南大西洋中段共轭盐盆盐构造变形期次对比及意义

Comparison of Salt Structure Deformation Periods of Conjugated Salt Basins in Central Segment of South Atlantic

南大西洋中段被动陆缘含盐盆地群是全球油气储量快速增长的地区,阿普特阶盐岩的分布及构造变形对该区油气成藏产生了重要影响.在前人地震及重磁资料基础上,沿热刺转换断裂带选取圣埃斯皮里图和宽扎这组典型共轭盐盆,平面上在Gplates软件中叠合布格重力异常数据进行板块构造重建,剖面上应用Move软件进行平衡恢复,根据反演结果设计正演模型,进行物理模拟对比实验.在重力滑脱及重力扩展作用下,盆地内形成了伸展-过渡-挤压的盐构造分带,而底板斜坡倾角、基底伸展量、前缘阻挡、盐上沉积载荷及同沉积速率等因素,造成了共轭盆地间盐构造变形过程的差异性.盐构造变形过程可分为盐构造初期变形阶段(阿普特期-阿尔比早期)和盐构造继承性发育阶段(两期,阿尔比晚期-古近纪早期和古近纪晚期现今):(1)阿普特期-阿尔比早期,圣埃斯皮里图和宽扎盆地初始为统一盐盆,盐盆东侧强烈伸展,形成伸展区断裂及盐筏,同时盐盆逐渐一分为二并形成外部高地带,盐岩重力变形逐渐减慢.(2)阿尔比晚期-古近纪早期,圣埃斯皮里图盆地外部高地带继承性隆起,盐岩垂向上浮迁移;宽扎盆地厚层盐高原隆起,盐岩越过外部高地带形成逆冲推覆体.(3)古近纪晚期-现今,圣埃斯皮里图盆地基底稳定热沉降,盐构造继承性发育,整个盆地盐岩呈现垂向迁移特征;宽扎盆地靠陆端基底一定规模构造抬升,盐岩向海运输重建,伸展区盐筏进一步发育成盐滚,过渡区及挤压区盐底辟持续上拱,逆冲推覆体继续发育并在盐上层形成大量断裂及褶皱,整个盆地盐岩不断向坡下聚集且构造分带性明显.

The passive-margin salt basins in the central segment of the South Atlantic are areas with rapid growth of petroleum reserves in the world,where the distribution and structural deformation of the Aptian salt rocks have significant impacts on hydrocarbon accumulation.According to the Hotspur transform fault zone,the typical conjugated salt basins of Espirito Santo and Kwanza are selected in this study.Based on the previous seismic profiles and gravity and magnetism data,the Bouguer gravity anomaly data are superimposed on the plane in the Gplates software for reconstruction of the plates,and the Move software is used to restore balanced profile on the section.Then forward models designed according to the inversion results,and the contrast analogue experments are carried out.According to the studies,under the superposition of gravity sliding and gravity spreading,salt tectonic zones of extension-transition-contraction developed in the passive-margin basins,while factors resulted in the of salt structure deformation process among the conjugated basins,including slope angle,basement extension,the outer high barrier,salt deposition load and syn-depositional rate.(1)From Aptian to early Albian,the Espirito basin and the Kwanza basin were initially a unified salt basin.The east side of the unified basin was strongly stretched,forming faults and salt rafts in the extensional domain.At the same time,the unified basin was gradually divided,accompanied by the formation of the outer high,and the gravity deformation of salt rocks gradually slowed down.(2)From late Albian to Early Paleogene,the inherited upflit of the outer high in the Espirito Santo basin resulted in the vertical upward migrations of salt rocks;the uplift of the thick salt plateau in the Kwanza basin caused salt rocks to cross the outer high and form the thrust nappe.(3)From Late Paleogene to the present,the basement of the Espirito basin had steady thermal subsidence,and the salt structures were successively developed,then the salt rocks in the whole basin exhibit vertical migration characteristics.In the Kwanza basin,the basement was uplifted on a large scale,and the transport of salt rocks to the sea started again.Salt rafts in the extensional domain were further developed into salt rollovers;salt diapirs in the transitional zone and the contractional domain continued to rise;and the thrust nappe continued to develop,forming a large number of faults and folds in the supra salt layers.Then the salt rocks of the entire basin are continuously accumulating downslope with obvious structural zonation.