根据对营尔凹陷下沟组岩心、测井和地震资料的分析，运用现代沉积学、层序地层学理论，将营尔凹陷下白垩统下沟组划分为4个三级层序。营尔凹陷下沟组主要发育辫状河三角洲、扇三角洲、近岸水下扇、远岸水下扇、滑溻浊积体、湖泊共6种沉积相类型，形成不同类型砂体。三角洲前缘亚相及远岸水下扇沉积相砂岩发育，砂岩物性较好，具备储集空间且储盖组合条件好，有利于油气成藏。三角洲平原亚相及近岸水下扇以粗碎屑沉积为主，砂泥分异差，储集物性差，难以形成油气藏。有利沉积相带空间展布与构造的配置关系决定了油气富集高产区的空间分布。%The Ying’er depression,Jiuquan Basin,Gansu appears as a Mesozoic-Cenozoic sedimentary depression on the northern margin of the Qilian fold zone. Four third-order sequences have been distinguished,on the basis of cores,well logs and seismic data,for the Lower Cretaceous Xiagou Formation as a main exploration target in the depression. Six sedimentary facies types are also discriminated,including the braided delta,fan delta,nearshore submarine fan,offshore submarine fan,fluxoturbidites and lake facies. The delta front subfacies and offshore submarine fan sandstones are believed to be the most favourable area for the hydrocarbon accumulation due to excellent physical properties and reservoir-seal associations. In contrast,the things are different for the delta plain subfacies and nearshore submarine fan deposits owing to coarser-grained desposits, and poor sand-mud differentiation and physical properties of the reservoir rocks. The spatial distribution and structural arrangement of the favourable facies zones play an important role in the spatial distribution of prolific areas.
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