The Entrada was deposited as part of a Late Jurassic erg that covered much of the western United States. Depositional environments of the Entrada include: (1) small-scale, coarse-grained eolian dunes, (2) eolian sand sheet, (3) large-scale eolian dunes, and (4) water-laid sands. Quartz and potassium feldspar overgrowths are the most abundant cements, followed by dolomite, calcite, and kaolinite.

Current reservoir models for eolian depositional environments suggest that well-sorted dune sands should have higher permeability than the finer grained and poorly sorted sand sheets. However, in the Entrada, no difference in permeability was

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detected between the facies, owing to the high percentage of preserved intergranular porosity in the sand sheet. Intergranular porosity is preserved by the development of thick clay coats, composed of illite and smectite, which inhibited porosity reduction by quartz and potassium feldspar overgrowths.

The clay coats are derived from mechanically infiltrated windblown clay deposited penecontemporaneously with the sand. This type of clay deposition is common in modern deserts. The thickest clay coats develop in facies where the grains undergo the least abrasion, such as sand sheets and interdunes. Therefore, upon lithification, sand sheets and interdunes will retain a high percentage of their intergranular porosity and will not act as permeability barriers to fluid migration.

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