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Reservoirs
Early Cretaceous late syn-rift reservoirs have been proven by the Kudu field. Interestingly, the original reservoir target for this field was in the Upper Cretaceous post-rift. However, poor reservoir results in the post-rift led to Chevron drilling deeper into the syn-rift. The reservoir at Kudu is complex, with development hampered by high pressures plus porosity and permeability reduction.
Reservoir lithology is Neocomian aeolian sandstones, with porosity and permeability values reaching up to 20% and 767 mD, respectively, and preservation at depth due to early calcite cementation and secondary porosity generation due to leaching.
Aptian carbonates are proven in Namibia by recent drilling. However, good-quality reservoir has proven difficult to find. A more detailed picture of facies distribution will begin to unfold, allowing more targeting of good-quality carbonate reservoirs.
The final, and perhaps most promising reservoirs, are the Late Cretaceous turbidite channel and fan systems with up-dip stratigraphic and structural traps. The secondary target of Murombe-1 was a Santonian age channel complex and although water-wet, the 36 m (118 ft) of net sand within a 242 m (792 ft) interval had an average porosity of 15%.
Seal and trap
An adequate seal is perhaps the final piece missing in the Namibian jigsaw puzzle. Many of the wells drilled offshore Namibia have had good evidence of source and reservoir, but have had only hydrocarbon shows. In some shelfal wells, thick clastic sequences have been found with very little shale to act as a seal for the sandstone reservoirs. On the other hand, some wells have encountered the opposite problem: thick shale sequences with little sandstone. There is clearly plenty of shale in the Namibian system, and as exploration is focused offshore and into deeper waters, the presence of shales will be almost guaranteed, and the risk of seal failure will be significantly reduced.
Conclusions
Broadband seismic has improved the imaging of syn-rift and post-rift structures, enabling more confident identification and mapping of prospects. These data clearly demonstrate the presence of thick syn- and post-rift packages and thick sediment in deepwater. The improved imaging and resolution provided by this type of seismic acquisition significantly de-risks exploration in frontier areas, something that is of high importance where well costs are extremely high.
