| The Yubei area of Tarim Basin located in the slope of Hetian ancient uplift,and the Ordovician carbonate karst reservoir develops very well.But due to multiperiods superimposed karst,the heterogeneity and the change of development scale and space distribution of fracture-cavecontained reservoir is very high.Based on core samples slices,the well drilling/logging/core data,geochemical data and the seismic interpretation of the Tarim basin,and combined the pre-study achievement,the developmental characteristics,main controlling factors and reservoir distribution of carbonate palaeokarst of Ordovician in the Yubei Area of Tarim basin has been studied.According to karst theory,palaeokarst has been defined and type of the carbonate palaeokarst has been developed.The research work has made clear that the the Yubei Area of Tarim basin has conditions to develop carbonate palaeokarst.The stage and subclass of syndepositional karst,palaeoweathering crust-type karst and buried karst are subdivided.The very complex superposition of karst is separated.The developmental characteristics and superimposed relationship of three types of karst from microfacies to macrofacies features have been conclude.Syndepositional karst role as the representative to dissolved pores in grains and moldic pores.It is only seen in grain limestone of Lianglitage Formation due to the superposition of the late karstification,and it is relatively limited distribution.Palaeoweathering crust-type karst is characterized by large-scale solution pores and dissolved fracture.It involves large depth with drilling venting and leakage phenomenon obviously.Vadose zone is Filled by directional cements,green mud,etc.Phreatic zone is characterized by granular calcite cementation.There are mainly episode Ⅰ and Ⅱ middle Caledonian and early Hercynian karst in the study area.It is obvious that early Hercynian karst transform early ones.Buried karst is characterized by dissolved pores and fracture which is filled by siliceous and organic matter,which develop along with pores formed in palaeoweathering crust-type karst,and it is very developed in fault and deeper place.It has been concluded that the main period of palaeoweathering crust-type karst is early Hercynian,and the main strata of solution pore is the Yingshan Formation.Finally,the developmental models of palaeoweathering crust-type karst of Yingshan Formation in early Hercynian have been summed up.In eastern Fault-Uplift,the supergene atmospheric water leaching model of the outcropped Yingshan Formation is considered.It is indicated that three younger upward cave horizons were developed during the intermissive rising of sea level in the early Hercynian,and the last one is the island model.In eastern low-lying,the supergene atmospheric water covering model is established.In other regions,Yingshan Formation is covered by Lianglitage Formation which is one hundred meters thick and argillaceous,so it is difficult to be affected by karst in early Hercynian.It has been discussed and verified that the climate,the Sedimentary Facies,the the paleogeomorphic environments,the fault and fracture are the important controlling factors for the Ordovician palaeokarst.Furthermore,the main controlling factors and the exploration of different types of karst reservoirs have been studied.According to the above study,Ⅰ to Ⅳ four karst reservoirs have been divided and their distribution areas have been delineated on the basis of multiperiods superimposed karst of Penglaiba Formation,Yingshan Formation and Lianglitage Formation of Ordovician. |
