Characteristics and Consequences of Red Bed Bleaching by Hydrocarbon Migration: A Natural Example From the Entrada Sandstone, Southern Utah

Extensive regions of yellow and white (“bleached”) sandstones within the terrestrial Jurassic red bed deposits Colorado Plateau reflect widespread interaction with subsurface reduced fluids which resulted in dissolution iron-oxide grain coatings. Reduced such as hydrocarbons, CO2, organic acids have been proposed bleaching agents. In this study, we characterize an altered section Slick Rock member Entrada Sandstone that exposes bleached sandstone bitumen-saturated pore spaces. We observe differences texture, porosity, mineralogy, geochemistry between red, pink, yellow, gray facies. facies quartz overgrowths, partially dissolved K-feldspar, calcite cement, fine-grained illite, TiO2-minerals, pyrite concretions. Clay mineral content is highest at margins section. Fe2O3 concentrations are up to 3× from but enriched 50× Metals Zn, Pb, rare-earth elements significantly Supported by a batch geochemical model, conclude hydrocarbon-bearing caused K-feldspar dissolution, precipitation quartz, calcite, clay, pyrite. Localized redistribution iron into concretions can account for most removed during bleaching. Pyrite carbonate stable isotopic data suggest hydrocarbons were sourced Pennsylvanian Paradox Formation. Bitumen spaces formed reductant trap required produce Cu, U, V enrichment all younger, oxidized saline brines.