Abstract: In order to properly evaluate the oil potential in overmature areas, microscopic observations are made on Ordovician-Silurian dark claystones. It is considered that there are secondary asphalts in overmature claystones. The content of asphalts should be excluded from the analytical data of organic carbon. The rocks with organic carbon content ranging from 0.4-1.0% are good source rocks of basinal facies, and those below 0.4% are poor source rocks of continental shelf facies.