Abstract：In the process of oilfield development, the variation of P-wave velocity and density of oil layer lead to the variations in pre- and post-developmen-t's interlayer time difference, waveform and amplitude of seismic reflections and cause seismic time-laps response. Taking the characteristic differential seismic time-lapse waveform as a starting point, the paper analyzed the relationship between the waveform feature of time-lapse seismic data and variation of velocity in oil layer and yielded following knowledge: ①the shape of differential waveform in thin oil layer is irrelative to the original velocity in surrounding bed and sandstone and only relative to variation of velocity in sandstone of reservoir. The differential waveform resulted from velocity change in thin oil layer has regularity: increasing velocity produces positive waveform and reducing velocity produces negative waveform; ②in practical studies, based on other research means, using differential waveform can directly interpret the variation of velocity in oil layer qualitatively: using positive waveform to judge velocity increasing and negative waveform to judge velocity decreasing in oil layers. The dynamic data of well development in SZ36-1 oilfield of Bohai oil zone proved that using differential waveform could directly interpret the variation of velocity in oil layer qualitatively.
