Page 173 - Geology and Geochemistry of Oil and Gas

142 ORIGIN OF OIL AND NATURAL GAS

Two options have been discussed by Nesterov et al. (1992). The first option
involved a continuous removal of reaction products from the reaction zone. This
enabled to model the generation and simultaneous expulsion of the volatile products
of decomposition. This thermal destruction model was referred to as a ‘‘tentatively
open’’ system. The second option involved heating the kerogen at the same rate of
temperature increase, but within vacuumed and then sealed quartz tubes. This op-
tion (‘‘closed system’’) involved the thermal destruction at a constant volume, with-
out the release of reaction products and with an increasing pressure, analogous to
the conditions of sealed pores or accumulations.
The study indicated that the thermal destruction of the kerogen and coaly in-
clusions with increasing temperature within a ‘‘tentatively open’’ system, regardless
of the type of the source organic matter, requires an input of thermal energy.
Continuous thermal alteration of kerogen under the ‘‘tentatively open’’ model,
with the continuous removal of reaction products, requires an input of thermal
energy. Thermal energy input for the decomposition of organic matter and for
the formation of products of the process occurs in the form of individual impulses
(endo-effects) associated with certain temperature intervals. Energy release due to
secondary recombination and molecule neoformation reactions (at the expense of
redistribution of hydrogen) is insignificant and does not compensate for the over-
whelming energy intake during the kerogen decomposition.
The thermal transformation of the organic matter and the catagenetic boundaries
of heat parameters coincide with the hydrocarbon generation phases. Thus, it is
likely that the hydrocarbon generation process in the source rocks increases to a
certain thermal boundary (to MC 4 –MC 5 stages), above which the thermodynamic
kerogen decomposition occurs at a lower level of heat expenditures.
The writers suggests the following conclusion about the speed of thermal effect on
the source rocks. If the oil generating sequence subsides into the heating zone slowly
enough, then the thermal decomposition process required to reach the oil window
will occur at a lower level of activation energy (up to 40 kcal/mol), and at a heat
expenditure of 120 kcal/kg of kerogen.
Neruchev et al. (1992) also observed thermodynamic alterations in the process of
organic matter transformation. According to their experiments, as the depth and
temperature increase in the catagenetic zone, the complex structural transformation
of organic matter is accompanied by several alternating maxima and minima of heat
release that causes this transformation. The maxima are mostly associated with the
stages of intense transformation of organic matter molecules and the generation of
substantial volumes of volatile products. The minima (endothermic zones) mainly
correlate with the reshaping and stabilization of the structure of organic matter with
a small amount of generated volatile substances. As the depth and temperature
increases, the scale of heat released from the organic matter transformation de-
creases. The process progresses toward endothermic reactions, which is in compli-
ance with general principles of thermodynamics. Usually, exothermal reactions
occur at relatively low temperatures. As the temperature increases, the process shifts
toward endothermic reactions.