Analyzing petroleum samples collected from around the world would show that their elemental compositions vary over only a narrow range: 82–87% carbon, 11–15% hydrogen, with the balance being oxygen, nitrogen, and sulfur. Oxygen and nitrogen seldom exceed 1.5% each, but sulfur can amount to as much as 6% in extreme cases. Yet these samples could show remarkable diversity in physical characteristics, ranging from lightly colored, free-flowing liquid to dark, smelly, highly viscous material. A consequence of the wide variability of physical properties coupled with a very narrow range of composition is that carbon content cannot be used as a simple predictor of properties; in this respect, petroleum is very different from coal (Chapter 17). If these same samples were analyzed to determine the specific chemical compounds present in each, any particular sample would be found to contain some 105 individual components, varying in concentration from one sample to another. That is, on an elemental basis, most oils have about the same composition, but on a molecular basis no two are exactly alike. These seemingly disparate properties arise because most components of petroleum belong to a small number of homologous series of compounds, of which the composition, on a weight percent basis, varies only little, even over a long span of the series. For example, pentane, C5H12, is 83.3% C and 16.7% H on a weight basis; pentadecane, C15H32, is 84.9% C and 15.1% H. All possible isomers of all the alkanes between pentane and pentadecane amount to 7666 compounds, yet their elemental compositions change by only 1.6 percentage units. The range of boiling temperatures of the components spans at least 550 °C; pentane, the smallest alkane liquid at room temperature, boils at 36 °C, and some oils contain material that does not boil even at 600 °C.

Composition

Petroleum contains four classes of compound: alkanes, cycloalkanes, aromatics, and heteroatomic compounds with one or more atoms of nitrogen, sulfur, and/or oxygen. In petroleum chemistry and technology, alkanes are called paraffins; cycloalkanes, naphthenes; and the heteroatomic compounds are lumped together as NSOs. Cycloalkanes, aromatics, and NSOs can all have one or more alkyl side chains.