## 2 Materials and Methods  
### Experimental Samples  
As shown in Figure 1a, the study area was the Xinchang gas field, which is located in the western depression of Sichuan Basin, China. Large-scale tight sandstone reservoirs are widely developed in the Upper Triassic Xujiahe Formation, and play an important role in natural gas exploration in the study area. The tight sandstone of the Xu-2 Member reservoir is formed in the delta facies of fluvial and marine interaction, which belong to a braided river delta sedimentary system. The reservoir sandstone is mainly clastic sandstone, followed by quartz sandstone and a small amount of feldspar sandstone. Due to long-term geological processes, the Xu-2 Member reservoir is characterized by deep burial, low porosity-permeability and complex reservoir space [26]. Due to the complex tectonic-sedimentary evolution process, diagenesis plays a strong role in controlling the quality and heterogeneity of reservoirs, making the Xu-2 Member reservoir belong to the fracture-pore pattern reservoir. It can be seen from Figure 1b that the higher structural curvature corresponds to the intersection of multiple faults and the bending of strata, which are usually the positions of reservoirs with good pore structure. Figure 1: Position, structure and fracture development model of the reservoir in study area. (**a**) The study area is located in the western Sichuan Basin, China; (**b**) the top surface structure and structural curvature distribution of the Xu-2 Member reservoir; (**c**) the fracture development model of the Xu 2 Member reservoir. According to the FMI imaging logging identification results in Figure 1c, the faults in the study area are mainly E-W direction, followed by the N-S direction. Meanwhile, the horizontal maximum principal stress direction of Xu-2 Member is in the E-W direction, and the fractures along this direction have a good opening degree. Therefore, the evaluation of pore structure and natural fractures is the key issue to predict the effective reservoir of Xu-2 Member. Sixteen rock samples were collected from the tight sandstone strata of Xu-2 Member. They were mainly composed of quartz, feldspar and debris. As shown in Table 1, quartz was in the range from 55% to 71% with an average of 61%, feldspar content varied from 11% to 31% with an average of 18%, and the rock debris was in the range from 8% to 31% with an average of 15%. High compositional maturity and feldspar content are favorable for the development of dissolution pores and micro-fractures in reservoirs. At the same time, it can be found from Table 1 that the porosity distribution range of the rock samples was 1.79-9.05%, with an average of 5.23%. However, the P-wave velocity (V\({}_{\rm P}\)) range of the rock samples was 4.5-5.4 km/s, and the relationship between velocity and porosity was relatively dispersed. In the case of small changes in porosity of rock sample, the velocity difference of each sample was obvious, which was very unfavorable for reservoir evaluation. Therefore, it was necessary to analyze the microscopic pore structure of the rock samples by micro-CT scanning. Detailed analysis of rocks at the core scale is essential because it helps to establish reliable models at the initial scale [27]. The micro-CT scanning images of rock samples were reconstructed using Avizo software to obtain a three-dimensional reconstruction map, and the three-dimensional reconstruction map of some samples was selected as shown in Figure 2. It can be seen from Figure 2 that the tight sandstone samples have the characteristics of different pore throat sizes and irregular pore shapes at the micron scale. The pores of rock samples mainly included contiguous pores and isolated pores, and the connectivity of the contiguous pores was better than that of the isolated pores. Due to the disordered distribution of the pores, the rock samples had obvious microscopic heterogeneity. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & \multirow{2}{*}{