Petroleum is not distributed evenly around the world. Slightly less than half of the world’s proven reserves are located in the Middle East (including Iran but not North Africa). Following the Middle East are Canada and the United States, Latin America, Africa, and the region made up of Russia, Kazakhstan, and other countries that were once part of the Soviet Union.

The amount of oil and natural gas a given region produces is not always proportionate to the size of its proven reserves. For example, the Middle East contains approximately 50 percent of the world’s proven reserves but accounts for only about 30 percent of global oil production (though this figure is still higher than in any other region). The United States, by contrast, lays claim to less than 2 percent of the world’s proven reserves but produces roughly 16 percent of the world’s oil.

Location of reserves

Oil fields

Two overriding principles apply to world petroleum production. First, most petroleum is contained in a few large fields, but most fields are small. Second, as exploration progresses, the average size of the fields discovered decreases, as does the amount of petroleum found per unit of exploratory drilling. In any region, the large fields are usually discovered first.

Since the construction of the first oil well in 1859, some 50,000 oil fields have been discovered. More than 90 percent of these fields are insignificant in their impact on world oil production. The two largest classes of fields are the supergiants, fields with 1 billion or more barrels of ultimately recoverable oil, and giants, fields with 500 million to 5 billion barrels of ultimately recoverable oil. Fewer than 40 supergiant oil fields have been found worldwide, yet these fields originally contained about one-half of all the oil so far discovered. The Arabian-Iranian sedimentary basin in the Persian Gulf region contains two-thirds of these supergiant fields. The remaining supergiants are distributed among the United States, Russia, Mexico, Libya, Algeria, Venezuela, China, and Brazil.

Although the semantics of what it means to qualify as a giant field and the estimates of recoverable reserves in giant fields differ between experts, the nearly 3,000 giant fields discovered—a figure which also includes the supergiants—account for 80 percent of the world’s known recoverable oil. There are, in addition, approximately 1,000 known large oil fields that initially contained between 50 million and 500 million barrels. These fields account for some 14 to 16 percent of the world’s known oil. Less than 5 percent of the known fields originally contained roughly 95 percent of the world’s known oil.

Sedimentary basins

Giant and supergiant petroleum fields and significant petroleum-producing basins of sedimentary rock are closely associated. In some basins, huge amounts of petroleum apparently have been generated because perhaps only about 10 percent of the generated petroleum is trapped and preserved. The Arabian-Iranian sedimentary basin is predominant because it contains more than 20 supergiant fields. No other basin has more than one such field. In 20 of the 26 most significant oil-containing basins, the 10 largest fields originally contained more than 50 percent of the known recoverable oil. Known world oil reserves are concentrated in a relatively small number of giant and supergiant fields in a few sedimentary basins.

Worldwide, approximately 600 sedimentary basins are known to exist. About 160 of these have yielded oil, but only 26 are significant producers, and 7 of these account for more than 65 percent of the total known oil. Exploration has occurred in another 240 basins, but discoveries of commercial significance have not been made.

Geologic study and exploration

Current geologic understanding can usually distinguish between geologically favourable and unfavourable conditions for oil accumulation early in the exploration cycle. Thus, only a relatively few exploratory wells may be necessary to indicate whether a region is likely to contain significant amounts of oil. Modern petroleum exploration is an efficient process. If giant fields exist, it is likely that most of the oil in a region will be found by the first 50 to 250 exploratory wells. This number may be exceeded if there is a much greater than normal amount of major prospects or if exploration drilling patterns are dictated by either political or unusual technological considerations. Thus, while undiscovered commercial oil fields may exist in some of the 240 explored but seemingly barren basins, it is unlikely that they will be of major importance since the largest are normally found early in the exploration process.

The remaining 200 basins have had little or no exploration, but they have had sufficient geologic study to indicate their dimensions, amount and type of sediments, and general structural character. Most of the underexplored (or frontier) basins are located in difficult environments, such as in polar regions, beneath salt layers, or within submerged continental margins. The larger sedimentary basins—those containing more than 833,000 cubic km (200,000 cubic miles) of sediments—account for some 70 percent of known world petroleum. Future exploration will have to involve the smaller basins as well as the more expensive and difficult frontier basins.

