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Tertiary to Cretaceous age karsts and collapse disturbances were observed from recent 3D seismic interpretation of an offshore Abu Dhabi oil-field. The seismic evidence of karst features was investigated using full-stack, spectral whitened, and discontinuity volumes. In addition, circular features were detected at specific sequence boundaries after examining curvature maps, disturbed amplitudes, velocity effects, and attenuation attributes. Several karsts and collapse disturbances tend to be associated with anhydrite beds overlaying thick carbonate intervals and seem to be limited to the Tertiary stratigraphic column; other karsts were observed to be limited to Cretaceous dolomite and limestone reservoir intervals. The Tertiary age karsts were observed to cause seismic image and amplitude disturbance at various depths, whereas the Cretaceous age karsts tend to be limited in radius and depth and have more limited effect on seismic response. In the attempt to better understand the limit of some karsts and collapse disturbances, well data (wireline log, conventional core, and thin-section) were investigated within the karst areas and integrated with 3D seismic data. 3D seismic based geometries and attributes were analyzed to evaluate the possibility of detecting the damage-zone limit versus velocity effect with depth due to fill material. Another effect investigated is that of fracture-fault zone distribution relative to karst localization. Analogue karst features of a similar age-range (Tertiary and younger) to the seismic examples occur within Jebel Hafit in the onshore UAE, where solution effects, debris fill, mineralization, and collapse effects can be observed and compared with the offshore examples. Here, structural discontinuities also enhance the karstic features. |
