The oil wettability of reservoir rock surfaces is a key issue affecting oil production and enhanced oil recovery. Understanding the factors that determine this wettability and how to change the rock wetting state is necessary to improve oil recovery efficiency. In this study, atomic force microscopy has been used to study the change of wettability of dolomites after exposure to crude oil. Oil aggregates are seen to adhere to particular areas of the surface in a manner dependent upon the surface electrical potential, with a higher affinity toward regions of the surface with negative charge, indicating that asphaltenes possess a native positive charge. The effect of exposure to brine has also been studied, with the key finding being that, via a modification of the surface potential, it leads to significantly less adsorption of the polar components of oil.