Abstract: Abstract：Based on data of logs, cores and outcrops, the characteristics of the stratigraphic sequence and sedimentary facies of the Yanchang Formation in the Longdong area, Ordos basin, are studied. Four third-order sequence boundaries are recognized in the formation. The authors divide the formation into four third-order depositional sequences and ten systems tracts. Four main depositional systems are identified： the fan delta, braided river delta, meandering river delta and lacustrine sedimentary systems. The distribution characteristics of the depositional systems in various systems tracts are analyzed and the depositional models of the systems tracts and the relationship between depositional sequences and petroleum are also discussed. The authors think that the sand bodies in channels of the delta plain and subaqueous distributary channels in the delta front in the lowstand systems tracts are main reservoirs, the sand bodies of turbidites in transgressive and highstand systems tracts are potential reservoirs, and the lacustrine mudstone and shale developed in transgressive and highstand systems tracts are good source rocks and cap rocks-all these show that the Longdong area is located in a favorable petroleum-bearing system with favorable source-reservoir-cap assemblages and has good petroleum prospects.