Based on a previous 2D seismic survey in offshore Sierra Leone and Liberia revealing the exploration potential of the margin, a 3D long offset survey in deepwater Sierra Leone was acquired in 2008. Only two exploratory wells were drilled in offshore Sierra Leone in shallow water in 1982 (Mobil A-1) and 1985 (Amoco A1-2), but they still provide information of petroleum systems (rich source rock, reservoir quality sands, presence of seal). Additionally, significant field analogues exist in other basins of the Gulf of Guinea (e.g. Cote d’Ivoire, Ghana). Across the region, Aptian to early Cenomanian sequences contain rich, oil-prone to mixed oil and gas-prone Type II or III source rock. In the Sierra Leone basin, the late Cretaceous interval thickens to more than 2000m and most likely includes mature Cenomanian to Turonian source beds, established as Type II throughout offshore West Africa. This study assesses the prospectivity of block SL-4 and SL-5 by interpreting approximately 2700 km² of 3D migrated data, tied to the existing regional 2D lines. This evaluation of the hydrocarbon potential of these blocks, the discovery of the Jubilee field in 2007 by Tullow in deepwater Ghana, the similar proven petroleum systems in offshore Cote d’Ivoire and the growing interest from major oil companies in offshore Sierra Leone and Liberia, suggest that this area offers significant exploration potential in one of the few remaining unexplored deepwater basins in West Africa.