## 5 Discussion  
### Joint Characterization of Pore-Fracture  
NMR is a technique that reflects the pore size of a sample by measuring the relaxation time of the fluid in the pore. The T\({}_{2}\) spectrum, which is measured when the rock is saturated with water, is the characteristic of the pore development inside the rock sample. In this study, NMR experiments were carried out on the water-saturated samples of four lithologies, and their relaxation time and signal strength were converted to obtain the relationship between the sample pore size and pore volume.  
As shown in Figure 8, the NMR curves of different lithologies are significantly different. Among the four lithologies, siltstone interlayers have the widest pore distribution range, with double peaks visible. The main development range is 10-800 nm, the pore volume is the largest, and the pore size is concentrated at the pore size above 100 nm, indicating that the pores above 100 nm are the main ones. Among the four lithologies, it has the highest porosity and the best reservoir capacity. Secondly, dolomitic felsic shale has obvious bi-peak characteristics, which are in the pore size range of 5-15 nm and 200-500 nm, respectively.  
Figure 7: Fractures in SEM of each lithology. (**a**) Felsic shale, 4755.67 m; (**b**) felsic shale (Part); (**c**) dolomitic felsic shale, 4857.57 m; (**d**) dolomitic felsic shale (Part); (**e**) clay-bearing felsic shale, 4587.92 m; (**f**) clay-bearing felsic shale (Part); (**g**) siltstone interlayers, 4635.30 m; and (**h**) siltstone interlayers (Part).  
Moreover, the NMR curve in the small pore size range has a higher peak and larger pore volume, indicating that the main pore size of dolomitic felsic shale is in the pore size range of 5-15 nm, and the porosity is lower than that of siltstone interlayers but higher than that of other lithologies. It has second-order storage capacity, and the small pore size is the main storage space. Felsic shale can also show double peak characteristics, but its pore volume is smaller than that of dolomitic felsic shale and siltstone interlayers, and its porosity is worse. The pore size range is mainly 5-400 nm, and pores with a smaller pore size are more developed from the perspective of pore volume. Only one peak is observed for clay-bearing felsic shale and it is located in the pore size range of 5-50 nm; its development pores are very small, the porosity is the worst, and the reservoir capacity is also the worst.  
From the overall distribution of the image, siltstone interlayers, dolomitic felsic shale, and felsic shale have visible peaks on the right side of the image, indicating that the internal fluid relaxation time of these three types of samples is relatively long, and the fluid is free. This state means that there may be cracks, large aperture pores in the sample, and there are more movable fluids. Among the three, the right peak of siltstone interlayers is the highest, followed by dolomitic felsic shale, indicating that siltstone interlayer fractures are the most developed, followed by dolomitic felsic shale fractures, and felsic shale fractures are the worst. The right peak is not observed for the clay-bearing felsic shale, so its cracks are almost not developed.