petroleous, adj.
/peuh troh"lee euhm/, n.
an oily, thick, flammable, usually dark-colored liquid that is a form of bitumen or a mixture of various hydrocarbons, occurring naturally in various parts of the world and commonly obtained by drilling: used in a natural or refined state as fuel, or separated by distillation into gasoline, naphtha, benzene, kerosene, paraffin, etc.
[1520-30; < ML: lit., rock oil, equiv. to L petr(a) rock ( < Gk pétra) + oleum OIL]

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Complex mixture of hydrocarbons derived from the geologic transformation and decomposition of plants and animals that lived hundreds of millions of years ago.

As a technical term, petroleum encompasses the liquid (crude oil), gaseous (natural gas), and viscous or solid (bitumen, asphalt) forms of hydrocarbons that occur in the Earth, but the meaning is often restricted to the liquid oil form. Crude oil and natural gas are the most important primary fossil fuels. Asphalt has been used since ancient times to caulk ships and pave roads. In the mid 1800s petroleum began to replace whale oil in lamps, and the first well specifically to extract it was drilled in 1859. The development of the automobile gave petroleum a new role as the source of gasoline. Petroleum and its products have since been used as fuels for heating, for land, air, and sea transport, and for electric power generation and as petrochemical sources and lubricants. Crude oil and natural gas, produced mostly in Saudi Arabia, the U.S., and Russia, now account for about 60% of world energy consumption; the U.S. is by far the largest consumer. At present rates of consumption, the known supply will be exhausted by the mid 21st century. Petroleum is recovered from drilled wells, transported by pipeline or tanker ship to refineries, and there converted to fuels and petrochemicals.
(as used in expressions)
Organization of Petroleum Exporting Countries
British Petroleum Co. PLC

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      complex mixture of hydrocarbons that occur in the Earth in liquid, gaseous, or solid forms. The term is often restricted to the liquid form, commonly called crude oil, but as a technical term it also includes natural gas and the viscous or solid form known as bitumen. The liquid and gaseous phases of petroleum constitute the most important of the primary fossil fuels.

      Liquid and gaseous hydrocarbons (hydrocarbon) are so intimately associated in nature that it has become customary to shorten the expression “petroleum and natural gas” to “petroleum” when referring to both. The word petroleum (literally “rock oil” from the Latin petra, “rock” or “stone,” and oleum, “oil”) was first used in 1556 in a treatise published by the German mineralogist Georg Bauer, known as Georgius Agricola (Agricola, Georgius).

History of use

Exploitation of surface seeps
      Small surface occurrences of petroleum in the form of natural gas and oil seeps have been known from early times. The ancient Sumerians, Assyrians, and Babylonians used crude oil and asphalt (“pitch”) collected from large seeps at Tuttul (modern-day Hīt) on the Euphrates for many purposes more than 5,000 years ago. Liquid oil was first used as a medicine (pharmaceutical) by the ancient Egyptians, presumably as a wound dressing, liniment, and laxative.

      Oil products were valued as weapons (weapon) of war in the ancient world. The Persians used incendiary arrows wrapped in oil-soaked fibres at the siege of Athens in 480 BC. Early in the Christian era the Arabs and Persians distilled crude oil to obtain flammable products for military purposes. Probably as a result of the Arab invasion of Spain, the industrial art of distillation into illuminants became available in western Europe by the 12th century.

      Several centuries later, Spanish explorers discovered oil seeps in present-day Cuba, Mexico, Bolivia, and Peru. In North America oil seeps were plentiful and were noted by early explorers in what are now New York and Pennsylvania, where the Indians were reported to have used the oil for medicinal purposes.

Extraction from underground reservoirs
      Until the beginning of the 19th century, illumination in the United States and in many other countries was little improved over that known by the early Greeks and Romans. The need for better illumination that accompanied the increasing development of urban centres made it necessary to search for new sources of oil, especially since whales, which had long provided fuel for lamps, were becoming harder and harder to find. By the mid-19th century kerosene, or coal oil, derived from coal was in common use in both North America and Europe.

