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A map of world oil reserves, 2013.

Oil reserves denote the amount of crude oil that can be technically recovered at a cost that is financially feasible at the present price of oil.[1] Hence reserves will change with the price, unlike oil resources, which include all oil that can be technically recovered at any price. Reserves may be for a well, for a reservoir, for a field, for a nation, or for the world. Different classifications of reserves are related to their degree of certainty.

The total estimated amount of oil in an oil reservoir, including both producible and non-producible oil, is called oil in place. However, because of reservoir characteristics and limitations in petroleum extraction technologies, only a fraction of this oil can be brought to the surface, and it is only this producible fraction that is considered to be reserves. The ratio of reserves to the total amount of oil in a particular reservoir is called the recovery factor. Determining a recovery factor for a given field depends on several features of the operation, including method of oil recovery used and technological developments.[2]

Based on data from OPEC at the beginning of 2013 the highest proved oil reserves including non-conventional oil deposits are in Venezuela (20% of global reserves), Saudi Arabia (18% of global reserves), Canada (13% of global reserves), and Iran (9%).[3]

Because the geology of the subsurface cannot be examined directly, indirect techniques must be used to estimate the size and recoverability of the resource. While new technologies have increased the accuracy of these techniques, significant uncertainties still remain. In general, most early estimates of the reserves of an oil field are conservative and tend to grow with time. This phenomenon is called reserves growth.[4]

Many oil-producing nations do not reveal their reservoir engineering field data and instead provide unaudited claims for their oil reserves. The numbers disclosed by some national governments are suspected of being manipulated for political reasons.[5][6]



Schematic graph illustrating petroleum volumes and probabilities. Curves represent categories of oil in assessment. There is a 95% chance (i.e., probability, F95) of at least volume V1 of economically recoverable oil, and there is a 5-percent chance (F05) of at least volume V2 of economically recoverable oil.[7]

All reserve estimates involve uncertainty, depending on the amount of reliable geologic and engineering data available and the interpretation of that data. The relative degree of uncertainty can be expressed by dividing reserves into two principal classifications—"proven" (or "proved") and "unproven" (or "unproved").[7] Unproven reserves can further be divided into two subcategories—"probable" and "possible"—to indicate the relative degree of uncertainty about their existence.[7] The most commonly accepted definitions of these are based on those approved by the Society of Petroleum Engineers (SPE) and the World Petroleum Council (WPC) in 1997.[8]

Proven reservesEdit

Proven reserves are those reserves claimed to have a reasonable certainty (normally at least 90% confidence) of being recoverable under existing economic and political conditions, with existing technology. Industry specialists refer to this as "P90" (that is, having a 90% certainty of being produced). Proven reserves are also known in the industry as "1P".[9][10]

Proven reserves are further subdivided into "proven developed" (PD) and "proven undeveloped" (PUD).[10][11] PD reserves are reserves that can be produced with existing wells and perforations, or from additional reservoirs where minimal additional investment (operating expense) is required.[11] PUD reserves require additional capital investment (e.g., drilling new wells) to bring the oil to the surface.[9][11]

Until December 2009 "1P" proven reserves were the only type the U.S. Securities and Exchange Commission allowed oil companies to report to investors. Companies listed on U.S. stock exchanges must substantiate their claims, but many governments and national oil companies do not disclose verifying data to support their claims. Since January 2010 the SEC now allows companies to also provide additional optional information declaring 2P (both proven and probable) and 3P (proven plus probable plus possible) provided the evaluation is verified by qualified third party consultants, though many companies choose to use 2P and 3P estimates only for internal purposes.

Unproven reservesEdit

An oil well in Canada, which has the world's third largest oil reserves.

