Wettability is an important parameter that influences the petrophysical properties of reservoir rocks. When drilling is doing, the penetration of mud filtration into formation causes considerable wettability alteration and affects the oil displacement by water. Hence the enhanced oil recovery methods will be affected because of formation damage. In this study, the effect of three samples (a bentonite commonly used in oil well drilling, a purified bentonite, and nanoclay) on wettability of sandstone and carbonate rocks has been studied using the contact angle method. The modification of surface properties of montmorillonite sheets by hydrophobic plug reformer changed the wettability of the clay particles from water-wet to oil-wet. This change resulted in the suitable dispersion of nanoclay (organic) into oil phase (as hydrophobic phase). The contact angle measurements of the prepared suspension from each of three samples showed that common and purified bentonites altered the wettability of the sandstone and limestone rocks to water wet, whereas the nanoclay changed contact angle to oil-wet behavior. Based on these observations, use of nanoclay, as an oil-based drilling mud additive, is useful due to its compatibility with oil zone wettability and suitable dispersion in oil-based drilling fluid.