      The Industrial Revolution brought on an ever-growing demand for a cheaper and more convenient source of lubricants as well as illuminating oil. It also required better sources of energy. Energy had previously been provided by human and animal muscle and later by the combustion of such solid fuels as wood, peat, and coal. These were collected with considerable effort and laboriously transported to the site where the energy source was needed. Liquid petroleum, on the other hand, was a more easily transportable source of energy. Oil was a much more concentrated and flexible form of fuel than anything previously available.

      The stage was set for the first well specifically drilled for oil, a project undertaken by Edwin L. Drake (Drake, Edwin Laurentine) in northwestern Pennsylvania. The completion of the well in August 1859 established the groundwork for the petroleum industry and ushered in the closely associated modern industrial age. Within a short time inexpensive oil from underground reservoirs was being processed at already existing coal-oil refineries, and by the end of the century oil fields had been discovered in 14 states from New York to California and from Wyoming to Texas. During the same period, oil fields were found in Europe and East Asia as well.

Significance of oil in modern times
      At the beginning of the 20th century the Industrial Revolution had progressed to the extent that the use of refined oil for illuminants ceased to be of primary importance. The oil industry became the major supplier of energy largely because of the advent of the automobile. Although oil constitutes a major petrochemical feedstock, its primary importance is as an energy source on which the world economy depends.

      The significance of oil as a world energy source is difficult to overdramatize. The growth in energy production during the 20th century is unprecedented, and increasing oil production has been by far the major contributor to that growth. Every day an immense and intricate system moves more than 60,000,000 barrels of oil from producers to consumers. The production and consumption of oil is of vital importance to international relations and has frequently been a decisive factor in the determination of foreign policy. The position of a country in this system depends on its production capacity as related to its consumption. The possession of oil deposits is sometimes the determining factor between a rich and a poor country. For any country, however, the presence or absence of oil has a major economic consequence.

      On a time scale within the span of prospective human history, the utilization of oil as a major source of energy will be a transitory affair of about 100 years. Nonetheless, it will have been an affair of profound importance to world industrialization.

Properties of oil

Chemical composition
Hydrocarbon content
      Although oil consists basically of compounds of only two elements, carbon and hydrogen, these elements form a large variety of complex molecular structures. Regardless of physical or chemical variations, however, almost all crude oil ranges from 82 to 87 percent carbon by weight and 12 to 15 percent hydrogen. The more viscous bitumens generally vary from 80 to 85 percent carbon and from 8 to 11 percent hydrogen.

      Crude oil can be grouped into three basic chemical series: paraffins, naphthenes, and aromatics. Most crude oils are mixtures of these three series in various and seemingly endless proportions. No two crude oils from different sources are completely identical.

      The paraffin series (paraffin hydrocarbon) of hydrocarbons, also called the methane (CH4) series, comprises the most common hydrocarbons in crude oil. It is a saturated straight-chain series that has the general formula CnH2n + 2, in which C is carbon, H is hydrogen, and n is an integer. The paraffins that are liquid at normal temperatures but boil between 40° and 200° C (approximately between 100° and 400° F) are the major constituents of gasoline. The residues obtained by refining lower-density paraffins are both plastic and solid paraffin waxes.

      The naphthene series has the general formula CnH2n and is a saturated closed-ring series. This series is an important part of all liquid refinery products, but it also forms most of the complex residues from the higher boiling-point ranges. For this reason, the series is generally heavier. The residue of the refinery process is an asphalt, and the crude oils in which this series predominates are called asphalt-base crudes.

      The aromatic series has the general formula CnH2n - 6 and is an unsaturated closed-ring series. Its most common member, benzene (C6H6), is present in all crude oils, but the aromatics as a series generally constitute only a small percentage of most crudes.

