Abstract: The carbonate weathering crust of the Upper Ordovician in Tazhong area is characterized by both late diagenetic and near-surfacial karst and penecontemporaneous karst for its outcrop without middle-deep burial or diagenetic environment and its minerals without completely stable conversion, namely, showing non-fabric selectivity dissolved fractures and large caves as well as fabric selectivity dissolved vugs, cast pores and intragranular dissolved pores. In vertical, the crust karst can be divided into superfacial karst zone, vertical percolation dissolved zone, radial flowdissolved zone and groundwater flowzone, with acting thickness of less than 100 m, controlled by reef-flat micro-relief and sea level variation in development. The vuggy layer scale formed by early diagenesis is more expanded by the weathering crust karstification, shaping several new fracture-vug bodies at the top. It is suggested that quality reservoir developed horizons may appear in the reef-flat paleo-relief highs of Lianglitag formation and within 100 mfromthe top.

Key words: Tarimbasin, Upper Ordovician, carbonate reservoir, weathering crust karst, identification mark