4
Organic Matter: The Driving Force for Early Diagenesis
146
and economically less valuable than light and sulfurlean crude oil. The difference between an oil shale and
a rock actively generating petroleum is only the thermal
history. An oil shale simply has not completed the phase
of diagenetic release of small molecules described
above. For more details on petroleum formation consult
the textbooks of Tissot and Welte (1984), Hunt (1996)
and Welte et al. (1997).
Molecular structural information about kerogen can
be inferred from elemental analysis, spectroscopic
methods and the results of pyrolysis and selected
chemical degradation experiments (see Rullkötter and
Michaelis 1990 for an overview). Yet, with the
understanding that kerogen is a complex heterogeneous macromolecular substance with contributions
from a variety of organisms and a wide range of chemical
alterations that occurred during diagenesis, it becomes
clear that there will be no single molecular structure of
kerogen, and only certain characteristic units can be
described at the molecular level.
Kerogens have been classified into types derived
from H/C and O/C atomic ratios in a van Krevelen
diagram (Fig. 4.11). The types indicated are related to
the hydrogen and oxygen richness, relative to carbon,
of the biogenic precursor material. Roughly, kerogen
Type I is related to hydrogen-rich organic matter as
occurring, e.g., in waxes and algal mats, kerogen Type
II represents typical oceanic plankton material, and
kerogen Type III is typical of land-derived organic
matter which has been transformed into lignite or coal.
A kerogen type IV not indicated in the diagram has
occasionally been defined to have very low H/C ratios
and to represent highly oxidized, largely inert organic
matter. The bold solid trend lines indicated in Figure
4.11 then represent the changes in elemental
composition initially occuring during diagenesis
(evolution grossly parallel to the x-axis due to loss of
oxygen functionalities) and later during oil and gas
formation (catagenesis; evolution grossly parallel to
y-axis due to loss of hydrogen-rich petroleum
hydrocarbons), until a carbon-rich residue is the
ultimate product (near origin in xy diagram). For more
details see Tissot and Welte (1984).
4.3.5
Early Diagenesis
at the Molecular Level
A small portion of sedimentary organic matter is soluble
in organic solvents and contains lipid compounds that
are either directly inherited from the biological
precursor organisms or cleaved by hydrolysis from
larger cellular units like cell walls or membranes (cf.
Figs. 4.9 and 4.10). The compounds include individual
substances as well as homologous series of structurally
related compounds. Most of them are functionalized
polar lipids that undergo decarboxylation (organic
acids) and dehydration reactions (alcohols) during early
diagenesis to produce saturated and olefinic hydrocarbons, of which the latter are progressively hydrogenated into their saturated analogs during later diagenesis. Alternatively, aromatic hydrocarbons are formed
by the loss of hydrogen. If these hydrocarbons
essentially have the same carbon skeletons and steric
configurations as their functionalized biogenic precursors, they are called biological markers or molecular
fossils (see Sections 4.3.2 and 4.3.5). Parallel to retention
of the biogenic carbon skeleton, structural rearrangements, catalyzed by clay minerals, partial cleavage of
substituents or ring opening may occur as side
reactions during diagenetic transformation of biogenic
lipids. During the earlier phases of diagenesis, including
processes occurring in the water column, such alterations appear to be mediated by microbial activity. With
increasing burial they are more and more driven by
thermodynamic constraints as temperature increases.
The following discussion of biological marker
reactions of course only applies to that fraction of lipid
compounds that have escaped the highly efficient
degradation in the uppermost sediment layer. It has
been established through quantitative assessment of
transformation reactions that degradation in this zone
may occur over timescales of days and that reaction
rates have often been underestimated by an order of
magnitude (Canuel and Martens 1996). It was
furthermore demonstrated in this study that the
degradation processes can be highly selective and
depend on the origin of the compounds (marine,
bacterial or terrestrial). Fatty acids apparently are
particularly sensitive to degradation whereas sterols
and hydrocarbons have a higher chance of entering
the deeper sediment. As a consequence, quantitative
assessment of the source and fate of organic matter
based on biological markers will be strongly limited as
long as diagenetic effects cannot be separated from
variations in organic matter supply.
