Saleh M. Billo
King Saud University
Riyadh
Oregon and Washington probably have a number of potentially productive areas such as Bellingham basin, Willamette trough, etc., that will require further exploration.
Many structural provinces may be compared with the major productive parts of the Western Overthrust Belt in the Great Basin, which covers most of Nevada, much of western Utah, and extends into California and Oregon.
Serious consideration should be given to deep drilling in order to test and understand the geometry of the structures that may hold petroleum reserves.
The Vale-Ontario region of Oregon should be explored for gas. Drilling costs are not too high, and the markets are available in Vale, Ontario, and Idaho district.
Very careful advertence should also be rendered for future exploration in the Puget Sound region of Washington for both oil and gas.
The depositional basins between the Miocene mountain ranges, especially in areas away from the coast in Washington deserve circumspection. Olympia, Bellingham, Tacoma, and Seattle provide a market for most of any oil discovered.
INTRODUCTION
Oregon and Washington rank 9th and 19th in size in the U.S., with total areas of about 96,981 and 68,192 sq miles each, respectively.
They lie within the Pacific Border province and consist of a series of mountains along the coast and a chain of troughs or lowlands east of the coastal mountain ranges (Fig. 1).
Oil and gas seeps occur along the Pacific Ocean side of the Olympia peninsula in northwestern Washington and in the Tertiary formations west of the Cascade Mountains in western Oregon and Washington. This paper strives to appraise the hydrocarbon potential of some of the structural provinces in Oregon and Washington.
The quality of the area will be selectively examined for hydrocarbon occurrences, promising source and reservoir rocks, modes of petroleum accumulations, and petroleum economics.
STRATIGRAPHIC SYNTHESIS
Eastern Oregon and Washington have physiographic and geologic characteristics different from those in western Oregon and Washington; likewise, in the north-south direction.
The Coast Range and the Cascade Mountains constitute two nearly parallel north and south structural upwarps with an intervening downwarp represented by the Puget Sound Basin in Washington and the Willamette Valley in Oregon (Fig. 1).
These topographic units extend southward into California as the Coast Range, the Sierra Nevada, and the Great Valley, the last being separated from the Willamette Valley by structural knot containing the Siskiyou Mountains.
SEDIMENTARY COLUMN
Approximately all the pre-Tertiary rocks of Oregon and Washington were metamorphosed before the end of Mesozoic time.
The only exclusions are: 1) about 36,000 indurated but unmetamorphosed marine Mississippian to Upper Cretaceous rocks exposed through fenestrations in basalts in Eastern Oregon, and 2) some thin Jurassic and Cretaceous sediments underlying Tertiary rocks of the Northern Coast Range.
The Mesozoic and Paleozoic rocks are apparently eugeosynclinal in character but altered by diastrophism, intrusion, and possibly regional metamorphism. The only expections are the areas of unmetamorphosed rocks mentioned above.
Much of the Tertiary in Oregon was characterized by volcanic activity in the areas of marine sedimentation as well as on land areas at the east.
Submarine basaltic eruptive rocks up to several thousand feet in thickness comprise much of the lower part of the Middle Eocene.
These are believed to occur throughout the Coastal Range province and probably are correlative with the Middle Eocene basalts of western Washington.
The Tertiary sediments and volcanics are separated from older rocks by a sharp angular and erosional unconformity. During Late Mesozoic and Tertiary time, uplift followed by erosion caused the removal of large areas of metamorphosed sediments and the creation of a pre-Miocene surface of great relief.
STRUCTURE
The area typifies the major tectonic features of the North American Cordillera which includes all the ranges and plateaus west of the Great Plains and the coastal lowlands of Mexico.
The tectonic map of the U.S. shows that actually the structure is that of a number of basins separated by uplifts or arches (Fig. 2).
The Cordilleran ranges and plateaus stand for a broad assemblage of mountain ranges related to orogenic belts of different ages, together with their associated plateaus and intermontane basins (Fig. 2).
The Cordilleran geosyncline had already started to develop before the Cambrian. Oscillating downwarps and the accretion of great thickness of sedimentary and volcanic deposits lingered nearly up to the end of the Devonian.
The upwarp began, attaining its climax during the Mississippian with the uplift of the Antler orogenic belt in central Nevada, north-northeast across the Great Basin, a part of the Basin and Range province, exhibiting alternations of uplifts and downwarps in meridional or near-meridional trends (McCaslin, 1987).
The Overthrust and Disturbed belt (Fig. 3) is a promising area for exploration because of the presence of structures for entrapping oil and gas (Woodward, 1981).
The Antler orogeny involved the sedimentary and volcanic beds in recumbent folds and in low-angle thrusts with horizontal displacements of tens of miles. The Antler orogenic belt was formerly known as the Manhattan geoanticline when its extent was unknown (Fig. 3).
Recent data reveal that the belt should swing eastward into central Idaho (Fig. 3).
There are thrusts of 100 miles, intrusions, Tertiary Basin and Range structures, Pennsylvanian folding, Tertiary volcanics, and Mesozoic intrusions to complicate the analysis.
These rocks are divided into a western assemblage or eugeosynclinal, and an eastern assemblage or miogeosynclinal.
The transitional assemblage includes elements of both in parautochthonous windows beneath the Robert Mountains thrust.
The Antler orogenic belt was the locus of intense folding and faulting that culminated in the Roberts Mountains thrust fault in Late Devonian or Early Mississippian time (Fig. 3).
Mississippian rocks probably once covered the entire region, but were thinnest over the transcontinental arch and the incipient Ancestral Rockies (Billo, 1985). Vertical uplift supervened during mid-Pennsylvanian.
