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Drilling offshore Southeast Sumatra in the area of the Widuri Field, Asri Basin has demonstrated that the oil producing Upper Gita sandstones within the Lower Miocene Talang Akar formation correlate with bright seismic amplitudes on 3-D stack data. Seismic amplitude maps at the Base Upper Gita horizon are interpreted as either a network of distributary channels or a meandering fluvial system. Channel fill lithologies vary considerably and are difficult to interpret from conventional seismic stack data. Carbonaceous shales, likely deposited during the abandonment phase of channeling, have acoustic impedances similar to sandstones. Dipole sonic log measurements of Poisson's ratio are significantly higher in carbonaceous shales than sandstones. Based on raytraced models, carbonaceous shale has a strongly decreasing AVO while sand has flat AVO response (no change in amplitude with offset). AVO attributes P and G or intercept and gradient inverted from 3-D prestack seismic data are used for detection of carbonaceous shale. Based on mapping, good correlation exists between well data and the AVO product attribute P*G. The stratigraphic model of shale abandonment fill in a meandering fluvial system is interpreted to be supported by the 3-D AVO data. This interpretation will be tested with future drilling.
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