SOME DATA ON THE ISOTOPIC COMPOSITION OF HYDROGEN IN OILFIELD WATERS OF TOMSK OBLAST

P. A. Udodov, V. G. Ivanov, A. D. Nazarov

(Tomsk Polytechnic Institute)

The isotopic composition of hydrogen in waters from oilfields was determined in the laboratory for isotopic analysis of water of the Scientific Research Institute of Hydrogeology at Tomsk Polytechnic Institute. All the necessary apparatus for analysis by the float method was manufactured during the period from 1964 to 1966 by the institute’s staff.

Water samples were taken at the Sovetskoye field (Kulemskaya, Vartovskaya, Pokurskaya suites) by A. D. Nazarov, assistant of the Department of Hydrogeology and Engineering Geology. The isotopic analysis of the waters was carried out under the supervision of V. G. Ivanov, a postgraduate student of this department. The float, or flotation, method, based on the difference in density between light and heavy water, makes it possible to determine the deviation in the content of heavy hydrogen isotopes in the water sample under investigation from water adopted as the standard. Usually water from large lowland rivers is taken as the standard, since it is characterized by the greatest constancy and an averaged content of hydrogen and oxygen isotopes. We used as the standard water from the Tom River, sampled in the summer of 1965.

The process of isotopic analysis of water by the flotation method consists in determining the temperature of flotation equilibrium, i.e., the temperature at which a float immersed in the water is in equilibrium—neither sinking nor rising. The temperature of flotation equilibrium is determined in one and the same quartz test tube first for the standard water and then for the water under investigation. The temperature difference makes it possible to calculate the deviation of the density of the water under investigation from the density of the standard.

Since the density of water depends not only on the content of isotopes, but also on the presence of dissolved substances and suspensions, before density measurements are made the water is carefully purified. This purification consists of fourfold distillation of the water with two oxidations of organic contaminants by potassium permanganate alternately in acidic and alkaline media. Phosphoric anhydride is used to create the acidic medium, and barium oxide to create the alkaline medium. The purification apparatus is made of quartz glass with ground joints.

The density of purified water depends both on the presence of heavy-hydrogen water, \(\mathrm{D_2O^{16}}\), and on the presence of heavy-oxygen water, \(\mathrm{H_2O^{18}}\). To eliminate the effect of heavy oxygen on density, normalization of the isotopic composition of the standard and investigated water is carried out according to

oxygen, which consists in passing water vapor and gaseous oxygen through a quartz tube with a catalyst. The catalyst is a specially prepared manganese oxide. The quartz tube is equipped with electric heating, ensuring that a temperature of 500°C is maintained. This procedure is described in greater detail in the literature (Brodsky, 1957; Shatenshtein et al., 1957; Grinberg and Petrikovskaya, 1965). We made only certain improvements. Determination of the isotopic composition of oxygen by this method, as our experience has shown, does not give satisfactory results, since in the process of purifying water, in order to oxidize organic contaminants, as has already been noted, oxygen-containing substances are used. Isotopic exchange occurs between the oxygen of these substances and the oxygen of the water (Alekseev and Kontsova, 1964), as a result of which the natural content of heavy oxygen in the waters under study changes.

Determination of deuterium by this method, however, is quite reliable, with an accuracy of 0.3–0.5γ. Oilfield waters containing admixtures of oil were additionally passed, prior to purification, through an adsorption-chromatographic column filled with granulated activated carbon of grade AR-3. In this column adsorption of oil admixtures on activated carbon took place (Grinberg and Petrikovskaya, 1965).

Table 1 presents the results of determining the isotopic composition of hydrogen in waters from oil fields.

As is evident from the table, in waters directly in contact with oil, the deuterium content is reduced. In waters not associated with oil, there is a regular increase in deuterium content with depth. Thus, groundwater (a well in Sosnino) is characterized by a reduced deuterium content (−0.9γ), which indicates its meteoric recharge, since atmospheric precipitation is depleted in deuterium. Waters of deeper horizons, however, are enriched in deuterium up to 2.5γ (well 318, well 3—r), which indicates a remote recharge area. Similar data were obtained by us in the southern part of Tomsk Oblast within the Kolyvan–Tomsk folded zone, where the waters of loose Quaternary deposits are characterized by a deuterium content close to that of atmospheric precipitation and surface watercourses (from −1γ to +1γ), whereas the waters of zones of rupture disturbances, having a remote recharge area and a great depth of circulation, are enriched in deuterium up to 3.5γ.

The first suggestion of a regular increase in deuterium content with depth was made by V. I. Vernadsky (1960) and was subsequently confirmed by a number of authors. A. E. Babinets (1961) indicates that, for groundwater in the southwest of the Russian Platform, a relationship is established between groundwaters of increased density and a longer residence in the strata of rocks.

