Abstract:

The reservoirs in the Xujiahe Formation in Western Sichuan Depression are characterized by ultradeep burial,ultra-tightness,ultra-low porosity,ultra-low permeability,ultra-late structures,ultra-high pressure, complex GWC,and complex anisotropy.Therefore,conventional seismic techniques are faced with great challenges when predicting and identifying such complex reservoirs and fracture systems,especially the fractures in Xujiahe Formation in the Western Sichuan Depression.Both conventional P-wave data,which could reflect the characteristics of matrix and fluid,and C-wave data,which could reflect matrix and anisotropy,could be achieved by using 3D-3C converted wave technique.This technique is of more advantages than simple P-wave technique in respect of identifying lithology and fluid property by using combined P-and S-wave data as well as predicting fractures by using azimuthal anisotropy of converted wave and split S-wave data.In view of the characteristics of the Xujiahe Formation gas reservoirs in Xinchang gas field,P-waves and converted waves 3D-3C data are fully utilized to perform fracture prediction with P-wave attributes,fracture detection with azimuthal anisotropy and fracture detection with split S-wave based on integrated interpretation of converted waves of 3D-3C data.Based on prediction of large-scale faults and fractured zones by only using P-wave attributes,small-scale fracture network is predicted by using azimuthal anisotropy and split S-waves.The problem of fracture prediction for the tight reservoirs in the Xujiahe Formation in Xinchang gas field is solved and the prediction accuracy is improved.

Key words: anisotropy, split S-wave, fracture detection, fractured reservoir, 3D-3C, seismic exploration