Based on the different responses of compensated neutron log (CNL) in the gas reservoir before and after mud filtrate invasion, the authors designed the test of time-lapsed CNLin cased hole. The result indicates that time-lapsed CNLis effective in gas identification of a reservoir whose permeability is higher than (1～3)×10^-3μm². The test has provided a new method for the interpretation of the questionable, low resistivity or mud contaminated reservoir and for the restudy of old well data.