Summary

This paper presents Western Geophysical seismic data from their Deepwater and Ultra Deepwater (1500-3000 m) surveys in the southern Congo and Kwanza basins, offshore Angola. Two composite seismic profiles, each nearly 300 km in length will be shown to illustrate; 1) the seismic expression of the continental-oceanic transition zone, 2) the relationship between crustal extension and pre-salt sedimentation, 3) seismic stratigraphic relationships used to infer pre-salt marine transgressions and 4) depositional thinning of the salt onto the rift flank where oceanic spreading was initiated.

Models for the Lower Cretaceous rifting of the South Atlantic and, in general, models for continental breakup, are likely to be modified based on recently acquired regional seismic profiles from offshore Angola. Seismic data from the Deepwater and Ultra Deepwater surveys are providing 200-300 km profiles that extend past the continental-oceanic transition zone and onto oceanic crust. The broad continental margin (>200 km) of the Kwanza basin generally consists of zones with moderate crustal extension (b =1.1), more highly extended abandoned rift segments (b =1.4), and the rift adjacent to the paleo-spreading center (b = 3 - 5 ). The new seismic data illustrates that preceding continental breakup, sag basins had developed overlying abandoned rift segments and they likely contained marine pre-salt sediments.

The identification of sag basins by Brumbaugh et al. (1994) and their relationship to crustal extension (Henry et al., 1995) applies to both sides of the South Atlantic. Recent paleogeographic mapping (Abreu, in press) has identified shallow seaways on both flanks of the Walvis Ridge / Rio Grande Rise during Barremian. Minor rises in sea level would fill the seaways and marine waters would flow into the sag basins. This created a restricted marine environment that provided an ideal setting for the development of marine source rocks. These source rocks may be a major component to the petroleum system associated with the recent Girrosal discovery.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah