Multiple Seismic Interpretation Scenarios for Pre-, and Syn-rift Structures in the Deep Ivorian Basin, Gulf of Guinea, West Africa

Schleder, Zsolt ¹; Zarshchikov, Alexander ¹; Akhmetzhanov, Andrey ¹; Peterhänsel, Arndt ¹; Toropov, Gennady ¹; Bradley, Christopher ²; Neese, Michael ²; Wallace, Ronald ²; Gauly, Gbomea Marcelle ³; N'Dri, Gérard ³
(1)Lukoil Overseas, London, United Kingdom. (2) Vanco Exploration Company, Houston, TX. (3) PETROCI, Abidjan, Cote d'Ivoire.

In the Cretaceous rift basins of offshore Côte d’Ivoire and Ghana hydrocarbon exploration focuses on Albian structural traps and Upper Cretaceous turbiditic plays with major discoveries (i.e. Baobab, Tweneboa, Jubilee and Dzata). Upper Cretaceous turbiditic flows pinch out against Albian topographic highs providing a stratigraphic component for hydrocarbon trapping. The Albian is a hydrocarbon source rock in the area with Albian fault network providing the migration pathway. Thus understanding the evolution of Albian and pre-Albian geometries is of importance that also helps to assess the spatial distribution of the ensuing turbiditic units.

In the Gulf of Guinea rifting initiated in Barremian time with the continental break-up marked by the Albian unconformity. During rifting, the Deep Ivorian strike-slip pull-apart basin was controlled by NE-SW trending dextral transform faults. The structural configuration is assessed on available industry standard 2D and 3D seismic data that spans from the coast to water depths of 3.5 km and reaches 8-10s TWT. On seismic the post-Albian geometries can be traced with great confidence, however, the interpretation of 2-4 km thick Aptian-Albian syn-rift strata and the pre-rift units is ambiguous owing to the attenuated seismic signal. An admissible, geometrically valid interpretation scenario for the Aptian-Albian is a single set of NW-SE striking listric normal faults with a displacement of 4 km or more and associated rotated fault blocks. According to this scenario the hanging wall consists of en-echelon tilted blocks and roll-over anticlines with associated crestal collapse grabens where the listric fault has a ramp-flat-ramp trajectory. In some cases, below the listric faults, reflectors resembling to rotated fault blocks can be seen. This led us to alternative interpretation scenarios either with extensional duplexes or with a number of a separate detachment levels. The latter model presumes subsequent normal faulting events that progressively steepens the strata and superimposes fault blocks with markedly different tilt. Geometrical reverse and forward modelling helped to better understand the implications of the scenarios. In the various scenarios the fault network and thus the migration paths are markedly different that may provide the input for a scenario-based basin modelling. The Aptian-Albian anticlines and rotated blocks provide viable structural targets and also act as obstacles for turbidity flows.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.