The elastic and petrophysical properties of the fluvial rocks of the Miocene in the western basin of the Gulf of Thailand display typical signatures of dispersed-clay sandstones and dispersed-quartz shales. That is, the relationships between various properties exhibit two strong trends associated with a grain-supported domain and a matrix-supported domain. The minimum porosity and maximum P-velocity occur at the transition between these domains at a clay volume of approximately 40–50%. This means that the hardest rocks occur at this transition. A simple rock-physics model has been developed that quantifies the elastic properties over a large depth range but does not explicitly account for the dispersed nature of the clay and quartz. It employs a single aspect ratio at each depth within an inclusion-based modeling scheme. The required aspect ratio at each depth can be estimated from the depth, the porosity, and the clay volume of the rock.