琼东南盆地深水区长昌凹陷构造演化及其对层序样式的控制

Tectonic evolution and its controlling on sequence pattern of Chang-chang sag,deepwater area of Qiongdongnan basin,South China Sea

运用平衡剖面技术对长昌凹陷的3条典型地震测线进行平衡复原,揭示长昌凹陷不同构造演化阶段的同沉积构造活动及其所形成的古构造格架,通过对凹陷内次级古构造单元及其演化类型的识别,综合分析构造演化对层序样式的控制,构建研究区沉积层序综合演化模式。研究结果表明:长昌凹陷的构造格局依次经历了始新世-早渐新世断陷期、晚渐新世断坳转换期和新近纪坳陷期3个发展阶段,由此产生了3个具有不同古构造格架背景的盆地原型相互叠加。在裂陷Ⅰ幕和裂陷Ⅱ幕主要发育上倾坡脚型断裂陡坡带层序样式和多级陡坡断阶带层序样式,而裂陷Ⅲ幕发育下倾坡脚型断裂陡坡带层序样式和多级缓坡断阶带层序样式,在裂后热沉降阶段主要发育断裂型挠曲带层序样式以及少量下倾坡脚型断裂陡坡带层序样式,而裂后加速沉降阶段则发育简单挠曲带层序样式。与断坡带和挠曲带相关的大型低位三角洲、低位扇以及下切谷充填是长昌凹陷构造岩性或岩性油气藏的有利发育区。

The synsedimentary tectonic activities and ancient structural frameworks in different evolutionary stages of Chang-chang sag were revealed by the restoration of 3 seismic lines across Chang-chang sag using balanced cross-section technique. On the basis of recognition and redefinition of secondary ancient structural units and its evolutionary types in association with further comprehensive analysis of the controlling of structure evolution on sequence pattern, the sedimentary filling model of Chang-chang sag was established.The results show that the tectonic evolution of Chang-chang sag has experienced three stages, namely the faulted period during Eocene to early Oligocene, the faulted-depressed transition period during late Oligocene and the depression period during Neogene. Correspondingly, there are three superimposed basin prototypes with different structural frameworks in Chang-chang sag. In rifting episode I and rifting episode II there were mainly updip-basalslope fault abrubt slope belt sequence pattern and step-fault abrubt slope belt sequence pattern, while in rifting episodeIII they gradually evolved to downdip-basalslope fault abrupt slope belt sequence pattern and step-fault gentle slope belt sequence pattern. And in the thermal subsidence period there were mainly faulting flexure belt sequence pattern and some downdip-basalslope fault abrubt slope belt sequence pattern, while in the accelerated subsidence period there was only simple flexure belt sequence pattern in Chang-chang sag. The large-scale low stand deltas, low stand fans and incised valley filling relating to fault slope belt and flexure belt should be favorable exploration areas of structural-lithologic reservoir and lithologic reservoir.