ABSTRACT: Bonga Field, Deepwater Nigeria: Comparison of Near-Surface, Well-Calibrated Submarine Channels with Reservoir Channel Sands

Abstract

Comparison of near-surface channels with channel reservoirs at Bonga, offshore Nigeria (OML118, 1000 m water depth), demonstrates some important lessons in using shallow analogs for reservoir prediction. Studies of shallow hazards and pipeline routing, integrating 3D and high-resolution 2D seismic with well logs, illustrate seafloor and nearsurface features including a large mud-draped canyon, mud volcanoes, and numerous pockmarks, which indicate fluid expulsion. Both the shallow overburden (Plio-Pleistocene) and the main reservoir intervals (Miocene) are characterized by deepwater channel geometries visible in seismic profiles, on map views, and within 3D volume views. Most near-surface channel features inhabit broad, relatively straight scours 1-3 km wide and 30-300 m deep, filled by composite packages of smaller, often sinuous channels 100500 m wide and 5-50 m deep. Although many of these near-surface channels display a chaotic to low-continuity, high-amplitude seismic facies character often associated with sandy fill, well logs through the near-surface section indicate these are mainly filled with mud. Channel geometries at reservoir level show different characteristics. The smaller-scale map-view geometries are consistent in size with the near-surface channels but the larger host scours are not as prevalent or obvious, and some thin, sheet-like sands are also present. A transition from lower slope to upper slope at the end of the Miocene probably accounts for this variation. A fundamental understanding of geologic setting and rock/fluid variation is critical before extrapolating seismic facies information from shallow analogues to deeper reservoirs.