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Seismic-Sedimentological Interpretation of Turbidite Sand Lobe Reservoirs. The Abo Field “B200 Sands,” Nigerian Deep Water

Fonnesu, Franco 1; Tarchiani, Cristiano 1; Ogunshote, Babatunde 2
1 Eni E&P, San Donato Milanese, Italy.
2 NAE, Lagos, Nigeria.

The proposed example is aimed to show the 3D seismic reconstruction and the related sedimentological interpretation of the main reservoir level of Abo field, a producing oil field in the Nigerian deepwater. The study, carried out by Eni and NAE personnel, is supported by a detailed horizons picking and interpretation of the derived seismic attribute maps (amplitude, AI, continuity, facies). It documents the relationships between small sized erosional channels (up to 300m wide) and their genetically associated depositional sand lobes (the B200 reservoir). On the attribute maps these channels show a low sinuosity thalweg and a fine-grained infilling, as suggested by their low amplitude and high AI response. They dissect a series of oil bearing sand prone lobes, logged and cored, showing low AI and high amplitude patterns on seismic maps. The available core data indicates that the lobe sands where periodically affected by bottom current activity capable to remove and redistribute fine sands and muds.

The overall areal distribution of the sand lobes suggests a basinward forestepping of the channel mouth, with younger lobes deposited downcurrent. Upcurrent, the sediment bypass within the persistent channel conduit, caused the dissection of the older lobes.

The proposed depositional model is based on the assumption that when the feeder channel is active its thalweg is virtually devoid of sediments. Some residual lags can locally deposited along the channel axis, but the gross of the turbidite flows have enough energy to bypass the channel region. The bulk of the sand load will be deposited downcurrent, at the channel mouth, forming a lobe accumulation.

If the clastic distribution pathway remains stable and the source provides a relatively continuous supply, the feeder channel is forced to erode the previously deposited lobe unit and the turbidite flows will form another younger lobe in a more downcurrent position. The process will interrupt when the clastic source is deactivated or, upcurrent, a major change in the channel pathway occurs (i.e. channel capture). In this case the channel conduit is abandoned and consequently filled by fine-grained deposits forming the major barriers to hydrocarbon transmissibility in the Abo field.

The authors thank Eni, SNEPCO and NNPC management for permission to present this study.


AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009