渤海湾盆地歧口凹陷构造演化及断裂带成因

以地震解释资料为基础,对渤海湾盆地歧口凹陷的构造演化及断裂带形成过程进行剖析,将歧口凹陷构造演化分为裂陷Ⅰ幕、裂陷Ⅱ幕和后裂陷期3个阶段。裂陷Ⅰ幕,歧口凹陷呈断陷结构,先存NNE向、NWW向断层发生斜向伸展,NE向断层为新生断层;裂陷Ⅱ幕,歧口凹陷呈断-坳结构,先存NNE向、NWW向、NE向断裂再活动,EW向断层为新生断层;后裂陷期,歧口凹陷呈坳陷结构,多期斜向伸展变形叠加形成现今不同类型的断裂带。断裂带可划分为5种类型：第1类多期伸展型断裂带由前古近系先存NNE向伸展断层叠加古近纪伸展变形而成;第2类变换-伸展型断裂带由前古近系先存NWW向变换断层叠加古近纪伸展变形而成;第3类单期伸展型断裂带由古近系伸展断层形成;第4类伸展-变换型断裂带由前古近系先存NNE向伸展断层叠加古近纪变换变形而成;第5类横向背斜型断裂带受新近系横向背斜控制而发育。在各演化阶段,主干断裂的分段生长特征及其与区域伸展应力方向的夹角共同控制了歧口凹陷中洼陷的形成、规模与迁移特征。歧口凹陷的构造演化对其烃源岩的发育、圈闭类型及油气运移有着重要影响。

Tectonic evolution and genesis of fault zones in Q ikou sag,Bohai Bay Basin

Based on seismic interpretation data, this paper performs an analysis on tectonic evolution and formation of fault zones in Qikou sag of Bohai Bay Basin, and divides tectonic evolution of Qikou sag into three stages: rifting episode I, rifting episode II and post-rifting episode. In the rifting episode I, Qikou sag showed a rift structure. The preexisting NNE- and NWW-trending faults extended obliquely, and the NE-trending faults were newborn faults. In the rifting episode II, Qikou sag had a fault-depression structure, and the preexisting NNE-, NWW- and NE-trending faults were reactivated, and the EW-trending faults were newborn faults. In the post-rifting episode, Qikou sag showed a depression structure, and different types of fault zones were formed by multiple phases of superimposed oblique extension and deformation. The fault zone can be divided into five types: the first type, i.e., multi-phase extending fault zone, is formed by the preexisting pre-Paleogene NNE-trending extensional faults superimposed with the Paleogene extensional deformation; the second type, i.e., transfer-extension fault zone, is formed by the preexisting pre-Paleogene NWW-trending transfer fault superimposed with the Paleogene extensional deformation; the third type, i.e., single-stage extensional fault zone, is formed by the Paleogene extensional fault; the fourth type, i.e., extension-transfer fault zone, is formed by the preexisting pre-Paleogene NNE-trending extensional fault superimposed with the Paleogene transfer deformation; the fifth type, i.e., transverse anticlinal fault zone, is developed under the control of the Neogene transverse anticlines. At each evolutionary stage, the formation, scale, and migration characteristics of subsags in Qikou sag were controlled by segmental growths of the main fault and its angle with the regional extension direction. The tectonic evolution of Qikou sag has an important influence on the development of source rocks, trap types, and hydrocarbon migration.