Electrical logging is now almost universally applied in oil and gas fields throughout the world. The usual practice is to record the self-potential and resistivity curves which, together, reveal the location and thickness of strata penetrated by the drill and a great deal as to their lithology and fluid content. The resistivity log thus obtained serves very well to disclose in detail formations penetrated and to distinguish between oil and water.With the device used for ordinary electrical logging purposes, the true resistivity of the formation is not measured. The recorded resistivity is only an apparent value which is governed by several influencing factors, besides the true resistivity of the formation and its fluid content. As applied to the oil content of a formation, it is a qualitative measurement, merely showing the vertical extent of the oil accumulation.Experience has shown that the electrical conductance of oil sands is due to connate water held by molecular forces to the sand grains. Research has disclosed a definite mathematical relationship between the true resistivity of an oil sand and the ratio of its oil and connate water.This leads the way to a new practical application of electrical logging. By using a suitable combination of measuring devices, the true resistivity of an oil sand may be measured in an oil well. The true resistivity measurement, used in conjunction with other field and laboratory data, may in certain cases be used quantitatively to make a determination of the net oil content of a productive layer.The method by which these measurements are obtained is described and actual examples are cited.