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Ta’nan sag was a sub-sag of Tamtsag Basin in Mongolia��in which developed delta facies and contained large-scale reservoirs��Through the comprehensive utilization of seismic��well logging and core data��the authors studied the sedimentary characteristics��distribution��evolution and reservoir property of delta in Nantun Formation of Ta’nan sag��The results showed that the delta in this area can be divided into three kinds of subfacies (delta plain��delta front��prodelta)and six kinds of microfacies��which are mostly located in the northwest of the basin��The scale of delta sand expanded gradually from the early stage to the last stage of Nantun Formation and the sedimentary range increased rapidly by aggradation and progradation��The porosity-permeability analysis of delta sandstone showed that the sand reservoir type of delta front in Ta'nan sag belonged to medium porosity and super-low permeability type. The porosity and permeability of underwater distributary channel sand ranged from 17% to 20% and 0.1×10^-3��10×10^-3μm² with the best reservoir property; the porosity and permeability of sheet sand ranged from 15% to 16%, 19% to 21% and 0.1×10^-3��5×10^-3μm² with better reservoir property; the porosity and permeability of river mouth bar sand ranged from 13% to 17% and 0.1×10^-3��5×10^-3μm² with the worst reservoir property.