Abstract: Deep-Marine Slump and Debris-Flow Dominated Reservoirs of the Zafiro Field, Offshore Equatorial Guinea

SHANMUGAM, G., S. B. FAMAKINWA, and R. J. HODGKINSON, and L. C. BLUNDELL

The Zafiro Field is the first significant oil discovery (1995) in Equatorial Guinea. Detailed study of over 1000 feet of conventional cores from the Intra Qua Iboe (IQI) interval (Pliocene) suggests that sandy reservoir intervals are dominated by sandy debris flow, slump, and bottom-current reworked facies. Deposits of turbidity currents are absent in sandy intervals. A deep-marine slope or base-of-slope setting is proposed; submarine-fan models with channels and lobes are not appropriate for these debris-flow dominated sands. The reservoir exhibits a mounded external seismic geometry with chaotic internal reflections. In wireline logs, these sands represent blocky, fining-up, and coarsening up trends. The fining-up log motif is a manifestation of abundant mud clasts in the upper part of the sand interval because actual sand size tends to increase upward. Dimensions of debris flow units estimated through seismic amplitude extraction work correlated to logs and cores are in agreement with empirical thickness:width ratios (1: 30-60) derived from modern and ancient examples. The IQI sands show random map-view trends that are characteristic of debris flows and slumps. Complex interfingering of reservoir sands suggests possible connectivity (both vertical and horizontal) between sands. In spite of their slump and debris flow origin, the principal reservoir sands commonly range in porosity from 25 to 30%, and permeability from 1000 to 3000 mD. DST flow rates are over 10,000 BOPD on a 2 3/8" choke. The IQI reservoir sands in the Edop Field (offshore Nigeria) that occur updip from the Zafiro Field also exhibit analogous depositional facies and reservoir properties. The Edop-Zafiro trend provides a test case for slope to base-of slope systems dominated by slumps and sandy debris flows.

AAPG Search and Discovery Article #90942©1997 AAPG International Conference and Exhibition, Vienna, Austria