Abstract: Ordos basin is a large sedimentary basin in the eastern China. In the inner of the basin，faults donut develop and strata are flat(the angle of inclination is less than one degree)，so the possibility that oil and gas can migrate along faults should be ruled out, and then oll and gas microleapage through the fractures and pores in strata should be main form for oil and gas vertical migration. A deep gas-producing well, which is located in the central ancient rift in the inner of the basin，is chosen for study. According to the characteristics of the vertical evolution of organic matter and the change of acidolysis hydrocarbon compositions in different clastic rock layers from the deep well， the chromatographic fractional effect of vertical oil-gas microleapage has been discussed. Owing to the chemical and physical difference among methane and its homologes in mass number，molecular diameter, and the solubility in water, etc.，methane and i-butane preferentially migrate to heavier hydrocarbon and n- butane，respectively. Because the concentation of organic matter is low and the amount of hydrocarbon generated in situ is accordingly little in the two sections which range from 0 to 1020m and 3080m to 3560m in depth，so the above chromatographic effect can be recorded. 1. In the section from 0 to 1020m depth, the result shows that the ratios of C₁/C₂⁺，C₁/C_tal, and iC₄/nC₄, of acidolysis hydrocarbon gases in sandstone are less than those in mudstone. It mainly results from the chromatographic fractional effect, i. e.，methane and i-butane are relativeiy easier to go through poorly permeabie mudstone barrier bed than heavier hydrocarbon and n- butane respectively. 2. In the section from 3080m to 3560m depth，the maturity of organic matter is in condensate to high mature stage，the composition of gas generated by such matter should be increasingly drier and therefore the ratios of C₁/C₂⁺，C₁/C_tal,tend to rise with depth. However, the change of the ratios shows a controversial tendency and an obvious inconsistence with the composition characteristics of gas generated by thermolization，suggesting that methane preferentially migrates to heavier hydrocarbon. In addition，the changing characteristics of C₁/C₂⁺，C₁/C_tal and C₂/C₃ ratios of the acidolysis hydrocarbon in the layers rich in organic matter ( the depth from 1020m to 3080m)，which decreases firstly and increases consequently with depth，is controlled by the generation of hydrocarbon in situ and hence the chromatographic fractional effect is concealed.