利用重磁资料研究西非中南段含盐盆地构造区划

西非中南段含盐盆地位于大西洋东岸浅海区,在构造演化的过渡阶段形成了巨厚的Aptian期蒸发盐岩,将该区分为盐上层、盐层和盐下层三个构造层序.而盐岩的遮蔽效应使得盐下层地震反射品质差、盐下层序研究程度低,致使裂谷主控断裂、裂谷规模及展布形态不清楚.本文利用重、磁资料覆盖面广、横向分辨能力强、受盐岩影响小的优势,结合地震及地质资料,研究了西非中南段含盐盆地盐下构造格局.通过研究表明,西非中南段含盐盆地整体呈现"东西分带,南北分块"的构造特征;重新厘定了里奥穆尼、加蓬、下刚果和宽扎4个含盐盆地边界;识别了2个盐下裂谷带及其内部28个盐下凹陷,盐下凹陷呈现垒堑相间的结构特征;推断了盐下一级断裂16条,二级断裂23条,呈现"东西拉张,北东走滑"的力学特征.该项研究成果为该区盐下地震层序再解释、盐下油气战略选区及下一步勘探部署提供了地球物理依据.

Tectonic division by gravity and magnetic anomaly data of salt-bearing basins,south-central section of West Africa

The salt-bearing basins in south-central section of West Africa are located on the east coast of the Atlantic Ocean. The thick salt rock formed during the Aptian period that happened at the transition stage of structural evolution, which is divided into three tectonic sequences:upper salt layer, salt layer and lower salt layer. It is a big challenge to research the major faults that control the boundaries of rifts and morphological dimensions of rifts based on the poor-quality seismic reflection data that is influenced by the shading effect of salt rock. In this paper, we study the subsalt structure of salt-bearing basins in this region using gravity and magnetic anomaly data which have several advantages such as wide area coverage, strong horizontal resolution and less effect from salt rock. In addition, combining the seismic reflection and geologic data, we further study the tectoniv pattern of subsalt in this area. Results suggest that the salt-bearing basins in south-central section of West Africa are characterized by "zones in east-west and blocks in south-north". The boundaries of Rio Muni Basin, Gabon Basin, Lower Congo Basin and Kwanza Basin are redefined. Two subsalt rifts and 28 subsalt sags are identiflied, where are present alternating uplifts and depressions. We also infer 16 first-order and 23 second-order subsalt faults, which exhibit "extension in east-west and strike-slip in north-east". The results of this study provide geophysical supports to the seismic sequence reanalysis, choosing exploration areas of subsalt oil and gas and the exploration deployment in the next step.