Logging and Log Interpretation - Spectral Gamma-Ray Logging

The introduction of the scintillation counter into field use for gamma-ray well logging has provided a new tool with special utility for the deternzination of natural radioactivity of sediments. Previous subsurface gamma-ray measurements have indicated only the total amount of radioactivity of the sediments. With the scintillation counter, the relative amounts of the elements which emit gamma radiation can be determined. Sediments logged included 35 zones in wells located in the Teras Gulf Coast, East Texas, and West Teras. Although the data obtained from measurements made to date are not sufficient to evaluate the possibilities of spectral logging, it appears likely that spectral measurements will yield information of geologic value. INTRODUCTION Although variations in the natural radioactivity of sediments were recognized as early as 1909,1 the appli- cation of such variations to borehole logging was not made until almost 30 years later. A group of Russian workers in the early 1930's were successful in making crude measurements of natural gamma activity in an oil well borehole. The first practical gamma-ray logging system was developed by Howell and Frosch2 in 1939. Induced radioactivity, or neutron, logging appeared soon after the introduction of gamma-ray logging. After early work had demonstrated the potentialities of radioactivity logging, development of sensitive and stable logging equipment for field use followed, so that neutron and natural gamma logs are now run routinely and have proved to be extremely valuable in characterizing subsurface formations. Even though tremendous strides have been made in developing radioactivity logging, it is apparent that the ultimate in the method of logging has not been reached, for, until very recently1 in none of the commercially available logging systems has advantage been taken of the fact that gamma radiation is composed of a spectrum of rays, each ray having a distinct and characteristic energy. An individual gamma ray originates in a single element and has energy determined by the element. The inten- sity of radiation, or number of rays emitted from unit volume in unit time, is determined by the concentration of the element. If it were possible to sort out these gamma rays and assign them to their parent elements, thereby essentially analyzing for the elements, a wealth of new information descriptive of formation material and fluid content would be at hand. For example, if in natural gamma logging, the gammas due to potassium and to the thorium and uranium series were measured individually, data would thus be available on the concentrations of each of the three elements, adding materially to the information content of the log. In contrast, conventional gamma logs measure only a quantity proportional to the combined concentrations of uranium, thorium, and potassium. The naturally radioactive elements occur in sediments in concentrations dependent on source material of the sediment, and through their chemical characteristics, on conditions of deposition,' so that the individual concentrations may be diagnostic for particular formations. These concentrations, particularly if measured in situ, could lead to a powerful tool for correlation and for reflecting environmental conditions under which sedimentation took place. As a correlation tool, individual uranium, thorium, and potassium measurements