According to A. G. Vasil’ev and co-workers (1958), infiltration waters usually have an isotopic composition close to that of waters of atmospheric precipitation or rivers, i.e., they do not contain heavy isotopes in elevated amounts. Relict waters, however, should have an isotopic composition close to that of marine waters. An increase in deuterium content with depth is also indicated by F. A. Alekseev and V. V. Kontsova (1964) and other investigators.

The chemical composition of waters of the Sovetsko-Sosninsko-Medvedevskoe field area (Table 1), determined in the Problem Geological Laboratory, is characterized by a regular increase in mineralization with depth.

Surface waters are fresh (0.3–0.4 g/l), bicarbonate–magnesium–calcium. Groundwaters (a well in the village of Sosnino) are fresh

Table 1

Chemical and isotopic composition of the waters studied

(0.17 g/l) of mixed composition with a slightly acidic medium (pH 6.2). Silicic acid (3.6 mg/l) and an elevated sulfate content (16 mg/l) are noted in them. Among the microcomponents determined by the TPI method, copper, zinc, strontium, titanium, and manganese were found in the groundwaters.

The waters of the Pokur Formation (well 318) are saline (17.5 g/l), sodium-chloride, slightly alkaline, with an iodine content of 0.9 mg/l, bromine 40.5 mg/l, and silicic acid 42.4 mg/l. Among the microcomponents in the waters, copper, zinc, silver, strontium, titanium, manganese, and barium were found.

The waters of the Vartov Formation, sampled in wells Nos. 19-r and 29-r, are characterized by a sodium-chloride composition, a neutral medium, elevated (in comparison with the waters of the Pokur Formation) contents of bromine (53.1 mg/l), iodine (3.4 mg/l), and silicic acid (150.1 mg/l), and increased mineralization (19.55 g/l).

The waters of the Kulomzino Formation (well 3-r) are characterized by still greater mineralization (22.5 g/l), a chloride–calcium–sodium composition, an alkaline medium (pH 7.7), elevated contents of iodine (10.9 mg/l) and bromine (66.0 mg/l), and a somewhat reduced content of silicic acid. Among the microcomponents in the waters, copper, zinc, silver, manganese, titanium, and barium were noted.

Thus, the chemical composition of the waters shows that, with depth, the degree of metamorphism of the waters and the proportion of sedimentation and pore waters increase. As the proportion of sedimentation waters increases, the deuterium content should also increase. This increase is also observed, as already noted above, in the region under study. However, in waters that are in direct contact with oil, this regularity is disturbed. These waters are characterized by a reduced deuterium content, which is undoubtedly caused by the influence of the oil pool.

Thus, for the waters of wells 19-r and 29-r the depletion in deuterium is, respectively, −0.6γ and −1.3γ. During testing of the indicated wells from the formation, inflows of water with a film of oil were obtained; from well No. 19-r, oil was even collected with a water content of only 25%, which after some time settled and for a still longer time remained in contact with the oil.

In addition, we studied water obtained as a result of separation of an emulsion. The emulsion was taken at the mouth of well No. 18-r of the Sovetskaya area from the upper part of horizon B-VIII (2159–2164). The water content in this emulsion was up to 2%. The well is located near the internal oil–water contact, and an inflow of bottom waters to it is quite possible; by their composition these should be close to the waters of well 3-r. This water is in the closest contact with oil, and therefore the influence of the latter is greatest here. It is very noteworthy that this water is characterized by the most reduced deuterium content (−2.2γ). It is possible that the degree of depletion increased in the process of separating the water from the oil. Separation was carried out in an autoclave at a pressure of 4–4.5 atmospheres (formation pressure 215.6 atm) and a temperature of 150°C (formation temperature about 69°C). Under these conditions, in the autoclave, with mechanical stirring, a significant part of the water and oil passed into the gaseous state; then, when the autoclave was cooled, the water condensed and accumulated at its bottom, from where it was then collected through a drain fitting. Thus, at high temperature during the separation process, additional isotopic exchange between the hydrogen of the water and the oil is possible. In the future the authors intend to isolate this water by another method, without heating and pressure, and to compare the results.

A significant change in the chemical composition of this water is evidenced—

there is low mineralization (1.48 g/l), an acidic medium (pH 5.75), and a chloride–magnesium–sodium composition.

The reduced deuterium content in waters that are in direct contact with oil is possibly explained by processes of separation of deuterium by oil, as also indicated by A. E. Babinets (1961). Unfortunately, the authors do not have data on the deuterium content in the oil itself from the Sovetsko-Sosninsko-Medvedevskoe field.

