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Seismic Interpretation Methodology for Fluvial Sandstone Reservoirs in Widuri Field, Offshore Southeast Sumatra, Indonesia
A wide range of seismic interpretation methods are used to detect fluvial sandstone reservoirs in Widuri and neighboring oil fields, offshore Southeast Sumatra. High quality 3D seismic data allow individual reservoirs to be imaged separately, and stratigraphic variations result in certain methods being suited to particular parts of the reservoir succession. Relevant techniques are reviewed in terms of both their general use in seismic interpretation, and specific application to the Widuri reservoir section. Amplitude variations result from changes in acoustic impedance, thin bed thickness and complex interference effects. Seismic two-way time differences are used to detect changes in the stratigraphic thickness of thick beds, lateral velocity variations, and lateral phase changes, which produce locally non-chronostratigraphic reflections. Time-shifted horizon slices, frequency variations and certain attribute ratios highlight subtle waveform variations above or below continuous marker horizons, such as coal seams.
Amplitude variation with offset (AVO) is a successful lithology indicator. In the upper reservoirs, amplitude at far offset distinguishes sandstone and carbonaceous mudstone, whereas deeper in the succession complex waveform variations with offset identify sandstone reservoirs closely above or below coal seams. Both range-limited and finite-angle stacks are used for 3-D AVO interpretation. In addition to external reservoir form, structural attributes, amplitude variations, coherence and trace shape attributes are used to image internal reservoir architecture.
Seismic interpretation methodology in Widuri Field has evolved over the past 10 years, from amplitude and isochrons used in initial field development to phase changes on far-angle stacks and 4D seismic used recently. The period from year 1996 to 2000 shows the effectiveness of these techniques in a late phase of field development. During this time over 150 wells have been drilled, with a success ratio of over 90%.
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