Status of the world oil supply

On several occasions—most notably during the oil crises of 1973–74 and 1978–79 and during the first half of 2008—the price of petroleum rose steeply. Because oil is such a crucial source of energy worldwide, such rapid rises in price spark recurrent debates about the accessibility of global supplies, the extent to which producers will be able to meet demand in the decades to come, and the potential for alternative sources of energy to mitigate concerns about energy supply and climate change issues related to the burning of fossil fuels. How much oil does Earth have? The short answer to this question is that nobody knows. In its 1995 assessment of total world oil supplies, the U.S. Geological Survey (USGS) estimated that about 3 trillion barrels of recoverable oil originally existed on Earth and that about 710 billion barrels of that amount had been consumed by 1995. The survey acknowledged, however, that the total recoverable amount of oil could be higher or lower—3 trillion barrels was not a guess but an average of estimates based on different probabilities. This caveatnotwithstanding, the USGS estimate was hotly disputed. Some experts said that technological improvements would create a situation in which much more oil would be ultimately recoverable, whereas others said that much less oil would be recoverable and that more than one-half of the world’s original oil supply had already been consumed.

There is ambiguity in all such predictions. When industry experts speak of total “global oil reserves,” they refer specifically to the amount of oil that is thought to be recoverable, not the total amount remaining on Earth. What is counted as “recoverable,” however, varies from estimate to estimate. Analysts make distinctions between “proven reserves”—those that can be demonstrated as recoverable with reasonable certainty, given existing economic and technological conditions—and reserves that may be recoverable but are more speculative. The Oil & Gas Journal, a prominent weekly magazine for the petroleum industry, estimated in late 2007 that the world’s proven reserves amounted to roughly 1.3 trillion barrels. To put this number in context, the world’s population consumed about 30 billion barrels of oil in 2007. At this rate of consumption, disregarding any new reserves that might be found, the world’s proven reserves would be depleted in about 43 years. However, because of advancements in exploration and unconventional oil extraction, estimates of the world’s proven oil reserves had risen to approximately 1.7 trillion barrels by 2015.

By any estimation, it is clear that Earth has a finite amount of oil and that global demand is expected to increase. In 2007 the National Petroleum Council, an advisory committee to the U.S. Secretary of Energy, projected that world demand for oil would rise from 86 million barrels per day to as much as 138 million barrels per day in 2030. Yet experts remain divided on whether the world will be able to supply so much oil. Some argue that the world has reached “peak oil”—its peak rate of oil production. The controversial theory behind this argument draws on studies that show how production from individual oil fields and from oil-producing regions has tended to increase to a point in time and then decrease thereafter. “Peak-oil theory” suggests that once global peak oil has been reached, the rate of oil production in the world will progressively decline, with severe economic consequences to oil-importing countries.

A more widely accepted view is that through the early 21st century at least, production capacity will be limited not by the amount of oil in the ground but by other factors, such as geopolitics or economics. One concern is that growing dominance by nationalized oil companies, as opposed to independent oil firms, can lead to a situation in which countries with access to oil reserves will limit production for political or economic gain. A separate concern is that nonconventional sources of oil—such as oil sand reserves, oil shale deposits, or reserves that are found under very deep water—will be significantly more expensive to produce than conventional crude oil unless new technologies are developed that reduce production costs.