Nonhydrocarbon content
      In addition to the practically infinite mixtures of hydrocarbon compounds that form crude oil, sulfur, nitrogen, and oxygen are usually present in small but often important quantities. Sulfur is the third most abundant atomic constituent of crude oils. It is present in the medium and heavy fractions of crude oils. In the low and medium molecular ranges, sulfur is associated only with carbon and hydrogen, while in the heavier fractions it is frequently incorporated in the large polycyclic molecules that also contain nitrogen and oxygen. The total sulfur in crude oil varies from below 0.05 percent (by weight), as in some Pennsylvania oils, to about 2 percent for average Middle Eastern crudes and up to 5 percent or more in heavy Mexican or Mississippi oils. Generally, the higher the specific gravity of the crude oil, the greater is its sulfur content. The excess sulfur is removed from crude oil during refining, because sulfur oxides released into the atmosphere during the combustion of oil would constitute a major pollutant (pollution).

      The oxygen content of crude oil is usually less than 2 percent by weight and is present as part of the heavier hydrocarbon compounds in most cases. For this reason, the heavier oils contain the most oxygen. nitrogen is present in almost all crude oils, usually in quantities of less than 0.1 percent by weight. Sodium chloride also occurs in most crudes and is usually removed like sulfur.

      Many metallic (metal) elements are found in crude oils, including most of those that occur in seawater. This is probably due to the close association between seawater and the organic forms from which oil is generated. Among the most common metallic elements in oil are vanadium and nickel, which apparently occur in organic combinations as they do in living plants and animals.

      Crude oil also may contain a small amount of decay-resistant organic remains, such as siliceous skeletal fragments, wood, spores, resins, coal, and various other remnants of former life.

Physical properties
      Oil consists of a closely related series of complex hydrocarbon compounds that range from gasoline to heavy solids. The various mixtures that constitute crude oil can be separated by distillation under increasing temperatures (temperature) into such components as (from light to heavy) gasoline, kerosene, gas oil, lubricating oil, residual fuel oil, bitumen, and paraffin.

      Crude oils vary greatly in their chemical composition. Because they consist of mixtures of thousands of hydrocarbon compounds, their physical properties such as colour, specific gravity, and viscosity also vary widely.

      Crude oil is immiscible with and lighter than water; hence it floats. Crude oils are generally classified as tar sands (tar sand), heavy oils (heavy oil and tar sand), and medium and light oils on the basis of specific gravity (i.e., the ratio of the weight of equal volumes of the oil and pure water at standard conditions, with pure water considered to equal 1) and relative mobility. Tar sands contain immobile oil, which does not flow into a well bore (see below). Heavy crude oils have enough mobility that, given time, they can be obtained through a well bore in response to enhanced recovery methods. The more mobile medium and light oils are recoverable through production wells.

      The widely used American Petroleum Institute (API) gravity scale is based on pure water, with an arbitrarily assigned API gravity of 10°. Liquids lighter than water, such as oil, have API gravities numerically greater than 10. Crude oils below 20° API gravity are usually considered heavy, whereas the conventional crudes with API gravities between 20° and 25° are regarded as medium, with light oils ranging above 25°.

Boiling and freezing points
      Because oil is always at a temperature above the boiling point of some of its compounds, the more volatile constituents constantly escape into the atmosphere unless confined. It is impossible to refer to a common boiling point for crude oil because of the widely differing boiling points of its numerous compounds, some of which may boil at temperatures too high to be measured.

      By the same token, it is impossible to refer to a common freezing point for a crude oil because the individual compounds solidify at different temperatures. However, the pour point—the temperature below which crude oil becomes plastic and will not flow—is important to recovery and transport and is always determined. Pour points range from 32° C to below -57° C.

Measurement systems
      In the United States, crude oil is measured in barrels (barrel) of 42 gallons each; the weight per barrel of API 30° light oil would be about 306 pounds. In many other countries, crude oil is measured in metric tons. For oil having the same gravity, a metric ton is equal to approximately 252 imperial gallons or about 7.2 U.S. barrels.

