just 1 cm
3 /s, equivalent to a tiny trickle out of a garden
hose. Thicker oil columns will probably build up
where the migration has to pass through less permeable sandstones and siltstones which require higher
capillary pressures.
The rate of migration is limited by the petroleum
generation in the source rocks in the drainage area.
The oil and gas can not flow faster than it is generated.
If the migration is slower than the generation, a trap
has formed somewhere along the migration pathway.
It the rate of oil generation increases, both the crosssection of oil-saturated rocks along the pathway will
increase and a thicker oil column produce higher oil
saturation and relative permeability. These are positive feedbacks that will make it possible for the migration rate to increase in order to adjust to the rate of
generation. Low permeability shales along the pathway will either fracture due to the high pressure or
form a cap rock. A trap which does not have pressure
communication with any higher structures will easily
reach fracture pressure and leak. This will not apply to
a lower trap if there is continuous permeable sandstone
up to a higher trap; the lower trap will not then reach
fracture pressure and leak (Fig. 15.4).
Migration of petroleum is very much influenced by
the composition of the petroleum, particularly the
density, viscosity and the gas/oil ratio. This depends
on the primary composition of the organic matter at
the time of deposition (organic facies). Starting with
different organofacies the composition of the oil and
gas can be modelled as a function of different PVT
conditions (di Primeo and Horsefield 2006). This may
explain changes in petroleum composition during
migration, trapping and remigration.
15.8 Regional Migration
In a basin with well-defined shales serving as cap
rocks the sandstones represent the migration routes.
In the North Sea basin, Cretaceous shales are
important cap rocks and the base of these Cretaceous
shales can be mapped out in great detail using 3D
seismics. The depth contours make the traps stand
out like mountains or hills on a topographic map.
Using that analogy, the migration of oil follows the
highest ridges and fills the structures down to the
lowest contour connecting it to the next mountain,
which is the spill point. The closure of the structure,
which defines the maximum thickness of the oil and
gas column, is represented by the vertical distance
from the valley up to the top of the mountain.
In an overpressured part of a sedimentary basin the
highest structures within one pressure compartment
are likely to leak. Deeper structures in the same compartment may have a high chance of retaining oil
because they will not reach fracture pressure and
leak (Fig. 15.4).
The evaluation of a trap depends very much on the
contours on this map, which are a function of the
accuracy of the depth conversion of the seismic data.
Faults offsetting the carrier bed and the shale seal
may obstruct migration but represent at the same time
a possible trapping mechanism.
Much of what has been said about oil migration
holds true also for gas migration. Gas molecules, particularly methane, are very small and may diffuse
through shales and mudrocks in a way which is not
possible for oil. The migration of gas above structures
is now easily seen on seismic sections, showing that
many gas fields do leak but are filled at a rate which
compensates for the loss.
15.9 Loss of Petroleum During
Migration
If we know the volume of source rocks in the drainage
area and how much each volume can yield, the total
volume of petroleum generated can be calculated.
Assuming a certain expulsion efficiency, the maximum amount of petroleum which could migrate into
a structure can be calculated. This has been done for
different sub-basins in the North Sea (Geoff 1983).
The percentage of oil and gas which was generated
but is not found in the major oil fields, could have been
lost through leakage from reservoirs or have
accumulated in smaller traps that have not been discovered. Some oil and gas is probably also left behind
along the migration route.
The properties of the sandstones which serve as
migration routes depend on the depositional environment and subsequent diagenesis. However, shallow
marine sandstones are normally coarsening-upwards
and have the cleanest and most permeable sand at the
top of the sequence. There will then often be a sharp
contact to the overlying shale and migration will be
very efficient with relatively little loss. In fining-up
382
K. Bjørlykke
3 /s, equivalent to a tiny trickle out of a garden
hose. Thicker oil columns will probably build up
where the migration has to pass through less permeable sandstones and siltstones which require higher
capillary pressures.
The rate of migration is limited by the petroleum
generation in the source rocks in the drainage area.
The oil and gas can not flow faster than it is generated.
If the migration is slower than the generation, a trap
has formed somewhere along the migration pathway.
It the rate of oil generation increases, both the crosssection of oil-saturated rocks along the pathway will
increase and a thicker oil column produce higher oil
saturation and relative permeability. These are positive feedbacks that will make it possible for the migration rate to increase in order to adjust to the rate of
generation. Low permeability shales along the pathway will either fracture due to the high pressure or
form a cap rock. A trap which does not have pressure
communication with any higher structures will easily
reach fracture pressure and leak. This will not apply to
a lower trap if there is continuous permeable sandstone
up to a higher trap; the lower trap will not then reach
fracture pressure and leak (Fig. 15.4).
Migration of petroleum is very much influenced by
the composition of the petroleum, particularly the
density, viscosity and the gas/oil ratio. This depends
on the primary composition of the organic matter at
the time of deposition (organic facies). Starting with
different organofacies the composition of the oil and
gas can be modelled as a function of different PVT
conditions (di Primeo and Horsefield 2006). This may
explain changes in petroleum composition during
migration, trapping and remigration.
15.8 Regional Migration
In a basin with well-defined shales serving as cap
rocks the sandstones represent the migration routes.
In the North Sea basin, Cretaceous shales are
important cap rocks and the base of these Cretaceous
shales can be mapped out in great detail using 3D
seismics. The depth contours make the traps stand
out like mountains or hills on a topographic map.
Using that analogy, the migration of oil follows the
highest ridges and fills the structures down to the
lowest contour connecting it to the next mountain,
which is the spill point. The closure of the structure,
which defines the maximum thickness of the oil and
gas column, is represented by the vertical distance
from the valley up to the top of the mountain.
In an overpressured part of a sedimentary basin the
highest structures within one pressure compartment
are likely to leak. Deeper structures in the same compartment may have a high chance of retaining oil
because they will not reach fracture pressure and
leak (Fig. 15.4).
The evaluation of a trap depends very much on the
contours on this map, which are a function of the
accuracy of the depth conversion of the seismic data.
Faults offsetting the carrier bed and the shale seal
may obstruct migration but represent at the same time
a possible trapping mechanism.
Much of what has been said about oil migration
holds true also for gas migration. Gas molecules, particularly methane, are very small and may diffuse
through shales and mudrocks in a way which is not
possible for oil. The migration of gas above structures
is now easily seen on seismic sections, showing that
many gas fields do leak but are filled at a rate which
compensates for the loss.
15.9 Loss of Petroleum During
Migration
If we know the volume of source rocks in the drainage
area and how much each volume can yield, the total
volume of petroleum generated can be calculated.
Assuming a certain expulsion efficiency, the maximum amount of petroleum which could migrate into
a structure can be calculated. This has been done for
different sub-basins in the North Sea (Geoff 1983).
The percentage of oil and gas which was generated
but is not found in the major oil fields, could have been
lost through leakage from reservoirs or have
accumulated in smaller traps that have not been discovered. Some oil and gas is probably also left behind
along the migration route.
The properties of the sandstones which serve as
migration routes depend on the depositional environment and subsequent diagenesis. However, shallow
marine sandstones are normally coarsening-upwards
and have the cleanest and most permeable sand at the
top of the sequence. There will then often be a sharp
contact to the overlying shale and migration will be
very efficient with relatively little loss. In fining-up
382
K. Bjørlykke