Impact of Seismic Loop-Scale Depositional Models on Reservoir Architecture in a Heavy Oil Accumulation, Santos Basin, Brazil

W. Keith Campbell, Ciaran O'Byrne, and Neil W. Stillman
Shell International E&P Inc, Houston, TX

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

A detailed loop-scale seismic interpretation was undertaken to better understand the regional context, paleo-slope position and depositional architecture of the BS-4 reservoirs. The main focus was on mapping of internal heterogeneities and understanding aquifer potential, both of which have direct impact on field development planning.

Study results demonstrate that considerable uncertainties remain with respect to distribution of discrete heterogeneities within these high net-to-gross reservoirs. Reservoir facies are interpreted to have been deposited within a dominantly channelised environment as part of a fan apron developed across an inherited stepped slope profile. An Eocene intra-slope break is evident 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 reservoir complex is situated in the distal part of this low gradient “step flat” with a higher gradient “exit ramp” immediately downdip. 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. Depositional models derived from this study provide constraints on static reservoir model inputs.