Major oil-producing countries

As mentioned above, petroleum resources are not distributed evenly around the world. Indeed, according to estimates published for 2015 by the U.S. Department of Energy, as few as 15 countries account for more than 75 percent of the world’s oil production and hold roughly 93 percent of its reserves. Significantly, those countries are projected to have a correspondingly large percentage of the world’s remaining undiscovered oil resources, which are estimated by the extrapolation of known production and reserve data into untested sediments of similar geology.

country	total production of petroleum and other liquids (thousands of barrels/day; 2017 estimate)	% of world production of petroleum and other liquids (2017 estimate)	total petroleum consumption (thousands of barrels/day; 2015 estimate)	proven reserves of crude oil, NGPL,*** and other liquids (billions of barrels; 2018 estimate)
Production and consumption of petroleum and other hydrocarbons
*2016 data.
**2017 data.
***Natural gas plant liquids (including ethane, propane, normal butane, isobutane, and pentanes).
Source: Energy Information Administration, U.S. Department of Energy, International Energy Statistics (2018).
Venezuela	2,174.3	2.23	676	302.25
Saudi Arabia	12,089.6	12.34	3,302	266.21
Canada	4,986.1	5.09	2,378.83*	170.54
Iran	4,668.6	4.76	1,850	157.2
Iraq	4,462.4	4.55	788	148.77
Kuwait	2,927.7	2.99	489	101.5
United Arab Emirates	3,720.5	3.80	849	97.8
Russia	11,200.4	11.43	3,512	80
Nigeria	2,037.2	2.08	325	37.45
United States	15,599.5	15.92	19,872.67**	35.21**
China	4,778.7	4.88	12,376.05	25.63
Qatar	2,068.3	2.11	255	25.24
Brazil	3,363.1	3.43	3,087	12.63
Mexico	2,260.5	2.31	2,026.75*	6.63
Norway	1,979	2.02	227.69*	6.38

Saudi Arabia

Saudi Arabia has the second largest proven oil reserves in the world—some 268 billion barrels, approximately 16 percent of the world’s proven reserves—not to mention significant potential for additional discoveries. The discovery that transformed Saudi Arabia into a leading oil country was the Al-Ghawār oil field. Discovered in 1948 and put into production in 1951, this field has proved to be the world’s largest, generating an estimated 55 billion barrels after 60 years of production. Saudi officials estimate that this field contains more than 120 billion barrels in recoverable reserves, if waterflooding (that is, water injection that forces oil from the oil reservoir) is considered. Another important discovery was the Saffāniyyah offshore field in the Persian Gulf in 1951. It is the third largest oil field in the world and the largest offshore. Saudi Arabia has eight other supergiant oil fields. Saudi fields, as well as many other Middle Eastern fields, are located in the great Arabian-Iranian basin.

Major oil fields of the Arabian-Iranian basin region.

Iraq, Kuwait, and Iran

The Middle Eastern countries of Iraq, Kuwait, and Iran are each estimated to have had an original oil endowment in excess of 100 billion barrels. Together they account for more than 23 percent of all proven reserves in the world. These countries have a number of supergiant fields, all of which are located in the Arabian-Iranian basin, including Kuwait’s field at Al-Burqān, which was discovered in 1938. Al-Burqān is the world’s second largest oil field, having originally contained 75 billion barrels of recoverable oil. Iraq possesses a significant potential for additional oil discoveries, primarily in its southwestern geographic region, where an estimated 45–100 billion barrels of crude oil are thought to reside. This resource has been slow to develop, because of the country’s involvement since 1980 in major wars and subsequent civil unrest.

Russia and the Caspian Sea region

Russia is thought to possess the best potential for new discoveries. It has significant proven reserves—some 80 billion barrels, approximately 6 percent of the world total—and is one the world’s leading petroleum producers. Russian oil is derived from many sedimentary basins within the vast country, and two supergiant oil fields, Samotlor and Romashkino, were discovered in 1964 and 1949 respectively. Production from these mature fields is on the decline, however, so that total Russian oil output is maintained by production at new fields. The best prospects for new Russian discoveries appear to exist in difficult and expensive frontier areas such as Sakhalin Island.

Sedimentary basins and major oil and gas fields of Europe, Russia, Transcaucasia, and Central Asia.

