Scour Lobe: Fundamental Depositional Element of Deepwater Distributary Systems

Vitor Abreu¹, John Van Wagoner², Rick Beaubouef¹, and D. C. J. D. Hoyal^2
1 ExxonMobil Exploration Company, Houston, TX
² ExxonMobil Upstream Research Company, Houston, TX

An ultra-high resolution 3-D seismic survey in the Basin 4 of the East Breaks mini-basins (offshore Texas, GOM), provides near outcrop scale resolution of a Pleistocene submarine fan. The fundamental depositional element in the fan is a Scour Lobe, which is strongly erosional and confined updip; lobate, and somewhat erosional and weakly confined downdip. Scour Lobes are 1 to 5 kilometers long, hundreds of meters wide and up to 25 meters deep in the scoured portion; and 1 to 3 kilometers wide and about 10 meters deep in the lobate portion.

Updip, the surface at the base of the Scour Lobe has a deep and narrow scour. Downdip, the scour broadens and shallows. Further upstream of the proximal part of the Scour Lobe, the depositional surface is conformable (a surface of bypass). The updip extent of the scours is interpreted to result from nickpoint migration in the upstream direction (headward erosion). Deposition evolves from prograding and then backstepping packages probably associated to flow avulsion, when a portion of the flow is diverted towards another location at the margin of the fan (region of maximum gradient) resulting in progressive abandonment of the previous sediment fairway.

The Scour Lobe model has strong implications for reservoir characterization of deepwater weakly confined and distributary systems. This model redefines EOD mapping in these systems, with implications for prediction of lateral and vertical connectivity as well as physical properties distribution. Importantly, the Scour Lobes seem to be the preponderant reservoir architecture in distributive producing fields.