A method for improving prediction of the viability of potential petroleum reservoirs, utilises a rock physics model appropriate for porous media, in which some of the solid material is "floating" or not involved in load support, and predicts permeability on the basis of compressional wave velocity vs. density trends, which may be determined by wireline log. In a further aspect, by introducing the concept of the capture fraction of smaller grains, another constraint is added to the model, which enables an improved estimate of permeability to be determined on the basis of seismic reflectivity measurements alone.