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The production of oil is accomplished as a result of its displacement fromthe reservoir by either gas or water, and the amount of oil recovery is limitedby the extent to which the displacing gas or water accumulates. This paperdescribes the mechanism by which the displacement is effected and theadvantages of water over gas as a displacing agent. In the light of the resultsof experimental observations of the flow of mixtures of oil and/or gas and/orwater through sands, certain conclusions are drawn relative to the changingcharacter of the displacement as depletion proceeds, and on the effects of theproperties of the fluids and of producing conditions on the ultimate oilrecovery. Introduction Crude oil has no inherent ability to expel itself from the pores of thereservoir rocks in which it is found; rather, it must be forcibly ejected ordisplaced by the accumulation of other fluids. Accordingly, a knowledge of themechanism by which one fluid is displaced by another is essential to anunderstanding of the fundamental process by which oil is recovered. The displacing fluids normally available are gas and water, either or bothof which may exist originally associated with the oil in a potentially usableform or may be supplied to the reservoir from external sources. Gas is presentin most oil reservoirs. If the quantity is relatively small, it may existoriginally completely dissolved in the oil, but if the quantity exceeds thatwhich may be held in solution by the oil at the prevailing pressure, the excesswill be found in the free state. Most reservoir sands appear to contain somewater, and in the majority of fields the oil is trapped in the structure andheld over water. In certain conditions the entrapping water may advance intothe oil zone and displace the oil therefrom. This paper describes in a qualitative manner some of the characteristics ofthe displacement of oil by either gas or water, with an attempt to elucidatesomewhat the mechanism by which such displacement is effected. T.P. 1337