Petroleum is a naturally occurring liquid found in rock formations. It consists of a complex mixture of hydrocarbons of various molecular weights, plus other organic compounds. It is generally accepted that oil is formed mostly from the carbon rich remains of ancient plankton after exposure to heat and pressure in Earth’s crust over hundreds of millions of years. Over time, the decayed residue was covered by layers of mud and silt, sinking further down into Earth’s crust and preserved.