Petroleum may be--after food--the most important substance consumed in modern society. It provides not only raw materials for the ubiquitous plastics and other synthetics, but also fuel for energy, industry, heating, and transportation. The word petroleum, derived from the Latin petra and oleum, means literally "rock oil" and refers to hydrocarbons that occur widely in the sedimentary rocks of the Earth's crust in the form of gases, liquids, semisolids, or solids. Mixtures of gases and liquids are the most common forms. From a chemical standpoint petroleum is an extremely complex mixture of \Thydrocarbon\t compounds, usually with minor amounts of impurities--nitrogen, oxygen, sulfur, and small quantities of trace metals. The fuels that are derived from petroleum supply more than half of the world's total supply of energy. Gasoline, kerosene, and diesel oil provide fuel for automobiles, tractors, trucks, aircraft, and ships. Fuel oil and natural gas are used to heat homes and commercial buildings, as well as to generate electricity. Petroleum products are the basic materials used for the manufacture of synthetic fibers for clothing and in plastics, paints, fertilizers, insecticides, soaps, and synthetic rubber. The uses of petroleum as a source of raw material in manufacturing are central to the functioning of modern industry. THE GEOLOGY OF PETROLEUM Geologists generally agree that petroleum is formed through the progressive chemical change of materials provided by microscopic aquatic organisms that were incorporated over aeons in marine sedimentary rocks, where most of the world's petroleum is found. Transformation of some of this sedimentary material to petroleum probably began soon after deposition, with bacteria playing a role in the initial stages and clay particles serving as catalysts. Heat within the Earth's crust provided energy for the reaction, temperatures increasing more or less directly with depth. Some evidence indicates that most petroleum has formed at temperatures not exceeding about 100 deg to 120 deg C (210 deg to 250 deg F), with the generation of petroleum hydrocarbons beginning as low as 65 deg C (150 deg F). At temperatures above 175 deg to 200 deg C (345 deg to 390 deg C) most liquid hydrocarbons are destroyed, although natural gas is not. The processes by which petroleum migrates into pools are also poorly known. Commercial deposits of oil and gas occur in pools or fields in which the oil or gas occupies pore space in the rock. Some pools are large, extending laterally over many square kilometers, with their vertical extent ranging from several meters to as much as several hundred meters. The oil or gas occupying the pore space in the pool has displaced the water that was initially present in the pores. The presence of a pool of oil or gas therefore implies that the oil or gas has migrated into the pool. When sediments such as mud and clay are deposited, they contain a large amount of water. As layer upon layer of sediment accumulates, the progressively increasing load of material causes the sediment to compact, expelling some of the water in the pores. Migration probably begins at this stage and may involve a substantial horizontal component. Initially, the oil hydrocarbons may be present in the water either suspended as tiny globules or dissolved, hydrocarbons being slightly soluble in water. Furthermore, the presence of soaplike substances in the water possibly enables the hydrocarbons to be taken up by the water, or solubilized. Sediments may accumulate to great thicknesses in the Earth's crust (see \Tsediment, marine\t). For example, in the Gulf Coast region of the United States, a thickness of sediments aggregating as much as 20 km (12 mi) has accumulated. With the deposition of such a thick sediment, the quantity of fluid expelled in the compaction process is large. Hydrocarbons are probably swept along by the moving water and subsequently partly extracted from the water by aggregation into thin films or small globules of oil or gas. The films or globules coalesce into progressively larger masses of oil or gas, eventually becoming large enough to enable the mass, propelled by buoyancy, to migrate in an upward direction through permeable rocks. Buoyancy results because the oil or gas has a lower density than the water. Reservoir Rocks and Traps An oil or gas pool requires a reservoir rock that is porous and permeable and a trap to contain the oil or gas. Migration conveys the oil or gas from its source into and through the reservoir rock and to a trap, where it finally accumulates. Most traps are created by a structural relationship in which a relatively permeable reservoir rock is bounded above by impermeable rock. Under conditions of buoyancy, the less dense oil or gas migrates into the trap, displaces the water, and remains in the trap. A common type of trap is a dome, or anticline, that dips away in all directions. An anticline trap is a form of structural trap because its trapping properties are