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Surface infiltration is not significant, however. It practically does not
affect the regional ion-salt composition, salinity and ground water type
distribution. The following reasons restrict the penetration of meteoric
waters into the Paleogene-Miocene rocks: the variability and lens-shaped
nature of the section; low permeability of the reservoirs; their exposure
over insignificant areas; the fault-block structure of local highs obstruct-
ing the infiltrated water migration downdip the reservoirs; low amount of
precipitation and others.
Based on this analysis, it is believed that the major factor in the ground
water desalination within the local highs is the low-salinity high-alkaline
waters that penetrated into the Paleogene-Miocene rocks from the under-
lying sediment complexes. Syngenetic nature of these waters with the
enclosing rocks cannot be accepted due to several factors: their continu-
ous association with hydrocarbons; their much lower salinity compared
with the hard waters; the peculiarity of their chemical composition; their
association with the maximum exposed areas; and the faulting and mud
volcanism.
The establishment of the mostly meteoric nature of the sulphate-sodium
waters and mostly depth origin of the alkaline waters determines the envi-
ronment for the existence of the chlorine-calcium waters in the region.
Taking into account their occurrence in the areas and on the structures
located outside the influence of surface factors and in conditions restrict-
ing the hydraulic communications with the underlying intervals, it is obvi-
ous that these waters, being sedimentary in their origins, create the major
hydrochemical background and cause its elevated salinity (up to 300 mg-
equiv.) and hardness ( S 2 up to 25%-equiv.).
Therefore, the conducted studies give reason to believe that under-
ground waters of the Oligocene-Miocene sediments are in effect mixtures
of waters of various origins. The basic mechanism defining the presence
of water of different geochemical types and salinities is mixing in vari-
ous proportions of the background high-salinity chlorine-calcium water
with depth-originated alkaline water. This mixing occurs under a geologic
environment facilitating the discharge of the latter (and is accompanied
by commercial or noncommercial oil and gas occurrences). Mixing also
occurs with the infiltration sulphate-sodium water in limited areas where
rocks are exposed on the surface.
For the region of a complex geology, with the maximum exposed and
eroded Paleogene-Miocene areas being most faulted and folded, it is
unlikely to identify in pure form the effect of each stated factor. Most likely,
under the actual environment of local structures both mechanisms work
simultaneously. Depending on the combination of specific conditions
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