285
Structural Group Analysis
of petroleum is a complex fraction in which the character of the constituents varies in terms of the
range of molecular weight and range of the polarity of the constituents, to mention only two parameters. However, the tendency by researchers to postulate average structures has been born again and
raised its head again in the 1990s and has continued to this day (at the time of writing). In addition,
the postulation of an average structure based on poorly planned and faulty experimental work has
also been applied to the issue.
Some researchers, after accumulating a large number of papers pursuing the myth of an average structure—at considerable expense and all to no avail without furthering petroleum science
one iota—have seen the light and finally denied that such structures exist while others continue
to pursue the dream but adding further mystery to the mythology of average structure or average
structural parameter by failing to understand (or by even denying) the reality of the limitations of
each method when applied to any extremely complex fraction of petroleum.
Finally, before embarking upon such a quest for the search for an average structure (or an average structural parameter), the investigator must ask himself or herself the question: How can the
chemical and physical properties of a complex mixture that contains not only hundreds (perhaps
thousands) of constituents of different chemical character and molecular weight be represented by
an average structure? The answer: It cannot.
It is the purpose of this chapter to present and review the historical methods that can be used
for the structural group analysis of complex petroleum fractions. Methods are also included that
are usually employed to emphasize a particular set of molecular parameters that are derived from
physical properties or from particular chemical methods. Each method should be recognized for the
particular capabilities that it offers and used in conjunction with one or more of the other methods
described in this chapter. In addition, the structural data is usually obtained from a combination of
two or more methods (Speight, 2001, 2002; Nasritdinov, 2003). This is preferred insofar as a multidimensional approach yields more valuable information about a particular fraction than the data
from a single method alone.
The readers are referred to other relevant chapters to place the subject matter of this chapter into
the proper perspective and to examine the assumptions for any form of logic and validity that are
used for each method. Indeed, it is not intended to advocate the use of one method over another. It
is, however, intended to show the methods as they were used historically, with some justification,
but it must be remembered that many of the methods culminate with the finding of an average
structure or an average structural parameter. No such structure or parameter exists and the use of
such information must be treated for what it is: a best but inaccurate guess that defies the science of
structural analysis.
11.2 METHODS FOR STRUCTURAL GROUP ANALYSIS
The precursors to petroleum are as diverse as the plant chemicals themselves, with the added recognition that there may have been some evolution of these chemicals over geological time as plants,
and their constituents, evolved to modern-day counterparts. This is not to suggest that petroleum is
a collection of plant chemicals and should be considered as a composite of carbohydrates, proteins,
fatty acids, and any other chemical that can be derived from the pages of a natural product textbook!
Petroleum (especially the higher molecular weight nonvolatile constituents) is an extremely complex material wherein the original precursors and the constituents of the ensuing protopetroleum
(Chapter 3) have undergone considerable change through chemical and physical interactions with
their environment.
Furthermore, local and regional variations in the nature of the original plant precursors and in
the maturation conditions offer a means of variation of petroleum from field to field and can be used
to explain the variations in petroleum types and constituents. What this means is that a derivative
of a chemical entity that is predominant in petroleum from one field may be an entity of somewhat
lesser importance in another crude oil, even in crude oils of one particular type.
Structural Group Analysis
of petroleum is a complex fraction in which the character of the constituents varies in terms of the
range of molecular weight and range of the polarity of the constituents, to mention only two parameters. However, the tendency by researchers to postulate average structures has been born again and
raised its head again in the 1990s and has continued to this day (at the time of writing). In addition,
the postulation of an average structure based on poorly planned and faulty experimental work has
also been applied to the issue.
Some researchers, after accumulating a large number of papers pursuing the myth of an average structure—at considerable expense and all to no avail without furthering petroleum science
one iota—have seen the light and finally denied that such structures exist while others continue
to pursue the dream but adding further mystery to the mythology of average structure or average
structural parameter by failing to understand (or by even denying) the reality of the limitations of
each method when applied to any extremely complex fraction of petroleum.
Finally, before embarking upon such a quest for the search for an average structure (or an average structural parameter), the investigator must ask himself or herself the question: How can the
chemical and physical properties of a complex mixture that contains not only hundreds (perhaps
thousands) of constituents of different chemical character and molecular weight be represented by
an average structure? The answer: It cannot.
It is the purpose of this chapter to present and review the historical methods that can be used
for the structural group analysis of complex petroleum fractions. Methods are also included that
are usually employed to emphasize a particular set of molecular parameters that are derived from
physical properties or from particular chemical methods. Each method should be recognized for the
particular capabilities that it offers and used in conjunction with one or more of the other methods
described in this chapter. In addition, the structural data is usually obtained from a combination of
two or more methods (Speight, 2001, 2002; Nasritdinov, 2003). This is preferred insofar as a multidimensional approach yields more valuable information about a particular fraction than the data
from a single method alone.
The readers are referred to other relevant chapters to place the subject matter of this chapter into
the proper perspective and to examine the assumptions for any form of logic and validity that are
used for each method. Indeed, it is not intended to advocate the use of one method over another. It
is, however, intended to show the methods as they were used historically, with some justification,
but it must be remembered that many of the methods culminate with the finding of an average
structure or an average structural parameter. No such structure or parameter exists and the use of
such information must be treated for what it is: a best but inaccurate guess that defies the science of
structural analysis.
11.2 METHODS FOR STRUCTURAL GROUP ANALYSIS
The precursors to petroleum are as diverse as the plant chemicals themselves, with the added recognition that there may have been some evolution of these chemicals over geological time as plants,
and their constituents, evolved to modern-day counterparts. This is not to suggest that petroleum is
a collection of plant chemicals and should be considered as a composite of carbohydrates, proteins,
fatty acids, and any other chemical that can be derived from the pages of a natural product textbook!
Petroleum (especially the higher molecular weight nonvolatile constituents) is an extremely complex material wherein the original precursors and the constituents of the ensuing protopetroleum
(Chapter 3) have undergone considerable change through chemical and physical interactions with
their environment.
Furthermore, local and regional variations in the nature of the original plant precursors and in
the maturation conditions offer a means of variation of petroleum from field to field and can be used
to explain the variations in petroleum types and constituents. What this means is that a derivative
of a chemical entity that is predominant in petroleum from one field may be an entity of somewhat
lesser importance in another crude oil, even in crude oils of one particular type.