64
The Chemistry and Technology of Petroleum
the two processes may occur independently, they are apparently in most cases parallel to each other.
Water washing results in the removal of more soluble hydrocarbons from crude oils. In general, light
hydrocarbons are more easily dissolved and selectively removed from petroleum than are the higher
molecular weight hydrocarbons.
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 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, that is, 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.
In summary, the ultimate composition of petroleum may be influenced strongly by alteration subsequent to the process of accumulation. The three major alteration processes are thermal maturation, deasphalting, and degradation associated with the action of surface-derived formation waters
(Bailey et al., 1974).
3.2.3 dIFFerenCes BetWeen ABIogenIC tHeory And BIogenIC tHeory
There are several differences between biogenic and abiogenic theories and these are listed as follows:
1. Raw material
a. Biogenic: Remnants of buried plant and animal life.
b. Abiogenic: Deep carbon deposits from when the planet formed or subducted material.
2. Events before conversion
a. Biogenic: Large quantities of plant and animal life were buried. Sediments accumulating over the material slowly compressed it and covered it. At a depth of several hundred meters, catagenesis converts it to bitumen and kerogen.
b. Abiogenic: At depths of hundreds of kilometers, carbon deposits are a mixture of
hydrocarbon molecules that leak upward through the crust. Much of the material
becomes methane.
3. Conversion to petroleum and methane
a. Biogenic: Catagenesis occurs as the depth of burial increases and the heat and pressure
breaks down kerogen to form petroleum. Significant advances in the understanding of
chemical processes and organic reactions and improved knowledge about the effects
of heating and pressure during burial and diagenesis of organic sediments support
biogenic processes.
b. Abiogenic: When the material passes through temperatures at which extremophile microbes can survive some of it will be consumed and converted to heavier
hydrocarbons.
4. Evidence supporting abiogenic theory
a. Cold planetary formation: In the late nineteenth century it was believed that the Earth
was extremely hot, possibly completely molten, during its formation. One reason for
this was that a cooling, shrinking planet was necessary in order to explain geologic
changes such as mountain formation. A hot planet would have caused methane and
other hydrocarbons to be out-gassed and oxidized into carbon dioxide and water; thus,
there would be no carbon remaining under the surface. Planetary science now recognizes that formation was a relatively cool process until radioactive materials accumulate together deep in the planet.
The Chemistry and Technology of Petroleum
the two processes may occur independently, they are apparently in most cases parallel to each other.
Water washing results in the removal of more soluble hydrocarbons from crude oils. In general, light
hydrocarbons are more easily dissolved and selectively removed from petroleum than are the higher
molecular weight hydrocarbons.
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 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, that is, 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.
In summary, the ultimate composition of petroleum may be influenced strongly by alteration subsequent to the process of accumulation. The three major alteration processes are thermal maturation, deasphalting, and degradation associated with the action of surface-derived formation waters
(Bailey et al., 1974).
3.2.3 dIFFerenCes BetWeen ABIogenIC tHeory And BIogenIC tHeory
There are several differences between biogenic and abiogenic theories and these are listed as follows:
1. Raw material
a. Biogenic: Remnants of buried plant and animal life.
b. Abiogenic: Deep carbon deposits from when the planet formed or subducted material.
2. Events before conversion
a. Biogenic: Large quantities of plant and animal life were buried. Sediments accumulating over the material slowly compressed it and covered it. At a depth of several hundred meters, catagenesis converts it to bitumen and kerogen.
b. Abiogenic: At depths of hundreds of kilometers, carbon deposits are a mixture of
hydrocarbon molecules that leak upward through the crust. Much of the material
becomes methane.
3. Conversion to petroleum and methane
a. Biogenic: Catagenesis occurs as the depth of burial increases and the heat and pressure
breaks down kerogen to form petroleum. Significant advances in the understanding of
chemical processes and organic reactions and improved knowledge about the effects
of heating and pressure during burial and diagenesis of organic sediments support
biogenic processes.
b. Abiogenic: When the material passes through temperatures at which extremophile microbes can survive some of it will be consumed and converted to heavier
hydrocarbons.
4. Evidence supporting abiogenic theory
a. Cold planetary formation: In the late nineteenth century it was believed that the Earth
was extremely hot, possibly completely molten, during its formation. One reason for
this was that a cooling, shrinking planet was necessary in order to explain geologic
changes such as mountain formation. A hot planet would have caused methane and
other hydrocarbons to be out-gassed and oxidized into carbon dioxide and water; thus,
there would be no carbon remaining under the surface. Planetary science now recognizes that formation was a relatively cool process until radioactive materials accumulate together deep in the planet.