Page 198 - Geology and Geochemistry of Oil and Gas

PRIMARY ACCUMULATION AND FREE PHASE MIGRATION 167
determined by the pressure gradients. Thus, the direction of flow may be either
upward or downward.
If two reservoirs are separated by a third one, with a higher or lower reservoir
pressure, the third reservoir hampers the cross-flow between the first two reservoirs.
However, it is possible to bypass the overpressured reservoir (AHFP) using the
concept developed by Eremenko (1961) which he called ‘‘the piston mechanism’’.
When a fracture is formed, vacuum is created and the fluids rush into the fracture.
Inasmuch as the fracture has a higher permeability than the bed with AHFP, the
fluids will flow along the fracture to reservoir with higher permeability, thus
bypassing the lower-permeability bed with AHFP. A similar concept was presented
later by Chepak (1984) and Solovyev (1986). This concept is not applicable to a bed
with ALFP (abnormally-low formation pressure), because such a bed is a receptacle
for the fluids. Thus, it will hinder the fluid movement in all directions.
The independent creation and existence of hydrodynamic systems may cause a
situation when the adjacent reservoirs in different parts of a basin exchange fluids in
the opposite directions. Elastic energy accumulated by the fluids (expressed in the
form of AHFP) cause fluid movements. This is especially evident in block-faulted
reservoirs. In such a situation, the fluids elastically compressed into a single phase
can cause their own movement. The separation of this phase into different phases
may occur later within a trap.
The diverse geological processes are not always mutually exclusive. These
processes and the associated forces often appear simultaneously within a single
geologic space. That is why it is very difficult to distinguish individual processes and
forces giving rise to them in a particular situation.
For example, in the process of formation of accumulations, buoyancy is most
active in the upper part of the Earth’s crust and within steeply dipping avenues of
migration (fractured zones, faults). With increasing depth, because the differences in
the physical properties of oil, gas and water tend to decrease, the action of buoyancy
declines up to a total disappearance (e.g., in ‘‘critical type’’ accumulations). The
capillary forces, which can either obstruct or cause migration (‘‘wick’’ effect, film
migration), increases to a depth of 700–1,000 m and then decline to a total
disappearance in ‘‘critical type’’ accumulations. Elastic forces, which are insignificant
or absent in sediments, increase upon burial and are predominant in the ‘‘critical
type’’ accumulations. Pressure and temperature gradients in the sediments increase
with increasing energy stress, i.e., with depth and, especially, with increasing tectonic
stress.
Mutual solubility of fluids, as a factor promoting migration, significantly varies
with increasing temperature and pressure: first, a rather high solubility of
hydrocarbon gases in water and oil; then, an increased mutual solubility of gas
and oil; and lastly, almost complete mutual solubility of gas, oil and water in
‘‘critical type’’ accumulations.
The role of sorption and hydrophilic or hydrophobic properties of rocks decreases
with increasing temperature (i.e., depth). The role of AHFP in the migration
processes (either constructive or obstructive) increases from a depth of 300–500 m up
to the emergence of energy-differentiated block-faulted reservoirs. Down the section,