Abstract: The Lower Xinhe Formation of Xiaohu sub-sag,Yabulai Basin is the key exploration horizon in Yumen Oilfield.Commercial oil flows found in well YT6,show a good exploration prospect.So,under the sequence framework,well understanding the distribution of contemporaneous sand bodies is favorable to the further exploration and exploitation.This paper aims to employ the classic sequence stratigraphy,to elaborately analyze the sequence characteristics,sedimentary facies types,and sediment evolution and distribution,and discusses the distribution of favorable sedimentary reservoir,based on the data of drilling cores,loggings and seismic data.The results show that the Lower Xinhe Formation develops a complete third-order sequence.Fan delta,braided river delta and lacustrine facies are developed in the Lower Xinhe Formation.The palaeogeography of the Lower Xinhe Formation is characterized by a clear difference between the southern part and its counterpart of the basin.Fan delta front is located along the south steep fault belt and braided river-delta front is developed in the northern and northwestern gentle slope,with lacustrine sediments in the central lake.From the period of lowstand system tract to highstand system tract,sedimentary characteristics and the sedimentary facies distribution are both correspondingly different in different stages.Overall,the changes of the depth of water from shallow to deep,and then to shallow cause the sedimentary area gradually to enlarge,and fan delta,braided river delta first to progradate,then retrogradate,and finally progradate.The analysis indicates that the distributary channels of highstand system tract in the northwest of YT6 well as favorable reservoir are the future exploration targets in the study area.