Abstract: The oil-gas-water system is rather complicated in the reef bank reservoir of the Changxing Formation of Permian in the Yuanba region of northeastern Sichuan. It is quite often to encounter water reservoirs or gas-water mixed reservoirs in drilling. It is highly required to develop a technique to effectively distinguish gas reservoirs from non-reservoirs. Based on the physical properties of different types of fluid, we studied the geophysical responding characteristics of gas reservoir, gas-water reservoir, water reservoir and non-reservoir respectively. It is found that the Poisson ratio and λρ in gas and gas-water mixed reservoirs are simply lower than those in non-reservoirs. Using the AVO inversion technique for inversion processing, we obtained the Poisson ratio and λρ of the reef bank deposits. And then the Poisson ratio and λρ were used to predict the distribution of gas in the Reef bank reservoir of the Changxing Formation in the region. The results of prediction fit well with the drilling data. Therefore, the AVO pre-stacking inversion technique could be promoted to similar marine reef bank reservoirs.