珠江口盆地白云主洼古近系文昌组沉积充填特征及演化*

受制于钻井资料少及地质条件复杂,珠江口盆地白云主洼古近系文昌组三级层序格架内的沉积体系发育特征及沉积充填演化关系一直缺乏系统研究,严重制约着白云主洼深部地层的油气勘探。基于三维地震和钻井资料,采用层序原型结构剖面恢复、增强地震相分析、砂体地震扫描解释等新技术方法,重建白云主洼文昌组三级层序的沉积体系,并结合主洼构造演化特征,还原文昌组的沉积充填演化过程。结果表明: 白云主洼文昌组发育4个“源-汇”体系,分别为西北缓坡带纯陆源碎屑的大型辫状河三角洲—湖泊相沉积、东北斜坡带陆源与火山碎屑混源的大—中型辫状河三角洲—湖泊相沉积、西南轴向陡坡带纯陆源碎屑的近岸水下扇和扇三角洲—湖泊相沉积、东南陡坡带陆源与火山碎屑混源的小型近岸水下扇及扇三角洲—湖泊相沉积。白云主洼文昌组沉积时期的演化过程可大致分为河流(WCSQ1下段)、河流—湖泊(WCSQ1上段)、超深湖发育(WCSQ2下段)、超深湖早期充填(WCSQ2上段)、超深湖充填(WCSQ3、WCSQ4)、浅湖发育(WCSQ5)等6个发育阶段,湖盆经历了初始成盆、强烈断陷为超深湖、被大规模沉积充填为深湖—半深湖,最后直至充填为浅湖的沉积演化过程。

Sedimentary filling characteristics and evolution of the Paleogene Wenchang Formation in Baiyun main sag,Pearl River Mouth Basin

Due to the lack of drilling data and complex geological conditions,characteristics of de positional system and associated sedimentary-filling evolution in the third-order sequence framework of Wenchang Formation in Baiyun main sag lack systematic study,which seriously restricts the oil and gas exploration in the deep strata of Baiyun main sag. Based on 3D seismic data and drilling data,this paper reconstructs the sedimentary system of the third-order sequence of Wenchang Formation by using new technologies and methods such as the restoration of original form structure section of sedimentary sequence,the analysis of enhanced seismic facies,and the seismic scanning interpretation of sand bodies,then restores the sedimentary-filling evolution process of Wenchang Formation in combination with the characteristics of tectonic evolution. The results show that there are four “source-to-sink” systems in the Wenchang Formation of Baiyun main sag. They are large-scale braided delta-lacustrine facies deposits developed in the northwest inherited gentle slope zone and derived from pure terrestrial clastics,large-medium scale braided delta-lacustrine facies deposits developed in the northeast gentle slope zone and derived from terrigenous-pyroclastic mixed sources,inshore subaqueous fan and fan delta-lacustrine facies deposits developed in the southwest abrupt slope zone and sourced from pure terrigenous clastics,small-scale nearshore subaqueous fan and fan delta-lacustrine deposits developed in the southeast steep slope zone and derived from terrigenous-pyroclastic mixed sources. The sedimentary evolution of the Wenchang Formation in Baiyun main sag can be divided into six stages: fluvial deposition(lower segment of WCSQ1),fluvial-lake deposition(upper segment of WCSQ1),ultra-deep lake occurrence(lower segment of WCSQ2),ultra-deep lake early filling(upper segment of WCSQ2),ultra-deep lake filling(WCSQ3,WCSQ4)and shallow lake occurrence(WCSQ5). The lake basin undergone a sedimentary evolution process of initial basin forming,intense rift into ultra-deep lake,large-scale sedimentary filling into deep lake and semi-deep lake,and finally sedimentary filling into shallow lake.