PSCovenant Oil Field, Central Utah Thrust Belt: Possible Harbinger of Future Discoveries*
Thomas C. Chidsey1, Michael D. Laine1, John P. Vrona2, and Douglas K. Strickland2
Search and Discovery Article #10130 (2007)
Posted July 30, 2007
*Adapted from poster presentation at AAPG Annual Convention, Long Beach, California, April 1-4, 2007.
Please refer to Search and Discovery Article #110014 (2005), “Structural Architecture, Petroleum Systems, and Geological Implications for the Covenant Field Discovery, Sevier County, Utah,” by Strickland et al.
1Utah Geological Survey, Salt Lake City, UT ( firstname.lastname@example.org )
2Wolverine Gas and Oil Corporation, Grand Rapids, MI
After over 50 years of 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. To date, 10 producing wells and one dry hole have been drilled from two surface pads. Covenant has produced over 2 million bbls of oil and no gas; the field averages 6400 BOPD.
The Covenant trap is an elongate, symmetric, northeast-trending anticline, with nearly 800 ft of structural closure and bounded on the east by a series of splay thrusts in a passive roof duplex. The eolian Jurassic Navajo Sandstone reservoir is effectively sealed by mudstone and evaporites in the overlying Jurassic Twin Creek Limestone and Arapien Shale. Oil analysis indicates a probable Mississippian source – oil derived and migrated from rocks within the 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 ft over a 1600-ac area. Porosity averages 12%; permeability is 100 mD. 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.
The central Utah thrust belt, or “Hingeline,” has seen cycles of petroleum exploration for the past 50 years because explorationists viewed the geology as a natural extension of productive thrust belt-style structures in northern Utah and southwestern Wyoming. Early unsuccessful efforts tested anticlines identified from surface mapping and seismic reflection data. The lack of a Cretaceous source seemingly was to blame for these failures. 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. To date, 10 producing wells and one dry hole have been drilled from two surface pads. Covenant has produced over 3 million bbls of oil and no gas; the field averages 5500 BOPD from the Jurassic Navajo Sandstone. The OOIP is estimated at 100 million bbls; the estimated recovery factor is 40%.
The Covenant trap is an elongate, symmetric, northeast-trending, fault-propagation anticline formed during the Sevier orogeny (Late Jurassic-Early Tertiary), with nearly 800 ft (240 m) of structural closure. 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 Twin Creek Limestone and Navajo Sandstone are repeated due to an east-dipping back thrust detachment within the structure. This back-thrust forms a hanging wall cutoff along the west flank and north-plunging nose of the fold. Only the first Navajo (and possibly the Twin Creek) is productive. The eolian Navajo Sandstone reservoir is effectively sealed by mudstone and evaporites in the overlying Jurassic Twin Creek Limestone and Arapien Shale. Oil analysis indicates a probable Mississippian source – oil derived and migrated from rocks within the Hingeline region.
Cores from the Navajo Sandstone display a variety of eolian lithofacies (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 ft (129 m) over a 960 acres (390 ha) area. Porosity averages 12%; permeability is ≤100 mD. 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.
Wolverine Gas & Oil Corporation
Kings Meadow Ranches No. 17-1
SENW Sec. 17, T.23S., R.1W, Sevier County, Utah
TD – 9382 ft
Completed – November 3, 2004
Producing reservoir – Jurassic Navajo Sandstone
IPF – 708 BOPD, 1 MCFGPD, 20 BWPD
Productive area – 960 acres
Gross pay – 487 ft
Net pay – 424 ft
Net to gross – 0.87
Hydrocarbon column – 450 ft
Average porosity – 12%
Permeability – up to 300 mD
Water saturation – 38%
Water resistivity – 0.279
Ohm-m @ 77oF, 26,035 TDS
BHT – 188oF
Type of drive – strong water drive
Initial reservoir pressure – 2630 psi
Production and Reserves
Producing wells – 10
Dry holes – 1
Abandoned producers – none
Monthly production (November, 2006) – 166,036 BO & 49,810 BW
Cumulative production (as of December 1, 2006) - 2,938,869 BO & 527,076 BW
OOIP – 100 million bbls
Estimated recovery factor – 40%
Covenant Field Reservoir: Eolian Jurassic Navajo Sandstone
Navajo Sandstone Lithofacies Recognized in Core from the Federal No. 17-3 Well
T – Thick: dune deposits containing the large-scale, trough, planar, or wedge-planar cross-beds (35 to 40°) commonly recognized as classical eolian dune features. Very well to well-sorted, dune and avalanche deposits. Sand layers greater than 0.5 cm thick in core. The brink to the toe of the dune slipface consists of thin, graded, tabular grainfall laminae (rarely preserved in the core) and thick, subgraded, avalanche laminae. The “thick” classification is correlated to avalanche deposits. Porosity ranging from 5 to 15%, and permeabilties typically from 7 to 300 mD.
