## 3 Result and Discussion
### Comparative Analysis of Shale Pore Throat Distributions  
Figure 11 displays the pore throat distribution for the Hechuan block H3-5 shale obtained through both NMR and MICP methods. Observations indicate that within the pore throat radius range of 0.001 to 0.075 \(\upmu\)m, the distribution frequency measured by NMR is significantly higher than that measured via the mercury intrusion method. There are primarily two reasons for this phenomenon:  
Limitations of the MICP Technique: Since shale has a lower compressive strength compared to sandstone [63], a lower pressure is used to ensure the authenticity of MICP results for shale. Consequently, with the application of a 100 MPa pressure, pore diameters smaller than 0.0075 \(\upmu\)m remain undetectable when using the MICP method. Characteristically, shales are highly compacted and contain an abundance of micro-pores; these intricate pores, due to their diminutive size, are challenging for the MICP technique to detect accurately. Figure 9: MICP schematic diagram: (**a**) MICP underestimates large pore porosity, (**b**) MICP overestimates small pore porosity. Figure 10: A comparison of pore-throat size distributions in sandstone as determined by NMR and MICP measurements: (**a**) S1-2, (**b**) S1-3, (**c**) S1-4, (**d**) S2-5, (**e**) S2-6, (**f**) S2-7, and (**g**) S2-8. NMR Signatures from Specific Components in Shale: Shales contain a significant amount of non-flowing hydrogen-containing components, such as kerogen, asphaltene, and structural water. These components produce signals in NMR measurements, and these signals mainly represent the smaller pores. This means that these micro-pores can only be detectable when using the NMR method. These two reasons are key to the notable differences in the micropore distributions obtained from the NMR and MICP methods. In research on core pore size distribution, it is essential to understand and consider these factors, as they directly influence the accuracy and reliability of the measurement results. Within the pore size range from 0.0075 to 0.19 \(\upmu\)m, the NMR method shows a slightly higher porosity distribution frequency than the MICP method. This phenomenon can be attributed to the high heterogeneity within the shale. Given the complex microstructure of the shale, which encompasses numerous micropores and nano-scale throats, it is challenging for the MICP method to achieve full saturation in this pore size range. Specifically, the capillary forces from the throats and the surface tension faced by the liquid mercury prevent it from fully entering or filling certain micro-pores. This disparity is especially pronounced in cores with low permeability. Taking the H2-3 core as an example, the extremely low permeability (0.0065 mD) and T\({}_{2}\) relaxation time distribution spectrum distribution suggest a significant presence of micro-pores and nano-scale throat structures. The maximum saturation measured via the MICP method for this core is only 59.246%, further confirming that in cores with low permeability, mercury struggles to saturate the micro pores completely. In contrast, the NMR method primarily relies on nuclear magnetic resonance signals to determine the presence and distribution of fluids within the core. It can more accurately detect the distribution of water or other hydrogen-containing fluids within the micro-pores, offering more detailed porosity distribution information. Thus, when dealing with highly heterogeneous rock samples with intricate microstructures, the NMR method may offer more advantageous insights into pore size distribution analysis. When the pore size exceeds 0.19 \(\upmu\)m, a phenomenon similar to that observed in sandstones occurs. Specifically, the cumulative frequency of MICP in region 1 is higher than that of NMR, while in region 2, it is lower than NMR. As shown in Figure 12, in the multi-sample pore throat characterization of the Hechuan shale core, a porosity distribution was observed that was similar to Figure 11 and the published data.