Several deepwater turbidite discoveries of Eocene age have been made in the northern Santos mobile salt basin, offshore Brazil. The Shell-operated BS-4 NE discovery (2001) shows significantly better reservoir development than in equivalent sections penetrated nearby. Both the discovery and appraisal wells encountered in excess of 100 m of predominantly clean sand.

A 3D seismically-based study covering some 4000 km2 was undertaken to better understand the sub-regional context, paleo-slope position and depositional architecture of the proximal, high net-togross reservoirs penetrated in the BS-4 NE. The main focus was on detailed loop-scale seismic interpretation of internal heterogeneities and understanding aquifer potential, both of which have direct impact on field development planning.

Study results demonstrate that considerable uncertainty remains with respect to the distribution of discrete heterogeneities within these highly amalgamated reservoirs, specifically the nature and configuration of predominantly silty material that drapes over incision surfaces. Reservoir facies were deposited within a dominantly channelized environment as part of a fan apron developed across an inherited stepped slope profile, with a depositional trend from NW to SE. An Eocene intra-slope break is evident about 15 km up-dip of the present-day closure, at the downward limit of several confined feeder canyons, outboard of which low- to moderate-aggradation channel complexes were deposited across a lower gradient slope. The BS-4 NE reservoir complex is situated in the outer part of this low gradient “step flat” with a higher gradient “exit ramp” immediately down-dip. Seismic mapping of complex erosional surfaces with evidence for multi-phase knick point migration and significant bypass, also supported by core and image log data, provide a framework within which to better understand the distribution of key heterogeneities. A key observation is that the canyon systems and channel complexes become progressively younger to the NE. There is a strong interplay between this migrating sedimentary system and tectonics that can be related to the amount of incision and bypass. The conceptual models derived from this front-end work provide key constraints to static reservoir model building in order to derive realistic infill architecture for testing the dynamic impact of reservoir heterogeneities. Results from the 2006 appraisal drilling on BS4-NE are consistent with the conceptual geological model.

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