Geology Reference
In-Depth Information
accompanied by the appearance, evolution and active functioning of
faulting and fracturing, which provide for vertical hydrodynamic com-
munications between different intervals including those with syngenetic
AHPP.
The proposed differentiation of AHPP and AHFP by the mechanism
of their emergence (for specific geologic environments) is to a certain
extent tentative, because the pore and formation pressure abnormality
in them is most common and integral effect of several syngenetic or/
and epigenetic processes. Nevertheless, the manifestation intensity in
drilling of their diagnostic attributes provides the opportunity to iden-
tify the prevailing mechanism of abnormality generation and to modify
the strategy and tactics of exploration, drilling technology and wells
testing.
A demonstrative example in this connection is a quantitative evalu-
ation of relationships between syn- and epigenetic AHFP in the for-
mation of abnormal pressure in a number of fields in the Lower Kura
Depression (South Caspian Basin). There, the Pliocene Productive
Sequence (the main oil-gas-saturated reservoir in the region) comprises
extremely nonuniform low-permeability lithofacies containing about
80-90% clay material. This provides for a wide commonality in the com-
plex of syngenetic AHFP caused by continuing (mostly gravitational)
compaction of clay beds and members. Table 3.1 includes excess of the
formation pressure over hydrostatic pressure for outside of the oil/water
contact areas on strongly faulted oil-gas-saturated southwestern flanks
of structures with clear manifestations of vertical fluid migration. (This
is local, exceptionally in intensely-faulted areas, presence of low-salinity
alkaline waters in the underlying section, thermobaric anomalies, etc.)
It also includes excess of the formation over hydrostatic pressure for the
water-bearing weakly-faulted northeastern plunges where, based on the
indicated attributes, vertical fluid cross-flows are limited. The lithofa-
cies character of the equivalent horizons over both flanks is identical.
Thus, it is possible to believe that the excess formation pressure of the
water-bearing flanks is mostly defined by syngenetic AHFP. At the same
time, in the outside of the oil/water contact on the oil-saturated flanks it
is defined by the integral effect of syn- and epigenetic AHFP caused by
penetration into the reservoir of high-pressure agents from the underly-
ing complexes. Thus, the fraction of syngenetic AHFP may be expressed
as its ratio to total AHFP. The Table shows that a relatively small portion
(5-30%) of total excess of formation pressure in the outside-of-the-oil-
outline areas is formed on the account of syngenetic AHFP (caused by
