65
Origin and Occurrence
b. Known hydrocarbon sources: Carbonaceous chondrite meteorites contain carbon and
hydrocarbons. Heated under pressure, this material would release hydrocarbon fluids
in addition to creating solid carbon deposits. Further, at least 10 bodies in our solar
system are known to contain at least traces of hydrocarbons. In 2004, the Cassini
spacecraft confirmed methane clouds and hydrocarbons on Titan, a moon of Saturn.
c. Unusual deposits: Hydrocarbon deposits have been found in places that are poorly
explained by biogenic theory. Some oil fields are being refilled from deep sources,
although this does not rule out a deep biogenic source rock. The White Tiger field in
Vietnam and many wells in Russia, in which oil and natural gas are being produced
from granite basement rock, are examples. As this rock is believed to have no oilproducing sediments under it, the biogenic theory requires the oil to have leaked in
from source rock dozens of kilometers away.
d. Deep microbes: Microbial life has been discovered 4.2 km deep in Alaska and 5.2 km
deep in Sweden.
5. Evidence supporting biogenic theory
It was once argued that the abiogenic theory does not explain the detection of various biomarkers
in petroleum. Microbial consumption does not yet explain some trace chemicals found in deposits.
Materials that suggest certain biological processes include tetracyclic diterpane, sterane, hopane,
and oleanane. Although microorganisms exist deep underground and some metabolize carbon,
some of these biomarkers are only known so far to be created in surface plants. This shows that
some petroleum deposits may have been in contact with ancient plant residues, though it does not
show that either is the origin of the other.
3.2.4 relAtIonsHIP oF Petroleum ComPosItIon And ProPertIes
The geochemistry of petroleum is an extremely complex subject. It is not the intention here to
examine in any detail the individual constituents of crude oil that have been reported elsewhere
(Evans et al., 1971; Deroo et al., 1974; Ho et al., 1974; Orr, 1977, 1986; Hunt, 1996; Brooks, 1981).
It is intended to indicate the general trends that occur when a wide range of crude oils is examined.
As has already been mentioned, the passage of whole, or fractions of, crude oils through adsorbents is well documented, and the changes in composition are authenticated. In terms of interaction
with minerals, one such example of alteration during migration exists in northern Iraq, close to the
city of Mosul. Qayarah heavy oil is produced (by steam stimulation) from shallow formations that
are located to the north of a sulfur bed. The sulfur is extracted commercially from the bed but must
have an extraneous bituminous material removed as part of the purification procedure. Qayarah
heavy oil has in excess of 8% w/w sulfur, of which one-quarter (i.e., approximately 2% w/w) is free
sulfur. The current conjecture is that the oil (or oil precursor) has migrated from the Kirkuk area
(to the south) through the sulfur bed, thereby contaminating the sulfur, which in turn causes chemical reactions to occur that produce the heavier constituents as well as picking up free sulfur during
the migration.
Of all the properties, specific gravity, or American Petroleum Institute gravity (Chapter 10), 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.
Origin and Occurrence
b. Known hydrocarbon sources: Carbonaceous chondrite meteorites contain carbon and
hydrocarbons. Heated under pressure, this material would release hydrocarbon fluids
in addition to creating solid carbon deposits. Further, at least 10 bodies in our solar
system are known to contain at least traces of hydrocarbons. In 2004, the Cassini
spacecraft confirmed methane clouds and hydrocarbons on Titan, a moon of Saturn.
c. Unusual deposits: Hydrocarbon deposits have been found in places that are poorly
explained by biogenic theory. Some oil fields are being refilled from deep sources,
although this does not rule out a deep biogenic source rock. The White Tiger field in
Vietnam and many wells in Russia, in which oil and natural gas are being produced
from granite basement rock, are examples. As this rock is believed to have no oilproducing sediments under it, the biogenic theory requires the oil to have leaked in
from source rock dozens of kilometers away.
d. Deep microbes: Microbial life has been discovered 4.2 km deep in Alaska and 5.2 km
deep in Sweden.
5. Evidence supporting biogenic theory
It was once argued that the abiogenic theory does not explain the detection of various biomarkers
in petroleum. Microbial consumption does not yet explain some trace chemicals found in deposits.
Materials that suggest certain biological processes include tetracyclic diterpane, sterane, hopane,
and oleanane. Although microorganisms exist deep underground and some metabolize carbon,
some of these biomarkers are only known so far to be created in surface plants. This shows that
some petroleum deposits may have been in contact with ancient plant residues, though it does not
show that either is the origin of the other.
3.2.4 relAtIonsHIP oF Petroleum ComPosItIon And ProPertIes
The geochemistry of petroleum is an extremely complex subject. It is not the intention here to
examine in any detail the individual constituents of crude oil that have been reported elsewhere
(Evans et al., 1971; Deroo et al., 1974; Ho et al., 1974; Orr, 1977, 1986; Hunt, 1996; Brooks, 1981).
It is intended to indicate the general trends that occur when a wide range of crude oils is examined.
As has already been mentioned, the passage of whole, or fractions of, crude oils through adsorbents is well documented, and the changes in composition are authenticated. In terms of interaction
with minerals, one such example of alteration during migration exists in northern Iraq, close to the
city of Mosul. Qayarah heavy oil is produced (by steam stimulation) from shallow formations that
are located to the north of a sulfur bed. The sulfur is extracted commercially from the bed but must
have an extraneous bituminous material removed as part of the purification procedure. Qayarah
heavy oil has in excess of 8% w/w sulfur, of which one-quarter (i.e., approximately 2% w/w) is free
sulfur. The current conjecture is that the oil (or oil precursor) has migrated from the Kirkuk area
(to the south) through the sulfur bed, thereby contaminating the sulfur, which in turn causes chemical reactions to occur that produce the heavier constituents as well as picking up free sulfur during
the migration.
Of all the properties, specific gravity, or American Petroleum Institute gravity (Chapter 10), 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.