Abstract: From analysis of the sedimentary setting and lithologic feature, Lianglitag formation of the Ordovician in Tazhong area can be divided into three reef formations, namely, framework reef, reef hill and chalky clay hill, within which fossils are so abundant that they play an important role in serving as a link between past and future. The reef-forming organisms are bacterial algae, calcareous algae, coral, sponge, polyzoa, stromatoporoid, etc., the reef-settled organisms are ostracoda, gastropo, brachiopoda and echinoderm, etc. In evolution stage of the organic reef, the reef-forming organism growing sequence is complete, existing advanced starting reproduction stage-broad reproduction stagedeclining stage. In the process of reef growth, the genus-species, abundance and assemblages appear obvious evolutionary and syngenetic characteristics. The analysis suggests that the reefs in Tazhong are characterized by big thickness of single reef, multiple cycles of development, large-scale multistage reef-flat assemblage, reef rmicrofacies well development and grouping-zoning distribution along the platform margin. The large scale of reef complex shaped by organic reef and grain flat combination provides good fundation for shaping the large scale of reefflat petroleum reservoir in this area.

Key words: Tarim basin, Tazhong area, Upper Ordovician, Lianglitag formation, organic reef flat, fossil assemblage, syngenetics