Abstract: Diagenetic process of c1.ay minerals in argillaceous rocks from Cretaceous in the Songliao basin is studied,with the following evolutionary models estabushed: a depth of burial of more than 1,200-1,300 meters and a temperature range of 65-70℃ are required for transformation of montmorillonite into montmorillonite/illite mixed-layer clay minerals; more than 1,6002,000 meters and 115-120℃ for transformation into montmorillonite/chlorite mixed-layer minerals; and more than 2,500-3,000 meters and 170-175℃ for transformation of the above-mentioned mixed-layer minerals into illite and chlorite.These are related to the extremely high geothermal gradient of the basin.Based on the evolutionary characteristics of clay minerals,the Cretaceous is divided into three diagenetic stages; the early,middle and late stages,each being related to some phase of oil and gas generation.The early stage is characterized by large amounts of dispersed montmorillonite,with organic materials not yet mature.The middle stage is marked with widespread occurrence of expanding layer of mixed-layer minerals,with mixed layer of M/I in the initial phase a.nd heavy crude formed of poor maturity.The crude oiI of Daqing oil field was mostly formed in this stage.The late stage is characterized by the o}urrence of M/Ch mixed-layer minerals,and hydrocarbans formed in this stage were light oil and wet gas of high maturity,with the appearance of dispersed chlorite in the last phase of diagenesis and formation of dry gas as a product of thermal decomposition of organic materials.Reasonable inference of the underground distribution of fluid may be made from the study of evolution of clay minerals.This is a useful means for appraising the prospectivity of a basin,determining the objectives of oil and gas exploration and making predictions about the favourable areas for oil and gas accumulation.