due to deformation or folding of the layers of rock after they were deposited. Other common structural traps are created by relationships involving faults. Stratigraphic traps are another form of trap, resulting from variations in the layers, or strata, that were created when the strata were deposited. Examples of stratigraphic traps include sandstone lenses, limestone reefs, and wedgeouts formed by depositional variations. The erosion of dipping strata along a former land surface that was subsequently buried by renewed deposition may also create a stratigraphic trap. The distinction between structural and stratigraphic traps is often blurred. For example, an anticlinal trap may be related to an underlying buried limestone reef. Beds of sandstone may wedge out against an anticline because of depositional variations or intermittent erosion intervals. \Lsalt dome\ls, formed by flowage of salt at substantial depths, also have created numerous traps that are both structural and stratigraphic. Virtually all types of sedimentary rock form reservoir rocks under certain conditions. Most reservoir rocks, however, consist of \Tsandstone\t or \Tlimestone\t. A good reservoir rock is one that is both permeable and porous (see \Tpermeability, rock\t). A sandstone composed of large, rounded grains that are more or less uniform in size is an ideal reservoir rock, because both its permeability and porosity are high. The permeability and porosity of sandstones, however, are highly variable, depending on grain sizes and usually related to solution of part of the rock, as water selectively dissolves some of it. \Lshale\ls, siltstones, and mudstones tend to be impermeable but sometimes serve as reservoirs if extensively fractured. The ages of rock that yield petroleum range from Precambrian to Recent (see \Tgeologic time\t). Rocks deposited during the Tertiary, Cretaceous, Permian, Pennsylvanian, Mississippian, Devonian, and Ordovician periods are particularly productive. By contrast, rocks of Jurassic, Triassic, Silurian, and Cambrian age are less productive. Rocks of Precambrian age yield only under exceptional circumstances. Although most petroleum is produced from underground reservoirs, petroleum occurs in a variety of forms at the surface. Surface seepages of oil and gas are common in many regions. Seeps range from mere traces, such as a film of oil or gas bubbling through water, to deposits large enough to be commercially valuable. Natural gas is almost instantly lost to the atmosphere when it seeps out, but the residue formed by seeping oil may form large deposits of heavy oil, tar, or asphalt. Some of the world's major asphalt deposits, containing millions of tons of asphalt, are simply giant oil seeps (see \Ttar, pitch, and asphalt\t). The presence of active seeps in an area is evidence that oil and gas are still migrating. Major Fields The great majority of oil reserves identified to date are located in a relatively small number of very large fields, known as "giants." In fact, the 300 largest oil fields contain about three-quarters of the world's discovered oil. This does not represent the real, worldwide distribution of oil, however, so much as the fact that the search for oil typically concentrates on the largest potential deposit. In the United States many fields produce only a few barrels of oil a day. Other petroleum provinces probably also contain large amounts in small pools, to be exploited in the future. Although most of the world's nations produce at least minor amounts of oil, the primary concentrations are in the Persian Gulf, North and West Africa, the North Sea, and the Gulf of Mexico. Out of about 90 oil-producing nations, five countries in the Middle East contain two-thirds of current, known oil reserves. PETROLEUM EXPLORATION, DEVELOPMENT, AND PRODUCTION Exploration for oil and gas is conducted on a worldwide basis, both on land and on the continental shelves. Seismic surveys are conducted in advance of exploratory drilling. These surveys essentially map an area's geology by measuring the reflection of sound waves from the underlying rock strata, in particular looking for traps where petroleum may have accumulated. Seismic surveys provide a remarkably detailed overview of the geologic, structural, and stratigraphic conditions beneath the surface and under exceptional conditions may reveal directly the presence of gas. New technologies, employing lasers and satellites, are also used to detect minute seepages that indicate underlying deposits. Normally, however, drilling is necessary to confirm the presence or absence of commercially producible amounts of gas and oil. Geologic information is also provided by drilling and is obtained through the use of modern borehole logging techniques. Drilling technology has advanced substantially in recent decades, particularly with respect to the capability of drilling from ships in water as deep as 1,800 m (6,000 ft), or from platforms whose legs are anchored to the seafloor. The development