Fluids derived from the compaction and dewatering of shale have relatively low salinity, similar to fluids derived from metamorphism of shale to form slate and schist. Fluid inclusions are trapped in mineral crystals as the solid minerals grow. The minimum temperature and salinity of the fluid can be determined by freezing and heating the inclusion. Fluids that interacted with previously dry basement rocks have high salinity. The fluids that form economically important oil and gas reservoirs in limestones represent mixing of low and high salinity fluids. Vertical flow between these reservoirs occurred along fault zones. Active faulting helps to pump fluids vertically.