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.