107
Kerogen
approach to minimize the formation of intractable residues—and the obliteration of important
structural features—has been to heat the kerogen at moderate temperatures for prolonged periods
and then to extract the degraded kerogen (Hubbard and Robinson, 1950). Thus it may be possible
to achieve complete conversion of the kerogen and recover >90% w/w as lower molecular weight
(liquid) products. However, product alteration is also a feature of thermal reactions, but there is the
distinct possibility that the thermal fragments that were allowed to escape from the reaction zone
(by virtue of their volatility) would preserve some of the original (skeletal) features of the kerogen.
Overall, the mild heat-soak-extraction method is a complementary method to the oxidation studies
for providing lower molecular weight products for structural studies.
The use of micropyrolysis coupled with gas chromatography–mass spectrometry (GC–MS)
can also provide valuable information about the structural units in kerogen (Schmidt-Collerus and
Prien, 1974). In addition to the online micropyrolysis-GC-MS studies, larger samples of kerogen
have been pyrolyzed to obtain products that were fractionated by chromatography (ion exchange,
complexation with ferric chloride (FeCl 3 ), and silica gel) into compound classes, and then by gel
permeation chromatography into fractions of increasing molecular weight. These samples can then
be investigated by conventional mass spectrometric techniques as well as by other spectroscopic
studies.
5.6.5 ACId-CAtAlyzed HydrogenolysIs
The use of hydrogenolysis in the presence of stannous chloride (SnCl 2 ) to degrade kerogen affords
good yields of liquid products for characterization purposes. Under these conditions, the majority
of the heteroatoms are generally removed: nitrogen as ammonia, oxygen as carbon dioxide or as
water, and sulfur as hydrogen sulfide. From these data it was concluded that the nitrogen, oxygen,
and sulfur functionalities comprise internuclear links (as opposed to their existence in ring systems)
in the kerogen structure. These conclusions may appear to be at variance with other data, but this
may be a result of the severe degradation conditions employed in the experiments, and examination
under milder conditions is warranted.
5.7 STRUCTURAL MODELS
The need to gather the very large mass of information about kerogen structure into a compact form
useful for guiding research and development has led to models for kerogen structure. These models
are not intended to depict the molecular structure of kerogen. At least not in the sense that the double helix describes the structure of deoxyribonucleic acid (DNA) or even in the sense that synthetic
polymers are described in terms of monomers joined to form chains that have well-defined structures. The kerogen models represent attempts, based on the available data, to depict a collection of
skeletal fragments and functional groups as a three-dimensional network in the most reasonable
manner possible.
However, it must be recognized that no single analytical technique provides sufficient information to construct a precise model of the macromolecular structure of kerogen. Thus, most workers
now use a multidimensional (multiple technique) approach, but as is quite often the case, the techniques used by different workers have different strengths and may emphasize different features of
kerogen structure. Sample-to-sample variation and the use of different isolation techniques also
complicate the issue.
Several models have been proposed for the structure of kerogen in which a multi-dimensional
approach has been employed. Such approaches are extremely valuable since they bring together the
results of several analytical methods. Indeed, the success of such approaches in the deduction of
structural types in kerogen is also paralleled by the use of a similar approach to the deduction of the
structural types that occur in the asphaltene fraction (Chapter 12). Such structural models (although
not absolute) are of interest to consider here because they give an overall picture of the perception
Kerogen
approach to minimize the formation of intractable residues—and the obliteration of important
structural features—has been to heat the kerogen at moderate temperatures for prolonged periods
and then to extract the degraded kerogen (Hubbard and Robinson, 1950). Thus it may be possible
to achieve complete conversion of the kerogen and recover >90% w/w as lower molecular weight
(liquid) products. However, product alteration is also a feature of thermal reactions, but there is the
distinct possibility that the thermal fragments that were allowed to escape from the reaction zone
(by virtue of their volatility) would preserve some of the original (skeletal) features of the kerogen.
Overall, the mild heat-soak-extraction method is a complementary method to the oxidation studies
for providing lower molecular weight products for structural studies.
The use of micropyrolysis coupled with gas chromatography–mass spectrometry (GC–MS)
can also provide valuable information about the structural units in kerogen (Schmidt-Collerus and
Prien, 1974). In addition to the online micropyrolysis-GC-MS studies, larger samples of kerogen
have been pyrolyzed to obtain products that were fractionated by chromatography (ion exchange,
complexation with ferric chloride (FeCl 3 ), and silica gel) into compound classes, and then by gel
permeation chromatography into fractions of increasing molecular weight. These samples can then
be investigated by conventional mass spectrometric techniques as well as by other spectroscopic
studies.
5.6.5 ACId-CAtAlyzed HydrogenolysIs
The use of hydrogenolysis in the presence of stannous chloride (SnCl 2 ) to degrade kerogen affords
good yields of liquid products for characterization purposes. Under these conditions, the majority
of the heteroatoms are generally removed: nitrogen as ammonia, oxygen as carbon dioxide or as
water, and sulfur as hydrogen sulfide. From these data it was concluded that the nitrogen, oxygen,
and sulfur functionalities comprise internuclear links (as opposed to their existence in ring systems)
in the kerogen structure. These conclusions may appear to be at variance with other data, but this
may be a result of the severe degradation conditions employed in the experiments, and examination
under milder conditions is warranted.
5.7 STRUCTURAL MODELS
The need to gather the very large mass of information about kerogen structure into a compact form
useful for guiding research and development has led to models for kerogen structure. These models
are not intended to depict the molecular structure of kerogen. At least not in the sense that the double helix describes the structure of deoxyribonucleic acid (DNA) or even in the sense that synthetic
polymers are described in terms of monomers joined to form chains that have well-defined structures. The kerogen models represent attempts, based on the available data, to depict a collection of
skeletal fragments and functional groups as a three-dimensional network in the most reasonable
manner possible.
However, it must be recognized that no single analytical technique provides sufficient information to construct a precise model of the macromolecular structure of kerogen. Thus, most workers
now use a multidimensional (multiple technique) approach, but as is quite often the case, the techniques used by different workers have different strengths and may emphasize different features of
kerogen structure. Sample-to-sample variation and the use of different isolation techniques also
complicate the issue.
Several models have been proposed for the structure of kerogen in which a multi-dimensional
approach has been employed. Such approaches are extremely valuable since they bring together the
results of several analytical methods. Indeed, the success of such approaches in the deduction of
structural types in kerogen is also paralleled by the use of a similar approach to the deduction of the
structural types that occur in the asphaltene fraction (Chapter 12). Such structural models (although
not absolute) are of interest to consider here because they give an overall picture of the perception