## 1 Introduction  
As the modern world moves towards a more sustainable future, the oil industry, as a leading industry, is undergoing a major transformation. The oil industry is being improved considering economic, social, and environmental factors of development. Since this industry carries impressive environmental risks, it must become more environmentally efficient, reducing pressure on the environment and human health. Special attention is paid to the problems of oil and oil product spills, reducing the impact on the climate, and protecting natural resources [1; 2; 3; 4]. During oil extraction, significant volumes of wastewater are generated, which are characterized as produced waters. For example, to reduce water consumption, oil companies are exploring ways to reuse water and improve the treatment of produced water, including neutralizing aggressive microbial agents. Monitoring microorganisms, both in the subsurface and introduced during oil reservoir operations, is one of the main challenges associated with water management in the oil industry since it is necessary to control and reduce the proportion of microorganisms that produce aggressive products for pipelines (for example, hydrogen sulfide and organic acids). However, these microbial associations may be of broad interest due to their physiological and biochemical characteristics [5; 6; 7]. Microorganisms associated with oil reservoirs are truly polyextremophiles since they are highly adapted to unfavorable conditions for life (high temperature, salinity, pressure, and toxicity). However, the oil reservoirs are rich in carbon and energy sources necessary for microorganisms. Among them, sulfate-, nitrate-, and iron-reducing, acetogenic, hydrocarbon-, iron-, and sulfur-oxidizing bacteria, as well as hydrogenotrophic and acetoclastic methanogenic archaea, are detected as important microbial groups. Many microorganisms can be associated with the metabolism of hydrocarbons, both individually and in syntrophic relationships [8; 9; 10; 11; 12]. Today, the issue of in-depth study of microbial life associated with oil and oilfield waters is becoming increasingly acute. Firstly, these microorganisms are important reservoirs of enzymes, and secondly, their metabolism directly affects the production and quality of oil. Active research in the field of petroleum microbiology is explained by the development of various biotechnologies, including Microbial Enhanced Oil Recovery (MEOR) processes, which use microbes that produce organic acids, surfactants, and gases [13; 14; 15; 16]. To achieve maximum oil recovery, reservoir flooding is often used, and this is one of the main reasons for the activation of microbial processes in oilfield ecosystems. Where surface or seawater is used for injection, microorganisms thrive and new microbial species join native ecosystems, which can have detrimental consequences for the oil industry. Various substrates for microorganisms present in the injected water and the abundance of electron acceptors such as sulfate, in combination with reservoir conditions, create a biotope, including a corrosive-hazardous one. The most common anaerobic microbial processes that can be initiated are fermentation, sulfate reduction, homoacetogenesis, and methanogenesis [8; 9; 12; 17]. It should be mentioned that the water quality and quantity have a direct impact on the microbial diversity in oil reservoirs and produced waters [18]. Thus, the microbial community of oil samples with a high water content is noticeably richer than that of oil samples with a low water content [19]. There are studies indicating a negligible effect of injected water on the native microbial community [20; 21], as well as the fact that the microbial life of oilfield ecosystems and formation waters is diverse and unstable [12; 22]. For example, [PERSON] et al.