--> --> Controls on Depositional Architecture and Reservoir Quality of a Large Deepwater Fan System Offshore South Africa: The Interplay of Provenance, Uplift and Oceanic Processes

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Controls on Depositional Architecture and Reservoir Quality of a Large Deepwater Fan System Offshore South Africa: The Interplay of Provenance, Uplift and Oceanic Processes

Abstract

Deep water fans are a significant focus for current hydrocarbon exploration. Acquisition of new data in emerging hydrocarbon provenances is allowing new insights into the presence and controls on clastic deep water systems. Sequence stratigraphic and seismic facies mapping on coarsely spaced 2D seismic data along the Transkei margin, South Africa shows, for the first time, the existence of a large-scale asymmetric submarine fan complex fairway. The late Aptian to Santonian aged fan system which measures 200 km in width is analogous in size to the modern Rhone fan or the ancient systems of the Lower Congo Basin. Late Cenomanian to early Campanian uplift of the southern African continental mass and a more humid climate created conditions for rapid erosion. The resultant large volume of eroded sediment was then deposited along the Transkei margin as a deep water clastic succession in excess of 1.5 km in thickness. Isochron maps indicate a long-lived stable entry point from the paleo-Mzimvubu River and compensational stacking of the fan lobes, with a pronounced depositional trend towards the southwest. Opening of a new sea due to onset of transform motion likely caused changes in ocean circulation and basin bathymetry in the Cretaceous. Seismically identified “shingling” bedforms and contourite drift deposits indicate that current activity was an important depositional process from Late Cretaceous through Tertiary deposition. These strong coast-parallel oceanic currents likely skewed deposition and reworked basin floor sediments. The integration of fieldwork has provided additional insight into the unroofing history and composition of the provenance area. Quartz-rich sandstones correlated to the Triassic Molteno Formation are expressed in outcrops of the Transkei coast and provide evidence that quartz-rich sediment existed across the whole Transkei catchment area. Early Cretaceous debrites contain clasts reworked from the Molteno. In contrast, Late Cretaceous outcrops indicate the landscape had largely equilibrated by the Campanian and that provenance lithologies were similar to those of the present-day, implying that the quartz-rich Molteno was being eroded between the Barremian and Campanian. Consequently, reservoir quality of the Santonian fan system is expected to be good. This paper will discuss the interplay between provenance, uplift and oceanic processes on the resultant depositional architecture and reservoir quality of the Transkei fan.