Petroleum, or "crude oil," is a liquid fuel that is present in various locations throughout the world. It has many uses, from the generation of electricity to the manufacture of medicines, plastics, and other commercial items.

Much like coal, petroleum is formed from the remains of biodegraded organic material. When animals that lived in the sea millions of years ago died underwater, their remains were gradually covered by layers of very fine dirt known as "silt" on the ocean floor. Then, as the years passed, pressure from the layers built up and compressed the organic material, forming the oil.

Petroleum has many different "viscosities," or thicknesses. The viscosity depends on the amount of gases and solids that are present in the oil. Often, natural gas is disolved in the liquid and can be extracted for other uses. Petroleum take three main forms: paraffin, asphaltic, and mixed-base. These forms are based upon the chemical makeup of the hydrocarbon-based oil.