New imaging technology, Reverse Time Migration, is improving subsalt and salt flank imaging in the Tupi field offshore Brazil.

Conventional wisdom has it that most of the big, easily located fields known as "elephants" have been discovered and the only fields left are smaller and more difficult to find. However, last year’s discovery by Petrobras of the large Tupi reservoir in Block BM-S-11 in the Santos Basin off the coast of Brazil put an end to this pessimism. This exciting find is ranked as one of the 50 largest fields in the world.

The Tupi reservoir is located 190 miles offshore in water depths of over 2000 m at approximately 4000 m to 5000 m below the seafloor. It is estimated to hold approximately 5-8 billion barrels of oil. The reservoir is pre-salt and is masked by a thick layer of salt which makes it very difficult to see the layers beneath. Since the discovery of Tupi, more large subsalt discoveries have been made within approximately a 50-mile radius. Until now, Brazil’s offshore discoveries have been found in post-salt formations.

A unique 20,000-sq km 3D seismic survey over the blocks covering the Tupi discovery was acquired in 2001-2002 by CGGVeritas as a multi-client survey. In 2008 a test area covering the Tupi discovery was reprocessed using our advanced Reverse Time Migration, which significantly enhanced the image of the reservoir beneath the salt. The test was performed on the CGGVeritas multi-client library, and based on its success CGGVeritas is now reprocessing the overall survey.

Imaging technologies such as Reverse Time Migration are increasingly being applied to new subsalt prospects found in deeper waters and in complex geologic settings where explorationists are constantly looking for ways to obtain an improved image of their prospects to lessen the associated risks before committing to drilling.

As depth imaging capabilities have expanded over the last few years with the introduction of more cost-effective PC clusters, CGGVeritas has led the development of new pre-stack depth imaging technologies such as Controlled Beam and Reverse Time Migration.

Reverse Time Migration provides the most accurate images of the subsurface in complex areas compared to other seismic migration schemes. By using a full two-way solution to the wave equation, and given an accurate velocity model, it will accurately construct an image using all available arrivals in the seismic record. This technique is particularly suited to areas where direct illumination of the subsurface is limited.

The improvements achieved in subsalt and salt flank imaging have been telling as demonstrated in the Reverse Time Migration result over Tupi. CGGVeritas is currently using this technology for imaging most of the projects it processes in the Gulf of Mexico, offshore Brazil and offshore West Africa. Besides subsalt imaging this technology also helps in imaging data in a wide range of geological settings which ultimately help in attribute and reservoir studies.

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