Origin of crude oil

Formation process
From planktonic remains to kerogen
      Although it is recognized that the original source of carbon and hydrogen was in the materials that made up the primordial Earth, it is generally accepted that these two elements have had to pass through an organic phase to be combined into the varied complex molecules recognized as crude oil. The organic material that is the source of most oil has probably been derived from single-celled planktonic (plankton) (free-floating) plants, such as diatoms and blue-green algae, and single-celled planktonic animals, such as foraminifera, which live in aquatic environments of marine, brackish, or fresh water. Such simple organisms are known to have been abundant long before the Paleozoic Era, which began some 540,000,000 years ago.

      Rapid burial of the remains of the single-celled planktonic plants and animals within fine-grained sediments effectively preserved them. This provided the organic materials, the so-called protopetroleum, for later diagenesis (i.e., the series of processes involving biological, chemical, and physical changes) into true petroleum.

      The first, or immature, stage of petroleum formation is dominated by biological activity and chemical rearrangement, which convert organic matter to kerogen. This dark-coloured, insoluble product of bacterially altered plant and animal detritus is the source of most hydrocarbons generated in the later stages. During the first stage, biogenic methane is the only hydrocarbon generated in commercial quantities. The production of biogenic methane gas is part of the process of decomposition of organic matter carried out by anaerobic microorganisms (those capable of living in the absence of free oxygen).

From kerogen to petroleum
      Deeper burial by continuing sedimentation, increasing temperatures (temperature), and advancing geologic age result in the mature stage of petroleum formation, during which the full range of petroleum compounds is produced from kerogen and other precursors by thermal degradation and cracking (the process by which heavy hydrocarbon molecules are broken up into lighter molecules). Depending on the amount and type of organic matter, oil generation occurs during the mature stage at depths of about 760 to 4,880 metres (2,500 to 16,000 feet) at temperatures between 65° and 150° C. This special environment is called the “oil window.” In areas of higher than normal geothermal gradient (increase in temperature with depth), the oil window exists at shallower depths in younger sediments but is narrower. Maximum oil generation occurs from depths of 2,000 to 2,900 metres. Below 2,900 metres primarily wet gas, a type of gas containing liquid hydrocarbons known as natural gas liquids (liquefied petroleum gas), is formed.

      Approximately 90 percent of the organic material in sedimentary source rocks is dispersed kerogen. Its composition varies, consisting as it does of a range of residual materials whose basic molecular structure takes the form of stacked sheets of aromatic hydrocarbon rings in which atoms of sulfur, oxygen, and nitrogen also occur. Attached to the ends of the rings are various hydrocarbon compounds, including normal paraffin chains. The mild heating of the kerogen in the oil window of a source rock over long periods of time results in the cracking of the kerogen molecules and the release of the attached paraffin chains. Further heating, perhaps assisted by the catalytic effect of clay minerals in the source rock matrix, may then produce soluble bitumen compounds, followed by the various saturated and unsaturated hydrocarbons, asphaltenes, and others of the thousands of hydrocarbon compounds that make up crude oil mixtures.

      At the end of the mature stage, below about 4,880 metres, depending on the geothermal gradient, kerogen becomes condensed in structure and chemically stable. In this environment, crude oil is no longer stable and the main hydrocarbon product is dry thermal methane gas.

The geologic environment
Origin in source beds
      Knowing the maximum temperature reached by a potential source rock during its geologic history helps in estimating the maturity of the organic material contained within it. Also, this information may indicate whether a region is gas-prone, oil-prone, both, or neither. The techniques employed to assess the maturity of potential source rocks in core samples include measuring the degree of darkening of fossil pollen grains and the colour changes in conodont fossils. In addition, geochemical evaluations can be made of mineralogical changes that were also induced by fluctuating paleotemperatures. In general, there appears to be a progressive evolution of crude oil characteristics from geologically younger, heavier, darker, more aromatic crudes to older, lighter, paler, more paraffinic types. There are, however, many exceptions to this rule, especially in regions with high geothermal gradients.

      Accumulations of petroleum are usually found in relatively coarse-grained, permeable, and porous sedimentary reservoir rocks that contain little, if any, insoluble organic matter. It is unlikely that the vast quantities of oil now present in some reservoir rocks could have been generated from material of which no trace remains. Therefore, the site where commercial amounts of oil originated apparently is not always identical to the location at which they are ultimately discovered.

