Abstract: The determination of gas saturation based on elastic modulus of formation enriches the methods of in-
terpreting and evaluating reservoir， improves the interpretation accuracy of gas saturation which is a very important
parameter. The elastic modulus can be determined by the velocities of compression and shear waves obtained from
array sonic logging as well as the information of density. According to Gassmann equation which describes the
relationship between the bulk modulus and shear modulus in a low frequency situation， an equation is established
between the gas saturation and bulk modulus as well as shear modulus in cases of rock matrix， gas saturated and
water saturated separately， thus gas saturation can be calculated. The processing software based on the model is
developed. The examples of Sichuan central field show that the methods are sufficiently accurate and reliable.

Key words: array sonic logging, gas saturation, bulk modulus, shear modulus