The Spectral Gamma-Ray Log
The counts recorded on a standard gamma ray log are the sum of all gamma rays, most of which are emitted by potassium-40 and isotopes in the uranium and thorium series. The energy levels of the gamma rays are characteristic of their source isotope and form distinctive spikes on a gamma-ray emission spectrum, although in practice these spikes are smeared into a diffuse train of peaks by scattering events. However, by subdividing the total energy range into restricted windows, estimates can be made of the separate contributions of potassium-40, uranium and thorium through computer processing.
The spectral gamma ray log is displayed as three curves of thorium (ppm), uranium (ppm), and potassium (%), while the gamma ray track records two gamma ray curves, the standard (SGR) and computed gamma ray (CGR) (Fig. 16). The SGR records the total gamma ray count from all sources, while the CGR results from the subtraction of the uranium contribution from the SGR. Because the CGR is the summation of thorium and potassium sources, this curve is an improved log to estimate clay content, free of perturbations by uranium. Common applications of the spectral gamma ray log are in the estimation of clay mineral volumes (and types), and the recognition of fractures that have had uranium salts precipitated in them by ground-water systems.
The potassium-thorium crossplot is useful for the recognition of clay minerals and distinction of micas and K-feldspars. The lines that radiate from the origin of the plot have gradients matched with values
Figures & Tables
This manual was created in 1994 to assist the geologist to interpret logs. In the not too distant past, the reading of geology from wireline logs was highly interpretive. The ability of a rock to conduct electrical current or sound waves is several steps removed from traditional outcrop descriptions based on the eye and hammer. However, the range of logging measurements has expanded markedly over the years. In particular, the addition of nuclear tools has introduced log traces that reflect both rock composition and geochemistry in a more direct manner. Taken together, both new and old logs contain a host of keys to patterns of rock formation and diagenesis. The majority of books on log analysis focus on the reservoir engineering properties of formations penetrated in the borehole. The promise of potential porous and hydrocarbon-saturated rocks generally pays for both the hole and the logging run. There are many examples of common log types from a variety of sequences.