## 4 Results
### NMR Fractal Characteristics  
During the investigation of the heterogeneity and non-intuitive characteristics of pore structure, \(T_{2 catoff}\) value (\(T_{2C}\)) was used as an effective point for relaxation time that partitioned the fractal curve into two sections [29]. It is believed that pores with relaxation time less than \(T_{2C}\) represent bound fluid pores, and pores with relaxation time exceeding \(T_{2C}\) are movable fluid pores. The \(T_{2}\) spectrum of the Wufeng-Longmaxi Formation shale samples mainly presented as double peaks (Figure 2). Therefore, the \(T_{2C}\) value of shale can be calculated based on \(T_{2}\) morphology, which means that the \(T_{2}\) value corresponding to the lowest point between the two peaks is used as the \(T_{2C}\). The fractal dimensions obtained from the \(T_{2}\) distribution's segments, where the \(T_{2}\) values exceeded the \(T_{2C}\), characterized the fractal dimensions of movable fluid pores (D\({}_{2}\)), and the fractal dimensions calculated from segments with \(T_{2}\) that were smaller than the \(T_{2C}\) referred to the fractal dimensions of bound fluid pores (D\({}_{1}\)). By analyzing the plots of lg(5) versus lg(\(T_{2}\)), the NMR fractal dimensions of the 16 shale samples in the study area are depicted in Figure 5, which demonstrates a clear  
Figure 4: The proportion of bound fluid saturation and movable fluid saturation in shale samples. characteristic of a two-section pattern. By linearly fitting the two segmented curves, D\({}_{1}\) and D\({}_{2}\), respectively, were calculated (Table 3). The fractal dimension (D\({}_{1}\)) of the bound fluid pores in the shale samples ranges from 1.6895 to 2.3821, with an average value of 2.0267. The fractal dimension (D\({}_{2}\)) of movable fluid pores ranges from 2.9914 to 2.9996, with an average value of 2.9962. Overall, the data indicates the relatively strong heterogeneity of the movable fluid pores of shale samples in the Z-3 well. According to the linear regression coefficient, R\({}^{2}\), the average  
The range of self-similarity in pore size is often used as the basis for the internal division of pore systems [30; 31; 32]. Based on the NMR fractal features of pores and the extent of fractal fitting, the pore sizes of tested samples from the Wufeng-Longmaxi Formation in the Z-3 well can be further classified into three categories: micropores (<0.1 \(\upmu\)m), mesopores (0.1 \(\upmu\)m-1.0 \(\upmu\)m), and macropores (>1.0 \(\upmu\)m). By plotting lg(S)\(-\)lg(R) curves, three segments of pores with differentiated pore structures can be observed, where R is the pore radius (Figure 6). This classification scheme represents the characteristics of the pore structure in different pore size ranges. The linear regression coefficient of movable fluid pores is smaller than that of bound fluid pores. Thus, the bound fluid pores exhibit stronger fractal characteristics than the movable fluid pores. In addition, the D\({}_{1}\) values are all smaller than D\({}_{2}\), which indicates that pores with larger diameters have a more heterogeneous pore structure. Also, there  
Figure 6: Pore aperture segmentation fitting results for 16 shale samples. Figures (**a-**p**) are fitting results for samples Q0–Q15.