petroleum, oily, flammable liquid that occurs naturally in deposits, usually beneath the surface of the earth; it is also called crude oil. It consists principally of a mixture of hydrocarbons, with traces of various nitrogenous and sulfurous compounds.

Origin and Natural Occurrence

During the past 600 million years incompletely decayed plant and animal remains have become buried under thick layers of rock. It is believed that petroleum consists of the remains of these organisms but it is the small microscopic plankton organism remains that are largely responsible for the relatively high organic carbon content of fine-grained sediments like the Chattanooga shale which are the principle source rocks for petroleum. Among the leading producers of petroleum are Saudi Arabia, Russia, the United States (chiefly Texas, California, Louisiana, Alaska, Oklahoma, and Kansas), Iran, China, Norway, Mexico, Venezuela, Iraq, Great Britain, the United Arab Emirates, Nigeria, and Kuwait. The largest known reserves are in the Middle East.

Exploration and Drilling of Wells

Because of the subterranean origin of petroleum it must be extracted by means of wells. Until an exploratory well, or wildcat, has been dug, there is no sure way of knowing whether or not petroleum lies under a particular site. In order to reduce the number of exploratory wells drilled, scientific methods are used to pick the most promising sites. Sensitive instruments, such as the gravimeter, the magnetometer, and the seismograph, may be used to find subsurface rock formations that can hold crude oil. Drilling is a fairly complex and often risky process. Some wells must be dug several miles deep before petroleum deposits are reached. Many are now drilled offshore from platforms standing in the ocean bed. Usually the petroleum from a new well will come to the surface under its own pressure. Later the crude oil must be pumped out or forced to the surface by injecting water, air, natural gas, steam, carbon dioxide, or another substance into the deposits. Enhanced recovery techniques have increased the percentage of oil that can be extracted from a field.

Composition and Refining of Petroleum

The physical properties and exact chemical composition of crude oil varies from one locality to another. The different hydrocarbon components of petroleum are dissolved natural gas, gasoline, benzine, naphtha, kerosene, diesel fuel and light heating oils, heavy heating oils, and finally tars of various weights (see tar and pitch). The crude oil is usually sent from a well to a refinery in pipelines (see under pipe) or tanker ships.

The hydrocarbon components are separated from each other by various refining processes. In a process called fractional distillation petroleum is heated and sent into a tower. The vapors of the different components condense on collectors at different heights in the tower. The separated fractions are then drawn from the collectors and further processed into various petroleum products. One of the many products of crude oil is a light substance with little color that is rich in gasoline. Another is a black tarry substance that is rich in asphalt.

As the lighter fractions, especially gasoline, are in the greatest demand, so-called cracking processes have been developed in which heat, pressure, and certain catalysts are used to break up the large molecules of heavy hydrocarbons into small molecules of light hydrocarbons. Some of the heavier fractions find eventual use as lubricating oils, paraffins, and highly refined medicinal substances such as petrolatum.

See also petrochemicals.

History and Development of Petroleum

Petroleum has been known throughout historical time. It was used in mortar, for coating walls and boat hulls, and as a fire weapon in defensive warfare. Native Americans used it in magic and medicine and in making paints. Pioneers bought it from the Native Americans for medicinal use and called it Seneca oil and Genesee oil. In Europe it was scooped from streams or holes in the ground, and in the early 19th cent. small quantities were made from shale. In 1815 several streets in Prague were lighted with petroleum lamps.

The modern petroleum industry began in 1859, when the American oil pioneer E. L. Drake drilled a producing well on Oil Creek in Pennsylvania at a place that later became Titusville. Many wells were drilled in the region. Kerosene was the chief finished product, and kerosene lamps soon replaced whale oil lamps and candles in general use. Little use other than as lamp fuel was made of petroleum until the development of the gasoline engine and its application to automobiles, trucks, tractors, and airplanes. Today the world is heavily dependent on petroleum for motive power, lubrication, fuel, dyes, drugs, and many synthetics. The widespread use of petroleum has created serious environmental problems. The great quantities that are burned as fuels generate most of the air pollution in industrialized countries, and oil spilled from tankers and offshore wells has polluted oceans and coastlines.

