Kaombo, from Exploration Towards Development : A Decade of Progress in Subsalt Imaging
Authors: Martin Victor , Douillard Anthony , Lemaistre Laurent , Clerget Vincent , Gerea Constantin 
Published: May 2013

Abstract
Kaombo is a multi-field development on block 32 in Angola. About two thirds of the block is covered with a salt canopy that strongly degrades visibility of the hydrocarbon traps. This lack of visibility prevented a good estimation of the resources and their distribution in several subsalt areas during the very first years of exploration. However, this last decade has seen a dramatic improvement in seismic processing, enabled by the increase in high performance computing capabilities of the industry. Through examples of subsalt fields of the Kaombo project, we show how new techniques have had a positive impact for exploration and development. These new techniques can be put into three categories that we illustrate in this presentation. First, use of more complex and accurate pre-stack depth migration (PSDM) algorithms through time was made possible by the increase of high performance computing capacities. Second, new acquisition techniques such as wide-azimuth have improved the illumination of the subsurface, filling gaps in the images. Lastly, progress in depth imaging led to a better understanding of the structures, which improved the description of the subsurface. This better description, in turn, had a positive impact on the production of new images. Today, new techniques continue to emerge. For instance, calculation of reverse-time migration (RTM) surface offset gathers enabled an improvement of the stack quality. Full waveform inversion was also helpful to define the shape of the salt in some areas where the imaging is still challenging. Progress in subsalt imaging is the product of many improvements. Although the contribution of any single step is sometimes not spectacular, the combination of all of these though time display a step change in depth imaging since the early days. Without it, development of subsalt fields such as Kaombo would not be possible.

Introduction
The Kaombo project is a multi-field development on block 32 in Angola. Tertiary turbiditic reservoirs are located below a salt canopy that has historically been a challenge for standard imaging techniques. 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 about two thirds of the block (figure 1). 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 (1999–2001) was focused on targets outside of the salt influence.
While its application was initially restricted to a few areas for cost reasons, PSDM rapidly became the standard exploration tool on the block (2002–2004). These first stages of subsalt explorations involved Kirchhoff and common azimuth wave equation migrations (CAM, Biondi and Palacharla, 1996), because of their relatively low computational cost, applied to a 3D narrow-azimuth dataset. Migrations were performed using 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 to standard time processing, but some work remained to increase the probability of success of subsalt prospects (figure 2) sufficiently enough to drill these prospects.