105
Kerogen
The Van Krevelen diagram (a plot of the atomic hydrogen–carbon ratio vs. the atomic oxygen–
carbon ratio) (Figure 5.2), derived from the elemental analysis of kerogen and coal, is a very practical means of studying kerogen composition. The position of kerogen in the H/C–O/C diagram is
related to the total quantity of hydrocarbons, which in turn is a function of the relative amounts of
aromatic hydrocarbon structures. The data for kerogen analysis in the H/C–O/C diagram can be
considered to describe the evolutionary path for kerogen from different precursors. Oil and gas are
believed to be formed during this evolutionary path. Analysis of the minor elements, sulfur and
nitrogen, is much more difficult to simulate and may require a more detailed framework.
Inferences regarding kerogen structure are drawn from the results of bitumen (kerogen extract)
analyses. The conclusions are generally based on the premise that the bitumen is analogous to
original organic matter, that is, the bitumen represents units of the precursor that did not become
bound to the insoluble three-dimensional macromolecular network of the kerogen. It is also assumed
that the bitumen is representative of units of the kerogen structure that have been cleaved more or
less intact, with little or no structural alteration, from the kerogen by thermal treatment.
Many different compound types have been identified (by extraction procedures) as part of the
kerogen matrix but their mode of inclusion in kerogen remains open to speculation. For example,
the compounds isolated from kerogen include paraffins (Cummins and Robinson, 1964), steranes, cycloalkanes, aromatics, and polar compounds (Durand, 1980). Kerogen generally contains only a small amount of bitumen (<15% w/w of the total organic matter), but the bitumen
usually has a higher hydrogen content than the corresponding kerogen. This corresponds to a
lower proportion of aromatics, as well as nitrogen-, oxygen-, and sulfur-containing compounds.
This is an obvious limitation to the due usefulness of structural inferences drawn from the bitumen composition.
The volatile oil generally represents a much larger fraction of the original organic material (usually 50% or more of the available organic carbon). The methods employed for analysis of the product oils are similar to those used for petroleum (Uden et al., 1978; Fenton et al., 1981; Holmes and
Thompson, 1981; Williams and Douglas, 1981; Regtop et al., 1982). As a consequence of their thermal treatment, the volatile oils are usually richer in aromatics and olefins than the starting kerogen
but are relatively deficient in nitrogen- and sulfur-containing compounds. As for petroleum coking
(Speight, 1987), the nitrogen- and sulfur-containing species are concentrated in the nonvolatile char.
The volatile oils are a greater reflection of the thermal treatment used to produce the oil and, consequently, are not too reliable in terms of an accurate picture of kerogen structure. Thus, although
many inferences about kerogen structure have been drawn from bitumen and oil analyses, their
limitations must be recognized.
5.6.2 FunCtIonAl grouP AnAlysIs
Attempts to characterize the oxygen functional groups in kerogen have focused on acid demineralization (successive treatments with hydrochloric acid and hydrofluoric acid) to prepare a kerogen
concentrate. The concentrate has then been treated by wet chemical methods to determine the distribution of oxygen functional groups (Robinson and Dineen, 1967).
5.6.3 oxIdAtIon
Oxidative degradation, one of the primary methods of structural determination used in natural product chemistry, has also been employed to examine kerogen structure (Vitorovic, 1980).
Alkaline permanganate and chromic acid have been the two most widely used oxidants, although
ozone, periodate, nitric acid, perchloric acid, air or oxygen, hydrogen peroxide, and electrochemical
oxidation (among many other reagents) have also been used.
Alkaline permanganate oxidation of kerogen has been carried out in two very different ways.
The products of this exhaustive oxidation are carbon dioxide, oxalic acid (HO 2 C–CO 2 H, from
Kerogen
The Van Krevelen diagram (a plot of the atomic hydrogen–carbon ratio vs. the atomic oxygen–
carbon ratio) (Figure 5.2), derived from the elemental analysis of kerogen and coal, is a very practical means of studying kerogen composition. The position of kerogen in the H/C–O/C diagram is
related to the total quantity of hydrocarbons, which in turn is a function of the relative amounts of
aromatic hydrocarbon structures. The data for kerogen analysis in the H/C–O/C diagram can be
considered to describe the evolutionary path for kerogen from different precursors. Oil and gas are
believed to be formed during this evolutionary path. Analysis of the minor elements, sulfur and
nitrogen, is much more difficult to simulate and may require a more detailed framework.
Inferences regarding kerogen structure are drawn from the results of bitumen (kerogen extract)
analyses. The conclusions are generally based on the premise that the bitumen is analogous to
original organic matter, that is, the bitumen represents units of the precursor that did not become
bound to the insoluble three-dimensional macromolecular network of the kerogen. It is also assumed
that the bitumen is representative of units of the kerogen structure that have been cleaved more or
less intact, with little or no structural alteration, from the kerogen by thermal treatment.
Many different compound types have been identified (by extraction procedures) as part of the
kerogen matrix but their mode of inclusion in kerogen remains open to speculation. For example,
the compounds isolated from kerogen include paraffins (Cummins and Robinson, 1964), steranes, cycloalkanes, aromatics, and polar compounds (Durand, 1980). Kerogen generally contains only a small amount of bitumen (<15% w/w of the total organic matter), but the bitumen
usually has a higher hydrogen content than the corresponding kerogen. This corresponds to a
lower proportion of aromatics, as well as nitrogen-, oxygen-, and sulfur-containing compounds.
This is an obvious limitation to the due usefulness of structural inferences drawn from the bitumen composition.
The volatile oil generally represents a much larger fraction of the original organic material (usually 50% or more of the available organic carbon). The methods employed for analysis of the product oils are similar to those used for petroleum (Uden et al., 1978; Fenton et al., 1981; Holmes and
Thompson, 1981; Williams and Douglas, 1981; Regtop et al., 1982). As a consequence of their thermal treatment, the volatile oils are usually richer in aromatics and olefins than the starting kerogen
but are relatively deficient in nitrogen- and sulfur-containing compounds. As for petroleum coking
(Speight, 1987), the nitrogen- and sulfur-containing species are concentrated in the nonvolatile char.
The volatile oils are a greater reflection of the thermal treatment used to produce the oil and, consequently, are not too reliable in terms of an accurate picture of kerogen structure. Thus, although
many inferences about kerogen structure have been drawn from bitumen and oil analyses, their
limitations must be recognized.
5.6.2 FunCtIonAl grouP AnAlysIs
Attempts to characterize the oxygen functional groups in kerogen have focused on acid demineralization (successive treatments with hydrochloric acid and hydrofluoric acid) to prepare a kerogen
concentrate. The concentrate has then been treated by wet chemical methods to determine the distribution of oxygen functional groups (Robinson and Dineen, 1967).
5.6.3 oxIdAtIon
Oxidative degradation, one of the primary methods of structural determination used in natural product chemistry, has also been employed to examine kerogen structure (Vitorovic, 1980).
Alkaline permanganate and chromic acid have been the two most widely used oxidants, although
ozone, periodate, nitric acid, perchloric acid, air or oxygen, hydrogen peroxide, and electrochemical
oxidation (among many other reagents) have also been used.
Alkaline permanganate oxidation of kerogen has been carried out in two very different ways.
The products of this exhaustive oxidation are carbon dioxide, oxalic acid (HO 2 C–CO 2 H, from