Abstract: The Ordovician palaeokarst carbonate reservoirs in the eastern Ordos Basin are poorly understood at present.Thus it will be fundamental for discovering gas reserves and evaluating its developmental prospects to analyze the formation mechanism of the palaeokarst reservoirs and the gas accumulation potentials of the Ordovician carbonates.We elaborated the reservoir characteristics,controlling factors for the reservoir formation,major controlling factors of gas accumulation and storage capacity of the Ordovician carbonates in the study area.The conclusion is that in the eastern Ordos Basin,the weathered crust reservoirs of carbonates are well developed,gas source is sufficient and the timing of source,migration and reservoir is good,so the area is favorable for large-scale gas accumulation. In terms of the reservoir characte-rization,it is featured by low porosity and low permeability as a whole,with the porosity of 3% and permeability of 0.05×10^-3 μm² being regarded as the lower limit of physical property for effective reservoir,and with the pore diameter of 30 μm and throat diameter of 5 μm regarded as the lower limit of reservoir pore-throat sizes.The generation of high-quality carbonate paleokarst reservoir is mainly effected by favorable sedimentary microfacies,effective karstification,and integrated diagenesi,etc.Dolomite with anhydrite concretions of half-filling type is the most important high-quality reservoirs.The controlling factors for gas accumulation are summarized as follows:① the development of high-quality reservoirs lays a substantial basis for gas accumulation;② the favorable timing of source,migration and reservoir is the key.In general,the Lower Paleozoic carbonate karst reservoirs in the eastern Ordos Basin are of great potential for exploration.

Key words: lower limit of physical properties for effective reservoirs, gas accumulation, palaeokarst reservoir, Ordovician, eastern Ordos Basin