Geoscience Reference
In-Depth Information
2.0
Algae
1.8
Cellulose
1.6
CO
2
Wood
1.4
H
2
O
CH
4
1.2
Peat
Kerogen
Peat
1.0
0.8
Coal
0.6
Petroleum
Gaseous hydrocarbons
0.4
0.2
0.0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Atomic O/C
Figure 8.1
A van Krevelen
(1950)
plot representing the relationship between the major types of organic
substances present in the geological record. The arrows indicate how loss of CH
4
(
x
=
0;
y
=
4),
CO
2
(
x
2) affects compositions in this diagram. Loss of H
2
OandCO
2
from the original organic products generates kerogen. Upon further heating, loss of liquid
hydrocarbons, and subsequently of gaseous hydrocarbons inclusive of methane, leaves coal as the
main residue (modified from Killops and Killops,
2005
).
=
2;
y
=
0), and H
2
O(
y
/
x
=
Reduced carbon is another important remnant of biological activity. Different types of
organic material are found in rocks:
•
Humic substances
are common components of soils and waters. The precursors of these
very complex compounds are diverse organic compounds resistant to biodegradation
such as lignin and tannins.
•
Kerogen
consists of a variety of polymers and macromolecules formed during diagen-
esis by microbial degradation and condensation of humic substances, which make the
products progressively more insoluble in diagenetic fluids. Different types of kerogens
are distinguished, which may eventually lead to the formation of liquid hydrocarbons
and coal. Bitumen is the fraction of carbon compounds that can be extracted from a rock
by liquid solvents.
•
Liquid hydrocarbons
(petroleum) evolve by loss of water, carbon dioxide, and methane
from some types of kerogen upon heating at temperatures of 60-150
◦
C (the oil
“window”). The nature of the end-product depends on the rate of transformation.
•
Gaseous hydrocarbons
such as methane evolve from kerogen and coal at temperatures
of 150-230
◦
C.
•
Coal
is the residual carbon-rich material left behind after hydrocarbons have left the
rock. The most abundant humic coals derive from vascular plants, whereas sapropelic
coal forms from fine-grained organic particles.
The maturation of carbon-rich material can be best presented in a van Krevelen
(1950)
plot representing the H/C ratio as a function of the O/C ratio (
Fig. 8.1
), in which loss of
