Complexity defies easy approach

Tricky Geology Is Utah Hallmark

Colorado's Piceance Basin
Mesaverde outcrop. Photo courtesy of Bill Barrett Corp
Colorado's Piceance Basin Mesaverde outcrop. Photo courtesy of Bill Barrett Corp
Few areas in the United States contain as much complexity thrust into prospectivity as central Utah.

“Central Utah is an area where there are stratigraphic changes happening from east to west. Then you complicate that by the Sevier orogeny, followed by the most recent episode of the Basin and Range normal faulting,” said AAPG member Doug Sprinkel, senior geologist for the Utah Geological Survey.

“You start with complex stratigraphy, then complicate it by telescoping the strata eastward during thrusting,” he said. “Then normal faulting extends the section westward, often along the pre-existing thrust faults.”

If the area’s geological complexity can’t be doubted, neither can its exploration potential.

Wolverine Gas & Oil Corp. lit a firestorm of interest in central Utah with its Covenant oil field discovery in Sevier County in 2004.

Estimates of original oil in place at Covenant range from 75 million to 150 million barrels.

Now reports of a new field discovery near Mayfield in Sanpete County are stoking industry interest again, primarily in Sanpete, Sevier, Carbon, Emery and Juab counties, about 100-170 miles south of Salt Lake City.

Studies, prospects, reviews and hopes all have been generated, but the complexity of the play area defies any easy approach.

Utah complexity: The intensely folded Arapien Shale forms the core of the Sanpete- Sevier Valley antiform that trends essentially north-south. The location of theWolverine Federal Arapien Valley 24-1 well is along an east tributary of the drainage that is about the center of the photograph, less than two miles away. Photograph by Doug Sprinkel.
Utah complexity: The intensely folded Arapien Shale forms the core of the Sanpete- Sevier Valley antiform that trends essentially north-south. The location of theWolverine Federal Arapien Valley 24-1 well is along an east tributary of the drainage that is about the center of the photograph, less than two miles away. Photograph by Doug Sprinkel.
“It’s something you have to be aware of – it’s something you can easily get tripped up on,” Sprinkel said. “A lot of geologists ignore the detail of this complexity.”

Stratigraphic Changes

Drilling in the play area generally reaches a maximum total depth of 14,000 feet – and trying to place the tops is anybody’s guess.

Right now, shallower formations don’t appear as promising. Pay zones in the new discovery could be between 8,000-9,500 feet, 10,000-11,000 feet and 12,500-14,000 feet.

Other central Utah prospects target potential pay zones at 6,500-9,000 feet and 11,000-13,000 feet, including both sandstone and limestone formations.

Commercial pay zone thickness could range from a healthy 300 feet to more than 1,000 feet, with closures covering four to six square miles.

According to reports, Wolverine has been producing 7,000 barrels of Covenant oil/day from 10 wells in the Navajo Sandstone at 5,700-6,200 feet on 80-acre spacing.

Thickness of the Jurassic Navajo Sandstone, the primary target, varies by location. Wolverine found about 500 feet of gross pay and 450 feet of oil column within the Navajo, in a sand up to 1,200 feet thick.

Stratigraphy changes are evident from both west-to-east and north-to-south across the central Utah region.

“For example, the Triassic section that underlies the Nugget Sandstone in northern Utah also is present on the hanging wall on the Nebo thrust plate in central Utah – but on the footwall the Triassic section is more typical of the Colorado Plateau geology,” Sprinkel said.

“As you go to the east (from the Nebo thrust in central Utah), you get into the Glen Canyon Group, which includes the Navajo Sandstone, the Kayenta Formation and the Wingate Sandstone, instead of Nugget,” he added.

On the Nebo thrust in central Utah, stratigraphic charts show the Upper Triassic-Lower Jurassic Nugget Sandstone is overlain by the Twin Creek Limestone (lowermost five members), Arapien Shale and Twist Gulch Formation.

Across the region in east-central Utah, the Glen Canyon Group (Wingate, Kayenta and Navajo) comprise a Nugget equivalent, and underlies the Jurassic Carmel Formation (Twin Creek and Arapien Shale equivalent). But the series also vary by location, with sometimes unpredictable changes in the Lower Jurassic.

“Above the Navajo in the western part of central Utah you have the Twin Creek overlain by the Arapien,” Sprinkel noted.

The Middle Jurassic thins to the east, going “from 3,500-4,000 feet depositional thickness down to 1,000 feet, fairly quickly,” he said.

An Open Question

Sprinkel was one of nine co-authors on a paper describing the petroleum geology of the Covenant field area.

It defined Utah’s central Sevier thrust, or Hingeline, as a section of thrust belt extending from the Uinta Mountains in northern Utah to the Marysville volcanic complex of south-central Utah.

This Hingeline coincides with a boundary between a thick succession of sediments in western Utah and a thinner succession in eastern Utah from the late Proterozoic to Triassic.

In the Cretaceous-early Tertiary, it influenced thrusts at the eastern margin of the Sevier belt. It also marks the general boundary between the Basin and Range and Colorado Plateau provinces.

