Geology Reference
In-Depth Information
compacted (fractured, low-plasticity, argillite-like and schistose) mem-
bers in individual intervals of thick clayey sequences either synchronous
with tectonic impulses or asynchronous but affected by them. The gravity
clay compaction occurs mostly at the immersion (compensated) stages of
sediment subsidence and the geotectonic compaction, most frequently at
inversion stages in the geologic evolution of the area.
Syngenetic AHPP of clearly gravitational origins in some uncompen-
sated by the deposition present-day (onshore) regions manifests their
fossilized nature (the preservation of previous compensated deposition
stages). Such AHPP unrelaxed over geologic time testifies to a slow relax-
ation, possibly long preservation and, hence, to a significant duration of
the clay sequence consolidation process.
There is another important formation mechanism of syngenetic pore
pressure in clays and formation pressure in substantially clayey reservoirs.
It is the dehydration of smectite group clay minerals, mostly montmorillon-
ite (М.Powers, J.Burst, D. Perry, J.Hower, K.Magara, W.Fertl, G.Chilingar,
N.A.Yeremenko, L.N.Kapchenko, A.A.Kartsev, M.Z.Rachinsky,
V.N.Kholodov, V.F.Simonenko, etc.). It is mostly controlled by the temper-
ature conditions. Its substance is in the release under certain environments
(mostly in the high-temperature zone over 105°С: Kartsev, Vagin, 1973;
Burst, 1969; Powers, 1976) of significant additional volumes of the consti-
tutional water from the clay minerals' lattice and interlaminar space with a
corresponding increase in the pore pressure in matrix. As the deposits sub-
side and the temperature grows, its role in the abnormal pressure genera-
tion in clay rocks progressively increases but, then, as the montmorillonite
transformation into hydro-micas ends, it gradually declines and eventually
the process dies out.
The formation of syngenetic AHPP in montmorillonite clays is a result
of two sequential processes. The major factor at the first stage (diagene-
sis-early catagenesis) is gravitational load from the overburden. It caused
compression in clays of the free syndepositional pore water and slowed
down pressing-out of fluids. These in turn determine a decrease in the pore
space volume. The dominating factor at the second stage (catagenesis) is
the generation in the clay's pore spaces of some dehydration “regenerated”
water excessive for this volume. This is accompanied sometimes by partial
decompaction.
In some cases, the extent of clay pore space volume increment may
reach 20-40% (Beka and Vysotsky, 1976; Bro, 1980; Kartsev and Vagin,
1973; Perry, 1969). In orogenic stages of the geologic evolution or dur-
ing its seismoactive cycles the stated processes can be superposed by the
geo(neo)tectonic AHPP generation. In the real geologic environment of
