These compounds are called unsaturated because they do not have the maximum number of hydrogen atoms. Olefins are not present in crude oil or natural gas but are formed when crude oil is processed in a refinery. These cyclic compounds are called cycloparaffins or naphthenes . These cyclical compounds can be modified even further by removing some hydrogen atoms and joining some of the carbon atoms with double bonds to form aromatics . The most important aromatics are benzene, toluene , and xylene .
Petroleum can also contain molecules containing oxygen, nitrogen, sulfur, and metals.
Petroleum also contains sulfur compounds that must be removed. Sulfur compounds in crude oil give the oil a distinctive sour odor. Therefore if a crude contains high levels of sulfur, it is called a sour crude and if it has little or no sulfur it is called a sweet crude . Crude oil also contains metals such as nickel and vanadium. Natural gas also contains sulfur, oxygen, and nitrogen that are removed before the gas is sold.