David L. Allin
Consulting geologist
Salt Lake City
Explorationists have been frustrated from the start in their efforts to discover and develop commercial petroleum reserves in the Colorado Plateau province of Utah. Can improvements be made, or is this a snipe hunt?
The region features large, Laramide structures such as the San Rafael swell, Monument uplift, Circle Cliffs uplift, Kaiparowits basin, Kaibab uplift, and myriad third order structures. Three of the four uplifts mentioned are hosts of extensive bituminous sandstone or tar deposits on outcrop that are relics of giant oil reservoirs in existence until exhumation within the last 1 million years.
EXPLORATION HISTORY
The pioneer oil finding team with Ohio Oil Co. must have been quite enthused when it mapped and leased the Circle Cliffs uplift just after World War I. This classic, sheepherder anticline has 2,000 ft of closure at the top of Permian encompassing 400,000 acres. Furthermore, it was covered with oil shows of bituminous sandstone in the lower Triassic and upper Permian outcrops that was later estimated to contain 1.3 billion bbl of oil.
Then, imagine the befuddlement the intrepid Ohio team must have experienced when it had to abandon a showless 3,212 ft Mississippian test on the crest of the uplift in 1921 surrounded by all that tar.
Post World War II exploratory efforts were rewarded with substantial shows of oil in both Permian Kaibab dolomites and Mississippian platform dolomitic rocks of the Mooney Falls member of Redwall limestone, but they lacked commercially exploitable volume. Upper Valley anticline was production tested by California Co. in 1951 until 10,000 bbl of 14 gravity oil were recovered from a zone below 8,777 ft.
Many operators found recurring subsurface characteristics from well to well in the Colorado Plateau. The entire stratigraphic column was saturated with fresh water. Hydrostatic heads of fluid columns were substantially subnormal, as much as 60% below normal. Porous zones encountered in wells drilled on anticline crests and axes contained dead oil, residual oil, glauconite, and pyrite. Natural gas was absent except carbon dioxide in structures beneath the volcanic fields of the High Plateaus near Escalante, Utah, north of the Kaiparowits basin (Table 1, stratigraphic column, and shows).
RED HERRING?
All exploration did not go unrewarded. A commercial petroleum discovery was made in this region by Tenneco on the Upper Valley anticline in 1964, 13 years after California Co.'s Mississippian oil production testing there. During the next few years, Tenneco developed a dolomitized reservoir in the Triassic Timpoweap and Permian Kaibab formations with 25 wells that have produced 20 million bbl of oil. As field development progressed, it became apparent that the densest oil in the reservoir was located 6 miles south of and 900 ft low to the crest of the structure, and a great portion of the potential reservoir area was flushed (Fig. 1).
The figure clearly points up some unusual characteristics of the Upper Valley field. Penetrations of crestal and axial areas of the structure found only fresh water, oil staining, and pyrite in the pool porosity zones. Reservoir pressures were 50% below normal, and no natural gas was encountered. It becomes apparent that hydrological conditions in the reservoir are not hydrostatic but are in fact strongly hydrodynamic.
HYDROLOGICAL CONSIDERATIONS
Fig. 1 suggests a vector for Permian water flow and what the location of the oil/water contact was prior to commencement of water washing. At Upper Valley field, the water movement is to the south-southwest, away from Permian recharge areas in the volcanic capped High Plateaus to the north and the Circle Cliffs uplift to the east. The fresh water has removed or reduced hydrocarbon molecules from the oil up to C15, which is in keeping with laboratory studies by LaFargue et al., 1988.
Exploratory drilling in the region by operators in the 20 years following development of Upper Valley field have failed to locate any other commercial occurrences of petroleum. A regional synthesis of hydrological information points up some of the reasons for these results.
Prior to exhumation of the Colorado Plateau in southern Utah and introduction of fresh water flow deep into the stratigraphic section, the crests of the indigenous structures or structural-stratigraphic traps were filled with petroleum as evidenced by the massive relic oil deposits on the Monument, San Rafael, and Circle Cliffs uplifts as well as the live oil at Upper Valley.
