Mineral phase transformations are clearly indicated from the XRD patterns. Ashing Oil Shale at 550 ̊C did not result in any major change in the mineralogical composition except that the intensity of the major peaks for calcite, phosphate, and quartz has increased due to the loss of OM, it is also expected that the clay minerals such as kaolinite were transformed into amorphous phases. On the other hand, ashing OS at 750 ̊C resulted in the appearance of calcium silicate, aluminium silicate and lime, and the disappearance of calcite. Above the temperature of 700 ̊C calcium carbonate present in OS starts to decompose to CaO and CO2 [14].