LOG INTERPRETATION IN SANDSTONE RESERVOIRS

Abstract

By comparison with carbonate rocks sandstones are texturally homogeneous; in consequence the interpretation from well logs of the fluid content and physical properties of sandstones is relatively simple. From an exploration standpoint one significant fact must be gleaned from the log: Will the formation of interest produce oil or gas? The precise porosity and thickness of the formation and its exact hydrocarbon saturation are, initially, secondary considerations. Some of the principal difficulties involved in determining interstitial water resistivity, porosity and formation resistivity are examined. It is confirmed that the so‐called “low zone” significantly decreases the resistivity of an oil‐bearing sand when this resistivity is found from an induction log. Theory and laboratory experiment confirm that diffusion and convective mixing of the filtrate and interstitial water do little to mitigate the problem. It is suggested that future correction charts for induction logs recognize this fact. New data bearing on the dirty sand problem show that if formations contain interstitial clay the interpretation procedure suggested by A. J. de Witte is substantially correct. It is pointed out that the real problem is to decide the manner in which the clay content of a typical dirty sand is disposed. Measurements on the effect of the porosity of sandstones on the difference between velocities through them when dry and fully water‐saturated disclose an interesting paradox: The velocity difference is a maximum when the porosity is a minimum. Significant differences in velocity between gas‐ and water‐saturated sandstones are not to be expected in the porosity range of practical significance.