There are several key factors proposed in the literature responsible for oil mobilization and recovery by nanofluids as enhanced oil recovery agents. These factors include interfacial tension (IFT) reduction, wettability alteration, inhibition of fines migration, and flow profile control. However, there is not a general consensus on the relative contribution and importance of each factor. Moreover, very little pore-scale evidence for the role of these mechanisms in a natural porous medium is available, especially for carbonate rocks. To fill the gaps, we directly studied mechanisms leading to oil mobilization by SiOx- and Al2O3-based nanofluids at the pore-scale. A set of spontaneous imbibition tests was performed in both Berea sandstone and Fond-du-Lac dolomite samples. Three-dimensional high-resolution fluid occupancy maps were captured using an X-ray micro-CT scanner, and in-situ contact angle distributions were obtained at different locations directly from the X-ray images. We found that wettability reversal rather than IFT reduction was the main pore-scale mechanism for oil recovery using simple nanofluids (without other chemical additives). Furthermore, in Berea, there was a synergistic effect between the non-ionic surfactant and SiOx nanoparticles for enhancing oil recovery through both IFT reduction (due to surfactant) and wettability reversal (due to nanoparticles). A set of statistical analyses of the trapped oil ganglia in Berea shows that the surfactant-augmented SiOx nanofluid generated smaller oil ganglia, which further proves that nanofluid can assist in oil mobilization.