Page 171 - Geology and Geochemistry of Oil and Gas

140 ORIGIN OF OIL AND NATURAL GAS

If the organic matter transformation stages are associated with, or may be related
to, the lithogenetic stages, then the oil and gas accumulation must be associated with
the geotectonic evolution of large regions of the Earth with sedimentary cycles.
The transformation process of a given organic matter within a given lithological
sequence is irreversible. The cyclic nature of the oil and gas generation process
implies the repetitiveness, or its possibility, of the oil and gas generation for a given
region of the Earth’s crust, but for different lithological (sedimentary) sequences.
The concept of the main phase (oil window) is widely accepted. Evaluation of the
hydrocarbon potential of a basin, and the timing of hydrocarbon generation and
formation of accumulations were determined based on this concept. Lopatin (1971)
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proposed to use a simplified form of the Arrhenius law, according to which the rate
of chemical reactions doubles when temperature increases by 101C.
The oil window concept is continuously evolving and improving, especially with
regard to the calculations of oil and gas source potential based on the elemental
composition. The main task is to develop a model of the dispersed organic matter
transformation that would realistically reflect the natural processes and balance the
mass of the original dispersed organic matter with the residual dispersed organic
matter and all gas, liquid, and solid generation products. In 1982, Neruchev and
Rogozina stated that such a model does not exist. Later, however, it was stated
(Neruchev and Rogozina, 1992) that such a model has been developed. In such a
model, however, one should consider the scatter of the elemental composition data,
even within a single sample. Thus, it would be easy to stack the analyses and show a
negative course for the catagenesis.
As Kontorovich (1991) indicated, the depositional rate and the heat flow change
in time depending on the processes occurring at depth. As a result, the rate of
hydrocarbon generation in oil and gas basins and the total mass of hydrocarbons
generated per unit of time sporadically change with time. Due to this, even within the
twin basins one may find somewhat different oil and gas areas and non-identical sets
of largest hydrocarbon accumulations. Besides, the transposition of coal thermolysis
laboratory experiments onto the dispersed organic matter in the natural environment
is quite tentative (Uspenskiy, 1970).
The above-presented ideas are similar to those mentioned before on the stress-
energy state of the sediment cover (and of the source rocks in particular). Such a
system tends to reach an equilibrium. Leveling of the energy non-uniformity occurs
by way of the energy redistribution within the system (source rocks) and its transfer
(together with fluids) into the adjacent beds with a lower potential energy. This
process is unavoidably accompanied by other plastic and fracture (hydrofracturing)
deformations.
In the case of a natural hydraulic fracture, the pressures are applied within the bed;
with all other conditions being equal, only the bed resistance to extension must be
overcome. Zheltov and Christianovich (1956, in: Eremenko and Chilingar, 1996, p. 159)

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Arrhenius’s rate equation provides the rate R of a thermally activated process: R ¼ R 0 expðE a =ksTÞ,
where R 0 ¼ a constant, E a ¼ the activation energy, k ¼ the Boltzmann constant, and T ¼ the absolute
temperature.