Abstract: 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 in
tight sands with very low permeability. We demonstrate that the variation in reflection coefficient at gas/water contacts is as
a transition zone within a sand formation 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.

Key words: poroelastic media, normal reflection coefficient, asymptotic equation, gas/water contact, chaotic inversion