42 PETROLEUM TECHNOLOGY, ECONOMICS, AND POLITICS
and water to segregate according to their relative densities in the
upper parts of the reservoir (Landes, 1959). If the pores in the reservoir rock are of uniform size and evenly distributed, there are
transition zones, from the pores occupied entirely by water to pores
occupied mainly by oil to those pores occupied mainly by gas. The
thickness of the water-oil transition zone depends on the densities
and interfacial tension of the oil and water as well as on the size of
the pores. Similarly, there is some water in the pores in the upper
gas zone, which has at its base a transition zone from pores occupied largely by gas to pores filled mainly by oil.
The cap rock and basement rock, which are generally impermeable (or have a much lower permeability than the reservoir rock to
oil and gas), act as a seal to prevent the escape of oil and gas from
the reservoir rock. Typical cap and basement rocks are clay and
shale, that is, strata in which the pores are much finer than those of
reservoir rocks. Other rocks, such as marl and dense limestone, can
also serve as cap and basement rocks provided that any pores are
very small. There are cases in which evaporites (salt, anhydrite, and
gypsum) act as effective sealants. The cap rock has a far lower permeability than the reservoir rock, but it is equally true that cap rocks
have very high capillary pressures while those of reservoir rocks
are much lower. The capillary pressure is the pressure required to
cause a fluid to displace from the openings in a rock by another
fluid with which it is not miscible. Capillary pressure is dependent
on the size of the openings, the interfacial tension between the two
fluids, and the contact angle for the system.
2.2.5 Transformation of Petroleum in the Reservoir
Petroleum is susceptible to alteration even after it has collected in
a reservoir or in sediments (Evans et al, 1971). The alteration process is ongoing and such alteration can change the quality of the oil
during the life of the reservoir, thereby affecting the price of the oil.
Therefore, it is important to briefly address the issue of the alteration of petroleum once it has accumulated in the reservoir.
Alteration of reservoir petroleum is accepted for most of the world
oil accumulations and may be related to the relative instability of
petroleum, or the traps may be susceptible to incursion to chemical agents, such as oxygen. Physical effects, such as those caused
when the level of burial of the trap changes as a result of further
subsidence or erosion, may also play a role. Examples of chemical
and water to segregate according to their relative densities in the
upper parts of the reservoir (Landes, 1959). If the pores in the reservoir rock are of uniform size and evenly distributed, there are
transition zones, from the pores occupied entirely by water to pores
occupied mainly by oil to those pores occupied mainly by gas. The
thickness of the water-oil transition zone depends on the densities
and interfacial tension of the oil and water as well as on the size of
the pores. Similarly, there is some water in the pores in the upper
gas zone, which has at its base a transition zone from pores occupied largely by gas to pores filled mainly by oil.
The cap rock and basement rock, which are generally impermeable (or have a much lower permeability than the reservoir rock to
oil and gas), act as a seal to prevent the escape of oil and gas from
the reservoir rock. Typical cap and basement rocks are clay and
shale, that is, strata in which the pores are much finer than those of
reservoir rocks. Other rocks, such as marl and dense limestone, can
also serve as cap and basement rocks provided that any pores are
very small. There are cases in which evaporites (salt, anhydrite, and
gypsum) act as effective sealants. The cap rock has a far lower permeability than the reservoir rock, but it is equally true that cap rocks
have very high capillary pressures while those of reservoir rocks
are much lower. The capillary pressure is the pressure required to
cause a fluid to displace from the openings in a rock by another
fluid with which it is not miscible. Capillary pressure is dependent
on the size of the openings, the interfacial tension between the two
fluids, and the contact angle for the system.
2.2.5 Transformation of Petroleum in the Reservoir
Petroleum is susceptible to alteration even after it has collected in
a reservoir or in sediments (Evans et al, 1971). The alteration process is ongoing and such alteration can change the quality of the oil
during the life of the reservoir, thereby affecting the price of the oil.
Therefore, it is important to briefly address the issue of the alteration of petroleum once it has accumulated in the reservoir.
Alteration of reservoir petroleum is accepted for most of the world
oil accumulations and may be related to the relative instability of
petroleum, or the traps may be susceptible to incursion to chemical agents, such as oxygen. Physical effects, such as those caused
when the level of burial of the trap changes as a result of further
subsidence or erosion, may also play a role. Examples of chemical