Gas Escape Features and Their Implication for Hydrocarbon Exploration in the Orange Basin, Offshore South Africa
Noluthando Jukuda1 and Christopher P. Davies2
1Southern Africa - West Coast, PetroSA, Cape Town, South Africa
2PetroSA, Cape Town, South Africa
The Orange Basin situated offshore west coast of Namibia and South Africa, extends from the Walvis Ridge in the north to the Agulhas Fracture Zone in the south. The basin boasts eight gas discoveries and only one oil discovery representing three separate plays. Of these the Albian Stratigraphic-Structural Play, established by the Kudu gas field in Namibia and the Ibhubesi gas field in South Africa is the most extensive and demonstrates the regional extent of the Petroleum System. The gas is probably sourced from Lower Aptian shales that are now gas prone due to maturity but some gas may come from interbedded Albian terrigenous shales.
The broad term ‘gas escape feature’ encompasses a wide range of surface or near-surface expressions that are a direct result of hydrocarbon migration from a deeper source. This study identifies and maps gas escape features, determines relationships with underlying structures and defines a classification system for the various types of features.
2D and 3D seismic data aid in the identification of these features, as well as in the establishment of whether underlying structures could influence the distribution geometries observed. Piston core data sampled at selected sites adjacent to gas escape feature locations confirm the existence of chemotropic communities, probably living on the escaping methane.
Mapping the geometry of the surface features and integrating it with the subsurface structures and their relationship with the basin and existing fields allows for a regional synthesis that provides more effective clues to possible exploration potential than from conventional evaluation of seep geometry.
Integrating regional mapping of gas escape features with traditional exploration methods will assist in high-grading the exploration potential of various areas of the Orange Basin.
AAPG International Conference and Exhibition, Cape Town, South Africa 2008 © AAPG Search and Discovery