## 6 Conclusions  
The petrophysical properties, pore structures, and effective movable fluid in pore space were analyzed by integrated optical and SEM petrographic and petrophysical analyses including poropperm, MICP, and NMR measurements. Various quantitative \(T_{2}\) spectral parameters derived from NMR experiments were employed to characterize and evaluate the pore structures of the Eocene sandstone reservoirs in the Dongying Depression, Bohai Bay Basin, China. Some key findings are as follows:  
1. Petrographic analysis reveals that the pore systems in the Eocene sandstone are composed mainly of residual intergranular pores, dissolution pores, micropores, and a minor amount of micro-fractures.
2. The \(T_{2\text{gm}}\), \(T_{2\text{mid}}\), \(S_{3}\), and \(\phi_{\text{em}}\) parameters have positive relationships with permeability, suggesting that the proportion of large and effective pores has a profound control on the overall permeability of the sandstones. Thus, the permeability of the Eocene sandstone is mainly dominated by large pore systems.
3. The NMR \(T_{2}\) spectral parameters exhibit excellent correlations with MICP capillary parameters, indicating that the NMR \(T_{2}\) spectral parameters also contain rich information on pore structures and movable fluids. Thus, pore structures, pore fluid distribution, and reservoir permeability can be effectively evaluated by using NMR \(T_{2}\) spectral parameters.
4. A comprehensive classification scheme of the Eocene sandstones is established based on NMR \(T_{2}\) spectral parameters while considering the petrophysical properties and macroscopic behaviors. The E\({}_{\text{s4}}\) sandstone reservoirs are classified into four types of pore structures (I, II, III, and IV). Type I and II pore structures exhibit good petrophysical properties and microscopic connectivity. Our study demonstrates the effectiveness and applicability of the NMR technique in characterizing low-permeability sandstone reservoirs and provides a basis for further improving reservoir quality evaluation in the Bohai Bay Basin, China. However, limited by the quantity of NMR experimental data, continuous evaluation of pore structure throughout entire reservoir  
\begin{table}
\begin{tabular}{l c c c c c c c c} \hline \hline
**Type** & \(T_{\text{mid}}\) **(ms)** & \(T_{\text{max}}\) **(ms)** & \(T_{\text{max}}\) **(ms)** & \(T_{\text{pm}}\) **(ms)** & \(\phi_{\text{em}}\) **(ms)** & \(S_{\text{s4}}\) **(s)** & \(\phi_{\text{em}}\) **(ms)** \\ \hline I & 11.22-71.80/43.96 & 505.26-2327.20/107 intervals would not be feasible. The combination of NMR experimental data and NMR logging or conventional logging data would be crucial for further extending these research findings.  
Conceptualization, Y.L. and K.L.; methodology, Y.L., K.L. and Y.W.; validation, Y.L. and Y.W.; formal analysis, Y.L.; investigation, Y.L., K.L. and Y.W.; resources, Y.L. and Y.W.; data curation, Y.L.; writing--original draft preparation, Y.L.; writing--review and editing, Y.L., K.L. and Y.W.; visualization, Y.L.; supervision, K.L.; project administration, K.L.; funding acquisition, K.L. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Natural Science Foundation of China (Grant No. 41821002), the Strategic Priority Research Program of the Chinese Academy of Sciences (No. XDA14010401) and the National Science & Technology Major Project of China (No. 2017 ZX05009001).  
Not applicable.  
The data presented in this study are available on request from the corresponding author.  
The authors declare no conflict of interest.