Petroleum See Also: Energy, Energy, Sources Of Or crude Oil, oily, flammable liquid (see States Of Matter) that occurs naturally in deposits, usually beneath the surface of the earth. The exact composition varies according to locality, but it is chiefly a mixture of Hydrocarbons. Petroleum is a fossil fuel thought to have been formed over millions of years from incompletely decayed plant and animal remains buried under thick layers of rock. Drilling for oil is a complex, often risky process. Scientific methods are used to locate promising sites for wells, some of which must be dug several miles deep to reach the deposit. Many wells are now drilled offshore from platforms standing on the ocean bed. Usually the crude oil in a new well comes to the surface under its own Pressure. Later it has to be Pumped or forced up with injected Water, Gas (see States Of Matter), or Air. Pipelines (see Specifications - Piping) or tankers transport it to refineries, where it is separated into fractions, i.e., the portions of the crude oil that vaporize between certain defined limits of Temperature. Fractions are obtained by a refining process called fractional Distillation, in which crude oil is Heated and sent into a tower. The vapors of the different fractions 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. Generally the fractions are vaporized in the following order: dissolved Natural Gas, gasoline, naphtha, kerosene, diesel fuel (see Diesel Engine), heating oils, and finally tars. Lighter fractions, especially gasoline, are in greatest demand and their yield can be increased by breaking down heavier hydrocarbons in a process called cracking. The leading producers of petroleum are Russia, the U.S., Saudi Arabia, Iran, Iraq, China, Mexico, the United Arab Emirates, Venezuela, Nigeria, Kuwait, and Norway. The largest reserves are in the Middle East. Modern industrial civilization depends heavily on petroleum for motive power, fuel, lubrication, and a variety of synthetic products, e.g., dyes, drugs, and Plastics. The widespread burning of petroleum products as fuels has resulted in serious problems of air pollution, and oil spilled from tankers and offshore wells has damaged ocean and coastline environments. The environmentally disruptive effects of oil wells have sometimes led to strong opposition to new drilling, as in wilderness areas of North Alaska.