Outcrop spectral gamma-ray signals of distal arid continental basins: Facies identification and controls

Abstract

Spectral gamma ray (SGR) studies can help link outcrop exposures to downhole data and provide useful case studies to reduce uncertainty in interpreting geophysical logs where core and other geological data is unavailable. However, this technique is underused in continental outcrop studies due to difficulties in determining facies comprising similarly sourced sediment, despite the fact that these deposits form prolific reservoirs for hydrocarbon and geothermal exploration, as well as carbon capture and hydrogen and storage. This work demonstrates the usefulness of spectral gamma ray measurements as an outcrop logging technique in determining quantifiable facies from the dryland continental sediments of the Cedar Mesa Sandstone Formation, Utah, USA. Complex sedimentary interactions between the aeolian, fluvial and lacustrine depositional environments are present, resulting in shared similar sedimentary characteristics within the preserved strata, such as mineralogy, grain size, sorting and provenance signals. Consequently, the geophysical signatures observed within the spectral gamma ray logs, have been suppressed, making it difficult to distinguish sedimentary facies from solely the spectral gamma ray responses. However, by combining K:Th cross-plot analysis and gamma ray log motif interpretation, this work demonstrates that individual facies within arid continental settings can be distinguished based on SGR outcrop logging, and unique spectral trends and values can be quantified and used to correlate facies across the depositional basin.