Solving Overburden Anomalies with Depth Imaging in Conjunction with Customized De-Multiple Flow in a Carbonate Field Offshore Abu Dhabi

Abstract

The seismic imaging of carbonate fields offshore Abu Dhabi is complicated by shallow overburden anomalies (e.g. channels, sink holes, karst features, etc.), strong anisotropy and complex multiple generation mechanisms. Noisy data and converted wave energy create further difficulties. All of these pose challenges for conventional time imaging (PreSTM), resulting in structural uncertainty and unreliable reservoir characterization. Successful imaging requires an accurate velocity model. This is important in the shallow overburden area where inaccurately modelled localized anomalies will amplify errors to deeper targets as waves pass through them, creating artificial pull-ups and push-downs. Seismic anisotropy has a key role in accurate subsurface imaging. In this region, anisotropy is complicated, with values ranging from negative delta to large positive epsilon. Inaccurately estimated anisotropy will result in over compensated velocities, and may cause cycle-skipping in diving-wave Full Waveform Inversion (FWI). Shallow water topography and strong impedance contrasts in the area lead to a substantial amount of free-surface and inter-bed multiples. The repetitiveness of the flat mega-scale geology makes it difficult to distinguish and attenuate multiples from primaries. In this paper, we demonstrate that extensive pre-stack depth migration (PreSDM) technologies including Dip Constraint Tomography (DCT), Structural Constraint Tomography (SCT), Vertical Seismic Profile (VSP) constraint anisotropy update and a well-designed de-multiple flow can successfully resolve the challenges mentioned above.