Abstract: Tertiary Reservoirs in Congo-Kwanza-Namibe Basins

Raposo, António and Murthy Inkollu - Sonangol

Conspicuous Tertiary age sediments of river delta system that extends to 4000-meter ocean water depth demarcate the basins Congo, Kwanza and Namibe (Figure-1). Years of exploration efforts have lead to recognition of sand reservoirs in channel, delta, pro-delta and, basin floor fan setting. The basin slope is extensively inundated by intricate channel network. Further, the basin Eocene-Oligocene-Miocene hinge zones are dominated by sediment depocenters in fault-bound troughs. Nearly twenty-five years of significant exploration in Tertiary reservoirs has an impetus for the current onset of major discoveries in deep offshore.

The sand supply system is well documented in the basins from the numerous well penetrations and, 2D-3D seismic vintages. As a result, exploration efforts have become lucrative in the basins. Prolific hydrocarbon system is recognizable in distinguishable traps, generative source, migration pathways and seals. A major trap is in channeled sandstone that is enveloped in a four-way structure closure or fault-bound closure or a drape against salt diapir. The defined trap has a structure and stratigraphic component. Channel paths are easily recognizable from 3D data sets and, sand-rich zones from the amplitude character. Oil composition in the discoveries points to marine sources. TOC varies from 10% to 2%. The timing of oil generation is estimated to be around mid-Miocene and reached at optimal levels in upper Miocene. Fault conduit paths have facilitated the vertical and lateral oil migration into the reservoir. The reservoir seals are invariably interbedded argillaceous claystone.

The reservoir sands are in amalgamated or stacked channels, having prolific parameters. Sand porosity is 20% to 40% and, permeability is 1 to 5 Darcy. Cumulative DST's range 10,000 to 15000 BOPD. Depth to the reservoir is between 700 to 1300 meters from the mud line. The reservoir crude is moderate to low in gravity and viscosity, which is mostly influenced by the reservoir temperature. Reservoir identification is feasible from AVO, DHI such as flat spots (Figure-2) and, phase change in amplitudes.

The Tertiary depocenter thickness widely differs in the basins. Maximum thickness is in Congo (@ 6,000 m), intermediate in Kwanza (@ 3,500 m), minimum in Namibe (@ 2,500 m). The variation is attributable to the role of tectonism at the margin and, to the influence of rivers on sediment supplies rate. Also, salt diapirism has played a key role on the sediment distribution pattern and localization. Sediment transport is effectively controlled by the vertical and horizontal spread of salt. From the discussed criteria, major anomalous zones for Tertiary reservoir exploration are indicated in the open concession areas of the basins.

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