Abstract In many mature fields, there is a growing debate as to how much of the remaining oil is mobile in the unswept regions versus immobile in the swept regions. Log measured oil saturations behind the flood front are substantially lower than the residual oil saturations measured from special core analysis. In order to explain this contradiction, data from several fields is analyzed with special emphasis on capillary pressure. The concept of irreducible water saturation, connate water saturation, residual oil saturation and "critical water saturation" are discussed. The conclusions which are supported by experiment and theory, have substantial commercial implications concerning the location of remaining reserves – in the immobile region or the unswept region. Based on these results, entirely different depletion strategies should be recommended. The concepts presented in this paper help to explain, in part, why reservoirs continue to produce past their forecasted abandonment, and why perfectly "history-matched" reservoir simulation models tend to grossly under-estimate incremental reserves through improved sweep efficiency. These issues are critical in optimizing the depletion strategy of oil and gas reservoirs. The discussion throughout this paper refers to an oil-water system in a water-wet or mixed-wet reservoir; however, these basic concepts can be applied to gas-water systems, three-phase systems, or other immiscible displacement systems and and oil-wet reservoirs.