Crude oil (also known as petroleum) is a type of fossil fuel found beneath the earth’s surface. It is formed by the gradual build-up of fossilised organic materials such as algae and plankton. As more layers build up, the bottom most layers are heated and subject to pressure, with the combination of heat and pressure leading to the matter transforming into the waxy substance kerogen. Following even more prolonged exposure to heat and pressure, the kerogen eventually becomes transformed into liquid and gases via the catagenesis process. The formation of crude oil occurs from this pyrolysis (heating) process. The range of heat at which kerogen becomes crude oil is called the oil window. Below this range the crude oil remains kerogen and above this point the crude oil becomes a natural gas.