14 Hydrocarbons and Coal
14.1 Source Rocks, Kerogen Types, and
Hydrocarbon Potential
14.1.1 Source Rocks
14.1.2 Kerogen Types and Hydrocarbon Potential
14.2 Generation and Migration ofHydrocarbons
14.2.1 The Evolution of Organic Matter
14.2.2 Kerogen-OiVGas Transformation
14.2.3 Expulsion and Migration ofOil
Expulsion of Oil (Primary Migration)
Secondary Migration (in Liquid Phase)
Other Transport Mechanisms
14.3 Examples ofHydrocarbon Habitats
14.3.1 Lacustrine Source Beds
14.3.2 Marine Deltas
14.3.3 Rift Basins and Shallow Epicontinental Seas
14.3.4 Carbonate Shelf: the Middle East
14.3.5 Hydrocarbons in Other Basin Types
14.3.6 Gas Hydrates
14.3.7 Conclusions
14.3.8 Summary (Hydrocarbons)
14.4 Evolution ofCoal
14.4.1 General Aspects ofCoal Formation
14.4.2 Preservation ofPeat
14.4.3 Coalification and Composition ofCoal
14.4.4 Example of a Coal Basin
14.4.5 Summary (Coal)
14.1 Source Rocks, Kerogen Types, and
Hydrocarbon Potential
14.1.1 Source Rocks
The source rocks, the generation and migration of
hydrocarbons and their exploration and production
are a very broad and extensive topic. Here, a few
basic principles and processes are briefly introduced.
Hydrocarbon source rocks may be defined as finegrained sediments which in their natural setting have
generated, are generating, or will generate and release enough hydrocarbons to form a considerable
accumulation of oil or gas (Brooks et al. 1987).
Whether such an accumulation is commercial largely
depends on economic considerations. A potential
source rock has the capacity to generate hydrocarbons in substantial quantities, but has not yet reached
sufficient organic maturation.
Oil shales contain thermally degradable organic
material, usually about 20% of the total organic matter, TOC, the remainder consisting of insoluble compounds (kerogen, see below). Oil shales and coal
must have a very high organic matter content to be of
economic interest. In contrast, hydrocarbon source
rocks may release only very small proportions of oil
or gas per unit rock volume to form an important accumulation in reservoir rocks. This is possible if the
source rocks represent a large and sufficiently subsided rock mass. The following brief discussion focuses on hydrocarbon source rocks, but also inc1udes
to some extent other types of organic-rich sediments.
The depositional environment of these sediments is
discussed in Sections 5.3 and 10.3.
The preservation of organic matter is, among other
factors, a function of the oxygen content of bottom
waters, the sedimentation rate, and the intensity of
benthic life (cf. Sect. 10.3.3). The infl.uence of these
factors is sumrnarized in Fig. 14.1. With water oxygenation and benthic activity decreasing, the zone of
methane fermenting bacteria expands upward at the
expense of the sulfate reduction zone. As a result,
more and also less stable organic matter can be preserved in the sediment.
Many so-called oil shales exposed in outcrops or drilled in
the subsurface are immature and have never delivered
thermocatalytic oil and gas. They may have produced some
biogenic methane at very shallow burial depth (microbial
methanogenesis). The situation is different for oil sands
which represent heavy oil deposits. The sands have trapped
oil which was provided by the normal type of hydrocarbon
source rocks and migrated via carrier beds into the sands.
The giant oil sand deposits of Alberta, Canada, are stored
in Cretaceous sands, but the oil comes from Devonian carbonate rocks and was transported by long-term meteoric
groundwater flow (Garven 1989). Increased solubility of
oil due to high water temperatures (around 100°C) facilitated the oil transport in aqueous solution. Later, the relatively light components of the oil sand were lost due to
biodegradation and dissolution at or elose to the erosional
lowering of the land surface.
