Petroleum is also called rock oil (Petra=rock, oleum=oil). It is a dark colored oily liquid, which occurs at various depths below the surface of the earth. The oil wells are generally called mineral oil or crude oil.
Composition of Petroleum
Petroleum is essentially a mixture of hydrocarbons particularly alkanes, cycloalkanes and aromatic hydrocarbons. Apart from hydrocarbons it may also contain compounds containing oxygen and nitrogen. The exact composition of crude oil varies from one place to another and from one oil field to another.
Origin and Occurrence of Petroleum
According to modern view, the oil has been produced because of bacterial decomposition of the remains of animals and plants (under high pressure) which were buried under the sea, millions of years ago. The hydrocarbons formed by decay of the microorganisms rise through porous rocks until they are trapped by impervious rocks. If the region is uplifted during the passage of time, the oil trap will be found to lie under land. Oil in these deposits generally floats over salt water. The oil in the petroleum field is generally covered with a gaseous mixture known as natural gas.