This thesis presents a systematic study on the effects of Ca2+, Mg2+ and SO4 2-ions on the wettability of carbonate rocks. The scope of this study covers two aspects:(1) An investigation on the specific effects of Ca2+, Mg2+ or SO4 2-on the wettability of calcite surfaces through interfacial tension, contact angle, zeta potential and direct force measurements using" soft tip" force spectroscopy technique in single electrolytes. The results suggest that the Ca2+ ions can weaken the water-wetness of calcite surface, whereas the Mg2+ or SO4 2-ions can enhance the water-wetness of calcite surface, especially Mg2+ ions have more impacts on the water-wetness of calcite surface than SO4 2-ions. In terms of disjoining pressure modeling, it shows that the attractive hydrophobic force in CaCl2 solution causes the oil droplet to wet the calcite surface and thus weaken the water-wetness, whereas the repulsive hydration force in MgCl2 solution or the repulsive electrostatic force in Na2SO4 solution can repel the oil droplet from calcite surface and thus enhance the water-wetness.(2) An examination on the symbiotic effects of Ca2+, Mg2+ and SO4 2-on the wettability of chalk samples through zeta potential, contact angle measurements and µ-CT imaging in seawater (SW) and ion tuned waters (ITWs). The results suggest that ITWs can increase the electronegativity as well as the water-wetness of chalk surfaces over SW. A surface complexation study reveals that the Ca2+, Mg2+ and SO4 2-ions in ITWs work symbiotically to determine the surface charges and thereby the wettability of chalk samples. Furthermore, the µ-CT study shows that SW4SO can improve the water-wetness of micropores especially that of nanopores, whereas SW0. 25Ca primarily improves the water-wetness of micropores. It is believed that the mechanism governing the wettability change in nanopores is a change in surface charges and that governing the wettability change in micropores is calcite dissolution. The µ-CT study also shows that the improvement of microscopic oil recovery by SW4SO and SW0. 25Ca over SW is 18.55% and 20.21%, respectively, hence SW0. 25Ca is more favorable over SW4SO in improving the oil recovery of chalk samples.