Andr MuntinghSoekor (Pty.) Ltd. Parow, South Africa
With the fast changing political situation in South Africa it has become possible for Soekor (Pty.) Ltd. to invite international companies to participate in oil and gas exploration in the South African part of the Orange basin.
The Orange basin, comprising a 130,000 sq km area off western South Africa that extends northwards into Namibia, represents a large frontier basin with known hydrocarbon accumulations and the potential for giant fields.
Comprehensive seismic coverage and a recent deepwater seismic survey in the Orange basin indicate exciting opportunities in the form of shallow and deepwater plays.
EXPLORATION HISTORY
Exploration in the Orange basin began in 1974 with the first borehole, Kudu 9A-1, being drilled in the Namibian portion.
Two more boreholes, Kudu 9A-2 and 3, were drilled in 1987 and 1988 and resulted in the discovery and confirmation of Kudu gas field (Fig. 1).
Since 1974 some 30 boreholes have been drilled in the South African portion of the basin in 136-744 m of water. Encouraging evidence of oil and gas were encountered in boreholes targeting structural and stratigraphic plays in rift and post-rift sediments.
Two boreholes intersected reservoirs with potentially commercial flow rates of gas and condensate, while a third borehole discovered oil; four boreholes encountered encouraging gas shows, and a further 11 had gas shows.
The hydrocarbon plays tested range from half grabens in the rift unit to domal and fault controlled closures, stratigraphically defined lowstand plays, and regional and stratigraphic pinchouts in the post-rift unit.
Before 1990, hydrocarbon exploration off South Africa's west coast was concentrated in less than 450 m of water. The construction of a sequence stratigraphic framework for the Orange basin during 1989 has led to the recognition of several hydrocarbon plays and fairways and to the recommendation to extend the seismic coverage into the deeper water areas of the Orange basin.
The seismic grid was therefore extended into water depths greater than 450 m to permit adequate delineation of prospective traps, reservoirs, and source and seal elements within depositional system tracts.
TECTONICS, GEOLOGY
The South African part of the Orange basin is bounded by the South African/Namibian border to the north and the Agulhas-Columbine arch in the south (Fig. 1).
The western margin of the Republic of South Africa is a divergent plate margin underlain by synrift grabens trending subparallel to the coastline and overlain by the siliciclastic sediments of the passive-margin Orange basin.
AU the key unconformities have been mapped over the Orange basin with seismic horizon T regarded as basement and the highly erosional 6At1 (117.5 million years old) regarded as the drift-onset/break-up unconformity (Fig. 2).
Rift onset and the formation of half-grabens occurred in the middle to late Jurassic. The graben zone comprises an inner graben complex and a central synrift wedge developed basinward approximately 50-150 km from the present coastline (Fig. 1).
The inner graben complex is filled with Upper Jurassic and Lower Cretaceous coarse continental clastics, fluvial/lacustrine sediments, and volcanics laid down in an area of active synsedimentary faulting and rugged paleotopography.
Termination of active rift sedimentation is marked by the 6At1 unconformity, which truncated the succession during upper Hauterivian times. The central synrift wedge is predicted to be filled by thick fluvial and lacustrine deposits with minor basic and alkaline lavas. A possible pre-rift unit is inferred from onshore deposits and may consist of Permo-Carboniferous and Jurassic Karoo sediments and lavas preserved in the basal portions of some half-grabens.
The overlying early drift sub-unit (horizon 6At1-to-13At1), Hauterivian to mid-Aptian, represents a proto-oceanic succession deposited during the transition between fully rifting, continental conditions and that of fully drifting, open marine conditions.
This succession consists generally of restricted marine and red continental sediments, occasionally interbedded with lavas (Fig. 3). The early drift sequence is terminated by the mid-Aptian 13At1 unconformity, signifying the onset of full drift, open oceanic conditions.
The drift stage sedimentary wedge is a thick succession punctuated by numerous erosional unconformities and has recently been subdivided into seismic-stratigraphic sequences and component systems tracts. A zone of gravity faults with inferred maximum displacement during the late Cretaceous characterizes the shelf-slope break (Fig. 2).
