We present a deterministic methodology for mapping the lithology, pore fluid, and porosity from seismic data. The input is the P- and S-wave data volumes that may come, e.g., from acoustic and elastic inversion or cross-well measurements. The output is the pore fluid type (hydrocarbon versus water), total porosity, and clay content. The key element of this methodology is a site-specific rock physics model that provides the needed transforms from the elastic rock properties to the reservoir properties. This model is established by comparing model-based predictions, such as impedance versus porosity, to the relations present in well log data. Once selected, the model is used to identify the presence of hydrocarbons from a combination of the P-wave impedance and Poisson's ratio. Then the P-wave impedance is used to map porosity and clay content assuming that a deterministic relation exists between the latter two properties. All deterministic equations are calibrated at a well and then are applied to upscaled well log data to confirm their validity at the seismic scale. This methodology is applied to log data from a fluvial environment. The results indicate that the relative error in porosity determination is 14%, which is acceptable for reservoir characterization purposes.