Sequence Stratigraphy of the Orange Basin, Western Offshore South Africa

MUNTINGH, ANDRE, SOEKOR (Pty) Ltd., Parow, South Africa

A seismic/sequence-stratigraphic framework for the siliciclastic fluvial to deepwater Cretaceous strata of the Orange basin has been constructed. Sequence-stratigraphic concepts developed by Exxon were used to interpret 10,000 km of seismic data and logs from 31 exploration boreholes within an area of 90,000 sq km.

The sequence stratigraphy of the western margin exhibits 34 cyclical depositional sequences intepreted to document the response of the passive margin to cyclical changes in relative sea-level from Mid-Aptian to Late Maastrichtian times. All but one of the sequence boundaries are type 1 unconformities displaying intense erosion on the shelf and interpreted to develop during periods of rapid fall in relative sea-level.

Basin-wide sequence boundaries (type 1 and 2 unconformities) were correlated with the third-order cycles of Exxon's global cycle chart utilizing SOEKOR'S paleontological dating of a limited number of marine condensed sections.

Higher-order cycles are recognized and best developed along the flanks of the structural arches where lower subsidence rates permitted impact of higher frequency cycles.

Component depositional systems tracts inferred to have resulted from changes in relative sea level were interpreted and paleogeographic maps were constructed outlining incised valley systems on the shelf, shelf edge canyons, prograding wedges, basin floor fan, and slope fan systems.

Distal marine shales and marine condensed sections provide both seal and hydrocarbon source.

The chronostratigraphic and depositional framework being used to predict prospective play areas for hydrocarbon exploration on the western offshore of South Africa is also applicable to the Namibian offshore.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)