66
The Chemistry and Technology of Petroleum
The variation in the character of crude oils with depth of burial is also of interest. An increase in
the lighter constituents and a decrease in the density and in the amount of heavier constituents with
increasing depth for sands of the same age have been noted. In sands of the same depth, an increase
in the lighter constituents and a decrease in density with age have also been noted. It has been suggested that the precursors of such oils are heavy naphthenic types, which under the influences of
pressure and temperature are slowly transformed into lighter, more paraffinic oils. However, there
do appear to be cases in which many oils are less paraffinic and of greater density than the overlying, younger crude oils; differences in the source materials or in regional geology or later chemical
alterations may be cited as possible controlling factors. There may also be a correlation between the
composition and the nature of the sediments. There is also the possibility that young, heavy oils are
converted into lighter crude oils over the course of geological time by mild thermal cracking, but if
such changes occur, catalysis may be necessary and thermal alteration is possible when the oil has
been exposed to significant temperatures.
For example, the solubility of the asphaltene constituents is susceptible to minor changes in the
nature of the solvent and also to minor amounts of insoluble organic material and other gases in the
solvent (Girdler, 1965; Mitchell and Speight, 1973). This is obviously very important in reservoirs
where a substantial gas cap exists above the oil; the constituents of the gas may not only dissolve
in the oil but may also cause asphaltic material (asphaltenes and resins) to precipitate within the
reservoir, thereby altering the character of the oil.
Thus there is the distinct possibility, if not certainty, that the original character of petroleum can
be substantially altered by events following the migration process. Each of the suggested mechanisms for this alteration (i.e., thermal alteration, deasphalting, and biogenic alteration) may all
play a role in the alteration of a particular crude oil. As for the choice of a migration mechanism,
the choice of the prevalent in situ alteration process depends upon the prevalent conditions to which
the petroleum is subject. A particular mechanism for alteration may be the prime operative, or all
may contribute to determining the ultimate character of the petroleum. The determining factor is
site specificity.
The processes involved in petroleum maturation are extremely complex but several trends are
notable in the maturation process:
1. Mature crude oils are low in asphaltic fraction (asphaltenes plus resins) and, consequently,
lower in nitrogen, oxygen, and sulfur.
2. Asphaltene constituents from mature crude oils are more aromatic (lower atomic H/C
ratio) than asphaltenes from immature crude oils.
3. A concentration effect (the extent of which depends upon the reactions involved in the
maturation process) causes the majority of the heteroelements (nitrogen, oxygen, and sulfur)
to be located in the higher molecular weight fractions.
In summary, the petroleum formation process is generally accepted as the transformation of
complex organic substances dispersed in source sediments. Like most typical organic reactions,
the rate of petroleum generation increases with increasing temperature and, consequently, with
increasing burial depths of the sediments. But the limits of burial and the temperatures to which the
petroleum precursors have been subjected are not well defined, except by assumptions leading to
laboratory experiments that are subject to question.
Whatever the conditions prevalent during maturation, nonhydrocarbon organic constituents also
experience compositional changes during burial diagenesis. The highly complex nonhydrocarbon
components, such as asphaltene constituents, are believed to be the products of condensation and
disproportionation reactions, which proceed simultaneously with those reactions responsible for
the formation of light hydrocarbons. In contrast to the hydrocarbon diagenesis pattern, asphaltenes
become more complex with increasing maturity. This is confirmed by the observed increase in
aromaticity.
The Chemistry and Technology of Petroleum
The variation in the character of crude oils with depth of burial is also of interest. An increase in
the lighter constituents and a decrease in the density and in the amount of heavier constituents with
increasing depth for sands of the same age have been noted. In sands of the same depth, an increase
in the lighter constituents and a decrease in density with age have also been noted. It has been suggested that the precursors of such oils are heavy naphthenic types, which under the influences of
pressure and temperature are slowly transformed into lighter, more paraffinic oils. However, there
do appear to be cases in which many oils are less paraffinic and of greater density than the overlying, younger crude oils; differences in the source materials or in regional geology or later chemical
alterations may be cited as possible controlling factors. There may also be a correlation between the
composition and the nature of the sediments. There is also the possibility that young, heavy oils are
converted into lighter crude oils over the course of geological time by mild thermal cracking, but if
such changes occur, catalysis may be necessary and thermal alteration is possible when the oil has
been exposed to significant temperatures.
For example, the solubility of the asphaltene constituents is susceptible to minor changes in the
nature of the solvent and also to minor amounts of insoluble organic material and other gases in the
solvent (Girdler, 1965; Mitchell and Speight, 1973). This is obviously very important in reservoirs
where a substantial gas cap exists above the oil; the constituents of the gas may not only dissolve
in the oil but may also cause asphaltic material (asphaltenes and resins) to precipitate within the
reservoir, thereby altering the character of the oil.
Thus there is the distinct possibility, if not certainty, that the original character of petroleum can
be substantially altered by events following the migration process. Each of the suggested mechanisms for this alteration (i.e., thermal alteration, deasphalting, and biogenic alteration) may all
play a role in the alteration of a particular crude oil. As for the choice of a migration mechanism,
the choice of the prevalent in situ alteration process depends upon the prevalent conditions to which
the petroleum is subject. A particular mechanism for alteration may be the prime operative, or all
may contribute to determining the ultimate character of the petroleum. The determining factor is
site specificity.
The processes involved in petroleum maturation are extremely complex but several trends are
notable in the maturation process:
1. Mature crude oils are low in asphaltic fraction (asphaltenes plus resins) and, consequently,
lower in nitrogen, oxygen, and sulfur.
2. Asphaltene constituents from mature crude oils are more aromatic (lower atomic H/C
ratio) than asphaltenes from immature crude oils.
3. A concentration effect (the extent of which depends upon the reactions involved in the
maturation process) causes the majority of the heteroelements (nitrogen, oxygen, and sulfur)
to be located in the higher molecular weight fractions.
In summary, the petroleum formation process is generally accepted as the transformation of
complex organic substances dispersed in source sediments. Like most typical organic reactions,
the rate of petroleum generation increases with increasing temperature and, consequently, with
increasing burial depths of the sediments. But the limits of burial and the temperatures to which the
petroleum precursors have been subjected are not well defined, except by assumptions leading to
laboratory experiments that are subject to question.
Whatever the conditions prevalent during maturation, nonhydrocarbon organic constituents also
experience compositional changes during burial diagenesis. The highly complex nonhydrocarbon
components, such as asphaltene constituents, are believed to be the products of condensation and
disproportionation reactions, which proceed simultaneously with those reactions responsible for
the formation of light hydrocarbons. In contrast to the hydrocarbon diagenesis pattern, asphaltenes
become more complex with increasing maturity. This is confirmed by the observed increase in
aromaticity.