      Oil is believed to have been generated in significant volumes only in fine-grained sedimentary rocks (sedimentary rock) (usually clays, shales, or clastic carbonates) by geothermal action on kerogen, leaving an insoluble organic residue in the source rock. The release of oil from the solid particles of kerogen and its movement in the narrow pores and capillaries of the source rock is termed primary migration.

      Accumulating sediments can provide energy to the migration system. Primary migration may be initiated during compaction as a result of the pressure of overlying sediments. Continued burial causes clay to become dehydrated by the removal of water molecules that were loosely combined with the clay minerals. With increasing temperature, the newly generated hydrocarbons may become sufficiently mobile to leave the source beds in solution, suspension, or emulsion with the water being expelled from the compacting molecular lattices of the clay minerals. The hydrocarbon molecules would compose only a very small part of the migrating fluids, a few hundred parts per million.

Migration through carrier beds
 The hydrocarbons expelled from a source bed next move through the wider pores of carrier beds (e.g., sandstones or carbonates) that are coarser-grained and more permeable. This movement is termed secondary migration. The distinction between primary and secondary migration is based on pore size and rock type. In some cases, oil may migrate through such permeable carrier beds until it is trapped by a permeability barrier and forms an oil accumulation (Figure 1—>). In others, the oil may continue its migration until it becomes a seep on the surface of the Earth, where it will be broken down chemically by oxidation and bacterial action.

      Since nearly all pores in subsurface sedimentary formations are water-saturated, the migration of oil takes place in an aqueous environment. Secondary migration may result from active water movement or can occur independently, either by displacement or by diffusion. Because the specific gravity of the water in the sedimentary formation is considerably higher than that of oil, the oil will float to the surface of the water in the course of geologic time and accumulate in the highest portion of a trap (petroleum trap).

Accumulation in reservoir beds
      The porosity (volume of pore spaces) and permeability (capacity for transmitting fluids) of carrier and reservoir beds are important factors in the migration and accumulation of oil. Most petroleum accumulations have been found in clastic reservoirs (sandstones and siltstones). Next in number are the carbonate reservoirs (limestones and dolomites). Accumulations of petroleum also occur in shales and igneous and metamorphic rocks because of porosity resulting from fracturing, but such reservoirs are relatively rare. Porosities in reservoir rocks usually range from about 5 to 30 percent, but all available pore space is not occupied by petroleum. A certain amount of residual formation water cannot be displaced and is always present.

      Reservoir rocks may be divided into two main types: (1) those in which the porosity and permeability is primary, or inherent, and (2) those in which they are secondary, or induced. Primary porosity and permeability are dependent on the size, shape, and grading and packing of the sediment grains and also on the manner of their initial consolidation. Secondary porosity and permeability result from postdepositional factors, such as solution, recrystallization, fracturing, weathering during temporary exposure at the Earth's surface, and further cementation. These secondary factors may either enhance or diminish the inherent conditions.

Oil traps
      After secondary migration in carrier beds, oil finally collects in a trap. The fundamental characteristic of a trap is an upward convex form of porous and permeable reservoir rock that is sealed above by a denser, relatively impermeable cap rock (e.g., shale or evaporites). The trap may be of any shape, the critical factor being that it is a closed, inverted container. A rare exception is hydrodynamic trapping, in which high water saturation of low-permeability sediments reduces hydrocarbon permeability to near zero, resulting in a water block and an accumulation of petroleum down the structural dip of a sedimentary bed below the water in the sedimentary formation.

Structural traps
 Traps can be formed in many ways (Figure 1—>). Those formed by tectonic events, such as folding or faulting of rock units, are called structural traps. The most common structural traps are anticlines, upfolds of strata that appear as ovals on the horizontal planes of geologic maps. About 80 percent of the world's petroleum has been found in anticlinal traps. Most anticlines were produced by lateral pressure, but some have resulted from the draping and subsequent compaction of accumulating sediments over topographic highs. The closure of an anticline is the vertical distance between its highest point and the spill plane, the level at which the petroleum can escape if the trap is filled beyond capacity. Some traps are filled with petroleum to their spill plane, but others contain considerably smaller amounts than they can accommodate on the basis of their size.

