The asymptotic equation of wave propagation in fluid-saturated porous media is available for calculating the normal reflection coefficient within seismic frequency band. This frequency-dependent reflection coefficient is expressed in terms of a dimensionless parameter  , which is the product of the reservoir fluid mobility (i.e. inverse viscosity), fluid density, and the frequency of the signal. In this paper, we apply this expression to Xinchang gas-field, China, where reservoirs are super tight sands with very low permeability. We demonstrate that the variation in reflection coefficient at gas/water contacts is observable within seismic frequency band. Then we conduct seismic inversion to generate attributes which first indicate the existence of fluid (either gas or water), and then discriminate a gas reservoir from a water reservoir.