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Zebra Prospect – Reading Between the Stripes
An amplitude anomaly within the Pliocene section was identified by BHP Billiton on vintage 2D and 3D seismic data in permit SC-41 of the Philippine Sandakan Basin. Reprocessing of the 3D data in 2003 allowed detailed mapping of the anomaly showing that it extended over several levels on a number of adjacent fault blocks. The anomalies were named "Zebra" because of the striped pattern in cross-sectional view.
In-house lithology and fluid prediction (LFP) software indicated that hydrocarbons would have no AVO response within this section and that seismic amplitude would be the only fluid discriminator. Quantitative analysis using rock physics data from wells within the permit was attempted and it appeared that this should give reasonably reliable results, despite only one well having actually penetrated similar Pliocene-age clastics. This pre-drill analysis strongly suggested that the amplitudes were a result of hydrocarbons, probably gas but possibly oil, in stacked reservoirs.
The amplitude anomalies showed a good fit to depth structure and strong hydrocarbon shows within the Late Miocene in a nearby well were further support for the presence of an effective source rock in the area.
The Zebra prospect was drilled by Unocal (as Operator of the Joint Venture) in June 2004. The well encountered numerous gas-bearing very fine sand to silt units that corresponded to the amplitude anomalies. Unfortunately the gas zones are thin with low hydrocarbon saturation and are uneconomic to develop.
Pre-drill models accurately predicted the presence of hydrocarbons in clastic sediments, however they failed to predict the very fine-grained nature of reservoir intervals and the low hydrocarbon saturations. The challenge now in this area is to locate good quality reservoirs within a hydrocarbon charge fairway using appropriately weighted geophysical and basin modelling techniques.
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