There are good biogenic gas accumulation conditions and exploration prospects in deepwater area of Rakhine Basin in Myanmar. Biogenic gas reservoirs，as unconventional natural gas reservoirs，form under very strict conditions. It is the primary issue to study the genesis of biogenic gas-source for analysis of factors of gas accumulation. It is proved by geochemistry analysis and seismic forward modeling that the seismic response created by gas chimneys，also known as biogenic gas reservoir，is obvious in Chinese exploration area of Rakhine Basin in Myanmar. Hydrocarbon source rocks of biogas is characterized by geologic features of“three high and one low，weak diagenesis”，indicating low lower organic carbon limit，low thermal maturation，low geothermal temperature environment and high deposition rate. The rock consolidation de? gree shows that it was in the earlier stage of weak diagenesis. Canyon shale，Miocene mudstones and Pliocene mudstones could provide food and material source for methane bacteria. The overlying mudstones on the Pliocene mudstones does not have the material conditions of biogas formation. It is speculated that natural gas hydrate might exist in the research area，and its stability changes with the variation of pressure and temperature caused by external factors. As a result，amounts of biogenic methane gas is expelled and entrapped to form biogenic gas reservoir. The process of gas expulsion and accumulation can be described as continuous gas generation and pressure increase-breaking pressure balance-gas leak-pressure rebalancing-gas regeneration and the second pressure increase-breaking pressure balance again-the second gas leak-pressure rebalancing.