Grobbler, Nico (PetroSA)



Deep marine basin floor channel and basin floor fan (bff) complexes are well developed in the third order Barremian (9A) to Aptian (13B) sequences in the Bredasdorp Basin, offshore the Southern Cape of South Africa. The bff com-plexes contain stratigraphically and structurally trapped hydrocarbons, within moderate to good reservoir quality turbidite channel sandstones. The Sable oil and gas fields are reservoired within the upper part of this turbidite system.

The sequences fall within a stage of early drift history with a progressively enlarging basin that flooded and integrated the initial post-rift embayments with connections to the proto- Indian Ocean. Deposition of well-defined systems tracts together with associated type1 erosional unconformities developed. The third-order (onlap-fill) sequences reflect both thermal subsidence along the ba-sin axis and episodes of re-activated faulting. Generalized facies distributions, determined from log patterns, core data and maximum grain size data have assisted in generating geological models for the region. Poorer quality channel overbank and sheet sand (distal fan) deposits are not resolvable from seismic and geological models must take this into account so that allowances can be made for these 'invisible' volumes.

Ideally, bff systems should be radial in shape but because deposition occurred in the relatively confined Bredasdorp Basin their shape is controlled by the ba-sin topography and as such are predominantly elongated.

The provenance for these sandstones consisted of Table Mountain quartzites and Cape granites sourced from the mainland and the Agulhas Arch. The ba-sin maintained it's strong northwest - southeast elongation, inherited from the synrift sub-basins and was open to relatively free marine circulation to the southeast with the Southern Outeniqua Basin and the Indian Ocean. Sedimen-tation into the Bredasdorp Basin thus occurred predominantly down the axis of the basin with main input direction from the west.


The Bredasdorp Basin, located on the southern continental margin off the coast of South Africa is mostly filled by marine Aptian to Maastrichtian deposits, which were deposited on pre-existing Late Jurassic to Early Cretaceous fluvial and shallow marine synrift deposits.

Drilling for hydrocarbons in the Bredasdorp Basin commenced in 1973, leading to the discovery in 1980 of the F-A gas field in the Berriasian to Valanginian shallow marine synrift sandstones along the northern flank of the basin (Figure 1). Further discoveries led to the gas-to-liquids (GTL) project at Mossel Bay commissioned in 1992, which produces syn-fuels from gas and condensate production from the F-A and E-M fields. Most recently the South Coast Gas Project (SCG) has been ratified to make available several central basin Barremian (9A) to Aptian (13B) gas and condensate discoveries in order to sustain and extend production of syn-fuels at the GTL plant. The generally thin pre-Aptian central basin gas charged reservoirs, confined to narrow channels, have proven to be a challenge to model, both geologically and commercially and their contribution to the basin's success will soon be realised.