Shaley sandstone is heterogeneous at a seismic scale.Gassmann's equation is suited for fluid substitution in a homogeneous medium.To study the difference between shaley sandstone effective elastic moduli calculated by mean porosity as a homogeneous medium,and those calculated directly from the sub-volumes of the volume as a heterogeneous medium,computational experiments are conducted on Han's shaley sand model,the soft-sand model,the stiff-sand model,and their combination under the assumption that the shaley sandstone volume is made up of separate homogenous sub-volumes with independent porosity and clay content.Fluid substitutions are conducted by Gassmann's equation on rock volume and sub-volumes respectively.The computational data show that at seismic scale,there are minor differences between fluid substitution on rock volume and that on sub-volumes using Gassmann's equation.But fluid substitution on sub-volumes can take consideration of the effects of low porosity and low permeability sub-volumes,which can get more reasonable data,especially for low porosity reservoirs.