Abstract: The geochemical characteristics of the existing natural gas in the Bohai Sea indicate that the biodegradation gas of heavy oil is generally associated with heavy oil reservoirs, and its proved reserves account for about 12 % of the total proven natural gas reserves. The complex tectonic evolution of Bohai Bay Basin led to the diversified background of biogas reservoirs, and the uncertainties in the genetic types of biogas in Bohai Sea and the geochemical characteristics and reservoir forming conditions. Compared with the biodegradation gas from other types of oil, the biodegradation gas of heavy oil has high methane content and large dry coefficient; it is dominated by dry gas and contains a certain amount of carbon dioxide and nitrogen. The methane carbon isotope is light, with δ¹³C₁ between -35 ‰ and -55 ‰ . The propane carbon isotope is heavy, with the phenomenon of isotope inversal. The carbon dioxide isotope is heavy, with δ¹³C_co2 between -10 ‰ and 20 ‰ . The degradation gas of crude oil is generated under the combined effect of aerobic and anaerobic microorganisms. The effective biodegradation of petroleum usually occurs in reservoirs with a temperature below 80℃. Therefore, the occurrence area of degradation gas generated by microorganism from crude oil is controlled by formation temperature. In addition, the formation water in the petroleum system is also an important geological influencing factor. NaHCO₃- or CaCl₂-type formation water with medium and low salinity is conducive to the reproduction of microorganisms and degradation of crude oil to generate methane. Biodegradation gas is the product of microbial metabolism, so the scale of degradation gas is determined by the size of reservoir. In the Bohai Sea, PL 19-3 oilfield, with optimal reservoir temperature and formation water type and huge reservoir scale, is the most suitable for large-scale generation of biodegradation gas of heavy oil. The proved geological reserves of crude oil in the Bohai Sea are about 36.4×10⁸t, of which the geological reserves of crude oil with a burial depth of less than 2 000 m are approximately 27.7×10⁸t. According to the simulation experiment of crude oil biodegradation, 0.14 to 0.62 m³ of methane gas can be produced by 1 m³ of crude oil per day. Annually, 52.01-255.5 m³ of methane gas can be generated. If 2 000 m is the critical limit for crude oil degradation, it is estimated that the maximum annual amount of degradation gas from crude oil in the Bohai Sea can reach 0.66×10¹²m³, showing huge exploration potential.