See also energy, sources of; oil industry.

liquefied petroleum gas or LPG, mixture of gases, chiefly propane and butane, produced commercially from petroleum and stored under pressure to keep it in a liquid state. The boiling point of liquefied petroleum gas varies from about -44°C; to 0°C; (-47°F; to 32°F;), so that the pressure required to liquefy it is considerable and the containers for it must be of heavy steel. When prepared as fuel, LPG is largely propane; common uses are for powering automotive vehicles, for cooking and heating, and sometimes for lighting in rural areas. LPG is an attractive fuel for internal-combustion engines; because it burns with little air pollution and little solid residue, it does not dilute lubricants, and it has a high octane rating.

cracking of petroleum: see petroleum.

Organization of Petroleum Exporting Countries (OPEC), multinational organization (est. 1960, formally constituted 1961) that coordinates petroleum policies and economic aid among oil-producing nations. Its Board of Governors and board chairperson are elected by member nations; OPEC's headquarters are in Vienna, Austria. Members consist of Algeria, Angola, Indonesia, Iran, Iraq, Kuwait, Libya, Nigeria, Qatar, Saudi Arabia, the United Arab Emirates, and Venezuela. Ecuador and Gabon suspended their memberships in 1992 and 1994, respectively. Saudi Arabia has traditionally dominated the organization, owing to its enormous oil reserves; the organization's members produce about 40% of the world's crude oil.

In 1973, as a result of the Arab oil embargo against Western nations who supported Israel during the Yom Kippur War (see Arab-Israeli Wars), OPEC was able to raise oil prices tremendously; the price hike caused inflation in oil-importing nations. Increases ensued from 1975 to 1980. However, as importing countries pursued alternate energy resources, OPEC was forced to lower prices by 1982. Oil prices remained low through most of the 1980s and 90s, with only a temporary hike during the Persian Gulf crisis of 1990-91 (see Persian Gulf War). With the cooperation of non-OPEC oil-exporting nations, OPEC was able to raise prices in 1999 by cutting production. As prices rose above $30 a barrel in early 2000, OPEC members agreed to increase production somewhat, cutting back production again a year later in an attempt to maintain prices. A worldwide economic slowdown caused oil prices to fall to near $20 by late 2001, but cutbacks by OPEC and non-OPEC nations, an economic rebound (including very strong economic growth in China), and the U.S. invasion and occupation of Iraq subsequently caused benchmark prices to rise and stay above $40 in mid-2004, with peaks above $50 at times. Efforts by OPEC to control prices, however, have generally been less influential than market forces.

National Petroleum Reserve, area, c.23 million acres (9.32 million hectares), Alaska North Slope, situated W of Prudhoe Bay and the Arctic National Wildlife Refuge. The reserve, which is the largest remaining parcel of unprotected public land in the United States, overlies a petroleum deposit estimated at 6-13 billion barrels. Designated the Naval Petroleum Reserve Number 4 in 1923 and run by the U.S. Navy, it passed to the federal Bureau of Land Management (BLM) in 1976, when its present name was adopted. From the mid-1990s to the mid-2000s the BLM sold oil leases for the NW and NE areas of the reserve. A plan for additional oil and gas drilling in the SW reserve was formulated in 1998 and later expanded (2004) to include opening most of a 9.2-million-acre (3.7-million-hectare) section of the area to drilling. Several environmental groups have objected to the plans, citing dangers to wetlands and wildlife and threats to the livelihood of indigenous people.

Nasby, Petroleum V., pseud. of David Ross Locke, 1833-88, American journalist and satirist, b. Vestal, N.Y. Locke was editor of the Findlay, Ohio, Jeffersonian when he first became prominent by publishing (1861) in it the Nasby letters. The writer, Petroleum Vesuvius Nasby, was ostensibly an ignorant, violently prejudiced, proslavery sympathizer, and the letters, which caught the fancy of readers from Lincoln down, were of aid to the Union cause in the Civil War. The letters soon appeared in the Toledo Blade, of which Locke became editor and part owner in 1865. He subsequently wrote Nasby letters as satiric propaganda for other causes. The Nasby letters were collected in various volumes including Swingen Round the Circle (1866) and The Nasby Letters (1893).

Subsurface reservoir of petroleum. The oil is always accompanied by water and often by natural gas; all are confined in porous rock. The natural gas, being lightest, occupies the top of the trap and is underlain by the oil and then the water. A layer of impervious rock, called the roof rock, prevents the escape of the petroleum. The part of the trap that is actually occupied by the oil and gas is called the petroleum reservoir.

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or crude oil

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 60percnt 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.

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