The evolution of overpressured pelite fractures in the Eogene overpressured zone in the Cangdong—Nanpi and Dongpu depressions，HUabei basin is estudied with log data，which shows that pelites with open fractures occur only in a certain depth interval and below this depth few Open fractures develop．The condition for the opening of overpressured pelite fractures is that thermocompression of pore fluids in pelites must exceed compression strength of pelites，which are controlled by porosities，subsurface temperature and burial depths of pelites．Primary migra— tion of hydrocarbons through overpressured pelite fractures may be the deepest and most effective primary migration in a basin，and its depths and。correspondence to organic maturation are con— nected with geothermal gradient．