The complex physicochemical interactions in the calcite-brine-crude oil system, triggered by the injection modified salinity water (MSW) into the reservoir, are modelled by several researchers. However, the proposed models are either not consistent with a wettability alteration mechanism or cannot explain the observed improved oil recovery in chalk. We propose a new methodology denominated “Available Adsorption Sites” (AAS) that assesses the wettability alteration as a combined effect of a chemical and electrostatic contribution. Thus, we describe mathematically the interactions between the polar groups in the oil phase and the chalk by considering analogy with the thermodynamics of adsorption of an ion on a charged surface. The chalk wetting properties depend on the number of sites available for the adsorption of oleic polar groups at the mineral surface and the electrical potential at the rock-brine and brine-oil interfaces. We evaluate how the AAS parameter correlates with the remaining oil saturation from spontaneous imbibition tests on chalk samples. This approach is not only useful for the predictive evaluation of the outcome of MSW in chalk reservoirs but can also be integrated in reactive transport models and assess the flow of organic contaminants (e.g., naphtenic acids) in chalk aquifers. The model can potentially be applied to other carbonates.