Management of carbonate reservoirs often needs to go beyond characterizing matrix properties to include fault and fracture effects, which can significantly impact water break-through and oil sweep efficiency. Predicting the flow impact of faults and fractures can benefit from the identification of these features from seismic data. Given their anisotropic nature, fracture trends in carbonate reservoirs have a reasonable chance of being detected with seismic methods. We will present the use of P-wave prestack seismic azimuthal anisotropy inversion for the detection and prediction of fractures in a Cretaceous Middle Eastern carbonate reservoir.
The study of azimuthal anisotropy using prestack P-wave seismic was shown to be useful in the identification of open fractures by Lynn et al, 1996. More recent work in Middle Eastern carbonate reservoirs, using prestack P-wave seismic for estimating azimuthal anisotropy to detect fractures, was done by Luo et al., 2005, Holmes and Thomsen, 2002, and Roberts et al., 2001.
Open fractures in the subsurface, when vertical and aligned, may be detected with seismic anisotropy using azimuthal AVO inversion of P-wave data. The azimuthal AVO inversion method presented here is based on Ruger's equation (Ruger and Tsvankin, 1997), which describes the observed reflectivity (R) as a function of normal incidence amplitude (A0), AVO slope (B0), anisotropy magnitude (B1), anisotropy orientation (f0), and noise (ni):
R(?i , fi) ~ A0 + [B0 + B1 cos 2(fi - f0)] sin2 ?i + ni
Inversion of Ruger's equation yields four attribute volumes (corresponding to A0, B0, B1, and f0). Additionally, we developed a confidence term for uncertainty analysis.