109
Kerogen
other hand, the shell is more tightly cross-linked and contains most of the heteroatom functional
groups and heterocyclic rings. This is interesting from the geochemical viewpoint since the outer
shell of this model is that part of the kerogen in contact with the mineral matrix; heteroatom functions tend to interact more strongly with minerals than do the hydrocarbon chains. Organic–mineral
interactions in the resulting composite would then be ideally situated to hinder physical separation
of minerals from kerogen. This picture is consistent with the data of other workers (Siskin et al.,
1987a,b) on chemically assisted oil shale enrichment.
Another hypothetical model for kerogen is based on the results of a multidimensional approach
to probing kerogen structure, which also included a detailed analysis of the functional groups in the
kerogen (Scouten et al., 1987; Siskin et al., 1995). A comparison with other kerogen models serves
to illustrate some of the key features of this model for kerogen. Aliphatic material is the most obvious feature of this model, and the aliphatic moieties are longer and more linear than those in other
kerogen models. In addition, the aliphatic moieties that are present both as alkylene bridges and
as alkyl side chains and secondary structure due to paraffin–paraffin interactions are important in
the kerogen. Naphthenic and partially hydrogenated aromatic rings also make an important contribution to the aliphatic moieties. The average ring system is only slightly larger than that in other
kerogen models, but the size distribution is appreciably broader; a significant number of the larger
four- to five-ring systems are present.
Yet another approach to deriving models for kerogen structure involves a more generalized
procedure in which models representative of the three types of kerogen and of the asphaltene
constituents from the corresponding oils as a function of maturity were developed (Tissot and
Espitalité, 1975; Béhar and Vandenbroucke, 1986). Emphasis in this work was placed on elucidating the chemistry of maturation for the three kerogen types and representing kerogen at the
beginning of diagenesis (excluding the early stages of diagenesis, which is probably dominated
by microbial action) (Béhar and Vandenbroucke, 1986). These models provide an interesting view
of the structural relationships between the three types of kerogen (Vandenbroucke, 2003). It is of
interest to note the similarities in the models of asphaltene constituents from kerogen (especially
in terms of the size of the polynuclear aromatic systems) with current postulates of the structures
of petroleum asphaltene constituents (Chapter 12).
5.8 KEROGEN MATURATION
The theory that the petroleum precursors form a mix that is often referred to as protopetroleum
(also referred to as primordial precursor soup or petroleum porridge) is an acceptable generalization. The continuation of the concept to the postulation of kerogen as a petroleum precursor has
received considerable attention. There is little doubt that kerogen is a descendent of organic detritus
(Cane, 1976), but the precise role played by kerogen in the generation of petroleum is still open to
speculation. Kerogen, when heated under a variety of conditions, produces oil. It does not follow,
however, that similar natural processes were responsible for the production of oil that is recovered
from various reservoirs today.
Some of the temperatures employed in modern investigations are usually much higher than those
anticipated on the basis of the geothermal gradient, that is, an increase in temperature of 0.012°C
(0.015°C) for every foot of depth, and that might have occurred during the maturation process
(Landes, 1966). The argument that increasing the temperature merely increases the rate of reaction
is certainly open to criticism. Increased temperatures are known not only to increase reaction rates
but also to alter the chemistry of the reactions. In summary there may be several inconsistencies in
the theory that kerogen is the direct precursor to petroleum.
After the relatively rapid alterations that take place shortly after the initial deposition of organic
matter in a sediment, the surviving organic matter undergoes additional changes. Kerogen is presumable produced from part of the organic detritus and oil from the other part. Kerogen maturation involves the loss of hydrogen and oxygen; hydrogen is lost primarily as methane and other
Kerogen
other hand, the shell is more tightly cross-linked and contains most of the heteroatom functional
groups and heterocyclic rings. This is interesting from the geochemical viewpoint since the outer
shell of this model is that part of the kerogen in contact with the mineral matrix; heteroatom functions tend to interact more strongly with minerals than do the hydrocarbon chains. Organic–mineral
interactions in the resulting composite would then be ideally situated to hinder physical separation
of minerals from kerogen. This picture is consistent with the data of other workers (Siskin et al.,
1987a,b) on chemically assisted oil shale enrichment.
Another hypothetical model for kerogen is based on the results of a multidimensional approach
to probing kerogen structure, which also included a detailed analysis of the functional groups in the
kerogen (Scouten et al., 1987; Siskin et al., 1995). A comparison with other kerogen models serves
to illustrate some of the key features of this model for kerogen. Aliphatic material is the most obvious feature of this model, and the aliphatic moieties are longer and more linear than those in other
kerogen models. In addition, the aliphatic moieties that are present both as alkylene bridges and
as alkyl side chains and secondary structure due to paraffin–paraffin interactions are important in
the kerogen. Naphthenic and partially hydrogenated aromatic rings also make an important contribution to the aliphatic moieties. The average ring system is only slightly larger than that in other
kerogen models, but the size distribution is appreciably broader; a significant number of the larger
four- to five-ring systems are present.
Yet another approach to deriving models for kerogen structure involves a more generalized
procedure in which models representative of the three types of kerogen and of the asphaltene
constituents from the corresponding oils as a function of maturity were developed (Tissot and
Espitalité, 1975; Béhar and Vandenbroucke, 1986). Emphasis in this work was placed on elucidating the chemistry of maturation for the three kerogen types and representing kerogen at the
beginning of diagenesis (excluding the early stages of diagenesis, which is probably dominated
by microbial action) (Béhar and Vandenbroucke, 1986). These models provide an interesting view
of the structural relationships between the three types of kerogen (Vandenbroucke, 2003). It is of
interest to note the similarities in the models of asphaltene constituents from kerogen (especially
in terms of the size of the polynuclear aromatic systems) with current postulates of the structures
of petroleum asphaltene constituents (Chapter 12).
5.8 KEROGEN MATURATION
The theory that the petroleum precursors form a mix that is often referred to as protopetroleum
(also referred to as primordial precursor soup or petroleum porridge) is an acceptable generalization. The continuation of the concept to the postulation of kerogen as a petroleum precursor has
received considerable attention. There is little doubt that kerogen is a descendent of organic detritus
(Cane, 1976), but the precise role played by kerogen in the generation of petroleum is still open to
speculation. Kerogen, when heated under a variety of conditions, produces oil. It does not follow,
however, that similar natural processes were responsible for the production of oil that is recovered
from various reservoirs today.
Some of the temperatures employed in modern investigations are usually much higher than those
anticipated on the basis of the geothermal gradient, that is, an increase in temperature of 0.012°C
(0.015°C) for every foot of depth, and that might have occurred during the maturation process
(Landes, 1966). The argument that increasing the temperature merely increases the rate of reaction
is certainly open to criticism. Increased temperatures are known not only to increase reaction rates
but also to alter the chemistry of the reactions. In summary there may be several inconsistencies in
the theory that kerogen is the direct precursor to petroleum.
After the relatively rapid alterations that take place shortly after the initial deposition of organic
matter in a sediment, the surviving organic matter undergoes additional changes. Kerogen is presumable produced from part of the organic detritus and oil from the other part. Kerogen maturation involves the loss of hydrogen and oxygen; hydrogen is lost primarily as methane and other