珠江口盆地开平凹陷古近系构造特征及构造演化分析

Paleogene tectonic evolution of Kaiping Sag,Pearl River Mouth Basin

开平凹陷位于珠江口盆地珠Ⅱ坳陷西南部深水油气勘探区,处于南海北部陆缘洋陆过渡带。由于该区构造特征极为复杂,加之油气勘探及研究程度较低,因此针对该区开展裂陷演化及沉积充填特征与烃源发育条件的分析研究,对于油气勘探及其油气地质评价等均具有重要的参考借鉴意义。基于开平凹陷研究区的高分辨率三维地震资料,重点对该区洼陷结构和断裂体系进行了深入解剖,系统阐述了古近纪断裂展布特征,并对研究区主干断裂活动特征进行了定量分析,探讨了古近纪裂陷期断裂体系演化规律及沉积中心迁移过程,并运用平衡剖面技术,恢复了开平凹陷古近纪以来的裂陷演化过程。研究表明开平凹陷古近系表现出“北断南超”的构造格局,裂陷期发育NE-NEE向和NWW向两组主要断裂,控制了凹陷构造变形及沉积充填特征,其控洼断层具有拆离伸展的断裂特征。尚须强调指出,开平凹陷主干断层活动强度整体表现为早始新世文昌期逐渐增强,晚始新世恩平期减弱并停止的特征,在始新世文昌组-恩平组沉积期发生过多次沉积中心迁移。因此,根据断裂体系展布特征和平衡剖面分析结果,综合判识确定开平凹陷是在南海北部陆缘晚中生代构造变形基础之上形成的新生代伸展断陷盆地,其裂陷演化过程经历了早文昌期的断陷伸展阶段、晚文昌期的拆离伸展阶段和恩平时期的底侵拗陷阶段。总之,开平凹陷古近系构造演化,总体上均受控于“先存构造约束、拆离薄化主导、岩浆作用参与”的珠江口盆地动力学机制。

The Kaiping Sag is located in the western part of Zhu II Depression,Pearl River Mouth Basin in the ocean-continent transition zone of the northern continental margin of the South China Sea.The tectonic characteristics of Kaiping Sag are very complex while the degree of research is relatively low.Therefore,we studied the evolution process of rifting in this area to reveal the conditions of sedimentation and hydrocarbon source development.Based on the high-resolution 3D seismic data of the Kaiping Sag,the geo-structure and fault system in this area were dissected,the distribution and development characteristics of the Paleogene faults were expounded systematically,the activity characteristics of main faults were quantitatively discussed,the evolution of fault system and the migration of sedimentary center in the Paleogene rift period were clarified,and the balanced profile of the rifting evolution of Kaiping Sag since the Paleogene was reconstructed.Results show that the Kaiping Sag shows a tectonic pattern of“north faulting and south surpassing”.Two groups of main fault sets,NE-NEE and NWW,are developed during the rifting period,which controlled the tectonic deformation and sedimentary filling of the sag,and its depression-controlling faults are characteristic of detachment and extension.The activity intensity of the main faults in Kaiping Sag gradually increased in the Wenchang Stage of the Early Eocene,and weakened-ceased in the post-Enping Stage of the Late Eocene.The sedimentary center migrated many times during the sedimentary period of Wenchang-Enping Formations in the Paleogene.The distribution of the fracture system and balance profile analysis suggest that Kaiping Sag is a Cenozoic extensional faulted basin formed based on the late Mesozoic tectonic deformation in the northern continental margin of the South China Sea,and its evolution can be divided into three stages:the early-Wenchang fault extension stage,the late-Wenchang detachment extension stage,and the Enping Formation underplating-depression stage.The tectonic evolution of Kaiping Sag is controlled by the dynamic mechanism of“Pre-existing tectonic constraints,detachment and thinning domination,and magmatism involvement”in the Pearl River Mouth Basin.