Play Analysis of the Natal Valley and Durban Basin, Off the East Coast of South Africa

Abstract

The Aptian source rock is a prolific hydrocarbon producer and has been proven in the Bredasdorp and Pletmos Basins along the south coast of South Africa. Although not yet encountered along the east coast, this study focuses on the modelling of this interval as the primary source rock. Basin modelling suggest an oil mature Aptian source during the Upper Cretaceous and Paleogene. The Aptian surface is buried deepest (9km) within the Natal Valley, and at it shallowest (4,5km) within the Durban Basin. Despite the Aptian interval being largely underlain by oceanic crust within the Natal Valley, it does not automatically preclude it from generating hydrocarbons within the region. Apart from basement heat flow history, sediment thermal properties together with depositional history are often ignored when considering source rock maturity. Studies have shown that source rock over oceanic crust can obtain maturity of 1% (Ro) at present day depths of approximately 5km. In addition to overburden and burial depth, using paleo heat flow and estimated erosion of roughly 2km, the postulated Aptian source rock is likely to be mature to generate hydrocarbons within the Natal Valley and Durban Basin. Several new play types were identified and mapped along the east coast including contourite drifts and a basement draping sedimentary wedge. The Oribi contourite drift, Eocene to Oligocene in age, is a substantial feature that covers an area of nearly 40000km2 and is up to 270m thick. A second, Miocene aged contourite drift, known as the M-drift has an aerial extent of 20000km2 and a thickness of 270m. These drifts are located in water depths of 3-4km at depths of 4-5km below the seabed. A sedimentary wedge overlying and downlapping onto basement may provide a likely new hydrocarbon target within the northern Natal Valley despite it being in ultra-deep water. It resides in water depths between 2.5km and 3.5km, about 8km below the seabed. This large feature has been mapped in a region extending across 15000km2 with thicknesses of up to 450m. Various leads are recognised and differ in size from 850km2 to 2500km2. Best estimates range between 887 MMbbl and 5186 MMbbl recoverable oil, and 1.76 Tcf of gas. In addition to the mapped leads, numerous other leads such as mounded sedimentary features, carbonate build-up and basin/slope fans are observed on seismic data along the margin. Major risks are source presence, migration distance and biodegradation.

AAPG Datapages/Search and Discovery Article #90332 © 2018 AAPG International Conference and Exhibition, Cape Town, South Africa, November 4-11, 2018