## 1 Introduction
[5] reported that the structure of microbial communities varies with time and environmental conditions and noted the predominance of bacterial taxa of potential operational significance, namely those associated with microbially induced corrosion (MIC; or biocorrosion) and biofouling. Accurate knowledge of microorganisms' characteristics of the oilfield itself and microorganisms associated with produced waters is noted by many scientific groups as critical to the control and development of certain technologies, including against MIC. To date, many of the world's oil reservoirs have been studied to analyze the taxonomic and metabolic diversity of microorganisms, with particular attention paid to sulfate-reducing bacteria (SRB), which are often associated with oil production problems such as souring, pipeline corrosion, and even reservoir plugging [11; 18; 23; 24; 25]. However, not only sulfate-reducing microorganisms are responsible for MIC; the activity of other representatives of the microbial world is also responsible for this process. Among the participants in this undesirable process, iron-reducing, iron-oxidizing, and organic acid-producing microorganisms are identified [8; 26]. In addition, the syntrophy of corrosive microbes with other representatives can also indirectly enhance corrosion [26; 27]. Thus, [PERSON] and coauthors [27] noted that representatives of the bacterial and archaeal communities from biofilms recovered from the inside of a corroded steel pipe complement each other and are connected by metabolic processes, such as the reduction of sulfates, iron, and elemental sulfur. MIC can be initiated by microbial metabolites or surface-associated biofilms, causing significant and costly damage to the environment [28]. Corrosion studies often involve experiments using specific species of bacteria cultured under controlled conditions [29]. Although these studies make a major contribution to elucidating the mechanisms of corrosion and protection against it by testing microbial growth inhibitors, the activity of microbesunder controlled laboratory conditions is very different from that in real conditions. One reason is that corrosion processes in natural conditions are usually associated with a mixed population of microorganisms [27]. Expanding current efforts to understand the microbial activity in important oilfields is not only of scientific interest but also critical to identifying economic and environmental impacts. Despite a number of studies focusing on microorganisms that inhabit the mysterious underground ecosystems around the world, in particular microbial communities at oil production sites, the true microbiological activity at these sites and its corrosive consequences often remain one-sided and poorly studied. Of particular interest are oilfields with a long history of exploitation that are on the verge of entering the final stage, such as oilfields in the Republic of Tatarstan (Russia). The objects of this study were bacterial communities associated with oil production sites, in particular, with existing biocorrosion damage. The biodiversity of oil-field waters and deposits on the surfaces of oil pipe fragments was studied using a 16S rRNA gene amplicon sequencing approach. A distinctive feature of the work is a study that directly evaluates the influence of the native microbial communities on biocorrosion patterns under laboratory conditions using real oilfield waters and under field conditions. Since the chemical properties of reservoir products determine their corrosion properties, the main parameters of the studied waters tested in immersion experiments were additionally evaluated.