## 5 Discussions
### Application of NMR \(T_{2}\) Spectral Analysis in Pore Structure Evaluation  
NMR \(T_{2}\) spectral analysis can provide quantitative parameters for reservoir classification and evaluation. According to the NMR \(T_{2}\) spectral parameters combined with petrography, MICP, and NMR \(T_{2}\) spectral analysis, four typical pore structures (I, II, III, and IV) of the sandstone reservoir in the Dongying Depression are classified (Figure 16). The results show that the \(T_{2}\) spectral parameters are sensitive to the characteristics of pore structures (Figure 17 and Table 2). From Type I to Type IV pore structures, the values of \(T_{\text{2 mid}}\), \(T_{\text{2 max}}\), \(T_{\text{2 gm}}\), and \(\varphi_{\text{em}}\) gradually increase, while the value of irreducible water saturation (S\({}_{\text{wi}}\)) decreases (Figure 17 and Table 2). The detailed information of pore structures are as follows. The capillary pressure cures of the Type I pore structure exhibit a long flat segment in the mercury intrusion stage, and it has the lowest displacement pressure (Figure 16I-A). The NMR \(T_{2}\) spectra are characterized as unimodal with longer relaxation time, implying a high proportion of large pores (Figure 16I-B). The \(T_{2}\) relaxation time ranges between 1 and 300 ms with a concentrated distribution (Figure 16I-B), and the corresponding pore size ranges from 0.6 to 4.0 \(\upmu\)m. This is usually typical of micro-fractures or large pore systems. The large pore systems are the combination of residual intergranular pores and dissolution pores (Figure 16I-C). Since the Type I pore structure has a larger pore throat radius and a better microscopic connectivity, it exhibits comparatively higher permeability. The capillary pressure cures show a relatively low displacement pressure and maximum mercury saturation in the Type II pore structure (Figure 16I-A). The NMR \(T_{2}\) spectra exhibit unimodal or bimodal behaviors with a higher right peak (Figure 16I-B). The \(T_{2}\) relaxation time is mainly between 1 and 200 ms (Figure 16I-B), with pore sizes being concentrated mostly between 0.1 and 1.6 \(\upmu\)m. The volume of the pore throat is lower than that in the Type I pore structure. The pore types of the Type II pore structure are composed mainly of intergranular pores and dissolution pores (Figure 16I-C), which show a good petrophysical properties and microscopic connectivity. Figure 16: (**I-A–IV-C**) Classification and evaluation of various types of pore structures in the E\({}_{84}\) sandstone reservoirs, showing the microscopic pore types and pore throat characteristics for different types of pore structures. Type III capillary pressure curves are characterized by a shorter flat segment and a higher displacement pressure (Figure 16 III-A). The NMR \(T_{2}\) spectra are typical of bimodal. The \(T_{2}\) relaxation time has a wide range of distribution, and the proportion of large pore throats is low (Figure 16 III-B). The pore types of the Type III pore structure are dominated by isolated dissolution pores with rare residual intergranular pores. Extensively, carbonate cementation is present in the Type III pore structure-dominated sandstones, forming a pore system dominated by small pore throats (Figure 16 III-C). The flat segments of Type IV capillary pressure curves are the shortest compared with all the other types of pore structures, and the microscopic connectivity is the poorest (Figure 16 IV-A). The NMR \(T_{2}\) spectra are characterized as unimodal or weak-bimodal with shorter relaxation time, indicating the presence of a high proportion of small pores (Figure 16 IV-B). The \(T_{2}\) relaxation time is mainly between 0.3 and 10 ms (Figure 16 IV-B), and the corresponding pore sizes are in the range of 0.004-0.1 \(\upmu\)m. Except for a small amount of isolated dissolution pores, micropores dominate the pore space of the Type IV pore structure.