Using AVO for fracture detection: Analytic basis and practical solutions

Tight, low-porosity reservoirs can produce significant amounts of hydrocarbons if fracture networks provide the necessary permeability. Characterization of naturally fractured reservoirs using surface seismic data is an important exploration problem that has attracted much attention in the literature. TLE, for example, had an entire issue (August 1996) on this subject. While the data and modeling in existing publications demonstrate that fracturing causes significant azimuthal variation in recorded seismic signatures, the problem is how to infer fracture parameters from these data. The goal of this paper is to complement recent experimental results with an analytic and modeling study of the amplitude-variation-with-offset (AVO) response of fractured reservoirs. The question we address is: What does P-wave reflectivity really tell us about the fractures? To find the answer, we introduce a convenient notation for azimuthally anisotropic models and present concise approximations for reflection coefficients in terms of the anisotropy parameters. Our analysis shows that P-wave amplitudes may be sensitive even to relatively weak anisotropy of the rock mass. We also discuss how the azimuthally dependent AVO signature can be interpreted and combined with normal-moveout and shear-wavesplitting analyses to constrain the crack density and other medium parameters. The simple relationships between reflection amplitudes and anisotropic coefficients can be regarded as rules of thumb to quickly evaluate the impact of anisotropy in a particular play, plan acquisition, and guide more advanced numerical inversion.