南海北部深水区靖海凹陷结构和构造演化分析

Analysis of the Geological Structure and Tectonic Evolution of Jinghai Sag in Deep-water Area,Northern South China Sea

靖海凹陷为南海北部深水区油气勘探的重点盆地之一,系统深入研究盆地结构特征及演化过程有助于深化对南海北部深水盆地的认识,为深水油气勘探提供新思路。本次研究以靖海凹陷2D地震资料的地质构造解释为基础,描述新发现的拆离断层的几何学特征,对比靖海凹陷东、西部结构样式差异性;通过沉降史模拟结果和典型拆离断层生长发育过程分析,恢复了靖海凹陷自新生代以来的沉降和构造演化过程;结合研究区深部资料与国外深水盆地最新研究进展,探讨了盆地结构和构造演化过程与南海北部陆缘地壳和岩石圈的伸展、减薄以及破裂作用的关系。研究结果表明,靖海凹陷东部与西部的盆地结构样式存在差异性,西部发育大型拆离断层及其所控制的拆离盆地系统,东部则呈现为由多个简单的南断北超的半地堑盆地所构成的复式地堑结构。盆地的构造演化划分为断陷期和坳陷期,其中断陷又可细分为断陷Ⅰ幕和断陷Ⅱ幕。盆地断陷Ⅰ幕对应于南海北部陆缘伸展阶段,高角度断层普遍发育;盆地断陷Ⅱ幕对应于南海北部陆缘减薄阶段—剥露阶段,拆离断层发育;当南海北部岩石圈完全破裂之后,盆地进入到坳陷期。

The Jinghai sag, which is located in southeast of the Pearl River Mouth Basin, is a deep-water sag with a great exploration potential. The fine description of the geological characteristics and the tectonic evolution of Jinghai sag is helpful to deepen understanding of the structural characteristics and evolution process of deep-water basin of northern South China Sea and provides a new idea for deepwater oil and gas exploration in China. Based on the tectonic interpretation of 2D seismic data covering the whole research area, this paper described the geometric features of detachment faults and compared differences of structure style in east and west Jinghai sag. In addition, by analysing subsidence result and the development process of detachment fault, we recovered the basin subsidence and filling progress. Integrated with previous research on deep basin and the dynamic information in adjoin areas this paper discussed the relationship between the extension thinning and break up of lithosphere of the northern margin of the South China Sea and the structure and tectonic evolution of the basin. The main research results are as follows ： （ 1 ） There are two different structural styles in the east and west part of Jinghai sag. In the west of the sag, there developed two large detachment faults, which control their detachment basin systems and the typical H block, and the two detachment faults are dipping landward and seaward, respectively. In the east, affected by the later volcanic activities, Jinghai sag deformed complicatedly and developed a series of landward dipping faults, showing the compound graben structure. （2） Combined with development of the typical detachment fault and basin subsidence history, the basin tectonic evolution has been divided into rift stage and post-rift stage. The syn-rift can be further divided into Rift- I period and Rift- II period. （ 3 ） Considering the extension development evolution of Jinghai sag and the extension and thinning of lithosphere under the background of spreading of the South China Sea, we argue that rift- I period of Jinghai sag is corresponding to the northern margin of the South China Sea extensional stage, while. Rift- II period of Jinghai sag is corres ponding to the northern margin of the South China Sea thinning and exhumation stage. After the spreading of the South China Sea the whole sag entered into the depression period which was characterized by thermal subsidence.