Deepwater Fold-Thrust Belts: Not All the Beasts Are Equal*

Deep-water fold thrust belts (DW-FTBs) are actually several different structural sub-styles considered together. There are at least four sub-styles in active and passive margins with very different driving mechanisms, structural and stratigraphic histories, hydrocarbon system characteristics, and therefore very different hydrocarbon potential. Subduction-driven active margin DW-FTBs were the first to be recognized and are geologically widespread. Active-margin DW-FTBs are accretionary prism created by shearing of sediments of the subducted plate. Though common, absence or dilution of abyssal source rocks, combined with commonly poor reservoir quality caused by volcanogenic sediments have often made them unrewarding exploration targets. On passive margins three types of gravity-driven extensional-compressional linked systems predominate. Among these systems we can further discriminate between salt- and shale-dominated systems. Gravitational collapse of shale-dominated sediment piles (Niger Delta, south Texas, and Amazon fan) produces linked extension-compression “couples, ” with a DW-FTB as the distal margin. Productivity is strongly dependent upon migration from deeply-buried source kitchens landward of the DW-FTB. Source kitchens are in turn dependent upon depth of burial, a problematic issue at the distal edge of a delta slope. Massive salt gliding DW-FTBs (Gulf of Mexico and South Atlantic salt basins) possess an added complexity in that they grade continuously updip into a salt-tectonic domain. This assemblage is only now being fully-tested, with encouraging results. Last are true gravitational slumps with a DW-FTB at the downdip margin (Equatorial Guinea, South Atlantic salt basins) which differ in that the detachment cuts through stratigraphy. This assemblage is largely untested but with considerable potentia