Chapter 15
Petroleum Migration
Knut Bjørlykke
The transport of petroleum from the source rock to the
reservoir rocks is called migration which occurs along
permeable carrier beds. It is important to understand
this process so that the direction of migration and
trapping of petroleum can be predicted. Many different theories have been proposed in the past but it is
now clear that petroleum is mainly transported as a
separated phase and that the process is mainly driven
by the buoyancy of petroleum relative to water. The
porewater is normally not moving much in relation to
the rocks. The upwards flow of the porewater is
constrained by the rate of compaction and it is
moved downwards by the basin subsidence during
sedimentation. The solubility of oil in water is very
low for most compounds. The solubility of gas, particularly methane, is much higher both in oil and water
and increases with depth (pressure). There is, however, very limited flow in sedimentary basins to transport petroleum.
Considerable amounts of gas can bubble out of
water or oil if the pressure is reduced due to uplift or
due to pressure reduction in a reservoir during
production.
15.1 Primary Migration
The expulsion of petroleum from a source rock into
adjacent rocks is called primary migration while the
migration further along carrier beds are referred to a
secondary migration.
Kerogen is a solid compound of very large
molecules (polymers) formed from organic matter
and may occur as dispersed particles in the sediments
or as laminae in a claystone. Kerogen may be loadbearing and capable of transmitting stress before it
generates petroleum. As the kerogen matures, much
of this solid matter breaks down to generate oil or gas
and is thus transformed into fluid phases. If the fluids
are not expelled immediately, this process increases
the volume of the fluid phase (porosity) compared to
the original volume of the solid phase in the source
rock.
The ratio between the volume of the fluid phase
(porosity) and the solid phase is often referred to as the
void ratio: V r ¼ φ= 1 À φ
ð
Þ.
Not all kerogen is transformed to fluids during
maturation. There is a residue of solids which is called
coke.
It has usually been assumed that there is a volume
expansion during maturation of kerogen because the
density of the oil and gas and the remaining solids in
the kerogen may be smaller than the density of the
primary kerogen, thus causing a volume expansion.
This expansion may not necessarily be very large in
the case of oil generation. Even if there was no overall
volume expansion, the generation of oil would contribute to the overpressure build-up, since the main
factor is the change in void ratio when solid kerogen
is altered to fluid petroleum. To illustrate this point we
may make the analogy of looking at frozen ground
with lenses of ice formed during the winter. Ice is a
solid that can carry the weight of the overburden, but
when it melts in the spring the ice becomes fluid and a
part of the porosity unless it is expelled. Overpressure
develops and sometimes small mud volcanoes may
K. Bjørlykke (*)
Department of Geosciences, University of Oslo, Oslo, Norway
e-mail: knut.bjorlykke@geo.uio.no
K. Bjørlykke (ed.), Petroleum Geoscience: From Sedimentary Environments to Rock Physics,
DOI 10.1007/978-3-642-34132-8_15, # Springer-Verlag Berlin Heidelberg 2015
373
Petroleum Migration
Knut Bjørlykke
The transport of petroleum from the source rock to the
reservoir rocks is called migration which occurs along
permeable carrier beds. It is important to understand
this process so that the direction of migration and
trapping of petroleum can be predicted. Many different theories have been proposed in the past but it is
now clear that petroleum is mainly transported as a
separated phase and that the process is mainly driven
by the buoyancy of petroleum relative to water. The
porewater is normally not moving much in relation to
the rocks. The upwards flow of the porewater is
constrained by the rate of compaction and it is
moved downwards by the basin subsidence during
sedimentation. The solubility of oil in water is very
low for most compounds. The solubility of gas, particularly methane, is much higher both in oil and water
and increases with depth (pressure). There is, however, very limited flow in sedimentary basins to transport petroleum.
Considerable amounts of gas can bubble out of
water or oil if the pressure is reduced due to uplift or
due to pressure reduction in a reservoir during
production.
15.1 Primary Migration
The expulsion of petroleum from a source rock into
adjacent rocks is called primary migration while the
migration further along carrier beds are referred to a
secondary migration.
Kerogen is a solid compound of very large
molecules (polymers) formed from organic matter
and may occur as dispersed particles in the sediments
or as laminae in a claystone. Kerogen may be loadbearing and capable of transmitting stress before it
generates petroleum. As the kerogen matures, much
of this solid matter breaks down to generate oil or gas
and is thus transformed into fluid phases. If the fluids
are not expelled immediately, this process increases
the volume of the fluid phase (porosity) compared to
the original volume of the solid phase in the source
rock.
The ratio between the volume of the fluid phase
(porosity) and the solid phase is often referred to as the
void ratio: V r ¼ φ= 1 À φ
ð
Þ.
Not all kerogen is transformed to fluids during
maturation. There is a residue of solids which is called
coke.
It has usually been assumed that there is a volume
expansion during maturation of kerogen because the
density of the oil and gas and the remaining solids in
the kerogen may be smaller than the density of the
primary kerogen, thus causing a volume expansion.
This expansion may not necessarily be very large in
the case of oil generation. Even if there was no overall
volume expansion, the generation of oil would contribute to the overpressure build-up, since the main
factor is the change in void ratio when solid kerogen
is altered to fluid petroleum. To illustrate this point we
may make the analogy of looking at frozen ground
with lenses of ice formed during the winter. Ice is a
solid that can carry the weight of the overburden, but
when it melts in the spring the ice becomes fluid and a
part of the porosity unless it is expelled. Overpressure
develops and sometimes small mud volcanoes may
K. Bjørlykke (*)
Department of Geosciences, University of Oslo, Oslo, Norway
e-mail: knut.bjorlykke@geo.uio.no
K. Bjørlykke (ed.), Petroleum Geoscience: From Sedimentary Environments to Rock Physics,
DOI 10.1007/978-3-642-34132-8_15, # Springer-Verlag Berlin Heidelberg 2015
373