It is widely believed that gas dissolved in water or a few percent of a separate gas phase in water can make the pore fluid mixture very compressible. The fluid bulk modulus (K), the inverse of compressibility, would drop significantly and, in turn, P-wave velocity and impedance will decrease. This suggests that seismic techniques (e.g., DHI and AVO) cannot distinguish a water zone with small amounts of gas either dissolved or as a free phase in water from economic gas reservoirs.

In exploration, the tendency is to consider natural gases as extremely light fluids with negligible modulus. Figure 1 shows,...

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