Abstract:

Taking the differential characteristics of the time-lapse seismic waveform as a breakthrough point, the relation between the characteristics and velocity of sand/mud interbeds in non-marine reservoirs is analyzed. Under the following conditions, (1)a five-layer model which consists of mud and shale, (2)Pwave velocity in mud is 2,400 m/s, (3)P-wave velocity in sand is 2,700 m/s, and (4)variation in shear wave velocity in sand is 150 m/s, the study simulates the time-lapse seismic waveform of the five-layer model by the use of convolution and time-lapse difference formula. It is revealed that the time-lapse seismic waveform of interbeds is a multi-peak complex wave, and the thickness of middle shale is a critical factor for the characteristics of the seismic waveforms, which has a critical value about to a quarter of wavelength. The masking effect could be created when the thickness of middle shale is less than this value, in the other word, the time-lapse seismic responses of the upper layers will mask those of the bottom lays.

Key words: time-lapse seismic waveform, sand/mud interbed, masking effect