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Rift Structures in the Angola Margin: Block 0


Angola Block 0 has long been a prolific asset for Angola and Chevron ever since the Cabinda Gulf Oil Company drilled the first well in the late 1950's. Recently, exploration activity has shifted its focus from finding traps in well-imaged post-salt sequences to exploring impact-sized traps in largely underexplored pre-salt rift structures. However, due to the presence of the thick Loeme salt layer and Toca carbonate layer the quality of seismic images for rift sequences has been generally poor. Thanks to modern imaging technology (long-offset, multi-azimuth PSDM seismic data), our understanding of pre-salt Geology has improved substantially so that many rift structures are clearly defined. Yet, in spite of the important advances in technology, poor data areas still exist making pure image-based interpretation on Block 0 rift structures quite challenging. In order to evaluate the main structural styles along the rifted Angolan margin and to define better trapping configurations on rift-related prospects, we have conducted a structural study, including key observations and understanding gained from constructing a series of balanced cross sections, to provide a basic structural model for rift structures in the area. Supported by constant bedding dip panels observed in syn-rift sediments from one recent well and widely observed a certain geometric relationship between folded hanging walls and partially-imaged fault reflections in various basement fault blocks, we conclude rift structures in the Angola margin have been mainly deformed by the extensional fault-related folding mechanism (Xiao and Suppe, 1992). This new interpretation framework with a restorable and testable structural model suggests we can use this model-based approach to both guide seismic interpretation through poor data areas and more precisely define trap geometry and structural timing for rift-related prospects. Furthermore this case study shows appropriate structural analyses can significantly reduce risk of exploring rift basins not only in Block 0 but along the entire West African margin.