Geology Reference
In-Depth Information
water squeezed out of the clayey deposits tends to move into higher per-
meability beds usually overlying and underlying the clayey deposits. It is
good to remember, however, that under specific conditions of the Pliocene
section the effect of this factor is relatively small. This happens because
the increased stratigraphic age of individual intervals of the section, zones
of their strong shale-out move toward the center of the depression. Thus,
regionally each shaled-out area of a particular stratigraphic interval is
underlain by a sandier area of the older interval.
This interrelation between permeable and low-permeability varieties in
the section results in obstructed water outflow conditions from the com-
pacting clays as they, in turn, are also overlain by clays. As a result, AHPP
is formed in them (see Chapter 3). This mechanism is manifested to its
maximum in predominantly clay facies and its intensity declines as this
facies is replaced by sand-clay alternation and the number of reservoir lay-
ers in the section increases.
The water may be partially squeezed-out into higher-permeability inter-
beds horizontally. The preconditions for this are much higher horizontal
permeability than the vertical one and parallel lamination surfaces of the
reservoirs and their replacing low-permeability varieties (Smith, Erdman
and Morris, 1972). If these conditions are not met regionally (the case of
the Pliocene section), the water release in lateral direction from the com-
pacting clay rocks can occur only at a small scale and only from limited
areas of their immediate contact with the reservoirs. Understandably, in
such a case the amount of the squeezed-out water per unit volume and per
unit time would hardly be capable of providing for and supporting large
elision flow.
Tectonic specifics of the South Caspian Depression support this conclu-
sion. As noted, the present-day structure of the depression is a tectonic
mega-element of a step-block type. The steps are separated by sublatitudi-
nal and sublongitudinal regional faults and sequentially subside from the
flank zones toward the central low (see Chapter 1, Figure 1.1). Within each
step-block, the Neogene-Quaternary interval is broken by relatively local
faults. These faults usually extend along the axes of anticlinal lines and belts
(the Kyurovdag-Neftechala, Alyat, Shubany-Karadag, Binagady-Yasma,
Fatmai-Makarov bank, Cheleken-Kumdag, Gograndag-Chikishlyar, etc.)
(See Figure 1.1). These faults are usually long, sometimes up to 100-150 km.
The amount of throw widely varies and in some cases reaches 2,000 m (the
Alyat Ridge). Morphologically, they are mostly overthrusts. Beside these
major faults, within individual anticlinal highs are developed small mostly
crosscutting faults, normal and reverse; they are found predominantly in
the crestal zones of the structures.
