Applying full-azimuth depth processing in the Local Angle Domain for Frequency Absorption versus Azimuth) (FAVAz) analysis to predict permeable, oil-saturated fractures
Predicting the permeability of fractured reservoirs is valuable for both reservoir assessment and drilling planning. Characterization of such systems requires advanced amplitude analysis, mainly based on seismic imaging results of the recorded wavefield. A significant amount of work has been done on the subject over the past few decades. Thomsen (1995) showed the effect of seismic amplitude variations for different fractured media. Pisetsky and Fedorov (1998) showed the influence of the size of cracks and the seismic wave length on the change of reflection coefficient. Rüger (1998) used the Zoeppritz equation to formalize the reflection coefficient variation as a function of the wavefront azimuthal direction with respect to the fracture azimuth. An approximation of the latter by Tsvankin and Grechka (1998) became the basis for AVAz inversion analysis. Studies have also been performed on frequency absorption properties of fluid-saturated and dry fractured layers. Goloshubin (2002) observed that reflections are stronger for fluid-saturated fractured layers than for dry fractured layers at low-frequency ranges. Geek (2008) studied high-frequency absorption vs. wavefront azimuth for dry fractures in the acoustic frequency range for P-waves, and showed that the absorption is insensitive to the azimuth for S-waves. Kozlov (2006) made a comprehensive generalization of the physics of frequency absorption effects in the case of dry and fluid-saturated fractures. In his study he showed that in fractured reservoirs, high fluid permeability has an observable effect of absorption of the low-frequency spectral range for reflected P-waves. This observation was also noted in several exploration areas of Western Siberia (Upper and Middle Jurassic deposits, Davydova, 2004). Despite the research detailed above, predicting and delineating oil-saturated fracture systems using low-frequency absorption analysis has not yet become a standard practice in the industry. This is in large part due to the approximation made by standard processing and imaging methods in creating azimuthal data. To make this process more effective, a true amplitude full-azimuth imaging procedure that decomposes the fully recorded seismic wavefield in the local angle domain is required. This procedure is carried out in-situ, in the depth migrated domain, preserving both reflection azimuth and structural azimuth. Such a technology is the ideal solution for extracting the main faults, building the Structural Tectonic Skeleton (STS), exposing the main fracture orientation, and eventually measuring anisotropic frequency absorption effects. This paper presents a study of carbonate reservoirs in an oilfield in the Middle Volga region of Russia, and suggests a workflow based on imaging and processing in the Local Angle Domain to predict prospective areas of oil-saturated permeable fractured reservoirs.