Geology Reference
In-Depth Information
the creation of each specific case (region, area, zone, etc.) and implementa-
tion of measures minimizing technical difficulties and economical risks in
conducting exploratory operations, well drilling and well testing.
Syngenetic AHPP is typical of stratigraphically young clay sequences,
thick, continuous and consolidating in an unbalanced way. They are associ-
ated predominantly with their gravity [geo(litho)static] and tectonic com-
paction and with dehydration processes. In the former case, we are dealing
with areas where low-permeability clayey formations were accumulat-
ing for a long time. There, intense compensated sediment subsidence and
rather restrained conditions for the release of pore fluid from rocks under
consolidation result in significant delay (restriction) in the outflow of the
pore water compared with the subsidence rate. This causes the cumulative
compression of pore fluids to pressures sometimes close to the geostatic
one. That in turn determines the preservation of high porosity (and elevated
water-saturation) in the clays at great depth. Thus, it forms in a rapidly sub-
siding unconsolidated clayey sequence, sometimes even with individual
quasi-liquid members, regionally developed abnormally-high pressured
pore space fluids without any association with structural conditions.
In tectonically active areas, subvertical motions of basement fault-
blocks and horizontal neotectonic stress project into the sediment cover.
This causes the geotectonic compaction of clayey formations accompa-
nied by geologically instantaneous compression of the enclosed fluids and
emergence of syngenetic AHPP.
Under the former option emergenceе of the AHPP is caused by par-
tial carrying by pore fluids the weight of the accumulating overburden.
It occurs under a compressing stress, which relatively slowly increases
with time and is directed mostly down. Under the latter option, AHPP is
a result of effective pressure (positive difference between geotectonic and
lithostatic pressures). The result is a spasmodic decrease in the pore space
volume, which occurs practically instantaneously according to a model of
the confining, vertical or lateral compression. Obviously, maximum com-
paction of clays occurs along the direction of applied force: under mostly
gravity compaction the upper intervals of clay sequences are most consoli-
dated and under vertical-fault-block tectonic motions, the lower ones, etc.
Under a mostly single-directional load, the clayey formations compact
rather monotonously and without substantial change in the pore space
geometry and clay texture. In conditions of a multi-directional tectonic
compression, their structure is usually deformed, sometimes with matrix
rearrangement (the so-called “crushed” clays). The presence of these clays
in the section is an indication of geotectonic consolidation superposed on
the normal process. However, it is so only if accompanied by abnormally
Search WWH ::




Custom Search