Nuclear Geophysics by V.I. Ferronsky

Author:V.I. Ferronsky
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

3.Groundwater recharge in the past;

4.Identification of an area of groundwater recharge;

5.Relationship between aquifers;

6.Mixing proportions of groundwater of different genesis;

7.Groundwater residence time in an aquifer;

8.Relationship of waters in conjugate hydrologic basins.

More details about solving the above problems can be found in Ferronsky and Polyakov (2012).

9.7 Isotopic Composition of Formation Waters

It follows from the previous chapters that the processes of water evaporation and condensation are of great importance in the fractionation of isotopes of natural waters. At the same time evaporation is primarily an attribute of surface conditions. It might occur in shallow underground waters but it is generally agreed by hydrogeologists that groundwater evaporation does not occur on a regional scale (Zaitsev 1967; Smirnov 1971).

But in local zones underground evaporation is likely to take place. An example of such a phenomenon is the evaporation of groundwater accompanying oil and gaseous deposits (Sultanov 1961). As a rule, these processes of water evaporation occur at elevated temperatures (~ 80 °C). In this case the isotopic fractionation factor s are αD = 1.032 and . The vapour phase differs insignificantly in isotopic composition from layer waters to a deposit. The water vapour that has been formed migrates with oil gases. During the migration of the vapour-gaseous mixture through porous layers at lower temperatures, underground fresh water deposits with mineralisation less than 1 g/l and δD and δ18O values greater than those that are characteristic of meteoric waters might form.

Thus, in the region of the Dnieper-Donets depression at depths exceeding 2000 m, waters with mineralisation of up to 4 g/l and values of δD from − 21 to − 53 ‰ and 18O from − 2.5 to − 4.6 ‰ were found. In this region, for oil waters with mineralisation ranging from 150 to 330 g/l the deuterium and oxygen-18 content varies within the limits δD from − 21 to − 54 ‰ and 18O from + 2.0 to − 4.6 ‰ (Yakubovsky et al. (1978).

Analogous waters were also found in the eastern part of the Terek-Sundzha oil and gas region at a depth of 4–5.5 км (Nikanorov et al. 1980). But naturally underground evaporation cannot result in considerable changes of the isotopic composition of the layer waters since the amount of evaporated water is always negligible in comparison with the amount of native water.

The basic process determining the isotopic composition of water undergoing underground circulation is that of isotopic exchange in the water –rock system. The isotopic exchange of water with gases (H2S, H2, CH4, CO2) and liquid hydrocarbons of oil also takes place. But these processes are considerably less effective than the exchange processes with water-bearing rocks. For example, according to Soyfer et al. (1967), the change in the isotopic composition of hydrogen in groundwaters due to exchange reactions with gaseous hydrogen and hydrogen sulphide is negligible.

The absence of the influence of isotopic exchange in the H2O–H2S system upon the isotopic composition of formation waters was demonstrated by Clayton et al. (1966) and Hitchon and Friedman (1969) By indirect means the possible scale of changes of hydrogen isotopic composition in groundwater due to exchange processes with liquid hydrocarbons of the oil series can be estimated.

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