Early Aptian to Late Eocene Paleogeography of the Orange Basin and its Implications for Facies Distribution, Offshore South Africa

Salomo, Jonathan
Promotion Department, Petroleum Agency SA, Cape Town, South Africa.

Paleogegraphic maps were constructed for selected stages of the Cretaceous and Paleogene periods, from early Aptian through late Eocene, off the west coast of South Africa. Regional patterns of facies distribution were examined using selected well and biostratigraphic data. These were combined with regional isopach and depth structure maps in order to generate paleogeographic maps including illustrations of sand distribution throughout the drift succession.

The aim of this study is to conceptualize the regional effect paleo geomorphological environments had on petroleum reservoir and source facies quality and distribution, through time.

Important influences on the development of the Orange Basin have been the location of depocentres and discharge points of paleo river systems, changes in climate of the hinterland since break-up, sea level changes and oceanic circulation. During the Early Aptian with the development of a broader fluvial plain as the margin became wider and with climates probably still very arid, meandering systems developed in areas where gradients were lower. Sediments were derived from the immediate metamorphic hinterland. In the Barremian, ramp-like shallow basin deltaic and coastal plain systems established. Anoxic conditions during the late to mid Aptian triggered the deposition of marine source shales. The late Aptian saw deltaic and coastal plain systems established. After the final separation of the Falkland plateau from southern Africa in the late Albian, the main sediment input switches from the Olifants to the Orange Rivers. The late Albian-lower Cenomaian constitutes the development of a steep shelf margin, especially in the southern parts of the Orange Basin. Late Turonian paleogeography constitutes free connection with the World Ocean and anoxic conditions, causing the deposition of marine source rocks. Prolonged aggradation throughout the Turonian led to the well documented gravity faulting event of the late Cretaceous, with the depocentre migrating westward during the Campanian and Maastrichtian. Sedimentation rates are greatly reduced in the Palaeocene and sequences are also much more complete. The Orange Basin's long and diverse paleoenvironmental history produced a variety of potential petroleum source rocks and reservoirs in terms of quality and geometry.

AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012