Analysis of High-Resolution Sequence Stratigraphy in Chang-8 Subsection of Yanchang Formation,Jiyuan Area of Ordos Basin

With the theory of high-resolution sequence stratigraphy,the base-level cycles in Chang-8 oil set of Yanchang Formation in Jiyuan area of Ordos Basin are recognized as 2 middle-term cyclic sequences(MSC1 and MSC2) and 6 short-term cyclic sequences(ssc1-ssc6) and the sedimentary environment is a shallow-water delta by utilizing the data of well logging,core and rock and mineral analysis.The results indicate that the delta front and predelta of expanded lake developed in Chang-82 and delta plain appeared in Chang-81 of Jiyuan area.The relationship among the cyclic stacking pattern of various grade of base-level and the distribution and evolution of sedimentary facies are analyzed and the delta of Jiyuan area is compared with the normal delta.A high-resolution sequence stratigraghic framework and stratigraphic-lithofacies palaeogeographic maps of Chang-81 and Chang-82 are established by taking the short-term cyclic sequence as unit.Compared with the normal delta,the shallow-water delta is charactered by its low A/S,no mouth bar and the microfacies where favorable reservoirs developed is the sandbodies of distributary channel which extended long and mainly developed in the period of the rising half cyclic sequences of short-term.