The complex transitional zone reflects the Jurassic-Eocene compressional Sevier orogeny, extensional tectonics and Oligocene-Neogene volcanism.

The Sevier thrust play area in central Utah is often called the Hingeline play. Well control shows high porosity within potential pay zones in the region and average permeability of 100 mD.

Tectonic history provides many structures for evaluation, and both the tectonic and depositional history have resulted in plentiful trapping mechanisms.

That leaves the question of source rock – which remains an open question.

“This is a Paleozoic oil, likely Carboniferous, and the most likely suspect is Mississippian. We haven’t been able to determine whether the source rock is Delle Phosphatic shale or Manning Canyon Shale,” Sprinkel said.

“Has the heat flow from Oligocene volcanic rocks in the area played a role?” he asked. “I don’t know the answer to that.”

Key to the question is whether the sourcing is generated on the local thrust sheet or is migrating in. And with many shale systems present, the sourcing for the Covenant field might not be the only source rock available.

“A couple of wells have had shows in the Triassic Sinbad Limestone member of the Moenkopi Formation,” Sprinkel noted.

And the Navajo is far from the only possible production target.

Some geologists “might not appreciate there are two potential reservoirs in the Twin Creek Limestone above the Navajo Sandstone,” Sprinkel said.

Thrusting, folding and faulting indicate a complex migration picture, which could lead to oil discoveries in unexpected places.

Topsy-Turvy

The Covenant oil field is believed to be the largest onshore oil province discovered in the U.S. Lower 48 since the East Anschutz Ranch field in the Absaroka thrust play.

While the on-trend fields in northern Utah reflect complex geology, they are no match for the topsy-turvy thrust and fold of central Utah.

“Northern Utah and southwest Wyoming do seem a bit simpler from that point of view, because there you’re thrusting your reservoir rock up and over the Cretaceous marine source rock,” Sprinkel said.

In proximity to the central Utah exploration region are major gas-producing areas:

The Clear Creek natural gas field, discovered in the 1950s, has produced more than 115 billion cubic feet of gas from the Cretaceous Ferron Sandstone.

The Drunkards Wash coalbed methane field is the largest gas-producing field in Utah.

The Joes Valley Gas Field in Sanpete County has produced more than three billion cubic feet of gas from the Cretaceous Ferron and Dakota sands.

Sprinkel said some earlier wells in the central Utah oil-play area produced “puffs” of gas that were flared, but quickly declined.

Petro-Hunt LLC, Clayton Williams Energy Inc., Yates Petroleum Corp. and Delta Petroleum Corp. are among the companies that have been active in the area recently.

In addition to 3-D seismic surveys, central Utah has seen a new gravity gradiometry study. ARKeX of Cambridge, England, conducted a multiclient, airborne survey of the Hingeline region.

ARKeX said its first-phase 150-squaremile survey was followed by acquisition of another 300 miles of gravity data. An additional 750 square miles are scheduled to be flown this summer.

Following the Hingeline

According to prospect descriptions, Cretaceous sands may be secondary Hingeline objectives in fault-trap closures and hanging-wall anticlines along the Wasatch Monocline, a 60-mile synchronous uplift formed during the late Cretaceous-late Eocene.

The north-south, high-angle, basementinvolved ancient Ephraim fault underlies the western margin of the Wasatch, defining the boundary of the Hingeline.

From Late Jurassic through early Tertiary, large-scale thrust sheets were emplaced in central Utah. Major thrust faults include the Canyon Range thrust, Pahvant thrust, Paxton thrust, the Charleston-Nebo thrust system and the Gunnison-Salina thrust.

To the west, the thrust systems are older and moved more than the eastern central Utah thrusts. The Ephraim and other middle Jurassic faults may have gone through additional Laramide-age, Maastrichtian through Eocene movement.

The structurally complex leading edges of the thrusts include numerous thrust splays, back thrusts, duplex systems, faultpropagation folds and ramp anticlines.

Jurassic shales, mudstone and evaporite beds served as glide planes along the hanging-wall flats of the thrust systems.

Most anticlinal closures in central Utah range from three to 12 miles long and one to three miles wide, with up to 4,000 feet of vertical closure.

Hanging-wall anticlines at the eastern margin of the Sevier belt show a similar structural style and reservoir development to the Absaroka thrust in northern Utahsouthwestern Wyoming.

In central Utah, the Jurassic extensional faulting may be a clue to identifying hydrocarbon migration paths and antiformal stacks that contain traps along thrusts.

At Covenant, the Navajo oil reservoir covers about 960 acres.

The field trap is a symmetric, northeasttrending, fault-propagation/fault-bend anticline with nearly 800 feet of structural closure and a 450-foot oil column.

As of early May, Wolverine had not released any details of the Sanpete County drilling results. Several other companies had Hingeline prospects either under study or in the funding stage.

By providence, central Utah should be one of the most interesting exploration areas to watch this year.

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Emphasis: Rocky Mountain Roundup