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Three-dimensional eolian bounding surface architecture of the Entrada sandstone, Utah: expanding our understanding of reservoir heterogeneity in wet eolian systems
Gross, Evan C.
Gross, Evan C.
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2020
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Eolian bounding surfaces represent significant changes in dune-field dynamics caused by allogenic and autogenic processes associated with erg evolution. Eolian deposits form noteworthy petroleum reservoirs (e.g., Norphlet Formation, Rotliegend Group, and the Tensleep Sandstone), but bounding surface architecture imparts significant reservoir heterogeneity. In order to understand dune morphodynamics, reconstruct ancient erg evolution, and evaluate the impact of flow-inhibiting bounding surfaces on hydrocarbon reservoir performance, the stratigraphic architecture of eolian systems must continue to be studied at multiple scales.This study pairs traditional field and laboratory methods (e.g., measured stratigraphic sections, handheld gamma-ray spectrometry, petrography) with advanced technologies (e.g, RTK GPS data, drone-derived digital outcrop models) to precisely resolve the kilometer-scale three-dimensional stratigraphic architecture of wet eolian Middle Jurassic Entrada Sandstone outcrops located ~60 km south of Moab, Utah at Rone Bailey Mesa (RBM). Six facies are identified, primarily based on sedimentary fabrics, and grouped into four facies associations named dune, sabkha, sand sheet, and ash fall. Statistical analyses and automated mineralogy data suggest that gamma-ray logs could potentially improve understandings of wet eolian bounding surface architecture and dune and sabkha facies association distributions in the subsurface. Seven modeled super bounding surfaces suggest that such surfaces are planar to somewhat undulatory, with no perceived trends. Five modeled interdune migration surfaces are also undulatory but exhibit a ~0.09° angle of climb toward the south-southwest (roughly parallel to the paleowind direction). Three modeled superimposition surfaces are concave up and some are quite sinuous in plan-view. Laterally-restricted sabkha facies association deposits observed in the upper-most portion of the Entrada Sandstone suggest that they were associated with closed damp interdune flats located between climbing sinuous transverse bedforms or patches of bedforms. The Entrada Sandstone at RBM is interpreted to have been deposited primarily within a central portion of a wet eolian system that experienced a long-term trend of available sediment supply increase. The erg was likely composed of barchan dunes, barchanoid ridges, larger sinuous transverse draas, and superimposed smaller bedforms. The sinuous transverse draas were ~11.3 m tall and had a wavelength of ~169 m, according to trigonometric calculations. Fluctuations in eustatic sea level, subsidence, and sediment supply were likely to be the primary allogenic drivers of Entrada erg evolution. The Entrada Sandstone observed within the field area overall preserves signals of allogenic forcings but also records localized evidence of autogenic bedform cannibalization of the substrate (e.g., scour-and-fill architecture).
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