A theoretical study is made of the dynamics of fluids in a horizontal layer of porous material subject to vertical compaction. It is shown that during the compaction process, fluid may be expelled from the layer both vertically and horizontally. When two immiscible fluids are present in the compacting layer, the degree to which each is expelled depends on several factors, most importantly their initial saturations and their viscosity ratio. It is shown that there is a trend toward an equilibrium saturation in the sense that large saturations tend to decrease during compaction, while low saturations tend to increase. Some potential implications for explaining hydrocarbon migration are suggested: in particular, that during compaction it is mainly water which is expelled from a sourcce bed, leaving behind a high hydrocarbon saturation. The hydrocarbon would subsequently migrate independent of water, driven by pressure gradients.