Sodium-bicarbonate groundwaters in southeastern West Siberia, Russia: Compositions, types, and formation conditions

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Highlights

  • Five main types of sodic waters in West Siberia.

  • Waters refer to meteoric and have biogenic carbon sources.

  • The formation scenario is the same: dissolution of sedimentary aluminosilicate minerals and concurrent precipitation of carbonates.

  • The diversity of HCO3-Na waters results from difference in their residence time, along with specific factors.

Abstract

Sodium-bicarbonate HCO3–Na (sodic) groundwaters exist throughout southeastern West Siberia at approximate depths from 50–300 m to 1.0–2.3 km in Mesozoic-Cenozoic sediments. They belong to five main types of fresh (I), brackish (II), high-pH low-saline (III), coal-related saline (IV), and carbonated (V) waters that differ in composition, as well as in depth and lateral extent. Waters of types I and II are of regional extent and common chemistry, while those of three other types have specific compositions and are restricted to local areas. Isotope data (δ18O, δD, δ13C) indicate that waters of all five types originated by the infiltration mechanism; type IV water has an enriched oxygen isotope composition; all water types except V have biogenic carbon sources. As shown by thermodynamic calculations, all HCO3–Na waters are nonequilibrium with many primary aluminosilicate minerals bur are equilibrated with carbonates and clay minerals. The number of minerals equilibrated with these waters increases progressively from type I to IV with salinity and pH. The obtained data allow reconstructing the formation of sodic waters of different types in the context of the evolution in the system ‘water – rock – gas – organic matter’. The formation scenario is the same for all types of water: dissolution of sedimentary aluminosilicate minerals which are not in equilibrium with the waters and concurrent precipitation of carbonates. Waters in the zone of slow water exchange at depths from 100 to 300 m acquire the HCO3–Na compositions, with TDS >0.7–0.8 g/L and рH >7.6. The diversity of the waters results from difference in their residence time, even during the formation of HCO3–Na chemistry (types I and II), and from environment effects: presence of inorganic CO2 (V) and organic carbon (IV) sources or their absence (III).

Introduction

Sodium-bicarbonate HCO3–Na (sodic) groundwaters are of broad natural occurrence and show great diversity in major-ion chemistry, compositions of their gas and organic components, as well as stable isotopes. The definitions and boundary parameters of sodic waters vary in different publications. The waters classified as sodium-bicarbonate in this study have a major-ion chemistry with (HCO3 + CO32-) and Na+ predominant among anions and cations, respectively, high pH (most often above 7.5), and a salinity of >0.6 g/L.

The origin and formation conditions of sodic waters have had a large literature (e.g., Blake, 1989; Matthess et al., 1992; Kimura, 1992; Appelo and Postma, 1994; May 1998; Jankowski and McLean, 2001; Gavrishin, 2005; Shvartsev and Wang, 2006; Shvartsev et al., 2007; Krainov et al., 2012; Popov and Abdrakhmanov, 2013; Christian et al., 2016; etc.) but remain controversial. The controversy can be overcome by viewing the problems in the context of interaction between the waters and the rocks they drain (Garrels and Mackenzie, 1967; Helgeson, 1968; Helgeson et al., 1969, 1984; Pačes, 1972; Aagaard and Helgeson, 1982; Drever, 1982; Helgeson and Murphy, 1983; Arnorsson et al., 1983; Alekseyev et al., 1997; Giggenbach, 1988; Tardy and Duplay, 1992; Nordstrom, Munoz, 1994; Grasby et al., 2000; Putnis, 2002; Hellmann et al., 2003; Fu et al., 2009; Zhu and Lu, 2009). According to the general geochemical aspect of the water-rock interaction theory (Shvartsev, 2008; Shvartsev et al., 2006, 2007, 2016, 2017, 2018), the HCO3–Na chemistry forms at a certain step of the process once water becomes saturated with respect to calcite. Water is never in equilibrium with magmatic minerals but is equilibrated with certain secondary (authigenic) minerals; the nonequilibrium causes dissolution of minerals and maintains continuous evolution of water (Shvartsev, 1991, 1994, 1995, 1997, 2010, 2012, 2013, 2014, 2015, 2016, 2017, 2019). The chemistry of groundwater interacting with rocks records the composition difference between dissolved primary minerals and precipitated authigenic phases and may include also organic compounds and gases. In this respect, HCO3–Na groundwaters widespread in thick Mesozoic-Cenozoic sediments of southeastern West Siberia are of special interest due to their diversity associated with the evolution of the ‘water – rock – gas (CH4 and CO2) – organic matter (coal, peat, etc.)’ system. This study focuses on chemical features of HCO3–Na waters and their formation conditions in southeastern West Siberia from the perspective of water-rock interaction.

