Page 109 - Geology and Geochemistry of Oil and Gas

WATER DRIVE 81
from the latter studies because some writers tied-in the very formation of oil and gas
accumulations with the development of artesian systems.
As long as the shallow and relatively young hydrodynamic basins were being
studied, there were no significant contradictions. Water intake (recharge) and
possible discharge areas were identified in most cases. Abundant rains in the
Caucasus Black Mountains resulted in raising the static water levels in wells of the
Old- and New-Grozny oilfields after 1–1.5 months. By adding tracers to water,
the movement of water sometimes could be recorded up to a few dozen kilometers.
Such basins have the water (infiltration) drive. Water moves from the recharge
areas (nearby hilly or mountainous regions) to the discharge areas (open: e.g.,
springs and rivers, or concealed: such as, cross-flow into different horizons). Water
velocity is quite high (in tens of meters per year), depending on the hydraulic head
drop from the recharge to the discharge area. No overpressure or abnormally-high
pressure is observed in the associated oil and gas accumulations. The water has low
salinity. The environment is oxidizing, especially in the first few hundred meters from
the surface (although the free oxygen disappears after less than a few tens of meters).
Some geologists believe that the presence of infiltration drive is unfavorable for
the oil and gas generation. The presence of numerous accumulations (some of them
quite large) in such cases is believed to be due to subsequent migration. The writers,
however, believe that a conventional view of the accumulations being destroyed by
the infiltration water is somewhat exaggerated.
The oil accumulations can be destroyed in several ways:
(1) Washing-out of the oil from accumulations by the way of tangential
transmission of water movement, layer-by-layer, up to the static oil accumula-
tion. This process is unlikely at reservoir environment because it requires a very
high speed of water movement. P. A. Antonov (in: Eremenko and Chilingarian,
1996, p. 53) calculated that velocity will be over 20 cm/s for a permeability of
500 mD, which is impossible in a basin.
(2) Chemical dissolution of liquid hydrocarbons in water and their delivery to the
surface. As previously mentioned, solubility of liquid hydrocarbons in water at
surface or near-surface environments is very low, so that it would take a few tens
of millions of years to destroy even a medium-size accumulation.
(3) Oxidation of the oil in accumulations through the action of aggressive water. In
such a case, a layer of oxidized oil forms between the oil and water. This layer
protects the accumulation from further destruction. It is similar to the asphalt
plug in an oil-bearing bed at its surface exposure; the difference is that the light
fractions of oil cannot be vented into the atmosphere.
(4) Squeezing-out the oil deposit out of the trap owing to the pressure drop at the
contour (Hubbert’s principle; see also Savchenko, 1977). Although this is
plausible, the process is very slow and is controlled by the reservoir permeability
and by the inclination angle of the barrier. In order for the accumulation to be
destroyed, the water/oil separation dip angle must be greater than that of the
inclination angle of the barrier.
A totally different situation occurred in the deeply buried basins, where
compaction and squeezing-out of water from the rocks are still continuing (see the