World-class outcrops of an outermost erg-margin can be observed within the Middle Jurassic Entrada Sandstone near Capitol Reef National Park, Utah. These erg-margin deposits contain isolated reservoir quality sandstone bodies that transition into a muddy tidal flat facies. These high quality reservoirs are dominated by eolian-influenced facies interbedded with sandy interdune facies. They are sealed vertically by muddy and silty facies of associated tidal flat deposits that act as excellent stratigraphic traps in the subsurface. A variety of approaches were used to characterize these Entrada erg-margin reservoirs including: annotated panoramas of outcrops, measured sections, scintillometer measurements of field sections, facies analysis, 2D high-resolution shallow seismic surveys, porosity and permeability analysis, and sedimentary petrography. Logs from the North Hill Creek/Flat Rock gas field were analyzed and correlated to the outcrop study. Eolian dune facies, along with an upper ripple laminated facies representing interdune deposits, display the highest porosities and permeabilities and are volumetrically the most important facies of the reservoir quality sandstones. Baffles and possible barriers within the sandstone bodies are limited to quartz filled fractures, deformation bands, silty and muddy interdune facies, and first order bounding surfaces. Many of the sandstone bodies within the outcrop belt are genetically related and in communication with each other. This relationship results from dune complex migration to the south and up section over time. Stratigraphic climb can potentially be imaged seismically and may serve as a key indicator of eolian dune complexes in the subsurface. The volumetric size of one of these complexes is estimated around 470 million cubic feet. Smaller outcrop sandstone bodies were often found to be isolated from the large dune complexes and ranged down to 1 million cubic feet in size.

The late Middle Jurassic (Callovian) Entrada Sandstone has been divided into two general facies associations consisting primarily of eolian sandstones in eastern Utah and "muddy" redbeds in central Utah. Sedimentary structures within the redbed portion are explained by the interfingering of inland sabkha, alluvial, and eolian depositional systems. A complete succession from the most basinward facies to the most terrestrial facies in the Entrada Sandstone consists of inland sabkha facies overlain by either alluvial or eolian facies. Where both alluvial and eolian facies interfinger, alluvial facies overlain by eolian facies is considered a normal succession. Sequence boundaries, often identified by more basinward facies overlying more landward facies, are observed in the Entrada Sandstone and are extrapolated for the first time across much of Utah, including both the eolian-dominated and redbed-dominated areas. Using these sequence boundaries as well as recent tephrochronologic studies, three time correlative surfaces have been identified in the Entrada. Based on the facies interpretations at each surface, five paleogeographic reconstructions and five isopach maps have been created, illustrating two major intervals of erg expansion and the location of the Jurassic retroarc foreland basin's potential forebulge. Eolian (erg-margin) sandstones pinch-out into muddy redbeds creating combination traps, as evidenced by dead oil (tar) and bleached eolian sandstone bodies within the Entrada. The Entrada Sandstone is a world-class analog for similar systems, such as the Gulf of Mexico's Norphlet Sandstone, where eolian facies grade into muddy redbed facies.