Abstract: Abstract：The influence of the pore structure on resistivity is much greater than that of fluid type for reservoirs with complex
pore structure，which has little difference in resistivity between gas reservoirs and water reservoirs. So，it is difficult to distin-
guish them with resistivity. Pore fluid bulk modulus is used to identify gas or water reservoirs based on the marked difference
betweengasbulkmodulusandwater（oroil）bulkmodulus. Theratioofcompressionalwaveandshearwavevelocityandelastic
parameters are used to determine the fluid type. The simulating results show that a small quantity of gas can cause significant vp
and vp/vsdecreas，and vsis insensitive to the gas content，and vp/vsof water reservoirs are much significantly greater than that
of gas reservoirs. A cross plot of vp/vsversus vscan be used to identify gas or water reservoirs. This cross plot can eliminate
the effects of porosity and lithology on vp/vs.

Key words: Key words：elastic parameters, vp/vs, pore fluid bulk modulus, fluid type