The Ordovician is the key period of tectonic movement and depositional evolution in the Tarim basin and its peripheral area.From the Early to Middle Ordovician

the sea level begun to rise slowly and the Tarim basin basically inherited the Late Cambrian sedimentary settings.In the early Late Ordovician

the regional tectonic movement converted from extension to compression

the peripheral area of the basin(mainly to its south) kept uplifting

and the differentiation of topography was enhanced in the platform.In Ordovician

Tarim basin developed two types of platform marginal facies and the paleoclimatic conditions were favorable for reef development.The marginal facies on the steep slop of platform was relatively narrow

the reef-bank complex was large in thickness and good in continuity in direction parallel with the platform margin and the high energy facies belt was located on the outer margin

with Tazhong I slope break zone as a typical example.In contrast

the marginal facies on the gentle slope of platform was relatively wide

the reef complex is relatively small in thickness

composed of small reefs and distributed in Lunnan

the edge of Bachu uplift and Hetian area.Platform marginal reefbanks

intraplateform banks and karst unconformities are main targets of exploration.