introduced air and mineral inorganic salts containing phosphorus and nitrogen sources into water injection.

The first step: in the aerobic fermentation stage, aerobic and facultative anaerobic hydrocarbon-oxidizing bacteria were activated in the near-well zone of injection wells to produce alcohols, fatty acids, surfactants, CO2, polysaccharides and others due to partial oxidation of hydrocarbons. On the one hand, these substances are crude oil release agents, on the other hand, they are used as nutrient sources for anaerobic microorganisms, including methanogenic bacteria.

The second step: in the anaerobic fermentation stage, methanogenic bacteria are activated in the anoxic layer to produce CH4 and CO2,. When dissolved in oil, the fluidity of the oil will be increased, and the recovery rate will then be improved. In this process, the ratio of isotope light methane to total methane produced by organisms increases.

Schematic Diagram of enhanced crude Oil recovery by Native microorganisms

Keywords 1, pump; 2, water injection well; 3, eutrophic water body; 4, residual oil zone; 5, formation microbial activity zone; 6, microbial metabolite (gas, biopolymer, surface activator, etc.); 7, front peak of oil layer; 8, oil-bearing rocks; 9, production wells