In the course of pilot trials of biotechnologies for the enhancement of oil recovery in formation waters of the Gangxi bed of the Dagang oil field (China), microbiological processes were investigated. The biotechnologies are based on injection into the petroleum reservoir of different oxygen sources (H2O2 solution or a water-air mixture) with nitrogen and phosphorus salts. The injection of water-air mixture with nitrogen and phosphorus salts resulted in an increase in the number of aerobic and anaerobic organotrophic bacteria, rates of sulfate reduction and methanogenesis in formation water and also the content of CO2 (from 4.8–12 to 15–23.2%) and methane (from 86–88 to 91.8%) in the gas. The preferential consumption of isotopically light bicarbonate by methanogens resulted in a higher content of the light 12C in methane; the δ13C/CH4 value changed from −45.1…−48.3 to −50.7…−59.3‰. At the same time, mineral carbonates of the formation water became isotopically heavier; the δ13C/Σcarbonates value increased from 3.4…4.0 to 5.4…9.6‰. Growth of hydrocarbon-oxidizing bacteria was accompanied by production of biosurfactants and decreased interfacial tension of formation water. Injection of H2O2 solution resulted in the activation of aerobic processes and in suppression of both sulfate reduction and methanogenesis. Methane content in the gas decreased from 86–88 to 75.7–79.8%, probably due to its consumption by methanotrophs. Due to consumption of isotopically light methane, the residual methane carbon became heavier, with the δ13C/CH4 values from −39.0 to −44.3‰. At the same time, mineral carbonates of the formation water became isotopically considerably lighter; the δ13C/Σcarbonates value decreased from 5.4…9.6 to −1.4…2.7‰. The additional amount of oil recovered during the trial of both variants of biotechnological treatment was 3819 t.