Abstract: Through physical simulation experiments and the statistical analyses of practical data from oilfields,the authors intended to make a systematic analysis of the influence of reservoir interlayer heterogeneity on the differential injection of hydrocarbon into a trap in secondary migration.It put forth for the first time a concept of critical permeability grade ratio for hydrocarbon injection.In the process of hydrocarbon injection,interlayer interference led to the shielding action of high permeability zones on low-permeability ones,thus making hydrocarbon injection take place only within definite bounds of interlayer permeability grade ratio (or critical grade ratio).The lower permeability limit value of a reservoir has a good relationship of log-log positive correlation with the highest permeability of sandbody within the reservoir.