60
The Chemistry and Technology of Petroleum
and even though the porous zones adjacent to a compacting mud are commonly sand (sands form
important reservoir rocks), almost any porous and permeable stratum will suffice.
Once the oil has accumulated in the reservoir rock, gravitational forces are presumed to be dominant, thereby causing the oil, gas, 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 (the gas cap), 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 have a far lower permeability than the reservoir rock
being impermeable 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 and
reservoir rocks have much lower capillary pressures. 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.
The distribution of the fluids in a reservoir rock is dependent on the densities of the fluids as
well as on the properties of the rock. If the pores are of uniform size and evenly distributed, there is
1. An upper zone where the pores are filled mainly by gas (the gas cap)
2. A middle zone in which the pores are occupied principally by oil with gas in solution
3. A lower zone with its pores filled by water
Such accumulations usually occur at folds in the Earth’s strata (anticlines), which may be several
kilometers in length. A certain amount of water (approximately 10%–30%) occurs along with the
oil in the middle zone. There is a transition zone from the pores occupied entirely by water to pores
occupied mainly by oil in the reservoir rock, and the thickness of this zone depends on the densities and interfacial tension of the oil and water as well as on the sizes of the pores. Similarly, there
is some water in the pores in the upper gas zone that has at its base a transition zone from pores
occupied largely by gas to pores filled mainly by oil.
The water found in the oil and gas zones is known generally as interstitial water. It usually occurs
as collars around grain contacts, as a filling of pores with unusually small throats connecting with
adjacent pores, or, to a much smaller extent, as wetting films on the surface of the mineral grains
when the rock is preferentially wet by water. The water may occur as wetting films, or collars,
around the sand grains as well as in some completely filled pores. The three-dimensional network
allows continuity to exist for the hydrocarbons by means of connections on every side of the sand
grains. The so-called gas–oil and oil–water contacts are generally horizontal but have been known
to exist as a very gentle incline. On occasion, part of an accumulation of the oil or gas has its lower
boundary marked, not by the water-bearing zone of the reservoir rock but by an adjacent sealing
rock that has characteristics similar to those of the cap rock. When the pressure and temperature
conditions are suitable in relation to the proportions and the nature of the gas and oil, there may be
no gas cap but only oil, with dissolved gas overlying the water.
Oil and gas cannot be retained as an accumulation unless there is a trap, and this requires
that the boundary between the cap rock or other sealing agent and the reservoir rock generally be
convex upward, but the exact form of the boundary varies widely. The simplest forms are the flat-lying
convex lens, the anticline, and the dome, each of which has a convex upper surface (Figure 3.2).
The Chemistry and Technology of Petroleum
and even though the porous zones adjacent to a compacting mud are commonly sand (sands form
important reservoir rocks), almost any porous and permeable stratum will suffice.
Once the oil has accumulated in the reservoir rock, gravitational forces are presumed to be dominant, thereby causing the oil, gas, 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 (the gas cap), 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 have a far lower permeability than the reservoir rock
being impermeable 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 and
reservoir rocks have much lower capillary pressures. 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.
The distribution of the fluids in a reservoir rock is dependent on the densities of the fluids as
well as on the properties of the rock. If the pores are of uniform size and evenly distributed, there is
1. An upper zone where the pores are filled mainly by gas (the gas cap)
2. A middle zone in which the pores are occupied principally by oil with gas in solution
3. A lower zone with its pores filled by water
Such accumulations usually occur at folds in the Earth’s strata (anticlines), which may be several
kilometers in length. A certain amount of water (approximately 10%–30%) occurs along with the
oil in the middle zone. There is a transition zone from the pores occupied entirely by water to pores
occupied mainly by oil in the reservoir rock, and the thickness of this zone depends on the densities and interfacial tension of the oil and water as well as on the sizes of the pores. Similarly, there
is some water in the pores in the upper gas zone that has at its base a transition zone from pores
occupied largely by gas to pores filled mainly by oil.
The water found in the oil and gas zones is known generally as interstitial water. It usually occurs
as collars around grain contacts, as a filling of pores with unusually small throats connecting with
adjacent pores, or, to a much smaller extent, as wetting films on the surface of the mineral grains
when the rock is preferentially wet by water. The water may occur as wetting films, or collars,
around the sand grains as well as in some completely filled pores. The three-dimensional network
allows continuity to exist for the hydrocarbons by means of connections on every side of the sand
grains. The so-called gas–oil and oil–water contacts are generally horizontal but have been known
to exist as a very gentle incline. On occasion, part of an accumulation of the oil or gas has its lower
boundary marked, not by the water-bearing zone of the reservoir rock but by an adjacent sealing
rock that has characteristics similar to those of the cap rock. When the pressure and temperature
conditions are suitable in relation to the proportions and the nature of the gas and oil, there may be
no gas cap but only oil, with dissolved gas overlying the water.
Oil and gas cannot be retained as an accumulation unless there is a trap, and this requires
that the boundary between the cap rock or other sealing agent and the reservoir rock generally be
convex upward, but the exact form of the boundary varies widely. The simplest forms are the flat-lying
convex lens, the anticline, and the dome, each of which has a convex upper surface (Figure 3.2).