Structural Evolution of the Eyan Salt Body, Offshore Gabon

Katie Milroy
Royal Holloway, Aberdeen, United Kingdom

The North Gabon sub-basin, offshore West Africa is an early Cretaceous rift basin overlain by Aptian post rift evaporites and a thick succession of Upper Cretaceous-Tertiary drift sediments which lead to the development of complex salt structures such as the Eyan salt body.

The Eyan salt structure is a composite of several structural styles. This includes two linear salt walls, a salt stock reaching the present day sea floor and an allochthonous plug fed extrusion, onto which a secondary minibasin has developed.

The results from 3D seismic interpretation show the occurrence of synkinematic growth packages. These sequences thicken into the underlying autochthonous salt and thin onto the flanks of structures, suggesting the early establishment of upright salt bodies, and minibasin subsidence. Stacked halokinetic sequences, flanking the structure record a complex history of passive diapirism. This is recorded by varying onlap relationships and angular unconformities; a function of the relative rates of salt rise, sediment accumulation and salt withdrawal. These relationships show an intricate history of vertical salt growth, expansion and reduction across the Eyan structure.

Time thickness maps demonstrate the changing pattern of depocentres, salt withdrawal and salt growth through time and together, with evidence from vertical seismic in the overburden, record a history of early salt mobilisation, periods of tectonic quiescence followed by further salt growth. A network of ridges and intersecting diapirs has developed over time which bares particular resemblance to analogue modelling experiments carried out by Rowan & Vendeville (2006).

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas