61
Origin and Occurrence
Many oil and gas accumulations are trapped in anticlines or domes, structures that are generally more easily detected than some other types of traps. Some traps are formed by the reservoir rock being cut off at its upper end by a fault that places sealing rock against the fractured
end. Alternatively, the upper end may have been eroded away during a period of unconformity,
resulting in the subsequent covering of the eroded top of the reservoir rock by a sealing rock.
There are also examples in which the reservoir rock wedges out at its upper end as an original
depositional feature due to lateral variation in deposition or abuts against an old land surface
(stratigraphic trap). Traps associated with salt intrusions are 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. Last, 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 with bitumen or
with 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 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 have created numerous traps that are
both a structural trap and a stratigraphic trap.
3.2.2.6 In Situ Transformation of Petroleum
Petroleum, as we know it, is a very complex mixture of organic compounds and is thermodynamically unstable under a variety of geological conditions. Therefore, petroleum is also susceptible to
alteration after it has collected in a reservoir or in sediments (Evans et al., 1971). Therefore, it is
important, at this point, to address the issue of the alteration of petroleum once it has accumulated
in the reservoir or as it accumulates in sediments on its journey to the reservoir.
Alteration of reservoir petroleum is accepted for most of the world oil accumulations. Alteration
may be related to the relative instability of petroleum or to the traps may be susceptible to incursion
to chemical agents, such as oxygen. Physical effects, such as those caused when the level of burial
of the trap changes as a result of further subsidence or erosion, may also play a role. Examples
of chemical alteration are thermal maturation and microbial degradation of the reservoir oil.
Examples of physical alteration of petroleum are the preferential loss of low-boiling constituents by
diffusion or the addition of new constituents to the oil-in-place by migration of these constituents
from a source outside the reservoir.
It is difficult to draw a precise distinction between chemical and physical processes since it is
often more than likely that the two processes are often interrelated and may even occur simultaneously (Evans et al., 1971). It is therefore pertinent at this point to present a brief acknowledgment of
the various means by which petroleum can be altered in a reservoir.
3.2.2.6.1 Thermal Alteration
The alteration of reservoir petroleum occurs over geological time; to what degree (no pun intended)
and to what extent this involves thermal forces remain speculative.
Although the geothermal gradient varies from place to place, it is generally on the order of
25°C–30°C/km (15°F/1000 ft or 120°C/1000 ft, i.e., 0.015°C/ft of depth or 0.012°C/ft of depth), that
is, approximately 1° for every 100 ft below the surface. Thus, with increasing depth of the reservoir, there is a tendency for crude oil to become lighter insofar as it contains increasing amounts
of low-molecular weight hydrocarbons and decreasing amounts of the higher molecular weight
constituents.
The pyrolysis of hydrocarbon waxes yields oily products, and it has been postulated that the formation of a waxy protopetroleum occurs. Prolonged exposure to moderate temperatures and high
pressures yields olefins, which then rearrange and/or polymerize to produce a wide variety of hydrocarbons containing branched chains and rings. However, it has been estimated that a temperature in
Origin and Occurrence
Many oil and gas accumulations are trapped in anticlines or domes, structures that are generally more easily detected than some other types of traps. Some traps are formed by the reservoir rock being cut off at its upper end by a fault that places sealing rock against the fractured
end. Alternatively, the upper end may have been eroded away during a period of unconformity,
resulting in the subsequent covering of the eroded top of the reservoir rock by a sealing rock.
There are also examples in which the reservoir rock wedges out at its upper end as an original
depositional feature due to lateral variation in deposition or abuts against an old land surface
(stratigraphic trap). Traps associated with salt intrusions are 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. Last, 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 with bitumen or
with 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 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 have created numerous traps that are
both a structural trap and a stratigraphic trap.
3.2.2.6 In Situ Transformation of Petroleum
Petroleum, as we know it, is a very complex mixture of organic compounds and is thermodynamically unstable under a variety of geological conditions. Therefore, petroleum is also susceptible to
alteration after it has collected in a reservoir or in sediments (Evans et al., 1971). Therefore, it is
important, at this point, to address the issue of the alteration of petroleum once it has accumulated
in the reservoir or as it accumulates in sediments on its journey to the reservoir.
Alteration of reservoir petroleum is accepted for most of the world oil accumulations. Alteration
may be related to the relative instability of petroleum or to the traps may be susceptible to incursion
to chemical agents, such as oxygen. Physical effects, such as those caused when the level of burial
of the trap changes as a result of further subsidence or erosion, may also play a role. Examples
of chemical alteration are thermal maturation and microbial degradation of the reservoir oil.
Examples of physical alteration of petroleum are the preferential loss of low-boiling constituents by
diffusion or the addition of new constituents to the oil-in-place by migration of these constituents
from a source outside the reservoir.
It is difficult to draw a precise distinction between chemical and physical processes since it is
often more than likely that the two processes are often interrelated and may even occur simultaneously (Evans et al., 1971). It is therefore pertinent at this point to present a brief acknowledgment of
the various means by which petroleum can be altered in a reservoir.
3.2.2.6.1 Thermal Alteration
The alteration of reservoir petroleum occurs over geological time; to what degree (no pun intended)
and to what extent this involves thermal forces remain speculative.
Although the geothermal gradient varies from place to place, it is generally on the order of
25°C–30°C/km (15°F/1000 ft or 120°C/1000 ft, i.e., 0.015°C/ft of depth or 0.012°C/ft of depth), that
is, approximately 1° for every 100 ft below the surface. Thus, with increasing depth of the reservoir, there is a tendency for crude oil to become lighter insofar as it contains increasing amounts
of low-molecular weight hydrocarbons and decreasing amounts of the higher molecular weight
constituents.
The pyrolysis of hydrocarbon waxes yields oily products, and it has been postulated that the formation of a waxy protopetroleum occurs. Prolonged exposure to moderate temperatures and high
pressures yields olefins, which then rearrange and/or polymerize to produce a wide variety of hydrocarbons containing branched chains and rings. However, it has been estimated that a temperature in