Minapuye I. Odigi and Prince Suka Momta
Two exhumed hydrocarbon traps crop out in the Afikpo Basin of the Southeastern Nigeria, each at the unconformity formed during the Santonian tectonic event. Oil accumulations are indicated by a pore-fill or pore lining of solid bitumen within the Campanian - Maastrichtian sandstone-dominated Ameta-Ozziza and Ebori Usumtong formations. Three types of trap geometries can be identified in the field: the wedge-out, channel and strike valley fills are found in the Amete-Ozziza and Ebori Usumtong areas of central Afikpo region. The Nkporo Formation (0 - 44m) consists of Alluvial fans, distributary and tidal channels; marine and fluviodeltaic deposits. The sandstones are dominantly quartzarenites and minor amount of subarkoses and arkoses, petrographic fabric, such as dissolved feldspar, clay and late quartz cements, are consistent with burial depths in excess of 3km. The sandstones generally have relatively good to moderate porosities, about one-half of which was primary. The distribution of each trap can be mapped out, allowing sealing elements to be defined. The two exhumed traps Ameta-Ozziza and Ebori Usumtong were stratigraphic and structural traps, with Campanian - Maastrichtian unconformable mudstone, and normal faults are the two top sealing elements. This article gives a preliminary description of two exhumed hydrocarbon traps in the Afikpo Basin; these traps became thermally degraded and exhumed during terminal Cretaceous tectonics involving magmatism and regional uplift. The stratigraphic relationship in the Afikpo basin is significant to hydrocarbon entrapment. The exhumed traps may indicate future plays on the largely unexplored lower Benue Trough of Nigeria where at a reasonable depth sandstone is overlain uncomfortably by the source rock of Nkporo shale which may be a favourable area of petroleum prospecting. The former ridges created during the Maastrichtian tectonics with unconformity surface may form collecting reservoirs for oil. These traps described are similar to that of oil fields in the North Sea. The author believes that Afikpo structures will provide useful analogs in detailed study of the subsurface reservoirs of the Niger Delta and Gulf of Guinea oil provinces in West Africa.
