UNEXPECTED RESERVOIR QUALITY SANDSTONE WITHIN THE MUDSTONE-RICH PORTION OF THE MIDDLE JURASSIC ENTRADA SANDSTONE, SAN RAFAEL SWELL, UTAH

Nestled within the understudied mudstone-rich portion of the Middle Jurassic (Callovian) Entrada Sandstone on the west flank of the San Rafael Swell, central Utah, is a laterally extensive reservoir-quality sandstone. This sandstone extends for at least 16 km (9.9 mi) along strike. The maximum width is unknown due to erosion and subsurface burial but locally is at least 2.0 km (1.2 mi) wide. The sandstone has an average thickness of 2.9 m (9.5 ft) with a maximum thickness of approximately 6.4 m (21 ft). Core plug porosity and permeability data indicate the sandstone is of good to excellent reservoir quality. The encapsulating mudstone-rich units represent an excellent seal. Provided the other aspects of a hydrocarbon system are present (source, generation, and migration), the sandstone has potential as a hydrocarbon reservoir within stratigraphic or combination traps.

Both basal and upper surfaces of the sandstone are relatively flat with rare occurrences of soft-sediment deformation at the basal contact. For most of the exposure, a single high-angle trough cross-set (TCS) extends from the base to the upper surface. Paleocurrent measurements taken from the exposed upper surface indicate a southward wind direction. We interpret this sandstone as an eolian deposit (dune) that migrated over and filled a supratidal sabkha pond. A relative sea-level transgression initiated a rise in the water table, saturating the lower portion of the dunes. This allowed the wetted sand to become cohesive or possibly cemented, thus preserving sedimentary structures (TCS). There are multiple possible modern analogs, the most promising located near the northern tip of the Gulf of California.