Controlling factors and genetic pattern of the Ordovician reservoirs in the Tahe area,Tarim Basin

The Middle-Lower Ordovician in the northern Tarim Basin experienced two phases of karstification in the Middle Caledonian and in the Early Hercynian,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 fractured-vuggy karst reservoirs of various types with strong heterogeneity and significant zonation.In the north zone,reservoir spaces are dominated by big caves and fractures; in the middle 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.Multi-stage karstification cycles occurred during the long time and multi-stage exposure in the north of Tahe area.The development of reservoirs is controlled by single-factor but multi-stage effects which are dominated 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.