GENETIC TYPES AND DISTRIBUTION FORECAST OF AVAILABLE CARBONATE RESERVOIRS IN ORDOVICIAN IN THE CENTRAL AREA OF TARIM BASIN

The available reservoirs of Ordovician carbonate in central Tarim can be classified into 3 types as epidiagenetic paleo-karst reservoir,burial karst reservoir and interlayer dolomite reservoir,and the epidiagenetic paleo-karst reservoir can be subdivided into 2 types as epidiagenetic karst reservoir formed later than Ordovician(EKR-1) and epidiagenetic karst reservoir formed from late Middle Ordovician to early Late Ordovician(Llandeilo to early Caradoc)(EKR-2).The key controlling factors to EKR-1 are paleo-karst morphology from Silurian to Devonian especially in Devonian and silicification by magma heat liquids in Permian.The favorable exploration regions are mainly distributed in areas from Tazhong-19 to Tazhong-9,Tazhong-1 to Tazhong-4 to Tazhong-403 and Tazhong-16 to Tazhong-15.The chief controlling factors to EKR-2 recently-discovered should be paleo-karst morphology from Llandeilo to early Caradoc and fault karstification of several stages.The reservoir is chiefly distributed in the middle-upper part of the Yinshan Fm.of the Lower Ordovician and in the area from the Tazhong Uplift to the Bachu Uplift.The burial karst reservoir mainly distributed in the Lianglitage Fm.of the Upper Ordovician is chiefly controlled by sedimentary facies(carbonate shoal) and the meteoric water dissolution produced from penecontemporaneous exposure,burial(organic or inorganic) karstification and fault-channeled fresh-water karstification.The available reservoir is chiefly distributed in shelf marginal high-energy shoal and reef of the Lianglitage Fm.developed in the southern belt near Tazhong-I Fault,forming a giant primary stratigraphy oil-gas pool traped by diagenetic carbonate heterogenetic karst reservoir.The key controlling factors to interlayer dolomite reservoir in the Lower Ordovician should be sedimentary facies(semi-restricted to restricted platform),burial recrystallization,burial karstification and fault-channeled fresh-water karstification.The reservoir is generally distributed by layers of larger area,and in the whole Tazhong-Bachu Uplift even in other more extensive regions,with small types of pore and cavity.