Abstract: Orderliness in the Midst of Chaos: Prediction of Deep-water Reservoir Facies in a Slump and Debris Flow-dominated System, Equatorial Guinea, West Africa

Famakinwa, Simeon Bolaji - Mobil Equatorial Guinea Inc.; and G. Shanmugam - Mobil Technology Co.

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 consists of a lower mudstone unit, often bioturbated, dark to dark gay in color with a smooth waxy feel to touch. This highstand, quiet-water mudstone was deposited primarily by hemepelagic settling. The overall microfloral and microfaunal assemblages in the unit indicate transgressive conditions. The Intra Qua lboe-Q (IQI-Q) sequence boundary separates this lower unit from the overlying deformed mudstone. The latter is usually found in association with pebbly mudstone, a consistent predictor of sandy reservoir facies within the overall stratigraphic cycle. The pebbly mudstone has absolutely no reservoir quality but is often succeeded by high-quality reservoir, massive sand which forms the major reservoir unit in the Zafiro Field complex. Both the pebbly mudstone and the massive sand were deposited by a process dominated by debris flows. The debris flow sands are thought to have occurred in pulses, and in between the pulses of sands are additional pebbly/deformed mudstone intervals. The reservoir horizons contain palynofloral assemblages characterized by Pediastrum, Botryococcus and Monoporates maxima indicating erosion of proximal areas and subsequent transport into a deeper marine setting.

The stratigraphic hierarchy continues with the deposition of fine-grained, well sorted, ripple-laminated sand and silt unit with interbedded mud layers, deposited mainly by a process of bottom-current reworking (BCR), during the last phase of deposition marked by relative quiescence. BCR sands are a latent reservoir unit in the Zafiro Field complex with fair to good reservoir quality. In our depositional model, reworking of the previously deposited sandy debris flow facies gives rise to the overlying BCR 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. Additional contorted and pebbly mudstone units may cap this sequence before the next highstand event comes in. Graded mudstone units are observed almost throughout the entire stratigraphic section. These units consist of numerous thin (1 - 5mm) silty to clayey discrete layers, almost like a set of varves, and are associated with virtually every process and sedimentologic facies. It is interpreted that the dominant process responsible for this facies is muddy turbidity current. Individual layers typically show grading, beginning with an underlying interval of light gray, silty sand to an overlying dark gray, clayey and muddy interval.

This notable stacking pattern is recognized spatially and temporally in the deepwater siliciclastic system of Block B, Equatorial Guinea, thus suggesting that although debris-flow deposits (sandy and muddy) may have a chaotic appearance at core scale, they are often stacked in a predictable vertical order.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil