Petroleum is the term for a complex mixture of hydrocarbons and lesser quantities of other organic molecules containing sulphur (S), oxygen (O), nitrogen (N) and some metals. Hydrocarbons are compounds that contain only hydrogen (H) and carbon (C) atoms and the number of carbon atoms in a compound determines its physical properties. For example, simple compounds such as methane (CH4), ethane (C2H6), propane (C3H8) and butane (C4H10) all have boiling temperatures below 0 °C and are therefore gases under ambient (surface temperature and pressure) conditions. Larger, more complex hydrocarbon compounds ranging from pentane (C5H12) to hexadecane (C16H34) are liquids under ambient conditions, whilst even larger compounds with a high molecular weight form waxy solids.

The molecular arrangement of hydrocarbon compounds is highly variable. The most commonly occurring forms are chemically stable, so-called saturated compounds known collectively as paraffins and cycloparaffins. A second group, in which the bonding arrangement is more complex and potentially less chemically stable, comprises unsaturated compounds called aromatics and alkenes. Aromatic compounds such as toluene (C6H5CH3) rarely amount to more than 15% of petroleum but they may impart a pleasant odour, hence their name.

Petroleum occurs naturally in several forms: natural gas – mainly gaseous hydrocarbons but also containing variable amounts of carbon dioxide; a liquid, called crude oil, that typically contains a very wide range of hydrocarbon compounds; and solid bitumen. Bitumen contains the heaviest (in the sense of high molecular weight) and most complex hydrocarbon compounds found in petroleum, and they are relatively enriched in oxygen, sulphur and nitrogen. The composition of typical petroleum samples are shown in Table 1. Note that oxygen is a significant impurity in bitumen, but is commonly found only in trace amounts in crude oil and natural gas. In contrast, nitrogen is negligible in bitumen and crude oil, but may constitute up to 15% in natural gas.

 Composition of typical petroleum samples.

ElementNatural gas/weight%Crude oil/weight%Bitumen/weight%