A given weight of water expands in volume with increase of subsurface temperature during burial. The expansion may contribute to water movement from deep sections to shallow, or from the basin center to the margins, and thus could be a contributing factor in hydrocarbon migration. The higher the geothermal gradient, the more water expansion can be expected; a high geothermal gradient favors fluid migration and possibly flushes more hydrocarbons.

Where the geothermal gradient is constant, the rate of water expansion increases with depth of burial. Therefore, water expansion becomes increasingly important at depths necessary for the generation of hydrocarbons.

In a geologic model of shale-sandstone interbeds, where the shales have the greater porosity and hence contain a greater volume of water, the expansion resulting from burial would cause water to move out of the shales and into the sandstones. This movement, therefore, could help the migration of hydrocarbons from shales to sandstones.