Sequence stratigraphic characteristic of Yanchang Formation of Upper Triassic in southern Ordos Basin and its control factors

According to the theory and approach of sequence stratigraphy and core,drilling,well-logging,seismic data,the Upper Triassic Yanchang Formation of the southern Ordos Basin was divided into 4 third-order sequences and 11 fourth-order sequences,and the sequence stratigraphic framework was established.Based on the analysis of the distribution characteristics of the sandbodies in the sequence stratigraphic framework,it is held that the reservoir is controlled by the sequence boundaries.It is also found that the beneficial reservoirs are the sandbodies in the early to middle stage of rising semi-cycle and the middle to late stage of falling semi-cycle of the base-level cycle.The development mode of the sandbodies in the sequence stratigraphy framework was established.The reservoir-forming combination in sequence stratigraphy framework is analyzed,and the characteristics and distribution of oil and gas reservoirs in the studied area are determined.The study demonstrates that there are three types of the oil-gas reservoirs in this area: generation and storage meandering river delta oil-gas reservoirs,upper generation and lower storage braided river delta reservoirs,and upper generation and lower storage or generation and storage deep-water gravity flow reservoirs.The beneficial areas in the studied area are forecasted.