Application of Seismic Sequence Concepts to Basin Evolution and Petroleum Exploration, Pletmos Basin, Offshore South Africa

John H. G. Keenan

As a result of excellent seismic data, shallow burial depths, and slow subsidence rates, 67 type 1 and type 2 unconformities were identified in the postrift Cretaceous section of the Pletmos subbasins. The purpose of this study was to develop a seismic sequence-stratigraphic framework for the mid-Valanginian to mid-Campanian sequences as a basis for future petroleum exploration. The study further developed the recent stratigraphic techniques proposed by Exxon. Sixteen of the 67 type 1 and type 2 unconformities could be correlated directly with Exxon's global third-order cycles; the remaining 51, interpreted to be fourth- and fifth-order cycles, could be grouped into megasequences comprising 2 to 6 sequences, bounded by major third-order type 1 unconformities. The study de onstrates the complex interplay of variations in sea level fluctuation, differential subsidence rates, sediment supply, and tectonic stability and the resultant wide range of geometries and styles in depositional environments. By using seismic sequence concepts, these variables can be accounted for and can be made with greater assurance. Further analyses of lowstand systems tracts are underway to localize stratigraphic trap prospects to be drilled in the near future.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.