Petroleum and natural gas are mixtures of hydrocarbons. On average, they consist of 85% C, 13% H and 2% N,S and (all weight %). The main components of natural hydrocarbons belongs to three groups :
Paraffins : or n-alkanes with general formula CnH2n-2. for n = 1 to 4 these are gases, from n = 5 to 15 liquids and above this solids (paraffin waxes). The gases methane, ethane, propane and butane form natural gas. They can be normal or branched chains as shown in below image. Isomers are compounds with the same chemical formula but a different molecular structure (and thus different chemical and PT properties). Iso-octane is the standard for motor fuel ratings.
Naphthenes : with general formula CnH2n , form saturated ring compounds in which n is 5,6,7. Cyclopentane and cyclohexane, are common components of crude oils, often in the methyl-form (together 2% or more of an average crude oil). In nature only naphthene rings with 5 or 6 carbons occur because of the range of bond angles that carbon can have.
Aromatics : generally a minor group of hydrocarbons that contain at least one benzene ring (C6H6) in which all carbons share the fourth bond. They are called undersaturated because they will react to add hydrogen or other elements to their rings. aromatics all contain at least one benzene ring in which the fourth bond is shared (ring on diagram).
Aromatics : This group of hydrocarbons usually constitute less than 1% of most crude oils. Their abundance correlates invasively with oil density. Among their compounds are organic solvents like toluene and polycyclic aromatics that are carcinogenic like 3,4-benzpyrene. The most complex aromatics can range up to the size of asphaltene particles.
Aromatics are highly valued by the petrochemical industry for their large variety of uses, but because of their low abundance refineries have to synthesize them form other hydrocarbon components.
Olefine : Like aromatics these are also undersaturated hydrocarbons, but they are much more reactive. They are very common in living matter (many natural oils, vitamins A, pigments, rubbers sterols etc.) but they do not occur in crude oil because they have been converted to more stable compounds during catagenesis. Their general formulae are CnH2n and CnH2n-2. Examples: Isoprene C5H8, Butylene C4H8, Ethylene C2H2.
Olefin hydrocarbon can also form during the refining process of crude oil. They form the basis for the manufacturing of the explosive tri-nitrotoluole TNT.
N, S and O compounds : These are non-hydrocarbons but are commonly found in crude oil, particularly in the heavier oils and in residue. If they contain at least one benzene ring, they are called aromatics. If they contain at least one cycloparaffin ring (the saturated ring), they are called naphthenic. And if they have neither, they are called paraffinic. Most common are combinations of these.
Waxes, Resins, Asphaltenes : These heavier HC compounds have complex polymeric or polycyclic compositions, often with NSO incorporated. At radicles, I.e. ends with several free electrons, metallic ions can attach. This high concentrations of elements such as vanadium.
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