      Another kind of structural trap is the fault trap. Here, rock fracture results in a relative displacement of strata that forms a barrier to petroleum migration. A barrier can occur when an impermeable bed is brought into contact with a carrier bed. Sometimes the faults themselves provide a seal against “updip” migration when they contain impervious clay gouge material between their walls. Faults and folds often combine to produce traps, each providing a part of the container for the enclosed petroleum. Faults can, however, allow the escape of petroleum from a former trap if they breach the cap rock seal.

      Other structural traps are associated with salt domes (salt dome). Such traps are formed by the upward movement of salt masses from deeply buried evaporite beds, and they occur along the folded or faulted flanks of the salt plug or on top of the plug in the overlying folded or draped sediments.

Stratigraphic traps
      A second major class of oil traps is the stratigraphic trap. It is related to sediment deposition or erosion and is bounded on one or more sides by zones of low permeability. Because tectonics ultimately control deposition and erosion, however, few stratigraphic traps are completely without structural influence. The geologic history of most sedimentary basins contains the prerequisites for the formation of stratigraphic traps. Typical examples are fossil carbonate reefs, marine sandstone bars, and deltaic distributary channel sandstones. When buried, each of these geomorphic features provides a potential reservoir, which is often surrounded by finer-grained sediments that may act as source or cap rocks.

      Sediments eroded from a landmass and deposited in an adjacent sea change from coarse- to fine-grained with increasing depth of water and distance from shore. Permeable sediments thus grade into impermeable sediments, forming a permeability barrier that eventually could trap migrating petroleum.

      There are many other types of stratigraphic traps. Some are associated with the many transgressions and regressions of the sea that have occurred over geologic time and the resulting deposits of differing porosities. Others are caused by processes that increase secondary porosity, such as the dolomitization of limestones or the weathering of strata once located at the Earth's surface.

World distribution of 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 exploration for oil began during the early 1860s, 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 5,000,000,000 or more barrels of ultimately recoverable oil, and world-class giants, fields with 500,000,000 to 5,000,000,000 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 (Figure 2—>). The remaining supergiants are distributed as follows: two in the United States, two in Russia, two in Mexico, one in Libya, one in Algeria, one in Venezuela, and two in China.

      The nearly 280 world-class giant fields thus far discovered, plus the supergiants, account for about 80 percent of the world's known recoverable oil. There are, in addition, approximately 1,000 known large oil fields that initially contained between 50,000,000 and 500,000,000 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 petroleum fields and significant petroleum-producing sedimentary basins 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 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 total known oil. Exploration has occurred in another 240 basins, but discoveries of commercial significance have not been made.

Geologic study and exploration (prospecting)
      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 polar regions or submerged continental margins. The larger sedimentary basins—those containing more than 833,000 cubic kilometres (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
      The first 200,000,000,000 barrels of world oil were produced in 109 years from 1859 to 1968. Since that time world oil production rates have stabilized at a rate of about 22,000,000,000 barrels a year.

       Recoverable Oil Resources of the World*, TableTable 1 (Recoverable Oil Resources of the World*, Table) shows the broad distribution of the world oil supply. Reserves are identified quantities of “in-place” petroleum that are considered recoverable under current economic and technological conditions. Estimated by petroleum engineers and geologists using drilling and production data along with other subsurface information, these figures are revised to include projected field growth as development progresses. Petroleum reserves are reported by oil companies and by some governments, and such data are compiled by the U.S. Department of Energy and the U.S. Geological Survey, as well as by oil industry trade journals. Undiscovered petroleum resources of the world have been estimated by the U.S. Geological Survey by the extrapolation of known production and reserve data into untested sediments of similar geology. A most likely consensus estimate was established, as was a range with upper and lower yield limits at 5 and 95 percent probabilities. The range for undiscovered oil resources assessed for the whole world is 275,000,000,000 to 1,469,000,000,000 barrels.

