The amplitude variation with offset (AVO) of seismic waves reflected from an interface between two geologic layers is a function of the P-velocity, S-velocity, and density of the layers and can be used to obtain information on lithology, porosity, fluid content, and other formation parameters. Density, however, often is poorly resolved. Parameterization of AVO in terms of P-impedance, S-impedance, and density is better than parameterization in terms of P-velocity, S-velocity, and density because uncertainty in the density estimate has a greater impact on errors in estimates of P- and S-velocity than on errors in P- and S-impedance. For this reason, quantitative seismic interpretation makes use of rock-physics templates formulated in terms of P- and S-impedance. An investigation of the ability of rock-physics templates to distinguish between organic-rich and organic-lean shales uses log data from the Eagle Ford Shale and shows the effect of kerogen on in situ stress determination.

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