The margin of the carbonate platform of the  Middle and Upper Ordovician, Tazhong area, Tarim Basin is characterized by a welldeveloped rimmed depositional system which consists of various types of grain banks and organic reefs. Through detailed study of sedimentary facies of individual wells, calcarenaceous, calcirudytic, bioclastic, and oolitic banks have been recognized in this rimmed system. Three types of reefs have been distinguished in the platform margin: cryptalgal limemud mounds, baffling reefs mainly constructed by branching bryozoa, sponges and/or corals, and framework reefs built up by sponges, calathium, stromatopora, corals, solenopora and others. The outer belt of the platform margin facing the deepwater basin is characterized by development of the assemblage of moderate and high energy grain banks and framework reefs. The assemblage of moderate and low energy grain banks, cryptalgal limemud mounds, and baffling reefs mainly occurs within the inner belt of the platform margin  . This rimmed system of platform margin is distributed along the western side of Tazhong No.1 Fault striking NWSE ,and is a belt more than 100 kilometers in the length. It is an important belt of oil and gas accumulation in this area. As far as the carbonate reservoir quality within this rimmed system is concerned, the outer belt of the platform margin is generally better than the inner one. The limestone beds in banks and reef cores are best, but sediments of interreef and interbank seas are poor.