Quantitative gas saturation estimation by frequency- dependent AVO analysis

Seismic amplitudes contain important information which can be related to fluid saturation. The amplitude-versus-offset (AVO) analysis of seismic data based on Gassmann’s theory and approximation of the Zoeppritz equations, has played an central role in reservoir characterization. However, this standard technique faces a long standing problem, its inability to distinguish between partial gas and “fizz-water” with little gas saturation. In this paper, we studied seismic dispersion and attenuation in partially saturated poroelastic media by using frequency-dependent rock physics model, through which the frequency-dependent AVO (FAVO) response is calculated as a function of porosity and water saturation. We propose a crossplotting of two attributes derived from FAVO response to differentiate partial gas saturation and “fizz-water” saturation. One of the attributes is a measure of “low frequency” or Gassmann, reflectivity, while the other is a measure of the “frequency-dependence” of reflectivity. This is in contrast to standard AVO attributes, where there is typically no such separation. A pragmatic FAVO inversion for rock and fluid properties is also established based on Bayesian theorem. Synthetic study is performed to explore the potential of the method to estimate gas saturation and porosity variations. An advantage of our work is that the method is in principle predictive, opening the way to further testing and calibration with field data. We believe that such work should guide and augment more theoretical studies of FAVO analysis.