110
The Chemistry and Technology of Petroleum
light hydrocarbon gases, water, and hydrogen gas; oxygen is lost primarily as water and carbon
oxides (Brooks, 1981; Brooks and Welte, 1984). Maturation increases with increased exposure of
the organic matter to time, temperature, and pressure. The catalytic effects of minerals in the sediment can accelerate and affect this process, as can the presence of water (Durand, 1980; Burnham,
1995; Siskin and Katritzky, 1995).
Kerogen is transformed into a variety of lower molecular weight compounds when subjected to
higher temperature and pressure. An ideal situation to study this transformation is in a sedimentary
sequence in which all geological and geochemical parameters except burial remain constant. The
microscopic, chemical, and physical methods of the progressive transformation of kerogen are a function of maximum burial depth (Louis and Tissot, 1967; Durand et al., 1972; Espitalité et al., 1973).
Transformations occurring over geological periods are in essence not accessible to human experiment, as we shall never be able to account for the millions of years involved in these natural processes. Therefore in the laboratory it has been found necessary, perhaps erroneously, to increase
temperatures to accelerate the reactions and compensate for time. Because a valid basis for comparison has not yet been established, it is not possible to determine the extent to which the laboratory processes are representative of natural transformations.
Although the degradation of kerogen and the associated weight loss are continuous and progressive, three successive stages may be distinguished as occurring in the laboratory heating studies of
kerogen (type II):
First, at approximately 350°C (660°F), there is a weight loss due mostly to the production of
water and carbon dioxide. Elemental analysis and infrared spectroscopy show a loss of oxygen
associated with a diminution of the carbonyl (C=O band). The elemental composition of the treated
sample is comparable to that of natural samples buried to depths of 1000–1500 m (3300–5000 ft).
This third statement should not be taken as being conclusive proof of the viability of the high-temperature maturation theory. After all, it is possible to burn toast to a product having the elemental
composition of a low-rank coal. And the action of concentrated sulfuric acid on sugar also gives a
product with the composition of a low-rank coal. But there is no claim that coal originated from the
immersion of the plant precursors in a lake of concentrated sulfuric acid. This might offer a new
experience in terms of the concept of acid rain (Chapter 29).
In the temperature range 350°C–500°C (660°F–930°F), kerogen encounters its major degradation stage. The products are mostly hydrocarbons (particularly aliphatic hydrocarbons), and the
elemental composition changes rapidly. The atomic H/C ratio decreases to approximately 0.5 and
dealkylation of aromatic systems occurs. The carbonyl (C=O) band progressively disappears: acids
are removed first and then esters, which are more stable, whereas ketones are progressively eliminated. This step of evolution has been correlated with the catagenesis occurring in sedimentary
basins. But again, and for reasons given in the preceding paragraph the viability of the comparison
is suspect.
Finally, at temperatures above 500°C (930°F), major structural rearrangements occur, such as
the formation of cluster or aggregates by aromatic. Such transformations are compared to the data
from the examination of samples from deep natural samples. This process has been compared to
the natural interval known as metagenesis. And, the comment at the end of the preceding paragraph
still stands.
It is necessary to recognize, however, that the quantitative aspects of kerogen evolution vary from
one type to the other as a result of differences in the original composition of kerogen and that these
generalizations may not reflect the true nature of the chemistry that occurs during the maturation
process. These conclusions are based on gross changes to analyzable chemical entities. The detailed
chemistry is more difficult to analyze.
The total amount of products generated upon heating, as measured by the weight loss, is greatest for type I kerogen and least for type III kerogen. The proportion of hydrocarbon products is
higher for type I kerogen and type II kerogen and lower for type III kerogen. In addition, the light
to medium hydrocarbon ratio (gas–oil ratio) is low for type I kerogen, moderate for type II kerogen,
The Chemistry and Technology of Petroleum
light hydrocarbon gases, water, and hydrogen gas; oxygen is lost primarily as water and carbon
oxides (Brooks, 1981; Brooks and Welte, 1984). Maturation increases with increased exposure of
the organic matter to time, temperature, and pressure. The catalytic effects of minerals in the sediment can accelerate and affect this process, as can the presence of water (Durand, 1980; Burnham,
1995; Siskin and Katritzky, 1995).
Kerogen is transformed into a variety of lower molecular weight compounds when subjected to
higher temperature and pressure. An ideal situation to study this transformation is in a sedimentary
sequence in which all geological and geochemical parameters except burial remain constant. The
microscopic, chemical, and physical methods of the progressive transformation of kerogen are a function of maximum burial depth (Louis and Tissot, 1967; Durand et al., 1972; Espitalité et al., 1973).
Transformations occurring over geological periods are in essence not accessible to human experiment, as we shall never be able to account for the millions of years involved in these natural processes. Therefore in the laboratory it has been found necessary, perhaps erroneously, to increase
temperatures to accelerate the reactions and compensate for time. Because a valid basis for comparison has not yet been established, it is not possible to determine the extent to which the laboratory processes are representative of natural transformations.
Although the degradation of kerogen and the associated weight loss are continuous and progressive, three successive stages may be distinguished as occurring in the laboratory heating studies of
kerogen (type II):
First, at approximately 350°C (660°F), there is a weight loss due mostly to the production of
water and carbon dioxide. Elemental analysis and infrared spectroscopy show a loss of oxygen
associated with a diminution of the carbonyl (C=O band). The elemental composition of the treated
sample is comparable to that of natural samples buried to depths of 1000–1500 m (3300–5000 ft).
This third statement should not be taken as being conclusive proof of the viability of the high-temperature maturation theory. After all, it is possible to burn toast to a product having the elemental
composition of a low-rank coal. And the action of concentrated sulfuric acid on sugar also gives a
product with the composition of a low-rank coal. But there is no claim that coal originated from the
immersion of the plant precursors in a lake of concentrated sulfuric acid. This might offer a new
experience in terms of the concept of acid rain (Chapter 29).
In the temperature range 350°C–500°C (660°F–930°F), kerogen encounters its major degradation stage. The products are mostly hydrocarbons (particularly aliphatic hydrocarbons), and the
elemental composition changes rapidly. The atomic H/C ratio decreases to approximately 0.5 and
dealkylation of aromatic systems occurs. The carbonyl (C=O) band progressively disappears: acids
are removed first and then esters, which are more stable, whereas ketones are progressively eliminated. This step of evolution has been correlated with the catagenesis occurring in sedimentary
basins. But again, and for reasons given in the preceding paragraph the viability of the comparison
is suspect.
Finally, at temperatures above 500°C (930°F), major structural rearrangements occur, such as
the formation of cluster or aggregates by aromatic. Such transformations are compared to the data
from the examination of samples from deep natural samples. This process has been compared to
the natural interval known as metagenesis. And, the comment at the end of the preceding paragraph
still stands.
It is necessary to recognize, however, that the quantitative aspects of kerogen evolution vary from
one type to the other as a result of differences in the original composition of kerogen and that these
generalizations may not reflect the true nature of the chemistry that occurs during the maturation
process. These conclusions are based on gross changes to analyzable chemical entities. The detailed
chemistry is more difficult to analyze.
The total amount of products generated upon heating, as measured by the weight loss, is greatest for type I kerogen and least for type III kerogen. The proportion of hydrocarbon products is
higher for type I kerogen and type II kerogen and lower for type III kerogen. In addition, the light
to medium hydrocarbon ratio (gas–oil ratio) is low for type I kerogen, moderate for type II kerogen,