Abstract: By contrasting two opposite seismic reflection features of hydrothermal conducting pathways (low continuity and weak seismic amplitudes) and hydrothermal zones (high continuity and strong seismic amplitudes) of data gathered from hypogenic karst systems in the Middle-Lower Ordovician of Shunnan area in Tarim Basin,we developed a new seismic characterization workflow consisting of seismic amplitude thresholding,fracture system interpretation,pick up,and merge display,to effectively describe the coupling relationship between spatial distribution of hydrothermal zones (hypogenic karst system' zones) and hydrothermal conducting pathways (fracture systems),and then to establish development modal of the hypogenic karsts.Characterization of the hypogenic karsts from the Yingshan Formation of Well SN4 in central Tarim Basin shows a close relationship between the formation and distribution of the areas affected by hydrothermal fluids and the migration pathways of the fluids.The migration pathway system is usually composed of primary and secondary faults,and lateral carriers (epigenic karst systems,porous layers and vuggy layers),among which,the primary and secondary faults together form vertical migration pathways.Fluids from the deep usually move upwards along the vertical pathways first and then flow along the lateral carriers-a criss-cross migration system.The accumulation of fluids from the deep at the top of the secondary faults makes it possible for the scattered epigenic karst systems of individual bead string type to communicate with one another and form a massive hypogenic karst system of "bead string commplex" type with such features as branched carriers,layered distribution and interfingered bead strings.

Key words: individual bead string, bead string complex, development model, hypogenic karst system, Ordovician, Tarim Basin