Controls of Salt Tectonics and Shortening on Hydrocarbon Prospectivity in Passive Margin Fold Belts of Offshore Gabon

摘要

Aptian-Albian salt basins underlying the modern slope and deep South Atlantic basin from southern Angola to Equatorial Guinea produce discontinuous, passive margin fold belts related to gravity sliding on the underlying salt layer. These passive margin fold belts are poorly mapped at a regional scale, yet locally form major structural traps for hydrocarbons. The Gabon Basin covers 130,300 km~2 and extends 550 km along the coast, as well as has recently been the location of major hydrocarbon discoveries. Seismic data were used, courtesy of Spectrum Geo Inc., which were correlated in 3D MOVE software to determine the significance of subsurface structural features in relation to productive hydrocarbon discoveries in offshore Gabon. Two mechanisms appear important for the formation of the passive margin fold belts: (1) steeper areas of seaward-dipping basement along the passive margin that act to promote sliding on the salt surface; and (2) increased sediment loading of the shelf and slope related to the modern Congo and Nyanga river systems. Seismic lines were used to map the major folds at the down-dip end, along with major normal faults at the updip end of the system. 2D MOVE software ws used to estimate the amount of shortening in the downdip direction. Wells tied to seismic indicate that the fold belt, and associated normal faults, began to form in the Eocene.