anoxic conditions and a high degree of preservation of
the organic matter produced in the surface waters. This
aspect is therefore of considerable interest in exploration for petroleum in freshwater basins, particularly in
Africa and China. The open oceans have normally had
oxygenated water, but during the Cretaceous most of
the Atlantic Ocean is believed to have been stagnant
during so-called “anoxic events”, and substantial
amounts of black shale were deposited in the deeper
parts of the ocean during these periods.
1.5
Early Diagenesis of Organic Matter
Microbiological breakdown of organic matter in
sediments is due to the activity of bacteria, fungi,
protozoa, etc. and under oxidising conditions these
are extremely effective. However, the porewater
quickly becomes reducing if the oxygen is not
replenished. In relatively coarse-grained sediments
(sand), oxygen may diffuse to depths of 5–20 cm
below the seabed, while in clay and fine-grained carbonate mud the boundary between oxidising and
reducing water (redox boundary) may be just a few
millimetres below the seafloor. The pores in the
sediments here are so small that water circulation
and diffusion are insufficient to replace the original
oxygen in the porewater as it gets used up by oxidation
of organic matter. Clay-rich sediments soon become a
relatively closed system, and the downward diffusion
of oxygen from the seabed is very slow in fine-grained
sediments.
Aerobic breakdown is therefore much more effective in coarse-grained sediments than in fine-grained
ones. In anaerobic transformation bacteria use organic
matter, e.g. short carbohydrate chains. Cellulose is
broken down by fungi, and finally by bacteria. The
end products are methane (CH 4 ) and carbon dioxide
(CO 2 ). Methane, however, is the only hydrocarbon
produced in any quantity at low temperatures by bacteria close to the surface of the sediment. Gas occurring at shallow depths (shallow gas) therefore consists
largely of methane (dry gas) unless there has been
addition from much deeper strata. Biogenic gas may
form commercial accumulations, as in Western
Siberia and also in the shallow part of the North Sea
basin. The presence of abundant shallow gas may
represent a hazard in the form of blowouts and fire
during drilling. Gas occurring at shallow depth may
also have a deeper source generated from a gas-prone
source rock (coaly sediments) or by cracking of oil,
but such gas has a very different isotopic signature
than biogenic gas.
1.6
Kerogen
As organic material becomes buried by the accumulation of overlying sediments, water is gradually
expelled during compaction.
Complex organic compounds like proteins are broken down into amino acids, and carbohydrates into
simpler sugar compounds. These are able to recombine to make larger compounds, for example by amino
acids reacting with carbohydrates (melanoid reaction).
As this type of polymerisation proceeds, the proportion of simpler soluble organic compounds diminishes
at depths of a few tens of metres down in the sediment.
It is these newly-formed complex organic structures
which are called kerogen.
Kerogen is a collective name for organic material
that is insoluble in organic solvents, water or oxidising
acids. The portion of the organic material soluble in
organic solvents is called bitumen, which is essentially
oil in a solid state.
Kerogen consists of very large molecules and is a
kind of polymer. When it has been exposed to sufficient time and temperature these large molecules will
crack into smaller molecules, mostly petroleum. When
the temperature is about 100
C a long period of geological time is required. In rapidly subsiding basins the
exposure time is shorter and oil generation may only
start at about 140–150
C. In the North Sea basin the
“oil window” may typically be between 130 and
140
C.
1.7
Migration of Petroleum
Petroleum migrates from low permeability source
rocks into high permeability reservoir rocks from
which the petroleum can be produced (Fig. 1.2b).
The main driving force for petroleum migration is
buoyancy because it is less dense than water. The
10
K. Bjørlykke
the organic matter produced in the surface waters. This
aspect is therefore of considerable interest in exploration for petroleum in freshwater basins, particularly in
Africa and China. The open oceans have normally had
oxygenated water, but during the Cretaceous most of
the Atlantic Ocean is believed to have been stagnant
during so-called “anoxic events”, and substantial
amounts of black shale were deposited in the deeper
parts of the ocean during these periods.
1.5
Early Diagenesis of Organic Matter
Microbiological breakdown of organic matter in
sediments is due to the activity of bacteria, fungi,
protozoa, etc. and under oxidising conditions these
are extremely effective. However, the porewater
quickly becomes reducing if the oxygen is not
replenished. In relatively coarse-grained sediments
(sand), oxygen may diffuse to depths of 5–20 cm
below the seabed, while in clay and fine-grained carbonate mud the boundary between oxidising and
reducing water (redox boundary) may be just a few
millimetres below the seafloor. The pores in the
sediments here are so small that water circulation
and diffusion are insufficient to replace the original
oxygen in the porewater as it gets used up by oxidation
of organic matter. Clay-rich sediments soon become a
relatively closed system, and the downward diffusion
of oxygen from the seabed is very slow in fine-grained
sediments.
Aerobic breakdown is therefore much more effective in coarse-grained sediments than in fine-grained
ones. In anaerobic transformation bacteria use organic
matter, e.g. short carbohydrate chains. Cellulose is
broken down by fungi, and finally by bacteria. The
end products are methane (CH 4 ) and carbon dioxide
(CO 2 ). Methane, however, is the only hydrocarbon
produced in any quantity at low temperatures by bacteria close to the surface of the sediment. Gas occurring at shallow depths (shallow gas) therefore consists
largely of methane (dry gas) unless there has been
addition from much deeper strata. Biogenic gas may
form commercial accumulations, as in Western
Siberia and also in the shallow part of the North Sea
basin. The presence of abundant shallow gas may
represent a hazard in the form of blowouts and fire
during drilling. Gas occurring at shallow depth may
also have a deeper source generated from a gas-prone
source rock (coaly sediments) or by cracking of oil,
but such gas has a very different isotopic signature
than biogenic gas.
1.6
Kerogen
As organic material becomes buried by the accumulation of overlying sediments, water is gradually
expelled during compaction.
Complex organic compounds like proteins are broken down into amino acids, and carbohydrates into
simpler sugar compounds. These are able to recombine to make larger compounds, for example by amino
acids reacting with carbohydrates (melanoid reaction).
As this type of polymerisation proceeds, the proportion of simpler soluble organic compounds diminishes
at depths of a few tens of metres down in the sediment.
It is these newly-formed complex organic structures
which are called kerogen.
Kerogen is a collective name for organic material
that is insoluble in organic solvents, water or oxidising
acids. The portion of the organic material soluble in
organic solvents is called bitumen, which is essentially
oil in a solid state.
Kerogen consists of very large molecules and is a
kind of polymer. When it has been exposed to sufficient time and temperature these large molecules will
crack into smaller molecules, mostly petroleum. When
the temperature is about 100
C a long period of geological time is required. In rapidly subsiding basins the
exposure time is shorter and oil generation may only
start at about 140–150
C. In the North Sea basin the
“oil window” may typically be between 130 and
140
C.
1.7
Migration of Petroleum
Petroleum migrates from low permeability source
rocks into high permeability reservoir rocks from
which the petroleum can be produced (Fig. 1.2b).
The main driving force for petroleum migration is
buoyancy because it is less dense than water. The
10
K. Bjørlykke