sequences, commonly found in fluvial sandstones and
turbidites, the buoyancy will force oil and gas up into
the finer-grained part of the sandstones where much of
it may be trapped in small pores.
It has been claimed that organic acids generated in
the source rocks could dissolve feldspar and mica in
the reservoir sandstones. Many of these acids are
rather water-soluble and will diffuse into the water
phase in the source rock, which often contains carbonate, and during migration would be neutralised before
reaching the reservoirs.
The sandstones adjacent to or interbedded with the
main source rock in the North Sea basin (Kimmeridge
shale) show little or no evidence of dissolution of
feldspar and precipitation of authigenic kaolinite.
This suggests that the generation of organic acids in
the source rocks has an insignificant effect on the
development of secondary porosity (Bjørlykke and
Aagaard 1992).
Compared to the total buffering system of, firstly,
the silicate mineral system and, secondly, the carbonate system, the addition of relatively small amounts of
comparatively weak acids (organic acids) will not
change the pH of the porewater significantly.
15.10 Petroleum Seepage and
Exploration
Oil exploration started by drilling where there already
was evidence of seepage. This was also the case in Oil
Creek in Pennsylvania, and in California.
When the reservoir is buried more deeply the
amount of leakage is highly variable and the migration
pathway to the surface may be very complex. The idea
that the presence of petroleum should be detected
directly has always seemed attractive and geochemical
surveys to detect oil have been carried out on a large
scale both on land and on the seafloor.
A leakage of petroleum may be a good sign because
it shows that some petroleum is present. On the other
hand the fact leakage occurs indicates that large
fractions of the petroleum generated may have been
lost to the surface. Many large oilfields have no visible
seeps directly above the structures (Thrasher et al.
1996).
A study of the Southeast Asian Basin showed that
leakage was concentrated over tectonic structures such
as faults and diapirs.
The probability of detecting petroleum on the land
surface or on the seabed is highest in areas of active
migration. Active seeps with gas may also be detected
by side-scan sonar (Hovland et al. 2002). Good
examples of active seeps are found in many basins in
California, the Gulf of Mexico, North Sea and
Indonesia (Abrams 1996).
Passive seeps where there is at present no active
seepage are more difficult to detect including by
refined geochemical methods.
In overpressured reservoirs the migration is likely
to be vertical due to hydrofracturing. However once
the petroleum is out of the overpressured compartment
further migration will tend to occur laterally, often
along gently dipping sandstones in a shaly sequence.
This is well illustrated from Haltenbanken, offshore
Norway, where petroleum is mostly leaking vertically
from overpressured Middle to Lower Jurassic
reservoirs (Karlsen et al. 1995) and migrating up-dip
through Palaeocene and Eocene sands up to the seafloor near the coast (Bugge et al. 1984, Thrasher et al.
1996).
Where there is oil migration into reservoirs that had
been buried to depths where the temperature has
exceeded 80
C the reservoir is pasteurised so there is
little biodegradation and oil can than flow to shallow
depth. If the reservoir has not been heated to this
temperature there is bacteria present in the reservoir
and oil may be biodegraded to several hundred metres
depth. (See Chaps. 1 and 21).
15.11 Summary
Primary migration of petroleum from a source rock to
the reservoir rock may occur through the rock matrix if
it contains sediments with sufficiently high permeability and low capillary entry pressure. If petroleum can
not flow through rock matrix the generation of petroleum will contribute to the build-up of the pore pressure until fracturing occurs.
Secondary migration has an upward component and
is driven by the buoyancy of the petroleum phases in
the porewater, and is resisted by the capillary forces.
The rate of migration is a function of the rate of
generation and primary expulsion. The rate of migration is therefore very low and the migration pathway
may have a very small cross-section, but with a high
degree of petroleum saturation. A high petroleum
15 Petroleum Migration
383
turbidites, the buoyancy will force oil and gas up into
the finer-grained part of the sandstones where much of
it may be trapped in small pores.
It has been claimed that organic acids generated in
the source rocks could dissolve feldspar and mica in
the reservoir sandstones. Many of these acids are
rather water-soluble and will diffuse into the water
phase in the source rock, which often contains carbonate, and during migration would be neutralised before
reaching the reservoirs.
The sandstones adjacent to or interbedded with the
main source rock in the North Sea basin (Kimmeridge
shale) show little or no evidence of dissolution of
feldspar and precipitation of authigenic kaolinite.
This suggests that the generation of organic acids in
the source rocks has an insignificant effect on the
development of secondary porosity (Bjørlykke and
Aagaard 1992).
Compared to the total buffering system of, firstly,
the silicate mineral system and, secondly, the carbonate system, the addition of relatively small amounts of
comparatively weak acids (organic acids) will not
change the pH of the porewater significantly.
15.10 Petroleum Seepage and
Exploration
Oil exploration started by drilling where there already
was evidence of seepage. This was also the case in Oil
Creek in Pennsylvania, and in California.
When the reservoir is buried more deeply the
amount of leakage is highly variable and the migration
pathway to the surface may be very complex. The idea
that the presence of petroleum should be detected
directly has always seemed attractive and geochemical
surveys to detect oil have been carried out on a large
scale both on land and on the seafloor.
A leakage of petroleum may be a good sign because
it shows that some petroleum is present. On the other
hand the fact leakage occurs indicates that large
fractions of the petroleum generated may have been
lost to the surface. Many large oilfields have no visible
seeps directly above the structures (Thrasher et al.
1996).
A study of the Southeast Asian Basin showed that
leakage was concentrated over tectonic structures such
as faults and diapirs.
The probability of detecting petroleum on the land
surface or on the seabed is highest in areas of active
migration. Active seeps with gas may also be detected
by side-scan sonar (Hovland et al. 2002). Good
examples of active seeps are found in many basins in
California, the Gulf of Mexico, North Sea and
Indonesia (Abrams 1996).
Passive seeps where there is at present no active
seepage are more difficult to detect including by
refined geochemical methods.
In overpressured reservoirs the migration is likely
to be vertical due to hydrofracturing. However once
the petroleum is out of the overpressured compartment
further migration will tend to occur laterally, often
along gently dipping sandstones in a shaly sequence.
This is well illustrated from Haltenbanken, offshore
Norway, where petroleum is mostly leaking vertically
from overpressured Middle to Lower Jurassic
reservoirs (Karlsen et al. 1995) and migrating up-dip
through Palaeocene and Eocene sands up to the seafloor near the coast (Bugge et al. 1984, Thrasher et al.
1996).
Where there is oil migration into reservoirs that had
been buried to depths where the temperature has
exceeded 80
C the reservoir is pasteurised so there is
little biodegradation and oil can than flow to shallow
depth. If the reservoir has not been heated to this
temperature there is bacteria present in the reservoir
and oil may be biodegraded to several hundred metres
depth. (See Chaps. 1 and 21).
15.11 Summary
Primary migration of petroleum from a source rock to
the reservoir rock may occur through the rock matrix if
it contains sediments with sufficiently high permeability and low capillary entry pressure. If petroleum can
not flow through rock matrix the generation of petroleum will contribute to the build-up of the pore pressure until fracturing occurs.
Secondary migration has an upward component and
is driven by the buoyancy of the petroleum phases in
the porewater, and is resisted by the capillary forces.
The rate of migration is a function of the rate of
generation and primary expulsion. The rate of migration is therefore very low and the migration pathway
may have a very small cross-section, but with a high
degree of petroleum saturation. A high petroleum
15 Petroleum Migration
383