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Implications of Variations in Salt Geometry for Exploration in the Brazil Salt Basins

Abstract

The Santos, Campos and Espirito Santo Salt Basins of central Brazil are characterized by an Aptian-age package of layered evaporites (“salt”). The salt provides the seal for and drove maturation within the world-class Pre-Salt play that has yielded many of the largest discoveries of the last 10-15 years (e.g., Lula, Libra, Buzios, Carcara). Salt was deposited across the basin with only the highest antecedent topography (e.g., the Sugarloaf and Cabo Frio highs) lacking depositional salt. Today, after salt movement, salt thickness ranges from less than a few meters in areas where the salt is welded to many thousands of meters. Along an inboard to outboard transect, the present day morphology of the salt can be characterized into four main provinces: (1) “Roller”, where evacuation of the salt is extensive and the post-salt sediments are thick; (2) “Mini-basin”, where large salt diapirs are flanked by deep mini-basins of post-salt sediment fill; (3) “Massive salt”, which exists primarily in the Santos Basin, where salt thickness typically exceeds 2 km and the post-salt sediment cover is thin; and, (4) “Canopy”, where salt displays complex compressional geometries involving post-salt sediments. The provinces reflect the gravitational drainage of original depositional salt from inboard to outboard under basin subsidence amplified by post-salt sediment loading. There are many implications of these variations of salt geometry for exploration in the highly competitive Pre-Salt play: (1) the need for 3D seismic acquisition, particularly in the “canopy” province; (2) the challenge of iterative velocity model scenario testing, especially during an active bid-round environment, to improve definition of the structure and image of the reservoir; (3) the uncertainty of the trap geometry resulting from poorly constrained velocity and anisotropy of the post-salt sediments; (4) the risking of top seal integrity; and, (5) the impact of salt restoration method on commodity prediction.