Silica Content and Gamma Ray Logs in Volcanic Rocks

Abstract

Chemical analyses of 254 rock samples from Iceland are used to establish a relationship between the natural gamma ray intensity (expressed as API gamma ray units, or API GU) and the silica concentration of Icelandic rocks (% SiO2). For each sample, the gamma response is calculated based on concentrations of K, Th, and U, using linear equations established by Belknap (1959). The SiO2 concentration in this sample population is shown to be linearly related to the K, Th, and U contents. Combining these equations, we obtain the following linear relation: API GU = (2.63 ± 0.10)*SiO2 - (102 ± 6). This result agrees with a previous calibration where actual gamma ray logs were compared with chemical analyses of cores and cuttings (Stefansson et al. 1982). The relation obtained makes it possible to express the natural gamma ray logs as percentage of the SiO2 in the Icelandic rocks. Furthermore, as the matrix thermal conductivity of volcanic rocks seems to correlate with their silica content, it might be possible to obtain continuous profiles of the thermal conductivity from the gamma ray logs together with a suitable porosity log.