## 5 Discussion
### Factors Influencing Tight Sandstone Wettability  
In order to investigate the factors influencing the wettability of tight sandstones in the lower Es4 sub-member of the Bonan Sag, we conducted a comparative analysis of the characteristics of pore size and mineral compositions between two oil-wetting samples and two water-wetting samples. SEM observations indicate that Sample C1 is composed predominantly of grain-coating illite (Figure 10a,b), which uniformly wraps the grains, creating a honeycomb-like structure, with individual illite crystals occurring as curved sheet perpendicular to the grain surfaces [46]. [PERSON] and [PERSON] (1998) utilized cryo-SEM to observe large illite crystals that stacked in a habit close to that of kaolinite, characteristic of oil-wetting [47]. [PERSON] (2000) also noted spherical water droplets with contact angles greater than 90\({}^{\circ}\) over illite regions using ESEM, signifying oil-wet properties [48]. Consequently, the weak oil-wetting tendency of Sample C1 may be attributed to the presence of grain-coating illite. The oil-wetting nature of Sample C2, as evidenced by its well-developed grain-coating chlorite and oil films (Figure 10c,d), is consistent with the oil-wetting characteristics of grain-coating chlorite reported by [PERSON] et al. (2020) [46]. Samples C3 and C4 are characterized by the predominance of dispersed sheet-like chlorite and rosette chlorite as the primary clay minerals. These chlorite forms are relatively young, primarily resulting from the transformation of montmorillonite and illite, with some crystallized directly from pore fluids [49]. The chlorite formation potentially occurred post oil and gas charge, which could account for the typical water-wetting characteristics of dispersed sheet-like chlorite and rosette chlorite. The presence of these chlorite morphologies may be a significant contributor for the water-wetting characteristics in samples C3 and C4. Figure 10: SEM image illustrating microscopic mineral types and morphology. Images (**a**,**b**) from Sample C1 showing grain-coating illite; images (**c**,**d**) from Sample C2 showing grain-coating chlorite and associated oil film; images (**e**,**f**) from Sample C3 showing dispersed sheet-like chlorite; images (**g**,**h**) from Sample C4 showing the occurrence of rosette chlorite. In order to investigate the effect of pore size on the wettability state, we calculated the percentage of pore volume in four typical tight sandstone samples with pore throat radii <0.1 \(\upmu\)m, 0.1-1 \(\upmu\)m, and >1 \(\upmu\)m to the total volume and conducted a correlation analysis between different pore volume fractions and the oil/water wettability index (Figure 11). The results showed that the more developed the pores with radii less than 0.1 \(\upmu\)m, the smaller their oil wettability index would be. The more developed the pores with radii of 0.1-1 \(\upmu\)m and greater than 1 \(\upmu\)m, the larger the oil wettability index would become. Pore sizes have a significant impact on the wetting state of tight sandstones, with larger pores being more likely to exhibit lipophilicity, while smaller pores tend to exhibit hydrophilicity. To characterize the wettability states of different pore sizes, a distribution model of wettability states across various pore sizes has been established (Figure 12). NMR measurements indicate that for the overall water-wetting 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 results indicate that, at the macroscopic scale, tight sandstone samples across different pore size ranges display a clear mixed wettability. This mixed wettability may be caused by the incursion of oil entering into an originally water-wet reservoir and causing wettability alteration, while the micropores remain water-filled due to high capillary forces [44,50].