What is the origin of oil?

Petroleum is a complex mixture of hydrocarbons, which in turn are natural chemical compounds, consisting essentially of hydrogen and carbon atoms. Hydrocarbons at normal temperature and pressure can occur in liquid (petroleum, in the common sense), solid (bitumen, asphalt, wax) or gaseous (natural gas) form, according to the complexity of their molecules.

Most men of science are inclined to assign it an organic origin. It is supposed to come from the decomposition and transformation of animal and plant remains, mainly microscopic animals and marine plants, that existed millions of years ago, in previous geological ages. The action of anaerobic bacteria on these residues led to the gradual decomposition until they were transformed into hydrocarbon.