E.F.A. I
Chemistry and Transport of Petroleum Hydrocarbons
Introduction
An understanding of the chemistry and transport of petroleum hydrocarbons provides the foundation for
forensically reviewing information dealing with petroleum hydrocarbon contamination. This chapter
provides basic terminology and concepts associated with the transport and fate of crude oil and refined
products in the subsurface.
Chemistry of Crude Oil
There are over one million types of hydrocarbons in crude oil, ranging from light gases to heavy residues.
No two crude oils are identical. Crude oil is thus defined as extremely complex mixtures of saturated and
aromatic hydrocarbons, ranging from C1 to C100 or higher, plus a wide variety of compounds containing
nitrogen, sulfur, and oxygen. In addition, there is also a fraction called the asphaltene fraction which is
basically insoluble in n-pentane and contains a very complex matrix of high molecular weight polar
compounds.
In most cases, 90 to 98% by weight of crude petroleum consists of hydrocarbons, while the remaining
materials include sulfur, oxygen, nitrogen, and other organic compounds. Variations in crude oil
composition occur due to the nature of the source of the organic material, the geologic and thermal
history, chemical changes that occur during oil formation and migration, and chemical alteration due to
biodegradation, oxidation, or selective dissolution. Despite wide variations in the chemistry of crude oil,
the elemental compositions fall within a narrow range of elements as shown below;
Elemental Composition of Crude Oil
Element Composition (%)
Carbon 84–87
Hydrogen 11–14
Sulfur 0–3
Nitrogen 0–1
Oxygen 0–2
Crude oils have normal paraffins (n-paraffins) ranging from C1 to C40. Although higher carbon numbers
exist in crude oils, most crude oils fall within the C5 to C30 range. The predominant hydrocarbon classes
that comprise crude oil are straight or branched chain alkanes, cycloalkanes, and aromatics. Alkanes
(paraffins) are saturated hydrocarbons. Linear or normal alkanes (n-alkanes) ranging from C1 to C40
have been identified in crude oil and usually comprise 15 to 20%. In general, the most abundant alkanes
in crude oil are the low-molecular-weight normal alkanes (C5–10). Normal alkanes (n-alkanes) are linear
chains of carbons linked by single covalent bonds.
1 Chemistry and Transport of Petroleum Hydrocarbons| Environmental Forensic
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, E.F.A. I
Isoalkanes are hydrocarbons containing branched carbon chains. The highest concentration of isoalkanes
in crude oils is in the C6 to C8 range. Crude oil can contain 10 to 15% isoalkanes. Cycloalkanes are similar
to alkanes except that cycloalkanes consist of rings of carbon atoms joined by single atomic bonds.
Cycloalkanes are abundant in crude oils and can comprise up to 30 to 40% by weight. The most abundant
cycloalkanes (also called naphthenes) are the single-ring cyclopentanes (C5H10) and cyclohexanes
(C6H12).
Steranes and triterpanes are complex cycloalkanes often used as markers to identify the source and age
of crude oil. Aromatic hydrocarbons consist of rings of six carbon atoms that are unsaturated (i.e., they do
not contain the maximum number of bonded hydrogen atoms). Aromatics include the BTEX (benzene,
toluene, ethylbenzene, and total xylenes) and polynuclear aromatic compounds (PNAs). Aromatic
hydrocarbons contain carbon atoms linked with double bonds, the simplest being benzene (C6H6). Each
hydrogen atom on the aromatic ring can be replaced with an alkyl group (CH3) which results in
compounds such as toluene with one alkyl group attached to the benzene ring.
Benzene rings can be linked to other benzene rings to form compounds such as biphenyls or terphenyls.
When two or more benzene rings are fused, polynuclear aromatic hydrocarbons (also known as
polycyclic aromatic hydrocarbons or PAHs) are formed. Polycyclic aromatic hydrocarbons are
compounds that originate from crude oil and many pyrolysis processes.
Polycyclic aromatic hydrocarbons are of concern because of their genotoxic properties. Naphthalene
(C10H8) is a lower molecular weight example and is generally considered to be a polycyclic aromatic
hydrocarbon, although it has only two aromatic rings. Other non-hydrocarbon components in crude oil
include sulfur, oxygen, and nitrogen.
Sulfur is typically the most abundant element and may be present in several forms, including elemental
sulfur, hydrogen sulfide, mercaptanes, and thiophenes (i.e., hydrogen molecules with bonded sulfur
atoms). The sulfur content in most crude oils varies from about 0.1–3% for some of the heavier oils to 5–
6% for bitumen. Sulfur does not decompose during the distillation process.
The majority of sulfur is, therefore, present predominately in the higher molecular weight fractions and
becomes concentrated in the higher weight refined products. The analysis of the sulfur content of crude
and refined products, such as diesel, can be used to provide evidence to distinguish between multiple
sources. The sulfur content of a petroleum hydrocarbon is determined using standards such as American
Society for Testing Materials (ASTM) D-124, D-1552, and D-4294.
Oxygen reacts with hydrocarbons to form compounds such as phenols, cresols, and xylenols. Nitrogen
can bond with hydrocarbon molecules in crude oil to form small concentrations of pyrrole, pyridine, and
quinoline. Metals are present in crude oils, although usually in small amounts.
Metals can occur as inorganic salts, metallic soaps, and organometallic compounds. In some instances,
sodium arsenite and arsenic trioxide are added to oil pumping wells to inhibit corrosion. The presence of
arsenic in crude oil may, therefore, provide a means for identifying the origin of the crude oil.
