Page 195 - Geology and Geochemistry of Oil and Gas

164 FORMATION OF HYDROCARBON ACCUMULATIONS

The above-described pattern is typical of the regions where reservoir pressure in
oil accumulations is equal to the saturation pressure even at the level of the
lowermost trap. If the gas-saturation pressure in the lowermost accumulations is
lower than the reservoir pressure, there is no separation of oil and gas in these
accumulations. According to Neruchev (in: Eremenko and Chilingar, 1996), the free
gas phase can form only when the organic matter concentration is sufficiently high.
The lower traps (Fig. 9.3) will be filled with oil and have a low saturation pressure.
As migration proceeds along the series of interconnected traps, the saturation
pressure deficit will decrease until it disappears. That is when gas is released out of
the solution and either forms gas caps or straight gas accumulations, thereby
squeezing oil into the higher located traps (see Fig. 9.3). The case (a) is a special
version of a more general case (b).
Some examples of differential entrapment were published by Savchenko (1952),
Maksimov (1954, 1962), Samedov and Buryakovsky (1962), Fyodorov (1962),
Buryakovsky (1973, 1974), and Bagir-zadeh et al. (1974).
The above-described phenomenon is just a simplification. The process in nature is
more complex and depends on the geologic scenario and changes in time. Various
factors may cause substantial changes, which include (1) time of trap formation, (2)
changes in the regional dip, (3) depth of burial and associated formation pressure
and temperature, and (4) exposure of the reservoir at the surface. Distribution of oil
and gas accumulations according to the principle of differential entrapment is
observed in many tectonic zones of the world.
Changes in the amount or direction of the regional dip occur in many oil and gas
provinces. They can cause a considerable redistribution of oil and gas accumulations.
Some traps may partially or completely open and consequently lose their hydrocarbons.

Fig. 9.3. Conceptual diagram of selective retention of oil and gas in a series of interconnected traps (after
Maksimov et al., 1981).