The High-resolution Sequence Stratigraphy Framework and Its Geological Significance of Triassic Yanchang Formation Ch8-Ch7,Ordos Basin

Triassic Yanchang Formation Ch8-Ch7 of Ordos Basin was situated in the middle and lower parts of Yanchang Formation,which represented maximum extension of lake basin and was chiefly ascribed to lacustrine delta facies.The character of basal level cycle of Triassic Yanchang Formation Ch8-Ch7 in Ordos Basin is analyzed by the theory of high-resolution sequence stratigraphy and is totally subdivided into 1 long-term base level cycle,3 middle-term cycles and 15 to 25 short-term cycles.Based on the above,the environment of Ch8 is delta and that of Ch7 evolved from semi-deep lake to deep lake.MSC3 and MSC4 of 3 middle-term cycles are unsymmetry cycles dominated by ascending half cycles.During the period of the top of MSC4 ascending half(cycle),maximum-flooding surface was formed.MSC5 is an unsymmetry cycle dominated by a drooping half(cycle).At the same time turbidite fan developed.According to the different wells' base-level cycles,it indicated that the reservoir developed in the sub-distributary channels,the debouchure bars and the turbidities in the half ascending short-term cycles,and it is crossed with the prodelta mud,so lithologic reservoir is easily formed.Establishment of isochronous stratigraphic framework based on the theory of high-resolution sequence stratigraphy should further direct significantly the exploration activity of Yanchang Formation in Ordos Basin.