Estimating rock porosity and fluid saturation using only seismic velocities

Evaluation of the fluid content in deep earth reservoirs or fluid contaminants in shallow earth environments has required the use of geophysical imaging methods such as seismic reflection prospecting. Interpretation of seismic velocities and amplitudes is based on theories of fluid-saturated and partially saturated rocks that have been available since the 1950s. Here we present a new synthesis of the same physical concepts that uses compressional-wave velocities together with shear-wave velocities in a scheme that is much simpler to understand and apply yet yields detailed information about porosity and fluid saturation magnitudes and spatial distribution. The key idea revolves around the fact that the density and the Lame elastic parameter a are the only two parameters determining seismic velocities that also contain information about fluid saturation. At low enough frequencies, Gassmann's well-known equations show that the shear modulus is independent of the fluid saturation level. We use these facts to construct saturation-proxy and data-sorting plots from seismic velocity data. The new method does not require reflectivity data. although it can use such information if available. The method can therefore be applied to a wide range of source-receiver configurations, including seismic reflection profiling (surface to surface), vertical seismic profiling (well to surface), and cross-well seismic transmission tomography (well to well), since availability of reflection data is not a requirement.