On Modern Mineral Formation from Deep Mineralized Waters of the West Siberian Oil-and-Gas-Bearing Province

A. F. Korobeinikov, A. D. Nazarov

(Submitted by the Department of Geology and Exploration of Mineral Deposits)

The question of mineral formation from underground waters and brines has been discussed in the literature, but for the region of Western Siberia modern mineral deposition from underground waters has not yet been specially studied. In this connection, we investigated newly formed carbonates from mineralized underground waters of the Tara Suite in the area of the village of Chazhemto, Tomsk Oblast (Kolpashevo well 5-R).

The study of the products of modern mineral formation pursued three main aims. First, to determine the composition of mineralized underground waters; second, to attempt to clarify the features of the process of mineral formation at the mouths of self-flowing wells in oil-and-gas-bearing areas; and, third, to carry out experimental studies of liquid and gas–liquid inclusions in newly formed minerals in order to confirm the applicability of the method of aqueous extracts from soluble minerals for determining the composition of mineral-forming solutions. The latter was prompted by the fact that, although the question of the possibility of determining the aggregate state of mineral-forming solutions, their temperature, and pressure is resolved positively [4, 5, 6, 7, 9, 10, 12, 15, 16], with respect to the use of information on the chemical composition of inclusions obtained from aqueous extracts for clarifying the chemistry of mineral-forming solutions, a number of researchers have doubts and objections [1, 8, 10, 11]. Therefore, the authors carried out analytical investigations of aqueous extracts of the contents of inclusions in calcites in order to determine the qualitative and quantitative reproducibility of mineral-forming solutions, using carbonates as an example.

Well 5-R was drilled in 1958 for oil-prospecting purposes. In the depth interval 2122–2126 m it penetrated thermal waters in deposits of the Tara Suite of Lower Cretaceous age, composed of calcareous sandstones with interbeds of aleurolites [3]. The initial yield of the well was 835 m³/day; later it decreased to 510 m³/day. The flow rate of the released gas was about 78 m³/day. The excess pressure at the wellhead reached 6 atm. The temperature of the self-flowing waters at the wellhead is 66°C, pH 7.5.

In chemical composition the waters are essentially sodium chloride waters, with a total mineralization of 5.05 g/L and a nitrogen–methane composition of dissolved gases (Table 1). In terms of the content of specific components the waters are sulfide-bearing. Over 10 years of free self-flow, the temperature, mineralization, sulfide content, and chemical and gas composition rema-

Table 1

Chemical composition of the calcite being formed (Kolpashevskaya well 5-R)

Composition of impurity elements: Mn—0.07, Cr—0.03, Ti, Cu—0.001, Co—traces.

remained constant. Their formation occurred as a result of processes of mixing and metamorphization of sedimentation and infiltration waters. The result of the action of infiltration waters may be considered the destruction of a small hydrocarbon deposit and the formation, in marine deposits, of modern sulfide waters of low mineralization.

When mineralized waters flow out at the wellhead, a large quantity of gas is released, as a result of which the jet of thermal water is sprayed onto the remaining base of the drilling derrick. On the walls and upper platform of the base of the drilling installation, boards, and other objects, knobby crusts of radially fibrous calcite up to 6–8 cm thick are formed (over 10 years of self-flowing discharge from the well). The area of modern mineral formation is about 10 m²; the average thickness of the carbonate layer being formed is about 3 cm. The surface of the carbonate crusts is usually knobby. In section, the newly formed calcites are thin-columnar aggregates of banded structure. The banding is caused by the presence of very thin (fractions of a millimeter to 1 mm) interlayers of organic matter and sulfur, separating the calcite aggregates into 3–4 layers, each 0.8–1.3 cm thick. At the same time, the distinctness of manifestation of these interlayers decreases from top to bottom. It appears that the lower interlayers of sulfur and organic matter, as the subsequent layers of columnar calcite aggregates accumulated, were gradually leached out.

The deposition of minerals is probably connected with the sharp temperature gradient of the mineralized waters at the wellhead (a temperature drop from 66°C at the wellhead to 22–37°C at the surface where they are sprayed). The formation of coatings of native sulfur on the surface of separate calcite layers can evidently be explained by the oxidation of hydrogen sulfide and hydrosulfides by atmospheric oxygen, or as a result of the reaction of sulfurous gases with one another, with the formation of elemental sulfur

\[ 4\mathrm{H}_2\mathrm{S} + 2\mathrm{SO}_2 = 4\mathrm{H}_2\mathrm{O} + 3\mathrm{S}_2 \ [2]. \]

Deposition of calcite from mineralized groundwaters is not observed either in the streams or in the Chaya River, into which the self-flowing groundwaters from the well drain. Before entering the river, the temperature of the thermal waters gradually decreases to 35–20°C, depending on the season of the year. In the river itself, a portion of the warm water for some time flows along the bottom as a separate jet, and only at a distance of 150–200 m is the difference in temperature between the thermal and river waters no longer detectable. The initial separation into jets of mineralized thermal and river waters is probably connected with the difference in their specific gravities.

In addition, modern deposition of carbonates from groundwaters in insignificant amounts was observed at the mouth of well Sosninskaya 318-bis, on the area of the Sovetskoye oil field. Here the thermal chloride-sodium, strongly saline (19 g/l), methane waters

come from deposits of the Pokur Formation of Cretaceous age, composed of fine-grained sands with interbeds of clays and siltstones [3]. At the wellhead the mineralized water has a temperature of 40°C.

