Subsalt Imaging in the Kaombo Development, Angola
Authors: Carpenter Chris 
Published: February 2015

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 24132, ’Kaombo, From Exploration Toward Development: A Decade of Progress in Subsalt Imaging,’ by V. Martin, A. Douillard, L. Lemaistre, V. Clerget, and C. Gerea, Total E&P, prepared for the 2013 Offshore Technology Conference, Houston, 6-9 May. The paper has not been peer reviewed. Copyright 2013 Offshore Technology Conference. Reproduced by permission.
Kaombo is a multifield development on Block 32 in Angola. Approximately two-thirds of the block is covered with a salt canopy that strongly degrades visibility, which prevented a good estimation of the resources and their distribution in several subsalt areas during the initial years of exploration. However, the last decade has seen a dramatic improvement in seismic processing, enabled by the increase in high-performance computing (HPC) capabilities. Through examples of subsalt fields of the Kaombo project, we show how new techniques have had a positive effect.
Introduction
In the Kaombo development, Tertiary turbiditic reservoirs are located below the salt canopy. Unlike other blocks of the lower Congo basin that are more proximal to the shore, a shallow canopy of complex-shaped salt bodies below the sea surface covers much of the block. The salt bodies create strong lateral seismic-velocity variations that are highly detrimental to classical time-processing approaches. Thus, prospect-maturation work during the early days of this block (1999 to 2001) was focused on targets outside of the salt influence.
While its application was initially restricted to a few areas for cost reasons, prestack depth migration (PSDM) rapidly became the standard exploration tool on the block during the period from 2002 to 2004. These first stages of subsalt explorations involved Kirchhoff and common azimuth wave-equation migrations, because of their relatively low computational cost, applied to a 3D narrow-azimuth data set. Migrations were performed by use of simple isotropic velocity models and a priori salt models where the salt geometry could not be assessed with certainty. The quality of the images was improved compared with standard time processing, but some work remained to increase the probability of success of subsalt prospects sufficiently to drill these prospects.
Application of depth-imaging techniques thus allowed a glance at the subsalt potential of the block. Depth imaging is a technique that takes advantage of modeling the propagation of waves in the space domain. Images are functions of data and a seismic-velocity model. In environments such as Block 32, complexity of the velocity model and uneven illumination of the subsurface by the data are severe challenges. The strategy of image improvement is therefore limited by three factors: the capability to accurately describe subsurface propagation parameters (velocity model), the capability to accurately model the wave propagation through the subsurface given the complexity level of the subsurface models (migration algorithms), and the capability to compensate for uneven illumination (acquisition). Progress in subsalt imaging can be achieved by improving methods in these three domains, which are unlocked by the increase of HPC capabilities and new acquisition designs.