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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° API and averages less than 10° 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° API and averages less than 10° 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>
The only other major tar-sand deposits may be located in the former Soviet Union, but they are poorly understood, because available information concerning their size and characteristics is still ambiguous and inconsistent.
The low commercial value assigned until recently to tar-sand deposits may be responsible for other large bitumen or extra-heavy-oil accumulations having remained undiscovered, unreported, or undeveloped. All important tar-sand deposits may therefore not have been discovered yet.
This situation will no doubt change as concern for the potential exhaustion of conventional lighter oil deposits increases; as technology for extracting and upgrading the bitumen contained in the tar sands to an economical feedstock for refineries becomes available; and as new uses for the bitumen are developed, as, for example, the elaboration in Venezuela of the [[Orimulsion]], a direct combustion product, an emulsion composed of 70% extra-heavy oil suspended in 30% water with the help of a chemical surfactant to stabilize the emulsion.<ref name<PachecoandAlonso_1995>Pacheco, L. A., and J. Alonso, 1995, Orimulsion-The growing pain of an idea, ''in'' R. F. Meyer, ed., Heavy crude and tar sands-Fueling for a clean and safe environment: 6th United Nations Institute for Training and Research International Conference on Heavy Crude and Tar Sands (Houston, Texas, February 1995), v. 1, p. 203-211.</ref>
Although problems of upgrading, transportation, and use need to be solved, the economics of producing bitumens and extra-heavy oils are improving, and attention is progressively being focused in the last few years on delineating and assessing the known bitumen and extra-heavy- and heavy-oil deposits and in searching for new ones. Many of the tar-sand deposits [[crop out]] and were identified early in the exploration for oil throughout the world. Most of those that do not crop out are at shallow depths along the rims of [[sedimentary basins]]
and were discovered when drilling for deeper, lighter oil prospects. Not many of those that crop out have remained unknown, but some of those in the shallow subsurface may be still waiting to be discovered.
==See also==
==See also==
* [[Historical sources of energy]]
* [[Historical sources of energy]]