At the same time, near other self-flowing wells in the southeastern part of the West Siberian artesian basin (Sosninskaya 3-R, Mariinskaya 1-OP, Tegul’detskaya 1-OP), no present-day mineral formation is observed. This may be connected with the fact that the wells are characterized by a small discharge and therefore have a low temperature of the outflowing waters at the wellhead as a result of their gradual cooling in the wellbores; the latter circumstance, naturally, did not favor the development of a sharp temperature gradient at the wellhead.

Mineralogical-petrographic, chemical, and thermal study of the products of present-day mineral formation showed that they are composed of calcite of scalenohedral habit with a slight admixture of sulfur and organic matter. Endothermic effects in four carbonate samples were established at temperatures of 870, 880, 890, and 900°C. In addition, individual endothermic effects appear at temperatures of 300, 330, 350, 540, and 690°C, which probably correspond to the decomposition of sulfur and organic matter in calcite (Fig. 1).

Individual crystals of calcite of scalenohedral habit have lengths of up to 2–3 cm, with thicknesses of 0.5–1.5 mm; i.e., the ratio of crystal length to width ranges from 20:1 to 40:1. The crystal heads are bounded by rhombohedra, but insufficiently, and therefore it was not possible to carry out complete goniometric measurements. The refractive indices of calcite from different samples are very constant and correspond to: \(N_o = 1.668\), \(N_e = 1.500 \pm 0.005\). Meigen’s reaction (cobalt nitrate) with the mineral gives a negative result, which is characteristic of calcite and not aragonite. In addition, simple twins and rhombohedral cleavage are observed in thin sections. All these data make it possible to confidently diagnose this mineral as calcite. The crystal-chemical formula of the calcite corresponds to

\[ (\mathrm{Ca}_{0.962}\ \mathrm{Mg}_{0.006}\ \mathrm{Fe}^{+2}_{0.03}\ \mathrm{Ba}_{0.001}\ \mathrm{Sr}_{0.001})_{1.0}\ \mathrm{CO}_3. \]

The results of chemical and spectral analyses of calcite are given in Table 1.

In order to assess the possibility of using aqueous extracts of liquid and gas-liquid inclusions from carbonates to determine the composition of mineral-forming solutions, several experiments were carried out. From two calcite samples, each weighing 100 g, monomineralic samples were prepared for producing extracts. The samples were ground in bidistilled water (in order to reduce losses of the gas phase of the inclusions) to a particle size of 0.1–0.01 mm, then diluted in 250 cm³ of bidistilled water and allowed to stand for 1 hour. The resulting solutions were filtered and analyzed for the following components: \(\mathrm{Na}^+\), \(\mathrm{K}^+\), \(\mathrm{Ca}^{+2}\), \(\mathrm{Mg}^{+2}\), \(\mathrm{Fe}^{+2}\), \(\mathrm{Fe}^{+3}\), \(\mathrm{NH}_4^+\), \(\mathrm{CO}_2\), \(\mathrm{Cl}^-\), \(\mathrm{HCO}_3^-\), \(\mathrm{SO}_4^{-2}\), \(\mathrm{NO}_2^-\), \(\mathrm{NO}_3^-\). The alkalis were determined by flame photometry, and the remaining ions by methods of analytical chemistry. The analytical results are given in Table 2. To take into account the effect of calcite solubility (the host mineral for liquid and gas-liquid inclusions), repeated aqueous extracts from the same samples were produced according to a known method [14, 17, 18, 19].

As can be seen from the table, the same components were recorded in the aqueous extracts as were established by analyses in the original mineralized waters. Moreover, the quantitative ratio of components in the extracts and in the mineralized thermal waters is close: the \(\mathrm{Cl}:\mathrm{SO}_4:\mathrm{HCO}_3\) ratio in the thermal waters is 125:1:7, and in the extracts—106:1:7.

Table 2

Chemical composition of mineralized waters and waters discharged from newly formed calcite

Chemical analyses were performed in the problem geological laboratory of the TPI; pH of bidistillate during the experiment was 6.5–6.6.

These data once again confirm that the water-soluble complex in the extracts, when the solubility of the host mineral is taken into account, is not a function of the dissolution of the rock minerals, but gives a general idea of the salt composition of the initial mineral-forming solution.

Fig. 1. Heating curves of calcites: a—endothermic curves, б—temperature-change curves, в—weight-loss curves.

All this makes it possible to draw the following conclusion: aqueous extracts of gas–liquid inclusions from newly formed minerals reflect not only the qualitative composition of the original solutions, but can also give an approximate idea of their quantitative proportions—for example, in the study of unmetamorphosed mineral complexes. The experiments carried out once again showed that liquid and gas–liquid inclusions in minerals are, in essence, true microvolumes of mineral-forming mother solutions and reflect the specific features of the chemistry of reactions during mineral deposition; therefore, aqueous extracts from minerals, rocks, and ores can be used to solve various genetic questions of natural mineral formation. The data from aqueous extracts can reliably illustrate relative changes in the composition and ratios of ions in solutions over the course of the mineral-forming process. However, when interpreting the results of aqueous extracts of inclusions in minerals, it is necessary to take into account the influence of superimposed hydrothermal processes on previously formed mineral complexes.

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