Abstract: Numerous studies show that shale brittleness has great influences on the anisotropy,pooling capacity,permeability and fracability of oil and gas reservoirs,thus making the prediction of shale brittleness one of the hot spots in shale gas reservoirs evaluation.Lab analyses also confirmed a close relationship between shale brittleness and several factors like the mineral content and the pore structure that affect mechanical properties of shale rocks.Rickman et al (2008) proposed a method for brittleness coefficient calculation based on elastic parameters.The method combined Young's modulus and the Poisson ratio-two mechanical parameters that can only be calculated with an assumption that the reservoirs in concern are isotropous.Therefore,the method can not accurately describe the brittleness of complex media.To deal with the problem,we introduced the elastic parameter calculation method of VTI media proposed by Thomsen (2013),and developed a brittleness calculation formula comprised of anisotropic parameters ε,γ and δ.Theoretically,the formulat links shale brittleness to anisotropy and offered a better way for anisotropic characterization of shale rocks.Application of the method to the study of the 5^th member of the Xujiahe Formation in Xinchang-Xiaoquan gas field of Sichuan Basin indicated that shale reservoirs in areas with abnormally high brittleness coefficients have strong brittleness,thus it can be used to better identify "brittle sweet spots" within "geologic sweet spots".The brittleness prediction results coincide well with the drilling results of the only two shale gas wells.

Key words: VTI media, elastic parameter, brittleness, fracture, shale gas