80
The Chemistry and Technology of Petroleum
from getting into the system and thereby effectively preserving organic matter. Further burial of the
organic matter occurs due to more deposition and pressures, and temperatures of the layers increase
due to increasing depth and the geothermal gradient 25°C–30°C/km or 15°F/1000 ft resulting in the
conversion or organic matter to petroleum.
Conversion is a breakdown of the organic matter that involves three main stages: diagenesis,
catagenesis, and metagenesis to form kerogen (Chapter 5), which is the high molecular weight
organic residue found in many types of sediment, particularly in oil shale. Migration relates to the
movement of oil or gas from its source rock, where it was formed, to a reservoir rock. Porosity and
permeability of the rocks control the migration of the hydrocarbon. Entrapment—the hydrocarbon
migrates into the formation that is effectively sealed due to a change in the lithology of the formation. For example, shale is commonly porous, but because of its fine grain size, it has very high
capillary forces that prevent fluid flow.
Oil (conventional or heavy) 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 convexly 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.
Many oil and gas accumulations are trapped in anticlines or domes, structures that are generally
more easily detected than some other types of traps.
The most common reservoir rocks are sandstone, limestone, and dolomite. The five basic elements of a reservoir system include (1) source rock—the rock containing the organic material that
is converted into petroleum reservoir fluid, (2) migratory pathway, (3) reservoir rock—rock that
can store and yield the fluid that has sufficient porosity and permeability, (4) a seal—impermeable
cap rock to prevent the upward escape of petroleum to the Earth’s surface, and (5) a trap—physical
arrangement or space that prevents migration of reservoir fluid whereby source, reservoir rock,
basement rock, and seal are arranged to trap petroleum.
The reservoir rock is a porous medium made up of pores conventionally envisaged as irregularly
shaped holes in rock some 1–100 μm in length and diameter, connected to maybe six other pores—
there can be some 10 6 pores in 1 cm 3 of rock and possibly over 10 22 pores in a typical reservoir.
4.2.1 struCturAl tyPes
Reservoirs (i.e., oil traps) are created by structural deformation of the geological strata whereby
these traps are formed by tectonic processes after the deposition of the beds involved. There are
three basic forms of a structural trap in petroleum geology: (1) anticline trap, (2) fault trap, and
(3) salt dome trap. The anticlinal trap is a typical structural trap that is produced by compressional
folding, by uplift, and by drape over older tectonically created features. An anticline is an example
of a rock that was previously flat but has been bent into an arch. The rocks have been folded or
bucked into the form of a dome and hydrocarbons accumulate in the hinge area of an anticline.
Another form of trap is the stratigraphic trap that is formed when other beds seal a reservoir
bed or when the permeability changes (through a change in facies, i.e., a change due to the presence
of rock with characteristics different to those of the reservoir rock or a change in lithology) within
the reservoir bed itself. The distinction between a structural trap and a stratigraphic trap is often
blurred. For example, an anticlinal trap may be related to an underlying buried limestone reef. Beds
of sandstone may wedge out against an anticline because of depositional variations or intermittent
erosion intervals. Salt domes, formed by flow of salt at substantial depths, also create numerous
traps that are both a structural trap and a stratigraphic trap.
In order to become a reservoir, the rock must possess fluid-holding capacity (porosity) and
also fluid-transmitting capacity (permeability); a variety of different types of openings in rocks
are responsible for these properties in reservoir rocks. The most common areas where fluids are
held are the pores between the grains of which the rocks are made or the cavities inside fossils,
openings formed by solution or fractures, and joints that have been created in various ways.
The Chemistry and Technology of Petroleum
from getting into the system and thereby effectively preserving organic matter. Further burial of the
organic matter occurs due to more deposition and pressures, and temperatures of the layers increase
due to increasing depth and the geothermal gradient 25°C–30°C/km or 15°F/1000 ft resulting in the
conversion or organic matter to petroleum.
Conversion is a breakdown of the organic matter that involves three main stages: diagenesis,
catagenesis, and metagenesis to form kerogen (Chapter 5), which is the high molecular weight
organic residue found in many types of sediment, particularly in oil shale. Migration relates to the
movement of oil or gas from its source rock, where it was formed, to a reservoir rock. Porosity and
permeability of the rocks control the migration of the hydrocarbon. Entrapment—the hydrocarbon
migrates into the formation that is effectively sealed due to a change in the lithology of the formation. For example, shale is commonly porous, but because of its fine grain size, it has very high
capillary forces that prevent fluid flow.
Oil (conventional or heavy) 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 convexly 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.
Many oil and gas accumulations are trapped in anticlines or domes, structures that are generally
more easily detected than some other types of traps.
The most common reservoir rocks are sandstone, limestone, and dolomite. The five basic elements of a reservoir system include (1) source rock—the rock containing the organic material that
is converted into petroleum reservoir fluid, (2) migratory pathway, (3) reservoir rock—rock that
can store and yield the fluid that has sufficient porosity and permeability, (4) a seal—impermeable
cap rock to prevent the upward escape of petroleum to the Earth’s surface, and (5) a trap—physical
arrangement or space that prevents migration of reservoir fluid whereby source, reservoir rock,
basement rock, and seal are arranged to trap petroleum.
The reservoir rock is a porous medium made up of pores conventionally envisaged as irregularly
shaped holes in rock some 1–100 μm in length and diameter, connected to maybe six other pores—
there can be some 10 6 pores in 1 cm 3 of rock and possibly over 10 22 pores in a typical reservoir.
4.2.1 struCturAl tyPes
Reservoirs (i.e., oil traps) are created by structural deformation of the geological strata whereby
these traps are formed by tectonic processes after the deposition of the beds involved. There are
three basic forms of a structural trap in petroleum geology: (1) anticline trap, (2) fault trap, and
(3) salt dome trap. The anticlinal trap is a typical structural trap that is produced by compressional
folding, by uplift, and by drape over older tectonically created features. An anticline is an example
of a rock that was previously flat but has been bent into an arch. The rocks have been folded or
bucked into the form of a dome and hydrocarbons accumulate in the hinge area of an anticline.
Another form of trap is the stratigraphic trap that is formed when other beds seal a reservoir
bed or when the permeability changes (through a change in facies, i.e., a change due to the presence
of rock with characteristics different to those of the reservoir rock or a change in lithology) within
the reservoir bed itself. The distinction between a structural trap and a stratigraphic trap is often
blurred. For example, an anticlinal trap may be related to an underlying buried limestone reef. Beds
of sandstone may wedge out against an anticline because of depositional variations or intermittent
erosion intervals. Salt domes, formed by flow of salt at substantial depths, also create numerous
traps that are both a structural trap and a stratigraphic trap.
In order to become a reservoir, the rock must possess fluid-holding capacity (porosity) and
also fluid-transmitting capacity (permeability); a variety of different types of openings in rocks
are responsible for these properties in reservoir rocks. The most common areas where fluids are
held are the pores between the grains of which the rocks are made or the cavities inside fossils,
openings formed by solution or fractures, and joints that have been created in various ways.