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Short wavelength velocity variations are observed within the Cretaceous chalk in some areas of the Southern North Sea. Depth imaging is required because of the strong lateral velocity variations but building a suitable high-resolution velocity model is not easy. Here we present a case history of a pre-stack depth migration project where five overlapping surveys with varying acquisition azimuths covering some 2000 sq km were input. The workflow uses hybrid grid tomography with dense multi-offset picks as input. Great care is taken to ensure the computed velocity variations are real. A side-effect of short wavelength velocity heterogeneity is that datasets with different azimuths may show differing moveout. However the tomography is able to make use of this and build a more robust model by making use of the additional raypaths. Without solving for the velocity anomalies the image at target is substantially degraded. Migrating with a high-resolution velocity model produced excellent imaging at the target reservoir level in the Rotliegend and Westphalian.

SummaryBlock 32, deep offshore Angola, is located in an area of the Lower Congo Basin heavily affected by the salt tectonism: salt domes or canopies are extensive. As a consequence having a good seismic image is critical in terms of exploration but also development of discoveries. Over the past decade the focus has moved towards the sub-salt play and the use of conventional seismic has reached its limits. As a consequence, Total E&P Angola embarked in 2012 in a project to address complex subsalt challenges in the Central North East part of the Block 32, using innovative technologies: from survey design, to acquisition, processing and imaging. The objective was to deliver interpreters a state-of-the-art and fit for purpose solution in a timely and cost-effective manner. This project is known as the Block 32 CNE Bi WATS. This abstract aims to give a full overview of the project, from its genesis to its execution, and finally present some of the results First EAGE/ASGA Workshop on Petroleum Exploration

We show here a novel velocity model building approach using the Beam-derived wavelet attributes through a 3D PSDM case study in the UK northern North Sea. There are several advantages by using the Beam-derived wavelet attributes to update the velocity model compared with the conventional method. The most important experience acquired from this project is the velocity model building lifecycle is significantly shortened without compromising the quality. Additionally, this Beam approach has its unique flexibility in pre-conditioning and post-processing seismic data, e.g. demultiples, as well as resolving local velocity anomalies. It is also demonstrated how the pre-BCU challenge was tackled in this project.

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