Other compounds, such as pentane, are even less soluble (2–3 ppm). However, solubility increases markedly with pressure. Many hydrocarbons have
solubilities of less than 1 ppm in water.
It is not possible that oil should be dissolved in
water and transported in an aqueous solution, both
because of the solubility and the low flow rates. It
would also be difficult to explain how the oil would
come out of solution in the reservoirs (traps). It is
therefore generally accepted that oil migrates as a
separate phase.
Gas, in particular methane, has a fairly high solubility in water, especially under high pressure. If
methane-saturated water rises to lower pressures,
large quantities of methane can bubble out of solution.
Oil is lighter than water, and oil droplets would be
able to move through the pores in reservoir rocks but
the caplliary restance is high for separate oil drops in a
water-wet rock (Fig. 1.5). In order to pass through the
narrow passage between pores (pore throat), the oil
droplets must overcome the capillary forces. When the
pores are sufficiently small in a fine-grained sediment,
these forces will act as a barrier to further migration of
oil. The small gas molecules, however, can diffuse
very slowly through extremely small pores and thus
escape from shales which form tight seals for oil. This
is however a very slow process when the cap rock is
tight and gas (methane) is often trapped as a separate
phase in reservoirs.
Oil can therefore not migrate as small discrete
droplets, but moves as a continuous string of oil
where most of the pores are filled with oil rather than
water (highly oil-saturated). The pressure in the oil
phase at the top is then a function of the height of the
oil-saturated column (string) and the density difference between oil and water.
The rate of migration is a function of the rate of
petroleum generation in the source rocks. The maturation of the source rock controlling the supply of petroleum which can migrate is a function of the
temperature integrated over time (Fig. 1.6).
The temperature history is a function of the burial
depth and the geothermal gradients.
Deep burial over long time will cause all oil to be
decomposed (cracked) into gas. In addition gas will be
generated directly from gas-prone kerogen in the
source rock. There is also usually a large amount of
gas dissolved in the porewater which can be released
when the pressure is reduced.
The degree of alteration of organic matter can be
measured in different ways. Plant material is altered
from a dull material to a material which becomes
more shiny with increasing temperature. This can be
quantified by measuring the amount of light reflected
from a piece of plant material (vitrinite) under the
microscope. This is the vitrinite reflectivity which
increases with higher temperatures and maturity. A
vitrinite reflectivity of 1.2 indicates that the source
rock has generated much of the oil that can be
generated. We will say that the source rock is in the
middle of the “oil window” (Fig. 1.7). Values
below 0.7–0.8. are found in source rocks which
have not been heated enough (immature source
rocks).
Sand grain
Oil flows
through reservoir
rocks
Water flows
through reservoir
rocks
Oil
Oil
H 2 O
Fig. 1.5 Most sandstones are water-wet and have a thin layer of
water around the grains. A continuous oil phase will flow easily
if the permeability is relatively high and the pore throats
between the pores are relatively wide. Isolated droplets of oil
will, however, be prevented from moving by capillary forces
12
K. Bjørlykke
solubilities of less than 1 ppm in water.
It is not possible that oil should be dissolved in
water and transported in an aqueous solution, both
because of the solubility and the low flow rates. It
would also be difficult to explain how the oil would
come out of solution in the reservoirs (traps). It is
therefore generally accepted that oil migrates as a
separate phase.
Gas, in particular methane, has a fairly high solubility in water, especially under high pressure. If
methane-saturated water rises to lower pressures,
large quantities of methane can bubble out of solution.
Oil is lighter than water, and oil droplets would be
able to move through the pores in reservoir rocks but
the caplliary restance is high for separate oil drops in a
water-wet rock (Fig. 1.5). In order to pass through the
narrow passage between pores (pore throat), the oil
droplets must overcome the capillary forces. When the
pores are sufficiently small in a fine-grained sediment,
these forces will act as a barrier to further migration of
oil. The small gas molecules, however, can diffuse
very slowly through extremely small pores and thus
escape from shales which form tight seals for oil. This
is however a very slow process when the cap rock is
tight and gas (methane) is often trapped as a separate
phase in reservoirs.
Oil can therefore not migrate as small discrete
droplets, but moves as a continuous string of oil
where most of the pores are filled with oil rather than
water (highly oil-saturated). The pressure in the oil
phase at the top is then a function of the height of the
oil-saturated column (string) and the density difference between oil and water.
The rate of migration is a function of the rate of
petroleum generation in the source rocks. The maturation of the source rock controlling the supply of petroleum which can migrate is a function of the
temperature integrated over time (Fig. 1.6).
The temperature history is a function of the burial
depth and the geothermal gradients.
Deep burial over long time will cause all oil to be
decomposed (cracked) into gas. In addition gas will be
generated directly from gas-prone kerogen in the
source rock. There is also usually a large amount of
gas dissolved in the porewater which can be released
when the pressure is reduced.
The degree of alteration of organic matter can be
measured in different ways. Plant material is altered
from a dull material to a material which becomes
more shiny with increasing temperature. This can be
quantified by measuring the amount of light reflected
from a piece of plant material (vitrinite) under the
microscope. This is the vitrinite reflectivity which
increases with higher temperatures and maturity. A
vitrinite reflectivity of 1.2 indicates that the source
rock has generated much of the oil that can be
generated. We will say that the source rock is in the
middle of the “oil window” (Fig. 1.7). Values
below 0.7–0.8. are found in source rocks which
have not been heated enough (immature source
rocks).
Sand grain
Oil flows
through reservoir
rocks
Water flows
through reservoir
rocks
Oil
Oil
H 2 O
Fig. 1.5 Most sandstones are water-wet and have a thin layer of
water around the grains. A continuous oil phase will flow easily
if the permeability is relatively high and the pore throats
between the pores are relatively wide. Isolated droplets of oil
will, however, be prevented from moving by capillary forces
12
K. Bjørlykke