PETROLEUM

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 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. also called Oil, or Crude Oil, complex mixture of hydrocarbons that occurs in the Earth in liquid, gaseous, or solid forms. The term is usually 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. Petroleum was known to many ancient peoples through surface seepages. Excavations in Iran, Iraq, and elsewhere show that bitumen, or asphalt, was used to caulk ships, to build roads, and for other purposes. Europeans of the age of exploration found similar seepages of the black liquid in the Americas and the Dutch East Indies (now Indonesia). The first important modern use of crude oil was as an illuminating fuel to replace whale oil in lamps. The first well drilled specifically for oil was that of E.L. Drake at Titusville, Pa., in 1859; within a few decades oil drilling was widespread in the United States, Europe, the Middle East, and the East Indies. The development of the automobile gave petroleum a new and swiftly expanding role as the primary source of gasoline. In subsequent decades oil and then natural gas replaced coal as the primary fuel for industrial and domestic heating. Petrochemicals derived from petroleum became the source of such chemical products as solvents, paints, plastics, synthetic rubber and fibres, soaps and cleansing agents, waxes and jellies, explosives, and fertilizers. Fuels derived from petroleum power the engines of automobiles, airplances, ships, tractors, trucks, and rockets. Petroleum fuels also generate a large portion of the world's electrical-power supply. Asphalt from petroleum is used to surface roads and highways. Petroleum is also used as a lubricant in a great variety of machines. Crude oil and natural gas are costly to locate, recover, and process, but together they have become the world's largest energy source, accounting for approximately 60 percent of all energy consumed. Saudi Arabia, the United States, and Russia are the world's leading producers; the United States is by far the largest consumer. About 30 percent of the world's original endowment of light and medium crude oil has been produced and consumed, as have about 14 percent of the heavy crude oil and 14 percent of the natural gas endowment. At current rates of consumption, the world supply of crude oil will be exhausted by the mid-21st century. If consumption of natural gas continues to grow, it too will be relatively short-lived as an energy source. Depletion of these resources will place greater urgency on extracting synthetic crude oil from bitumen and oil shale, deposits of which have been little exploited as long as oil and gas have remained plentiful. Petroleum is derived from aquatic plants and animals that lived and died hundreds of millions of years ago. Their remains mixed with mud and sand in layered deposits that, over the millennia, were geologically transformed into sedimentary rock. Gradually the organic matter decomposed into petroleum, which migrated from the original source beds to more porous and permeable rocks, such as sandstones and siltstones, where it finally became entrapped. Such entrapped accumulations of petroleum are called reservoirs. A series of reservoirs within a common rock structure or a series of reservoirs in separate but neighbouring formations is commonly referred to as an oil (or gas) field. A group of fields is often found in a single geologic environment known as a sedimentary basin or province. Bitumen is formed by crude oil that has migrated toward the Earth's surface and has been stripped of its lighter fractions by descending water. The substance is found immobilized in deposits commonly known as tar sands. The major components of petroleum are hydrocarbons, compounds of hydrogen and carbon that display great variation in their molecular structure. The simplest hydrocarbons are a large group of chain-shaped molecules known as the paraffins. This broad series extends from methane, which forms natural gas, through liquids that are refined into ordinary gasoline, to crystalline waxes. A series of ring-shaped hydrocarbons, known as the naphthenes, ranges from volatile liquids such as naphtha to tarry substances such as the asphaltenes, the main constituents of bitumen. Another group of ring-shaped hydrocarbons is known as the aromatics; the chief compound in this series is benzene, a popular raw material for making petrochemicals. Nonhydrocarbon constituents of petroleum include sulfur, oxygen, nitrogen, and the metals nickel and vanadium. Most of these impurities are removed during refining-especially sulfur, which forms a noxious air pollutant upon combustion. In oil exploration, geologic techniques can determine only the existence of rock formations that are favourable for oil deposits, not whether oil is actually there. Drilling is the only sure way to ascertain the presence of oil. With modern rotary equipment, wells can be drilled to depths of more than 9,000 m (30,000 feet). Once oil is found, it may be recovered (brought to the surface) by the pressure created by natural gas or water within the reservoir, by injecting water or steam into the reservoir to raise the pressure artificially, or by injecting such substances as carbon dioxide, polymers, and solvents to reduce oil viscosity and capillarity. Thermal recovery methods are frequently used to enhance the production of heavy crude oils, whose extraction is impeded by viscous resistance to flow at reservoir temperatures. Crude oil is transported to refineries by pipelines, which can often carry more than 500,000 barrels per day, or by oceangoing tankers. The basic refinery process is distillation, which separates the crude oil into fractions of differing volatility. After the distillation, other physical methods are employed to separate the mixtures, including absorption, adsorption, solvent extraction, and crystallization. After physical separation into such constituents as light and heavy naphthas, kerosene, and light and heavy gas oils, selected petroleum fractions may be subjected to chemical conversion processes, such as cracking and reforming. In the most general terms, cracking breaks the large molecules of heavier gas oils into the smaller molecules that form the lighter, more valuable naphtha fractions. Reforming changes the structure of straight-chain paraffin molecules into branched-chain isoparaffins and ring-shaped aromatics. The process is widely used to raise the octane number of gasolines obtained by distillation of paraffinic crude oils. 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