The Tengiz field on the northeast border of the Caspian Sea is a supergiant with up to 9 billion barrels recoverable reserves. It was originally discovered in 1979; however, it was not actively developed until the American oil company Chevron gained equity in the region in 1993. Operating equipment and producing oil in this field are extremely complex because of the oil’s high levels of hydrogen sulfide gas, extremely high well pressure, and large volumes of natural gas.  Kazakhstan’s Kashagan field in the northern Caspian Sea was discovered in 2000. It was the largest and newest conventional field discovered since the finding of Alaska’s Prudhoe Bay field in 1968. Kashagan is estimated to have already produced 7 billion to 9 billion barrels out of its proven 30-billion-barrel reserves.

Sub-Saharan Africa

Sub-Saharan Africa, primarily West Africa, holds a rich resource base with multiple supergiant and giant fields. Beginning to the north, Ghana boasts the most recent potential supergiant, the Jubilee field, with potential reserves of 2 billion barrels. It was discovered in 2007 and produced more than 110,000 barrels per day by 2011. However, the majority of sub-Saharan African recoverable reserves and supergiant or giant fields are in Nigeria, Angola, Equatorial Guinea, and Gabon.

Nigeria’s Niger delta harboured the country’s first commercial oil discovery, the Oloibiri oil field, which is now referred to as Oil Mining Lease (OML) 29. The Niger delta province spans from onshore to deepwater offshore and holds upward of 37.4 billion barrels of oil and 193 trillion cubic feet of gas reserves. Several reservoirs make up the total play, with the giant Agbami light sweet crude field having over 1 billion barrels of recoverable reserves. Agbami was discovered in 1998 and began to produce some 10 years later. Outside the Niger delta is the giant deepwater oil field Bonga, or OML 118, discovered in 1996 southwest of the Niger delta. With recoverable reserves of 600 million barrels, OML 118 began to produce in 2005.

Angola and its Cabinda province have recoverable reserves totaling more than 9.5 billion barrels of oil and 10.8 trillion cubic feet of natural gas. Block 15 is the largest producing deepwater block in Angola. The offshore petroleum development zone is located in the Congo basin and has estimated total recoverable hydrocarbon reserves of 5 billion barrels. Discovered by ExxonMobilaffiliate Esso Exploration Angola in 1998, the giant Kizomba field with over 2 billion barrels of recoverable reserves launched Angola’s commercial production rise and the country’s membership in OPEC in 2007. The development of the Kizomba field was a phased-in process, with production beginning in 2004 and full development occurring in 2008. Angola’s Block 17 includes the Dalia and Pazflor fields. Dalia was first discovered in 1997. It began production in 2006 and has estimated recoverable reserves of 1 billion barrels. Pazflor field, discovered in 2000 and located northeast of Dalia, is estimated by the operator, Total, to contain recoverable reserves of 590 million barrels. The field first began to produce in 2011.

Equatorial Guinea has an estimated 1.1 billion barrels of recoverable reserves and boasts the first deepwater field brought online in West Africa. The giant Zafiro field was discovered in 1995 by ExxonMobil and Ocean Energy. It is located northwest of Bioko island, and it contained the bulk of the country’s recoverable reserves. Zafiro began production using a floating production storage and offloading vessel in 1996. Equatorial Guinea’s major hydrocarbon contribution, however, is its natural gas resources. The Alba field is estimated to have up to 4.4 trillion cubic feet of reserves or an equivalent 759 million barrels of oil. This enormous supply allowed government officials to justify significant infrastructure development on Bioko island for exporting liquefied natural gasand oil.

Gabon is West Africa’s second largest reserves holder with 2 billion barrels of recoverable reserves. The giant Rabi-Kounga field was discovered in 1985 and began production in 1989. Originally, Rabi-Kounga was estimated to have 440 million barrels of reserves, but this was increased in 1993 to 850 million barrels following a reappraisal, the creation of additional facilities, and infill drilling by the Shell petroleum company. By the early 21st century, however, only a fraction of this amount remained for further production.

