345
13
Structure of Petroleum
13.1 INTRODUCTION
The structure of petroleum has been a topic of some interest for at least five decades when the first
theory of petroleum structure was published (Pfeiffer and Saal, 1940). And, the structure of petroleum has become a much more important issue with the increased use of the heavier crude oils.
Studies of petroleum structure present indications of the interactions and interrelationships of the
constituents. In fact, the implications of petroleum structure as it relates to recovery operations as
well to refinery operations have become of significant importance.
However, in order to discuss the chemical and physical structure of petroleum, it is necessary
to give consideration to the chemical and physical nature of the constituents of petroleum. The
chemical nature of the various constituents of petroleum has been addressed elsewhere with some
reference to the physical nature of petroleum (Chapter 9) and it is not the intent to repeat these discourses here. It is, however, necessary to understand the interrelationships of the various constituents. Furthermore, it is essential that the concept of petroleum being a continuum is accepted not
only among the lower molecular weight ranges but that the continuum is complete and continues
into the higher molecular weight ranges. It is for this reason that the chemical and physical nature
of the resin and gas oil fractions also should be given consideration.
The concept that petroleum is a continuum of structural types that may be further defined in
terms of polarity and molecular weight is not new and has been generally accepted. The initial focus
in this concept was on the hydrocarbon types that occurred in the volatile saturates and aromatics
fractions, leaving the much less volatile polar aromatics fraction (Figure 13.1), leaving the nature
of the nonvolatile polar aromatics fraction subject to structural group analysis (Chapter 11) and,
therefore, open to speculation about the true nature of these constituents (Chapter 12). Further work
on nonvolatile gas oils has shown that the continuum is ubiquitous, at least sufficient to include such
fractions. By inference, and considering the processes by which petroleum is formed, it is to be
anticipated that a continuum is the operative factor in petroleum structure.
The concept that asphaltene fraction is located at the high-molecular-weight end of this continuum has also been postulated and is now becoming accepted (Speight, 1994). Similar reasoning is
appropriate to the constituents of the resin fraction. In addition, there is evidence that the continuum
of hydrocarbon types, or more correctly hydrocarbon fragments, continues from the gas oil fractions into the resins and asphaltenes.
Data from pyrolysis investigations and from high-performance liquid chromatography-ultraviolet
(HPLC-UV) investigations show that the condensed aromatic hydrocarbon fragments in the resins
and asphaltenes match those in the gas oil fraction. It now appears that the asphaltene constituents
are not different types (i.e., polynuclear aromatic sheets containing 12 or more condensed rings) to
the remainder of the oil. The asphaltene constituents are compatible with the structural types with
the lower molecular weight fractions and also compatible with natural product origins of petroleum
(Speight, 1986, 1994). It is, however, in addition to the increasing molecular weight of the resin and
asphaltenes constituents, the frequency of occurrence of the heteroatom functions that increases.
This is the main difference between the constituents of the resin and asphaltenes fractions and the
remainder of the constituents in petroleum.
Thus, there is no reason to refute or deny the existence of the continuum from the lower molecular
weight species to the highest molecular weight, highest polarity species. Indeed, there is sufficient
13
Structure of Petroleum
13.1 INTRODUCTION
The structure of petroleum has been a topic of some interest for at least five decades when the first
theory of petroleum structure was published (Pfeiffer and Saal, 1940). And, the structure of petroleum has become a much more important issue with the increased use of the heavier crude oils.
Studies of petroleum structure present indications of the interactions and interrelationships of the
constituents. In fact, the implications of petroleum structure as it relates to recovery operations as
well to refinery operations have become of significant importance.
However, in order to discuss the chemical and physical structure of petroleum, it is necessary
to give consideration to the chemical and physical nature of the constituents of petroleum. The
chemical nature of the various constituents of petroleum has been addressed elsewhere with some
reference to the physical nature of petroleum (Chapter 9) and it is not the intent to repeat these discourses here. It is, however, necessary to understand the interrelationships of the various constituents. Furthermore, it is essential that the concept of petroleum being a continuum is accepted not
only among the lower molecular weight ranges but that the continuum is complete and continues
into the higher molecular weight ranges. It is for this reason that the chemical and physical nature
of the resin and gas oil fractions also should be given consideration.
The concept that petroleum is a continuum of structural types that may be further defined in
terms of polarity and molecular weight is not new and has been generally accepted. The initial focus
in this concept was on the hydrocarbon types that occurred in the volatile saturates and aromatics
fractions, leaving the much less volatile polar aromatics fraction (Figure 13.1), leaving the nature
of the nonvolatile polar aromatics fraction subject to structural group analysis (Chapter 11) and,
therefore, open to speculation about the true nature of these constituents (Chapter 12). Further work
on nonvolatile gas oils has shown that the continuum is ubiquitous, at least sufficient to include such
fractions. By inference, and considering the processes by which petroleum is formed, it is to be
anticipated that a continuum is the operative factor in petroleum structure.
The concept that asphaltene fraction is located at the high-molecular-weight end of this continuum has also been postulated and is now becoming accepted (Speight, 1994). Similar reasoning is
appropriate to the constituents of the resin fraction. In addition, there is evidence that the continuum
of hydrocarbon types, or more correctly hydrocarbon fragments, continues from the gas oil fractions into the resins and asphaltenes.
Data from pyrolysis investigations and from high-performance liquid chromatography-ultraviolet
(HPLC-UV) investigations show that the condensed aromatic hydrocarbon fragments in the resins
and asphaltenes match those in the gas oil fraction. It now appears that the asphaltene constituents
are not different types (i.e., polynuclear aromatic sheets containing 12 or more condensed rings) to
the remainder of the oil. The asphaltene constituents are compatible with the structural types with
the lower molecular weight fractions and also compatible with natural product origins of petroleum
(Speight, 1986, 1994). It is, however, in addition to the increasing molecular weight of the resin and
asphaltenes constituents, the frequency of occurrence of the heteroatom functions that increases.
This is the main difference between the constituents of the resin and asphaltenes fractions and the
remainder of the constituents in petroleum.
Thus, there is no reason to refute or deny the existence of the continuum from the lower molecular
weight species to the highest molecular weight, highest polarity species. Indeed, there is sufficient