Abstract: The phase state of hydrocarbons formed in carbonate source rocks at overmatured stage change from oil into wet and dry gases. When the carbonate rocks at overmatured stage act as gas-prone source rocks,their lower limit values of organic carbon content depend not only on the content of absorbed hydrocarbons,organic types and thermal evolution levels,but on the mechanisms and conditions of hydrocarbon expulsion to a certain extent. With the overmatured carbonate rocks of Ordovician Majiagou in the Ordos Basin as an example,first the paper made quantitative analyses on absorbed gaseous hydrocarbons generated,using the methods of hydrocarbon acidolysis from tanked cuttings,residual gas saturation,and geochemical analyses of whole hydrocarbons;then estimated the lower limit values of organic carbon content to be 0.03%-0.06% and further approached on the lower limit of organic carbon content combined with different hydrocarbon expulsion mechanisms.