THE ROLE OF SEDIMENTARY FACIES, PETROLOGY, AND TEXTURES IN RESERVOIR CONTINUITY OF THE EMERGING JURASSIC ENTRADA ERG-MARGIN GAS PLAY, UTAH

Recent discovery of high btu gas-charged Jurassic reservoirs in the southern Uinta basin, Utah have ignited interest in below-Tertiary reservoirs within the basin and other areas of Utah. An outcrop analog has been studied in an effort to better understand subsurface reservoir conditions. This case study is located within the Entrada erg-margin trend and specifically just east of Capitol Reef National Park.

The case study illustrates the stratigraphic trap potential of the Entrada erg-margin. Eolian-dominated sandstones are relatively isolated and enveloped within mudstones of the tidal flat facies. We have identified nine sedimentary facies within or associated to these eolian-dominated reservoir quality rocks. Four of these facies act as the primary reservoir within the section, two of which (SDS - small dune sets and LDS - large dune sets) constitute approximately seventy percent of the reservoir rock. All sandstones within the reservoir rock are feldspathic arenites. The basal ripple laminated (BRL) facies displays high porosity (23-26 percent) but relatively low permeability (7.4-264 mD). The low permeability likely results from clay fibers that cross open pores forming a “bridge” between grains. Hence, this facies might act as a baffle to fluid flow within the reservoir. Another potential baffle exists in the form of quartz-filled fractures. Thin section analysis shows a dramatic decrease in open pores and pore connectivity across these filled fractures. Finally, cataclasis exists along fault planes that cut through the reservoir sandstones. The reduction in grain size along the fault plane effectively baffles fluid flow. On the lamina scale, the alternating very fine to fine sand in lamina of eolian dunes likely displays preferential fluid flow in the coarser lamina. In summary, sedimentatary facies, petrology, and textures must be considered if one is to maximize fluid flow within these reservoir quality rocks.