Page 160 - Geology and Geochemistry of Oil and Gas

STUDIES OF SOLUBLE AND INSOLUBLE PORTIONS OF ORGANIC MATTER 129
Regardless of the age of rocks (therefore, the duration of geochemical processes),
all sedimentary basins of the world exhibit the bituminosity peak, which determines
the ‘‘main phase’’ of oil and gas generation (i.e., oil and gas windows), at about the
same depth. Why is that? It was found that the depth may change depending on the
heat flow. Some regions display a compressed, and some others, stretched geother-
mal zonation (changes in the geothermal gradient). This clarification, however, does
not answer the question.
Why is it that in different regions, regardless of the age of rocks, there is only one
‘‘main phase’’ of oil generation, occurring at present time (present-day sample col-
lection and analyses)?
Based on the restoration of depositional and geochemical processes, there must
have been several such phases in various basins. The present-day geochemical and
thermodynamic status of sedimentary basins is usually not unique and does not
contradict the appearance in the times past of similar geochemical and thermody-
namic environments. Lastly, why is it that the theory of the ‘‘main phase’’ considers
the heat flow as the only energy source? Are not there other energy sources in the
Earth crust?
In an attempt to evade all these questions, alternative concepts have been de-
veloped. One example is a concept of the oil and gas generation cycles. The entire
process of generation, existence of oil, and destruction of these accumulations was
called an oil- and gas-generation cycle (Maksimov et al., 1972). This concept is
discussed in the chapter devoted to the hydrocarbon generation.
The bitumen extracted from the samples of different ages reflects the depositional
environment of organic matter, diagenetic and catagenetic alterations, and the ex-
pulsion of most volatile-substances from it. The theory of the ‘‘main phase’’ stresses
the transformations during diagenesis and catagenesis. At the same time, the initial
conditions (the depositional environment) and the final conditions (the expulsion of
volatile substances) remain in the background.
If one accepts the alternative view of oil- and gas-generation cycles and its treat-
ment of the process as continuous, but varying in intensity, most of the above ques-
tions can be answered. Currently observed bitumen maxima may not reflect the ‘‘main
phase’’ but may reflect any other phase, and the main phase must be established for
each particular area. As stated before, temperatures in excess of 100 1C are most
favorable for the hydrocarbon desorption. Therefore, the hydrocarbons generated
within the corresponding rock intervals are easily removed. This is supported by the
coinciding minimum on Akramkhodzhayev’s (1973) curves reflecting the recovery of
additional hydrocarbons from the previously extracted rock samples. Neruchev and
Eremenko (1968) suggested a great role of the expulsion processes in such a situation.

7.3. COMBINED STUDIES OF SOLUBLE AND INSOLUBLE PORTIONS OF ORGANIC
MATTER

Determination of soluble (bitumen) and insoluble (kerogen) portions of the dis-
persed organic matter is necessary for its complete characterization. Chemical and