东、西非大陆边缘比较及其油气意义

Geological characters of the East and West Africa continental margins and their significance for hydrocarbon exploration

通过比较,分析了东、西非大陆边缘在形成时间、形成方式和边缘结构等方面存在的差异,并对东、西非边缘的油气成藏条件进行了探讨。东非边缘的裂谷作用时间早,且结束时间也早;形成了以右行剪切边缘、左行剪切边缘及它们之间的斜向剪切为特点的大陆边缘形式。东非边缘也具有陆架窄、陆坡较陡,沿大型海洋转换断层形成了一系列地形高的特点,对沉积物延伸具有明显的限制作用。而西非被动大陆边缘的裂谷作用开始时间较晚,海底扩张开始的时间亦较晚;边缘为拉分型,具有陆架宽、倾角缓的特点,发育了与油气关系密切的厚层蒸发岩及相应的盐构造,形成了大量盐构造及泥底辟,有利于油气聚集,并形成了很多大型油气圈闭。

Through comparing the geological characteristics between the East and West Africa margins, some differences in their evolution history, structural styles and textures can be concluded. The hydrocarbon-forming conditions of these two margins are discussed, too. The rifting on the East Africa margin commenced and ended earlier. It mainly happened from Jurassic to Early Cretaceous. Under the influence of this rifting and subsequent drifting, the continental margin in East Africa were partitioned to several different segments represented by the dextral- and sinistral-type shear margins and the oblique-type shear margin. Therefore, the shelves were narrower and the slopes were steeper and a series of ridges or arches formed and controlled by the major transform zones on the East Africa margin, which definitely confined or limited the extension of sediments seaward. In another way, not only the rifting but the sea spreading on the West Africa margin started later and mainly happened during the Early Cretaceous. The West Africa margin was mainly a pull-apart margin with the broader shelf and gentle-dipping slope. The thick evaporite rocks depositing on the West Africa margin generated numbers of large scale halokinesis and mud-diapire structures beneficial to hydrocarbon accumulation. In fact, lots of large or giant hydrocarbon plays have been confirmed on the West Africa margin.