Abstract
Amplitude versus offset (AVO) measurements for deep hydrocarbon-bearing sands can be compromised when made in close proximity to a shallow salt piercement structure. Anomalous responses are observed, particularly on low acoustic impedance bright spots. Common-midpoint (CMP) data from key seismic profiles traversing the bright spots do not show the expected Class 3 offset responses as defined by Rutherford and Williams (1989). On these CMPs, significant decrease of far trace energy is observed. CMP data from other seismic profiles off-structure do exhibit the Class 3 offset responses, implying that structural complications may be interfering with the offset response. A synthetic AVO gather was generated using well log data, which supports the off-structure Class 3 responses, further reinforcing the concept of structurally-biased AVO responses. Acoustic, pseudo-spectral modelling of the structure substantiates the misleading AVO response. Pseudo-spectral modelling results suggest signal degradation observed on the far offsets is caused by wavefield refraction — a shadow zone, where the known hydrocarbon-bearing sands are not completely illuminated. Such shadow zones obscure the correct AVO response, which may have bearing on exploration and development.
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