Unproven reserves are based on geological and/or engineering data similar to that used in estimates of proven reserves, but technical, contractual, or regulatory uncertainties preclude such reserves being classified as proven.[12] Unproven reserves may be used internally by oil companies and government agencies for future planning purposes but are not routinely compiled. They are sub-classified as probable and possible.[12]

Probable reserves are attributed to known accumulations and claim a 50% confidence level of recovery. Industry specialists refer to them as "P50" (i.e., having a 50% certainty of being produced). The sum of proven plus probable reserves is also referred to in the industry as "2P" (proven plus probable).[9]

Possible reserves are attributed to known accumulations that have a less likely chance of being recovered than probable reserves. This term is often used for reserves which are claimed to have at least a 10% certainty of being produced ("P10"). Reasons for classifying reserves as possible include varying interpretations of geology, reserves not producible at commercial rates, uncertainty due to reserve infill (seepage from adjacent areas) and projected reserves based on future recovery methods. The cumulative amount of proven, probable and possible resources are referred to in the industry as "3P" (proven plus probable plus possible).[9]

Russian reserve categoriesEdit

In Russia, and reserves categories A, B, and C1 correspond roughly to proved developed producing, proved developed nonproducing, and proved undeveloped, respectively; the designation ABC1 corresponds to proved reserves. The Russian category C2 includes probable and possible reserves.[13]

Strategic petroleum reservesEdit

Many countries maintain government-controlled oil reserves for both economic and national security reasons. According to the United States Energy Information Administration, approximately 4.1 billion barrels (650,000,000 m3) of oil are held in strategic reserves, of which 1.4 billion is government-controlled. These reserves are generally not counted when computing a nation's oil reserves.


Unconventional oil resources are greater than conventional ones.[14]
Cumulative oil production plus remaining reserves and undiscovered resources. United States not included.

A more sophisticated system of evaluating petroleum accumulations was adopted in 2007 by the Society of Petroleum Engineers (SPE), World Petroleum Council (WPC), American Association of Petroleum Geologists (AAPG), and Society of Petroleum Evaluation Engineers (SPEE). It incorporates the 1997 definitions for reserves, but adds categories for contingent resources and prospective resources.[7]

Contingent resources are those quantities of petroleum estimated, as of a given date, to be potentially recoverable from known accumulations, but the applied project(s) are not yet considered mature enough for commercial development due to one or more contingencies. Contingent resources may include, for example, projects for which there are no viable markets, or where commercial recovery is dependent on technology under development, or where evaluation of the accumulation is insufficient to clearly assess commerciality.

Prospective resources are those quantities of petroleum estimated, as of a given date, to be potentially recoverable from undiscovered accumulations by application of future development projects. Prospective resources have both an associated chance of discovery and a chance of development.

The United States Geological Survey uses the terms technically and economically recoverable resources when making its petroleum resource assessments. Technically recoverable resources represent that proportion of assessed in-place petroleum that may be recoverable using current recovery technology, without regard to cost. Economically recoverable resources are technically recoverable petroleum for which the costs of discovery, development, production, and transport, including a return to capital, can be recovered at a given market price.

"Unconventional resources" exist in petroleum accumulations that are pervasive throughout a large area. Examples include extra heavy oil, oil sand, and oil shale deposits. Unlike "conventional resources", in which the petroleum is recovered through wellbores and typically requires minimal processing prior to sale, unconventional resources require specialized extraction technology to produce. For example, steam and/or solvents are used to mobilize bitumen for in-situ recovery. Moreover, the extracted petroleum may require significant processing prior to sale (e.g., bitumen upgraders).[7] The total amount of unconventional oil resources in the world considerably exceeds the amount of conventional oil reserves, but are much more difficult and expensive to develop.

Estimation techniquesEdit

Example of a production decline curve for an individual well

The amount of oil in a subsurface reservoir is called oil in place (OIP).[11] Only a fraction of this oil can be recovered from a reservoir. This fraction is called the recovery factor.[11] The portion that can be recovered is considered to be a reserve. The portion that is not recoverable is not included unless and until methods are implemented to produce it. [12]

Volumetric methodEdit

Volumetric methods attempt to determine the amount of oil in place by using the size of the reservoir as well as the physical properties of its rocks and fluids. Then a recovery factor is assumed, using assumptions from fields with similar characteristics. OIP is multiplied by the recovery factor to arrive at a reserve number. Current recovery factors for oil fields around the world typically range between 10 and 60 percent; some are over 80 percent. The wide variance is due largely to the diversity of fluid and reservoir characteristics for different deposits.[15][16][17] The method is most useful early in the life of the reservoir, before significant production has occurred.