Upper Mississippian and Lower Pennsylvanian rocks were stripped from the Antler orogenic belt. Permian rocks go entirely over the orogenic belt (Fig. 3).
Crustal movements of Late Miocene age yield northwest-southeast folds in Washington, and north-south folds in the Coast Range of Oregon.
Major north-south upwarps and downfolds at the close of the Pliocene were superimposed on the earlier structures with the resultant development of the Coast Range, Cascade Mountains, and the intervening trough.
OIL AND GAS OCCURRENCES
Traces of oil in the form of very small seepages occur in the Tertiary formations west of the Cascade Mountains.
There is no conclusive evidence that commercial amounts of oil are present in these formations although licensed drilling tests under geologic surveillance may be favored.
EXPLORATION RESULTS
Benton County gas field in the Rattlesnake Hills, eastern Washington, produced methane gas (99.35%) from a large, northwest trending, faulted asymmetrical anticline in basalts. Gas apparently burst forth from buried terrestrial deposits.
Bellingham field in western Washington produces methane from Pleistocene glacial debris. The beds below the glacial till are continental in origin and contain coal seams.
High methane gas at 600 psi and shows of oil have also been reported from Black Diamond field in Washington 20 miles south of Seattle.
Exploration was preliminary for beds of Eocene coal in the Kummer anticline.
Traces of oil and gas were detected from nine wells that were drilled on a faulted anticline in Ocean City field in Grays Harbor County, Wash. The structure map shows 400 ft of closure.
Sunshine Mining Co.'s 1 Medina, the only producer, obtained production from a Blakely sandstone at 3,95258 ft.
The initial flow of 400 b/d of oil was down to 157 b/d in August 1957 and 4-5 b/d at the end of 1960. Some 12,000 bbl of oil had been produced by the end of 1960.
Methane is produced from deep artesian wells in the northwest part of the Snake River downwarp near Vale and Ontario, Ore. Although the gas is used locally, no commercial production has been attained.
EXPLORATION METHODS
Some of the exploration methods (excluding wildcat drilling) presently in use would not be of much profit in a region covered with glacial debris and where the sediments are intercalated with basalt flows and sills.
There is no difficulty in getting drilling rigs to areas and then in keeping them supplied; besides, markets are readily available. However, drilling costs are very high.
The basaltic flows, sills, dikes, in addition to the overburden of glacial till cause obstacles for seismic exploration, and gravity methods are encumbered by laccoliths.
Drilling through the unconsolidated boulder till will be expensive, and keeping and bracing the holes through the Ocean City shales may be both arduous and costly.
Deep zones of weathering, vegetation cover, and forests of huge trees will also raise the charges of exploration.
PETROLEUM POTENTIAL
Paleozoic rocks in Eastern Oregon are 3,300 ft thick.
The Mesozoic rocks are 32,000 ft thick, of which 65% is black shale, 30% is sandstone, and 5% is limestone; should they be found in relatively undisturbed condition, they would make an excellent subject for petroleum exploration.
These rocks are closely folded into a narrow trough that appears not to extend northward much beyond the southerly extent of the outcrops.
The unmetamorphosed rocks are limited southwestward by metamorphism in the Blue Mountains to the east but seem to trend southward beneath the lavas toward the Klamath Mountains. The extent of these unmetamorphosed rocks is very restricted.
Outcrops of Cretaceous and Jurassic unmetamorphosed sedimentary rocks are bounded on the south by the extensively metamorphosed rocks of the Klamath Mountains, on the east by Cascade Mountains volcanics, on the west by the Pacific Ocean, and on the north by an irregular line where they plunge beneath Tertiary and sedimentary rocks.
The pre-Tertiary rocks outcropping in the Olympic and Cascade Mountains in western Washington are all highly metamorphosed and unfavorable for hydrocarbons.
Similarly, in eastern Washington, all present evidence shows that nothing but metamorphic rocks devoid of oil and gas underlie the Tertiary rocks of the eastern part of the state.
The outcrops around the edges of the Columbia River Plateau, are all extensively metamorphosed.
PETROLEUM RECOVERY
Exploration in Oregon has recently been concentrated on development of Mist gas field in Columbia County.
Mist field lies on the highly faulted nose of a northeast-plunging anticline (Fig. 1). The geology is such that most pools are small, and almost every successful well is considered to be a discovery (Mason, 1988).
The annual production at Mist field averaged 4.6 bcf, and the trend appears on the rise.
While drilling in Oregon and Washington on the whole continued to decline, leasing propensity in the latter state has significantly increased.
State leasing currently surmounts federal lands acquisition.
Shell and Exxon triumphed as the most active operators in Washington, and drilling performance in Oregon scored its best results ever.
ACKNOWLEDGMENTS
This paper has been selected for poster presentation during the AAPG 1991 Circum-Pacific Conference in Honolulu. King Saud University supported the work.
BIBLIOGRAPHY
Billo, Saleh M., The Transcontinental arch and its relation to the Colorado oil and mineral belt, Journal of Petroleum Geology, Vol. 8, No. 3, 1985, pp. 343-352.
Mason, T.D., Oil and Gas Developments in West Coast in 1987, AAPG Bull., Vol. 72/10B, 1988 pp. 149-157.
McCaslin, John C., Eastern geologist gathers Great Basin data, OGJ, June 29, 1987, p. 73.
Woodward, Lee A., Hydrocarbon potential of the Montana thrust and fold belt, World Oil, Vol. 193/2, 1981, pp. 97-105.
Copyright 1990 Oil & Gas Journal. All Rights Reserved.