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By the use of modern electrical logging methods and evaluation techniques accurate values of porosity and water saturation can be computed in most reservoir formations. While these values are essential in the quantitative definition of in-place hydrocarbons, they do not, alone, predict type of fluid production to be expected from the formation.The product of water saturation times porosity tends to be constant for homogeneous, hydrocarbon-bearing formations above the transition zone, where as in the transition zone this constancy is not maintained, the product Porosity.Sw tends to track the porosity. Through computer processing of log data an appropriate display of continuous Porosity.Sw values are recorded in log form. From the shape of the curve an estimate of type fluid production can be made. For intervals known to be above the transition zone variations in the Porosity.Sw curve are indicative of changes in the physical properties of the formations. This knowledge is useful in prediction of formation producibility. The technique is applicable to carbonates and sandstone formations both clean and shaly, that are characteristic of reservoirs in Indonesia and is compatible with standard logging program presently being utilized here.Examples illustrating the method in several of Indonesia's hydrocarbon bearing basins are included.