Abstract: The accumulative proved geologic reserves of the gas fields in central Ordos Basin is about 337.73 billion cubic meters,of which more than 80% comes from carbonate reservoirs(mostly the karst reservoirs) in the Majiagou Formation of the Middle Ordovician.On the basis of a systematic and detailed description and study of the key core samples as well as the microscopic observation of cast thin sections,the authors of the paper put forward the formation environment,genetic types,mechanisms of the Ordovician karstification and the karstic re-servoir characteristics.They also suggest that coarse medium crystalline dolomite with intercrystalline pores and the nodular dolomite with anhydrite deposited in particular environment are the material bases of reservoir deve-lopment,while the development and preservation of the secondary pores,the fractures and the vugs formed by the karstification in the epidiagenetic stages and burial diagenesis stages are crucial for the formation of a good reservoir.The karstification in the burial diagenesis stages and structural fissures mainly overprinted on the pore system formed in the epidiagenetic stage.The reservoirs can be further classified into the fracture-dissolved pore(vug) type,the fracture-pore-dissolved pore type and the pore type,among which the fracture-dissolved pore(vug) type is predominant.Quality reservoirs are mainly distributed vertically in the M₁³ and M₁² sub-members with well-developed anhydrite nodular medium crystalline dolomite and horizontally in the central basin-marginal flat facies belt with anhydrite nodules.

Key words: karst, reservoir space, reservoir type, Majiagou Formation, Ordos Basin