"From metamorphic and igneous petrology, an analysis is made of what minerals should be present in spent shale Many of the traditional methods used in metamorphic petrology are inappropriate because fluid pressures are so low and spent shales are highly undersaturated with respect to Si02 However, decarbonation reactions found in the metamorphism of carbonate rocks allow more possibilities than are typically suggested for oil shale . From igneous petrology, the pyroxenes and olivines found in spent shale are confirmed, but nepheline and leucite should be present but are not usually discovered. Melilite, which is ubiquitous to spent shale, is found in metamorphic and igneous rocks formed under similar circumstances."