Page 193 - Geology and Geochemistry of Oil and Gas

162 FORMATION OF HYDROCARBON ACCUMULATIONS

use it repeatedly, because along this route they experience the least capillary
resistance and the least loss of matter (all sorption sites have already been saturated).
Along the way, heavier molecules tend to displace the lighter ones from the surfaces
of mineral particles, which results in some change in fluid composition.
Any migration along a relatively long distance results in the separation of oil and
gas from the oil–gas mixture. (The opposite phenomenon, i.e., mixing of the oil and
gas, also occurs in certain cases.) When the migration is dominated by buoyancy, the
oil and gas separation is most distinct. This is particularly distinct in the presence of
a chain of interconnected traps within a single structural trend. The principle of the
‘‘differential entrapment’’ of oil and gas has been presented simultaneously by
Gussow and Maksimov in 1954.
The following relationships may be observed as a result of regional uplift of a
reservoir bed with local highs separated by significant lows (Figs. 9.1 and 9.2).
Observations made by Gussow and Maksimov concern possible explanations for
the difference in the oil and gas content of adjacent or nearby traps. They suggest
that the nature of the hydrocarbons trapped in the reservoirs depends on the
proximity of the reservoirs to the source rocks. As oil migrates across basins and
encounters a series of traps, the process of differential entrapment becomes
operative. The theory states that a trap filled with oil and gas may eventually lose the
oil to another trap updip as oil and gas continue to migrate into the area. In Fig. 9.1,
during stage 1, trap A is an oil reservoir with a free gas. After trap A is filled and
additional oil cannot enter the trap, it spills updip to trap B, which contains oil only.

Fig. 9.1. Theory of differential entrapment: different stages in the migration and accumulation of oil and
gas in interconnected traps (after Gussow, 1968; courtesy of AIME).