Gamma ray log to estimate clay content in the layers of water boreholes

Abstract

This work describes a complete procedure to estimate the clay content of water-bearing layers in boreholes from gamma-ray logs using an optimised clay index, which conventionally is the normalised gamma-ray value in each layer with respect to the maximum and minimum values throughout the borehole. In addition to the geological description of the basin, the interest of knowing the clay content of the layers lies in its importance for the estimation of the permeability of them, given the high influence that this content has on permeability, and also in its application for the determination of the real formation factor. To use the clay index, the gamma-ray log particularities require extensive processing including background radiation removal and the filtering of random oscillations in radioactivity, among other factors, to ensure reliable results; so specific algorithms are presented to carry out this processing. The direct utilisation of the maximum gamma-ray value in clay index is problematic as it depends on the presence of media with a certain radioactive emission that is independent of the clay content, and whose emissivity and abundance are not constant throughout a hydrological basin. For this reason, a statistical criterion, developed to replace the maximum gamma-ray logged by the value that corresponds to 100% of the clay content, is presented. The procedures developed in this study was applied to a twenty-seven borehole series located in the southern Duero Basin, in the Iberian Peninsula, which can be considered representative of continental basins with multi-layered detrital lithologies. Two relevant conclusions were derived from this study. First, the distribution of the layers in each borehole within a detrital basin followed a beta distribution based on their clay content. Second, the mode of this distribution was the best estimator of the sedimentary environment, which clarifies its hydrological potential.