Abstract: Oil samples of different migration distances were obtained from the extraction of the simulation expe riment.The saturated and aromatic hydrocarbon fractions in these oil samples were made GC/MS analysis.The results of the stuey on the fractionation effects of n-alkane,alkyl phenanthrene,alkyl sulfur fluorene and alkyl fluorene in the early stage of the secondary petroleum migration revealed that the n-alkane with very little polarity was subjected greater influences from the physical adsorption of clay mineral after entering drainage layers full of water.This resulted in the decreasing trend of nC_18-/nC₁₉₊ peak value ratio within a certain migration distance.When the equilibrium of physical adsorption was reached,the peak value ratio of nC_18-/nC₁₉₊ had an obvious increasing trend.Affected by geologic chromatographic effects,alkyl phenanthrene,alkyl sulfur fluorene and alkyl fluorene in the aromatic fractions with greater polarity presented obvious migration fractionation effects of high molecular hysteresis.The n-alkane and aromatic fractions were subjected physical adsorption at first in the early stage of the secondary petroleum migration,and the action on n-alkane was far greater than on aromatic fractions either in time or in space.The knowledge obtained from the experiment provided powerful research basis for the rational and effective evaluation of hydrocarbon migration processes.