Abstract: For a long time, there is an argument about the lower limit of the richness of organic matter for a carbonate to be source rocks. It is a key problem in evaluating oil and gas prospects in carbonate rock area. However, the richness indicators for carbonate source rocks used in China are set on lower levels. Based on a preliminary study on the richness and loss of organic carbon during thermal evolution, considering the difference of diagenesis between shales and carbonate rocks, it is suitable to take 0.3-0.5% as the lower limit of organic carbon and 100 ppm or more as that of hydrocarbon content. Thermal simulation experiments on several samples showed that the threshold of carbonate source rocks is at about 420℃, but the lower limit is at 470℃ which was acquired from the maximum pyrolysls temperature. The experiment also shows that the hydrocarbon content decreases with thermal evolution and 8-9% organic carbon was lost during the whole maturation stage. Organic carbon loss increased to 16% In the mid-period of overmaturation stage and 20% at the later stage.