Crude oil - as petroleum directly out of the ground is called - is a remarkably varied substance, both in its use and composition. Crude oil is formed from the preserved remains of prehistoric zooplankton and algae, which have been settled to the sea (or lake) bottom in large quantities under anoxic conditions. It was formed over millions of years from the remains of tiny aquatic plants and animals that lived in ancient seas due to compression and heating of ancient organic materials over geological time. The oldest oil-bearing rocks, date back to more than 600 million years, the youngest being as old as about 1 million years.
Although various types of hydrocarbons - molecules made of hydrogen and carbon atoms - form the basis of all crude oils, they differ in their configurations. The chemical structure of petroleum is composed of hydrocarbon chains of different lengths. Because of this, petroleum may be taken to oil refineries and the hydrocarbon chemicals separated by distillation and treated by other chemical processes, to be used for a variety of purposes. It can be a straw-colored liquid or tar-black solid. Red, green and brown hues are not uncommon.