of horizontal drilling techniques has increased productivity, allowing producers access to deposits that were previously left untapped. Drilling Most wells are now drilled by the rotary method. A steel bit attached to a drill pipe is revolved at the bottom of a hole. This action breaks up the rock by chipping and cutting it. Meanwhile a special mud mixture runs down through the multichannel drill pipe, lubricating the bit, carrying rock cuttings upward to the surface, and creating a pressure that prevents subsurface water from infiltrating the well area. When the drill reaches oil-bearing formations, a casing pipe containing special tubing is lowered into the hole and used to withdraw the fluid. If necessary, explosives or special acid or sand solutions may be injected through the casing pipe to break up the formation and increase the flow of oil and gas. The rate of the flow and the pressure and volume of the well are controlled by special pipings and gate valves--called a "Christmas tree"--installed on the drilling rig above ground. Once it has been established that a formation contains oil or gas, the precise limits of the field must be delineated through further drilling, since only a portion of any given geological formation will contain petroleum. Production begins once it is known that the extent of the field will allow economical exploitation, and the precise placement of wells is determined after analysis of the size of the field and the geology of the rock. Drilling continues after a field enters production. Extension wells are drilled to further define the boundaries of the oil, infill drilling is conducted within the field to increase recovery rates, and service wells are used to reopen wells that have become clogged. Additionally, wells are often drilled at the same location but to different depths, to test other geological structures for the presence of petroleum. The chemistry of the oil, and especially the presence of gas, can require additional aboveground processing equipment. When natural gas is present, it must be separated from the liquid petroleum, and lighter liquids, like ethane, butane and propane, must be condensed out of the gas, to be sold as fuels or to the petrochemical industry. At gas fields carbon dioxide and sulfur gases may exist as contaminants and are stripped out before the gas is shipped. Initial production is usually through the mechanism of primary recovery--that is, an initial reliance on the field's own pressure to bring the oil or gas to the surface. As the field ages and pressure drops, this is followed, however, by pumping the oil up to maintain production levels. Most fields will yield only one-quarter to one-third of their oil through pumping, depending on the porosity of the rock and the viscosity of the petroleum. The proportion of natural gas recovered is usually much higher, on the order of three-quarters. Secondary recovery, consisting of the pumped injection of water or gas into the field, is used to restore pressure and increase the proportion of petroleum removed. Tertiary, or enhanced, recovery has also been used on some oil fields. This involves the injection of steam into the well to heat the oil, especially where it is "heavy" and flows poorly. Carbon dioxide or detergents are also used to speed the rate of oil flow. Sometimes more than half of the oil in a deposit may be recovered through these secondary and tertiary methods. Oil and gas are shipped directly from the wellhead. At small, isolated fields, oil may be stored in tanks and picked up by truck. More often, networks of increasingly larger pipelines bring the production from many fields together at one distribution point. Offshore Drilling Approximately one-third of the world's oil is produced from offshore fields, usually from steel drilling platforms set on the ocean floor. In shallow, calm waters, these may be little more than a wellhead and workspace. The larger ocean rigs, however, include not only the well equipment but processing equipment and extensive crew quarters. Recent developments in ocean drilling include the use of floating tension leg platforms that are tied to the sea floor by giant cables and drill ships, which can hold a steady position above a seafloor well using constant, computer controlled adjustments. Sub-sea satellite platforms, where all of the necessary equipment is located on the ocean bed at the well site, have been used for small fields located in producing areas. In Arctic areas islands are built from dredged gravel and sand to provide platforms capable of resisting drifting ice fields. Transportation Large-scale transport of crude oil, refined petroleum products, and natural gas is usually accomplished by pipelines (see \Tpipe and pipeline\t) and \Ltanker\ls, while smaller-scale distribution, especially of petroleum products, is carried out by barges, trucks, and rail tank cars. Pipelines can be used for crude oil and for light oil products, but they are not usually practicable for