Petroleum Prospectivity of the Deepwater Orange Basin, South Africa

Agnes N. Jikelo
Petroleum Agency SA, Parow, South Africa

The Orange basin, straddling the South African - Namibian border, is part of the South Atlantic passive margin and was formed during the rifting of the South American and African plates.

The underlying structure of the Orange basin consists of synrift half - grabens trending sub parallel to the coastline. Post rift features of the basin include growth faults and toe thrusts within the distal part of the drift succession. Recently acquired seismic data reveals that growth faults and toe-thrust structures extend into deep and ultra deep water.

The deepwater geology has been inferred from shallower areas, and analogues from other parts of the world, in particular the South Atlantic have been used. The deepest wells drilled in the Orange basin, O-A1 and DSDP 361, confirm the existence of a regional Aptian source, with TOC values exceeding 4%. The Cenomanian/Turonian source, has also been predicted on a regional basis but has not been tested.

Potential sandstone reservoirs are predicted in the Lower Cretaceous and have been intersected by some boreholes. Four relatively deeper boreholes, showed poor poro-perm properties due to cementation by quartz and clay minerals. However these properties are expected to improve basinward and more so in the deepwater areas where depositional environment is different and favourable. An analogy of reservoir features is displayed by Tertiary channels offshore Angola, some of which host hydrocarbons.