Petroleum, as well as the major petroleum-producing nations, octane rating, petrochemical industry, and petroleum and environment. It combined old and new reports using the table research technique. Many millions of years ago, organic substances found underground or as a result of chemical reactions in the atmosphere likely gave rise to petroleum or rock oil. During refinement, fractional distillation separates the mixture of gaseous, liquid, and solid hydrocarbons into several products. Petroleum has been a source of energy since the Middle Ages and has recently dominated the synthesis of chemical molecules.