Abstract: The lower member of the Palaeogene Xiaganchaigou Formation in the Wunan-Lucaotan region,western Qaidam Basin,Qinghai comprises two sequence boundaries including one long-term base-level cycle(third-order sequence) LSC1 and four medium-term base-level cycles(fourth-order sequence) MSC1,MSC2,MSC3 and MSC4 on the basis of cores,well logs and three-dimensional seismic data.The study of the reservoir-seal associations shows that the lower member of the Xiaganchaigou Formation may be highly prospective for hydrocarbon exploration.The favourable reservoir rocks consist of distributary channel sandstones and channel-mouth bar sandstones on the braided delta front in MSC3,where the composite tectonic-lithologic oil pools and lithologic oil pools are hosted.