Geology Reference
In-Depth Information
5.3.2
Abnormally-High Pore and Formation Pressures as
Indicators of Fluid Migration
Present-day natural status of the geo-fluid-dynamic systems is in cause-
and-effect relationship with the spatial position in them of areas with dif-
ferent levels of the reservoir energy and hydraulic potential. This enables
the use of pore and formation pressure and their gradients' vertical and
lateral distributions as indicators of energy resources in natural reser-
voirs, of the direction and extent of their realization at the current stage of
a regions' geologic evolution. In a general case pore and formation pres-
sure and its gradients equal or close to the hydrostatic correspond with the
environment of relative fluid mobility and its fulfilled (or in the process of
fulfillment) migration. Abnormally high pore and formation pressure in
geo-fluid-dynamic5
5
systems and high gradients indicate a stressed hydrau-
lic field, obstruction (limitation) or absence of discharge in the system.
At the same time what should be kept in mind is that AHPP and AHFP
development in some areas and section intervals, and as a result the exis-
tence in the systems of normalized pressure gradients, is only the neces-
sary but not in all cases sufficient factor for the implementation of regional
or interformational ground water and accompanying hydrocarbonate
migration. These factors describe mostly system's potential capabilities and
should not be interpreted as the indication of water medium mobility. The
implementation is only possible when the complex of sufficient factors is
available. Most significative among those are:
t
Favorable lithofacies and reservoir environment providing
for the possibility of migration.
t
No tectonic, lithological and/or hydrodynamical barriers to
the ground water flow.
t
The existence and conductivity of vertical communica-
tion channels (faults, hydrogeological “windows”, zones of
elevated fracturing, mud volcano eruption throats, contact
zones of exo- and cryptodiapir plugs with emplacing rocks).
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It is necessary to distinguish between AHFP in a hydrodynamic system (outside
of the accumulation outlines, in the aquifer areas of reservoirs) and the excess
pressure within the accumulations caused by the difference in specific gravity
between oil, gas and water. Here and thereafter the writers treat AHFP (except
when stated otherwise) as abnormally-high formation pressure outside the out-
lines of productive horizons and in wet intervals.
