The Study On Lower-Middle Ordovician Karst Reservoir Analysis Of The Shunnan Area In The Central Tarim Basin

Karst reservoir is an important type of reservoir in Lower-Middle Ordovician of the Shunnan area in the central part of Tarim Basin,has an important oil and gas exploration and development prospects.The basic characteristics,diagenesis,formation mechanism and the development and distribution rule of the karst reservoir be discussed through the methods of core observation,thin sections identification,cathodoluminescence analysis,carbon,oxygen,strontium isotope analysis,rare earth element analysis,fluid inclusion test.(1)The result shows that the karst reservoir development mainly in the micrite,less development in grainstone,dolomite and algal boundstone.Reservoir space types are mainly solution pores,karst caves and fractures which were not filled.Reservoir type mainly has two kinds,respectively is karst-fractured-cavity reservoir and karst-cave reservoir.The total thickness of karst reservoir is thick,the karst-fractured-cavity reservoir accounted for 92.4%,and the karst-cave reservoir accounted for 7.6%.Karst reservoir from more to less developed in the Yijianfang formation,Lower Yingshan formation and Upper Yingshan formation.(2)There are two phases of atmospheric water karstification,respectively are the first act of Middle Caledonian karstification and the Early Hercynian karstification.The first act of Middle Caledonian calcites for karstification contain many single-phase aqueous inclusions,the values of ?13C of this calcites are positive,and the values of ?18O of this calcites are higher,and the diagenetic fluid of this calcites were lower than the seawater's.And the analysis results of strontium isotope and rare earth element are similar to the values of normal marine limestone,and it have less rare earth elements.The values of ?13C of the calcites of Early Hercynian karstification are negative,and the values of ?18O of this calcites are very negative,and the analysis results of strontium isotope are greater than the values of normal marine limestone,and the analysis results of rare earth element are more enriched than the values of normal marine limestone.(3)The first act of Middle Caledonian karstification controlled by the plane of unconformity.In the Platform edge the fracture-cavity systems of the first act of Middle Caledonian karstification were packed and destructed,it has a limited reservoir significance.Inside the Platform the fracture-cavity systems of the first act of Middle Caledonian karstification can effectively save,it has some reservoir significance.Vertically,the development depthof this karst reservoir is relatively stable.This kind of karst reservoirs are more development near the inner platform direction.(4)The Early Hercynian karstification controlled by fault.The fracture-cavity systems of the Early Hercynian karstification have a lot of residual spaces.The Early Hercynian karstification is the important mechanism of the karst reservoir formation,it is of great importance.The southeast of Shunnan area follow the karstification pattern of permeabilit along the fault,and this karst reservoir distribute along the fault,especially on the segment of pull-apart.Lengthways,the distribution of this karst reservoir is saltatory,and it apart from the unconformity surface.The northwest of Shunnan area follow the karstification pattern of permeabilit along the fault and the karstification pattern of fault induced.The characteristics of development and distribution of this karst reservoir are more complicated.And this karst reservoir may not distribute along the fault.Vertically,it can distribute along the bedding,and sometimes it is saltatory.