63
Origin and Occurrence
The thermal maturation process is essentially a series of disproportionation reactions, for
example, there are hydrogen transfer reactions, and in the more polar constituents of the oil,
decarboxylation, dehydration, and desulfurization reactions produce carbon dioxide, water, and
hydrogen sulfide. There is also a simultaneous production of gases, such as methane and other light
hydrocarbons (Evans et al., 1971). Thus, the presence of the more thermodynamic pyrobitumen
types of materials plus low molecular weight hydrocarbon gases and the absence of intermediate
oil constituents are usually an indication of disproportionation reactions during thermal alteration.
In fact, the thermal alteration process is essentially a disproportionation reaction in which there is
hydrogen transfer from, presumably, the more aromatic polar species to the more aliphatic species.
3.2.2.6.2 Deasphalting
Another relatively common method of petroleum alteration involves deasphalting in the reservoir.
Deasphalting (Chapter 9) is the precipitation of asphaltenes 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. Other hydrocarbons, having up to 16 carbon atoms
(hexadecane, C 16 H 34 ), are also known to precipitate an asphaltene fraction or an asphaltic fraction
(asphaltene constituents plus resin constituents) from crude oil (Chapter 9) but are not known to
have substantial effects in the reservoir. Such deasphalting can occur as a natural process among
the heavier crude oils 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 since both processes usually occur concomitantly and the net change in composition is
that the oils become lighter (Evans et al., 1971). The yield of asphaltenes is related to the amount of
the liquid hydrocarbon used for the deasphalting (Mitchell and Speight, 1973; Speight et al., 1984),
the amount of asphaltenes precipitated in a reservoir can also be correlated with the amount of gas
dissolved in the oils. The solution gas–oil ratio (GOR) for these oils is a measure of the amount of
gas injected into the reservoir. In addition, the API gravity of the oil is an approximate measure of
asphaltene content insofar as the API gravity decreases with increasing asphaltene content.
There has also been the suggestion that gravity segregation occurs in connection with reservoir
deasphalting. For example, in reservoirs that have a high vertical profile, assuming a small temperature differential between the top and the bottom of the reservoir, the crude oil may be progressively
heavier with increasing depth. The material at the higher points of such reservoirs could conceivably contain much more gas in solution than oil in the lower parts of the reservoir. Thus, more
asphaltenes are precipitated at the top of the reservoir than at the bottom of the reservoir (Evans
et al., 1971). That gravity segregation occurs is not in doubt since there is evidence that the bitumen
in the lower regions of oil sand formations contains higher proportion of asphaltenes than the bitumen at the top of the oil sand formation. Whether this is due to some past occurrence during the
maturation of the oil, similar to the mechanism already suggested, or to some other means is not
clear at this time.
3.2.2.6.3 Biodegradation and Water Washing
The microbial alteration of crude oil (biodegradation) and alteration due to water washing, that
is, the removal of water-soluble compounds, are common methods of petroleum alteration. Both
processes are frequently observed in combination since they are both due to the action of moving
subsurface water. The biodegradation of crude oil is a selective utilization of certain types of hydrocarbons by microorganisms (Evans et al., 1971; Bailey et al., 1973a,b; Deroo et al., 1974; Connan
et al., 1975).
The processes and effects of water washing are less well described than the biodegradation effects
since there is a distinct lack of specific examples of the water washing process. Furthermore water
washing normally should have less severe effects upon the composition of a crude oil, and although
Origin and Occurrence
The thermal maturation process is essentially a series of disproportionation reactions, for
example, there are hydrogen transfer reactions, and in the more polar constituents of the oil,
decarboxylation, dehydration, and desulfurization reactions produce carbon dioxide, water, and
hydrogen sulfide. There is also a simultaneous production of gases, such as methane and other light
hydrocarbons (Evans et al., 1971). Thus, the presence of the more thermodynamic pyrobitumen
types of materials plus low molecular weight hydrocarbon gases and the absence of intermediate
oil constituents are usually an indication of disproportionation reactions during thermal alteration.
In fact, the thermal alteration process is essentially a disproportionation reaction in which there is
hydrogen transfer from, presumably, the more aromatic polar species to the more aliphatic species.
3.2.2.6.2 Deasphalting
Another relatively common method of petroleum alteration involves deasphalting in the reservoir.
Deasphalting (Chapter 9) is the precipitation of asphaltenes 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. Other hydrocarbons, having up to 16 carbon atoms
(hexadecane, C 16 H 34 ), are also known to precipitate an asphaltene fraction or an asphaltic fraction
(asphaltene constituents plus resin constituents) from crude oil (Chapter 9) but are not known to
have substantial effects in the reservoir. Such deasphalting can occur as a natural process among
the heavier crude oils 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 since both processes usually occur concomitantly and the net change in composition is
that the oils become lighter (Evans et al., 1971). The yield of asphaltenes is related to the amount of
the liquid hydrocarbon used for the deasphalting (Mitchell and Speight, 1973; Speight et al., 1984),
the amount of asphaltenes precipitated in a reservoir can also be correlated with the amount of gas
dissolved in the oils. The solution gas–oil ratio (GOR) for these oils is a measure of the amount of
gas injected into the reservoir. In addition, the API gravity of the oil is an approximate measure of
asphaltene content insofar as the API gravity decreases with increasing asphaltene content.
There has also been the suggestion that gravity segregation occurs in connection with reservoir
deasphalting. For example, in reservoirs that have a high vertical profile, assuming a small temperature differential between the top and the bottom of the reservoir, the crude oil may be progressively
heavier with increasing depth. The material at the higher points of such reservoirs could conceivably contain much more gas in solution than oil in the lower parts of the reservoir. Thus, more
asphaltenes are precipitated at the top of the reservoir than at the bottom of the reservoir (Evans
et al., 1971). That gravity segregation occurs is not in doubt since there is evidence that the bitumen
in the lower regions of oil sand formations contains higher proportion of asphaltenes than the bitumen at the top of the oil sand formation. Whether this is due to some past occurrence during the
maturation of the oil, similar to the mechanism already suggested, or to some other means is not
clear at this time.
3.2.2.6.3 Biodegradation and Water Washing
The microbial alteration of crude oil (biodegradation) and alteration due to water washing, that
is, the removal of water-soluble compounds, are common methods of petroleum alteration. Both
processes are frequently observed in combination since they are both due to the action of moving
subsurface water. The biodegradation of crude oil is a selective utilization of certain types of hydrocarbons by microorganisms (Evans et al., 1971; Bailey et al., 1973a,b; Deroo et al., 1974; Connan
et al., 1975).
The processes and effects of water washing are less well described than the biodegradation effects
since there is a distinct lack of specific examples of the water washing process. Furthermore water
washing normally should have less severe effects upon the composition of a crude oil, and although