Based on the sectioned quantitative study of XRD data of clay minerals in argillites from two wells in Zhujiangkou Basin

it is proved that the increase of the ratio of illite and smectite layer contents in illite-smectite mixed layer clay(I/S)is related mainly to the dissolution of potassium feldspar.When deposited organic substances and potassium feldspar are abundant enough

the ratio increase would proceed by the net increase of illite layers at the temperature roughly corresponding to or slightly higher than that of the oil generation threshold;when the temperature is lower than that of threshold

smectite layers would be converted to illite according to dissolution precipitation mechanism;if no supply of K

+

the ratio would increase by net dissolution of smectite layers.In general

the increase ways are independent of the permeability because it could usually allow the ions to transport smoothly.Only when it as low as to filter merely the big organic complex aluminium ions

the illitization reaction would substitute for the net addition of illite layers.Permeability even lower than that or the system being closed would stop the trend of the increase in all ways.