Page 170 - Geology and Geochemistry of Oil and Gas

STAGEWISE NATURE AND CYCLIC TRANSFORMATION OF ORGANIC MATTER 139
The processes of oil and gas generation can be subdivided into four stages. During
the first two stages, the organic matter exhibited energy change from the higher forms
(biological and chemical energy) to the lower forms (heat and mechanical energy).
The initial organic matter for the oil generation was in a dispersed state. Its further
migration and the formation of accumulations represent the energy concentration
from the dispersed organic matter, which cannot occur due to its own internal energy.
If the first two stages of oil and gas generation may have occurred as a result of
mobilization of the internal energy, then the third stage required an external energy.
The source of this energy lies in geological processes (tectonic movements). Thus, the
third stage must be associated with (1) significant restructuring of the oil and gas
basins, (2) disruption of equilibrium in these basins, and (3) emergence of forces
causing changes in the potential energy of fluids that appeared within reservoirs by
that time. In such a case, the process of formation of accumulations may be taken as
a process accompanying the energy equalization in beds, in a general direction of the
process toward attainment of the surface of World Ocean (assumed zero level of the
potential energy of fluids).
The stagewise nature of the organic matter transformation cannot be compared
with the cyclicity of the oil and gas generation process any more than a ‘‘part’’ that
can be compared to the ‘‘whole’’: one is part of the other. The stages of the organic
matter transformation can be compared with the catagenetic cycles identified by
Vassoyevich (1975).
Vassoyevich used the coal ‘‘metamorphism’’ in Donyets Basin of Ukraine for his
classification of the oil-generating processes. Initially, the main generation phase was
placed between 801C and 1501C. Later, the temperature ranges have been modified.
Vassoyevich stated that the main stage (main phase) of the oil and gas generation
occurs as the source rocks reach the mesocatagenesis zone (minimum temperature of
601C). Generation of new hydrocarbons is significantly increased (including, for the
first time, low-molecular-weight hydrocarbons). The mesocatagenesis usually occurs
at a depth between 2.7 and 8 km and at a temperature between 2101C and 2801C
(Vassoyevich, 1975).
It is interesting to note that the depth/temperature of the mesocatagenetic zone
corresponding to the hydrocarbon window (main phase) does not coincide with the
zone of oil reserves encountered at a depth of 1–2 km. The gap is between 0.7 and
7 km. The thickness of some oil- and gas-producing formations does not exceed
300 m. Thus, the proponents of the main phase would have to take position of the
mandatory vertical migration for the formation of hydrocarbon accumulations. The
writers do not rule out the possibility of formation of oil accumulations due to a
distant vertical migration, but believe, however, that the vertical migration usually is
limited to the oil- and gas-producing formation.
The writers agree with Vassoyevich (1975) that of primary interest to the petro-
leum geologists are depositional cycles, because the sedimentogenesis, i.e., birth
(genesis) of the sediment, is a crucial stage of lithogenesis. During sedimentogenesis,
sediment may become a source rock, good or poor reservoir, or a seal. Changes
during the diagenesis, catagenesis, methagenesis, and hypergenesis greatly depend on
the initial type of the deposit.