Stratton field in south Texas produces from multiple dispersed and concentrated sandstone pays in the Oligocene Frio Formation. The middle Frio Formation is the most important gas reservoir in the Frio Reservoir gas play in the Texas Gulf Coast region. For better understanding of exploitation and exploration of this reservoir it is important to determine its rock physics properties in order to relate them to seismic observations such as amplitude and travel time. In this paper, the middle Frio clean sandstone reservoir rock density, bulk modulus, and velocity at Stratton field have been calculated. The empirical constants “free parameters" a, b, c were statistically estimated from the observed sonic velocity (Vpsonic) and porosity (φ).The values of these best fit parameters are used to calculate the modulus of the dry porous rock frame (mdry) which in turn is used to calculate the saturated plane-wave modulus (msat) and the velocity of the saturated porous rock frame (Vpsat). The changes in the middle Frio rock parameters are predicted by the changes in the reflection coefficient (R0) calculated at the top of the middle Frio reservoirs. The introduction of gas significantly decreases the rock velocity and density resulting in a negative reflection coefficient “amplitude dim spot effect".