Page 167 - Geology and Geochemistry of Oil and Gas

136 ORIGIN OF OIL AND NATURAL GAS

Strakhov (1960) stressed the need for strongly reducing hydrogen sulfide geo-
chemical facies. As a result of his idea, pyritized sequences have been included
among the source rocks. Later, Strakhov changed his mind.
The quantitative presence of organic matter in rocks was reviewed first by Trask
and White (1915, in: Eremenko and Chilingar, 1996, p. 153). According to Arkh-
angelskiy (1954), sediments may be classified as source rocks if they contain organic
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matter at least 2% or 43 kg/m . The absolute amount of organic matter buried in
various genetic types of sediments depends on many factors, but mainly on the
biological productivity of source organisms and by the facies environment during
burial. Relative organic matter concentration also depends on the depositional
speed. For example, due to a very slow rate of deposition, organic matter content in
the Central Polar Basin reaches 1%. Generally, organic matter concentration in
sediments widely ranges between trace amounts and 100% (in peat).
Trofimuk et al. (1980, 1982) believed that the content of a dispersed organic
matter in source rocks may be even lower, because part of it had been spent to
generate oil and gas that have been expelled. Some other investigators believe that
less than 1% of organic matter is converted into oil; thus, the expulsion loss could
not significantly affect the residual carbon distribution in sediments.
Many scientists (e.g., Kontorovich et al., 1975; Nazarkin, 1994; Bordovskiy, 1974)
recognize the effect of depositional rate on the qualitative and quantitative outcome
of the organic matter transformation. First, the relative increase in the organic
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matter concentration is observed. Then, the rate reaches 50–200 tons/km /year and
the depositional rate decreases due to the organic matter dilution by minerals
(Nazarkin, 1994). Rieke and Chilingarian (1974) found that, with other conditions
being equal, the value and duration of AHFP also directly correlates with the de-
positional rate. The depositional rate in most known oil and gas basins is between
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150 and 1000 tons/km /year.
The major geologic factor in transforming organic matter into bitumen is the
compaction of rocks under the overburden pressure. Brod and Eremenko (1950)
stated that the oil and gas generation process is lengthy and continuous: ‘‘y Hy-
drocarbon compounds arise as a result of competition between two opposite trends:
when subsidence prevails over uplift, during small as well as large oscillations of a
given Earth’s crust area.’’
Oscillations of the Earth’s crust are the cause of relationships among the depos-
itional processes, rock formation, and structural development. Sedimentation is re-
sponsible for the accumulation of organic matter in deposits. Lithification is
responsible for the organic matter transformation, whereas the structural develop-
ment (tectonic activity) is responsible for the formation of combustible fossil fuel
accumulations, their metamorphism, and destruction.
‘‘Oil and gas generation is, therefore, an unalienable part of the Earth’s crust
evolution and involves dynamic processes. It is not just a simple mechanical
displacement but consists mainly of complex transformations. These transforma-
tions consist of biologic, geochemical, physicochemical, and other changes. They
manifest themselves jointly, but play different roles at different stages of oil and
gas generation’’.