Measured shear velocities in clastic reservoir rocks have been shown to be independent of the type of fluid present in the pore space while being influenced by the porosity. There is promise that in hydrocarbon (particularly gas) reservoirs one may derive an independent estimate of porosity from the shear velocity of the rock. It has been shown that sonic velocities are best interpreted when decomposed into moduli that are intrinsic measures of rock frame and pore fluid mechanical properties. While the effect of porosity on the shear modulus has been documented for clean sandstone, these relationships break down in the presence of even limited amounts of clay mineral in the sandstone. Existing correlations between shear velocity, porosity and clay content appear to poorly represent the physics of acoustic transport in the rock. We propose a method to estimate total porosity in shaly sands from the shear modulus and a knowledge of the clay content of the rock. The formula accounts for the clay effect in a manner consistent with the known behavior of clay in rock. The method is successfully demonstrated in two well examples with porosity ranging from 10 p. u. to 38 p. u. , dry clay volumes from 0% to 60%, and in zones with water, oil or gas in the pore space. Clay concentrations are determined with an accuracy of +/-2%. Comparisons are also presented with existing correlations showing the superior fit of the proposed technique over a wide range of porosity and clay content.