Spectral Gamma-Ray Logging for Facies Discrimination in Mixed Fluvial-Eolian Successions: A Cautionary Tale

Abstract

Accurate assessment of downhole natural gamma-ray logs requires a reference database built up from well-exposed outcrops where the recognition and interpretation of lithofacies is unequivocal. Although only a few cases have been published for spectral gamma-ray logs, some of these show there is a strong correlation to depositional environment. We present detailed spectral gamma-ray data, within an interpretative framework of the sedimentology and petrography, for a mixed fluvial-eolian succession in the Cutler Group of southeastern Utah, United States. In this case, these data did not help identify depositional environment. Discrimination of eolian from fluvial deposits is not reliable because there is much overlap between the gamma-ray emissions of each lithofacies, although the eolian sediments tend to have lower values than the fluvial sediments. Interdune deposits cannot be discriminated from flood-plain sediments. The thorium/potassium ratio is approximately constant for all lithofacies, and is of no use as a facies discriminator. The thorium/uranium ratio is less than 7, much lower than might be expected for continental lithofacies from a review of the literature. These results have general application to mixed fluvial-eolian successions because they reflect the typical interactions between eolian and fluvial sedimentary processes, and do not depend especially on the character of the original sediment.