The recent hydrocarbon exploration in Baorao Trough of Jierg alangtu Sag shows that Aershan Formation has huge resource potential.In this paper,the sequence stratigraphic theory based on base-level cycles is used to divide Aershan Formation into one secular cycle(the third-level sequence) and three corresponding middle-term stratigraphic circles(the forth-level sequence)MSC1,MSC2 and MSC3.Within the isochronal framework of the Sag,the distribution of reservoir rock and caprockand the development charactersof reservoir-cap rock assemblages are analyzed.The authors point out that two series of reservoir-cap rock assemblages, MSC1+MSC2 and MSC3, are developed in the Formatio n,but MSC3 is more favorable.The subtle reservoir exploration analysis indicates that such areas as the dropped side of the II fault of Xilin in the southeast glacies area and the developmental region of inshore subsea apron and sublacustrine fan in the northeast Gubiliang of the J68 well field are favorable regions for subtle reservoir exploration.