Quantitative gas saturation estimation by frequency-dependent amplitude-versus-offset 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 approximate of the Zoeppritz equations, has played an important role in reservoir characterization. However, this standard technique faces a long standing problem, which is unable 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 can be calculated as a function of porosity and water saturation. We propose a crossplotting of two attributes derived from FAVO response to differentiate amongst the three cases of full water, full gas and fizz-water saturation. One of the attributes measures a “low frequency” or Gassmann, reflectivity, while the other measures the “frequency-dependence” of reflectivity. This is in contrast to traditional AVO attributes, where there is typically no such separation. A rather pragmatic FAVO inversion scheme is also established by matching the theoretical FAVO response to the observed ones for quantitative estimation of gas saturation. The inversion scheme has the merit of being readily applicable to field data. A synthetic study is performed to explore the potential of the method to image 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.