Sequence Stratigraphic Framework and Sedimentary Facies of Yanchang Formation,Triassic System in Ordos Basin

In terms of the principle of high resolution sequence stratigraphy,sequence boundary marks are determined and two kinds of sequence boundaries of unconformity and shift surface of deposition are identified according to outcrop,core,logging and seismic data in Yanchang Formation,Upper Triassic System in Ordos Basin.There are seven sequence boundaries which classify six long-term base level cycles further in the basin.Based on stacking modes of short-term to medium-term base-level cycles,stratum mode of sequence SQ1 to the rising stage of sequence SQ3 is mainly composed of rising stage of long-term base level cycles which composed rising asymmetry cycle configuration.Stratum mode of the falling stage of sequence SQ3 to sequence SQ6 is mainly composed of falling stage of long-term base level cycles which composed falling asymmetry cycle configuration.Sedimentary facies and its distribution are mainly controlled by the change of base level.Sedimentary micro-facies are mainly composed of(braided) river and distributary channel of delta front.Favorable sandbo-dies are mainly distributed in the lower to middle stage of long-term base level rising cycles and the middle to upper stage of long-term base level falling cycles.