Eocene Depositional Model for the Brazil Santos Basin in the Vicinity of the BS-4—NE Discovery
Funmi Roberts Ebiwonjumi, Daniel Schwartz, 2013. "Eocene Depositional Model for the Brazil Santos Basin in the Vicinity of the BS-4—NE Discovery", Shelf Margin Deltas and Linked Down Slope Petroleum Systems–Global Significance and Future Exploration Potential, Harry H. Roberts, Norman C. Rosen, Richard H. Fillon, John B. Anderson
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The BS-4-NE discovery in Santos Basin, located in water depths of about 1500 meters, was drilled in March, 2001, by the consortium formed by Shell, Petro-bras, and ChevronTexaco and operated by Shell. The discovery well encountered nearly 100 meters of very high net-to-gross clean sand in what appears to be a sand-rich fan system in a toe-of-slope setting of middle Eocene age.
Seismic sequence stratigraphic interpretation across the shelf-to-slope transition indicates that the lower Eocene consisted of low stand systems tract strata that are successively overlain by southeastward prograding then aggrading shelf and slope strata of middle Eocene. The late Eocene strata are predominantly progradational in nature.
Seismic volume interpretation on good quality 3D seismic data revealed that the structural and stratigraphic evolution of the BS-2 and BS-4 area were closely linked. Two counter regional fault systems, a southern east-west and a northern northeast-southwest trending system, were active throughout the early Tertiary. The resultant southward dipping tilted blocks had significant accommodation space in the southern portions of the blocks. Northwesterly sourced sediments in-filled some of the accommodation space throughout the early Eocene, culminating in the flooding of the accommodation space in the late-early Eocene and the northeast-southwest fault system. Middle Eocene shelf-slope progradation and aggradation, in concert with renewed movement on the faults, resulted in the back stepping of fan deposition toward the northeast, contained between the fault systems. Late Eocene shelf-slope progradation, coupled with a shift in slope geometry, resulted in fan building from the northeast, into the central fault block.
The regional Eocene deposition system is dominated by mixed sand-mud systems in both BS-2 and BS-4 blocks. Synchronous fault movements and sedimentation has resulted in shifting of fan depocenters and compensation stacking of basin floor fans, which influenced subsequent Eocene sand distribution. Lower Eocene sedimentation, controlled by southeastward shelf and slope prograding and a late Paleocene turbidite channel delivery system, is characterized by a series of southeastward trending submarine channel systems displaying sinuous channel character. The upper lower Eocene and middle Eocene is characterized by more southerly trending channels and associated fans, which in-filled northern and central portions of the study area. Upper middle Eocene and upper Eocene sedimentation is dominated by northeastern sourced fans feeding southwestward, infilling the northeast portion of the central fault block. Slope channel length and width appears to be related to slope morphology; i.e., longer and wider channels associated with planar morphologies, and shorter and narrower channels associated with a more sigmoidal morphology. Smaller more short-lived channels appear to be associated with mixed sand-mud depositional systems. More long-lived channels are associated with sandy depositional systems. The thick, moderately sorted, high net-to-gross sand package penetrated in BS-4-NE discovery is part of a sandy fan system.
Geometric measurements of depositional elements of the slope channel complexes and toe-of-slope fans are comparable to similar deep-water depositional systems found in offshore Angola.