4
Organic Matter: The Driving Force for Early Diagenesis
144
condensation pathway for geomacromolecule
formation. Neither adsorption nor condensation alone
may be a satisfactory process for preservation of labile
organic substances. Adsorption of monomers can
merely retard their biodegradation, and condensation
is not favored in (pore water) solution. However, if the
processes operate in concert – adsorption promoting
condensation and condensation enhancing adsorption
of further reactants – a plausible mechanism for the
preservation of organic matter arises. Condensation
reactions between adsorbed compounds would lead
to the formation of very strongly bound macromolecules resulting in a marked divergence in the
diagenetic history of the adsorbed monolayer and
nonmineral-bound organic matter. More direct evidence
is, however, still required to establish the quantitative
importance of this process relative to other processes,
such as selective preservation.
4.3.4
The Formation of Fossil Organic
Matter and its Bulk Composition
The geomacromolecular organic matter surviving
microbial degradation in the early phase of diagenesis
consists of three fractions, termed fulvic acids, humic
acids and humin. They are ill-defined in their chemical
structures, but are operationally distinguished by their
solubilities in bases (humic and fulvic acids) and acids
(fulvic acids only). All three are considered to be
potential precursors of kerogen, which designates the
type of geomacromolecules formed at a later stage of
diagenesis. Kerogen is also only operationally defined
as being insoluble in non-oxidizing acids, bases and
organic solvents (Durand 1980; Tissot and Welte 1984).
Besides this high-molecular-weight organic material,
sediments contain low-molecular-weight organic
substances that are collectively called bitumen and are
Fig. 4.10
The selective preservation pathway model of kerogen formation (after de Leeuw and Largeau 1993 and
Tegelaar et al. 1989).
Organic Matter: The Driving Force for Early Diagenesis
144
condensation pathway for geomacromolecule
formation. Neither adsorption nor condensation alone
may be a satisfactory process for preservation of labile
organic substances. Adsorption of monomers can
merely retard their biodegradation, and condensation
is not favored in (pore water) solution. However, if the
processes operate in concert – adsorption promoting
condensation and condensation enhancing adsorption
of further reactants – a plausible mechanism for the
preservation of organic matter arises. Condensation
reactions between adsorbed compounds would lead
to the formation of very strongly bound macromolecules resulting in a marked divergence in the
diagenetic history of the adsorbed monolayer and
nonmineral-bound organic matter. More direct evidence
is, however, still required to establish the quantitative
importance of this process relative to other processes,
such as selective preservation.
4.3.4
The Formation of Fossil Organic
Matter and its Bulk Composition
The geomacromolecular organic matter surviving
microbial degradation in the early phase of diagenesis
consists of three fractions, termed fulvic acids, humic
acids and humin. They are ill-defined in their chemical
structures, but are operationally distinguished by their
solubilities in bases (humic and fulvic acids) and acids
(fulvic acids only). All three are considered to be
potential precursors of kerogen, which designates the
type of geomacromolecules formed at a later stage of
diagenesis. Kerogen is also only operationally defined
as being insoluble in non-oxidizing acids, bases and
organic solvents (Durand 1980; Tissot and Welte 1984).
Besides this high-molecular-weight organic material,
sediments contain low-molecular-weight organic
substances that are collectively called bitumen and are
Fig. 4.10
The selective preservation pathway model of kerogen formation (after de Leeuw and Largeau 1993 and
Tegelaar et al. 1989).