One of the major challenges of unconventional shale reservoirs is to understand the effects of organic richness (total organic carbon, TOC), mineralogy, microcracks, pore shape, and effective stress on elastic properties. The generation of petrophysical parameters, such as TOC, and quantification of total and organic porosities through a physically consistent petrophysical model are described. Rock-matrix density, which is a key parameter in determining total porosity, is estimated as a function of the amount of TOC and its level of maturity. Then the petrophysical parameters are used as inputs for rock-physics modeling to constrain the bedding-normal compressional-wave velocity as a function of various parameters (e.g., TOC, porosity, mineralogy, pore shapes, and microcrack density) in combination with effective stress. Modeling results on three shale plays from North America show that compressional-wave velocity in these specific formations is controlled mainly by variations in TOC, mineralogy, and pore shape. Shear-wave velocity in organic shales also was refined as a function of compressional-wave velocity and amount of TOC.