## 4 Results  
### NMR \(T_{2}\) Spectrum and Related Pore Structure Parameters  
Through NMR experiments, the connections between \(T_{2}\) relaxation time and signal intensity of 16 shale samples were obtained (Figure 2). The \(T_{2}\) spectrum mainly exhibited distinct double peaks, and the left peak was the major peak for most of the tested samples. The \(T_{2}\) relaxation times varied from 0.2466 ms to 3.8606 ms for the left peak signals, indicative of pores with diameters from 1 nm to 200 nm. The right peak signals exhibited relaxation times from 11.0094 ms to 331.8049 ms, corresponding to larger pores ranging in size from 0.2 \(\upmu\)m to 20 \(\upmu\)m. The plots in Figure 2 display a wide range of pore sizes, indicating strong complexity in pore size distribution. In addition, for most of the tested samples, the values in the interval between the two peaks approached zero, indicating the discontinuity between the two groups of pores represented by the left and right peaks. The \(T_{2}\) spectra of most shale samples show peaks at 0.1-1 ms, and the left peaks are much more significant than the right peaks, showing that the NMR \(T_{2}\) curves' morphology of shale samples are dominantly contributed by pores with pore size ranging from 1 nm to 200 nm. The proportion of the storage capacity provided by pores with pore sizes ranging from 1 nm to 200 nm is much larger than the proportion provided by pores with pore sizes ranging from 0.2 \(\upmu\)m to 20 \(\upmu\)m.  
NMR is an effective approach to obtain pore size distribution of shale samples [28]. The pore size distribution of the Wufeng-Longmaxi Formation in the Z-3 well is shown in Figure 3 based on \(T_{2}\) spectrum data. The pore size of the Wufeng-Longmaxi shale samples is mainly within the 1-100 nm range, with a notable secondary distribution from 100 nm to 1 \(\upmu\)m.  
NMR characterizes the pore structures in rocks by generating relaxation signals from hydrogen nuclei under an external magnetic field and measuring \(T_{2}\) relaxation time spectra established by fluid hydrogen nuclear relaxation signals in pores of different radii. Therefore, analyzing the \(T_{2}\) spectrum is a feasible way to characterize the pore structure of shale and investigate non-intuitive features, such as fractal features [11].  
The integral area of the \(T_{2}\) spectrum is directly proportional to the hydrogen content of the fluid in the core pores, and the correlation between the nuclear magnetic signal and the porosity can be obtained by calibrating the nuclear magnetic porosity of the rock samples to be tested. The results (Table 2) show that the NMR porosity of the Wufeng-Longmaxi Formation in the Z-3 well ranges from 2.61% to 4.81%, with an average value of 4.33%. The \(T_{2}\) cutoff value of 16 tested samples ranges from 2.2861 ms to 68.8991 ms, averaging at 24.89 ms. The permeability of the samples obtained by [PERSON] model varies from 0.00011 mD to 0.235 mD, and the average value is 0.01566 mD.  
Figure 2: \(T_{2}\) spectrum of representative samples of Wufeng–Longmaxi Formation in the Z-3 well. (**a**) Sample Q0 from layer 1; (**b**) Samples Q1–Q2 from layer 2; (**c**) Samples Q3–Q5 from layer 3; (**d**) Samples Q6–Q7 from layer 4; (**e**) Samples Q10–Q12 from layer 5; (**f**) Samples Q13–Q15 from layer 6.