Rift Architecture and Its Control from Pre-Salt Plays in the South Atlantic: Lessons from Offshore Brazil
Oscar R. Lopez-Gamundi and Roberto Barragan
Hess Corporation, Houston, TX
The initial rifting in the South Atlantic is dominated by asymmetric half-graben geometries. This structural asymmetry due to differential, tectonically-induced subsidence created optimal conditions for the development of lacustrine to brackish source rocks on the fault-margin of the half-graben. In contrast the fault margin is characterized by low potential for clastic reservoir facies due to underfilled conditions and provenance (volcanic basement). The ramp margin of the half-graben is characterized by low accommodation space and the development of high-energy carbonates (oolitic calcarentites and/or coquinas). This intial phase of confined, tectonically-induced subsidence is followed by a sag phase controlled by thermal subsidence and therefore more laterally extensive. The sag phase sediments expand over the rift shoulders on the fault margins creating potential for pinchout traps and also on the ramp margin of the pre-existing half-graben. Key targets in this phase are shallow marine, high-energy carbonates and possibly late sag microbial limestones. Although sag facies commonly extend beyond the underling rift facies onlapping or draping over basement, the key risk in this type of traps is the possibility for the overlying salt to rest directly on basement (bold high), restricting the areal extent of the traps. The final configuration of the traps may be also modified by some important basin-scale factors such as uplift due to magmatic underplating (i.e. Santos basin).
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