Characteristics and genetic mechanism of sandbody architecture in Chang-8 oil layer of Yanchang Formation,Ordos Basin

A typical shallow-water delta deposit developed in Chang-8 oil layer of Yanchang Formation,Ordos Basin.Based on previous studies of sedimentary conditions such as provenance analysis,basin bottom configuration,clastic supply capacity and water energy,we examined sedimentary characteristics of Chang-8 oil layer of Yanchang Formation and concluded that Chang-8 oil layer has a developed braided-river delta deposit in the southwestern and northwestern areas of Ordos Basin,a meandering river delta deposit in the northeastern area and a fan delta deposit in the western area.Distributary-channel sandbody and subaqueous distributary-channel sandbody are main reservoir types of these deposits.The practical exploration proved that different sedimentary system areas have significant differences in reservoir property.For instance,the reservoir property of sandstones is better in the northwestern and southwestern areas of the basin,where a considerable scale of reserves has been discovered.However,the reservoir property of sandstones is poor in the northeastern area,where no commercial scale of reserves is found.Herein the main reason for reservoir property differences is attributed to different sandbody architectures.Based on vertical and horizontal anatomy of these sandbodies,we divided the upper part sandbodies of Chang-8 oil layer in Yanchang Formation into distributary channel and distributary sand-dam types by analyzing the genetic mechanism of sandbodies.The study demonstrated that the distributary channel sandbody,developing mainly in the western area of the basin,is formed under conditions of proximal material source,high-energy and sufficient material supply when the channel energy is greater than the jacking force of lake water upon a river entering a lake,and the river channel continues extending forward to form banded,vertical-stacking thick sandbodies.While the distributary sand-dam sandbody,developing mainly in the northeastern area of the basin,is formed under conditions of slow slope,shallow water,distal material source,low energy and slow deposition when the channel energy entering a lake is alternately equivalent to that of lake water,and the river channel forms lobate,discontinuous and thin-layered sandbodies.