Page 182 - Geology and Geochemistry of Oil and Gas

HYDROCARBON EXPULSION (‘‘PRIMARY MIGRATION’’) 151
Olenin (1984) presented a review of the definitions of ‘‘oil and gas province’’ term.
Mirchink et al. (1976) defined a petroliferous province as an oil and gas basin or the
association of adjacent basins, the structural position and tectonic evolution of
which were controlled by the same major structural element of the Earth’s crust. This
term (petroliferous province) can be considered equivalent to ‘‘geologic province’’
with oil and gas fields.

9.2. HYDROCARBON EXPULSION (‘‘PRIMARY MIGRATION’’), HETEROGENEITY OF THE
MEDIUM, DISSOLUTION IN WATER AND GAS, AND DIFFUSION

The formation of hydrocarbon accumulations occurs in the process of migration
of hydrocarbons. K. P. Kalitskiy (in: Eremenko and Chilingar, 1996) proposed an
idea of the in situ formation of oil and gas accumulations. This idea was soon
rejected due to the absence of substances that would be able, after their
transformation, to form an oil accumulation within the original volume. Thus,
accepting that concentration (gathering) of hydrocarbons was needed to form
accumulations, the petroleum geologists turned their attention to finding out which
area, or volume, would be involved in such migration. Usually, the energy-related
side of the issue remained ignored.
Starting with Illing (1934), it is believed that migration process includes an
independent stage: the expulsion (‘‘primary migration’’). The expulsion is a
detachment of most volatile components from organic matter and mineral matter
and their movement within the poorly permeable source rocks.
The secondary migration involves the movement of oil and gas in permeable rocks
through pores and fractures. In this case, displacement of matter is governed by the
laws of mechanics, whereas the role of the law of the matter transfer during
the expulsion processes is unclear. The detachment of newly-formed molecules from
the organic matter occurs at the molecular level and is governed by the laws of
molecular physics.
Where and how does that quantum leap occur from the molecular level to the
level of matter subjected to the laws of mechanics? The dissipation of newly-formed
molecules may occur spontaneously, due to the internal energy of the changing
matter. The temperature will accelerate the process. On the one hand, it will enable
the breakage of the bonds between the newly-formed molecules and organic matter.
Also, by increasing the unencumbered run of the molecules, it will help their
distancing from the organic matter.
Can we call these molecules detached from organic matter (mostly not
hydrocarbons) oil? Oil is a complex chemical mixture. Its components, mostly
hydrocarbons, continuously interact between themselves and with the surrounding
medium. Their specificity is defined by their common occurrence in the same matter.
In order to gather the individual molecules into a single mass, some additional
energy must be applied. What kind of energy is it? Unfortunately, there is no clear
answer to this question. In 1950, Brod and Eremenko proposed that the mobile
hydrocarbon and non-hydrocarbon substances that form in the rocks migrate into