Reservoir fluids from the Tamsagbulag oilfield (Mongolia) have been studied to develop an integrated physicochemical method for intensification of oil production and enhanced oil recovery using micro-organisms. The reservoir conditions of the oilfield facilitate the formation of a diverse biotic community represented by microorganisms of the Arthrobacter, Pseudomonas, Bacillus, Micrococcus, and Flavobacterium genera. The microflora responds positively to the introduction of an oil-displacing composition at a concentration of 5% as a source of nitrogen nutrition. The reservoir microflora exhibits a hydrocarbon biodegradation activity with the degree of degradation of viscous oil of 8.15 and buildup of metabolic products, which reduce the surface tension from 70 to 42 mN/m. Chromatographic and IR analyses have confirmed the fact of severe destructive changes in viscous-oil hydrocarbons. In this case, the oxidation factor (C=O) has increased by a factor of 16. In modeling the displacement of viscous oil using the integrated method, a relative growth in the oil displacement efficiency by 14.4% has been obtained.