The Relationship of Salt-Tectonic Structures to Pre-Evaporite Depositional and Structural Regimes and Evaporite Facies in the Campos Basin, Offshore Brazil

Tomasso, Mark ¹; Wright, Wayne ²; Costa, Fabio O.³; Araujo, Alfredo D.³; Sant'Anna, Marilia ³; Machado, Elizabeth C.³; Hudec, Mike ²; Jackson, Martin ^2
1 Enhanced Oil Recovery Institute, The University of Wyoming, Laramie, WY.
² Bureau of Economic Geology, University of Texas at Austin, Austin, TX.
³ Petrobras E&P-EXP, Rio de Janeiro, Brazil.

Using a database of 3-D seismic, wire-line logs, well cuttings, and core, covering a 9000 km^2 area of the offshore Campos Basin, salt-related structures were analyzed, and a depositional model developed 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 present. Areas dominated by thin laminated anhydrite and halite have little deformation relative to areas dominated by thick halite. Rift-age basement horsts provide a buttressing effect during salt movement resulting in along-strike and down-dip 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. This allows a compare and contrast approach to be taken with previous studies that interpret salt rafting as the main agent of down-slope movement. As such, lessons learned in the Campos Basin study have direct application to these areas.

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009