Paul L.A. Burden
Soekor (Pty.) Ltd.
Parow, South Africa
The Bredasdorp basin is situated off the south coast of the Republic of South Africa, southeast of Cape Town and west-southwest of Port Elizabeth (Fig. 1).
Both cities have modern deepwater harbor facilities. Infrastructure along the coast includes well developed road, air, and rail links. A petrochemical plant for the conversion of offshore gas and condensate to hydrocarbon fuels as well as certain chemical feedstocks has recently been completed at Mossel Bay about 100 km north of the Bredasdorp basin. Refineries are situated at Cape Town and Durban.
The basin covers about 18,150 sq km and lies beneath the broad continental shelf known as the Agulhas bank, where water depths range from 80 m in the northwest to 230 m in the southeast at the present-day shelf edge.
The Agulhas current, which flows southwesterly, is concentrated along the shelf edge. The climate is temperate, and weather conditions are seldom extreme.
Soekor (Pty.) Ltd., the state funded oil exploration company, is actively exploring for hydrocarbons in the basin, where 135 wells have been drilled since 1970.
GEOLOGY, STRATIGRAPHY
The Bredasdorp basin is the most southwesterly of four rift basins that first developed during middle to late Jurassic and probably extended onto the adjacent Falkland plateau, which at that time lay immediately to the south and formed part of southern Gondwanaland.
The major fault trend during rifting followed the southeasterly trend of older compressional structures of the Permo-Triassic Cape fold belt. This resulted in the development of the four southeasterly-elongated rift basins separated by basement highs. The other three basins are the Pletmos, Gamtoos, and Algoa basins (Fig. 1).
During late Valanginian, right-lateral shear stresses developed along the Agulhas-Falkland fracture zone, a major Gondwanaland breakup structure. Resultant plate movement at the onset of drifting separated the Falkland plateau from the African plate.
The pre-existing rift basins were dissected and subsided, and marine conditions prevailed thereafter. Bredasdorp basin stratigraphy is therefore divided into synrift and drift episodes of sedimentation (Fig. 2).
Economic basement in the Bredasdorp basin consists of slates and quartzites of the Ordovician to Devonian Cape supergroup that are extensively faulted and fractured. The northern and southern flanks of the basin are bounded by the Infanta and Agulhas arches, respectively.
These features are elongate basement highs bounded by major fault scarps between which the synrift/drift depocenters were developed (Figs. 1, 3).
The Bredasdorp Jurassic-Cretaceous sediments directly overlie basement and fill a number of large southeasterly-trending half grabens. The succession is divided into two sequences separated by an angular type 1 unconformity-the drift-onset unconformity 1At1.
The graben-fill comprises a thick accumulation of alluvial fanglomerates and braided and meandering fluvial sediments that were deposited contemporaneously with faulting. Tuffaceous interbeds in this sequence indicate intermittent volcanic activity. The early graben fill is of Kimmeridgian or Portlandian age (Fig. 2).
The first marine sandstones occur within the synrift succession, where they are interbedded with lagoonal and fluvial claystones. A thick marginal marine sandstone complex caps the sequence. This complex is well known from boreholes on the flanks of the basin and is an extensive regressive coastal barrier bar system of early Cretaceous age.
These sandstones form the major gas reservoirs of the north flank gasfields (Fig. 1) where the F-A gas field is currently being developed. The rift fill is truncated by the rift/drift unconformity (1At1), which is most erosive on the basin flanks. The synrift succession is intensively faulted and folded in places.
The drift sequence, deposited after the major rift/drift unconformity (1At1) is characterized by deep-marine poorly-oxygenated conditions. The early drift succession from seismic horizons 1At1 to 5At1 is an onlap, infill succession of deep marine claystones and sandy turbidites of Hauterivian to Barremian age (Figs. 2, 3).
Gas prone source rocks occur in some north flank wells and are expected to be present in the center of the basin but have not been intersected.
Barremian to mid-Aptian sedimentation was characterized by strong shelf progradation from the northern margin of the basin (Fig. 3). Marine claystones are interbedded with shelf shoal sandstones around the northern rim and with deep marine fan lobe and channel sandstones in the central basin,
During the Barremian low sedimentation rates resulted in the deposition of dry to wet gas prone and locally oil prone organic rich shales in the 5A and 9A sequences that sourced gas accumulations in the basin (see table).
The mid-Aptian unconformity (13At1) was followed by a period of sediment starvation over much of the basin during which time organic rich shales were deposited in the central basin area. These wet gas and oil prone shales are the thickest and best quality source rocks in the Bredasdorp basin (Table 1).
The late Aptian to Cenomanian sequences (Fig. 2) below sequence 15A show strong progradation from the northwestern rim of the basin. In the northwest, shelf sandstones are interbedded with marine claystone while submarine fanlobe/channel sandstone development occurs, especially in the 14A sequence in the central basin area.