14.1.2 Kerogen Types and
Hydrocarbon Potential
During burial and under increasing temperature, the
organic matter undergoes aseries of geochemical
14.1 Source Rocks, Kerogen Types, and
Hydrocarbon Potential
14.1.1 Source Rocks
14.1.2 Kerogen Types and Hydrocarbon Potential
14.2 Generation and Migration ofHydrocarbons
14.2.1 The Evolution of Organic Matter
14.2.2 Kerogen-OiVGas Transformation
14.2.3 Expulsion and Migration ofOil
Expulsion of Oil (Primary Migration)
Secondary Migration (in Liquid Phase)
Other Transport Mechanisms
14.3 Examples ofHydrocarbon Habitats
14.3.1 Lacustrine Source Beds
14.3.2 Marine Deltas
14.3.3 Rift Basins and Shallow Epicontinental Seas
14.3.4 Carbonate Shelf: the Middle East
14.3.5 Hydrocarbons in Other Basin Types
14.3.6 Gas Hydrates
14.3.7 Conclusions
14.3.8 Summary (Hydrocarbons)
14.4 Evolution ofCoal
14.4.1 General Aspects ofCoal Formation
14.4.2 Preservation ofPeat
14.4.3 Coalification and Composition ofCoal
14.4.4 Example of a Coal Basin
14.4.5 Summary (Coal)
14.1 Source Rocks, Kerogen Types, and
Hydrocarbon Potential
14.1.1 Source Rocks
The source rocks, the generation and migration of
hydrocarbons and their exploration and production
are a very broad and extensive topic. Here, a few
basic principles and processes are briefly introduced.
Hydrocarbon source rocks may be defined as finegrained sediments which in their natural setting have
generated, are generating, or will generate and release enough hydrocarbons to form a considerable
accumulation of oil or gas (Brooks et al. 1987).
Whether such an accumulation is commercial largely
depends on economic considerations. A potential
source rock has the capacity to generate hydrocarbons in substantial quantities, but has not yet reached
sufficient organic maturation.
Oil shales contain thermally degradable organic
material, usually about 20% of the total organic matter, TOC, the remainder consisting of insoluble compounds (kerogen, see below). Oil shales and coal
must have a very high organic matter content to be of
economic interest. In contrast, hydrocarbon source
rocks may release only very small proportions of oil
or gas per unit rock volume to form an important accumulation in reservoir rocks. This is possible if the
source rocks represent a large and sufficiently subsided rock mass. The following brief discussion focuses on hydrocarbon source rocks, but also inc1udes
to some extent other types of organic-rich sediments.
The depositional environment of these sediments is
discussed in Sections 5.3 and 10.3.
The preservation of organic matter is, among other
factors, a function of the oxygen content of bottom
waters, the sedimentation rate, and the intensity of
benthic life (cf. Sect. 10.3.3). The infl.uence of these
factors is sumrnarized in Fig. 14.1. With water oxygenation and benthic activity decreasing, the zone of
methane fermenting bacteria expands upward at the
expense of the sulfate reduction zone. As a result,
more and also less stable organic matter can be preserved in the sediment.
Many so-called oil shales exposed in outcrops or drilled in
the subsurface are immature and have never delivered
thermocatalytic oil and gas. They may have produced some
biogenic methane at very shallow burial depth (microbial
methanogenesis). The situation is different for oil sands
which represent heavy oil deposits. The sands have trapped
oil which was provided by the normal type of hydrocarbon
source rocks and migrated via carrier beds into the sands.
The giant oil sand deposits of Alberta, Canada, are stored
in Cretaceous sands, but the oil comes from Devonian carbonate rocks and was transported by long-term meteoric
groundwater flow (Garven 1989). Increased solubility of
oil due to high water temperatures (around 100°C) facilitated the oil transport in aqueous solution. Later, the relatively light components of the oil sand were lost due to
biodegradation and dissolution at or elose to the erosional
lowering of the land surface.
14.1.2 Kerogen Types and
Hydrocarbon Potential
During burial and under increasing temperature, the
organic matter undergoes aseries of geochemical