44 PETROLEUM TECHNOLOGY, ECONOMICS, AND POLITICS
based on crude oil samples from the western Canada sedimentary
basin (Deroo et al., 1974). Systematic changes have been observed,
from what are believed to be normal, unaltered oils in the deeper
reservoirs, to heavier oils in the more shallow reservoirs, and to
drastically altered oils very close to the surface — the sandstone
reservoirs impregnated with heavy oils, such as the Athabasca oil
sands. There are also instances of an increase in sulfur content of
the crude oil along with increasing bacterial degradation. It is not
yet clear whether sulfur is added to the crude oils by the action
of bacteria or whether the sulfur compounds are not attacked by
the bacteria and thus remain after the bacteria have preferentially
eliminated the hydrocarbon moieties.
2.2.6 Relationship of Petroleum Composition and
Properties
Of all the properties, specific gravity, or American Petroleum
Institute (API) gravity, is the variable usually observed. The changes
may simply reflect compositional differences, such as the gasoline
content or asphalt content, but analysis may also show significant
differences in sulfur content or even in the proportions of the various hydrocarbon types.
Elemental sulfur is a common component of sediments, and, if
present in the reservoir rock, dissolves in the crude oil and reacts
slowly with it to produce various sulfur compounds and/or hydrogen sulfide, which may react further with certain components of
the oil. These reactions are probably much the same as those that
occur in the source bed and are presumed to be largely responsible
for the sulfur content of petroleum; these reactions are accompanied by a darkening of the oil and a significant rise in specific gravity and viscosity.
The variation in the character of crude oil with depth of burial is
also of interest. An increase in the lighter constituents and a decrease
in the density and a decrease in the amount of higher molecular
weights constituents with increasing depth of the reservoir often
occurs. Thus, there is the distinct possibility, if not certainty, that
the original character of petroleum can be substantially altered
by events following the migration process. Each of the suggested
mechanisms for this alteration (i.e. thermal alteration, deasphalting, and biogenic alteration) may all play a role in the alteration of
a particular crude oil. As for the choice of a migration mechanism,
based on crude oil samples from the western Canada sedimentary
basin (Deroo et al., 1974). Systematic changes have been observed,
from what are believed to be normal, unaltered oils in the deeper
reservoirs, to heavier oils in the more shallow reservoirs, and to
drastically altered oils very close to the surface — the sandstone
reservoirs impregnated with heavy oils, such as the Athabasca oil
sands. There are also instances of an increase in sulfur content of
the crude oil along with increasing bacterial degradation. It is not
yet clear whether sulfur is added to the crude oils by the action
of bacteria or whether the sulfur compounds are not attacked by
the bacteria and thus remain after the bacteria have preferentially
eliminated the hydrocarbon moieties.
2.2.6 Relationship of Petroleum Composition and
Properties
Of all the properties, specific gravity, or American Petroleum
Institute (API) gravity, is the variable usually observed. The changes
may simply reflect compositional differences, such as the gasoline
content or asphalt content, but analysis may also show significant
differences in sulfur content or even in the proportions of the various hydrocarbon types.
Elemental sulfur is a common component of sediments, and, if
present in the reservoir rock, dissolves in the crude oil and reacts
slowly with it to produce various sulfur compounds and/or hydrogen sulfide, which may react further with certain components of
the oil. These reactions are probably much the same as those that
occur in the source bed and are presumed to be largely responsible
for the sulfur content of petroleum; these reactions are accompanied by a darkening of the oil and a significant rise in specific gravity and viscosity.
The variation in the character of crude oil with depth of burial is
also of interest. An increase in the lighter constituents and a decrease
in the density and a decrease in the amount of higher molecular
weights constituents with increasing depth of the reservoir often
occurs. Thus, there is the distinct possibility, if not certainty, that
the original character of petroleum can be substantially altered
by events following the migration process. Each of the suggested
mechanisms for this alteration (i.e. thermal alteration, deasphalting, and biogenic alteration) may all play a role in the alteration of
a particular crude oil. As for the choice of a migration mechanism,