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URANIUM AND THORIUM IN THE WATER-ROCK SYSTEM (ON THE EXAMPLE OF OIL-AND-GAS BEARING DEPOSITS IN NORTHERN REGIONS OF WEST SIBERIA)

D. A. Novikov 1,2, L. D. Vakulenko 1,2, P. A. Yan 1,2, A. A. Khvashchevskaya 3

1 A.A.Trofimuk Institute of Petroleum Geology and Geophysics SB RAS, Novosibirsk, Russia; 2 Novosibirsk State University, Novosibirsk, Russia; 3 Tomsk Polytechnic University, Tomsk, Russia

The paper presents the first data on the distribution of uranium and thorium in the underground waters of Mesozoic sediments of northern regions of the West Siberian oil and gas basin. The studied waters are very diverse in chemical composition and the amount of total mineralization (from ultrafresh to weak brines). The contents of natural radionuclides vary in a wide range (mg/cu dm): U (4,90•10–7–5,14•10–3), Th (1,90•10–6–8,93•10–3). The Th/U ratio varies from 0.07 to 93.1 and its maximum values characterize the waters of the Valanginian aquifers. The Th/U ratio in water – resistant rocks is in a rather narrow range, it amounts to 1.82–5.24, with an average of 3.56. Against this background, rocks of the Bazhenov Formation are distinguished by the lowest values of Th/ U = 0.07–1.71, its nature of radioactivity is associated with uranium. The formation of the radionuclide composition of underground waters of oil-and-gas-bearing deposits with high Th/U ratios took place in a sharply reducing medium, where thorium has a greater migration capacity than uranium. The evolution of the sedimentary rock basin has led to the widespread development of erosion processes and the redistribution of U and Th in the water-rock system.

Keywords: hydrogeochemistry, uranium, thorium, water-rock system, northern regions of West Siberia.

DOI 10.20403/2078-0575-2023-3-78-88

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