During late Albian to Coniacian (Fig. 2), another episode of sediment starvation occurred that resulted in the development of a further organic rich shale interval, but this is thin and mostly immature.
Late Cretaceous and Tertiary sediments (Fig. 2) are of shelf origin and are generally interbedded calcareous sandstones and marine claystones. By the end of Cretaceous, open shelf conditions prevailed over the entire Bredasdorp basin. Equivalent deepwater sediments are present to the southeast in the Southern Outeniqua basin (Fig. 1).
The main tectonic feature of the Bredasdorp basin is extensive extensional faulting that cuts the synrift and in some cases the drift sequences up to the 5At1 level (Fig. 3). There is some evidence for uplift, inversion, and folding having occurred during pre-5A time (early drift).
During early Tertiary, numerous lampropyric intrusions were emplaced especially in the western and eastern areas of the basin. During the same period at least 500 m of uplift occurred along the Agulhas arch along the southwestern rim of the basin.
DISCOVERIES, POTENTIAL
The first exploratory well in the Bredasdorp basin, F-1, drilled during 1970, encountered gas shows within shallow marine sandstones directly beneath the rift/drift unconformity (1At1) and below 1OA51 in the drift succession. However, reservoir quality was poor and DST results disappointing.
During the late 1970s Soekor drilled various other synrift faulted structural highs throughout the basin, some of which had encouraging gas shows beneath 1At1.
In 1980 Soekor followed up the gas discovery at F-1 with the F-A2 borehole on the same structure. F-A2 encountered porous and permeable shallow marine reservoir sandstones beneath 1At1 that flowed as much as 26 MMscfd of gas with a total of 1,200 st-tk b/d of natural gas liquids.
Today F-A gas field and its satellites form the Mossel Bay Gas Project, which will produce 25,000 b/d of petrol and diesel for more than 25 years. The F-A platform has begun supplying gas and condensate to the large petrochemical plant at Mossel Bay, where these are converted to liquid fuels.
Soekor currently manages the offshore activities of the project, which include operation of the platform, the monitoring of production boreholes, and the geological and geophysical interpretation of data from these boreholes.
Various synrift gas prospects have also been drilled along the southern flank of the basin. Some of these produced substantial gas and NGL on test, but reservoir qualities are generally poor in this area.
Play types are faulted domal closures where oil to wet gas prone source shales within the overlying early drift sequence onlap pre-1At1 shallow marine, synrift sandstones.
Exploration in the drift succession began with the drilling of borehole E-AA1 in 1986, located in the south central basin and drilled to test domal closures at 1At1 and 13At1.
The borehole discovered oil in Albian deep marine turbidites in the 14A sequence and gas and condensate in geologically similar sandstones of Barremian to Albian age in the 5A to 12A sequences. Combined flow rates for the intervals tested were 5,250 st-tk b/d of light oil and NGL and 27 MMscfd of gas.
The E-AA1 discovery heralded a period of intensive drilling in the central basin that still continues. The targets comprise submarine fan and channel sandstones that are developed on the type 1 unconformity surfaces of Barremian to Aptian age.
Throughout this period an important factor in the recognition of such deep marine fan complexes has been the understanding of seismic-stratigraphic principles and their application to the basin.
During 1988 a team of Soekor explorationists worked with Dr. L.F. Brown at the University of Texas at Austin. Their task was to construct a seismic-stratigraphic framework and geological history for the Bredasdorp basin. Major unconformities were identified as being type 1, and areas of high prospectivity were identified and successfully drilled.
Since the initial discovery a number of similar fields have been discovered, with oil flow rates as high as 8,500 st-tk b/d. Soekor currently has an exploration partner in Engen, the energy division of Gencor, a South African corporation.
Engen's participation to date has been exclusively in boreholes targeting oil and NGL in the drift sequences in the 13A and 14A sequences.
Soekor is currently looking for more partners to explore for similar fields in this and the other South African offshore basins.
EXPLORATION DATABASE
Since 1970, the beginning of Soekor's offshore exploration in the Bredasdorp basin, 46,000 km of multichannel seismic data have been acquired over the area.
In recent years this has been processed by Soekor's own processing department. In addition three 3D seismic surveys have been acquired over F-A gas field, E-M gas field, and the E-AA and EAR oil fields in the central basin.
A total of 135 boreholes have been drilled in the basin as of August 1992. Most have full suites of electric and geological logs.
In addition Soekor routinely carries out a wide variety of studies on data from each well including petrophysics, petrography, core analysis, paleontology (foraminifera, ostracods, dinoflagellates), geochemistry, and vitrinite reflectance.
Soekor also routinely applies its own proprietary amplitude versus offset techniques, which assists in the detection of hydrocarbons, to seismic data in the Bredasdorp basin.
ACKNOWLEDGMENTS
I would like to thank my colleagues at Soekor for their support in providing data for this article as well as the management of Soekor for their permission to publish it.
Copyright 1992 Oil & Gas Journal. All Rights Reserved.