The pre-exhumation petroleum pool map for the Paleozoic section in the area with hydrostatic ground water conditions is reconstructed on a Permian structure map by Irwin, 1976 (Fig. 2, Part A).
Exhumation in the last million years has caused radical changes to the depicted hydrostatic picture. At least 50% of the oil that migrated vertically upsection in the uplifts to the Permo-Triassic unconformity was lost to the Colorado River drainage, and the remainder was moved back down dip and the light ends were washed away by the influx of fresh water.
This process left 13 billion bbl in the Tar Sand Triangle on the plunge of the Monument uplift, 500 million bbl on the San Rafael swell, and 1.3 billion bbl on the Circle Cliffs uplift.
Simultaneously, the oil columns in unbreached structures such as the Upper Valley anticline underwent washing and displacement into structural sites not normally envisioned by petroleum wildcatters, even though analogs exist in the Laramide structures of the Big Horn basin of Wyoming. This brings us to the present day Paleozoic oil pool map (Fig. 2, Part B).
Part B vividly illustrates that the few dozen wildcat wells drilled on structures in the region of Garfield and Kane counties, Utah., missed oil accumulations in the Permian section as well as potentially prolific Mississippian and Devonian reservoirs where similar hydrodynamic conditions exist. Part B also indicates water flow directions from recharge areas supported by sparse drillstem test data used to calculate hydrostatic head in Carboniferous rocks (from Teller et al., 1986).
FUTURE EXPLORATION
Prediction of subsurface water flow and the direction and relocation of oil columns will greatly enhance the return from exploratory drilling on the Colorado Plateau. There appears to be great potential for missed petroleum reservoirs sited on structural flanks that are not breached into Permian.
Additional targets lie on structures that contain seal sequences that impeded vertical migration of hydrocarbons to the present day surface. The largest structure in the Colorado Plateau that has not leaked and covered itself with bituminous sandstone is the Kaibab uplift in Kane County, Utah, and Coconino County, Ariz.
It is of further importance to note that current research being conducted on the Precambrian Chuar group could be on the road to identifying the source for the prodigious volumes of relic oil seen on Colorado Plateau structures in Utah.
It would appear that vertical migration from the hundreds of meters of laminated mudstones in the Proterozoic Chuar group (as mapped by Rauzi, 1990) could be a source rock of the proper richness and volume to charge these massive structures with petroleum that was later relocated or breached in recent time.
Palacas et al. 1989 reported that 281 m of a member of the Chuar group averaged 3% total organic carbon content (7% peak) with genetic potential averaging 6,000 ppm (16,000 ppm peak) were within the oil generation window in the Grand Canyon of Arizona.
It is time to give the Colorado Plateau of Utah another long, hard look, past the minable bituminous sandstones and to learn and profit from Tenneco's experience at Upper Valley field by using a new concept derived from a new data set.
BIBLIOGRAPHY
Dahlberg, E.C., 1982, Applied Hydrodynamics in Petroleum Exploration: New York, Heidelberg, Berlin, Springer-Verlag, 161 p.
Irwin, C.D., 1976, Permian and Lower Triassic Reservoir Rocks of Central Utah, in J.G. Hill, ed., Symposium on Geology of the Cordilleran Hingeline: Rocky Mountain Association of Geologists, pp. 193-202.
LaFargue, E. and C. Barker, 1988, Effect of Water Washing on Crude Oil Compositions: AAPG Bull., Vol. 72, No. 3, pp. 263-276.
Palaces, J.G. and M.W. Reynolds, 1989, Preliminary Petroleum Source Rock Assessment of Upper Proterozoic Chuar Group, Grand Canyon, Ariz. (abs.): AAPG Bull., Vol. 73, No. 3, p. 397.
Rauzi, S.L., 1990, Distribution of Proterozoic Hydrocarbon Source Rock in Northern Arizona and Southern Utah: Phoenix, Arizona Oil & Gas Conservation Commission, 38 p.
Teller, R.W. and D.T. Chafin, 1986, Selected Drillstem Test Data for the Upper Colorado River Basin: Lakewood, USGS WRI Report 84-4146, 112 p.
Copyright 1990 Oil & Gas Journal. All Rights Reserved.
