Petroleum is a complex mixture of hydrocarbons in gaseous, liquid, or solid form. The composition and properties of crude oil are dependent on the origin, age, and conditions of the source geologic formation. Geochemists use the composition of light hydrocarbons (C1—C9), which constitute about 50% of the carbon in crude oil, to identify its origin and determine how, why, and where petroleum exists as well as its migration pattern. The carbon number and structure of hydrocarbons are both highly variable in different crude oils. Usually very small hydrocarbons (C1-C4) are gaseous at ambient temperatures and can be used as fuel (e.g., methane, propane, butane) or for making specific products such as polymeric materials and plastics. Larger hydrocarbons (C5-C12) are liquids which can be easily heated to become gaseous and are thus used as fuels and solvents. Still larger hydrocarbons (C13-C17) are liquids and are used as fuels and lubricants. Hydrocarbons with carbon numbers larger than 17 are usually solids. The compositional differences explain why some crude oils contain more low-boiling components, while others contain more high-boiling components.

Petroleum hydrocarbons (e.g., aliphatic and aromatic hydrocarbons) can also be classified according to their structures; they usually are divided into three classes: paraffins, naphthenes, and aromatics (Figure 1). Paraffins are saturated hydrocarbons (alkanes) of straight or branched chains and without ring structure (a saturated hydrocarbon


Straight chain: CH4, CH3-CH3, CH3-CH2-CH3

Branched chain

Naphthenes ch3


Figure 1 Three classes of petroleum hydrocarbons (paraffins, naphthenes, and aromatics).


Figure 1 Three classes of petroleum hydrocarbons (paraffins, naphthenes, and aromatics).

means no additional hydrogen atoms can be added to the carbons). Examples of paraffins with straight chains are methane, ethane, propane, and butane (all gaseous), and pentane and hexane (which are liquids). Branched-chain paraffins are usually present in the heavier fraction, and they possess a higher octane rating. Paraffins account for 2-50% of the composition of crude oils.

Naphthenes are saturated hydrocarbons possessing ring structure; they may feature a paraffinic side chain. Naphthenic hydrocarbons are the most abundant class present in most crude oils (accounting for 25-75%). As their amount increases, the boiling point of the fraction proportionately increases - with the exception of heavy oil fractions. Monocyclic naphthenes are distributed in the light fractions of crude oil, while polycyclic naphthenes are mainly in the heavier oil fractions. So far no evidence exists for the structures of napthenes with ring numbers in excess of five. However, polycyclic naphthenes with ring numbers of seven or eight have been suggested by current mass spectrometry techniques.

Aromatic hydrocarbons contain aromatic nuclei such as benzene, naphthalene, or phenanthrene which may link with paraffin and/or naphthene ring side chains. The amount of aromatics usually varies from 15% to 50% in crude oils, and it tends to be concentrated in the high-boiling fractions. The most abundant compounds in this group are benzols, naphthalenes, phenanthrenes, pyrenes, and anthracenes, and their properties are very different from those of the aliphatic hydrocarbons. Aromatic hydrocarbons are believed to provide the largest contribution to the toxicity of crude oil. The sum total concentration of benzene, toluene, ethylbenzene, and the xylenes (o, m, and p; referred to as BTEX) is commonly used to represent the total amount of volatile monocyclic aromatic hydrocarbons (MAHs) in petroleum.

Besides hydrocarbons, petroleum contains small amounts of sulfur-, nitrogen-, and oxygen-containing compounds, as well as trace amounts of metallic constituents. These heteroatomic constituents heavily influence the properties of crude oil, even though their concentrations are relatively low. Asphalthenes (high molecular mass hydrocarbons) and resins are generally soluble in aromatic solvents but not in paraffinic solvents. Asphalthenes containing numerous heteroatomic components are the most complex known substances in crude oils, and both asphalthenes and resins (as emulsification agents) stabilize water-in-oil emulsification.

Sulfur-containing hydrocarbons are the most important of the heteroatomic constituents in petroleum (they account for c. 0.1-5% of the total composition). Each crude oil has its own distinctive type and proportion of sulfurous hydrocarbons, depending upon geological environment and time. Compounds may include H2S, mercaptans, thiophenols, thiophenes, and benzothiophenes. In general, the higher the density of crude oil, the higher its sulfur content. Most sulfur-containing compounds are highly toxic to marine organisms.

The amount of nitrogen in different crude oils can account for 0.02-1.5% of the total content. There are many nitrogen-containing compounds in oil, including pyridines, hinolines, acredines, indols, and carbozoles. Nitrogen-containing hydrocarbons significantly contribute to the properties of natural surfactants; they also greatly influence the physical-chemical properties and toxicity of crude oils.

In most crude oils, the amount of oxygen-containing hydrocarbons varies from 0.1% to 3% of the total composition; it is also directly proportional to boiling point. Over 20% of these compounds are concentrated in the asphalthene and resin fractions, and may include neutral compounds such as ethers, anhydrides, furans, and acidic oxygens.

The proportions of both hydrocarbons and metals in crude oils can also vary greatly. Therefore, volatility, specific gravity, and viscosity can also vary just as greatly. These variable components help to determine physical-chemical characteristics and biodegradation processes, and further determine the bioavailability to, and toxicity in, marine organisms.

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