Abstract: On the basis of analyzing clay minerals of more than 8, 000 shale samples collected from 20 oil-bearing basins, we summarize six transformation types of smectite in Chinese oil-bearing basins: I .Continual conversion type from smectite to illite; H .Noncontinous conversion type from smectite to illite; Ⅲ.Conversion type from smectite to chlorite; Ⅳ. Conversion type from minor smectite to illite; V . Nonconversion type of smectite and Ⅵ.Smectite-absent type.We discuss different effects of these types on the primary migration of petroleum in this paper.First of all in the basin of continual conversion type, the mineral transformation does obvious effect to the primary migration.For in this kind of basin.the dehydration stage in the conversion from smectite to illite corresponds to the stage of the primary migration of petroleum. When organism transform into petroleum, the mineral smectite coexisted with organism transforms into interstratified illite / smectite-and furthur illite.The shale microfractures emerged in the process of transformation provides channel for the primary migration of petroleum. The dehydrated water in this process contributes as a carrier and the unusual high presure produces power to the primary migration. Whearas, the second, in the basins of noncontinous conversion or non-conversion type, the effect is not obvious, since mineral tranformation does not match with the stage of petroleum generation or there is not enough transformative smectite. And the third, if there isn't smectite's transformation or smectite itself in the basins, the trans formation and dehydration of smectite seldom occur.Conseouently.there is no effect of clay minerals on the primary migration.