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.



College and Department

Physical and Mathematical Sciences; Geological Sciences



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erg-margin, Entrada Sandstone, reservoir characterization



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Geology Commons