TC – Thin Continuous: sand layer continuously bedded, trough, planar, or wedge-planar cross-beds less inclined than the thick lithofacies (20 to 35°), and less than 0.5 cm thick in core. Moderately well to poorly sorted with more clay cementation than the “TD” lithofacies. The TC lithofacies can occur within the thick and Thin Discontinuous lithofacies making the TC lithofacies a transitional phase. All porosities are under 10% and permeabilites range from 1 to 30 mD
TD – Thin Discontinuous: flat lying bedding less than 0.5 cm thick in core containing wind ripples, and some cross-bedding (0 to 20°). Moderately to poorly sorted with greater carbonate cement than TC layer. Thin discontinuous tightly packed, reworked ripple strata are representative of dune toe lithofacies. Low porosities and permeabilites are characteristic.
WBH – Interdunal: low-angle to horizontal laminae or distorted bedding consisting of very fine to fine-grained, thin, poorly sorted sandstone, siltstone, and shale dominated by carbonate cement. Beds may contain wind ripples or fluvial characteristics (scour). Very low porosities and permeabilities. Interdunal fluvial characteristics indicate sheet flow or flooding events in a fluvial/wadi while other deposits suggest wet, playa or lacustrine conditions.
M – Massive: homogenized sandstone layers showing no distinct sedimentary structures or laminations. This lithofacies probably formed by water-saturated sand.
FB – Fault Breccia: Breccia resulting from a fault and fracture zones running through the core of the Federal No. 17-3 well.
Lithology – very fine- to medium-grained (1/16 mm to ½ mm), quartz sandstone; 97%, white or clear quartz grains with varying amounts of K-feldspar.
Sand Grains – subangular to subrounded, very well to well-sorted, usually frosted.
Pore Types – primary intergranular, fracture.
Grain Density – 2.651 g/cm3
Diagenesis – minor overgrowths of quartz; some authigenic clay mineralization has occurred in the form of grain-coating, pore-bridging, and fibrous illite; some ferroan (?) dolomite and fractured, corroded K-feldspar are also present.
Oil gravity – 40.5º API
Color – dark brown
Viscosity – 4.0 centistokes @ 77ºF
Pour point – 2.2ºF
Sulfur – 0.48%
Nitrogen – 474 parts per million
Stable carbon-13 isotopes – -29.4‰ (saturated) and -29.0‰ (aromatic) hydrocarbons
Pristane/phytane ratio – 0.96
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Funding for this ongoing research was provided, in part, under the Preferred Upstream Management Program of the U.S. Department of Energy, National Energy Technology Laboratory, Tulsa, Oklahoma, contract number DE-FC26-02NT15133. Support was also provided by the Utah Geological Survey (UGS) and Wolverine Gas & Oil Corp (WGO).
Field cores were described by L.F. Krystinik, geologic consultant. Jake DeHamer, WGO, interpreted core petrophysical data. The poster design was by Liz Paton of the UGS. James Parker, Liz Paton, Sharon Wakefield, and Cheryl Gustin of the UGS prepared the figures; Brad Wolverton of the UGS Core Research Center assisted with core photography.