## 5 Discussion
### Reservoir Classification Based on Mercury Injection and Fractal Dimension  
Based on the complexity of the pore-throat structure in tight sandstone reservoirs, a multivariate classification coefficient is used. The fractal dimension and the pore-throat combination types and pore-throat size distribution characteristics are combined to classify the pore structure of the tight sandstone reservoirs in the study area into three categories. The classification method of the multivariate classification coefficient is modified from previous classification methods [40]. In this study, seven parameters, including porosity, permeability, median pore-throat radius, sorting coefficient, displacement pressure, maximum mercury injection saturation, and mercury withdrawal efficiency, were selected for quantitative evaluation. The multivariate classification coefficient for tight reservoirs (F\({}_{\text{eci}}\)) was then constructed as follows:  
\[\text{F}_{\text{eci}}=\ln\frac{\frac{\varphi_{i}}{\varphi_{\text{max}}}\times \frac{\text{K}_{\text{i}}}{\text{K}_{\text{max}}}\times\frac{\text{R}_{\text{ min}}}{\text{R}_{\text{max}}}\times\frac{\text{S}_{\text{i}}}{\text{S}_{\text{ max}}}\times\frac{\text{R}_{\text{H}i}}{\text{R}_{\text{H}i}\text{max}}}{\frac{\text{X} _{\text{P}_{\text{eci}}}}{\text{X}_{\text{P}_{\text{max}}}}\times\frac{\text{ P}_{\text{eci}}}{\text{X}_{\text{P}_{\text{max}}}}\times\frac{\text{P}_{\text{eci}}}{\text{X}_{ \text{P}_{\text{max}}}}\times\frac{\text{P}_{\text{eci}}}{\text{X}_{\text{max}}}} \tag{8}\]  
where \(\varphi_{i}\) and \(\varphi_{max}\) represent porosity and its maximum value; \(K_{i}\) and \(K_{max}\) represent permeability and its maximum value; \(R_{mi}\) and \(R_{max}\) represent the median pore-throat radius and its maximum value; \(S_{i}\) and \(S_{max}\) represent sorting coefficient and its maximum value; \(R_{Hi}\) and \(R_{Hmax}\) represent maximum mercury injection saturation and its maximum value; \(X_{Pi}\) and \(X_{Pmax}\) represent sorting coefficient and its maximum value; \(P_{cdi}\) and \(P_{cdmax}\) represent displacement pressure and its maximum value. Based on calculations, the multivariate classification coefficient of the tight sandstone reservoirs in the study area ranges from \(-\)11.64 to 2.48. As shown in Figure 9, statistical analysis revealed that the interquartile range (25-75%) of the F\({}_{\rm{eci}}\) values lies between \(-\)6 and 0. Therefore, \(-\)6 and 0 were selected as the classification thresholds for the reservoir. Considering additional reservoir parameters, the tight sandstone reservoirs in the study area are classified into three categories (I, II, III) from high to low quality. The classification and evaluation standard for the tight sandstone reservoirs of the Xuijaweizi Fault Depression in the study area was established, considering the distribution of each parameter's value (Table 4). Type I reservoirs include samples S3, S12, S13, S14, and S16. The reservoir storage space in these reservoirs is mainly composed of small-to-medium intergranular pores and narrow plate-like throat channels, with a small number of dissolution pores and micropores. \begin{table}
\begin{tabular}{c c c c} \hline
**Pore Structure Classification** & **I** & **II** & **III** \\ \hline \multirow{4}{*}{Pore Typology} & Predominantly & Dominantly intragranular & Primarily microporous \\  & fine-to-medium intergranular & pores with subordinate & networks with spatially \\  & pores & intergranular pores & limited dissolution voids \\  & pores & & \\  & pores & & \\  & Narrow lamellar & Narrow lamellar & Tubular structures \\  & & & \\  & & & \\  & & & \\  & & & \\  & & & \\ \end{tabular}
\end{table} 
Table 4: Classification of pore structure in tight sandstones from the Denglouk Formation within the study area. Figure 9: Scatter plot of the F\({}_{\rm{eci}}\) index for each sample overlaid with box plot statistics. Therefore, Type I reservoirs have high storage and flow capacity. Type II reservoirs include samples S1, S2, S4, S6, S7, S12, S15, S17, and S18.