Not being unique, the model of S. Shvartsev provides a universal explanation for the geological evolution of the rock-water system and the regional-scale diversity of sodium-bicarbonate waters, while the ion exchange or geological models account for specific mechanisms in this evolution.

Section snippets

Sampling and methods

The study is based on data collected by a joint team from the A.A. Trofimuk Institute of Petroleum Geology and Geophysics (Tomsk Branch) and the National Research Tomsk Polytechnical University (Tomsk) in 2000–2015 and published evidence. Groundswaters were sampled at 157 wells and 118 springs, altogether 460 samples (Fig. 1).

Highly changeable parameters (pH, temperature, and electrical conductivity) were measured in the field using a portable digital AMTAST AMT03 m (USA). In the field, the

Study area

Sodium-bicarbonate waters were studied in the southeastern part of the West Siberian artesian basin (including two smaller basins of Chulym-Yenisei and Middle Ob) and in the northern Altai-Sayan fold area (including the Kuznetsk artesian basin and the Salair and Kolyvan-Tom fold area). The Kuznetsk and Chulym-Yenisei basins (Fig. 1) are especially advantageous for studying these waters.

Chulym-Yenisei basin is located in the southeastern West Siberian artesian basin, mainly within the Tomsk

Results

Sodium-bicarbonate waters are of regional extent in southeastern West Siberia and occur in Mesozoic-Cenozoic sediments at depths from 30 to 300 to 1000–2300 m in the zone of mainly slow water exchange, between fresh HCO3–Ca waters above and saline Cl–Na waters below. They are chemically diverse (with 70–100% NaHCO3), with TDS from 0.2 g/L to 25 g/L; рH from 6.3 to 10.3; the gas phase consisting of methane, nitrogen, and carbon dioxide; high contents of organic components and low dissolved

Discussion

The obtained results were used to model the formation of HCO3–Na waters with regard to specificity of their different types (Fig. 7). Water percolating through rocks continuously dissolves (hydrolyzes) nonequilibrium minerals. These reactions release elements (Cа, Mg, Na, Fe, K, Al, Si, etc.) at different rates and produce assemblages of authigenic phases. Water molecules are likewise involved into hydrolysis: they decompose into H+ and OH and form a particular chemical environment. H+ binds

Conclusions

1. Sodium-bicarbonate groundwaters occur at depths from 50 to 300 to 1000–2300 m in Mesozoic-Cenozoic sediments throughout southeastern West Siberia. They are compositionally diverse and belong to five different types distinguished according to their compositions, depths, and extent. All waters are of infiltration origin and all have a biogenic source of CO2, except for type V where some CO2 comes from an inorganic source below the sodic water zone.

  • 2.

    The formation conditions of HCO3–Na waters

Acknowledgment

The paper is dedicated to the memory of Professor Stepan L. Shvartsev whose valuable advice helped me to prepare my doctor thesis and whose ideas lie at the base of this study.

The work was supported by grant 17-17-01158 from the Russian Science Foundation, by grant 20-05-00127 from the Russian Foundation for Basic Research.

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