4
Organic Matter: The Driving Force for Early Diagenesis
160
polished blocks in reflected light under a petrographic microscope. When sediments are lean in
organic matter, the organic particles have to be
concentrated by dissolution of the mineral matrix
in consecutive treatments with hydrochloric and
hydrofluoric acid. The concentrates are then
analyzed as smear slides in transmitted light or
embedded in araldite
®
resin and subsequently
studied as polished blocks similar to whole-rock
samples.
Organic particles visible under the microscope (>1
µm) are called macerals. The most important groups in
the order of increasing reflectance are liptinite, vitrinite
and inertinite. Liptinites are lipid-rich parts of aquatic
(e.g. alginite) or land plants (e.g. cutinite, suberinite,
sporinite, resinite), the terms indicating the origin of
these organoclasts. Many liptinites are probably related
to nonhydrolyzable, highly aliphatic biopolymers
found in algae (algaenan) and land plants (cutinan,
suberan) (see Sect. 4.3.3). These biopolymers serve as
cell wall components of the organisms and their stability
allows the morphological shapes of plant material to
be preserved after sedimentation and burial so that
they can be identified under the microscope. Vitrinites
derive from the woody parts of higher plants. Inertinites
are highly reflecting particles of strongly oxidized or
geothermally heated organic matter of various origin,
most commonly from higher plants. Non-structured,
often (incorrectly) called amorphous, organic matter is
known as bituminite or sapropelinite. Lipid-rich organic
matter, even if finely dispersed, can be recognized under
the microscope after UV irradiation by its bright
fluorescence.
In addition to maceral distribution, organic petrographers determine vitrinite reflectance as a measure
of geothermal evolution of sedimentary organic matter.
For more details of microscopic analysis see Taylor et
al. (1998).
4.5.5
Bitumen Analysis
The larger part of sedimentary organic matter is
insoluble in organic solvents. The proportion of
the soluble fraction (bitumen) can be relatively
high in surficial sediments, then decreases in
amount with increasing depth of burial due to
formation of humic substances and kerogen. It
only increases again when temperatures become
high enough for thermal kerogen cracking to generate petroleum (Tissot and Welte 1984).
The most common solvent used for extraction of
bitumen from dried sediments is dichloromethane
(CH 2 Cl 2 ) with a small admixture (e.g. 1%) of methanol,
although more polar mixtures like chloroform/methanol
or chloroform/toluene are also used. Occasionally, when
very polar lipids from surficial sediments are to be
extracted, wet sediment samples are preferred, and
extraction starts with acetone or methanol or a mixture
of these two because they mix with water. Extraction is
then repeated with dichloromethane or a solvent of
similar polarity. Extraction in a Soxhlet apparatus usually
takes one to two days, whereas reasonably complete
extracts can be obtained within minutes with the support
of ultrasonication or blending. After filtration, the
solvent is removed by rotary evaporation and the total
extract yield determined gravimetrically, as is commonly
done for any subfraction after further separation.
Internal standards for quantitation of single compounds are added either before or after extraction,
occasionally only after liquid chromatographic
separation (see later).
The most polar and highest-molecular-weight
extract components (called asphaltenes, a term derived
from petroleum geochemistry, but often also applied to
surficial sediments or even biological material) may
interfere with many subsequent separations and
analyses. For this reason, these components are
frequently removed from the total extract by dissolving
it in a small amount of, e.g., dichloromethane and adding
a large excess of a nonpolar solvent like n-hexane (or
n-pentane, n-heptane). The n-hexane-soluble extract
fraction can be further separated into compound
classes (or fractions of similar polarities) either by
column liquid chromatography, medium-pressure liquid
chromatography (MPLC: Radke et al. 1980; HMPLC:
Willsch et al. 1997), high-performance liquid chromatography (HPLC), or thin-layer chromatography (TLC),
depending on the sample quantity used and the
sophistication of the separation required. In MPLC
separation, as indicated in Figure 4.21, the polar lipids
are withheld by a pre-column filled with deactivated
silica gel, and only the hydrocarbons are separated on
the main silica gel column into nonaromatic and
aromatic hydrocarbon fractions. Because the main
separation system is only operated with n-hexane, the
difficult separation between the two hydrocarbon
fractions is very reproducible, an important prerequisite
for some applications, particularly in petroleum
geochemistry. The polar lipids can be removed from
the pre-column with a polar solvent. If emphasis is
placed on the subfractionation of the polar heterocompounds, then HMPLC may be the method of choice.
There are, however, many variations of this separation
scheme. The nature of the geological samples and the
scientific objectives will determine the type and extent
of separation required.
