Abstract: Tectonostratigraphy of the Rio Muni Basin, Equatorial Guinea: Implications for Future Exploration Potential

Loader P. R.; Goh, K.; Taylor, M. K.; Dailly P. - Triton Energy

The Rio Muni Basin of Equatorial Guinea is effectively the most northerly extension of the Aptian Salt Basin of West Africa exhibiting similar stratigraphy and hydrocarbon play potential to the southern parts, proven prolific from Gabon to Angola.

The onshore and shelfal parts of the Rio Muni Basin are relatively narrow, respectively 20km and 30km wide, whilst Tritons 1997 seismic data demonstrates its extension as a thick sedimentary wedge some 100kms into the deep water Atlantic margin.

The Rio Muni Basin represents the conjugate margin to the Sergipe Alagoas Basin of Brazil and is bounded to the north and south by the Campo and Ascension Fracture Zones, separating it respectively from the South Douala and North Gabon Basins. Trending through the Rio Muni basin is a subtle subordinate ("Bata") Fracture Zone which appears to be a reactivation of the Congo Cratonic Margin.

The Basin lies at the boundary of the Southern Atlantic, dominated by simple extensional tectonics, and the Equatorial Atlantic in which lateral motion on transforms accomodates Atlantic opening. Within the Basin one can see the extensional rift to drift basin development on which are superimposed the effects of final separation along the Equatorial Zone transforms, locally expressed as reactivation of transform faults.

Rift sediments have been penetrated in Rio Muni, only the top of which is imaged on seismic. A Post-Rift Late Aptian to Recent section at least 6000m thick is described on seismic. During the drift phase, Late Cretaceous transform reactivation caused transpressional uplift and the resulting oversteepening on the shelf led to gravity sliding of shelf sediments, detaching either in Aptian saliferous horizons or in younger shale layers. A later Miocene compressional phase is also apparent, broadly contemporaneous with asymmetric uplift and volcanism on the Cameroon Volcanic line.

The effects of the structural evolution of Rio Muni can be recognized in distinct structural provinces; en echelon anticlines along reactivated transforms i.e., Calatrava and Campo highs, intervening areas with detached rafts, often carbonate cored, at the toes of which thrust structures are developed and intervening stable platform areas.

Just six wells have been drilled in the basin, all located on the present day continental shelf, three of which have been located on prominent inversion features where Aptian clastics are either at, or near surface. Live oil has been encountered in four of the wells, together with an excellent Early Albian source rock. The deep waters of the Rio Muni Basin remains totally unexplored.

Four play types are recognized along the shelf: Cenomanian-Turonian clastics, Albian carbonates, Aptian clastics and Neocomian (?) Synrift.

New seismic data has indicated the potential of the deep water turbidite plays of Late Cretaceous to Tertiary. A combination of thick sediment pile, linked to channel systems on the shelf and the recognition of a variety of seismic amplitude anomalies has significant implications for future exploration.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil