Sequence stratigraphic framework of the Triassic Yancang Formation in the Ordos basin and petroleum exploration

Through a comprehensive analysis of the outcrops,rock cores,well logging and trace element data,five sequence boundaries of different origins are identified,and on that basis,the Upper Triassic Yanchang Formation in the Ordos basin is divided into four long-term base-level cycles(LSC1,LSC2,LSC3 and LSC4) that are persistent on a regional scale.Then the relationship between the source-reservoir-cap associations and the base-level cyclic sequences of the formation is discussed in detail and it is pointed out that the long-term base-level cycles bear close relation to the source-reservoir-cap rock association.The main reservoir sand bodies occur in the early and middle stages of the ascending half cycle and the middle and late stages of the descending half cycle of the long-term base-level cycle,and lacustrine mudstone and shale developed at positions of ascending and descending transition of the long-term base-level cycle are good hydrocarbon source rocks and cap rocks.