heavy fuel oil, which does not flow well without being heated. The high capital costs involved in building a pipeline require a large and guaranteed oil-flow volume to be economical. Increasingly, opposition to their passage makes it difficult to build new pipelines, especially in the United States. Tankers, on the other hand, can be sent to any destination where a port can accommodate them and can be shifted to different routes according to need. Although large tankers are more economical than small ones, the number of ports available to handle the larger vessels is limited. Until the mid-1970s the size of tankers grew significantly, the largest reaching 500,000 deadweight tons, but the fragmentation of the market has meant more numerous, smaller shipments and a resurgence of smaller tankers. The shipment of petroleum products by tanker has become more common; but unlike crude oil from different fields, which can be mixed, petroleum products--gasoline and heating oil, for example--must be kept separate and uncontaminated by each other, requiring the compartmentalization of the tanker. Due to its lower density, natural gas is much more expensive to ship. Most \Tnatural gas\t moves by pipeline, but in the late 1960s tanker shipment of cryogenically liquefied natural gas (\TLNG\t) began, particularly to Japan. Special alloys are required to prevent the tanks from becoming brittle at the low temperatures (-161 deg C/-258 deg F) required to keep the gas liquid. Refining Crude oil is rarely used in its raw form but must instead be processed into its various products. Aside from contaminant minerals such as sulfur and small amounts of trace metals--which are removed during refining-- petroleum is composed of hydrocarbons, essentially varying combinations of carbon and hydrogen atoms; any hydrocarbon can be converted into any other given the appropriate application of energy, chemistry, and technology. The smaller the molecule and the lower the ratio of carbon to hydrogen, the lighter the hydrocarbon, the lower the evaporation temperature, and, usually, the more valuable the product. Every crude oil contains a mix of these different hydrocarbons, and the two tasks of a refinery are to separate them out into usable products and to convert the less desirable hydrocarbons into more valuable ones. The tall metal towers that characterize petroleum refineries are distillation, or fractionating, towers. Distillation is the primary method used to refine petroleum. When the heated crude oil is fed into the lower part of a tower, the lighter oil portions, or fractions, vaporize. Losing temperature as they rise, they condense into liquids, which flow downward into the higher temperatures and are revaporized. This process continues until the various fractions have achieved the appropriate degrees of purity. The lighter fractions, like butane, gasoline and kerosene, are tapped off from the top of the tower; heavier fractions, like fuel and diesel oils, are taken from the lower half. At more complex refineries the less valuable products of distillation are refined once again through various conversion processes, broadly referred to as "cracking." Through the application of vacuum, heat, and catalysts, larger, heavier molecules are broken down into lighter ones. Thermal cracking, for instance, uses heat and pressure, while catalytic cracking employs a finely powdered catalyst, and hydrocracking involves the addition of hydrogen to produce compounds with lower carbon to hydrogen ratios, such as gasoline. Other processes produce high-octane products for blending with fuels, remove undesirable constituents, or make special petroleum compounds, including lubricants. Petroleum products are usually distributed from the refinery in the form in which they are to be used. Depending on the geographical location, customer demand, and seasonal needs, refiners can substantially alter their product slate. In winter, for example, less gasoline and more heating oil is produced. The chief refinery products are liquefied petroleum gas (\TLPG\t); \Tgasoline\t and jet fuel; petroleum solvents; \Tkerosene\t; the so-called middle distillates, including heating oil and diesel fuel (known as gasoil outside the United States); residual fuel oil; and asphalts (bitumens), the heaviest fractions. In the United States, with its high demand for gasoline, refineries typically upgrade their products much more than in other areas of the world, where the heavy end products, like residual fuel oil, are used in industry and power generation. Petroleum products are used to produce \Tpetrochemicals\t, which are the costliest of all petroleum derivatives. In petrochemical production, oil distillation products are broken down by cracking them into light, unsaturated gases, which are then recombined in a variety of ways to yield intermediate products, such as ethanol, styrene, ethyl chloride, butadiene, and methanol. These intermediates are used for the manufacture of plastics, synthetic