188
The Chemistry and Technology of Petroleum
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.
This concept has resulted in the consideration of petroleum as a continuum of molecular types and
the nature of continuum is dictated by the proportions of the precursors that form the protopetroleum
after which the prevalent conditions become operational in the formation of the final crude oil product.
With this in mind, it might be anticipated that similar molecular types occur in heavy oil and bitumen
as occur in conventional petroleum. It then becomes a question of degree as well as molecular weight.
8.2 ULTIMATE (ELEMENTAL) COMPOSITION
With few exceptions, the proportions of the elements (carbon, hydrogen, nitrogen, oxygen, sulfur,
and metals) in petroleum (whatever and wherever the source) vary over fairly narrow limits:
Carbon
83.0%–87.0%
Hydrogen
10.0%–14.0%
Nitrogen
0.1%–2.0%
Oxygen
0.05%–1.5%
Sulfur
0.05%–6.0%
Metals (Ni and V) <1000 ppm
The narrow range of variation is quite surprising when the variation of the precursors is considered
(Chapter 3) and even more surprising when one considers the wide variation in physical properties
from the lighter, more mobile crude oils at one extreme to the heavier asphaltic crude oils at the
other extreme (see also Charbonnier et al., 1969; Draper et al., 1977). In addition, when the many
localized or regional variations in maturation conditions are assessed, it is perhaps surprising that
the ultimate compositions are so similar. Perhaps this observation, more than any other observation,
is indicative of the similarity in nature of the precursors from one site to another.
Because of the narrow range of carbon and hydrogen content, it is not possible to classify petroleum on the basis of carbon content as coal is classified; carbon contents of coal can vary from as
low as 75% w/w in lignite to 95% w/w in anthracite (Speight, 1994a). Of course, other subdivisions
are possible within the various carbon ranges of the coals, but petroleum is restricted to a much narrower range of elemental composition.
The elemental analysis of oil sand bitumen has also been widely reported (Speight, 2009),
but the data suffer from the disadvantage that identification of the source is too general and is
often not site specific. In addition, the analysis is quoted for separated bitumen, which may have
been obtained by any one of several procedures and may therefore not be representative of the
total bitumen on the sand. However, recent efforts have focused on a program to produce sound,
reproducible data from samples for which the origin is carefully identified (Wallace et al., 1988).
It is to be hoped that this program continues as it will provide a valuable database for tar sand and
bitumen characterization.
Like conventional petroleum, of the data that are available, the elemental composition of oil sand
bitumen is generally constant and, like the data for petroleum, falls into a narrow range (Speight,
1990 and references cited therein):
Carbon
83.4% ± 0.5%
Hydrogen
10.4% ± 0.2%
Nitrogen
0.4% ± 0.2%
Oxygen
1.0% ± 0.2%
Sulfur
5.0% ± 0.5%
Metals (Ni and V) >1000 ppm
The Chemistry and Technology of Petroleum
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.
This concept has resulted in the consideration of petroleum as a continuum of molecular types and
the nature of continuum is dictated by the proportions of the precursors that form the protopetroleum
after which the prevalent conditions become operational in the formation of the final crude oil product.
With this in mind, it might be anticipated that similar molecular types occur in heavy oil and bitumen
as occur in conventional petroleum. It then becomes a question of degree as well as molecular weight.
8.2 ULTIMATE (ELEMENTAL) COMPOSITION
With few exceptions, the proportions of the elements (carbon, hydrogen, nitrogen, oxygen, sulfur,
and metals) in petroleum (whatever and wherever the source) vary over fairly narrow limits:
Carbon
83.0%–87.0%
Hydrogen
10.0%–14.0%
Nitrogen
0.1%–2.0%
Oxygen
0.05%–1.5%
Sulfur
0.05%–6.0%
Metals (Ni and V) <1000 ppm
The narrow range of variation is quite surprising when the variation of the precursors is considered
(Chapter 3) and even more surprising when one considers the wide variation in physical properties
from the lighter, more mobile crude oils at one extreme to the heavier asphaltic crude oils at the
other extreme (see also Charbonnier et al., 1969; Draper et al., 1977). In addition, when the many
localized or regional variations in maturation conditions are assessed, it is perhaps surprising that
the ultimate compositions are so similar. Perhaps this observation, more than any other observation,
is indicative of the similarity in nature of the precursors from one site to another.
Because of the narrow range of carbon and hydrogen content, it is not possible to classify petroleum on the basis of carbon content as coal is classified; carbon contents of coal can vary from as
low as 75% w/w in lignite to 95% w/w in anthracite (Speight, 1994a). Of course, other subdivisions
are possible within the various carbon ranges of the coals, but petroleum is restricted to a much narrower range of elemental composition.
The elemental analysis of oil sand bitumen has also been widely reported (Speight, 2009),
but the data suffer from the disadvantage that identification of the source is too general and is
often not site specific. In addition, the analysis is quoted for separated bitumen, which may have
been obtained by any one of several procedures and may therefore not be representative of the
total bitumen on the sand. However, recent efforts have focused on a program to produce sound,
reproducible data from samples for which the origin is carefully identified (Wallace et al., 1988).
It is to be hoped that this program continues as it will provide a valuable database for tar sand and
bitumen characterization.
Like conventional petroleum, of the data that are available, the elemental composition of oil sand
bitumen is generally constant and, like the data for petroleum, falls into a narrow range (Speight,
1990 and references cited therein):
Carbon
83.4% ± 0.5%
Hydrogen
10.4% ± 0.2%
Nitrogen
0.4% ± 0.2%
Oxygen
1.0% ± 0.2%
Sulfur
5.0% ± 0.5%
Metals (Ni and V) >1000 ppm