Geology Reference
In-Depth Information
fields) with practically nonexistent infiltration are the preserved deposi-
tional chlorine-calcium solutions.
The initial chlorine-calcium water medium in all intervals of the Middle-
Upper Miocene complex is objectively supported by the exactly this type
waters in the local stagnant (dead-end) areas of some reservoirs; they are
fault-trapped, which preserved them from being replaced by the infiltra-
tion agents. The background salinity values in the syndepositional chlo-
rine-calcium solutions are: for the Chokrackian horizon 50-60 mg-equiv.
(14.7-17.6 g/l), for the Karaganian horizon 40-50 mg-equiv. (11.8-14.7
g/l), for the Sarmatian Stage 180-200 mg-equiv. (52.9-58.8 g/l), for the
Meothic Stage 40-50 mg-equiv. (11.8-14.7 g/l).
The formation of ground water zoning in the Middle and Upper
Miocene is strongly affected by interformational fluid crossflows through
faults. They involve a wide stratigraphic range from the Jurassic to the
Chokrackian and determine the appearance of injection type positive
hydrochemical. Total salinity values in these anomalies substantially exceed
the background values. One example is a case described by Sulin (1935) of
high salinity ground water (964.4 mg-equiv. or 285.4 g/l) derived from the
Chokrackian in the Datykh field, which is typical of Upper Jurassic evapo-
rite facies (Volobuyev and Sokirko, 1982).
A general pattern in all horizons of the complex is the continuous salin-
ity and secondary salinity factor growth (in the syndepositional chlo-
rine-calcium solutions) with the stratigraphic depth. Also typical is the
association with hard water areas and positive hydrochemical anomalies of
the cross-flow genesis at maximum hydrocarbon saturation.
A specific feature in the ground waters of all waterhead complexes
in the Tersk-Caspian Foredeep (trough) is the presence (beginning at
about 1,500 m) and gradual increase in the sulphate concentration with
the stratigraphic and physical depth from 0.5 mg-equiv. in the upper to
4.0 mg-equiv. in the lower intervals (see Figure 2.22). This correlation is
approximated by the following equation: SO
2
−
= 2.28
H
1.639
.
The correlation between the sulphate contents and the formation tem-
perature (which restricts the activity of the sulphate-reducing bacteria) is
as follows: SO
2
−
= 2.51
⋅
10
-6
⋅
t
2.750
(see Figure 2.23).
The following conclusions are proposed based on the hydrochemistry
field data in the region:
⋅
10
-6
⋅
t
The vertical hydrochemical zoning within the drilled strati-
graphic range (Pliocene through the Upper Jurassic) is
normal.
