ORIGIN AND OCCURRENCE OF OIL 43
alteration are thermal maturation, deasphalting, 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.
The thermal maturation of petroleum in the reservoir due to the
geothermal gradient is a series of disproportionate reactions resulting in the production of gases, such as methane and other light
hydrocarbons (Evans et al, 1971). The geothermal gradient is generally on the order of 25 to 30°C/km (15°F/1000 ft or 120°C/1000 ft,
i.e. 0.015°C per foot of depth or 0.012°C per foot of depth), i.e.
approximately one degree for every 100 feet below the surface. The
temperature gradient that exists, while not seemingly high enough
for many organic reactions, may be sufficient for the task, given
geologic time. 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.
Deasphalting is the precipitation of asphaltene constituents
from crude oils by admixture with or dissolution in the oil of large
amounts of light hydrocarbon and/or gaseous hydrocarbons that
vary from methane to the various heptane isomers (Mitchell and
Speight, 1973; Speight et al, 1984; Speight, 2007a). Such deasphalting
can occur whenever considerable amounts of the lower molecular
weight hydrocarbons are generated in substantial quantities because
of thermal alteration of the oil or as a result of gas incursion from
secondary migration. The overall effects of gas deasphalting are often
difficult to distinguish from those of thermal maturation because
both processes usually occur concomitantly and the net change in
composition is that the oils become lighter (Evans et al., 1971).
The microbial alteration of crude oil and alteration due to water
washing — the removal of water-soluble compounds — are common methods of petroleum alteration. Both processes are frequently
observed in combination because they are both due to the action
of moving subsurface water. The biodégradation of crude oil is a
selective utilization of certain types of hydrocarbons by microorganisms (Evans et al, 1971; Bailey et al, 1973a; Bailey et al, 1973b;
Deroo et al, 1974; Connan et al, 1975).
Biodegradation and water washing can be expected wherever
reservoirs are close to the surface and where they are accessible to
surface-derived waters. An example of such crude oil alteration is
alteration are thermal maturation, deasphalting, 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.
The thermal maturation of petroleum in the reservoir due to the
geothermal gradient is a series of disproportionate reactions resulting in the production of gases, such as methane and other light
hydrocarbons (Evans et al, 1971). The geothermal gradient is generally on the order of 25 to 30°C/km (15°F/1000 ft or 120°C/1000 ft,
i.e. 0.015°C per foot of depth or 0.012°C per foot of depth), i.e.
approximately one degree for every 100 feet below the surface. The
temperature gradient that exists, while not seemingly high enough
for many organic reactions, may be sufficient for the task, given
geologic time. 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.
Deasphalting is the precipitation of asphaltene constituents
from crude oils by admixture with or dissolution in the oil of large
amounts of light hydrocarbon and/or gaseous hydrocarbons that
vary from methane to the various heptane isomers (Mitchell and
Speight, 1973; Speight et al, 1984; Speight, 2007a). Such deasphalting
can occur whenever considerable amounts of the lower molecular
weight hydrocarbons are generated in substantial quantities because
of thermal alteration of the oil or as a result of gas incursion from
secondary migration. The overall effects of gas deasphalting are often
difficult to distinguish from those of thermal maturation because
both processes usually occur concomitantly and the net change in
composition is that the oils become lighter (Evans et al., 1971).
The microbial alteration of crude oil and alteration due to water
washing — the removal of water-soluble compounds — are common methods of petroleum alteration. Both processes are frequently
observed in combination because they are both due to the action
of moving subsurface water. The biodégradation of crude oil is a
selective utilization of certain types of hydrocarbons by microorganisms (Evans et al, 1971; Bailey et al, 1973a; Bailey et al, 1973b;
Deroo et al, 1974; Connan et al, 1975).
Biodegradation and water washing can be expected wherever
reservoirs are close to the surface and where they are accessible to
surface-derived waters. An example of such crude oil alteration is