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Role of Halokinesis in the Evolution of the Cabo Frio Fault and the Ilha Grande Gravitational Cell in the Santos Basin, Brazil: Insights from Multi-Section Previous HitBalancedTop Restoration

Guerra, Marta1
 Underhill, John R.2

1E&P-EXP, PETROBRAS, Rio de Janeiro, Brazil.
2
School of GeoSciences,
University of Edinburgh, Edinburgh, United Kingdom.

The Santos Basin holds the major volume of rock salt in the Brazilian margin. Salt not only acts as a very efficient seal, but it also plays a significant role in creating structures that can either serve as migration fairways or traps for petroleum.

The role of halokinesis in controlling structural styles and sediment dispersal in the basin was investigated using evolved restoration performed along regional cross-sections integrated with 2-D and 3-D seismic interpretation. Proprietary software permitted the simultaneous restoration of seven cross-sections, with 3-D visualisation of different stages of the post-salt structural evolution, depicting the active structures that control depocentres at each stage.

Gravitational gliding and spreading, driven primarily by the flow of ductile Aptian salt in response to differential load and thermal subsidence, account for the main deformation within the post-salt sequence. During the Late Cretaceous and early Cenozoic, the basin received a very thick sedimentation, which greatly affected halokinesis. As a result, a large landward-dipping listric fault, the Cabo Frio Fault, formed offshore, detached on the salt layer. It has been active controlling the major depocentres in the area, since the end of the Turonian. The restoration demonstrates that this fault has migrated offshore throughout its evolution and consequently its present-day position differs from the original one. It possibly resulted from the differential load imposed by a thick prograding wedge, independent of any structure that may have affected the base salt.

A gravitational cell detached on salt, consisting of up-dip extension and down-dip contraction, is proposed here ("Ilha Grande Gravitational Cell"), with the Cabo Frio Fault in the transition zone and salt withdrawal basins in the contractional domain. Due to halokinesis, the post-salt sequence has been moving basinwards; therefore it is of utmost importance that cross-section restoration be applied to produce reliable scenarios for the petroleum system assessment not only in
Santos but also in other salt-prone sedimentary basins, for a promising present-day correspondence between pre- and post-salt structures may not attest a favourable scenario in the past.

AAPG Search and Discover Article #90100©2009 AAPG International Conference and Exhibition 15-18 November 2009, Rio de Janeiro, Brazil