Using time-lapse seismics as a reservoir-monitoring tool, geophysics can help distinguish different reservoir production scenarios. For example, Eiken et al. (2000) successfully detected fluid-saturation changes after CO2 injection using time-lapse seismics at Sleipner Field. Over the cycle of a reservoir life, oil saturation usually decreases, reservoir pressure declines, and gas breakout may occur. These changes cause rock property changes that are detectible in time-lapse seismics. Therefore, it is important to understand the effects of pressure and saturation changes on rock properties. While the effects of saturation changes are often well described by Gassmann (1951), Brown and Korringa (1975), and Mavko (1975), the effects of pressure changes are less understood. Here we focus on understanding the effects of fluid pressure on velocities.