Geology Reference
In-Depth Information
dies out and is replaced by a different relaxation process,
i.e., by “resorption”, which is stretched in time. This process
involves the mass transfer via mechanisms of diffusion, cap-
illarity, osmosis, etc. and can occur independently of the rate
and direction of tectonic motions (see below).
A combined study of the above-identified correlations opens the oppor-
tunity for a quantitative evaluation of the following effects: (1) time effect
on clay compaction; (2) amount of outflow and rate of emigrating fluids;
and (3) probability of functioning, boundary conditions, and place and
role of the elision water-exchange at great depths.
The best type-model for the solution of the presented problems is the
South-Caspian Basin. The complete clay compaction (porosity < 2%) is
recorded only in the Lower Cretaceous and Middle Jurassic intervals,
which comprise a 2,000-m thick fractured argillite and shale sequence. The
overlying Santonian-Lower Campanian complexes are composed of plastic
shale varieties with up to 10% porosity at a depth of 3 to 3.5 km.
In its geologic evolution, the SE plunge of the Greater Caucasus
Meganticlinorium of Mesozoic age had undergone a significant tectonic
inversion. The inversion resulted in a large, at least 3 to 6 km, uplift between
the end of Jurassic and through Cretaceous time (Khain, 1950; Milanovsky
and Khain, 1964). Thus, it appears that clay porosity values in these inter-
vals reflect not their current depths of burial but the paleo-depths, i.e., 8
to 12 km. The compaction process of thick clay sequences in this region is
long-lasting and is completed at a depth of 10 to 12 km.
The section described above is devoid of major intraformational hia-
tuses. The tectonic movements maintained their direction over long geo-
logic periods. Under such conditions, the total compaction of the Pliocene
clays from the initial 13% porosity at a depth of 4 km until the comple-
tion of total compaction in Jurassic and Cretaceous clays have a rate of
0.09*10
−
6
%/year. This is based on the average age of the Post-Miocene
complex in the Baku Archipelago of 13 MMY and the average age of the
Jurassic and Cretaceous clays of 140 MMY at a depth of 12 km. The afore-
mentioned depth of 4 km is the upper limit of the prevalent “resorption”
process.
Inasmuch as the areal extent of Pliocene clay facies is 70,000 km
2
, clay
thickness is 4,000 m, and clay porosity changed from 13 to 1%, the amount
of squeezed-out water is 0.25 MMCM/year, 250 MMCM over 100 years,
and 250 MMCM over 1,000 years. This estimate is approximate. The change
in clay sequence thickness with the subsidence was not taken into account
because it does not affect the order of magnitude.