Materials balance methodEdit

The materials balance method for an oil field uses an equation that relates the volume of oil, water and gas that has been produced from a reservoir and the change in reservoir pressure to calculate the remaining oil. It assumes that, as fluids from the reservoir are produced, there will be a change in the reservoir pressure that depends on the remaining volume of oil and gas. The method requires extensive pressure-volume-temperature analysis and an accurate pressure history of the field. It requires some production to occur (typically 5% to 10% of ultimate recovery), unless reliable pressure history can be used from a field with similar rock and fluid characteristics.[12]

Production decline curve methodEdit

Decline curve generated by decline curve analysis software, utilized in petroleum economics to indicate the depletion of oil & gas in a petroleum reservoir. The Y axis is a semi log scale, indicating the rate of oil depletion (green line), and gas depletion (red line). The X axis is a coordinate scale, indicating time in years and displays the production decline curve. The top red line is the gas decline curve, which is a hyperbolic decline curve. Gas is measured in MCF (thousand cubic feet in this case). The lower Blue line is the oil decline curve, which is an exponential decline curve. Oil is measured in BBL (Oil barrels). Data is from actual sales, not pumped production. The dips to zero indicate there were no sales that month, likely because the oil well did not produce a full tank, and thus was not worth a visit from a tank truck. The upper right legend (map) displays CUM, which is the cumulative gas or oil produced. ULT is the ultimate recovery projected for the well. Pv10 is the discounted present value of 10%, which is the future value of the remaining lease, valued for this oil well at $1.089 million USD.

The decline curve method uses production data to fit a decline curve and estimate future oil production. The three most common forms of decline curves are exponential, hyperbolic, and harmonic. It is assumed that the production will decline on a reasonably smooth curve, and so allowances must be made for wells shut in and production restrictions. The curve can be expressed mathematically or plotted on a graph to estimate future production. It has the advantage of (implicitly) including all reservoir characteristics. It requires a sufficient history to establish a statistically significant trend, ideally when production is not curtailed by regulatory or other artificial conditions.[12]

Reserves growthEdit

Experience shows that initial estimates of the size of newly discovered oil fields are usually too low. As years pass, successive estimates of the ultimate recovery of fields tend to increase. The term reserve growth refers to the typical increases in estimated ultimate recovery that occur as oil fields are developed and produced.[4]

Estimated reserves by countryEdit

Trends in proved oil reserves in top five countries, 1980-2013 (date from US Energy Information Administration)

BBL = barrel of oil

Countries with largest oil reserves
Most of the world's oil reserves are in the Middle East.[18]
Summary of Proven Reserve Data as of 2012[3]
Country Reserves[19]
109 bbl
109 m3
106 bbl/d
103 m3/d
Reserve/ Production Ratio1
1   Venezuela 296.50 47.140 2.1 330 387
2   Saudi Arabia 265.40 42.195 8.9 1,410 81
3   Canada 175.00 27.823 2.7 430 178
4   Iran 151.20 24.039 4.1 650 101
5   Iraq 143.10 22.751 3.4 540 163
6   Kuwait 101.50 16.137 2.3 370 121
7   United Arab Emirates 97.80 15.549 2.4 380 156
8   Russia 80.00 12.719 10.0 1,590 22
9   Libya 47.00 7.472 1.7 270 76
10   Nigeria 37.00 5.883 2.5 400 41
11   Kazakhstan 30.00 4.770 1.5 240 55
12   Qatar 25.41 4.040 1.1 170 63
13   China 25.40 4.038 4.1 650 15
14   United States 25.00 3.975 7.0 1,110 10
15   Angola 13.50 2.146 1.9 300 19
16   Algeria 13.42 2.134 1.7 270 22
17   Brazil 13.20 2.099 2.1 330 17
Total of top seventeen reserves 1,540.43 244.909 59.5 9,460 64
1 Reserve to Production ratio (in years), calculated as reserves / annual production. (from above)

It is estimated that between 100 and 135 billion tonnes (which equals between 133 and 180 billions m³ of oil) of the world's oil reserves have been used between 1850 and the present.[21]

OPEC countriesEdit

Since OPEC started to set production quotas on the basis of reserves levels in the 1980s, many of its members have reported significant increases in their official reserves.[22][23] There are doubts about the reliability of these estimates, which are not provided with any form of verification that meet external reporting standards.[22] The following table illustrates these rises.

