Permian Basin Oil Recovery Conference, 7–8 May, Midland, Texas
This paper presents a review of current methods of interpreting and applying gamma ray-density logs in evaluating fundamental reservoir data. The basic theories and physical principles underlying the techniques of density logging with gamma rays are discussed along with some of the problems associated with the instrumentation of such a tool. An effort has been made to evaluate the instrument calibration techniques, and discussion and figures are presented which describe the effect of borehole conditions, pressure, temperature, etc. The relationship of natural density, grain density, and interstitial fluid density to porosity is presented. Field data illustrate how density logs can be correlated with porosity and grain density data. With the aid of adequate core analysis data, a reasonably accurate estimate of formation porosity can be obtained from density log interpretation. The paper also offers a discussion of the limitations of the density log as a porosity tool. The current applications of the density log to geologic work are cited, and the prospective uses and future developments are briefly explored.
The principle of gamma ray absorption as a function of density was adapted to the logging of petroleum formations through the utilization of a phenomenon known as "back-scattering". This technique uses a tool which contains both a gamma ray source and detector. The detector measures the intensity of the gamma rays emitted from the source that have been back-scattered to the detector by the formation. After proper instrument calibration, the log response can be used to compute the natural density of the formation. Since there is a definite relationship between natural density and porosity, it is possible to predict formation porosity from density logs. It is necessary to correlate carefully density log data with core analysis data before the direct correlation of density log response with porosity can be made.