Abstract

After over 50 years of unsuccessful exploration in the central Utah thrust belt, or “Hingeline,” the 2004 discovery of Covenant oil field proved that this region contains the right components (trap, reservoir, seal, source, and migration history) for large accumulations of oil. Covenant has produced over 3.1 million barrels (500,000 m³) of oil and no gas, an average of 5400 barrels (860 m³) of oil per day, from the eolian Jurassic Navajo Sandstone. The original oil in place is estimated at 100 million barrels (15.9 million m³), with an estimated recovery factor of 40% to 50%.

The Covenant trap is an elongate, symmetric, northeast-trending anticline with nearly 800 feet (270 m) of structural closure and bounded on the east by a series of splay thrusts. The structure formed above a series of splay thrusts in a passive roof duplex along the “blind” Gunnison-Salina thrust and west of a frontal triangle zone within the Jurassic Arapien Shale. The Jurassic Navajo Sandstone and overlying Twin Creek Limestone are repeated due to an east-dipping back-thrust detachment within the structure. Only the Navajo in the hanging wall of the back thrust (and possibly the Twin Creek) is productive. The Navajo Sandstone reservoir is effectively sealed by mudstone and evaporite beds in the overlying Jurassic Twin Creek Limestone and Arapien Shale. Oil analysis indicates a probable Carboniferous source—oil derived and migrated from rocks within the Utah Hingeline region.

Cores from the Navajo Sandstone display a variety of eolian facies (dune, interdune, lake/playa, fluvial/wadi), fracturing, and minor faults, which in combination, create reservoir heterogeneity. Reservoir sandstone is 97% frosted quartz grains (bimodal grain size), with some quartz overgrowths and illite. The net reservoir thickness is 424 feet (129 m) over a 960-acre (390 ha) area. Porosity averages 12%; permeability is ≤100 millidarcies. The drive mechanism is a strong water drive; water saturation is 38%. A thorough understanding of all the components that created Covenant field will determine whether it is a harbinger of additional, large oil discoveries in this vast, under-explored region.