       Recoverable Oil Resources of the World*, TableTable 1 (Recoverable Oil Resources of the World*, Table) indicates that the most likely total world oil endowment is about 2,390,000,000,000 barrels. Of this amount, 77 percent has already been discovered and 30 percent has already been produced and consumed. If this estimate proves to be reasonably accurate, current relatively stabilized world oil-production volumes could be sustained to about the middle of the 21st century, at which time a shortage of conventional oil resources would force a production decline.

      The Middle East is thought to have had an estimated 41 percent of the world's total oil endowment. North America is a distant second but has already produced almost half of its total oil. Eastern Europe, because of the large deposits in Russia, is well endowed with oil. Western Europe is not, with most of its oil under the North Sea. Likewise, Africa, Asia, and South America are thought to have only relatively moderate amounts of oil. It is interesting to note that a large undiscovered oil resource is believed to exist in North America, which has many frontier basins. Both the Middle East and eastern Europe, however, are also thought to contain significant oil prospects.

Major oil-producing countries
       Leading Oil Countries, TableTable 2 (Leading Oil Countries, Table) lists the 18 countries believed to have had an original oil endowment exceeding 20,000,000,000 barrels. It also serves to show the concentration of world oil. These 18 countries have accounted for 86 percent of the world's oil production. They hold 94 percent of its reserves. Significantly, they are projected to have 82 percent of the world's remaining undiscovered oil resources. As can be seen, regions geologically favourable to the generation and deposition of oil are fairly rare. The 18 countries listed are estimated to have contained 89 percent of the world's original oil endowment.

 Saudi Arabia, shown in Figure 2—>, is thought to have had the largest original oil endowment of any country. The discovery that transformed Saudi Arabia into a leading oil country was the Al-Ghawār field. Discovered in 1948, this field has proved to be the world's largest, containing 82,000,000,000 barrels. Another important discovery was the Saffānīyah offshore field in the Persian Gulf. It is the third largest oil field in the world and the largest offshore. Saudi Arabia has eight other supergiant oil fields. Thus, it has the largest oil reserve in the world, not to mention significant potential for additional discoveries.

  Russia is thought to possess the best potential for new discoveries. Also, it has significant reserves. Russian oil is derived from many sedimentary basins within the vast country, while Saudi Arabian fields, as well as many other Middle Eastern fields, are located in the great Arabian-Iranian basin (Figures 2—> and 3—>). Russia has two supergiant oil fields, Samotlor and Romashkino. Production from these fields is on the decline, bringing total Russian oil output down with them. The best prospects for new Russian discoveries appear to exist in the difficult and expensive frontier areas.

 North America also has many sedimentary basins; they are shown in Figure 4—>. Basins in the United States have been intensively explored and their oil resources developed. More than 33,000 oil fields have been found, but only two are 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 but 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 United States, has contributed to a significant drop in domestic oil output. Mexico has produced only about one-fifth of its estimated total oil endowment. With two supergiant fields (Cantarell offshore of Campeche state and Bermudez in Tabasco state) and with substantial remaining reserves and resources, it will be able to sustain current production levels well into the 21st century. Conversely, Canada, with considerably smaller oil reserves and most of its undiscovered resource potential in remote regions, is unlikely to be able to sustain current production levels beyond the 1990s. Canada's largest oil field is Hibernia, discovered off Newfoundland in 1979. This giant field has yet to be developed.

 The Middle Eastern (Middle East) countries of Iraq, Kuwait, and Iran are each estimated to have had an original oil endowment in excess of 100,000,000,000 barrels. These countries have a number of supergiant fields, all of which are located in the Arabian-Iranian basin, including Kuwait's Al-Burqān field (Figure 2—>). Al-Burqān is the world's second largest oil field, having originally contained 75,000,000,000 barrels of recoverable oil. Iraq possesses a significant potential for additional oil discoveries.

      The United Kingdom is an important North Sea exporter; however, as its undiscovered resource potential appears somewhat limited, it may require more of its oil output for internal use in the future.

