The oil-producing reservoir units in the deepwater basin of Equatorial Guinea, Northwest of Bioko Island, are dominated by deposits of slump and debris flow processes. These gravity-flow processes, induced by relative sea level falls and local shelf margin tectonics, have led to a predictable stratigraphic hierarchy in the Zanclean (Lower Pliocene) of the Zafiro Field complex, Equatorial Guinea.

The stratigraphic succession begins with a highstand, quiet-water, pelagic and hemipelagic mudstone overlain by deformed mudstone. The latter is often overlain by pebbly mudstone, a consistent predictor of sandy reservoir facies which caps this stratigraphic hierarchy. Deposition of the sandy facies is from pulses of debris flows, generally interbedded with and finally capped by additional pebbly mudstone units. The last phase of deposition is marked by relative quiescence when the previously deposited sandy facies is reworked by bottom currents to give rise to the overlying bottom-current reworked (BCR) reservoir facies. Bottom-current reworking of sediments is a persistent phenomenon in deepwater marine settings. Each of the stratigraphic facies may contain varying amounts of reworked sands but the dominant amount of BCR sands is usually found overlying the main sandy reservoir facies of debris-flow origin.

Although debris-flow deposits (sandy and muddy) have a chaotic appearance at core scale, they are stacked in a predictable vertical order.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas