Abstract: Seismic data analysis is one of the key technologies for characterizing subtle reservoirs and monitoring subsurface pore fluids. Fundamental rock physics can help us understand seismic response of rocks and fluid properties. Fluid substitution is an important part of rock physics and is a tool of identifying fluid and quantifying formation, and it also plays an important role in AVO analysis. The basis of fluid substitution is Gassmann's relations, which predict how the rock modulus changes with a change of pore fluids. According to the analysis of rock physics, reservoir characterization and hydrocarbon detection in some deepwater survey have been done.