Oil sand
A loose-to-consolidated sandstone or a porous carbonate rock, impregnated with bitumen, a heavy asphaltic crude oil with a viscosity under reservoir conditions greater than 10,000 millipascal seconds (10,000 centipoise); also known as tar sand or bituminous sand. See also: Asphalt and asphaltite
Heavy-oil reservoirs are distributed throughout the world and were formerly categorized in many different and sometimes conflicting ways. Under the leadership of the United Nations Institute for Training and Research (UNITAR), modern definitions for these heavy substances have emerged: (1) Bitumens (oil sand hydrocarbons) possess viscosities greater than 10,000 mPa · s (10,000 cP) and cannot be produced by conventional methods. (2) Extra-heavy crude oils possess viscosities equal to or less than 10,000 mPa · s (10,000 cP), and an American Petroleum Institute (API) gravity less than 10°. They are heavier than water but have some mobility under reservoir conditions. (3) Heavy crude oils possess API gravities ranging from 10 to 20°.
About 175 experimental projects for the enhanced recovery of heavy oil and bitumen are in operation around the world and, because similar technologies are being used, reserve and recovery estimates for the three categories—heavy crude, extra-heavy crude, and bitumen—frequently are lumped together.
Estimates of the combined world reserves of heavy crude and bitumen exceed 5 × 1012 bbl (8 × 1011 m3), with bitumen accounting for about half the total. The seven largest of these heavy crude and bitumen deposits contain almost as much oil in place as the world's 300 largest conventional oil fields. The largest proven accumulation of bitumen-containing oil sands occurs in Alberta, Canada, but deposits in Russia rank close behind. Much of the huge accumulation in Venezuela's Orinoco Basin has been reclassified as heavy and extra-heavy crude oil, but this area also, seemingly, contains substantial reserves of bitumen. Smaller reserves occur, in descending order, in the United States, Madagascar, Italy, Albania, Trinidad, and Romania. Estimates of the bitumen that will prove ultimately to be recoverable—generally of the order of 5–25%—are speculative and depend on the development of successful technologies at competitive costs.
Geological factors
The major oil sand and heavy oil deposits are remarkable not only for their size but also for their unconventional geologic settings. The deposits generally exhibit certain characteristics. (1) They tend to occur in a deltaic or nonmarine environment, featuring extensive, fingerlike, fluvial sands with large porosities and permeabilities. Porosities, often of the order of 25–35%, tend to be considerably greater than in most petroleum reservoir sandstones (5–20%). A lack of mineral cement, which occupies the void space in most sandstones, permits these high porosities and gives rise to the term oil sands. (2) The deposits are covered by a widespread regional shale cap which restrained upward escape of the oil migrating into the ancient river channels, the efficient systems that gathered the subsurface fluids. (3) The deposits are emplaced in a gently dipping homocline with updip stratigraphic convergence. (4) Degradation of the original, lighter crude to heavy oil has been effected by water washing and bacterial action. Considerable debate continues about the origin of the heavy oils and whether they are thermally mature or immature. The evidence suggests, however, that the oils were degraded in place.
While most of the world's heavy-oil reservoirs are found in deltaic settings, there are significant exceptions. These exceptions include the Cambrian and Lower Permian carbonate rocks, which host the bulk of Russia deposits, and the Paleozoic Carbonate Triangle in northern Alberta. See also: Petroleum geology
Alberta deposits
Canada's Alberta Oil Sands, comprising four enormous and a number of lesser deposits, contain the best known of the world's oil sand reservoirs. Established resources are of the order of 1.25 × 1012 bbl (2 × 1011 m3). The Athabasca Tar Sands deposit is the largest known petroleum accumulation in the world, with a total area of 13,000 mi2 (34,000 km2) and in-place reserves of 9.19 × 1011 bbl (1.46 × 1011 m3). Cold Lake, rapidly gaining prominence for its in-place projects, contains 2.05 × 1011 bbl (3.26 × 1010 m3); Peace River, 7.5 × 1010 bbl (1.2 × 1010 m3); and Wabasca, 4.26 × 1010 bbl (6.8 × 109 m3). The deposits range across the northern part of the province (see illus.).
Tar sand deposits in Alberta. 1 mi = 1.6 km.
The Athabasca Oil Sands are located in the McMurray Formation, of Lower Cretaceous age. The formation outcrops along the Athabasca River, north of the city of Fort McMurray. Elsewhere, the deposit is buried under a variable layer of overburden which reaches up to 1720 ft (525 m) in thickness. Fortunately, some of the richest parts of the deposit are covered by the thinnest overburden.
Russian deposits
Hundreds of bitumen occurrences have been charted in Russia; about 70% are found in carbonate rock. Estimates of reserves approximate 1.15 × 1012 bbl (1.8 × 1011 m3). Of considerable interest is the Olenek anticline in northeastern Siberia where, it is speculated, reserves total 6 × 1011 bbl (9.5 × 1010 m3). Huge deposits have also been discovered near Baku in Kazakhstan and at Melekess in Volga-Ural. Yarega, in Siberia's Timan-Pechora province, is the only place in the world where underground mining for commercial production of heavy oil is carried out.
