Oil shale is seen as a potential replacement for our rapidly declining petroleum reserves. Oil shale is neither oil nor shale, but is rather a solid organic material tied up in marle, a form of limestone or siltstone. An oil is produced by retorting the shale to volatize the oil. Shale is either retorted in situ or mined and retorted above ground. Water is produced during retorting and liberating from the shale and as a product of combustion. In situ retort water production can be considerably increased by groundwater inflow.