Implications of Slope Failures for the Hydrocarbon System in the Orange Basin, Offshore South Africa

Gesa Kuhlmann¹, Rolando di Primio¹, Dave van der Spuy², and Brian Horsfield^1
1GFZ Potsdam, Potsdam, Germany
²Petroleum Agency SA, Cape Town, South Africa

Along the south-west African passive continental margin different episodes of huge slope failures have been documented. Our study is aimed at studying the impact and timing of observed mass failures on the petroleum systems within the southern Orange Basin.

Our investigations of basin evolution make use of 2D seismic interpretation covering the continental margin from the shelf to the deep marine domain of the southern Orange Basin within the time-frame from post-rift to present day. The main depocentre locations developed from east to west, with siliciclastic basin infill and aggradation during the Cretaceous and progradation during the Tertiary. A Late Cretaceous episode of margin instability occurred in the north east and a second phase of Tertiary mass movement occurred in the south east of the study area. At the location of the Cretaceous mass failure an increase in sediment load generates a profound effect on the underlying source rocks seen in modelled maturation and petroleum generation potential. Through the enhanced thickness generated by the synsedimentary Tertiary slope failure hydrocarbon generation rates increase again but do not contribute substantially to the maturation history.

Today’s gas leakage features fed by the active hydrocarbon system are observed in the shelf area (<400 m waterdepth) east of the slope failures. The location of gas generation in the outer basin compared to the gas leakage sites, however, implies subsequent migration towards the inner part of the basin. Further modelling will constrain the migration pathways, timing and duration of the events and lead to a basin-scale quantification of thermogenic gas contributions into the hydrosphere and atmosphere as a function of geologic time

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas