351
Structure of Petroleum
from the original protopetroleum. Oxidation experiments certainly indicate that the reaction
sequence is as follows (Moschopedis and Speight, 1976c):
Oils → Resins → Asphaltenes
The oxygenated water passing through petroleum sediments can have some effect on the aforementioned relationship, but the effect of oxidation during the maturation processes is unknown and, at
best, speculative.
Other than high-temperature experiments, which are subject to wonder and criticism (Chapter 5),
there is no hard evidence that petroleum formed directly from kerogen. It may also be considered
just as probable that petroleum formed (Chapter 3) simultaneously with kerogen (Chapter 5) at
which time the petroleum being mobile moved away and carried with the other kerogen relative, the
dispersible asphaltenes.
A relatively high degree of aromaticity is generally prevalent in the asphaltenes and the resins.
In some resins, however, the hydrocarbons show an increase in aliphatic material (more side chains,
for example) until, with considerable saturation, the oils, which contain numerous alkyl chains of
varying length, are reached. The degree of aromaticity is important when the resins are desorbed.
A high aromaticity of the maltenes (i.e., that part of petroleum remaining after the asphaltenes have
been removed, often referred to as the deasphalted or deasphaltened oil) indicates good solvency
for the asphaltenes. In fact, the solvent power of the maltenes is one of the most important factors
in determining the physicochemical behavior of the petroleum colloid system (Speight, 1994; Storm
and Sheu, 1994; Sheu and Storm, 1995; Espinat et al., 1998; Merdrignac and Espinat, 2007).
The means by which asphaltenes associate in solution and in crude oil has been the subject of
many investigations. Much of the work related to asphaltene association has arisen from molecular
weight measurements from which deductions have been made about the asphaltene micelle, but
there is still considerable conjecture about the true nature of the micelle. Specifically, there is some
debate about whether the micelle in petroleum is composed of homogeneous material, insofar as it is
composed only of asphaltene molecules, or is composed of asphaltene and resin molecules (Pfeiffer
and Saal, 1940; Dickie and Yen, 1967; Koots and Speight, 1975; Speight, 1994).
The pronounced tendency of asphaltenes to form aggregates in hydrocarbon solution is one of
their most characteristic traits. Thus, while a number of experimental methods indicate that isolated
asphaltene monomers have an average molecular weight on the order of 1500–2500 (vapor pressure
osmometry in polar solvents), numerous methods show that asphaltenes associate spontaneously in
most hydrocarbon media, resulting in observed molecular weights of 10,000 or more. Thus, it is not surprising that this tendency for asphaltenes to aggregate in hydrocarbon solvents has been applied to the
existence of asphaltenes in crude oil. The debate is about whether or not this line of thinking is correct!
However, dilute solutions (0.01%–0.5% by weight) of an asphaltene fraction in a nonpolar organic
solvent (e.g., carbon tetrachloride, CCl 4 ) exhibit the free hydroxyl absorption band (∼3585 cm −1 ) in
the infrared. At higher concentrations (>1.0% w/w), this band becomes less distinguishable, with
concurrent onset of the hydrogen-bonded hydroxyl absorption (3200–3450 cm −1 ). On addition of
a dilute solution (0.1%–1% w/w) of the corresponding resins to the asphaltene solutions, the free
hydroxyl absorption is reduced markedly or disappears, which indicates the occurrence of preferential intermolecular hydrogen bonding between the asphaltenes and resins.
Furthermore, molecular weight data show that apparent molecular weights of asphaltenes determined in benzene or carbon tetrachloride are reduced from the range 4800 ± 200 to the range
1800 ± 100 when phenol is added to the solutions before molecular weight determination. The
reduced value (1800 ± 100) is near that calculated on the basis of the single theoretical asphaltene
structure and is significantly higher than the calculated molecular weight (∼900) of the phenol–
asphaltene mixture. Addition of measured quantities of phenol to suspensions of asphaltenes in
either a petroleum naphtha fraction or in the oil fraction from the bitumen causes solubilization of
part of the asphaltenes.
