北部湾盆地涠西南凹陷流沙港组岩性圈闭形成条件

Analysis on Forming Conditions of Lithologic Trap in Liushagang Formation,Weixinan Depression of BBW Basin

涠西南凹陷为北部湾盆地一典型的箕状断陷,古近纪经历3次构造运动,地层表现为多旋回沉积特征,发育多套生储盖组合。通过对古近系沉积地层进行层序地层格架分析,认为始新统流沙港组二段(流二段)层序、流沙港组一段(流一段)下层序和流一段上层序是岩性圈闭发育的主要层序。古近系三级层序中不同体系域类型严格控制着岩性圈闭的发育和形成,具体表现为流二段层序的岩性圈闭主要发育于低位体系域和湖侵体系域,流一段下层序的岩性圈闭主要发育于低位体系域,流一段上层序的岩性圈闭主要发育于高位体系域。凹陷的多物源充填所形成的多种沉积体系类型也是形成多种类型岩性圈闭的重要原因。发育于陡坡带的近岸水下扇岩性圈闭、扇三角洲岩性圈闭、缓坡带浊积水道岩性圈闭和洼槽带湖底扇岩性圈闭为有利的圈闭目标。

Weixinan Depression is a typical dustpan-like rift in BBW Basin,which has experienced three tectonic movements in Paleogene,displays polycyclic sedimentary characteristics,develops several sets of source-reservoir-cap rock associations.According to the analysis on Paleogene stratigraphic framework using sequence stratigraphy,sequences in the second section of Liushagang Formation and in lower and upper sequences of the first section of Liushagang Formation are the main sequences in which lithologic traps were developed. The development and formation of lithologic traps were strictly controlled by different system tract types in third order sequences of Paleogene.The detailed performances are： lithologic traps mainly developed in lowstand system tract and lacustrine transgressive system tract of sequence in the second section of Liushagang Formation,in lowstand system tract of lower sequences in the first section of Liushagang Formation and in highstand system tract of upper sequences in the first section of Liushagang Formation.Abundant sedimentary system types resulted from multi-source filling were important to the formation of various lithologic traps.Nearshore underwater fan lithologic trap,fan delta lithologic trap which developed in steep slope belt,turbidite channel lithologic trap in gentle slope belt and sub-lacustrine fan lithologic trap which developed in sag belt are favorable targets.