United States, Mexico, and Canada

North America has many sedimentary basins. Basins in the United States have been intensively explored, and their oil resources developed. More than 33,000 oil fields have been found, but there are only two supergiants (Prudhoe Bay, in the North Slope region of Alaska, and East Texas). Cumulatively, the United States has produced more oil than any other country. Its proven oil reserves amount to some 40 billion barrels, representing approximately 2 percent of the world total, but the country is still considered to have a significant remaining undiscovered oil resource. Prudhoe Bay, which accounted for approximately 17 percent of U.S. oil production during the mid-1980s, is in decline. This situation, coupled with declining oil production in the conterminous U.S., contributed to a significant drop in domestic oil output through the end of the 20th century. In the early 21st century, however, advancements in unconventional oil recovery resulted in skyrocketing production, and by 2015 the U.S had become the world’s leading petroleum-producing country.

Sedimentary basins and major oil and gas fields of North America

Mexico has more than 10 billion barrels of proven oil reserves and is one of the top 10 oil producers in the world. However, its principal supergiant oil field (Cantarell, offshore of Campeche state), which is one of the largest conventional oil fields discovered in the Western Hemisphere, peaked at more than 2 million barrels per day in 2003, making it difficult to sustain current production levels well into the 21st century. Mexico’s potential supergiant, Chicontepec, which contains roughly 40 percent of the country’s reserves, is estimated to hold 17 billion barrels of oil equivalent. However, most of the oil is extra-heavy crude, and this circumstance has hampered development.

Canada has less than 10 billion barrels of proven reserves of conventional liquidoil, but huge deposits of oil sands in the Athabasca region of Alberta in western Canada bring the country’s total proven oil reserves to more than 170 billion barrels, behind only oil giants Venezuela and Saudi Arabia. Canada’s largest oil field is Hibernia, discovered in the Jeanne d’Arc basin off Newfoundland in 1979. This giant field began producing in 1997 and was soon joined by two other fields, Terra Nova (first production 2002) and White Rose (first production 2005).

Venezuela and Brazil

Venezuela is the largest oil exporter in the Western Hemisphere and has long been an important country in the world oil market. With approximately 298 billion barrels of proven oil reserves, it has the world’s largest oil endowment. Most of the estimated 500 billion barrels of potential reserves, however, are in the form of extra-heavy oil and bitumen deposits located in the Orinoco belt in the central part of the country, which have not been exploited to a large extent. The country’s most important producing field is the Bolivar Coastal field. Discovered in 1917, this complex of large and small reservoirs is found in the Maracaibo basin in the west. These mature fields have produced over 70 percent of the estimated recoverable reserves, but they are declining in production.

Since the late 20th century, Brazil has emerged as an important energy producer. Its 15.5 billion barrels of proven oil reserves are the second largest in South America. Most of those reserves are located in the Atlantic Ocean, in the Campos and Santos basins off the coasts of Rio de Janeiro and São Paulo states respectively. Carioca-Sugar Loaf, Lula (formerly Tupi), and Jupiter make up the primary fields to be developed in very deep waters. Lula alone is thought to contain between 5 and 8 billion barrels of recoverable reserves. Total estimated potential reserves are greater than 120 billion barrels of reserves for the offshore area.

North Sea

The United Kingdom is an important North Sea producer, and its proven oil reserves of some 3 billion barrels are the largest in the European Union. The supergiant Forties field, identified in 1970, was the second commercial discovery in the North Sea following Norway’s supergiant Ekofisk in 1969. Crude oil production, which peaked in the late 1990s, has declined to less than half of its peak level, however, and Britain, once a net oil exporter, is now a net oil importer. The broader North Sea, however, is under potential rejuvenation, similar to mature assets elsewhere in the world. The original Ekofisk is expanding again after 40 years of production. In 2011 the operating partners were given approval by Norway to develop and manage Ekofisk South and Eldfisk II, which increases reserves recovery by more than 470 million barrels. Of the five countries with divided interests in the North Sea, Norway holds the most recoverable reserves. In addition, it has the most recent giant field discovery, the Johan Sverdrup, 2010. The field has an estimated 1.7 to 3.3 billion barrels total reserves.