A Review of Current Techniques in Gamma-Ray And Neutron Log Interpretation

Abstract

Introduction The radioactivity log may present any of several combinations of radioactivity curves depending upon the purpose for which the log is run. The log normally consists of a gamma-ray curve and one or more neutron curves which are selected to provide the specific information desired. Radioactivity logs can provide information useful in exploration for and the development of petroleum reservoirs. They are valuable also in the maintenance of wells by providing information useful to guide completion, repair, recompletion and secondary-recovery operations. The Gamma-Ray Curve Basis for Geological Significance The development of the radioactivity log began with the recording of the natural gamma-ray curve. The amplitude of this curve is a measure of the gamma-ray intensity in the borehole. Variations in the amplitude of this curve in a general way follow the concentration of the radio-elements in the formation. Experience has shown that the great majority of the radio-elements of the sedimentary column are carried by the shales. The gamma-ray curve is thus able to identify the sandstones and the carbonate rocks in which petroleum reservoirs can occur, principally by their low content of shales which carry the radio-elements. The development of a clean sandstone formation requires, during its deposition, erosional conditions which wash the sand grains free of the fine clay-mineral particles and carry them away to be deposited in another section of the sedimentary basin. This can happen on a grand scale only if erosional and sedimentation conditions are relatively stable over a long period of time. More commonly, the sorting action of the erosional forces is inadequate to fully separate the clay minerals from the sand so that a great majority of sandstones contain clay-mineral fractions.