The northern margin of the Mesozoic-Cenozoic Lusitanian (Western Portuguese) Basin is known mostly from borehole sections. Mineralogical studies of the subsurface Cretaceous are based on XRD, XRF, TEM and Flame-spectroscopy of cuttings. Clay mineralogy varies stratigraphically, with kaolinite dominating in the Aptian/Albian/Cenomanian and late Turonian-early Coniacian. Illite is common in the late Cenomanian and Campanian-Maastrichtian. Smectite increases in abundance through the Late Cretaceous. Lateral variations in this division characterize palaeogeography and specific tectonic controls. The overall clay supply depended upon the varying influence of sea level, climate and palaeogeography.