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Tar sands (also called bituminous sands and oil sands) have, in general, been defined as reservoirs containing oil too [http://pubs.usgs.gov/fs/fs070-03/fs070-03.html viscous] to flow into a well in sufficient quantities for economic production or, in other words, oil essentially immobile in the reservoir. Several more specific definitions have been proposed, but none has been generally accepted. A United Nations Institute for Training and Research (UNITAR) Working Group on Definitions<ref name=Danyluketal_1984>Danyluk, M., B. Galbraith, and R. Oma&ntilde;a, 1984, Toward definitions for heavy crude oil and tar sands, ''in'' R. F. Meyer, J. C. Wynn, and J. C. Olson, eds., The future of heavy crude and tar sands: Second International United Nations Institute for Training and Research International Conference (Caracas, Venezuela, February 1982): New York, McGraw Hill, p. 3-6.</ref><ref name=Martinez_1984>Martinez, A. R., 1984, Report on Working Group on Definitions, ''in'' R. F. Meyer, J. C. Wynn, and J. C. Olson, eds., The future of heavy crude and tar sands: Second International United Nations Institute for Training and Research International Conference (Caracas, Venezuela, February 1982): New York, McGraw Hill, p. Ixvii-Ixviii.</ref> stated that, in determining the resource volumes of heavy oil and bitumen, viscosity of the oil should be used first to differentiate between crude oils, on one hand, and bitumens on the other. Density (specific gravity) should be considered next. Tar sands were therefore said to be characterized as containing bitumen, liquids, or semisolids with viscosities greater than 10,000 [http://www.merriam-webster.com/medical/centipoise centipoise] (cp) at original reservoir temperature, generally corresponding to a specific gravity of less than 10&deg; [http://www.glossary.oilfield.slb.com/en/Terms.aspx?LookIn=term%20name&filter=API%20gravity API] at 60&deg;F (16&deg;C). Carrigy<ref name=Carrigy_1983>Carrigy, M. A., 1983, [https://www.onepetro.org/journal-paper/SPE-12556-PA Thermal recovery from tar sands]: Journal of Petroleum Technology, v. 35, no. 13, p. 18.</ref> was critical of this definition and preferred to define tar sands as "reservoirs that contain low-gravity oil (~10&deg; API or less) and need a large thermal input to reduce the oil viscosity to a level that will allow it to be produced through a well at economic rates." Other authors have defined tar sands as reservoirs containing oil with a specific gravity of less than 10&deg; API (bitumen) and immobile in the reservoir.

Tar sands (also called bituminous sands and oil sands) have, in general, been defined as reservoirs containing oil too [http://pubs.usgs.gov/fs/fs070-03/fs070-03.html viscous] to flow into a well in sufficient quantities for economic production or, in other words, oil essentially immobile in the reservoir. Several more specific definitions have been proposed, but none has been generally accepted. A United Nations Institute for Training and Research (UNITAR) Working Group on Definitions<ref name=Danyluketal_1984>Danyluk, M., B. Galbraith, and R. Oma&ntilde;a, 1984, Toward definitions for heavy crude oil and tar sands, ''in'' R. F. Meyer, J. C. Wynn, and J. C. Olson, eds., The future of heavy crude and tar sands: Second International United Nations Institute for Training and Research International Conference (Caracas, Venezuela, February 1982): New York, McGraw Hill, p. 3-6.</ref><ref name=Martinez_1984>Martinez, A. R., 1984, Report on Working Group on Definitions, ''in'' R. F. Meyer, J. C. Wynn, and J. C. Olson, eds., The future of heavy crude and tar sands: Second International United Nations Institute for Training and Research International Conference (Caracas, Venezuela, February 1982): New York, McGraw Hill, p. Ixvii-Ixviii.</ref> stated that, in determining the resource volumes of heavy oil and bitumen, viscosity of the oil should be used first to differentiate between crude oils, on one hand, and bitumens on the other. Density (specific gravity) should be considered next. Tar sands were therefore said to be characterized as containing bitumen, liquids, or semisolids with viscosities greater than 10,000 [http://www.merriam-webster.com/medical/centipoise centipoise] (cp) at original reservoir temperature, generally corresponding to a specific gravity of less than 10&deg; [http://www.glossary.oilfield.slb.com/en/Terms.aspx?LookIn=term%20name&filter=API%20gravity API] at 60&deg;F (16&deg;C). Carrigy<ref name=Carrigy_1983>Carrigy, M. A., 1983, [https://www.onepetro.org/journal-paper/SPE-12556-PA Thermal recovery from tar sands]: Journal of Petroleum Technology, v. 35, no. 13, p. 18.</ref> was critical of this definition and preferred to define tar sands as "reservoirs that contain low-gravity oil (~10&deg; API or less) and need a large thermal input to reduce the oil viscosity to a level that will allow it to be produced through a well at economic rates." Other authors have defined tar sands as reservoirs containing oil with a specific gravity of less than 10&deg; API (bitumen) and immobile in the reservoir.

The general term ''tar sands'' will be used in this article to include both bitumen and extra-heavy-oil accumulations.

The general term ''tar sands'' will be used in this article to include both bitumen and extra-heavy-oil [[accumulation]]s.

Tar-sand deposits are known from many parts of the world, but present knowledge indicates that two giant deposits, one in western Canada and the other in eastern Venezuela, may contain as much as 80-85% of the approximately 3800 billion bbl of total bitumen and extra-heavy oil in place in the presently known tar-sand deposits. Following the definitions discussed above, the [https://en.wikipedia.org/wiki/Athabasca_oil_sands tar sands of western Canada] are considered bitumen deposits, while the [http://geology.com/usgs/venezuela-heavy-oil/ Orinoco Oil Belt] of eastern Venezuela falls into the category of heavy- and extra-heavy-oil accumulation, because although the specific gravity of the oil ranges from 4 to 17&deg; API and averages less than 10&deg; API, the reservoir temperatures are high enough to lower the viscosities to less than 10,000 cp, making the oil mobile under reservoir conditions and allowing some of the oil in place to be recovered by primary production methods; in other words without the need for thermal stimulation.<ref name=Mastersetal_1983>Masters, C. D., D. H. Root, and W. D. Dietzman, 1983, [http://pubs.er.usgs.gov/publication/ofr83728 Distribution and quantitative assessment of world crude-oil reserves and resources]: U.S. Geological Survey Open-file Report 83-728, 11p.</ref>

Tar-sand deposits are known from many parts of the world, but present knowledge indicates that two giant deposits, one in western Canada and the other in eastern Venezuela, may contain as much as 80-85% of the approximately 3800 billion bbl of total bitumen and extra-heavy oil in place in the presently known tar-sand deposits. Following the definitions discussed above, the [https://en.wikipedia.org/wiki/Athabasca_oil_sands tar sands of western Canada] are considered bitumen deposits, while the [http://geology.com/usgs/venezuela-heavy-oil/ Orinoco Oil Belt] of eastern Venezuela falls into the category of heavy- and extra-heavy-oil accumulation, because although the specific gravity of the oil ranges from 4 to 17&deg; API and averages less than 10&deg; API, the reservoir temperatures are high enough to lower the viscosities to less than 10,000 cp, making the oil mobile under reservoir conditions and allowing some of the oil in place to be recovered by primary production methods; in other words without the need for thermal stimulation.<ref name=Mastersetal_1983>Masters, C. D., D. H. Root, and W. D. Dietzman, 1983, [http://pubs.er.usgs.gov/publication/ofr83728 Distribution and quantitative assessment of world crude-oil reserves and resources]: U.S. Geological Survey Open-file Report 83-728, 11p.</ref>