REACTIVATION OF BASEMENT FAULTS AND ITS IMPACT ON HYDROCARBON ACCUMULATION IN LOWER CONGO-CONGO FAN BASIN
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摘要:
下刚果-刚果扇盆地位于西非被动大陆边缘，是典型的裂谷与被动陆缘叠合盆地。该区以往的研究多集中于盐上层系，对基底断裂研究较少。以构造分析为基础，探讨了基底断裂复活及其对盐上层系构造变形和油气分布的影响。结果表明，研究区盐下裂谷层系具有“两坳夹一隆”的构造格局，大西洋枢纽带夹持于内裂谷带和外裂谷带之间；基底断裂复活主要发生在紧邻大西洋枢纽带西侧、呈狭长带状分布的基底断裂复活带，断裂复活形式主要表现为继承性，其次为构造反转；基底断裂复活不仅对盐上层系的构造变形具控制作用，而且为油气从盐下层系向盐上层系运移提供了良好的垂向通道条件，是外裂谷带盐下油气向盐上层系运移的关键部位。
Abstract: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.
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