Characteristics and evaluation of carbonate reservoir in the southern Qilian Basin

Based on the analysis of rock thin section,scanning electron microscope and properties,the characteristics of the carbonate reservoir in the southern Qilian Basin were studied.The result shows that reservoir rocks are dominated by oolith limestone,bioclastic limestone,micrite limestone and little dolomite,and the best oolith limestone reservoir mainly developed within Middle Triassic Dajialian Formation.The reservoir space is mainly secondary pores developed in intragranular dissolved pores,intergranular dissolved pores,intercrystal dissolved pores and fractures.The reservoir properties are poor,with ultra-low porosity and low permeability.Dolomitization,dissolution and tectonism improve the reservoir properties,while cementation and compaction play destructive roles to reservoir properties and pores of reservoir,which results in poor porosity and permeability.According to reservoir properties and pore structure parameters,the reservoirs in the study area are divided into type Ⅲ and type Ⅳ.