OPEC countries
oil reserves of OPEC 1980–2005
Declared reserves of major OPEC Producers (billion of barrels)
BP Statistical Review - June 2009
OPEC Annual Statistical Bulletin 2010/2011
Year Iran Iraq Kuwait Saudi Arabia UAE Venezuela Libya Nigeria
1980 58.3 30.0 67.9 168.0 30.4 19.5 20.3 16.7
1981 57.0 32.0 67.7 167.9 32.2 19.9 22.6 16.5
1982 56.1 59.0 67.2 165.5 32.4 24.9 22.2 16.8
1983 55.3 65.0 67.0 168.8 32.3 25.9 21.8 16.6
1984 58.9 65.0 92.7 171.7 32.5 28.0 21.4 16.7
1985 59.0 65.0 92.5 171.5 33.0 54.5 21.3 16.6
1986 92.9 72.0 94.5 169.7 97.2 55.5 22.8 16.1
1987 92.9 100.0 94.5 169.6 98.1 58.1 22.8 16.0
1988 92.9 100.0 94.5 255.0 98.1 58.5 22.8 16.0
1989 92.9 100.0 97.1 260.1 98.1 59.0 22.8 16.0
1990 92.9 100.0 97.0 260.3 98.1 60.1 22.8 17.1
1991 92.9 100.0 96.5 260.9 98.1 62.6 22.8 20.0
1992 92.9 100.0 96.5 261.2 98.1 63.3 22.8 21.0
1993 92.9 100.0 96.5 261.4 98.1 64.4 22.8 21.0
1994 94.3 100.0 96.5 261.4 98.1 64.9 22.8 21.0
1995 93.7 100.0 96.5 261.5 98.1 66.3 29.5 20.8
1996 92.6 112.0 96.5 261.4 97.8 72.7 29.5 20.8
1997 92.6 112.5 96.5 261.5 97.8 74.9 29.5 20.8
1998 93.7 112.5 96.5 261.5 97.8 76.1 29.5 22.5
1999 93.1 112.5 96.5 262.8 97.8 76.8 29.5 29.0
2000 99.5 112.5 96.5 262.8 97.8 76.8 36.0 29.0
2001 99.1 115.0 96.5 262.7 97.8 77.7 36.0 31.5
2002 130.7 115.0 96.5 262.8 97.8 77.3 36.0 34.3
2003 133.3 115.0 99.0 262.7 97.8 77.2 39.1 35.3
2004 132.7 115.0 101.5 264.3 97.8 79.7 39.1 35.9
2005 137.5 115.0 101.5 264.2 97.8 80.0 41.5 36.2
2006 138.4 115.0 101.5 264.3 97.8 87.3 41.5 36.2
2007 138.2 115.0 101.5 264.2 97.8 99.4 43.7 36.2
2008 137.6 115.0 101.5 264.1 97.8 172.3 43.7 36.2
2009 137.0 115.0 101.5 264.6 97.8 211.1 46.4 36.2
2010 151.2 143.1 101.5 264.5 97.8 296.5 47.1 36.2

The sudden revisions in OPEC reserves, totaling nearly 300 bn barrels, have been much debated.[24] Some of it is defended partly by the shift in ownership of reserves away from international oil companies, some of whom were obliged to report reserves under conservative US Securities and Exchange Commission rules.[22][25] The most prominent explanation of the revisions is prompted by a change in OPEC rules which set production quotas (partly) on reserves. In any event, the revisions in official data had little to do with the actual discovery of new reserves.[22]

Total reserves in many OPEC countries hardly changed in the 1990s.[22] Official reserves in Kuwait, for example, were unchanged at 96.5 Gbbl (15.34×10^9 m3) (including its share of the Neutral Zone) from 1991 to 2002, even though the country produced more than 8 Gbbl (1.3×10^9 m3) and did not make any important new discoveries during that period.[22] The case of Saudi Arabia is also striking, with proven reserves estimated at between 260 and 264 billion barrels (4.20×1010 m3) in the past 18 years, a variation of less than 2%,[22] while extracting approximately 60 billion barrels (9.5×109 m3) during this period.

