The origin of oil or petroleum is not well understood. There are several theories, but the matter is still one of scientific controversy. It is generally accepted however that the origin of oil begins with biological fossils, just as with coal - but that they are the remains of sea creatures rather than trees. The study of fossils is called paleontology. The study of the creation of oil is part of geology.

Oil is a mixture of liquid hydrocarbons (chemical compounds containing only hydrogen and carbon) plus various impurities such as sulphur. They can be separated out by using the fact that they have different boiling points. This is done at an oil refinery. (see below)

Unprocessed oil or petroleum is usually called crude oil or mineral oil. The name petroleum (that petrol comes from!) is from a combination of Latin words meaning "rock oil".

Oil may have a variety of appearances and constitution. Some forms are black, others dark green, and some light like kerosene. The liquid oils in the mixture that we call crude oil ranges from very viscous to easy-flowing ones. Each oil constituent is a hydrocarbon - they have varying molecular weights and differ from one another in structure and properties. These various species are separated into groups, or fractions, by a process of distillation called refining. Oil fuel, in all of its usable forms, is a refined product, unlike coal and natural gas which can often be burned in their natural condition.

Oil has been known and used since the most ancient times. It was used chiefly as a liniment or medicine, not as a fuel. The Bible refers to pitch being used for building purposes - cementing walls - in Babylon.

Oil flows from natural springs in many localities. It was obtained from such springs in what is now Western Pennsylvania by the Seneca Indians, who used it for medicinal purposes. Drilling for oil was begun in the 1800s and this is now the commonest way in which it is extracted.

As a fuel, oil was originally used as kerosene for lighting, replacing animal, vegetable and coal oils. It also came to be used in furnaces. Its biggest use, however, came with the development of the car. Today almost all forms of transport - cars, lorries, buses, trains, ships and aeroplanes - are fueled by oil - mainly diesel or petrol. Oil has also been burned to produce electricity in oil fired power stations.

Oil Refining

When crude oil arrives at an oil refinery it first undergoes a process of fractional distillation (separating the mixture according to boiling point). The crude oil is heated and piped into a large tower with perforated trays welded to the walls every few feet. The heated vapours rise, cool and condense at different levels (trays) in the tower. The lighter hydrocarbons condense on the upper trays. In this way the crude is separated into fractions or distillates.