Third Time Lucky? Imaging the Dentale Formation Offshore Gabon

Abstract

Rapid lateral and vertical velocity and thickness variations within Albian carbonates and Aptian evaporites in West African salt basins present considerable problems for seismic imaging and depth modelling. Failure to address these problems has effectively restricted sub-salt exploration success offshore southern Gabon, even with 3D PSDM data processing. Using new 3D data from Dussafu and employing a range of velocity model updating techniques, migration algorithms and geological data we have built a very detailed and accurate velocity model of the complex, stratified Albian carbonate layer. This enabled us to remove some of the artificial deformation evident in earlier processing at Base Salt and below, which resulted from inaccurate velocity models. This successful processing project relied upon close collaboration between the geology and geophysics teams within the operator and the geophysical contractor. This collaboration led to the careful selection of methods to develop a reliable earth model, producing clear seismic images which will be used as a basis for field development.

Tutorial: RTM image enhancement; bad models and mad physics

“…Here we consider data from an offshore shallow water environment (courtesy of Harvest Natural Resources) where we have a complex salt canopy below a very high velocity carbonate layer: so here the salt is the low-velocity medium (Pavlov et al, 2016). Figure 6a shows the final RTM image from the production project with a colour overlay of the interval velocity model.…”