Geology Reference
In-Depth Information
The former function most often corresponds with the environments
of the old and stabilized uplifted young platforms. The latter one corre-
sponds with the Alpine “non-equilibrium” environment and present-day
mobile belts, activated subsiding zones of epi-Hercynian platforms and
orogens.
Upon intense and short-lived (“avalanche”) deposition of thick, facially-
uniform monomineral clay series isolated from the discharge zones, the
rate of discharge of pore fluids lags appreciably behind the speed (and
depth) of clay subsidence. This gives rise to a limited extent of consoli-
dation and the preservation at great depths of “unstable” highly plastic
high-porosity clays with high water-saturation and low density (up to
quasi-liquefaction). Sometimes thin, lithologically variable, lens-shaped
sandy clays are in contact with regionally continuous reservoirs over a
long period of time, which provides for good drainage. Under such cir-
cumstances, the clay compaction may occur rapidly, with relatively drastic
decrease in porosity.
The lag in pore fluid emigration from the subsiding clay deposits gives
rise to a high porosity and intense compression of pore fluids with resulting
abnormally-high formation pressures (sometimes of a lithostatic magni-
tude). There is a continuous outflow of pore fluids from pores in hydraulic
communication with the discharge zones. If this outflow continues for a
substantial duration of geologic time, the maximum clay compaction will
result with total loss of plasticity of clays and their conversion into frac-
tured tabular argillite-like shales.
The aforementioned is supported by the field data from a number of
the Alpine geosynclines (Padan, South Caspian, Los-Angeles, Maracaibo,
Indolo-Kuban and Terek-Caspian Troughs) and platforms of various ages
(Scythian Plate, Gulf Coast) (Tables 5.1, 5.2, 5.3, 5.5, 5.6).
The patterns of clay compaction in these regions are represented by the
porosity
vs
. depth correlation diagrams (Figure 1, 2, Tables 5.1, 5.2, 5.3, 5.5,
5.6). Some curves have been considered as the most characteristic (type
curves):
1.
Weller's curve
, which represents the practically complete
platform compaction of Paleozoic clays that during a long
geologic time experienced almost complete discharge of
their interstitial fluids (Weller, 1959).
2.
Vassoyevich curve
, which shows the effect of some lag
between the outflow of pore fluids and the subsidence
rate of the Oligocene-Miocene clays in an environment of
mildly active area of the Fore-Caucasus Hercynian platform
(Vassoyevich, 1960).
