Keywords: 
• Fuxian area /
• Yanchang Formation /
• sandy debris flows /
• sedimentary characteristics /
• significance of petroleum exploration

Abstract: According to the analysis for cores, seismic interpretation and welllogging , the authors have thoroughly studied the sedimentary facies types and characters of Yanchang Formation of the Upper Triassic in the south Ordos Basin with thin sections and grain size analysis and a great deal of indoor testing data. The authors have proposed that the sedimentary facies type of Chang 9 to Chang 6 of Yanchang Formation is sandy debris flows. With basin sedimentary and evolutionary background,
 the genetic mechanism of the sand debris flows have been discussed, and detailed studies have been made on the rock types and sedimentary structures of the sandy debris flows, build  up relevant sedimentary models. The results indicate that the sandy debris flows deposit is composed of two genetic facies of massivebedded siltsand and packsand and mud pebble siltsand and packsand, which indicate deposition from deltafront sand during slip and slump under outside function of energy.Relationship between sandy debris flow depositional systems and distribution of oil and gas are elaborated in detail. It is considered that sandy debris flow deposits are injection migration oilgas accumulation types and classic lithologic hydrocarbon reservoir, which are the most favorable sedimentary facies belt for the generation of Yanchang Formation reservoirs.