38 PETROLEUM TECHNOLOGY, ECONOMICS, AND POLITICS
of various kinds; limestone reefs can also serve as reservoir rocks
and give rise to overlying traps of anticlinal form as a result of differential compaction. Lastly, examples are also known in which the
reservoir rock extends to the surface of the earth but oil and gas
are sealed in it by clogging of the pores by bitumen or by natural
cements. Many reservoirs display more than one of the factors contributing to the entrapment of hydrocarbons.
The distinction between a structural trap and a stratigraphie trap
is often difficult to define. 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 have created numerous traps that
are both a structural trap and a stratigraphie trap.
2.2.2 Accumulation in Reservoirs
To all intents and purposes and to the casual observer, a rock is
solid. Yet a rock can possess the fluid holding capacity by virtue
of the uneven contact between the mineral grains and the ensuing
space (porosity) within the rock.
Reservoir rock (usually, but not always, sandstone) 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 of which 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 relative proportion of the different kinds of openings
varies with the rock type, but pores usually account for the bulk of
the storage space. The effective porosity for oil storage results from
continuously connected openings that provide the property of permeability (the capacity of the rock to allow fluids to pass through),
and although a rock must be porous to be permeable, there is no
simple quantitative relationship between the two.
In reservoir rock, porosity is generally in the range of 5 to 30%;
while permeability is commonly between 0.005 Darcy and several
Darcys. It should be noted that pores may be, at best, only a millimeter or so in width, whereas fossil and solution cavities may
sometimes be 30 to 50 times wider. Many joints and fractures are
probably only a millimeter across, although they may extend for
considerable distances.
of various kinds; limestone reefs can also serve as reservoir rocks
and give rise to overlying traps of anticlinal form as a result of differential compaction. Lastly, examples are also known in which the
reservoir rock extends to the surface of the earth but oil and gas
are sealed in it by clogging of the pores by bitumen or by natural
cements. Many reservoirs display more than one of the factors contributing to the entrapment of hydrocarbons.
The distinction between a structural trap and a stratigraphie trap
is often difficult to define. 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 have created numerous traps that
are both a structural trap and a stratigraphie trap.
2.2.2 Accumulation in Reservoirs
To all intents and purposes and to the casual observer, a rock is
solid. Yet a rock can possess the fluid holding capacity by virtue
of the uneven contact between the mineral grains and the ensuing
space (porosity) within the rock.
Reservoir rock (usually, but not always, sandstone) 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 of which 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 relative proportion of the different kinds of openings
varies with the rock type, but pores usually account for the bulk of
the storage space. The effective porosity for oil storage results from
continuously connected openings that provide the property of permeability (the capacity of the rock to allow fluids to pass through),
and although a rock must be porous to be permeable, there is no
simple quantitative relationship between the two.
In reservoir rock, porosity is generally in the range of 5 to 30%;
while permeability is commonly between 0.005 Darcy and several
Darcys. It should be noted that pores may be, at best, only a millimeter or so in width, whereas fossil and solution cavities may
sometimes be 30 to 50 times wider. Many joints and fractures are
probably only a millimeter across, although they may extend for
considerable distances.