Abstract: The huge gas potential in the 5^th member of the Xujiahe Formation has made it one of the hottest exploration targets in China.However,the formation has not been well explored and many geological “mysteries” remained unsolved-anisotropy of the gas reservoirs there is one of them.A research was carried out on the anisotropy to better understand the reservoirs features and accumulation patterns,so as to improve the precision of reservoir prediction and success rate of well drilling.Previous study shows that the member had been controlled by sedimentary environment and formed lithologic assemblage dominated by thick mudstone and shale and thin tight sandstone.Looking horizontally,the shale varies drama-tically and shows strong anisotropy due to complicated mineral combination,directional alignment of detrital grains,cyclic veneer and other heterogeneous features.There have had no effective method for analyzing and predicting the anisotropy so far.Available methods are mostly based on seismic data to obtain anisotropy information of rocks through analyses of seismic reflection and the direct responses of azimuth attributes upon rock types,physical features and structure configurations.Comparatively speaking,anisotropy feature of shale formations is likely to be ignored by isotropy analyses based solely on stacked wide-azimuth seismic data.By combining both the azimuth and wide-azimuth stacked seismic data,we compared and analyzed the seismic reflection responses,curvature attributes,coherence attributes and impedances of the two sets of data,and then captured features of reservoir space under the guidance of the anisotropy information.A fine interpretation of sedimentary facies,lithology and fracture of the reservoirs and an accurate prediction of sweet spots in the member were then achieved by applying the above-mentioned method and data yield from it.

Key words: isotropy, anisotropy, wide-azimuth, azimuth, shale gas, 5^th member of the Xujiahe Formation, western Sichuan Basin