Sadad al-Huseini, former head of exploration and production at Saudi Aramco, estimates 300 Gbbl (48×10^9 m3) of the world's 1,200 Gbbl (190×10^9 m3) of proven reserves should be recategorized as speculative resources, though he did not specify which countries had inflated their reserves.[26] Dr. Ali Samsam Bakhtiari, a former senior expert of the National Iranian Oil Company, has estimated that Iran, Iraq, Kuwait, Saudi Arabia and the United Arab Emirates have overstated reserves by a combined 320–390bn barrels and has said, "As for Iran, the usually accepted official 132 billion barrels (2.10×1010 m3) is almost one hundred billion over any realistic assay."[27] Petroleum Intelligence Weekly reported that official confidential Kuwaiti documents estimate reserves of Kuwait were only 48 billion barrels (7.6×10^9 m3), of which half were proven and half were possible. The combined value of proven and possible is half of the official public estimate of proven reserves.[23]

In July 2011, OPEC's Annual Statistical Review showed Venezuela's reserves to be larger than Saudi Arabia's.[28][29]

Prospective resourcesEdit

Arctic prospective resourcesEdit

Location of Arctic Basins assessed by the USGS

A 2008 United States Geological Survey estimates that areas north of the Arctic Circle have 90 billion barrels (1.4×1010 m3) of undiscovered, technically recoverable oil and 44 billion barrels (7.0×109 m3) of natural gas liquids in 25 geologically defined areas thought to have potential for petroleum. This represented 13% of the expected undiscovered oil in the world. Of the estimated totals, more than half of the undiscovered oil resources were estimated to occur in just three geologic provinces—Arctic Alaska, the Amerasia Basin, and the East Greenland Rift Basins. More than 70% of the mean undiscovered oil resources was estimated to occur in five provinces: Arctic Alaska, Amerasia Basin, East Greenland Rift Basins, East Barents Basins, and West Greenland–East Canada. It was further estimated that approximately 84% of the oil and gas would occur offshore. The USGS did not consider economic factors such as the effects of permanent sea ice or oceanic water depth in its assessment of undiscovered oil and gas resources. This assessment was lower than a 2000 survey, which had included lands south of the Arctic Circle.[30][31][32]

Unconventional prospective resourcesEdit

In October 2009, the USGS updated the Orinoco tar sands (Venezuela) value to 513 billion barrels (8.16×1010 m3).[33]

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. Estimated technically recoverable shale oil resources total 335 to 345 billion barrels.[34]

