Abstract:

TheMiddle=LowerOrdovician in the northern Tarim Basin experienced two phases of karstification in theMiddle Caledonian and in the EarlyHercynian，respectively.The karstification mainly took place under low temperature meteoric water conditions at surface or near the surface，and was controlled by five factors;that is，structures（prototype basins and tectonic transformation），sequences（configuration and patterns），lithofacies（sedimentary facies and diagenetic facies），source of fluids（surface，formation and deep fluids）and time.This resulted in fracturedˉvuggy karst reservoirs of various typeswith strong heterogeneity and significant zonation.In the north zone，reservoir spaces are dom inated by big caves and fractures;in the m iddle zone，reservoir spaces are pores，vuggs and fractures;in the south zone，reservoir spaces are mainly fractures with local vuggs and pores.The major controlling factors vary in different areas and different layers.Multistage karstification cycles occurred during the long time and multistage exposure in the north ofTahe area.The development of reservoirs is controlled by single-factor but multistage effectswhich are dom inated by surface karstification，and three sets of cavern-type reservoirs are formed.In the south where the Upper Ordovician is relatively thick，because of short-time exposure and relatively weak karstification，the development of favorable reservoirs are jointly controlled by peleotopography，exposure time，high-energy facie zones，distribution of fractures and faults as well as later burial and dissolution.

Key words: joint control, genetic mechanism, karst reservoir, carbonate rock, Ordovician, Tahe oilfield, Tarim Basin