Geology Reference
In-Depth Information
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Syngenetic generation of AHFP due to geostatic load and
tectonic stress.
The first of these mechanisms is supported by the continuous decline of
the formation pressure abnormality factor down the section of local struc-
tures, which are hydrodynamicly isolated from the near-surface discharge
foci.
Available information gives reason to suggest epigenetic, caused by the
injection of high-pressure fluids, AHFP origin in reservoirs of the flyshoid
Paleocene-Oligocene part of the section. It also suggests their mostly syn-
genetic nature caused by tectonic compression of closed natural reservoirs
in the Mesozoic and Miocene complex. Under the neotectonically-active
environment in the region the latter mechanism was especially important
for the upper (molasses) part of the section. It is composed of plastic clay-
evaporite rock sequence with syngenetic AHPP and includes individual
lens-shaped reservoir beds and members.
Additional factor in the formation of syngenetic AHPP and AHFP in
the deeply subsided areas of the productive series apparently were baric
effects caused by catagenetic dehydration of clay minerals and generation
of the “regenerated” water. This is appropriate for zones of elevated tem-
perature in the deep subthrust folds of the II and III structural stages. The
participation of this mechanism in the formation of abnormal pressure in
the deeply buried intervals of the subthrust structures is supported by the
presence there of maximum abnormal values with elevated hydro-mica
content in the clay's mineral composition (Gabinet, 1980). At the same
time, lower abnormality factor values in the equivalent intervals typical of
the less heated upper structural stage coincide there with elevated primary
montmorillonite content in clays.
On the whole, the hydrodynamic system of the internal mobile belt
zones in the Carpathian trough displays a highly stressed state with an
average formation pressure abnormality factor value of 1.46.
3.2.2.2
The Indolo-Kuban Foredeep
The basin includes abnormally-high pore and formation pressure in all
penetrated stratigraphic components of the sedimentary section (Upper
Jurassic through Plioceneа).
Abnormally-high pore pressure in clay intervals is diagnosed while
drilling through formations with intense oil-gas-water shows, blowouts,
swelling shales and tubing extrusion, gas-cutting of drilling fluid, tubing
sticking and overpull, narrowing of borehole, etc. Abnormally high pore
