Abstract: The Ordovician in Tazhong area is characterized by large buried depth and stable lateral distribution before the Middlle-Lower Ordovician underwent erosion. By means of seismic stratigraphy, thickness tendency analysis and Ro data, the palaeogeomorphology in middle period of Caledonian tectonic movement in this area is well described as follows: Tazhong uplift is an ellipse shape region around No.2 structural belt with smooth topography and undeveloped local structures; existing slope break zone from surrounding sags to major uplift, and gradual variation of relief by features of sag→slope→platform margin break zone→inner-platform topographic low and ramp→ swell. The Upper Ordovician reef flat mainly distributed in the platform margin break zone and the inner-platform swell ramp zone, with feature of shifting with the platform margin migration. Study shows that the palaeogeomorphology is of remarkable effect on controlling the distribution, vertical evolution and lateral migration of reef flat reservoir body. It is predicted that there existed the platform margin facies in Mid-Late Caledonian tectonic movement in the area between TB-2 well and Z-3 well in southern Tazhong area, meaning that this area has the favorable conditions for biobermal reservoir development.

Key words: Tarim basin, Ordovician, palaeogeomorphology, reef flat, karstification