This paper describes a new method for measuring the oil saturation of coresduring the course of fluid-flow experimentation. The method depends upon theaddition to the oil of a soluble tracer material that is the primary absorberof the X-rays incident upon the core. The paper includes a description of theX-ray equipment and intensity-measuring apparatus, calibration data for twocores, and results obtained with additional cores for various conditions of oiland water-flooding. The data and results are presented to show thepotentialities of the X-ray method at this stage of development rather than toestablish particular concepts pertaining to fluid-displacement mechanisms ofpermeable media. Further development of this method will include adequatepressure and flow-rate control in the liquid and gas-flow systems,.as well asadditional stabilization of the X-ray power supply. Introduction: In the last several years a type of application of the' fundamentalpermeability saturation relationships has been developed which others greatpromise in the interpretation of general reservoir performance. In thesedevelopments no attempt is made to solve the detailed flow dynamics in actualoil and gas-producing systems. That problem still remains unsolved. But thepermeability-saturation phenomena are incorporated in the analysis of oilproducing reservoirs in a manner such as to reflect the true role they play incontrolling the gross reservoir behavior. In particular, they are introduced insuch a manner as to determine the gas-oil ratio of the production at any stageof depletion and, when combined explicitly with the so-called material-balanceprinciples in differential form, one can obtain a differential equationdescribing completely the average pressure and gas-oil-ratio history ofoil-producing reservoirs. While thus far they have been applied directly onlyto gas-drive reservoirs, they are susceptible of extension to partialwater-drive systems, and qualitatively the same principles are of value in thestudy of complete water-drive reservoirs. Moreover, in addition to giving ameans for computing the variation of the reservoir pressure and gas-oil ratiosvs. the cumulative production, they automatically lead to predictions of thegross ultimate recovery under natural depletion-and when properly extended theyprovide means for predicting the ultimate recovery under gas or waterinjection. The data required for such applications are the initial fluid distribution inthe producing section, the pressure-volumetemperature data on the gas andliquid phases, and the permeability-saturation relationship for theoil-saturated zone. The former can be obtained with reasonable accuracy bymethods that are well established and available throughout the industry. Thepermeability-saturation data, however, involve techniques that are not at allstandardized. T.P. 2124