Abstract: Mound seismic anomaly with resemblance to anticline has been identified in seismic profile of Mid-Upper Ordovician over Mannan area of Tarim basin, which was intepreted previously as buildup formed in sedimentary process. However, reef was widely accepted following comparism with seismic reflection characteristics of reef in Mid-Upper Ordovician of Tabei area and elsewhere in the world. The reef belonged to platform marginal barrier reef of carbonate sedimentary system in hinge zone, extending 90 km from east to west with width of 2 ~ 4 km, distributed “S” type along with the reef of Mid-Upper Ordovician. It formed the horseshoe shaped reef group in Tabei area, with the thickness up to 500 m and good reservoir quality. The reef in Mannan area should be an important target for oil and gas exploration in the future.

Key words: Tarim basin, Mangar sag, Middle Ordovician, Late Ordovician, Organic reef, Seismic interpretation