b. Crude petroleum is a complex mixture of various chemical compounds, commonly known as
hydrocarbons. A typical hydrocarbon consists of:
(1) 11 to 13 weight percent hydrogen.
(2) 84 to 87 weight percent carbon.
(3) Traces of nitrogen, sulfur, oxygen, and helium may be found as impurities in crude petroleum. A
low sulfur content results in a "sweet" crude oil and a high sulfur content is classified as a "sour" crude oil.
(4) Although all petroleum is constituted primarily of carbon and hydrogen, the molecular
constitution of crude oils differs widely. About 18 series of hydrocarbons have been recognized in crude
petroleum. Of these series, the most commonly encountered are the parrafins, napthenes, olefins and aromatics.
(a) Parrafins are hydrocarbons that are richest in hydrogen and the most stable hydrocarbon.
There are two types, straight-chain parrafins and branched-chain parrafins. Of the two types, the branched-chain
parrafins are preferred for their higher octane number and anti-knock characteristics in gasolines.
(b) Napthenes are closed-chain hydrocarbons, generally very stable and are primarily found as a
component of lubricating oils.
(c) Olefins are characterized by having at least one double bond between some of the carbon
atoms. This means that at least two carbon atoms are joined with each other rather than a hydrogen atom. As
with paraffins, olefins can be either straight chained or branch-chained molecules. Because of their double bonds,
olefins are highly reactive with other elements and make them unsatisfactory for use in fuels that need a high
degree of stability. However, olefins are an essential part of petrochemicals such polypropylene and butyl rubber,
used in tire inner tubes.
(d) Aromatics are characterized by at least one six-membered ring of carbon atoms. These
molecules are known as aromatics because of their pleasant odor. Aromatics are naturally occurring in crude oil
and are desired for gasolines because of their anti-knock qualities. However, the aromatics also tend to dissolve
some types of rubber. Aromatics such as toluene were used during World War II to manufacture TNT and
aviation gasolines. Today, toluene is used in solvents ad other petrochemicals.
(e) No two crude oils are alike. Even crude oils taken from different wells in the same oil field
can differ significantly. Therefore, a method of classifying crude oils was designed. The simplest and most
widely accepted system classifies crude oils according to the relative quantities of parrafin and asphalt. There are
(f) Another method of classification is according to a crude oil's physical properties. Of these,
the most important properties for classification are the density and the viscosity. Since the density of a liquid is a
function of temperature and pressure, it is necessary to designate standard conditions for reporting density and,
hence, specific gravity ad API gravity.