Paleokarstification and Reservoir Distribution of Ordovician Carbonates in Tarim Basin

Three types of paleokarstification,including syndepositional,weathering-crust,and buried ones,are distinguished in Ordovician marine carbonate within Tarim basin.It is synthetically considered that the paleokarstification is a key factor of the formation of Ordovician carbonate reservoirs.Intragranular and intergranular solution spaces and moldic pores formed by syndepositional meteoric fresh-water selective dissolution,are provided matrix porosity for the carbonate reservoir.These reservoirs are controlled by meteoric diagenetic lens in the distribution and are generally developed along the high-energy platform margin facies in the shape of discontinuous lenticle.The carbonate reservoirs related to weathering karstification are regionally distributed within ancient buried hills of exposed Ordovician carbonate,and are vertically limited within 200 meters below erosional unconformity interface of Ordovician carbonate.Based on vertical and lateral development characteristics of weathering karstification,it is directed that karst-type reservoirs are largely distributed within cap rubblerock of surface karst,vertical vadose and horizontal phreatic zones in the vertical direction,and generally developed at karst upland margin,karst slope,upstream karst valley,and karst hammock in the region.Burial karstification is generally related to the dissolution of carbonate minerals resulted from organic acid by which the thermal evolution of organic matter accompanied,is commonly proceeded along previous pore and fissure system,is characterized by more stages and various scales,and is a key factor for the optimized reworking of carbonate reservoir.