57
Origin and Occurrence
spends a long time in the oxic zone where aerobic bacteria are active. Such environments occur even
at great ocean depths with circulation, as well as in shallow, circulating seas with a high energy
input. Low organic contents in the rocks reflect efficient degradation even where the initial organic
productivity was high.
3.2.2.5 Accumulation in Reservoir Sediments
At an early date it was recognized that hydrocarbon accumulations require the existence of a reservoir rock to store the fluids and a cap to prevent their escape. In addition to these readily appreciated
requirements, there must also be a source rock in which the oil and gas are formed.
Reservoir 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 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. These openings alone provide the
property of permeability, and although a rock must be porous to be permeable, there is no simple
quantitative relationship between the two.
Reservoir rocks tend to show far greater variations in permeability than in porosity, and in addition, these two properties, as measured on core samples from reservoir rocks, are not always identical with the values indicated for the rock in bulk underground. The differences arise from the
nonrepresentative nature of cores, especially when there are wide variations in the sizes of the
openings in the rocks and irregularities in their distribution. Porosity is generally in the range
of 5%–30%; while permeability is commonly between 0.005 D and several Darcys as measured
on small samples. 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 30–50 times wider. Many joints and fractures are probably only a millimeter across, although they may extend for considerable distances.
The processes by which petroleum migrates into pools are not well defined. However, it is known
that crude oil and gas occur in pools or fields in which the oil or gas occupies pore space in the rock.
Some pools are large, extending laterally over many square kilometers, with their vertical extent
ranging from several meters to as much as several hundred meters. The crude oil or gas occupying
the pore space in the pool has displaced the water that was initially present in the pores. The presence of a pool of oil or gas therefore implies that the oil or gas has migrated into the pool.
Petroleum accumulations are generally found in relatively coarse-grained porous and permeable
rocks that contain little or no insoluble organic matter. It is highly unlikely that petroleum originated
from organic matter of which now no trace remains. Rather, it appears that petroleum constituents
are generated through geochemical action on organic detritus that is usually found in fine-grained
sedimentary rocks. In addition, it may be anticipated that some of the organic material would also
change sufficiently from the original organic material to remain in the sediment through, and
beyond, the oil-generating stage. Thus it is generally assumed that the place of origin of oil and gas
is not identical to the locations where it is found. The oil has had to migrate to present reservoirs
from the place of origin.
When sediments such as mud and clay are deposited, they contain water. As the layers of sediment accumulate, the increasing load of material causes compaction of the sediment and part of
the water is expelled from the pores. Migration probably begins at this stage and may involve a
substantial horizontal component. Initially, the oil hydrocarbons may be present in the water either
suspended as tiny globules or dissolved, hydrocarbons being slightly soluble in water.
Petroleum constituents may migrate through one or more formations that have permeability and
porosity similar to those of the ultimate reservoir rock. It is the occurrence of an impermeable, or a
very low permeability, barrier that causes the formation of oil and gas accumulations. Since practically all pores in the subsurface are water saturated, the movement of petroleum constituents within
Origin and Occurrence
spends a long time in the oxic zone where aerobic bacteria are active. Such environments occur even
at great ocean depths with circulation, as well as in shallow, circulating seas with a high energy
input. Low organic contents in the rocks reflect efficient degradation even where the initial organic
productivity was high.
3.2.2.5 Accumulation in Reservoir Sediments
At an early date it was recognized that hydrocarbon accumulations require the existence of a reservoir rock to store the fluids and a cap to prevent their escape. In addition to these readily appreciated
requirements, there must also be a source rock in which the oil and gas are formed.
Reservoir 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 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. These openings alone provide the
property of permeability, and although a rock must be porous to be permeable, there is no simple
quantitative relationship between the two.
Reservoir rocks tend to show far greater variations in permeability than in porosity, and in addition, these two properties, as measured on core samples from reservoir rocks, are not always identical with the values indicated for the rock in bulk underground. The differences arise from the
nonrepresentative nature of cores, especially when there are wide variations in the sizes of the
openings in the rocks and irregularities in their distribution. Porosity is generally in the range
of 5%–30%; while permeability is commonly between 0.005 D and several Darcys as measured
on small samples. 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 30–50 times wider. Many joints and fractures are probably only a millimeter across, although they may extend for considerable distances.
The processes by which petroleum migrates into pools are not well defined. However, it is known
that crude oil and gas occur in pools or fields in which the oil or gas occupies pore space in the rock.
Some pools are large, extending laterally over many square kilometers, with their vertical extent
ranging from several meters to as much as several hundred meters. The crude oil or gas occupying
the pore space in the pool has displaced the water that was initially present in the pores. The presence of a pool of oil or gas therefore implies that the oil or gas has migrated into the pool.
Petroleum accumulations are generally found in relatively coarse-grained porous and permeable
rocks that contain little or no insoluble organic matter. It is highly unlikely that petroleum originated
from organic matter of which now no trace remains. Rather, it appears that petroleum constituents
are generated through geochemical action on organic detritus that is usually found in fine-grained
sedimentary rocks. In addition, it may be anticipated that some of the organic material would also
change sufficiently from the original organic material to remain in the sediment through, and
beyond, the oil-generating stage. Thus it is generally assumed that the place of origin of oil and gas
is not identical to the locations where it is found. The oil has had to migrate to present reservoirs
from the place of origin.
When sediments such as mud and clay are deposited, they contain water. As the layers of sediment accumulate, the increasing load of material causes compaction of the sediment and part of
the water is expelled from the pores. Migration probably begins at this stage and may involve a
substantial horizontal component. Initially, the oil hydrocarbons may be present in the water either
suspended as tiny globules or dissolved, hydrocarbons being slightly soluble in water.
Petroleum constituents may migrate through one or more formations that have permeability and
porosity similar to those of the ultimate reservoir rock. It is the occurrence of an impermeable, or a
very low permeability, barrier that causes the formation of oil and gas accumulations. Since practically all pores in the subsurface are water saturated, the movement of petroleum constituents within