REACTIVATION OF BASEMENT FAULTS AND ITS IMPACT ON HYDROCARBON ACCUMULATION IN LOWER CONGO-CONGO FAN BASIN

The Lower Congo-Congo Fan Basin, located on the passive continental margin of west Africa, has experienced three evolutionary stages, including a rifting stage (Early Cretaceous Valanginian to Barremian), a transitional stage (Aptian to Early Albian) and a passive continental marginal stage (Early Cretaceous Albian to present), as a typically superimposed salt basin consisting of a lower rifted sequence and an upper continental marginal sequence. Previous studies, however, focused mainly on the post-salt sequence, and basement faults are ignored, that has badly influenced our understanding on the hydrocarbon migration and distribution in the study area. This time, structural analysis is carried out and the reactivation of basement faults and its effects on structural deformation and hydrocarbon accumulation in the post-salt sequences are discussed. It reveals that the structural framework of the pre-salt rift sequence in the study area is composed of two depressions, the inner and the outer rift zones, and an uplift called the Atlantic hinge zone sandwiched between the inner and outer rift zones. Reactivation of basement fault is mainly inherited and followed by structural inversion. The activation is strong on the west side of the Atlantic hinge zone, where formed a narrowly distributed reactivity belt by basement faults. The basement fault reactivity exerted a strong influence on the structural deformation of the post-salt strata, including the controls over the location of post-salt graben and half-graben, the occurrence of the "thick-skinned" detachment structures directly linked to the basement fault, the formation of the fault salt welding, which promoted the development of salt diapirs. The reactivity belt of the basement faults may provide vertical pathways for hydrocarbon migration from the pre-salt to the post-salt sequences in the outer rift zone.