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Diversity, geochemical activity, and biotechnological potential of the microorganisms from oil bed 302 of the Romashkinskoe oilfield (Tatarstan, Russia) are reported. The microbial community contained almost no aerobic microorganisms. Sulfatereducing (10 3 -10 6 cells/mL) and fermentative bacteria (10 2 -10 5 cells/mL) pre� dominated in the oilfield. Sulfate reduction was the predominant process in formation water with the rates up to 26.6 µg S 2- L/day. The number of methanogens and methanogenesis rate in formation water did not exceed 10 4 cells/mL and 8.19 µg CH4 L/day, respectively. Analysis of the 16S rRNA gene clone library revealed the sequences of denitrifying bacteria of the genera Sulfurimonas and Thauera. The oil recovery tech� nique combining the stimulation of fermentative bacteria and suppression of sulfate reducers in the oilfield was proposed for development of the bed 302. Fermentative bacteria could be activated by the traditional method, i.e., injection of molasses and nitrogen and phosphorus mineral salts through the injection wells. Introduction of high concentrations of nitrate will activate the growth of denitrifying bacteria, suppress the growth of sulfidogenic bacteria, and result in decreased sulfide concentration in formation water. The pro� posed biotechnology is technologically simple and environmentally friendly.

The existing methods of oil field exploitation give the opportunity to extract no more than half of the geological resources of oil; from carbonate oil collectors, only 20% of oil is extracted [1]. Furthermore, the portion of recovered oil is tending to decrease due to the development of deposits of viscous oils under difficult geological and physical conditions. Thus, new methods for enhancing oil recovery are needed to at least maintain oil extraction at a constant level. There are various chemical and physicochemical methods for enhancing oil recovery. The method of secondary flooding, developed by the Soviet academician A.P. Krylov in the 1930s, is widely used in many oilproducing countries. Surface water is pumped into the oil stratum through a system of water-injection wells, which results in an increase in the stratal pressure. This technology is highly efficient. Chemical methods, used in combination with secondary flooding, belong to the so-called tertiary methods of oil recovery enhancement. These include immiscible flooding (alkaline or polymeric) and miscible flooding with solvents and acids, as well as flooding with the use of surface-active compounds (surfactant flooding, or microemulsion flooding). The surfactants widely used in Europe and North America are chemically synthesized oil sulfonates. Gas injection is also applied, with CO 2 , N 2 , and hydrocarbon gases being mainly used. Thermal methods include injection of a heat carrier (hot water or vapor) or intrastratal combustion. Selective plugging of highly permeable layers for the improvement of flooding is little used in Europe and Siberia but is widely applied in North America, Canada in particular [2]. The development of methods for microbial enhancement of oil recovery started in the middle of the 20th century. They are highly efficient and diverse, environmentally safe, and relatively cheap, although science-intensive. Three substantial prerequisites exist that allow us to believe in the prospects for the development of new efficient biotechnologies for enhancing oil recovery based on the geochemical activity of microorganisms. First, oil fields exploited with water flooding are widely inhabited by aerobic and anaerobic microorganisms belonging to various physiological groups, which retain viability and biochemical activity in oil strata. Second, microorganisms are able to degrade oil hydrocarbons to smaller and more labile organic molecules and to synthesize such oil-releasing agents as ee 2 , ec 4 , fatty acids, alcohols, polysaccharides, surfactants, and other technologically active substances. Third, microorganisms are capable of in situ production of oil-releasing substances directly in zones and microzones containing residual oil. Present research that aims at the development of biotechnologies for oil recovery enhancement should be concerned with study of the regularities of microbial distribution and geochemical activity in oil fields under different geophysical conditions, as well as with advanced investigation of physiological and biochemical characteristics of microorganisms and development of methods for regulation of microbial processes in oil fields [3].

In field experiments, we tested an original technology to enhance oil recovery based on the activation of the stratal microflora of flooded oil fields. The technology includes the injection of aerated fresh water with added mineral salts. The activity of microflora (aerobic and anaerobic) increased sharply in the near-bottom zone of the injection well. The microbiological processes occurred in two stages. The first stage includes the activation of aerobic oxidation of organic matter in the oil, which leads to the formation of oil displacement agents, such as organic acids, surfactants, polysaccharides, and carbonic acid. During the second, anaerobic stage gases, methane, and carbonic acid were formed. The field tests were carried out in three pilot tests of different waterflooded areas of the Romashkino field. The additional oil that was recovered reached a total of 41.08 t.t, that was 32.9% of the total pilot production. The enhancement of oil recovery correlated with the rate of methanogenesis. The carbon isotope composition of the carbonates of stratal waters and the concentration of one of the main degradation products (acetate) were changed.