Geology Reference
In-Depth Information
the section of the invading fluids and the by the capacity
of the enclosing rocks.
t
In accumulations of the third zone the balance between
the influx of waters and hydrocarbons is distorted in favor
of the waters. This phenomenon may be explained as the
hydrocarbon washout from traps by the invading alkaline
waters and for this reason by a partial destruction of the
accumulations. High absolute values of per-unit volume
oil-saturation in the accumulations of this zone give rea-
son to believe that they are at a relatively early stage of
dissipation
3
.
t
There is a wide range of the conductivity values within
which the destruction (re-formation) of the accumula-
tions occurs. It evidences that this process is determined
not only by active circulation of ground water in high-
permeability reservoirs but is apparently caused to a sub-
stantial degree, especially in low-permeability intervals,
by high pressure gradients during the fluid invasion from
the overlying sediments (a high waterhead most likely
does not facilitate hydrocarbon accumulation and pres-
ervation in the traps.
Significance of the latter circumstance is indicated by the correla-
tion (Figure 7.4) between per-unit volumes of oil-saturation in the
3
It is noteworthy that all, without exception, accumulations in this zone display
high apparent resistivity in the aquifer area. Studies showed that high appar-
ent resistivity in the aquifer area of the Productive Sequence is caused by some
residual oil. Lab core studies did not find any indications of oil in the aquifer of
Upper Productive Sequence. These intervals have low apparent resistivities from
logs (up to 1-2
ohmm
).
As opposed to this, the aquifers of Productive Sequence's
basal horizons contain significant residual oil amounts (on the average 10-15%
of the pore volume), which increases apparent resistivities to 15-25 ohmm and
greater. The nature of this phenomenon is viewed differently depending on the
specific geologic situation. In some cases, the oil in the aquifer is interpreted as
the migration traces. In some other cases, it is viewed as the indication of a partial
destruction and shrinking of larger paleo-accumulations through in-reservoir oil
cross-flow into the upper intervals. Incidentally, no indications of residual oil in
the aquifer of the Upper Productive Sequence in combination with other known
geologic factors disagree with a view of syngenetic nature in the upper portion of
its section (Ali-Zadeh, Putkaradze and Salayev, 1968).
