Geology Reference
In-Depth Information
the Middle Pliocene interval is also a certain salinity value of no greater
than 180-210 mg.-equiv. (51.5-60 g/l) (Table 2.8, see Figure 2.8). Also, in
all areas they are associated with certain depth not dependent on stratig-
raphy and positioned 900-1,200 m above the Base of the Middle Pliocene.
The formation of so specific a composition and the water presence only
in a narrow depth interval cannot be explained by any known metamorphic
process of the source marine (ooze) waters. Chemically, they can only be
a result of mixing hard and alkaline waters. It is supported by the presence
in the transitional zones in some cases of class
S
1
a
waters (Abramovich
et al
., 1934; Sulin, 1935). Typically hard waters in various formations and
horizons convert to alkaline ones at about equal salinity values, i.e., water
conversion from one type to another occurs at certain proportions of the
mixing ingredients.
We believe that the water-mixing mechanism works as follows. When
chlorine-calcium waters interact with hydrocarbonate-sodium, carbonates
of the alkaline enter into an exchange reaction with calcium and magnesium
chlorides, the primary salinity and secondary alkalinity factors increase
and secondary salinity decrease. The result is waters primary salinity factor
of about 92
8%-equiv.
Waters of various hydrochemical types are found in transitional zones.
The reason is that in the mixing process form formation water mixes with
different ratios of source components. A range of intermediate “transi-
tional” types is created. Each is determined by the fractions of components
in the mix. When the chlorine-calcium component dominates, “transi-
tional” chlorine-calcium and chlorine-magnesium waters form, when
hydrocarbonate-sodium prevails. “Transitional” sulphate-sodium and
alkaline varieties occur.
The dependence of the ground water chemistry on the lithology of
enclosing rocks (
the direct correlation between the rocks' clay content and
waters' total
salinity) (Mekhtiyev and Rachinsky, 1968; Samedov, Akhundov
and Rachinsky, 1963) can also be explained only through the mixing pro-
cesses. It is noteworthy that the water salinity increase in higher-clay areas
is always accompanied by changes in their ion-salt composition (increase
in the secondary salinity factor in hard waters or decrease in the primary
alkalinity factor in hydrocarbonate-sodium ones). This association occurs
due to the mixing because of low probability of its occurrence due to cation
exchange (which does not result in salinity increase) and leaching of clay
rocks by filtering waters (the filtration flow inversely depends on the clay
content).
In our view, alkaline waters invading the rocks through the faults mostly
developed in the crestal zones of structures displace syndepositional
−
99%-equiv. and secondary alkalinity - 6
−
