Edgar B. Heylmun
Tucson
The Coconino plateau of Arizona lies south of the Grand Canyon at an average elevation of about 6,000 ft above sea level and includes more than 2 million acres of rangeland.
In this entire expanse, only four tests have been drilled for oil and gas, and of these, three had shows of oil or gas, and the other was a shallow, inadequate test (Fig. 1).
Parts of the plateau lie within the Kaibab and Coconino National Forests, and the northern fringes lie within Grand Canyon National Park. Areas on the northeast and northwest lie within the Navajo, Havasupai, and Hualapai Indian Reservations, but the rest of the vast region consists of federal, state, railroad, and private lands that can be leased for oil and gas if not already under lease. There is a large amount of state land which, in Arizona, is not put up for competitive bid and can be leased over-the-counter.
Arbitrarily, the Coconino plateau extends from the Little Colorado River on the east to the Aubrey Cliffs on the west and from the Grand Canyon on the north to the San Francisco lava fields on the south. Humphreys Peak, an extinct volcano, is in the San Francisco Mountains and reaches an elevation of 12,670 ft, the highest point in Arizona. Most of the Coconino Plateau consists of undulating rangelands with junipers and pines at elevations above 6,000 ft. Stream-courses have cut canyons that drain northward into the Grand Canyon, but most of the plateau is accessible to 4 x 4 vehicles in dry weather. There are summer thunderstorms and winter snows, but overall, the region remains quite dry. Flagstaff, population 35,000, is the commercial center for the region, and food and lodging are available in Flagstaff, Williams, Grand Canyon, and other localities. The Grand Canyon Railroad operates steam powered passenger trains from Williams to Grand Canyon, through the heart of the Coconino plateau.
GEOLOGY
The Kaibab formation of Permian age forms the surface over most of the Coconino plateau. The unit is dolomitic and cherty and is very resistant to erosion. It forms the rimrock at Grand Canyon and much of the Coconino Plateau is, in fact, a stripped surface on the Kaibab. Due to the massive nature of the formation, it is difficult to obtain accurate dips and strikes, and slumping due to solution further complicates matters. However, the general structure of the region is fairly accurately reflected in the topography of the stripped surface. Topographic highs often indicate upwarps or anticlinal structures, many of which trend in a north or northwest direction. Numerous normal faults are found in the Coconino plateau, but only the north-striking Aubrey Cliffs fault to the west, which is downthrown on the west with several hundred feet of displacement, can be considered a major fault.
Over much of the Coconino plateau, dips are less than 2, and most upwarps and anticlinal features are very subtly expressed on the surface. There are no major angular unconformities in the stratigraphic section, so that structures mapped on the surface should extend to depth. The various structures extend southward beneath the lava flows so that possibilities for oil and gas exist beyond the Coconino Plateau itself.
The Sinclair 1 Santa Fe, drilled in 1951-52, in 35-28n-1w, Coconino County, was located in the middle of the Coconino plateau and provides a typical stratigraphic section for the region. Formation tops are shown in the table.
The total Paleozoic section under the central part of the Coconino plateau is about 3,600 ft in thickness. This could be underlain by a thick section of thermally mature (but not overmature) sedimentary rocks of Precambrian (Proterozoic) age which have recently been recognized by the U.S. Geological Survey in Grand Canyon. These old rocks contain sequences of black, organic shale, so the heretofore ignored Precambrian could be a prime drilling objective under the Coconino Plateau as well as elsewhere in the region.
DRILLING OBJECTIVES
The surface of the Coconino plateau is formed on the Kaibab and Toroweap formations of Permian age, but where these units are adequately buried, they could be prime drilling objectives. They have been responsible for live oil shows at tests in other regions and are productive in southern Utah. The formations were deposited in marine shelf environments and, in the case of the Toroweap formation, tidal flat and sabkha environments as well. In areas of outcrops, the Kaibab often exhibits vuggy and cavernous porosity and is responsible for the presence of sinkholes and collapsed structures. The Kaibab and Toroweap thicken toward the northwest.
The Kaibab and Toroweap formations are underlain by red sandstones, siltstones, and shales of the Coconino, Hermit, and Queantoweap formations which are not considered to be prime drilling objectives despite the presence of porous and permeable sandstones. Dead oil has been observed in these formations in tests drilled elsewhere, but such occurrences are the exception rather than the rule.
The Supai formation of Permian age is a redbed sequence that has interbedded limestone units that may correlate, in part, with the petroliferous Fort Apache limestone to the southeast, and with the Pakoon formation to the northwest. The Supai formation is believed to have been deposited in a regressive marine tidal flat or sabkha environment which could be very favorable for the occurrence of oil.
