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Timing of Late Pre-Salt Faulting and Salt Mobilization in the Santos Basin, Brazil

Abstract

Timing and tectonic context of fault activity at the base of the Aptian layered evaporite sequence (LES) in the Santos Basin (and elsewhere) impacts the pre-salt petroleum system, in particular trap formation and hydrocarbon charge, while the prolific pre-salt carbonate reservoir facies is associated with a fault controlled paleo-relief.

The fault analysis at the LES base and underlying pre-salt reservoir sequences is based on high-resolution seismic images from a recent broadband least Square RTM migration of a seamless 34000 sq.km area covering most of Santos basin. The high fidelity images of the pre-salt geometries and fault architecture benefit from a focused and integrated velocity model building process, including refraction and reflection FWI across the entire depth section, well tie analysis and rigorous geological constraints.

A rugose relief of -6000 to -7000m at the LES base (BoS) determines the San Paulo Plateau (Santos Basin, Brazil), which is bound to the West by a prominent half-graben system that downthrows the BoS by up to 3500m along NNE-SSW trending faults (Merluza Graben). The faults align with the dominant fault system on the San Paulo Plateau where individual fault throw rarely exceeds 500m.

The LES architecture subdivides the plateau into 3 domains: The Albian Cap domain at the inboard NW sector lacks the post-salt Albian carbonates and the LES is restricted to salt rollers under counter-regional faults. The outboard domain comprises predominantly grounded mini basins floored by Albian carbonate mega flaps, separated by up to 4000m thick salt walls and diapirs with complex internal structural geometries. At the distal sector of the plateau, the mini basins are transient into a fold & thrust belt domain of well-stratified LES revealing an original thickness of ~1500m and covered by 300-500m of Albian carbonates.

The lower limit of fault activity is set by well-imaged volcanic layers interbedded with the pre-salt reservoir section, whereas younger growth strata at adjacent half grabens indicate subsequent fault movement. Talus signatures along fault scares indicate exposure during pre-salt carbonate build-ups. At the same time footwall uplift at the Meluza graben shoulders are documented by lacking sag-phase sediments and erosion into the rift sediment section. It appears that the final faulting stages are synchronous with the development of the pre-salt carbonates and prolonged into at least the initial cycles of salt deposition.