The mineralogy of the whole rock and clay size fractions has been determined for 12 boreholes from the Magellan Basin, Chile. The degree of clay diagenesis is assessed and compared to the organic maturity indicator vitrinite reflectance. The whole rock mineralogy is dominated by quartz, albite, high Fe-chlorite with minor illite/muscovite. Diagenetic zeolites are found in the Tertiary strata deposited in a platform environment in the eastern basin area. Laumontite is concentrated in albite rich sediments in the eastern basin area. Laumontite is concentrated in Cretaceous and Tertiary sediments of the Andean foreland. Expandable layers in illite/smectite are reduced to < 10% at 4900 m depth in Eocene shales; a general reduction of expandability from 70 to 40% occurs between 1500 and 2500 m throughout the basin. Ro data from the Early Cretaceous Springhill Formation reflect the deepening of the platform towards the West. Clay diagenesis is considered to be due to Tertiary overburden, whereas vitrinite reflectances in the Springhill Formation may additionally have been affected by a terminating Jurassic volcanism.