The dependence of wettability on brine ionic composition in organic-brine-mineral systems, which is denoted as ion-tuned wettability in this paper, has important industrious applications but is still not well understood. The dominant mechanisms and their relative importance are still under debate. This paper uses molecular dynamics to study three possible mechanisms of ion-tuned wettability in an oil-brine-quartz system, including electrical double layer (EDL) repulsion, cation bridging, and hydration repulsion. We compare the contact angle and COO- distribution of the molecular system under different interface charging conditions and the contact angle predicted by EDL repulsion theory. The results indicate the existence of Ca2+ bridging and K+ bridging, and that medium ionic strength favors the form of K+ bridging most. The three mechanisms are all proved to have impact on wettability, of which Ca2+ bridging is the strongest, EDL repulsion and hydration repulsion the weaker, K+ bridging the weakest. Based on the results, we suggest that all the three mechanisms should be evaluated to predict the ion-tuned wettability, and conclude several possible brine-modifying strategies to make sandstone reservoir more water-wet. (C) 2019 Elsevier Inc. All rights reserved.