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A crude oil is a liquid mixture of hydrocarbons, that is to say, molecules consisting of carbon atoms and hydrogen. A crude oil generally contains also impurities, especially sulfur, but also trace metals (nickel and vanadium), nitrogen, etc.
As many deposits as crude! The proportions of different molecules are extremely variable gross to time, and the amounts of impurities. Hydrocarbon molecules that are encountered range from methane (C1) to the heavier molecules (several tens of carbon atoms). When the number of carbon atoms exceeds some units, the carbon atoms combinations give very different structures. Therefore many isomers can be distinguished (molecules having the same number of carbons but different structures):
• Linear paraffins (normal paraffins) where the carbon atoms are "aligned". Their main features: a good cetane number, a mouvais octane, low resistance to cold;
• The iso paraffins and branched paraffins, in good octane;
• The naphthenes where the carbon atoms form a ring;
• The aromatic molecules characterized by the existence of a ring of6 carbon atoms with linkages of a special nature. The aromatics are characterized by a high density, excellent octane but a very mouvais cetane number, good resistance to cold.
