Elastic modelling of reflectivity and AVO at the Elang Formation, Laminaria East

Abstract

The Elang formation produces a weak event which is virtually impossible to pick on conventionally processed seismic sections. Its top is difficult to define on wireline logs. A rigorous approach to modelling its reflectivity is as a superposition of reflections from a sequence of layers at £2 m thickness. This formation response verifies that Amplitude Variation with Offset (AVO) can be derived for interfaces near the top of the Elang if Vp, Vs and r are averaged over at least 10 m above and 10 m below this interface. Using a depth of 3264 m for Top Elang this method predicts a polarity reversal at ray parameter, p = 0.105 s/km (about mid-streamer). However, at near-to-mid streamer offsets (p < 0.105 s/km) the reflectivity is too sensitive to depth to derive an AVO representative of the Elang. At Laminaria East, the full depth model predicts a peak/trough wavelet at the TWT near top Elang (3215 ms) which is grossly influenced by the sidelobes of boundaries beneath the reservoir. At far offsets (p > 0.105 s/km) a consistent trough develops in the t-p formation response which extends out to an equivalent 8 km source/receiver offset. This trough is clear on the full model seismogram when multiples and guided waves are suppressed. It may provide the basis for defining a more interpretable signature for the Elang by weighting far offsets traces and perhaps acquiring data out to very long offsets.