Page 191 - Geology and Geochemistry of Oil and Gas

160 FORMATION OF HYDROCARBON ACCUMULATIONS

River Formation of the United States) is an unfavorable factor for the formation of
hydrocarbons. Still, the hydrocarbon accumulations form because the uniformity of
these sequences and of their organic matter content is quite relative. However, the
accumulations turn out to be small compared with the huge volume of organic
matter concentrated in such sequences. In such cases, the bituminous sequences
formed in a sedimentary basin can contain relatively small hydrocarbon reserves
(relative to the organic matter content in the basin).
All of the above-described primary migration mechanisms can develop in nature
and act simultaneously. Relative predominance of a mechanism depends on the
specific subsurface temperature and pressure and geochemical environment.
Laboratory studies and field observations on the subject should be continued.
The same mechanisms can be operative during the secondary migration phase.
Changes in the intercommunication of pores occur during compaction. As a result,
part of the pores becomes isolated. As pointed out by Beletskaya (1990) and
Nazarkin (1994), organic matter transformation processes in such pores are unique.
Organic matter in the closed pores is enriched in all kinds of bitumens low in alkanes.
Nazarkin (1994) stated that this is one of the reasons why sequences (such as
bazhenites, domanikites, etc.) with abundant isolated pores do no generate large
volumes of hydrocarbons.
A heterogeneous sedimentary sequence with alternating reservoir and sealing
rocks exhibits differences in energy potentials. The other phenomena prevail in such
a case: (1) first of all buoyancy (the Archimedes principle), (2) hydraulic heads, and
(3) the elastic forces of fluids and rocks.
Oil and gas migration due to buoyancy force can occur only through fractures or
large pores. A number of forces resist the gravity-induced movement of oil and gas.
They include (1) friction, (2) interphase friction (caused by motions of oil, gas and
water relative to one another), (3) relative permeability, (4) viscosity (or internal
friction), and (5) molecular attraction between the rock walls and moving fluids
(capillary forces).
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Droplets of oil or gas moving through the pores are subject to the Jamin effect .
In order for a globule (minimum surface) to move through a narrow passage its
shape must change. Consequent increase in its surface results in the expenditure of
additional energy. The capillary pressure difference Dp (needed for the droplet
movement) at the opposite ends of the droplet is equal to
Dp ¼ sð1=R 1 1=R 2 Þ (9.1)

where R 1 and R 2 are the radii of curvature of advancing and receding end of gas
bubble and s is the interfacial tension.
Although the Dp may be small for a single bubble, the cumulative resistance of
many bubbles may be large. Additional resistance to flow is created by the
polymolecular layers of oriented molecules of surface-active components in the oil,
3
which are adsorbed on the rock surface and may be thick (10 –10 4 cm).
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Jamin effect is manifested as the restrictive force exerted upon the fluids flowing through narrow
passages by successive bubbles of gas.