蒙古国塔木察格盆地塔南凹陷早白垩世构造古地貌控砂机制*

构造活动是源-汇系统中形成古地貌和砂分散体系的重要影响因素。塔木察格盆地塔南凹陷在早白垩世是一个典型的断陷湖盆,经历了多期幕式断陷活动,但同沉积构造活动及其配置所产生的古地貌对层序结构特征、沉积体系及砂体分布的控制作用仍不明确。文中以地震、测井、岩心资料为基础,以层序地层学理论和古地貌控砂理论为指导,建立了塔南凹陷下白垩统层序地层格架;结合断层活动性分析,阐明了不同类型古地貌对沉积体系和砂体分布的控制作用和规律。构造演化和沉积充填的综合研究表明: 受控盆主断裂的幕式断陷活动及同沉积断裂的差异断陷活动影响,塔南凹陷发育横向凸起和走向斜坡2种类型的构造调节带,这些构造调节带是水系入盆的通道,控制着盆地的主体物源方向、沉积体系类型与分布特征;根据断层的几何形态及发育部位,共识别出4种类型构造坡折带,分别是陡坡断崖型坡折带、陡坡断阶型坡折带、缓坡反向断阶型坡折带和盆内坡折带,它们各自控制了不同类型沉积体系的形成与分布;沉积物入盆后,沉积物分散体系和砂体的分布特征受同沉积断裂的组合样式控制,研究区共发育梳状断裂系、叉状断裂系和平行断裂系3种同沉积断裂平面组合;与构造调节带相对应的断裂坡折带低部位是大型储层砂体,特别是低位扇三角洲或盆底扇砂体发育的有利部位,是寻找砂岩油气藏的有利区带。研究成果对于深化断陷湖盆源-汇系统理论、指导断陷湖盆砂体分布预测具有重要的理论意义和实际价值。

Sedimentary characteristics and controlling factors of large-scale longshore bar in the Oligocene Huagang Formation,Xihu sag

The Xihu sag is located in the eastern depression of the East China Sea Shelf Basin,which is the largest hydrocarbon-bearing sag in this basin. It has been debated on sedimentary environments and sedimentary systems of the Oligocene Huagang Formation as the major reservoir. On the basis of cores,well-logging response,and seismic reflection characteristics,three typical lithofacies types of thick sandstone in the Huagang Formation are identified in the central-eastern Xihu sag,namely as massive gravelly medium sandstone,fine-medium sandstone with swash beddings,and silt-fine sandstone with ripples. Statistics of sedimentary structures parameters indicate the thick-bedded sandstone of the Huagang Formation were developed in washing zone along shoreline with strong hydrodynamic condition and wave action. On the gamma logging curve,the thick sandstone is characterized by low-amplitude,blocky pattern in the lower part,gradually changing to high-amplitude,jagged pattern in the top part. On the seismic profile,mound reflections with flat bottom and convex-up top show the medium amplitude and continuity,indicating the large-scale longshore bar systems. Stacked pattern and spatial distribution were depicted combined with well-logging patterns and matching of seismic attributes and sand content. The longshore bar systems of the Huagang Formation in the Xihu sag were mainly developed at edge of the delta front in the eastern part of the study area,which is formed from the earlier braided-river delta front reworked by wave action during base level fall.