petroleum

petroleum (crude oil) Fossil fuel that is chemically a complex mixture of hydrocarbons. It accumulates in underground deposits and the chemical composition of petroleum strongly suggests that it originated from the bodies of long-dead organisms, particularly marine plankton. Petroleum is rarely found at the original site of formation, but migrates laterally and vertically until it is trapped. Most petroleum is extracted via oil wells from reservoirs in the Earth's crust sealed by upfolds of impermeable rock or by salt domes which form traps. In the first stage of petroleum refining, the heavier hydrocarbons, which usually have higher boiling points than lighter ones, are distilled. The next stage is cracking, which breaks the heavy hydrocarbons down into more economically useful products, such as petrol and paraffin. Purification of the various products to remove impurities, such as sulphur and nitrogen compounds, completes the refining process. The most versatile end products are ethene and propene, which are widely used in the plastics and chemical industries. See also natural gas