Undiscovered resources
      With an estimated 77 percent of the world's total recoverable oil endowment having already been discovered, the remaining 23 percent, mostly located in smaller fields or in more difficult environments, is expected to become ever more expensive to find and to recover. More than 11,000 man-years were required to construct the largest of the North Sea gravity platforms, making capital costs per daily oil production as much as 40 times the costs in the Middle East. A guyed tower constructed in more than 300 metres of water in the Gulf of Mexico has been estimated to produce oil at about 65 times the production cost in the Middle East. As oil exploitation moves into deeper waters or under Arctic ice, the cost will further escalate and will be reflected in the world economy.

Joseph P. Riva, Jr. Gordon I. Atwater

Additional Reading
Robert H. Dott, Sr., and Merrill J. Reynolds (comps.), Sourcebook for Petroleum Geology (1969), is an excellent collection of petroleum-geology scientific concepts. Edgar Wesley Owen, Trek of the Oil Finders: A History of Exploration for Petroleum (1975); and Harold F. Williamson and Arnold R. Daum, The American Petroleum Industry, 2 vol. (1959–63, reprinted 1981), relate the history of petroleum geology and the development of the oil industry. Kenneth K. Landes, Petroleum Geology, 2nd ed. (1959, reprinted 1975); L.W. LeRoy, D.O. LeRoy, and J.W. Raese (eds.), Subsurface Geology: Petroleum, Mining, Construction, 4th ed. (1977); B.P. Tissot and D.H. Welte, Petroleum Formation and Occurrence, 2nd rev. ed. (1984); and John M. Hunt, Petroleum Geochemistry and Geology (1979), are sources of information on theories of the origin and accumulation of petroleum, as well as on the practical applications of scientific knowledge to petroleum problems. E.N. Tiratsoo, Oilfields of the World, 3rd ed., rev. (1986), describes the nature and distribution of the major oil fields of the world. C.D. Masters, D.H. Root, and E.D. Attanasi, “Resource Constraints in Petroleum Production Potential,” Science, 253(5016):146–152 (July 12, 1991), contains U.S. Geological Survey assessments of world oil and gas resources. Joseph P. Riva, Jr., Exploration Opportunities in Latin America (1992), provides information on Latin American oil and gas reserves, resources, and production potential, and Petroleum Exploration Opportunities in the Former Soviet Union (1994), contains similar information for each of the former Soviet republics. Michel T. Halbouty (ed.), Future Petroleum Provinces of the World (1986), contains an annotated list of the world's giant oil and gas fields. Kenneth K. Landes, Petroleum Geology of the United States (1970), describes in detail petroleum geology on a state-by-state basis. Joseph P. Riva, Jr., John J. Schanz, Jr., and John G. Ellis, U.S. Conventional Oil and Gas Production: Prospects to the Year 2000 (1985), assesses U.S. domestic reserves, resources, and production potential. Joseph P. Riva, Jr., World Petroleum Resources and Reserves (1983), treats the formation and accumulation of conventional and unconventional petroleum, exploration and production methods, and petroleum basin geology. G.D. Hobson (ed.), Modern Petroleum Technology, 5th ed., 2 vol. (1984), discusses petroleum engineering, including production, refining, and transport. Fillmore C.F. Earney, Petroleum and Hard Minerals from the Sea (1980), examines worldwide development of seabed resources with emphasis on offshore petroleum.Collections of scientific papers may be found in G.D. Hobson (ed.), Developments in Petroleum Geology, 2 vol. (1977–80); and in publications of the American Association of Petroleum Geologists, including the AAPG Bulletin (monthly), the October issue of which contains an annual review of significant exploration and production activity; and the AAPG Memoir (irregular). Basic Petroleum Data Book (three per year); and Minerals Yearbook, prepared by the U.S. Bureau of Mines, include annual statistical reviews of the petroleum industry. Each year maps, production figures, and geologic data are published in August by World Oil and in December by the Oil and Gas Journal.Joseph P. Riva, Jr. Gordon I. Atwater

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Universalium. 2010.

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