Results of Recent Drilling of Specific Lowstand Systems Tracts Targets, the Implications to Future Exploration, and the Seismic-Sequence Stratigraphic Model Pletmos Basin, Offshore South Africa

KEENAN, JOHN HEWSON GRANT, Soekor Ltd, Republic of South Africa

The Pletmos basin is a complex series of subbasins off the south coast of South Africa. Depositional styles displayed in the postrift mid-Valanginian to recent sediments reflect the complex interplay of variations in subsidence, sediment supply, eustatic change, and climate. An extensive seismic sequence stratigraphic framework study carried out in 1988 served as the basis for further oil exploration and the evaluation of lowstand systems tracts.

Wells were drilled to intersect shelf-margin wedges, basin-floor fans, slope-front fans, prograding lowstand wedges, and incised valley-fill successions depicted by detailed mapping of recent seismic data, integrated with well data. Sequences that apparently resulted from both major and minor downward shift in relative sea level were tested. Although there appears to be adequate sand accumulations on the relict shelf edge in the provenance area, there was little sand in the lowstand target. Seismically, the interpreted lowstand elements are located beyond and below the relict shelf and reflect geometries clearly indicative of component elements of a lowstand systems tract. However, there seems to be some contention about the processes that brought about their deposition.

It is important to distinguish between lowstand systems that have been derived by slumping and slope failure, and those that have developed as a result of sediment removal from the shelf and subareal regions during relative sea-level fall. Sand prediction and the relationships between source rock, migration pathways, and reservoir can, therefore, be ascertained with a higher degree of certainty.

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