2 Chemistry and Transport of Petroleum Hydrocarbons| Environmental Forensic
Analysis I
Chemistry and Transport of Petroleum Hydrocarbons
Introduction
An understanding of the chemistry and transport of petroleum hydrocarbons provides the foundation for
forensically reviewing information dealing with petroleum hydrocarbon contamination. This chapter
provides basic terminology and concepts associated with the transport and fate of crude oil and refined
products in the subsurface.
Chemistry of Crude Oil
There are over one million types of hydrocarbons in crude oil, ranging from light gases to heavy residues.
No two crude oils are identical. Crude oil is thus defined as extremely complex mixtures of saturated and
aromatic hydrocarbons, ranging from C1 to C100 or higher, plus a wide variety of compounds containing
nitrogen, sulfur, and oxygen. In addition, there is also a fraction called the asphaltene fraction which is
basically insoluble in n-pentane and contains a very complex matrix of high molecular weight polar
compounds.
In most cases, 90 to 98% by weight of crude petroleum consists of hydrocarbons, while the remaining
materials include sulfur, oxygen, nitrogen, and other organic compounds. Variations in crude oil
composition occur due to the nature of the source of the organic material, the geologic and thermal
history, chemical changes that occur during oil formation and migration, and chemical alteration due to
biodegradation, oxidation, or selective dissolution. Despite wide variations in the chemistry of crude oil,
the elemental compositions fall within a narrow range of elements as shown below;
Elemental Composition of Crude Oil
Element Composition (%)
Carbon 84–87
Hydrogen 11–14
Sulfur 0–3
Nitrogen 0–1
Oxygen 0–2
Crude oils have normal paraffins (n-paraffins) ranging from C1 to C40. Although higher carbon numbers
exist in crude oils, most crude oils fall within the C5 to C30 range. The predominant hydrocarbon classes
that comprise crude oil are straight or branched chain alkanes, cycloalkanes, and aromatics. Alkanes
(paraffins) are saturated hydrocarbons. Linear or normal alkanes (n-alkanes) ranging from C1 to C40
have been identified in crude oil and usually comprise 15 to 20%. In general, the most abundant alkanes
in crude oil are the low-molecular-weight normal alkanes (C5–10). Normal alkanes (n-alkanes) are linear
chains of carbons linked by single covalent bonds.
1 Chemistry and Transport of Petroleum Hydrocarbons| Environmental Forensic
Analysis I
, E.F.A. I
Isoalkanes are hydrocarbons containing branched carbon chains. The highest concentration of isoalkanes
in crude oils is in the C6 to C8 range. Crude oil can contain 10 to 15% isoalkanes. Cycloalkanes are similar
to alkanes except that cycloalkanes consist of rings of carbon atoms joined by single atomic bonds.
Cycloalkanes are abundant in crude oils and can comprise up to 30 to 40% by weight. The most abundant
cycloalkanes (also called naphthenes) are the single-ring cyclopentanes (C5H10) and cyclohexanes
(C6H12).
Steranes and triterpanes are complex cycloalkanes often used as markers to identify the source and age
of crude oil. Aromatic hydrocarbons consist of rings of six carbon atoms that are unsaturated (i.e., they do
not contain the maximum number of bonded hydrogen atoms). Aromatics include the BTEX (benzene,
toluene, ethylbenzene, and total xylenes) and polynuclear aromatic compounds (PNAs). Aromatic
hydrocarbons contain carbon atoms linked with double bonds, the simplest being benzene (C6H6). Each
hydrogen atom on the aromatic ring can be replaced with an alkyl group (CH3) which results in
compounds such as toluene with one alkyl group attached to the benzene ring.
Benzene rings can be linked to other benzene rings to form compounds such as biphenyls or terphenyls.
When two or more benzene rings are fused, polynuclear aromatic hydrocarbons (also known as
polycyclic aromatic hydrocarbons or PAHs) are formed. Polycyclic aromatic hydrocarbons are
compounds that originate from crude oil and many pyrolysis processes.
Polycyclic aromatic hydrocarbons are of concern because of their genotoxic properties. Naphthalene
(C10H8) is a lower molecular weight example and is generally considered to be a polycyclic aromatic
hydrocarbon, although it has only two aromatic rings. Other non-hydrocarbon components in crude oil
include sulfur, oxygen, and nitrogen.
Sulfur is typically the most abundant element and may be present in several forms, including elemental
sulfur, hydrogen sulfide, mercaptanes, and thiophenes (i.e., hydrogen molecules with bonded sulfur
atoms). The sulfur content in most crude oils varies from about 0.1–3% for some of the heavier oils to 5–
6% for bitumen. Sulfur does not decompose during the distillation process.
The majority of sulfur is, therefore, present predominately in the higher molecular weight fractions and
becomes concentrated in the higher weight refined products. The analysis of the sulfur content of crude
and refined products, such as diesel, can be used to provide evidence to distinguish between multiple
sources. The sulfur content of a petroleum hydrocarbon is determined using standards such as American
Society for Testing Materials (ASTM) D-124, D-1552, and D-4294.
Oxygen reacts with hydrocarbons to form compounds such as phenols, cresols, and xylenols. Nitrogen
can bond with hydrocarbon molecules in crude oil to form small concentrations of pyrrole, pyridine, and
quinoline. Metals are present in crude oils, although usually in small amounts.
Metals can occur as inorganic salts, metallic soaps, and organometallic compounds. In some instances,
sodium arsenite and arsenic trioxide are added to oil pumping wells to inhibit corrosion. The presence of
arsenic in crude oil may, therefore, provide a means for identifying the origin of the crude oil.
2 Chemistry and Transport of Petroleum Hydrocarbons| Environmental Forensic
Analysis I
