Daniel J. Brennan
Regulatory Compliance Specialists
Scottsdale, Ariz.
A recent application by United Gas Search Inc., Tulsa, for a permit to drill in extreme northwestern Arizona has sparked interest in that area, largely unknown in the oil business.
The area is known locally as the Arizona strip. It lies north and northwest of the Grand Canyon (Fig. 1).
There is not even a road to the strip from the rest of Arizona. Access is by road from Utah or Nevada.
Just what is there to cause a company to go to the trouble of drilling an 11,000 ft well so far from production and from sources of equipment and supply? With oil and gas prices as low as they are, do potential reserves economically justify stepping out so far?
At a recent U.S. Bureau of Land Management lease sale, one that saw the first competitive bidding on federal land in Arizona, United Gas Search bid successfully on three tracts totaling 5,122.32 acres. The company paid $7/acre for the largest tract of 2,082 acres and an average of $3.26/acre for the remaining 3,040 acres.
United in June filed an application to drill a 6,500 ft wildcat in 11-41n-11w, in Mohave County, Ariz., and later extended the depth objective. The site is about 10 miles south-southeast of St. George, Utah.
OIL, GAS POTENTIAL
Shows of oil or gas are very common in the area, as are reports of surface seeps.
Giardina listed 11 wells in Mohave County with reported shows, including oil stain in samples, fluorescence and cut, or tests of noncommercial amounts of oil.
Thirteen deep wells and their shows are listed (Table 1). Many shallower wells have been drilled in the area but were not used in compiling this report.
Virgin field, in 41s-12w (Fig. 1), was Utah's first oil field. Discovered in 1907, it continues to produce. Production is from the basal member of the Permian Timpoweap formation at 500-800 ft.
Cumulative production from the more than 150 wells drilled in the field area is probably somewhere in excess of 200,000 bbl of paraffin base oil.
The first drilling in the Virgin field area was stimulated by the presence of several oil seeps and by outcrops of oil bearing sands.
Anderson Junction field, in 40s-13w, was discovered in 1968. Cumulative production through 1988 was only 2,733 bbl of oil and 16.388 MMcf of gas. Pennsylvanian Callville limestone is the producing horizon.
The wells were apparently situated on an east-dipping fault block, as is United Gas Search's proposed location.
STRATIGRAPHY
Geologic maps of Mohave County, Ariz., and Southwest Utah show that Triassic Moenkopi and Permian Kaibab limestone cover most of the surface.
Permian Coconino sandstone is exposed on the east side in the Hurricane Cliffs as is the Permian Hermit shale.
In Utah, to the north and west, younger Triassic, Jurassic, and Cretaceous rocks are exposed at the surface.
This region is near or over the Paleozoic shelf and slope edge, between shallow water deposits on the east and deepwater sediments on the west. As a consequence of the sparse drilling, facies relationships have not been worked out in detail.
A typical stratigraphic column is shown where it is related to a composite log representative of the region (Fig. 3).
It may be said that in general rocks on the west are thicker and more nearly totally marine, while to the east the rocks grade into shoreline and continental facies. Evaporites are common in the transition from marine to continental.
Over time the shoreline shifted back and forth from east to west with changes in relative sea level but with an overall westerly movement. As a result lateral and vertical facies relations are complex and are greatly simplified here.
Triassic Moenkopi, which crops out over most of this area, is composed of six members. Only the lowermost member is of interest in this report.
That member, the Timpoweap, is productive in Virgin field to the north. It is composed of a basal chert pebble conglomerate with overlying red siltstone and gray limestone. Other members consist of similar facies, with local gypsum beds.
PERMIAN FORMATIONS
The Kaibab formation is predominantly limestone with nodular and layered chert. Near the top and at the base evaporates are present, Electric logs show some interbedded shale layers (Fig. 2).
Beneath the Kaibab is found the Toroweap formation, also of Permian age. The Toroweap resembles the Kaibab in the types of rocks it contains but is much more variable vertically as shown on the electric log. Once again, the base is largely evaporitic.
Both Kaibab and Toroweap contain discontinuous limestone and dolomitic beds, some capped by shale, others by evaporates. Any of these could serve as reservoirs for oil or gas. Numerous shows have been reported from this part of the section from wells drilled in the area.
Underlying the Toroweap, the Permian Coconino sandstone, 50-100 ft thick, is a consistent marker over much of the area. While it may be interpreted as a blanket sandstone, in fact, the Coconino contains within itself numerous massive crossbeds that may serve as seals for individual sand compartments.
Each sand compartment has the potential to trap oil and gas and give them up in vast amounts. The porosity of the Coconino is unusually high, and permeability has been excellent in the few fields where it is productive.
The Hermit "shale" underlies Conconino. It, too, is Permian. Thickness varies from about 350 ft in the eastern part of the map area to about 1,000 ft to the west in Nevada.
The Hermit is composed of red-brown shale, shaly sand, and sandstone. It is probably too impermeable to offer good oil and gas objectives.
The Queantoweap formation, which is the western equivalent of the Supai of the Paradox basin, is present under the Hermit. The Queantoweap consists of thick sequence of horizontal and cross bedded yellow-brown to dull red sandstones. Thickness ranges from about 400 ft to perhaps more than 1,000 ft on the west.
The lowermost Permian formation in the area is the Pakoon.
PENNSYLVANIAN ROCKS
It is difficult to separate the Pakoon from the underlying Callville formation of Pennsylvanian age.
