- For another use of the term "shale oil", meaning synthetic oil derived from oil shale, see shale oil
Tight oil (also known as shale oil or light tight oil, abbreviated LTO) is petroleum that consists of light crude oil contained in petroleum-bearing formations of low permeability, often shale or tight sandstone. Economic production from tight oil formations requires the same hydraulic fracturing and often uses the same horizontal well technology used in the production of shale gas. It should not be confused with oil shale, which is shale rich in kerogen.
Inventory and examples
Tight oil formations include the Bakken Shale, the Niobrara Formation, Barnett Shale, and the Eagle Ford Shale in the United States, R'Mah Formation in Syria, Sargelu Formation in the northern Persian Gulf region, Athel Formation in Oman, Bazhenov Formation and Achimov Formation of West Siberia in Russia, in Coober Pedy in Australia, Chicontepec Formation in Mexico, and the Vaca Muerta oil field in Argentina. In June 2013 the U.S. Energy Information Administration published a global inventory of estimated recoverable tight oil and tight gas resources in shale formations, "Technically Recoverable Shale Oil and Shale Gas Resources: An Assessment of 137 Shale Formations in 41 Countries Outside the United States." The inventory is incomplete due to exclusion of tight oil and gas from sources other than shale such as sandstone or carbonates, formations underlying the large oil fields located in the Middle East and the Caspian region, off shore formations, or about which there is little information. Amounts include only high quality prospects which are likely to be developed.
In 2012, at least 4,000 new producing shale oil (tight oil) wells were brought online in the United States. By comparison, the number of new producing oil and gas wells (both conventional and unconventional) completed in 2012 globally outside the United States and Canada is less than 4,000.
Tight oil shale formation are heterogeneous, they vary widely over relatively short distances, thus even in a single horizontal drill hole the amount recovered may vary, as may recovery within a field, or even between adjacent wells. This makes evaluation of plays and decisions regarding the profitability of wells on a particular lease difficult. Production of oil requires some natural gas, oil cannot be produced from a portion of a formation which contains only oil. Formations which formed under marine conditions contain less clay and are more brittle, and thus more suitable for fracking than formations formed in fresh water which may contain more clay.[dubious ] Formations with more quartz and carbonate are more brittle.
Prerequisites for exploitation include being able to obtain rights to drill, easier in the United States and Canada where private owners of subsurface rights are motivated to enter into leases; the availability of expertise and financing, easier in the United States and Canada where there are many independent operators and supporting contractors with critical expertise and suitable drilling rigs; infrastructure to gather and transport oil; and water resources for use in hydraulic fracturing.
Outside the United States and Canada, development of shale oil (tight oil) resources may be limited by the lack of available drilling rigs: 2/3 of the world's active drill rigs are in the US and Canada, and rigs elsewhere are less likely to be equipped for horizontal drilling. Drilling intensity may be another constraint, as tight-oil development requires far more completed wells than does conventional oil. Leonardo Maugeri considers this will be "an insurmountable environmental hurdle in Europe". 
Estimated technically recoverable shale oil resources
Not all oil which is technically recoverable using current technology may be economically recoverable at current or anticipated prices. Sums include only high quality prospects which are likely to be developed.
- Russia: 75 billion barrels
- United States: 48 to 58 billion barrels
- China: 32 billion barrels
- Argentina: 27 billion barrels
- Libya: 26 billion barrels
- Venezuela: 13 billion barrels
- Mexico: 13 billion barrels
- Pakistan: 9 billion barrels
- Canada: 9 billion barrels
- Indonesia: 8 billion barrels
- World Total 335 to 345 billion barrels
- Mills, Robin M. (2008). The myth of the oil crisis: overcoming the challenges of depletion, geopolitics, and global warming. Greenwood Publishing Group. pp. 158–159. ISBN 978-0-313-36498-3.
- IEA (29 May 2012). Golden Rules for a Golden Age of Gas. World Energy Outlook Special Report on Unconventional Gas (PDF). OECD. p. 21.
- Bloomberg (May 17, 2013). "Chevron says shale to help make Argentina energy independent". FuelFix. Retrieved May 18, 2013.
- "Technically Recoverable Shale Oil and Shale Gas Resources: An Assessment of 137 Shale Formations in 41 Countries Outside the United States" (PDF). U.S. Energy Information Administration (EIA). June, 2013. Retrieved June 11, 2013.
- "The Shale Oil Boom: a US Phenomenon" by Leonardo Maugeri, Harvard University, Geopolitics of Energy Project, Belfer Center for Science and International Affairs, Discussion Paper 2013-05
- Shale oil and tight oil
- The Shale Oil Boom: a US Phenomenon by Leonardo Maugeri, Harvard University, Geopolitics of Energy Project, Belfer Center for Science and International Affairs, Discussion Paper 2013-05