AVO analysis in finely layered azimuthally anisotropic media

Abstract

Over the years, amplitude-variation-with-offset (AVO) analysis has proved its usefulness in exploration of oil and gas reservoirs. However, the model conventionally used to interpret AVO anomalies - a single isolated interface between two isotropic half-spaces is often too simplistic. Here, I examine what can be obtained from AVO responses for a significantly more complicated reservoir model - a stack of plane azimuthally anisotropic layers. This model can be used to simulate AVO signatures over finely layered fractured reservoirs. I describe a processing technique which takes seismic data in the frequency - slowness domain, properly corrects it for slant wave propagation in a finely layered medium, and produces an instantaneous AVO intercept and an azimuthally varying AVO gradient as functions of vertical traveltime. Ideally, an obtained AVO intercept and gradient are those which would be recorded in the case of isolated interfaces and in the absence of interference between closely spaced reflections. I use an azimuthally dependent AVO gradient to obtain an instantaneous AVO azimuth, which corresponds to the direction of the greatest AVO gradient. In fractured reservoirs, the AVO azimuth is related to the orientation of vertical cracks. Therefore, fracture characterization is one of the potential applications of the described technique. I perform a numerical study to examine the stability of the azimuthal AVO with respect to errors in the velocity model of the reservoir, inaccuracies in wavelet estimation, and random noise in the data. The results of numerical examples indicate that azimuthal AVO is reasonably stable and suggest the possibility of detecting principal directions of azimuthal anisotropy in layers which are thinner than half of the dominant seismic wavelength.