Salt tectonics and its relationship to hydrocarbon accumulation in salt basins with a lower rifted section and an upper continental marginal section: A case study of the Lower Congo-Congo Fan basins and the Kwanza Basin in West Africa

The West African coastal basins, such as the Gabon Coastal Basin, Lower Congo Basin and Kwanza Basin, are typical oil-bearing salt basins with a lower rifted section and an upper continental marginal section. Three evolution stages can be identified in those basins, including a rifting period (the Early Cretaceous Valanginian to Barremian), a transitional period (Aptian to the early Albian) and a passive continental margin period (the early Cretaceous Albian to present), thus developing three tectono-sedimentary sequences from base to top: pre-salt, evaporites, and post-salt. On one hand, under the influence of the uplifting African craton and the continuing subsidence of the Atlantic passive margin from Paleogene to present, and taking the evaporite layer as a detachment, a gravity gliding tectonic deformation layer was formed in the post-salt sequence. This layer represents a cover-detached structure with complex deformation, various salt structural styles, and obvious tectonic zonation from east to west. On the other hand, tectonic deformation in the pre-salt tectono-sedimentary sequence was very weak, therefore, basement-related geological structures generally remained unchanged since South America and Africa separated some 165 million years ago. Salt tectonics controlled the formation and distribution of post-salt reservoirs and traps. At the same time, the salt layer works as a good regional seal for the subsalt sequence that can be interpreted as a boundary between post-salt and pre-salt petroleum systems. Therefore, in general terms, oil and gas generated in the subsalt petroleum system can only migrate and accumulate locally, particularly in deep to ultra-deep water areas where the salt strata have been thickened. However, in shallower water areas, where salt thickness was dramatically thinned or missing, subsalt oil and gas may migrate into the post-salt sequence through salt windows and/or connecting faults. Existing near-shore post-salt fields in West Africa provide support for this migration and accumulation process.