It should be noted that the literature data on the deuterium content in oil and oilfield waters are rather contradictory. One group of researchers (Alekseev and Kontsova, 1964; Grinberg and Petrikovskaya, 1965; Uklonskii et al., 1965; Filippova, 1935) presents data indicating an elevated deuterium content in oils. In their opinion, isotopic exchange occurs at the contact between water and oil, as a result of which the water is enriched in deuterium.

Thus, for example, according to the data of I. V. Grinberg and M. E. Petrikovskaya (1965), the excess density due to deuterium of oilfield waters of the Carpathian region reaches 7.6γ (Table 2).

As can be seen from the table, the excess density of oilfield waters is lower than the excess density of the “combustion waters” of the corresponding oils. Oils with a higher deuterium concentration are accompanied by oilfield waters enriched in deuterium. These authors state that in the oilfield waters of the Carpathian region no clear dependence is observed of the deuterium content on the stratigraphic age of the enclosing rocks, on the specific gravity of the waters, or on their mineralization.

Table 2

Isotopic composition of hydrogen in oils and oilfield waters of the Carpathian region
(I. V. Grinberg and M. E. Petrikovskaya, 1965)

According to other authors (Soifer et al., 1967), oil does not contain deuterium in increased amounts and is even sometimes depleted in it. The authors named used different methods of isotopic analysis of water. The first group of authors bases itself on the results of the flotation me-

In the case of isotopic analysis of hydrogen in oil, it was first combusted and “combustion water” was obtained. Further determination of deuterium was carried out by the flotation method.

Another group of authors uses the photoneutron method. In their opinion, combustion of oil and flotation analysis of “combustion waters” give overestimated results.

On the other hand, the possibility cannot be excluded that different oil fields, formed under different conditions, are characterized by different deuterium contents. R. G. Pankina (1967) indicates that, if oil formation occurred in isolated basins or in lagoons with elevated salinity and intensive sulfate-reduction processes, then such oils should be characterized by sulfur enriched in \(S^{34}\) and by hydrogen depleted in deuterium. Oils whose genesis is associated with freshened basins should be characterized by a higher content of \(S^{32}\) and deuterium. In this connection, the study of the isotopic composition of hydrogen in oil and oilfield waters of Western Siberia is of great theoretical and practical interest.

Thus, the investigations carried out on the isotopic composition of waters of oil fields in Tomsk Oblast make it possible to note the influence of the oil pool on the deuterium content in the waters.

Study of the isotopic composition of hydrogen in oil, of the process of isotopic exchange between the hydrogen of water and oil, and determination of the equilibrium constants of this reaction will make it possible to explain the reason for the lowered deuterium content in the oilfield waters of Tomsk Oblast and to outline criteria for the practical use of this phenomenon in the course of prospecting and exploration for oil fields.

REFERENCES

1. Alekseev F. A., Kontsova V. V. 1964. On the geochemistry of deuterium in groundwaters, in the collection Direct Prospecting for Oil and Gas. Nedra.

2. Babinets A. E. 1961. Groundwaters of the Southwest of the Russian Platform. Distribution and Conditions of Formation. Publishing House of the Academy of Sciences of the Ukrainian SSR.

3. Brodskii A. I. 1957. Chemistry of Isotopes. Publishing House of the Academy of Sciences of the USSR.

4. Vasil’ev A. G. et al. 1958. Investigation of groundwaters by the method of isotopic analysis. Proceedings of the Kharkov Department of the All-Union Chemical Society named after D. I. Mendeleev, vol. 1.

5. Vernadskii V. I. 1960. The History of Natural Waters. Selected Works, vol. IV, book 2.

6. Gavrilenko E. S. 1965. Hydrochemical Indicators of Oil-Bearing Potential According to the Salt and Isotopic Compositions of Groundwaters. Kiev. Naukova dumka.

7. Grinberg I. V. and Petrikovskaya M. E. 1965. Investigation of the Isotopic Composition of Organic Matter of Combustible Minerals. Kiev. Naukova dumka. 1965.

8. Molochnova V. A., Sokolov M. M., Gorev A. V., Butrov N. M. 1967. On the question of the deuterium content in natural waters. Geochemistry, No. 5.

9. Pankina R. G. 1967. The isotopic composition of sulfur and hydrogen of oils as an indicator of certain paleogeochemical features of a petroleum source basin. Geochemistry, No. 5.

10. Soifer V. N., Brezgunov V. S., Vlasova L. S. 1967. The role of stable hydrogen isotopes in the study of geological processes. Geochemistry, No. 5.

11. Uklonskii A. S. et al. 1965. Isotopic Composition of the Waters of Uzbekistan. Nauka, Uzbek SSR.

12. Shatenshtein A. I. et al. 1957. Isotopic Analysis of Water. Publishing House of the Academy of Sciences of the USSR.