oil

Flammable substance, usually insoluble in water, and composed chiefly of carbon and hydrogen. Oils may be solids (fats and waxes) or liquids. Various plants produce vegetable oils; mineral oils are based on petroleum.

The crude oil (unrefined petroleum) found beneath the Earth's surface is formed from the remains of dead plants and animals. As these plants and animals died they were buried with mud near the sea floor. Over millions of years, heat from the Earth's interior and pressure from overlying rocks slowly changed the dead remains into hydrocarbons (substances containing hydrogen and carbon). The hydrocarbons, being light molecules, moved upwards and became trapped beneath impermeable rocks.

Oil reservoirs are often found beneath the seabed and drilling technology is used to locate these supplies of oil. Crude oil extracted from the ground is refined in fractional distillation columns to produce more useful products such as petrol, diesel, kerosene, plastics, and chemicals for pharmaceuticals.