See alsoEdit


  1. ^ Society of Petroleum Engineers, Petroleum reserves and resources definitions, accessed 24 Feb. 2017.
  2. ^ "Oil reserve definitions". BP. Retrieved 4 December 2013. 
  3. ^ a b "OPEC Share of World Oil Reserves 2010". OPEC. 2011. 
  4. ^ a b David F. Morehouse (1997). "The Intricate Puzzle of Oil and Gas Reserves Growth" (PDF). U.S. Energy Information Administration. Archived from the original (PDF) on August 6, 2010. Retrieved 2014-08-19. 
  5. ^ "Proven Oil Reserves". 2008. Retrieved 2008-04-17. 
  6. ^ The Asylum, Leah McGrath Goodman, 2011, Harper Collins
  7. ^ a b c d e "Petroleum Resources Management System". Society of Petroleum Engineers. 2007. Retrieved 2008-04-20. 
  8. ^ "Petroleum Reserves Definitions" (PDF). Petroleum Resources Management System. Society of Petroleum Engineers. 1997. Retrieved 2008-04-20. 
  9. ^ a b c d "Glossary of Terms Used in Petroleum Reserves/Resources" (PDF). Society of Petroleum Engineers. 2005. Retrieved 2008-04-20. 
  10. ^ a b Wright, Charlotte J.; Rebecca A Gallun (2008). Fundamentals of Oil & Gas Accounting (5 ed.). PenWell Books. p. 750. ISBN 978-1-59370-137-6. 
  11. ^ a b c d e Hyne, Norman J. (2001). Nontechnical Guide to Petroleum Geology, Exploration, Drilling and Production. PennWell Corporation. pp. 431–449. ISBN 9780878148233. 
  12. ^ a b c d e Lyons, William C. (2005). Standard Handbook Of Petroleum & Natural Gas Engineering. Gulf Professional Publishing. pp. 5–6. ISBN 9780750677851. 
  13. ^ Society of Petroleum Engineers, SPE Reserves Committee,
  14. ^ Alboudwarej; et al. (Summer 2006). "Highlighting Heavy Oil" (PDF). Oilfield Review. Retrieved 2008-05-24. 
  15. ^ "Defining the Limits of Oil Production". International Energy Outlook 2008. U.S. Department of Energy. June 2008. Retrieved 2008-11-22. 
  16. ^ E. Tzimas, (2005). "Enhanced Oil Recovery using Carbon Dioxide in the European Energy System" (PDF). European Commission Joint Research Center. Retrieved 2008-08-23. 
  17. ^ Green, Don W.; Willhite, G. Paul (1998), Enhanced Oil Recovery, Society of Petroleum Engineers, ISBN 978-1555630775 
  18. ^ "World Proved Reserves of Oil and Natural Gas". US Energy Information Administration. 2007. Retrieved 2008-08-19. 
  19. ^ PennWell Corporation, Oil & Gas Journal, Vol. 105.48 (December 24, 2007), except United States. Oil includes crude oil and condensate. Data for the United States are from the Energy Information Administration, U.S. Crude Oil, Natural Gas, and Natural Gas Liquids Reserves, 2006 Annual Report, DOE/EIA-0216(2007) (November 2007). Oil & Gas Journal's oil reserve estimate for Canada includes 5.392 billion barrels (857,300,000 m3) of conventional crude oil and condensate reserves and 173.2 billion barrels (2.754×1010 m3) of oil sands reserves. Information collated by EIA
  20. ^ U.S. Energy Information Administration (EIA) – U.S. Government – U.S. Dept. of Energy, September, 2011 EIA - International Energy Statistics
  21. ^ How Much Oil Have We Used?, Science Daily, 8 May 2009. Retrieved Mar 2014.
  22. ^ a b c d e f g WORLD ENERGY OUTLOOK 2005:Middle East and North Africa Insights (PDF). INTERNATIONAL ENERGY AGENCY. 2005. pp. 125–126. 
  23. ^ a b "Oil Reserves Accounting: The Case Of Kuwait". Petroleum Intelligence Weekly. January 30, 2006. Retrieved 2008-08-23. 
  24. ^ Adam, Porter (15 July 2005). "How much oil do we really have?". BBC News. 
  25. ^ Maugeri, Leonardo (January 23, 2006). "The Saudis May Have Enough Oil". Newsweek. 
  26. ^ "Oil reserves over-inflated by 300bn barrels – al-Huseini". October 30, 2007. Retrieved 2008-08-23. 
  27. ^ "On Middle Eastern Oil Reserves". ASPO-USA's Peak Oil Review. February 20, 2006. Retrieved 2008-08-20. 
  28. ^ Faucon, Benoit (18 July 2011). "Venezuela Oil Reserves Surpassed Saudi Arabia In 2010-OPEC". Fox Business. Retrieved 18 July 2011. 
  29. ^ "OPEC Share of World Crude Oil Reserves". OPEC. 2010. Retrieved June 3, 2012. 
  30. ^ United States Geological Survey, (USGS) (July 27, 2008). "90 Billion Barrels of Oil and 1,670 Trillion Cubic Feet of Natural Gas Assessed in the Arctic". USGS. Retrieved 2008-08-12. 
  31. ^ MOUAWAD, JAD (July 24, 2008). "Oil Survey Says Arctic Has Riches". New York Times. 
  32. ^ Alan Bailey (October 21, 2007). "USGS: 25% Arctic oil, gas estimate a reporter's mistake". Vol. 12, No. 42. Petroleum News. Retrieved 2008-07-24. 
  33. ^ Christopher J. Schenk; Troy A. Cook; Ronald R. Charpentier; Richard M. Pollastro; Timothy R. Klett; Marilyn E. Tennyson; Mark A. Kirschbaum; Michael E. Brownfield & Janet K. Pitman. (11 January 2010). "An Estimate of Recoverable Heavy Oil Resources of the Orinoco Oil Belt, Venezuela" (PDF). USGS. Retrieved 23 January 2010. 
  34. ^ "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. 

External linksEdit