New geological and petroleum insights gained from geophysical imaging along South Africa’s western margin

Abstract

This presentation will illustrate how new, until now hidden, messages can be interpreted from gravimetric, magnetic and conventional 2-D seismic reflection data imagery along the western continental margin of South Africa (Fig.1). The advent of satellite derived offshore gravity together with sea floor and land topography has opened up a refreshing way of looking at our continental margins, and has re-emphasised the link between the latter and their continental interiors and oceanic regions. Reprocessing of old magnetic records, and the integration of land and marine data sets, is also rapidly expanding our understanding of basement architecture. Expanding knowledge of sand-rich deep-water canyon-fan ~systems (largely as the result of pioneering petroleum exploration and 3-D seismic surveys), has given new insight into the predictive interpretation of 2-D seismic data in frontier areas where no boreholes or 3-D seismic data exist. The direct effects that hydrocarbons may have on seismic data have helped reduce exploration risks world -wide. For example, the ‘chimney’ effect that natural gas escaping through the rock column can have on seismic data may point to exploration fairways for gas in the Orange Basin (Fig.1). This is particularly the case when viewed together with gas shows, surface gas seeps, ‘bright spots’, AVO anomalies, etc. in the same vicinity, or along the predicted migration routes away from known source rock ‘kitchens’. The increasing attention given to the deep-water portions of margins has led to the recognition of possible gas hydrate zones in many areas. This margin is no exception.