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Polarity of Asymmetric Half-Grabens in the South Atlantic and Its Influence on Trap Integrity: Examples from Offshore Brazil and Uruguay

Lopez-Gamundi, Oscar1 and Morales, Ethel
1[email protected]

The initial phase of the diachronous opening of the South Atlantic is characterized by Jurassic to Cretaceous synrift deposits confined to half-grabens followed by more areally extensive, early postrift (sag) sedimentation. Recent exploratory efforts in the South Atlantic have been focused on offshore Brazil, particularly in the pre-salt, postrift transitional (sag) section of the Greater Campos Basin (Santos, Campos and Espirito Santo basins). The underlying asymmetric half-graben configuration, with a clearly defined border fault margin and a ramp margin, is absent south of the Santos basin in the Pelotas basin but can be identified again further south in the Punta del Este basin in offshore Uruguay. Differential compaction due to the contrasting nature of rift fills and adjacent basement highs is the main control of the four-way traps in the pre-salt discoveries of the Santos and Campos basins. These rifts show variable (landward or basinward) polarity but in some cases there is a dominant landward (westward) vergence, opposite to the present deepening of the continental margin. In this latter case differential compaction at the half-graben border fault margin has been a critical factor to accentuate and/or create counter-regional dips necessary to form structural (four-way) closures at the sag level. Seismic evidence of differential compaction is provided by the presence of hangingwall compaction synclines over basement footwall cutoff points. The synclines are characterized by approximately vertical fold axes immediately above the hanging-wall cutoff of the basement. In poorly imaged areas the termination of the divergent seismic configuration of the synrift strata can be used to place the master fault of the half-graben. Trap integrity of the four-way closures is then enhanced in asymmetric half-grabens with landward dipping master border faults. Conversely, half-grabens with basinward vergence, defined by basinward dipping master border faults and landward dipping ramp margins, create gently dipping, counter regional dips and therefore contribute to the formation of riskier four-way traps.

 

AAPG Search and Discovery Article #90166©2013 AAPG International Conference & Exhibition, Cartagena, Colombia, 8-11 September 2013