超伸展陆缘盆地深部结构及油气勘探意义——以琼东南盆地为例

Deep architecture of hyperextended marginal basin and implications for hydrocarbon exploration:A case study of Qiongdongnan Basin

南海北部被动陆缘盆地是中国海域重要的油气基地。以琼东南盆地为例,通过大量地震、钻井资料的综合分析,从被动陆缘深部地壳形变与盆地充填耦合的角度,揭示了超伸展陆缘裂谷盆地沉积充填特点及地层样式。研究表明,位于南海西北次海盆西延长线上的琼东南盆地属于夭折型陆缘裂谷盆地,沿中央坳陷带岩石圈拆离减薄形成了细颈带和远端带,盆地裂陷期经历3幕伸展作用,早期断陷幕发育以高角度正断裂为特征的孤立断陷,中期拆离幕发育以低角度拆离断裂为特征的拆离断陷,且拆离断裂作用具有明显自东向西迁移的特点,晚期断-拗幕沉积中心位于凹陷中央。正是由于变形构造活动迁移特征构成了琼东南盆地独特的深部构造格局,进而导致不同构造单元的沉积充填及地层样式存在明显差异,其主拆离断裂侧以发育扇三角洲为主,而相向的滚动背斜侧则以发育辫状河三角洲为特征。这些因素制约裂陷期烃源岩分布以及深部储层展布,进而约束油气地质条件的差异。因此,基于岩石圈非瞬时破裂过程所建立的超伸展陆缘裂谷盆地深部结构及沉积充填特点认识,对琼东南盆地油气勘探具有重要的指导意义,同时也为被动陆缘盆地深层地层样式及沉积充填研究提供可参考范例。

The passive continental margin basin in the northern South China Sea is an important oil and gas base in China’s offshore area.This paper reveals depositional filling characteristics and stratigraphic patterns of hyperextended continental margin rift basins based on coupling analysis of the deformation of deep crust and basin filling in the passive continental margin through comprehensive analyses of massive seismic and drilling data from the Qiongdongnan Basin of South China Sea.The results show that the Qiongdongnan Basin,located on the western tip of the northwest sub-basin of the South China Sea,is once a failed continental margin rift basin and presents now as a neck and distal zone along the central depression belt after lithospheric detachment and thinning.The rifting period contains three stretching stages.During the early stage,the formation of isolated rifted basin is characterized by high-angle positive faults.During the detachment active stage,the formation of detachment rifted basin is characterized by low-angle detachment faults with their active time obviously characterized by an east-west migration.And during the late rifting-depression stage,the depocenter is located in the center of the depression.The migration characteristics of differential deformation tectonic movement define the unique deep structure pattern of the Qiongdongnan Basin and lead to obvious differences in depositional fillings and stratigraphic patterns in different tectonic units,especially in depositional systems,where fan delta deposits dominate the main detachment fault side and braided delta deposits occupy the opposite rolling anticline.These factors restricted the distribution of source rocks and deep reservoirs during the rifting period,which in turn constrained geological conditions of hydrocarbon generation and accumulation.Therefore,the deep architecture of hyperextended continental marginal rift basins established based on non-transient breaking-up process of the lithosphere and depositional filling characteristics have important guiding significance for hydrocarbon exploration in the Qiongdongnan Basin,and may serve as reference for the study of deep stratigraphic pattern and depositional filling in passive continental margin basins.