Petroleum oil is generally considered to be formed from animal and vegetable debris buried by sand and silt in sea basins or estuaries. Most scientists believe that anaerobic bacteria under reducing conditions have decomposed the debris, so that most of the oxygen and nitrogen has been removed, to form crude oil. Probably the sequence of bacterial decay, loss of oxygen and nitrogen, enrichment in carbon and hydrogen v/ere aided by alternate exposure to heat and pressure.

The resulting petroleum consists essentially of hydrocarbons. The primary hydrocarbon constituents of oil are:

In addition to the hydrocarbons, crude oil still contains some organic nitrogen, sulfur, and oxygen compounds.

Crude oils come from thousands of feet below the earth's surface. They are usually mixed with mud, sand, brine, and other contaminants. Most of these contaminants are eventually separated from the crude, first on the field and again in the refinery, however, small amounts still remain suspended and emulsified with the crude. Some of these sediments and impurities cannot be burned.