## 3 Result and Discussion  
### T\({}_{2}\) Relaxation Time Distribution Spectrum  
In the state where the sandstone core is fully saturated with formation brine (TDS: 4500 ppm), the characteristics of the NMR T\({}_{2}\) relaxation time distribution spectrum distribution can effectively elucidate the distribution features of its pore throats. The greater the number of peaks in the spectrum, the more complex the distribution of the pore throats [56]. Concerning the Tuha oilfield, the fully water-saturated T\({}_{2}\) spectra of the eight tight sandstones are shown in Figure 4. Based on the T\({}_{2}\) spectra, the sandstone cores can be categorized into two types: monomodal (comprising S1-1, S1-2, S2-5, S2-6, S3-7, and S3-8) and bimodal (S1-3 and S1-4). The six monomodal cores demonstrate a wide distribution range for T\({}_{2}\) relaxation time, with nuclear magnetic signals mainly detected from 0.03 ms to 1000 ms. However, their peak values are relatively concentrated, primarily distributed between 1.5 and 5 ms, indicating that they predominantly feature micropores. The T\({}_{2}\) spectra of the two bimodal cores are strikingly similar, both showcasing a left peak with a signal intensity slightly higher than the right one, and the two peaks are continuous, suggesting that the reservoir's overall pore throat distribution is continuous. Both bimodal cores exhibit a left peak corresponding to a T\({}_{2}\) relaxation time of 0.8 ms, which is lower than the peak T\({}_{2}\) time observed in the monomodal state. The right peak's T\({}_{2}\) relaxation time is 20 ms, aligning with the peak T\({}_{2}\) time of the monomodal state. It is noteworthy that the porosity distribution range for these eight cores is between 13.66% and 15.88%; however, there is a considerable variation in the total amount of nuclear magnetic signals. This variation is not due to errors in core saturation but is attributable to the disparities in the core volume, which leads to significant differences in signal magnitude. Figure 5 illustrates the 100% formation brine (TDS: 174,000 ppm) saturated T\({}_{2}\) relaxation time distribution spectra of shales from eight different core samples of four wells in the Hechuan area. From the T\({}_{2}\) spectra, it is evident that the T\({}_{2}\) distribution of the Hechuan shale primarily spans from 0.02 ms to 1000 ms. The T\({}_{2}\) relaxation time distribution spectra of cores H1-2, H2-4, and H3-5 exhibit a distinct bimodal distribution. Notably, core H2-3 is considerably distinct from the other seven cores, with its signal peak T\({}_{2}\) relaxation time being 0.6 ms, which is lower than the rest. Moreover, the peak position of the H2-3 core has shifted significantly to the left compared to the other cores, suggesting that this specific core mainly comprises smaller pore sizes than the others, which implies a relatively lower permeability. Consulting the physical properties of the eight Hechuan shale samples (Table 2), it is evident that the H2-3 core exhibits the lowest permeability amongst the eight, measuring 0.0065 mD, which aligns with the T\({}_{2}\) spectra measurements. [PERSON] pointed out that the horizontal anisotropy of porous media and the chemical reaction of fluid and core in the reservoir will affect core permeability. [PERSON] pointed out that the horizontal anisotropy of porous media and the chemical reaction of fluid and core in the reservoir will affect core permeability [57, 58]. Further observation reveals a trend in the NMR signal peak corresponding to T\({}_{2}\) relaxation times for the following core samples: H2-3 \(<\) H4-8 \(<\) H3-6 \(\approx\) H1-2 \(\approx\) H4-7 \(<\) H1-1. This pattern aligns with the distribution of core permeabilities, indicating that for the Hechuan shale samples, as permeability increases, the peak value of T\({}_{2}\) also rises. However, a notable exception is observed in the case of the H3-5 sample.