Multiple Commercial Gas Accumulations Along a Single Third Order Sequence Turbidite Complex: The Barremian 10 as Reservoirs of the Bredasdorp Basin, Offshore South Africa

Vuyokazi P. Godongwana, Varsha Singh, Jim R. Turner, and M. Mlaba
New Ventures, PetroSA, Cape Town, South Africa

A number of gas and gas-condensate discoveries are present in the Bredasdorp Basin, offshore South Africa. The South Coast Gas (SCG) development targeted production from five Barremian gas accumulations to be tied back to existing infrastructure. All five reservoirs produce from one 3rd order low stand wedge Turbidite system in the Bredasdorp basin. Several facies representing slope, upper fan and midfan are demonstrated in log correlations.

The proposed depositional model for the Barremian-Aptian reservoir sandstones is a series of stacked deep-marine turbidite sands separated by shale interbeds. These sands were sourced from the shelf areas and funneled through a submarine canyon or transported by mass flows down to the slope and out across the basin floor.

The inferred style of lateral transition from thick amalgamated channel deposits to supra-fan argillaceous sediments is demonstrated by the log correlation of the Barremian sandstone in each discovery. Drilling has proved that stacked amalgamated sandstones exhibit rapid lateral thinning and thus lateral variation in petrophysical properties.

The Barremian sandstones are poorly imaged on the seismic data because of the low acoustic impedance contrast between the sandstones and encasing shales. In addition, the reservoirs are difficult to map due to the relatively thin sandstone beds.
Fourteen wells have penetrated the five Barremian discoveries with approximately 70m of core. Channel widths and heterogeneity as well as the effect of the thin sandstone beds on the hydrocarbon trapping mechanism are relatively unknown.

However, some of the follow-up wells drilled in these discoveries were characterised by thin tight sandstones and overbank claystones, demonstrating the difficulty in determining preserved channel dimensions.

AAPG International Conference and Exhibition, Cape Town, South Africa 2008 © AAPG Search and Discovery