Albian Channel Fairway Mapping, Orange Basin, Offshore West Coast South Africa
Answa N. de Lange, Jeff Aldrich, and Candice Sissing
New Ventures, PetroSA, Cape Town, South Africa
The offshore passive margin Orange Basin, extends from the Luderitz Arch in Namibia, to the Agulhas Fracture Zone. It covers an area of about 250 000km2 with the focus of this study being the Albian Orange River depocentre about 200km south of the Namibian border, offshore South Africa.
During the Albian, marine and deltaic conditions were dominant, depositing reservoir quality incised fluvial channel sandstones between second-order unconformities, 14At1 (103Ma) and 15At1 (93Ma)*. At the beginning of this 3rd order super sequence cycle, the principal drainage system suddenly shifted about 300km towards the north. This caused the Orange deltaic system to prograde generally westwards under increasing sediment supply and accommodation rates, developing a depocentre near the mouth of the Albian Orange River.
The Albian distributary meandering channels were deposited on the lower to middle shoreface of a delta front, and form the reservoir of the Albian stratigraphic-structural play. This is currently the most significant play in the South African part of the Orange Basin and includes the Ibhubesi gas field. Sandstones in this gas field show good porosity-permeability qualities and extremely good flow rates (30+ MMcfgpd) for relatively thin (10 - 20m) sandstone reservoirs.
The objective of the poster is to show the individual channel complexes mapped from 2D and 3D seismic data, making use of the characteristic trough over peak amplitude anomalies that correspond to porous sandstones. Incorporating this data with borehole and regional data an analogue is made to present-day river systems.
Building a geological model of the channel fairways, and incorporating all seismic, regional and well data gives a good overview of the most prospective areas for continued gas exploration in this play.
* Haq et al.
AAPG International Conference and Exhibition, Cape Town, South Africa 2008 © AAPG Search and Discovery