Mineralogical and organopetrographic analysis were carried out on core samples of four geothermal wells from the Aliaga geothermal field located in western Turkey. Whole rock mineralogy and clay minerals were determined, and thermal maturity of the different levels of the formation was defined by means of vitrinite reflectance measurements. The measured maturity gradients compiled from analytical data for four wells and outcrops ranged from low to very high values.