Quantitative Estimation Of Gas Saturation By Frequency Dependent AVO: Numerical, Physical Modelling And Field Studies

It is well known that seismic amplitudes contain important information which can be related to fluid saturation. Most interpretation is based on Gassmann’s theory and studies of amplitude variations with offset (AVO) with the Zoeppritz equations. However, this traditional AVO technique is often unable to make quantitative estimation of the gas saturation, hence unable to distinguish between commercial and non-commercial gas deposits. Recent attention has focussed on if the frequency response of reflections can also be used to reveal fluid information through the application of frequency-dependent AVO (FAVO), although many of these studies are based on empirical relationships and are lack of a thorough understanding of the underline mechanism and rock physics principles. In this study, we extend the “squirt-flow” model to include the effect of gas saturation on seismic attenuation and dispersion. Combining with Wood’s formula, the FAVO response can be calculated as a function of porosity and gas saturation. Therefore a model-based inversion scheme can be established by matching the calculated FAVO response to the observed ones for quantitatively estimating the gas saturation. There are three technical contributions in this approach: 1) a theoretical frame work for modelling the FAVO response and the effects of gas saturation on seismic attenuation and dispersion; 2) an efficient spectral decomposition algorithm for extracting the FAVO response from real data; 3) an optimization work flow for estimating the gas saturation. In our examples, the numerical modelling is able to predict the FAVO response to changing gas saturation which is seen in the physical modelling data, and the 3D P-wave field seismic data. Consequently we argue that with a careful model-based approach, it is possible to invert the FAVO response in terms of gas saturation. This may have important implications in the exploration of tight and unconventional gas resources.