GEO ExPro

A New Exploration Frontier

The sub-salt discoveries recently reported in the Santos basin, offshore Rio de Janeiro, Brazil could be the largest in over 30 years and has captured the world's attention.
This article appeared in Vol. 5, No. 4 - 2008

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During 2007 and 2008, Fugro acquired the Brazil Deep Focus (BDF) seismic survey that was designed with long offsets (10 km) and deep records (14 to 18 seconds). The survey is approximately 15,000 km long, covering 300,000 to 400,000 km² in the Campos and Santos basins offshore Brazil. The seismic line across the Tupi area was acquired in the Santos basin. Gravity and magnetic data were also recorded with the survey. Data courtesy of Fugro Data Services, AG The regional framework given by the Brazil Deep Focus will enable explorationists to evaluate the extent of salt as well as study the relationship of salt and basin tectonics. Below is a preliminary pre-stack depth migrated line across the giant Tupi structure which is reported to have tested over 4,800 barrels of oil per day.


See the preliminary pre-stack depth migrated

Recent sub-salt discoveries in the Santos basin, offshore Brazil, have opened up a new frontier play that may expand into the adjoining basins to the north and south. In an effort to help explorationists understand the regional framework of the salt basin, as well as the potential extent of this play, Fugro began acquisition of the Brazil Deep Focus survey in 2007.  

The Brazil Deep Focus survey is a new long offset 2-D seismic survey that covers a large part of the Santos and Campos basins offshore Brazil (compare survey map on the preceding seismic foldout). Both pre stack time and pre stack depth data are being produced for the survey. A regional interpretation of existing potential field data was created to assist with the regional basin model.

Processing Challenges and the Salt Model

Accurate interpretation of salt is important for a pre stack depth migration project. There are several challenges both geological and geophysical in producing a good pre stack depth migration image in this region. Pre stack depth processing uses predicted sediment and salt interval velocities combined with a salt interpretation to provide a more accurate image of the sub salt section.  

Solutions

A new interpretation was created taking the layered salt into consideration. Also a series of velocity scans of salt floods were produced to evaluate the optimum salt velocity. The results were an interpretation that maps salt as both layered, and what appears to be opaque, or has no internal structure, using a velocity for salt that is different from other salt regions. In the early work, it is believed the salt velocity may be variable across the region of this survey depending upon the mineral content of the evaporate layer along with the purity of the salt. A consistent salt velocity was used for the final salt model.  

In the more difficult areas, different processing methods were used to try to get a better image of the base of salt and sub-salt section.  

The seismic line shown on the foldout is a regional look at the Tupi field area. Preliminary images of the pre-rift section suggest there are at least three discernable units in some areas as shown on the close up of the Tupi structure. We are currently working on the sub-salt tomography and the final base of salt models at this time to refine the pre-stack depth migration velocities.  

Only time will tell us the full extent and value of the sub-salt play. During this time, Fugro will continue to provide data and interpretations to assist in the exploration and development of existing and new play concepts.

  • Regional interpretation of existing potential field data overlain with preliminary base of salt assists with the regional basin model. Shown on the map are volcanic features such as seamounts (in pink) and transform faults on oceanic crust (black).rnttttThe Tupi discovery is 300 km offshore in 2,200 km water depth. Potential flow rates are 20,000 bopd of 28 to 30º API gravity oil. Nearly 2,000 m of salt overlay the 76 m of oil saturated reservoir rocks believed to be coquinas made of pelecipods.



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