Geology Reference
In-Depth Information
The fluid migration, as stated above, is of a pulsating nature and is acti-
vated during individual tectonic phases, especially at inversion neotectonic
stages. A definitive migration factor there, the same as in the previous
cases, is the realization of the AHFP gradients directed from the areas of
greater normalized pressure into the zones of its smaller value. This causes
the fluid to flow into the fault zones connecting intervals of oil and gas
generation with those of oil and gas accumulation. Pressure gradients' dis-
tribution in hydrodynamically-open (up to the surface) systems will show
their decline from the bottom upward through local structure sections and
their decline from the crestal, most faulted areas to the plunges and flanks.
In the hydrodynamically closed systems (a reservoir bed overlain by a seal)
pressure gradients are at maximum at the top of an interval “infected” with
AHFP. Downward, toward the source of high-pressure fluids, they decline.
Establishment of the spatial relationship between charge and discharge
areas enables, under these options of the accumulation formation, to adjust
the direction of drilling and spud wells in the areas optimal in terms of the
anticipated productivity.
An important factor in describing the conditions and formation mech-
anism of accumulations, therefore rightly directing their exploration and
appraisal, is the genetic association between AHFP and AHPP. As part of
this association, the identification of the relationships between pressure
abnormality in reservoirs and the extent of compaction of clay sequences
in contact with them is extremely important. Of special interest in this
respect is a case of permeable intervals with certain AHFP values over-
lain and underlain with maximum compacted clays totally devoid of
AHPP. The causes of such a combination of formation and pore pressures
(excluding the effect of the artesian pressure gradient or the presence of
underlying salt-bearing sequence) may be the result of invasion into the
reservoirs by a high-pressure fluid from the underlying intervals through
sequences of highly fractured and faulted argillites. Obviously, abnormal
pressure in the reservoirs, in such a case of a recent origin as the possibil-
ity of its preservation in permeable beds, is determined by the functioning
of a continuously acting “replenishment” mechanism. With such situa-
tions is also associated a possible role of endogenous processes in AHFP
formation.
Scientists have suggested that in the described combination of forma-
tion and pore pressures abnormality the former is inherited and was pre-
served because of the preceding discharge by the contacting clay members
at the time of their compaction. This suggestion is questionable because of
low probability of lengthy preservation of AHFP in permeable rocks under