4.3.6
Biological Markers
(Molecular Fossils)
Molecules with a high degree of structural complexity
are particularly informative and thus suitable for
studying geochemical reactions because they provide
the possibility of relating a certain product to a specific
precursor. For example, specific biomarkers have been
assigned to some common groups of microalgae. These
compounds include long-chain (C 37 -C 39 ) n-alkenones,
Organic Matter: The Driving Force for Early Diagenesis
146
and economically less valuable than light and sulfurlean crude oil. The difference between an oil shale and
a rock actively generating petroleum is only the thermal
history. An oil shale simply has not completed the phase
of diagenetic release of small molecules described
above. For more details on petroleum formation consult
the textbooks of Tissot and Welte (1984), Hunt (1996)
and Welte et al. (1997).
Molecular structural information about kerogen can
be inferred from elemental analysis, spectroscopic
methods and the results of pyrolysis and selected
chemical degradation experiments (see Rullkötter and
Michaelis 1990 for an overview). Yet, with the
understanding that kerogen is a complex heterogeneous macromolecular substance with contributions
from a variety of organisms and a wide range of chemical
alterations that occurred during diagenesis, it becomes
clear that there will be no single molecular structure of
kerogen, and only certain characteristic units can be
described at the molecular level.
Kerogens have been classified into types derived
from H/C and O/C atomic ratios in a van Krevelen
diagram (Fig. 4.11). The types indicated are related to
the hydrogen and oxygen richness, relative to carbon,
of the biogenic precursor material. Roughly, kerogen
Type I is related to hydrogen-rich organic matter as
occurring, e.g., in waxes and algal mats, kerogen Type
II represents typical oceanic plankton material, and
kerogen Type III is typical of land-derived organic
matter which has been transformed into lignite or coal.
A kerogen type IV not indicated in the diagram has
occasionally been defined to have very low H/C ratios
and to represent highly oxidized, largely inert organic
matter. The bold solid trend lines indicated in Figure
4.11 then represent the changes in elemental
composition initially occuring during diagenesis
(evolution grossly parallel to the x-axis due to loss of
oxygen functionalities) and later during oil and gas
formation (catagenesis; evolution grossly parallel to
y-axis due to loss of hydrogen-rich petroleum
hydrocarbons), until a carbon-rich residue is the
ultimate product (near origin in xy diagram). For more
details see Tissot and Welte (1984).
4.3.5
Early Diagenesis
at the Molecular Level
A small portion of sedimentary organic matter is soluble
in organic solvents and contains lipid compounds that
are either directly inherited from the biological
precursor organisms or cleaved by hydrolysis from
larger cellular units like cell walls or membranes (cf.
Figs. 4.9 and 4.10). The compounds include individual
substances as well as homologous series of structurally
related compounds. Most of them are functionalized
polar lipids that undergo decarboxylation (organic
acids) and dehydration reactions (alcohols) during early
diagenesis to produce saturated and olefinic hydrocarbons, of which the latter are progressively hydrogenated into their saturated analogs during later diagenesis. Alternatively, aromatic hydrocarbons are formed
by the loss of hydrogen. If these hydrocarbons
essentially have the same carbon skeletons and steric
configurations as their functionalized biogenic precursors, they are called biological markers or molecular
fossils (see Sections 4.3.2 and 4.3.5). Parallel to retention
of the biogenic carbon skeleton, structural rearrangements, catalyzed by clay minerals, partial cleavage of
substituents or ring opening may occur as side
reactions during diagenetic transformation of biogenic
lipids. During the earlier phases of diagenesis, including
processes occurring in the water column, such alterations appear to be mediated by microbial activity. With
increasing burial they are more and more driven by
thermodynamic constraints as temperature increases.
The following discussion of biological marker
reactions of course only applies to that fraction of lipid
compounds that have escaped the highly efficient
degradation in the uppermost sediment layer. It has
been established through quantitative assessment of
transformation reactions that degradation in this zone
may occur over timescales of days and that reaction
rates have often been underestimated by an order of
magnitude (Canuel and Martens 1996). It was
furthermore demonstrated in this study that the
degradation processes can be highly selective and
depend on the origin of the compounds (marine,
bacterial or terrestrial). Fatty acids apparently are
particularly sensitive to degradation whereas sterols
and hydrocarbons have a higher chance of entering
the deeper sediment. As a consequence, quantitative
assessment of the source and fate of organic matter
based on biological markers will be strongly limited as
long as diagenetic effects cannot be separated from
variations in organic matter supply.
4.3.6
Biological Markers
(Molecular Fossils)
Molecules with a high degree of structural complexity
are particularly informative and thus suitable for
studying geochemical reactions because they provide
the possibility of relating a certain product to a specific
precursor. For example, specific biomarkers have been
assigned to some common groups of microalgae. These
compounds include long-chain (C 37 -C 39 ) n-alkenones,