rubbers, synthetic fibers, and other products. There are many advantages to making the first conversion in a refinery, and the oil industry has undertaken chemical manufacture on a large scale (see \Tchemical industry\t). The refinery industry originally was concentrated in or near the oil fields, in part because natural gas, which could not then be economically transported long distances, was available to fuel the highly energy-intensive refining process. The combination of cheap oil, larger crude tankers, and concern about refinery security--particularly after the 1951 nationalization in Iran shut down the world's largest refinery at Abadan--led to the shifting of the refining industry to the major consuming nations. After the oil price shocks of the 1970s relatively cheaper natural gas and the exporting nations' desire to export the highest value-added product possible has meant a new movement of the refining industry back to the oil-producing areas. THE GROWTH OF THE PETROLEUM INDUSTRY Petroleum from ground seepages--often in the form of asphalt or tar--has been known and used since ancient times for boat caulking, road mending, as a medicine and liniment, as an incendiary substance in war, and as an illuminant. In the 1850s the rising price of lamp oil, derived from the overhunted whale population, created incentives to find substitutes, including materials made from shale oil and coal. In Romania, particularly, oil began to be dug from surface seeps and refined by heating into kerosene, which was used as a lamp oil. The first well to be actually drilled was completed on Aug. 27, 1859 at Titusville, Pennsylvania. For the rest of the century, the United States and Russia dominated world production. In the United States the industry was controlled at first by small operators, individuals who owned a well or a refinery where kerosene was distilled from crude oil, distributed in barrels, and sold by the gallon in retail stores. By the late 1870s, John D. \TRockefeller\t had purchased most of the nation's refineries, effectively controlling the industry. The \TSherman Anti-Trust Act\t (1890) resulted in the breakup (1911) of Rockefeller's Standard Oil Trust into its various components, three of which evolved into Mobil, Chevron, and Exxon (Esso). The U.S. industry has since been divided into three branches: the large companies, or "majors"; medium-size companies called "independents"; and a host of small producers, refiners, and distributors. Production was at first concentrated in the Appalachian oil fields until the Spindletop discovery in Texas in 1901 produced the nation's first "gusher," and more oilfields were discovered in the South and Southwest. The industry's original goal was kerosene for lighting; there were no uses for other products--such as gasoline--which were usually burned or discarded. By the turn of the century, however, the development of the internal-combustion engine and the growing popularity of the automobile ensured a market for gasoline, especially when Henry Ford began mass production of the Model T. Demand for gasoline has grown especially rapidly since World War II. Even after the slowdown following the two oil crises of the 1970s, gasoline still comprises 30% of oil consumption. Even as demand soared, however, new discoveries continued to ensure that supply would outpace demand. In 1908 the first field was found in Persia (Iran). In 1938 oil was discovered in Saudi Arabia, which is now the world's second-largest producer and holds the largest reserves in the world. In the United States the discovery of massive quantities of oil in giant fields such as East Texas, found in 1930, kept prices low. Even as late as the late 1930s the United States was producing 60% of the world's oil, and its companies controlled the bulk of the international trade. Overseas, the industry came to be dominated by the so-called Seven Sisters, five major U.S. oil companies that competed with one another and with Royal Dutch Shell--which had developed out of an 1885 oil-field discovery in Indonesia--and the Anglo-Persian Oil Company (now British Petroleum), formed after the 1908 oil find in Persia. From 1920 to 1970 most of the world's oil trade outside the United States was dominated by the Seven Sisters, which controlled not only the oil fields but also refineries, pipelines, and tankers. Before 1900, Russia and the United States were the world's largest producers. The turmoil following the 1917 Revolution, however, significantly reduced the ability of the \TUSSR\t to export oil, and despite discoveries in the 1960s of supergiant fields in western Siberia, Soviet production still falls far short of matching its capacity. Natural gas occupies a special niche within the petroleum industry. Before World War II the field was limited by the inability to ship natural gas long distances, and unless energy-intensive industries moved to exploit it, the gas found in oil fields was frequently burned off. Unassociated, or dry, gas, found in fields