32
The Chemistry and Technology of Petroleum
The molecular species in petroleum vary from simple hydrocarbon species (in the majority) to
very complex organic molecules containing atoms of carbon, hydrogen, nitrogen, oxygen, and sulfur (in the minority), as well as trace amounts of metals such as vanadium, nickel, iron, and copper
(Chapter 8). The complexity of petroleum is further illustrated by the number of potential isomers,
that is, molecules having the same atomic formula, that can exist for a given number of paraffin
carbon atoms, which increases rapidly as molecular weight increases:
Carbon Atoms in Hydrocarbon
Number of Isomers
4
2
8
18
12
355
18
60,523
This same increase in number of isomers with molecular weight also applies to the other molecular types present. Since the molecular weights of the molecules found in petroleum can vary from
that of methane (CH 4 ; molecular weight = 16) to several thousand (Chapters 9 and 10), it is clear that
the heavier nonvolatile fractions can contain virtually unlimited number of molecules. However, in
reality the number of molecules in any specified fraction is limited by the nature of the precursors
of petroleum, their chemical structures, and the physical conditions that are prevalent during the
maturation (conversion of the precursors) processes.
The foregoing comments lead to the conclusion that a detailed analysis may apply to a specific
petroleum sample. However, the generic name petroleum describes more accurately what could be
in a sample of petroleum rather than what is actually in it.
The original methods of classification arose because of commercial interest in petroleum type
and were a means of providing refinery operators with a rough guide to processing conditions.
It is therefore not surprising that systems based on a superficial inspection of a physical property,
such as specific gravity or API (Baumé) gravity (Chapter 10), are easily applied and are actually
used to a large extent in expressing the quality of crude oils. Such a system is approximately
indicative of the general character of a crude oil as long as materials of one general type are
under consideration. For example, among crude oils from a particular area, an oil of 40°API
(specific gravity = 0.825) is usually more valuable than one of 20°API (specific gravity = 0.934)
because it contains more light fractions (e.g., gasoline) and fewer heavy, undesirable asphaltic
constituents.
As already pointed out (Chapter 1), the definition of petroleum-associated materials has been
varied, unsystematic, diverse, and often archaic and it is only recently that some attempt has been
made to define these materials in a meaningful manner. However, attempts to classify petroleum
have also evolved and it is the purpose of this chapter to review these methods and present them for
further consideration.
2.2 CLASSIFICATION SYSTEMS
2.2.1 ClAssIFICAtIon As A HydroCArBon resourCe
Petroleum is referred to generically as a fossil energy resource and is further classified as a hydrocarbon resource and, for illustrative (or comparative) purposes in this text, coal and oil shale kerogen
have also been included in this classification. However, the inclusion of coal and oil shale under the
broad classification of hydrocarbon resources has required (incorrectly) that the term hydrocarbon
be expanded to include the macromolecular nonhydrocarbon heteroatomic species that constitute
coal and oil shale kerogen. Use of the term organic sediments would be more correct (Figure 2.1).
The Chemistry and Technology of Petroleum
The molecular species in petroleum vary from simple hydrocarbon species (in the majority) to
very complex organic molecules containing atoms of carbon, hydrogen, nitrogen, oxygen, and sulfur (in the minority), as well as trace amounts of metals such as vanadium, nickel, iron, and copper
(Chapter 8). The complexity of petroleum is further illustrated by the number of potential isomers,
that is, molecules having the same atomic formula, that can exist for a given number of paraffin
carbon atoms, which increases rapidly as molecular weight increases:
Carbon Atoms in Hydrocarbon
Number of Isomers
4
2
8
18
12
355
18
60,523
This same increase in number of isomers with molecular weight also applies to the other molecular types present. Since the molecular weights of the molecules found in petroleum can vary from
that of methane (CH 4 ; molecular weight = 16) to several thousand (Chapters 9 and 10), it is clear that
the heavier nonvolatile fractions can contain virtually unlimited number of molecules. However, in
reality the number of molecules in any specified fraction is limited by the nature of the precursors
of petroleum, their chemical structures, and the physical conditions that are prevalent during the
maturation (conversion of the precursors) processes.
The foregoing comments lead to the conclusion that a detailed analysis may apply to a specific
petroleum sample. However, the generic name petroleum describes more accurately what could be
in a sample of petroleum rather than what is actually in it.
The original methods of classification arose because of commercial interest in petroleum type
and were a means of providing refinery operators with a rough guide to processing conditions.
It is therefore not surprising that systems based on a superficial inspection of a physical property,
such as specific gravity or API (Baumé) gravity (Chapter 10), are easily applied and are actually
used to a large extent in expressing the quality of crude oils. Such a system is approximately
indicative of the general character of a crude oil as long as materials of one general type are
under consideration. For example, among crude oils from a particular area, an oil of 40°API
(specific gravity = 0.825) is usually more valuable than one of 20°API (specific gravity = 0.934)
because it contains more light fractions (e.g., gasoline) and fewer heavy, undesirable asphaltic
constituents.
As already pointed out (Chapter 1), the definition of petroleum-associated materials has been
varied, unsystematic, diverse, and often archaic and it is only recently that some attempt has been
made to define these materials in a meaningful manner. However, attempts to classify petroleum
have also evolved and it is the purpose of this chapter to review these methods and present them for
further consideration.
2.2 CLASSIFICATION SYSTEMS
2.2.1 ClAssIFICAtIon As A HydroCArBon resourCe
Petroleum is referred to generically as a fossil energy resource and is further classified as a hydrocarbon resource and, for illustrative (or comparative) purposes in this text, coal and oil shale kerogen
have also been included in this classification. However, the inclusion of coal and oil shale under the
broad classification of hydrocarbon resources has required (incorrectly) that the term hydrocarbon
be expanded to include the macromolecular nonhydrocarbon heteroatomic species that constitute
coal and oil shale kerogen. Use of the term organic sediments would be more correct (Figure 2.1).