## 6 Conclusions  
On the basis of a comprehensive evaluation of the wettability of tight sandstone reservoirs in the lower-fourth member of the Shahejie Formation (Es4x), Bonan Sag, and Bohai Bay Basin, using a combination of ESEM, CA measurements, SI, and NMR experiments, the following conclusions can be drawn:  
1. The comprehensive wettability investigation including ESEM, CA, SI, and NMR indicate that the four typical tight sandstone samples, C1, C2, C3, and C4, in the study area show weak oil wettability, strong oil wettability, water wettability, and strong water wettability, respectively. The ESEM analysis has revealed distinct wettability characteristics among the four tight sandstone samples, with samples displaying a spectrum of wettability at pore (micro) scales. The CA measurements indicate that at mesoscales only, Sample C2 exhibits intermediate wettability, while the other samples display water-wet characteristics. The SI experimenting results align with ESEM observations, confirming the wettability behavior observed at the micro-scale.
2. The SI and NMR experiments showed that pore sizes can significantly affect wettability, with a defined pore-size threshold between water-wet and oil-wet conditions. For the overall water-wet samples, the pore size threshold between water-wet and oil-wet conditions is 1 \(\upmu\)m, while for the overall oil-wet samples, this threshold is at 0.1 \(\upmu\)m. The wetting state of pore sizes in the range of 0.1 \(\upmu\)m to 1 \(\upmu\)m controls the overall wettability of the rock.
3. The morphology of clay minerals significantly influences wettability states. Grain-coating illite and grain-coating chlorite tend to exhibit oil-wet characteristics, while dispersed sheet-like chlorite and rosette chlorite are more likely to manifest as water-wet. The morphology of clay minerals is an important factor leading to the difference in wettability of different wells in the Bonan Sag.
4. Our study demonstrates that any individual wettability measurement methods would struggle to fully reflect the true wettability of tight sandstone reservoir. In contrast, an integrated multi-scale wettability measurement approach can take full advantage of the strengths of each technique, thereby enhancing the accuracy and reliability of wettability assessments, providing deeper insights into fluid behavior within the tight sandstone pore network.  
Conceptualization, J.C., K.L., G.L. and T.M.; Data curation, H.L., X.F. and X.H.; Formal analysis, J.C. and K.L.; Funding acquisition, K.L. and G.L.; Investigation, J.C. and L.Y.; Methodology, K.L. and Z.G.; Project administration, G.L., H.L. and T.M.; Resources, K.L. and H.L.; Software, Z.G.; Supervision, K.L.; Validation, J.C., K.L., L.Y. and T.M.; Visualization, X.F.; Writing--original draft, J.C.; Writing--review & editing, K.L. All authors have read and agreed to the published version of the manuscript.  
This project is funded by a project from the Shandong Provincial government (No. 2020 ZLYS08), a research-service project from the Shengli Oilfield Company, SINOPEC (No. 30200027-23-ZC0613-0010), and by the NSFC Innovation Group (No. 41821002).  
Not applicable.  
The data presented in this study are available on request from the corresponding author. The data are not publicly available due to privacy.  
The Shengli Oilfield Company, SINOPEC, provided core and oil samples used for the experiments and various analyses, as well as regional geological data. Most of the analyses were carried out in the State Key Laboratory of Deep Oil and Gas, China University of Petroleum.  
Conflicts of Interest:Authors [PERSON], [PERSON], [PERSON], and [PERSON] were employed by the Shengli Oilfield Company. Author [PERSON] was employed by the Wuxi Institute of Petroleum Geology, SINOPEC. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.