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Origin and Occurrence
Primary migration is the movement of hydrocarbons (oil and natural gas) from mature,
organic-rich source rocks to a point where the oil and gas can collect as droplets or as a continuous phase of liquid hydrocarbon. Secondary migration is often attributed to various aspects
of buoyancy and hydrodynamics (Schowalter, 1979; Barker, 1980) and is the movement of the
hydrocarbons as a single, continuous fluid phase through water-saturated rocks, fractures, or
faults followed by accumulation of the oil and gas in sediments (traps) from which further
migration is prevented.
However, it must be emphasized that the various mechanisms that have been proposed for
petroleum migration do not compete with each other but are in fact complementary to each
other. The prevailing conditions in the subterranean strata may dictate that a particular migration
mechanism is favored and this is therefore the dominant mechanism, with alternative migration
mechanisms playing lesser roles. In another given situation, the role of the dominant migration
mechanism is reversed.
It is believed that, during migration, petroleum does not move through the bulk of the nonsource rock and shale bodies but through faults and fractures that may be in the form of a channel
network that permits leakage of oil and gas from one zone to another. During this migration,
the composition of the oil may be changed through physical causes such as filtration and adsorption, in a manner analogous to the chromatographic separation of petroleum (Chapter 9). Reactions
with minerals such as elemental sulfur or even with sulfur-containing minerals (e.g., sulfates) may
also occur.
Many examples of the effect of adsorption on crude oils exist (Chapter 9). The passage of
whole, or fractions of, crude oils through adsorbents is well documented, and the changes in
composition are authenticated. In terms of interaction with minerals, one such example of alteration during migration exists in northern Iraq, close to the city of Mosul. A heavy oil (Qayarah)
is produced (by steam stimulation) from shallow formations that are located to the north of a
sulfur bed. The sulfur is extracted commercially from the bed but must have an extraneous
bituminous material removed as part of the purification procedure. The Qayarah crude oil has
in excess of 8% w/w sulfur, of which one-quarter (i.e., approximately 2% w/w) is free sulfur.
The current conjecture is that the oil (or oil precursor) migrated from the Kirkuk area (to the
south) through the sulfur bed, thereby contaminating the sulfur, which in turn causes chemical
reactions to occur that produce the heavier constituents as well as picking up free sulfur during
the migration.
Any porous and permeable stratum will suffice as the reservoir, and a very common reservoir
rock is a porous or fractured limestone (especially of the reef, bioherm type); several such reservoir
structures are found throughout the Earth. Most reservoir rocks are sedimentary rocks, almost
always the coarser grained of the sedimentary rocks: sand, sandstone, limestone, and dolomite.
A less common reservoir is a fractured shale or even igneous or metamorphic rock. It is only rarely
that shales act as reservoir rocks, and again fractures and other relatively wide openings are believed
to confer the required reservoir properties on an otherwise unsuitable rock. It may be that petroleum
found in at least some reef structures is indigenous because of the large concentration of organisms
in reefs, and there is often no other obvious source of the oil.
Source beds do not generally coincide with reservoir rocks and the belief is that petroleum,
before it comes to rest in a trap, migrates large distances. Various examples are known of vertical migration, and migration upward for some kilometers is considered possible. Nevertheless, the
converse may also be true, and the in situ theory (juxtaposition of source bed and reservoir rock)
advocates that petroleum migrates very little, if at all, and that even the amount of vertical migration is negligible.
Once the oil has been transferred to the reservoir rock, it is free to move under any force that
may be applied. 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 porous stratum.
Favorable locations where the oil can accumulate may be anywhere along the path of fluid travel,
Origin and Occurrence
Primary migration is the movement of hydrocarbons (oil and natural gas) from mature,
organic-rich source rocks to a point where the oil and gas can collect as droplets or as a continuous phase of liquid hydrocarbon. Secondary migration is often attributed to various aspects
of buoyancy and hydrodynamics (Schowalter, 1979; Barker, 1980) and is the movement of the
hydrocarbons as a single, continuous fluid phase through water-saturated rocks, fractures, or
faults followed by accumulation of the oil and gas in sediments (traps) from which further
migration is prevented.
However, it must be emphasized that the various mechanisms that have been proposed for
petroleum migration do not compete with each other but are in fact complementary to each
other. The prevailing conditions in the subterranean strata may dictate that a particular migration
mechanism is favored and this is therefore the dominant mechanism, with alternative migration
mechanisms playing lesser roles. In another given situation, the role of the dominant migration
mechanism is reversed.
It is believed that, during migration, petroleum does not move through the bulk of the nonsource rock and shale bodies but through faults and fractures that may be in the form of a channel
network that permits leakage of oil and gas from one zone to another. During this migration,
the composition of the oil may be changed through physical causes such as filtration and adsorption, in a manner analogous to the chromatographic separation of petroleum (Chapter 9). Reactions
with minerals such as elemental sulfur or even with sulfur-containing minerals (e.g., sulfates) may
also occur.
Many examples of the effect of adsorption on crude oils exist (Chapter 9). The passage of
whole, or fractions of, crude oils through adsorbents is well documented, and the changes in
composition are authenticated. In terms of interaction with minerals, one such example of alteration during migration exists in northern Iraq, close to the city of Mosul. A heavy oil (Qayarah)
is produced (by steam stimulation) from shallow formations that are located to the north of a
sulfur bed. The sulfur is extracted commercially from the bed but must have an extraneous
bituminous material removed as part of the purification procedure. The Qayarah crude oil has
in excess of 8% w/w sulfur, of which one-quarter (i.e., approximately 2% w/w) is free sulfur.
The current conjecture is that the oil (or oil precursor) migrated from the Kirkuk area (to the
south) through the sulfur bed, thereby contaminating the sulfur, which in turn causes chemical
reactions to occur that produce the heavier constituents as well as picking up free sulfur during
the migration.
Any porous and permeable stratum will suffice as the reservoir, and a very common reservoir
rock is a porous or fractured limestone (especially of the reef, bioherm type); several such reservoir
structures are found throughout the Earth. Most reservoir rocks are sedimentary rocks, almost
always the coarser grained of the sedimentary rocks: sand, sandstone, limestone, and dolomite.
A less common reservoir is a fractured shale or even igneous or metamorphic rock. It is only rarely
that shales act as reservoir rocks, and again fractures and other relatively wide openings are believed
to confer the required reservoir properties on an otherwise unsuitable rock. It may be that petroleum
found in at least some reef structures is indigenous because of the large concentration of organisms
in reefs, and there is often no other obvious source of the oil.
Source beds do not generally coincide with reservoir rocks and the belief is that petroleum,
before it comes to rest in a trap, migrates large distances. Various examples are known of vertical migration, and migration upward for some kilometers is considered possible. Nevertheless, the
converse may also be true, and the in situ theory (juxtaposition of source bed and reservoir rock)
advocates that petroleum migrates very little, if at all, and that even the amount of vertical migration is negligible.
Once the oil has been transferred to the reservoir rock, it is free to move under any force that
may be applied. 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 porous stratum.
Favorable locations where the oil can accumulate may be anywhere along the path of fluid travel,