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