Organic Matter: The Driving Force for Early Diagenesis
160
polished blocks in reflected light under a petrographic microscope. When sediments are lean in
organic matter, the organic particles have to be
concentrated by dissolution of the mineral matrix
in consecutive treatments with hydrochloric and
hydrofluoric acid. The concentrates are then
analyzed as smear slides in transmitted light or
embedded in araldite
®
resin and subsequently
studied as polished blocks similar to whole-rock
samples.
Organic particles visible under the microscope (>1
µm) are called macerals. The most important groups in
the order of increasing reflectance are liptinite, vitrinite
and inertinite. Liptinites are lipid-rich parts of aquatic
(e.g. alginite) or land plants (e.g. cutinite, suberinite,
sporinite, resinite), the terms indicating the origin of
these organoclasts. Many liptinites are probably related
to nonhydrolyzable, highly aliphatic biopolymers
found in algae (algaenan) and land plants (cutinan,
suberan) (see Sect. 4.3.3). These biopolymers serve as
cell wall components of the organisms and their stability
allows the morphological shapes of plant material to
be preserved after sedimentation and burial so that
they can be identified under the microscope. Vitrinites
derive from the woody parts of higher plants. Inertinites
are highly reflecting particles of strongly oxidized or
geothermally heated organic matter of various origin,
most commonly from higher plants. Non-structured,
often (incorrectly) called amorphous, organic matter is
known as bituminite or sapropelinite. Lipid-rich organic
matter, even if finely dispersed, can be recognized under
the microscope after UV irradiation by its bright
fluorescence.
In addition to maceral distribution, organic petrographers determine vitrinite reflectance as a measure
of geothermal evolution of sedimentary organic matter.
For more details of microscopic analysis see Taylor et
al. (1998).
4.5.5
Bitumen Analysis
The larger part of sedimentary organic matter is
insoluble in organic solvents. The proportion of
the soluble fraction (bitumen) can be relatively
high in surficial sediments, then decreases in
amount with increasing depth of burial due to
formation of humic substances and kerogen. It
only increases again when temperatures become
high enough for thermal kerogen cracking to generate petroleum (Tissot and Welte 1984).
The most common solvent used for extraction of
bitumen from dried sediments is dichloromethane
(CH 2 Cl 2 ) with a small admixture (e.g. 1%) of methanol,
although more polar mixtures like chloroform/methanol
or chloroform/toluene are also used. Occasionally, when
very polar lipids from surficial sediments are to be
extracted, wet sediment samples are preferred, and
extraction starts with acetone or methanol or a mixture
of these two because they mix with water. Extraction is
then repeated with dichloromethane or a solvent of
similar polarity. Extraction in a Soxhlet apparatus usually
takes one to two days, whereas reasonably complete
extracts can be obtained within minutes with the support
of ultrasonication or blending. After filtration, the
solvent is removed by rotary evaporation and the total
extract yield determined gravimetrically, as is commonly
done for any subfraction after further separation.
Internal standards for quantitation of single compounds are added either before or after extraction,
occasionally only after liquid chromatographic
separation (see later).
The most polar and highest-molecular-weight
extract components (called asphaltenes, a term derived
from petroleum geochemistry, but often also applied to
surficial sediments or even biological material) may
interfere with many subsequent separations and
analyses. For this reason, these components are
frequently removed from the total extract by dissolving
it in a small amount of, e.g., dichloromethane and adding
a large excess of a nonpolar solvent like n-hexane (or
n-pentane, n-heptane). The n-hexane-soluble extract
fraction can be further separated into compound
classes (or fractions of similar polarities) either by
column liquid chromatography, medium-pressure liquid
chromatography (MPLC: Radke et al. 1980; HMPLC:
Willsch et al. 1997), high-performance liquid chromatography (HPLC), or thin-layer chromatography (TLC),
depending on the sample quantity used and the
sophistication of the separation required. In MPLC
separation, as indicated in Figure 4.21, the polar lipids
are withheld by a pre-column filled with deactivated
silica gel, and only the hydrocarbons are separated on
the main silica gel column into nonaromatic and
aromatic hydrocarbon fractions. Because the main
separation system is only operated with n-hexane, the
difficult separation between the two hydrocarbon
fractions is very reproducible, an important prerequisite
for some applications, particularly in petroleum
geochemistry. The polar lipids can be removed from
the pre-column with a polar solvent. If emphasis is
placed on the subfractionation of the polar heterocompounds, then HMPLC may be the method of choice.
There are, however, many variations of this separation
scheme. The nature of the geological samples and the
scientific objectives will determine the type and extent
of separation required.