Venezuela deposits
Enormous reserves of heavy oil occur in a 36-mi-wide (60-km) band stretching along the northern bank of the Orinoco River for a distance of 400 mi (700 km). The deposits, for many years known collectively as the Orinoco Tar Belt, contain between 7 × 1011 bbl (1.1 × 1011 m3) and 1 × 1012 bbl (1.6 × 1011 m3) of heavy hydrocarbons. Some of the crude possesses an API gravity of between 10 and 20°, but most is between 4 and 10°. Most of the heaviest oil, however, shows some mobility because of the below-normal asphaltene content, the temperature of the reservoir, and the presence of dissolved gases. For this reason, the Venezuelan government encourages the use of the term Orinoco Heavy Oil Belt, instead of Tar Belt, to describe these deposits. It is believed, though, that substantial reserves, perhaps as much as one-third of the total hydrocarbons, are too viscous to flow to the well bore and may therefore be categorized as bitumen, or tar oil.
United States deposits
In the United States, 550 tar sand occurrences are known to exist in 22 states, and it is expected the number will increase significantly in future surveys. Seven states (Alabama, California, Kentucky, New Mexico, Texas, Utah, and Wyoming) possess deposits aggregating 1 × 106 bbl (1.6 × 105 m3) or more. Total United States reserve estimates have remained relatively constant since the mid-1970s at approximately 3 × 1010 bbl (4.8 × 109 m3), but considerable changes have occurred in the amounts ascribed to the various states.
Utah is the most important tar sand state with an estimated 1.47 × 1010 bbl (2.3 × 109 m3) of bitumen in place, or about half of the United States total. At one time, though, it was believed that more than 90% of United States tar sand reserves were located there. Over 50 deposits have been identified, but the vast bulk of the oil occurs in six giant deposits, four of which are in the Uinta Basin of northeastern Utah. Three of these (P. R. Spring, Hill Creek, and Sunnyside) occur in the Green River Formation (Eocene), but Asphalt Ridge, probably the best-known tar sand deposit in the country, occurs in older strata.
The Tar Sand Triangle, with reserves of approximately 6.1 × 109 bbl (9.6 × 108 m3), the largest deposit in the United States, lies in a remote and very rugged area of northeastern Utah, in the White Rim Sandstone Formation. Most of the deposit underlies Federal Recreation and Wilderness areas. Federal agencies also control major sections of the P. R. Spring, Hill Creek, and Sunnyside deposits. The other major ore body, Circle Cliffs, is also located in southeastern Utah.
The thickness of the Utah reservoirs ranges between 0 and 330 ft (0 and 100 m), and the depth ranges from 0 to 2200 ft (0 to 670 m). Some interesting characteristics distinguish the reservoirs: the bitumen occurs in consolidated sandstone; the connate water, at 0.6%, and the sulfur content of the hydrocarbon, at 0.5%, are about one-tenth of the corresponding values in the Canadian and Venezuelan oil sands. However, the porosity of the formations and the continuity and magnitude of the bitumen saturation are far less. Economic exploitation is therefore in question.
In terms of United States reserves, Alabama has moved into second place, with 4.3 × 109 bbl (6.8 × 108 m3) attributed to the North Area. Texas (Uvalde area) and California are next, each with 3 × 109 bbl (4.8 × 108 m3).
In California, the reserves are contained in a number of well-known deposits, Santa Maria, with 2 × 109 bbl (3.2 × 108 m3), being the largest. The Edna deposit, located midway between Los Angeles and San Francisco, is a rarity among oil sand deposits in that it is considered a marine facies. The Sisquoc deposit is amenable to the hot water extraction process used by the large Canadian projects. The fossiliferous McKittrick deposit is more properly considered a diatomaceous oil shale. See also: Oil shale
Other major world deposits
The Bemolanga deposit in western Madagascar is noteworthy. Covering approximately 154 mi2 (400 km2), it contains reserves estimated at 2.1 × 1010 bbl (3.3 × 109 m3). Another country with multibillion-barrel oil sand deposits is Italy; the Ragusa area in Sicily contains 1.4 × 1010 bbl (2.2 × 109 m3) of bitumen.
Bibliography
C. J. Borregales, Production characteristics and oil recovery in the Orinoco Oil Belt, UNITAR 1st International Conference on the Future of Heavy Crude and Tar Sands, Edmonton, Alberta, 1979
M. Danyluk, B. Galbraith, and R. Omana, Department of Defense, Toward definitions for heavy crude oil and tar sands, United Nations Institute of Training and Research (UNITAR) 2d International Conference on the Future of Heavy Crude and Tar Sands, Caracas, Venezuela, 1982
G. J. Demaison, Tar Sands and Supergiant Oil Fields, Can. Inst. Min. Metallurgy Spec. Vol. 17, 1977
V. A. Kuuskraa, S. Chalton, and T. M. Doscher, The Economic Potential of Domestic (U.S.) Tar Sands, U.S. Department of Defense, January 1978
R. F. Meyer, P. A. Fulton, and W. D. Dietzman, A preliminary estimate of world heavy crude oil and bitumen resources, UNITAR 2d International Conference on the Future of Heavy Crude and Tar Sands, Caracas, Venezuela, 1982
G. D. Mossop, Geology of the Athabasca Oil Sands, SCIENCE Reprint Series, January 1980
Ali Fazeli = egeology.blogfa.com
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