Pre-salt Depth Imaging of the Deepwater Santos Basin, Brazil

摘要

Several discoveries, such as Tupi, Bem-Te-Vi, Parati, and Guara, have been announced in the Santos Basin off the coast of Brazil. Most of these recent discoveries have been in pre-salt layers. These layers were well imaged by a salt flood volume produced in 2003, but distortions in the base of salt (BOS) and pre-salt layers were still obvious. Therefore, a depth migration with a complete salt model is necessary for correctly positioning the reservoir structures. In order to obtain high-quality subsurface images, building an accurate velocity model and using the optimal migration algorithm for the geology is paramount. For pre-salt imaging, not only the pre-salt velocity, but also the overburden velocities, i.e. sediment velocities, salt velocities and salt geometry, are critical. The unique aspects of the local geology in the Santos Basin make building the velocity model challenging. The three major issues are: 1) the presence of a thin Albian layer above the salt, 2) the sensitivity of the top of salt (TOS) picking to pre-salt imaging, and 3) the existence of evaporite layers within the salt. The following techniques are introduced to improve the accuracy of the velocity model and the pre-salt image: Albian layer tomography, iterative TOS interpretation, and intra-salt tomography.