The author's correlations show a range of 280-500 ft of thickness for the Pakoon. Lithologically, it is composed of marine limestone and dolomite, with some evaporates to the west. Regionally the Pakoon seems to thin out and disappear in central Arizona.
Pennsylvanian Callville carbonates are found beneath the Pakoon and extend further east before thinning out to a feather edge. The Caliville contains bedded and nodular chert in part. A few limited occurrences of evaporites are known.
The Pakoon and Callville may be considered more than marine-influenced lateral equivalents of the Permian Supai and Pennsylvanian Hermosa formations of the Paradox basin.
The Pakoon the Callville formations are believed to have been deposited near the edge of a Pennsylvanian shelf.
They have the potential to include shallow water carbonate bars and biohermal mounds, which produce in the Paradox basin.
MISSISSIPPIAN, DEVONIAN
The top of the Mississippian Redwall limestone is a profound unconformity in eastern and central Arizona, but it may diminish in time significance in this region.
There is a suggestion that Chester (uppermost Mississippian) rocks may be present in the western part of the area. If this is so, it implies the potential for numerous stratigraphic traps at the Mississippian-Pennsylvanian unconformity.
With deeper water and shaly facies known to have been present to the west in Nevada, the potential for updip migration and trapping is enormous.
The Mississippian of the Arizona subsurface is usually subdivided into four units with a thickness of about Boo ft.
The uppermost contains porosity related to surface exposure and karstification. The second unit is usually identified on electric logs by a thick zone of carbonate porosity. This is usually described in sample logs as crystalline, vuggy dolomite. Both of these zones are potential reservoirs.
Devonian and older rocks are not well known in this area. Only a few wells have been drilled deep enough to encounter tham.
The Devonian is called Temple Butte or Martin by different operators. It is usually described as marine limestone that rests on varicolored shales on a surface eroded into Cambrian deposits.
Ordovician and Silurian have not been identified in this area.
Cambrian formations in the area are believed to include the Muay limestone, Bright Angel shale, and Tapeats sandstone.
Their potential as reservoir beds depends on the presence of source rocks, migration paths, and seals.
STRUCTURE
The surface in this region is composed of a series of cliffs, the Hurricane cliffs on the east and several smaller cliffs stepping the topography generally down from the west.
Even though there are several cliffs, the surface bedrock units are nearly the same across the region. Permian Kaibab-Toroweap and Triassic Moenkopi comprise most of the outcrops.
Over much of the area bedrock is concealed by talus from the cliffs and by several stages of Tertiary and Quaternary deposits and locally by lava flows.
Nonetheless, outcrops are more than adequate to define surface structure and stratigraphy.
A simplified surface structure map of the area shows an abundance of mostly north-trending, mostly down to the west faults (Fig. 2). The principal faults in the area are the Hurricane fault zone and the Washington fault.
United's location lies just east of the Washington fault. Surface dips are easterly, making the prospect appear to be a simple tilted fault block.
Closure is provided by regional dip to the east, northeast, and southeast and by the fault on the west.
The location of the fault zones near or at the margin of the Colorado plateau suggests that the faults are related to the uplift of the plateau and may be simple, planar normal faults.
The consistent tilting of the fault blocks to the east brings up the possibility that the faults may indeed be marginal to the plateau and related to the uplift, but instead of planar faults we may be seeing a series of listric faults whose dip becomes less and less with increasing depth.
THRUST FAULT HYPOTHESIS
Another possibility is that the faults are somehow an indication of a concealed series of thrust faults: the Laramide thrust belt.
This could be so if the faults were originally thrusts and have undergone subsequent reversal of apparent movement by crustal extension after the conclusion of the Laramide compressional event (Fig. 3).
The thrust faulting hypothesis receives much support in the form of a sample log in the files of the Utah Department of Oil, Gas, and Minerals. The log is interpreted to show a repetition of Mississippian in the California Co. 1 St. George Unit, in 19-43s-15w, Washington County, Utah.
This interpretation differs markedly from the tops released by the California Co. in 1951 when the well was abandoned.
At that time the well was the only deep test for many miles, and knowledge of the geology was still relatively unrefined.
The thrust faulting interpretation is shown on the accompanying cross section (Fig. 3). The interpretation may not be significant in regard to the proposed United Gas Search test, which appears to be in a separate fault block.
But so much is unknown in this region that it is conceivable that additional thrust faults may be present almost anywhere. The cross section has a vertical exaggeration of about five times, and that makes the faults as drawn have apparent dip steeper than exists in nature. Flatter dips would invite a stronger bias toward the thrusting hypothesis.
EXPLORATION RESURGENCE?
The industry went through one round of excited exploration of the thrust belt in the early 1980s, with considerable disappointment in southern Utah and in Arizona.
Perhaps with United's wildcat we are seeing the beginning of a new round of exploration that will shed more light on the whereabouts of the thrust belt.
Developments in Nevada have built up a store of optimism for the potential of Paleozoic objectives in the whole region.
The development of fields in complexly faulted terrain there may presage similar developments in northern Arizona and southern Utah.
REFERENCE
Giardina, S.i. Jr., Geologic Review of Northwestern Arizona for Petroleum Exploration Investigators, Oil & Gas Conservation commission Special Publication 4, Arizona oil & Gas Conservation commission, Phoenix, 1979, 72 p.
Copyright 1991 Oil & Gas Journal. All Rights Reserved.