During the Cretaceous drift phase, episodes of varying accommodation rates can be recognized. Sequences above the drift onset unconformity 6At1 (117.5 million years old) and below 13At1 (112 million years old) display well-developed aggradational as well as pro-gradational third order sequence sets deposited in a ramp setting.
Low tectonic and/or eustatic accommodation rates and predominantly progradational stacking patterns are characteristic of the sequences between 13At1 (112 million years old) and 14At1 (103 million years old).
Sequences between 14At1 and 15 At1 (93 million years old) are characterized by alternating progradational and aggradational stacking patterns.
Deposition of the highly aggradational, mostly third order sequences between unconformities 15At1 and 16Dt1-17At1 (85-80 million years old) indicate increased tectonic and/or eustatic accommodation rates. Sequence stacking along a very steep depositional margin was terminated at about 85 million years old by intensive gravity faulting and folding, followed by intensive Type 1 erosion.
Between the erosion of 16Dt1 and the end of the Cretaceous at unconformity 22At1 (67 million years old), reduced accommodation rates resulted in well developed, progradational stacking patterns.
The 22At1 unconformity marks the end of Cretaceous sedimentation with the Cenozoic sediments represented by a well developed wedge, thickening from a few hundred meters on the shelf to 1,500 m basinward.
Growth and gravity faulting are common throughout the Tertiary succession (Figs. 2, 3), and intrusives occur sporadically.
The major part of the Orange basin is located offshore, although recent aero-magnetic data suggest possible limited extension of the basin onshore.
GEOLOGY, PROSPECTIVITY
Results from the initial round of exploratory drilling proved the existence of source rock at three levels in the Orange basin (see table).
A borehole testing a graben play within the inter graben zone proved the existence of an oil prone lacustrine source in the pre-6At1 interval. Only three grabens have been tested by boreholes. The central graben zone will have expelled large amounts of oil generated from lacustrine source rock throughout the burial history of the Orange basin, and it is expected that this oil could be trapped in overlying structures.
The 13A sequence (mid-Aptian) source exhibits a rapid transition from gas-prone on the shelf to a good quality oil-prone source basinward. The 15A source interval, deposited during the global Cenomanian/Turonian oceanic anoxic event, is expected to improve from wet gas-prone to locally oil prone on the shelf to oil-prone beyond the shelf-break. Due to the lack of boreholes, this has not been proved.
Half of the 30 boreholes drilled in the Orange basin encountered some form of hydrocarbons. Two of the boreholes are classified as having potentially commercial flow rates of gas and condensate, and one borehole is classified as an oil well with encouraging oil shows.
Reservoir sandstone is developed throughout the stratigraphic column, and when intersected within favorable traps could yield large volumes of reservoired hydrocarbons. Reservoirs in the graben unit comprise fluvio-deltaic and lacustrine sandstones and conglomerates, while those in the post-rift unit range from fluvio-deltaic to marine sandstones.
Hydrocarbon bearing reservoirs exhibit excellent poro-perm characteristics at depths of as much as 4,300 m as indicated at Kudu 9A1.
Favorable sand-shale ratios were encountered in the drift stage sediments on the shelf with numerous valleys and channels indicating possible transport of sandstone onto the basin floor where it could be present in the form of turbidite and basin-floor fans.
Updip migration of the expelled hydrocarbons along sandstone beds, faults, and fractures is assumed for the different hydrocarbon plays not in direct contact with source rocks. Shales interbedded with reservoir sandstones in the different hydrocarbon plays provide the seals. Major faults may also act as seals.
Several play concepts remain untested in the Orange basin due to the sparse borehole control and large untested areas. These comprise graben plays, large domal structures, rotated fault blocks, fault traps, stratigraphic pinchouts, basin-floor fans, incised valleys, prograding wedge plays, and structurally enhanced highstand plays.
The Sequence stratigraphic approach followed by Soekor provides a sequence stratigraphic framework for the Orange basin and delineates hydrocarbon fairways in the basin.
This integrated approach to oil exploration, the recent discovery of good quality oil-prone source rocks, numerous gas and oil shows, and the large amount of untested plays have created renewed interest in the Orange basin.
Opportunities for continued exploration have prompted Soekor to extend seismic coverage of the Orange basin into 500-1,500 m of water.
Copyright 1993 Oil & Gas Journal. All Rights Reserved.
