Multi-wave seismic based depiction of subtle channel sandbody:A case study of the Shaximiao Formation in the ZT area,northwestern Sichuan Basin
Tontinental tight sandstone gas is abundant in the Sichuan Basin,and it is an important option for incremental gas reserves/production.The channel sandbodies in the first member of the Jurassic Shaximiao Formation(Sha 1 Member)in the basin are vertically superimposed in multiple periods and laterally crisscrossed in the same period.Moreover,the channel sandbodies are diverse and subtle.They reflect complex and variable seismic responses,making their depiction very challenging.In the ZT area,northwestern Sichuan Basin,the multi-period channel sandbodies and reservoirs of Sha 1 Member are described unclearly with a low accuracy.To address this problem,using the newly collected three-dimensional three-component converted transverse wave(PS wave)seismic data,combined with the pre-stack and post-stack PP-wave seismic data,this paper develops a key technique of multi-wave seismic based depiction of channel sandbodies on the basis of forward simulation.With the support of this technique,the multi-wave seismic responses of channel sandbodies with the variation of sandbody porosity are analyzed.The results show that the PP-wave seismic responses at the sandbody top change from strong wave crest to strong wave trough with the increase of porosity,while the polarity of PS wave seismic responses does not reverse with the change of physical properties.The lithological characterization based on PS wave can effectively identity multi-period subtle channel sandbodies.Compared with the pre-stack P-G attribute that reflects the rate of change of PS wave impedance,the amplitude energy of PS wave is more focused,and the sandbodies depicted by PS wave have clearer lateral distribution with more visible boundaries.The pre-stack P-G attribute has a higher vertical resolution,which is conducive to distinguishing the stacked sandbodies and determining their periods.The proposed multi-wave seismic based technique makes full use of the high resolution of pre-stack P-G attribute to define the vertical periods of sandbodies and the lateral continuity of PS wave to delineate the lateral boundaries of sandbodies.It enables a complete and fine depiction of channel sandbodies in the Sha 1 Member in the ZT area.The technique provides a great support to well deployment and reserves upgrade in the ZT area,lays a foundation for extensive and beneficial exploitation of continental tight sandstone gas in the Sichuan Basin,and drives the advancement of multi-wave seismic exploration technology.
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