The Callville formation of Pennsylvanian age consists of interbedded limestone and sandstone units which were deposited in an oscillating marine shelf and lagunal environment. The formation correlates, in part, with the Hermosa formation in the Paradox basin to the northeast, where it is considered to be a prime drilling objective. The thick evaporate units found in the Pennsylvanian to the northeast have not been recognized under the Coconino plateau, but their presence cannot be ruled out. The underlying red Molas shale is widespread and marks a regional unconformity.
Beneath the unconformity, the Redwall limestone of Mississippian age consists of massive crystalline pink and bluff limestones and cherty dolomites that were deposited in a marine shelf environment. The Redwall is responsible for oil production in Utah and must be considered a prime drilling objective. The unit often exhibits vuggy and cavernous porosity. The Redwall correlates, at least in part, with the Madison and Leadville formations to the north and northeast, and with the Escabrosa limestone to the southeast.
The Martin formation consists of interbedded shale and limestone, with a basal sandstone unit, It is Late Devonian in age and is of marine shelf and tidal flat origin. At numerous outcrops in central Arizona, the Martin contains petroliferous limestones, and in many drill holes, it has been responsible for live oil and gas shows. Therefore, the Martin must be considered a prime drilling objective in the region. The basal sandstones and conglomerates of the Martin formation mark a regional unconformity.
Neither Silurian nor Ordovician units have been recognized in northern Arizona, either inoutcrops or in wells. Rocks of Cambrian age underlie the Martin formation in the Coconino Plateau region and consist of mottled marine limestone and dolomite of the Muav formation which, in turn, are underlain by glauconitic and micaceous shales and sandstones of the Bright Angel formation. The basal Cambrian unit in the region is the coarse-grained arkosic and conglomeratic sandstone of the Tapeats sandstone, which unconformably overlies Precambrian sedimentary, metamorphic, and igneous rocks. Whereas oil and gas shows have been encountered in Cambrian units in northern Arizona and southern Utah, they are considered to be only secondary drilling objectives. However, with the discovery of organically rich and thermally mature black shales of Precambrian age in the Grand Canyon, tests drilled in the region should penetrate the Cambrian and test the top of the Precambrian, in order to determine the nature of the older units.
OIL AND GAS SHOWS
At the Sinclair 1 Santa Fe, there were live oil shows in a number of limestone and dolomite zones in the Supai, Callville, Redwall, Martin, and Muav formations between 1,560 ft and 2,902 ft in depth. Drill-stem tests were conducted on oil shows in the Martin formation with negative results and low bottomhole pressures. The low pressures that plague Arizona are not due to the downcutting of the Grand Canyon but are partly due to the lack of intense folding and structural deformation in the region. At the Black Mesa 1-3 Fee, drilled to a depth of 3,670 ft in 27-25n-2e, Coconino County, in 1969, small amounts of gas were encountered in the Martin formation at depths between 2,880 ft and 3,050 ft. The Black Mesa test, just off of Arizona State Highway 64, bottomed in the Tapeats sandstone and was ultimately plugged back and completed as a water well. The Lockhart-1 Babbitt test was drilled to a depth of 3,624 ft in 1948-49 in 21-27n-9e, Coconino County, in the eastern part of the Coconino plateau. The operator reported oil shows in the Martin formation at depths between 3,290 ft and 3,328 ft, but no attempt was made to complete the well as a producer. The test bottomed in Precambrian granite, thus proving that sedimentary rocks containing black shales of Precambrian age are not present in that area. A nearby test did not go to an adequate depth to evaluate the principal objective zones.
It can be seen that of the few tests drilled in the Coconino plateau, most had oil or gas shows. It would certainly appear that additional testing is warranted in such a vast region. Some might contend that the deep downcutting of the Grand Canyon has released all of the oil and pressures, but the writer knows of prolific oil production from the Tensleep sandstone in Wyoming within 7 miles of Tensleep outcrops. The Paleozoic stratigraphic section under the Coconino plateau thickens gradually toward the northwest, thus increasing the number of potentially productive oil and gas zones in that direction.
CONCLUSIONS
The Coconino plateau is a vast region of more than 3,000 sq miles that remains virtually untested for oil and gas. Numerous subtle upwarps and anticlinal features, fault traps, and zones could hold possibilities for fractured oil reservoirs. Stratigraphic traps could be present, especially in formations deposited in tidal flat, lagunal, or sabkha environments. The presence of low bottomhole pressures in one test does not necessarily indicate that pressures would be low in all other tests drilled in the region. Drilling depths are relatively shallow inasmuch as a 4,000 ft drill hole would test the entire stratigraphic section between the Triassic and Precambrian. It would appear that this enormous region deserves further testing for oil and gas.
Copyright 1990 Oil & Gas Journal. All Rights Reserved.
