Geology Reference
In-Depth Information
Fault orientation, their just described role as regional water-barriers
and variation patterns in the Pliocene lithofacies testify against large-
scale elision water-exchange in this stratigraphic complex, hence, against
ground water flow from the central depression to its periphery. Such
migration may occur under the specific geologic environment of flank
areas. It may occur over relatively short distances within individual
tectonic steps or parts thereof (given the needed litho-reservoir condi-
tions) and up to the water flow intersection with faults refracting this
flow. Preliminary modeling shows that under such conditions the rate of
regional lateral ground water flow enforced by the natural pressure gradi-
ents cannot exceed 0.01 cm/year. This makes the Pliocene water medium
substantially stagnant.
This conclusion is supported by the production practices in the
region's oil and gas (gas-condensate) fields. The experience with over
300 production target intervals indicates that the drives with actively
contracting fluid contacts (i.e., realizing the energy resources of the eli-
sion waterhead system), i.e., of the regional ground water mobility, are
atypical for the Pliocene complex. Major manifestations of the reservoir
energy are solution gas drive and the combination solution gas + elas-
tic water drive (with the former one dominating). Pressure gradients
between the productive and outside-the-accumulation-outline areas
arising in the process of field development are a thousand times greater
than over the natural hydraulic slopes. Nevertheless, they cannot provide
for the occurrence of a strong water drive. This leads to the conclusion
of extremely limited lateral water mobility within the Pliocene sequence
in the region.
The above data substantiate stagnant regime of the regional hydrogeo-
logical background in all intervals of the penetrated sedimentary section in
the South Caspian Depression and the domination of it by vertical ground
water cross-flows. This in turn proves objective existence of a new type
regional geo-hydrodynamical system, the cross-flow/injection one. Its
main feature is mostly through-passing vertical - from the bottom upward
directed - ground water flow through tectonic channels against the back-
ground of substantially restricted lateral water-exchange.
Two reasons add sufficient substantiation to the reviewed model and
determine its reliable match with the natural environment.
t
One, the data on the deuterium content and distribution
in syngenetic and injected ground waters of the region's
Pliocene show that the alkaline varieties are noticeably
