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Diversity in Salt-Tectonic Structures and Their Relationships to Evaporite Facies and Pre-Evaporite Depositional and Structural Regimes in the Campos Basin, Offshore Brazil

Mark Tomasso1, Wayne R. Wright2, Previous HitFábioTop O. Costa3, Alfredo D. Araújo3, Marilia V. Sant' Anna3, Elizabeth Machado3, Michael R. Hudec2, and Martin P. Jackson2
1Enhanced Oil Recovery Institute, The University of Wyoming, Laramie, WY
2Bureau of Economic Geology, Jackson School of Geoscience, University of Texas at Austin, Austin, TX
3Petrobras E&P-EXP, Rio de Janeiro, Brazil

In a 9000 km2 area of the offshore Campos Basin we analyzed salt-related structures and developed a depositional model for the overlying Albian carbonates. The presumed upper Aptian evaporites in this small area of the greater Campos Basin display a wide variety of structural styles, dimensions and geometries. Salt geometries range from essentially flat to small pyramidal forms to large salt diapirs and walls.

Analyses of this salt succession indicate that at least three separate evaporite sub-basins formed in response to the depositional geometry of the underlying Barremian-Aptian sag-fill and Neocomian-Barremian rift basins. Evaporite structures in the basin show distinct geographic separation from up- to down-dip. Salt structures having distinct wavelength and amplitude characteristics throughout the study area reflect different amounts of deformation relative to both paleotopography and type of evaporite facies. Areas dominated by thin anhydrite/halite have little deformation relative to areas dominated by thick halite. Rift-age basement horsts provided a buttressing effect during salt movement resulting in along-strike separation of areas having negligible salt movement from those with larger scale translation. Pivoting of salt around the buttress yielded thrust structures oblique to the main translation direction. Analysis of listric faults controlled by salt movement indicates that the up-dip areas moved earlier than the down-dip areas. The variety of salt structures, facies and their associated styles of deformation and timing all played a critical role in defining the depositional and structural model for the overlying Albian carbonate succession.
The study area forms a mirror-image analog for the Pinda Formation in the Lower Congo and Kwanza basins of West Africa. As such, lessons learned in the Campos Basin study have direct application to these areas.

AAPG International Conference and Exhibition, Cape Town, South Africa 2008 © AAPG Search and Discovery