北部湾盆地海中凹陷与涠西南凹陷构造特征及成因机制

北部湾盆地是中国南海北缘主要的含油气盆地之一,盆地内部油气富集差异明显。利用北部坳陷内高精度的地震资料,对坳陷内的两个重要凹陷的结构、断裂体系及构造样式进行了解剖,并讨论了造成两个凹陷构造特征差异明显的主要原因。研究结果表明,研究区发育NE向、NEE向及近EW向多组断裂体系,受伸展应力由NW—SE向转变为近NS向的影响,断裂体系在晚始新世发生转变,由NE—NEE向为主转变为近EW向,并沿NE—NEE向雁列展布。海中凹陷整体表现为简单半地堑,断裂体系简单,构造样式以伸展构造样式为主,局部发育断展型反转构造;涠西南凹陷则整体表现为多米诺式复杂半地堑,断裂体系复杂,构造样式同样以伸展构造为主,但沿2号断裂带发育伸展-走滑构造样式。造成海中凹陷和涠西南凹陷构造特征差异的影响因素包括NW向隐伏断裂、NE向先存断裂的差异及构造反转动力源等。

Structure characteristics and genetic mechanism of Haizhong sag and Weixinan sag in Beibu Gulf Basin

Beibu Gulf Basin is one of the major petroliferous basins in the northern margin of South China Sea, with significant differences in oil and gas enrichment within the basin. Based on the high-precision seismic data in the Northern depression, this paper analyzes the structures, fault systems, and structural styles of two important sags in the depression, and discusses the main reasons for obvious differences in structural characteristics of the two sags. The research results show that NE-trending, NEE-trending, and nearly EW-trending fault systems are developed in the study area. Influenced by the transition of extensional stress from NW-SE trending to nearly NS trending, the fault systems changed in the Late Eocene from the dominant NE-NEE trending to nearly EW-trending, and showed an NE-NEE en echelon pattern. The Haizhong sag as a whole is a simple half-graben, the fault system is also simple, the structural style is dominated by the extensional structure, and positive inverted structures are developed in partial area. Overall, the Weixinan sag is a complex domino-type half-graben, the fault system is complex, the structural style is also dominated by extensional structure, and the extensional strike structure is developed along the No. 2 fault zone. The influencing factors that cause the tectonic differences between Haizhong sag and Weixinan sag include the differences in NW-trending buried faults, NE-trending preexisting faults, and dynamic source of structural inversion.