without oil, was usually abandoned. New steel alloys permitted the laying of large-diameter pipes in the United States after World War II. The discovery of the Groningen field in the early 1960s in the Netherlands and the exploitation of huge deposits in Soviet Siberia in the 1980s led to a similar expansion of pipelines and natural gas use in Europe. While cryogenics to liquefy gas was long used for storage, overseas shipment of liquefied natural gas (\TLNG\t) by special tankers did not begin commercially until the late 1960s. Japan is still the primary customer for \TLNG\t. Most natural gas continues to move by pipeline. From 1951 to 1970 demand for oil grew by 7% per year, with world production growing from 12 to 46 million barrels per day (mb/d). Most of this growth occurred in the Third World, especially the Middle East. The Middle East Oil Powers The postwar development of the enormous Middle Eastern oil fields had a profound influence not only on the societies in the producing countries but also on the world at large. Vast quantities of cheaply produced oil poured out of Iran, Iraq, Kuwait, Saudi Arabia, and the United Arab Emirates. In consuming areas oil replaced coal for industrial uses, and as automobile use grew in the United States, Europe, and Japan, gasoline consumption soared. The increasing concentration of production in the Middle East left the industry subject to political instability--evidenced by such events as the 1951 oil-field nationalization in Iran, the 1956 Suez Crisis, which shut off supplies to Europe through the Suez Canal, and the 1967 Arab oil embargo. Low oil prices in the years following the 1956 Suez Crisis caused the oil companies to announce unilateral reductions in payments to the producing governments. In response Venezuela joined with several Middle Eastern producers to create the \TOrganization of Petroleum Exporting Countries\t (\TOPEC\t) in an effort to increase their bargaining power against the companies and consumers. In the late 1960s the producing countries began to raise taxes and royalties on oil production. This effort was greatly facilitated by the 1973 Arab oil embargo--sparked by the October Arab-Israeli War (see \TArab-Israeli Wars\t)--which caused prices to triple in a few short months and created widespread panic in world oil markets. The Iranian oil crisis, touched off by the struggle to overthrow the shah of Iran in late 1978 and early 1979, caused prices to triple again. The \TIran-Iraq war\t, begun in 1980, caused the cessation of both countries' production, extending higher prices for another five years. In the Middle East, oil profits were so enormous that per capita incomes almost overnight rivaled those in the industrialized nations, transforming the societies of the richest oil producers. Confident of continued oil demand and even higher prices in the future, many governments undertook massive borrowing to pay for a variety of social programs, military spending, and other investments. With their new power the \TOPEC\t countries took greater control of their domestic industries, nationalizing the oil operations of the multinationals and attempting to determine world oil prices by declaration rather than negotiation. By the late 1970s, however, worldwide demand for oil had fallen for the first time since the Depression of the 1930s. Oil, which had been the fuel of choice for half a century, declined from 46% of total world energy use in 1979 to 39% in 1985. New fields in Mexico, Alaska, and the North Sea further reduced \TOPEC\t's sales. In an effort to support oil prices Saudi Arabia had steeply lowered production. After prices fell, briefly, below $10 per barrel, the other \TOPEC\t members agreed to accept some of the burden of cutting production in order to stabilize markets. This agreement ushered in a new era of expanding oil consumption. The continuing conflict over each member country's share of \TOPEC\t production was a primary factor behind Iraq's invasion of Kuwait in August 1990 (see \TPersian Gulf War\t). ENVIRONMENTAL IMPACTS OF PETROLEUM CONSUMPTION Both the production and processing of oil involve the use of a variety of substances, some toxic, including lubricants in oil wells and catalysts and other chemicals in refining. The amounts used, however, tend to be small and relatively easy to control. More detrimental to the environment is the spillage of oil, which has been a particularly common event. Minor losses from truck and car accidents can affect rivers and streams. Leakage from underground gasoline storage tanks, many abandoned decades ago, has contaminated some local water supplies and usually requires expensive operations either to clean or seal off. Oil spills (see \Toil spill\t) during petroleum transportation have been the most visible problem. These include losses into the environment from the flushing of ships' tanks into the open ocean--although new regulations have reduced this significantly. There have also been instances of oil wells at