Structure of Petroleum
from the original protopetroleum. Oxidation experiments certainly indicate that the reaction
sequence is as follows (Moschopedis and Speight, 1976c):
Oils → Resins → Asphaltenes
The oxygenated water passing through petroleum sediments can have some effect on the aforementioned relationship, but the effect of oxidation during the maturation processes is unknown and, at
best, speculative.
Other than high-temperature experiments, which are subject to wonder and criticism (Chapter 5),
there is no hard evidence that petroleum formed directly from kerogen. It may also be considered
just as probable that petroleum formed (Chapter 3) simultaneously with kerogen (Chapter 5) at
which time the petroleum being mobile moved away and carried with the other kerogen relative, the
dispersible asphaltenes.
A relatively high degree of aromaticity is generally prevalent in the asphaltenes and the resins.
In some resins, however, the hydrocarbons show an increase in aliphatic material (more side chains,
for example) until, with considerable saturation, the oils, which contain numerous alkyl chains of
varying length, are reached. The degree of aromaticity is important when the resins are desorbed.
A high aromaticity of the maltenes (i.e., that part of petroleum remaining after the asphaltenes have
been removed, often referred to as the deasphalted or deasphaltened oil) indicates good solvency
for the asphaltenes. In fact, the solvent power of the maltenes is one of the most important factors
in determining the physicochemical behavior of the petroleum colloid system (Speight, 1994; Storm
and Sheu, 1994; Sheu and Storm, 1995; Espinat et al., 1998; Merdrignac and Espinat, 2007).
The means by which asphaltenes associate in solution and in crude oil has been the subject of
many investigations. Much of the work related to asphaltene association has arisen from molecular
weight measurements from which deductions have been made about the asphaltene micelle, but
there is still considerable conjecture about the true nature of the micelle. Specifically, there is some
debate about whether the micelle in petroleum is composed of homogeneous material, insofar as it is
composed only of asphaltene molecules, or is composed of asphaltene and resin molecules (Pfeiffer
and Saal, 1940; Dickie and Yen, 1967; Koots and Speight, 1975; Speight, 1994).
The pronounced tendency of asphaltenes to form aggregates in hydrocarbon solution is one of
their most characteristic traits. Thus, while a number of experimental methods indicate that isolated
asphaltene monomers have an average molecular weight on the order of 1500–2500 (vapor pressure
osmometry in polar solvents), numerous methods show that asphaltenes associate spontaneously in
most hydrocarbon media, resulting in observed molecular weights of 10,000 or more. Thus, it is not surprising that this tendency for asphaltenes to aggregate in hydrocarbon solvents has been applied to the
existence of asphaltenes in crude oil. The debate is about whether or not this line of thinking is correct!
However, dilute solutions (0.01%–0.5% by weight) of an asphaltene fraction in a nonpolar organic
solvent (e.g., carbon tetrachloride, CCl 4 ) exhibit the free hydroxyl absorption band (∼3585 cm −1 ) in
the infrared. At higher concentrations (>1.0% w/w), this band becomes less distinguishable, with
concurrent onset of the hydrogen-bonded hydroxyl absorption (3200–3450 cm −1 ). On addition of
a dilute solution (0.1%–1% w/w) of the corresponding resins to the asphaltene solutions, the free
hydroxyl absorption is reduced markedly or disappears, which indicates the occurrence of preferential intermolecular hydrogen bonding between the asphaltenes and resins.
Furthermore, molecular weight data show that apparent molecular weights of asphaltenes determined in benzene or carbon tetrachloride are reduced from the range 4800 ± 200 to the range
1800 ± 100 when phenol is added to the solutions before molecular weight determination. The
reduced value (1800 ± 100) is near that calculated on the basis of the single theoretical asphaltene
structure and is significantly higher than the calculated molecular weight (∼900) of the phenol–
asphaltene mixture. Addition of measured quantities of phenol to suspensions of asphaltenes in
either a petroleum naphtha fraction or in the oil fraction from the bitumen causes solubilization of
part of the asphaltenes.