On the nature of a large anticlinal fold in the Upper Kama salt deposit, its geomechanical and gas-geochemical zonality

I.I. Chaikovskiy^*, O.V. Ivanov^**, I.L. Pan’kov^***, E.P. Chirkova^****

Mining Institute, Ural Branch, Russian Academy of Sciences, Perm, 614007 Russia

E-mail: ^*ilya@mi-perm.ru, ^**miner200@mail.ru, ^***ivpan@mi-perm.ru, ^****zaitseva_59@mail.ru

Received July 29, 2021

ORIGINAL ARTICLE

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DOI: 10.26907/2542-064X.2021.3.490-499

For citation: Chaikovskiy I.I., Ivanov O.V., Pan’kov I.L., Chirkova E.P. On the nature of a large anticlinal fold in the Upper Kama salt deposit, its geomechanical and gas-geochemical zonality. Uchenye Zapiski Kazanskogo Universiteta. Seriya Estestvennye Nauki, 2021, vol. 163, no. 3, pp. 490–499. doi: 10.26907/2542-064X.2021.3.490-499. (In Russian)

Abstract

Large (more than 10 m in height) folds complicate the longwall mining of potash salts and are accompanied with the formation of foci of gas-dynamic phenomena. In this study, to identify the folding nature, changes in the physical properties of its rocks, and the distribution of gases, we analyzed the structural plan of the mined formation, performed structural geological observations, determined the amount and composition of associated gases, and identified the deformation parameters of salts.

It was found that a separate flexure-like fold with a height of more than 30 m developed at the intersection of two fold systems of the salt stratum. During the formation of this dislocation, the sylvinite layer underwent folding catagenesis, which led to the boudinage and the appearance of a directive texture, the intensity of which decreases with the distance from the fold. In terms of the geomechanics, the following two zones were formed: near, slightly weakened rocks with subzones of stiffer (about 0 m) and more plastic (about 58 m) rocks, as well as distant, heavily weakened rocks with subzones of more viscous and less elastic (about 131 m) rocks and less viscous and more elastic (about 241 m) rocks. Their formation, along with the flattening of grains, can be associated with the squeezing and spatial redistribution of gas-liquid inclusions. In the process of gas-phase diffusion, zonality in the distribution of gases was observed (methane and its homologues → nitrogen → carbon dioxide). It turned out to be consistent with their migration ability. The high value of the C₂H₆/i-C₄H₁₀ index suggests that a part of the hydrocarbon gases could have been generated in situ from the organic matter of salts during the fold catagenesis. It was established that the zone of influence of a 30-m fold is 380–500 m, which makes it possible to predict the formation of free gases foci in this interval and the adoption of appropriate protection measures.

Keywords: Upper Kama salt deposit, folding, gas content, physical and mechanical properties

Acknowledgments. This study was supported by the Russian Foundation for Basic Research (project no. 20-45-596017 r_NOTs_Permskii krai).

Figure Captions

Fig. 1. Structural map of the surface of the KrII layer of the Usolskiy mine: 1 – boundaries of the panels and blocks; 2 – isolines of the top of the KrII formation; 3 – synclinal kinks; 4 – the investigated section of the mine.

Fig. 2. Structural map of the KrII stratum surface in the area of the anticlinal fold: 1 – isolines of the KrII stratum roof; 2 – mine workings; 3 – axes of the synclinal folds; 4 – sampling locations and sample numbers.

Fig. 3. General view and details of the structure of rocks at different distances from the fold. Red – sylvinite, gray – interlayers of rock salt, black – fragments of clay interlayers.

Fig. 4. Changes in the physical and mechanical properties of rocks (a), their gas content and the composition of bound gases (b) at different distances from the fold.

Fig. 5. Comparison of rocks from the area of the fold with the rocks of the KrII layer and the underlying rock salt of the Usolskiy mine: a – the ratio of gas content and the content of hydrocarbon gases; b – the ratio of geochemical indicators on A. Prinzofer and E. Pernaton’s diagram [5].

References

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5. Prinzhofer A., Pernaton E. Isotopically light in natural gas: Bacterial imprint or diffusive fractionation? Chem. Geol., 1997, vol. 142, nos. 3–4, pp. 193–200. doi: 10.1016/S0009-2541(97)00082-X.
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