when analyzing the differences in the pore and fracture development of the three types of shale, felsic shale is taken as the benchmark. It can be found that when the clay content is high, the fracture porosity is basically similar to that of felsic shale, but the porosity decreases a lot. This may be due to the increase in clay content, which leads to the filling of some pores in the shale by clay minerals. Therefore, the fracture contribution rate of the shale is much higher than that of the feldspar sheet, and even exceeds that of the siltstone interlayers, reaching 57.25%. Compared with that of felsic shale, the porosity of dolomitic felsic shale decreases somewhat, but the fracture porosity far exceeds it. This may be due to the increased britteness of shale caused by the increase in the dolomite content, which makes it more likely to fracture and form fractures when subjected to tectonic movement or external pressure. Therefore, the contribution rate of the fracture is much higher than that of other \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Lithology** & **Porosity\%** & **Fracture** & **Fracture** & **Total** \\  & & **Porosity\%** & **Contribution Rate \%** & **Porosity \%** \\ \hline Felsic shale & 3.85 & 1.48 & 27.76 & 5.33 \\ Dolomitic felsic shale & 2.57 & 10.37 & 80.14 & 12.94 \\ Clay-bearing felsic shale & 1.12 & 1.5 & 57.25 & 2.62 \\ Silstone interlayers & 6.69 & 7.2 & 51.84 & 13.89 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Porosity statistics of each lithology. Figure 9: Multi-scale coupling development model of pores and fractures. lithologies, up to 80.14%. Compared with siltstone interlayers, the development of fractures and pores in shale is much lower than that of siltstone interlayers due to its formation factors. These conclusions are basically consistent with the previous NMR measurement results, and the two are mutually corroborated, indicating the reliability of the research.