sea "blowing out," or flowing uncontrollably, although the amounts from blowouts tend to be smaller than from tanker accidents. The 1979 Ixtoc I blowout in the Gulf of Mexico was an exception, as it flowed an estimated 3 million barrels over many months. Tanker accidents typically have a severe impact on ecosystems because of the rapid release of hundreds of thousands of barrels into a small area. The largest single spill to date is believed to have occurred during the 1991 Persian Gulf War, when as much as 10 million barrels were dumped in the Persian Gulf by Iraq, apparently intentionally. More typical is the spill from tanker Exxon Valdez, where a quarter of a million barrels were lost. While oil, as a hydrocarbon, is at least theoretically biodegradable, large-scale spills can overwhelm the ability of the ecosystem to break the oil down. Over time the lighter portions of crude oil evaporate, leaving the heavy, tarlike portion. Oil itself breaks down the protective waxes and oils in the feathers and fur of birds and animals, resulting in a loss of heat retention and causing death by freezing. Ingestion of the oil can also kill animals by interfering with their ability to digest food. Some crude oils contain toxic metals as well. The impact of any given oil spill is determined by the size of the spill, the degree of dispersal, and the chemistry of the oil. Spills at sea are thought to have a less detrimental effect than those in shallow waters. Emissions from petroleum combustion create a number of environmental problems. Locally, the combination of hydrocarbons, nitrogen oxide and sunlight results in low-level ozone, or smog, particularly in large urban areas and especially when air does not circulate well. Automobiles contribute to much of the problem in some areas, such as New England and the Los Angeles area in California. The primary effects are on the health of those exposed to the ozone, but plant life has been observed to suffer as well. On a regional level the emission of sulfur and nitrous oxides can also cause the formation of acid particles at high altitudes, which eventually precipitate in the form of \Tacid rain\t, damaging plants, wildlife, and property. The main source of acid rain, however, is the combustion of coal by electric utilities. Most oil products are low in sulfur or are desulfurized, and while natural gas sometimes includes sulfur as a contaminant, it is typically removed at the production site. Nitrous oxides are produced by both oil and gas. At the global level there is concern that the increased use of hydrocarbon-based fuels will ultimately raise the temperature of the planet, as carbon dioxide reflects the infrared or thermal emissions from the Earth, preventing them from escaping into space (see \Tgreenhouse effect\t). THE FUTURE Although some experts suggest that the bulk of the world's oil and gas has already been discovered and that declining production is inevitable, others believe that substantial amounts of oil and gas remain to be found and, furthermore, that unconventional sources will eventually be exploited. Unconventional sources include methane dissolved in subsurface waters, which will possibly provide an immense source of natural gas; the extraction of oil from \Ttar sands\t--which contain billions of barrels of the fuel--and from oil shales (see \Tshale, oil\t); and the liquefaction and gasification of coal (see \Tsynthetic fuels\t). All attempts to utilize these sources have proved so far to be uneconomic compared to the costs of producing oil and natural gas. The increased use of methanol and ethanol is being promoted for environmental reasons, but major uncertainties about both their economics and their emissions remain. Future technologies may, however, find ways of creating viable fuels (see \Tfuel\t) from these various substances. The oil industry faces the challenge of planning for an increasingly uncertain future. Although oil is now recognized as likely to be abundant for the next decade, concern about political instability in producing areas remains. In addition, environmental problems, both local and global, will see not only requirements to use expensive control equipment, but increased restrictions on both the production and use of oil. Natural gas, however, is being advocated as the alternative to oil, given its much lower emissions and greater abundance. Michael Lynch Bibliography: Anderson, Robert O., Fundamentals of the Petroleum Industry (1984); Chapman, R. E., Petroleum Geology (1983); Griffin, James M., and Steele, Henry B., Energy Economics and Policy (1986); Kinghorn, Robert R., An Introduction to the Physics and Chemistry of Petroleum (1983); Leffler, William L., Petroleum Refining for the Non-Technical Person (1979); National Academy of Engineering, Energy: Production, Consumption, and Consequences (1990); Wheeler, R. R., and Whited, Maurine, Oil from Prospect to Pipeline, 5th ed. (1985); Yergin, Daniel, The Prize: The Epic Quest for Oil, Money & Power (1990).