© 2014 SEG. In clay-bearing formations, clay, as a matrix component, has much lower elastic moduli than the other common matrix minerals (e.g. quartz, calcite, or dolomite). This contrast in elastic properties between clay and other matrix materials results in inaccuracies in the estimation of the effective moduli of the total matrix when using traditional averaging schemes. An integrated two-stage Gassmann-Sun model is proposed to improve the estimates of effective elastic moduli of clay-bearing formations. In the first stage, we introduce \"clay pore space\" to the original solid matrix and saturate the solid rock frame with clay. In the second stage, we incorporate porosity into the clay-saturated total matrix and saturate the clay-bearing rock frame with fluids (water and/or hydrocarbon). The Gassmann-Sun model utilizes a pore structure parameter (PSP) to quantify the effect of pore structure on the effective moduli of a dry rock frame. This two-stage model considers the pore structures of both the \"clay pore space\" and the fluid-filled pores. As first-order approximations, the PSPs for these two different pore systems are assumed to be the same. The two-stage model is tested using a well log dataset from a clastic reservoir in the North Sea. S-wave PSP values can be used to explain the scatterings of effective shear modulus at fixed clay volume or porosity. S-wave PSPs are also found to be strongly correlated to clay content. Synthetic shear modulus logs generated with the proposed model are in much better agreement with acoustic measurements than those generated from the Voigt-Reuss- Hill average.