CLAY MINERAL DIAGENESIS AND THERMAL HISTORY OF THE THONEX WELL, WESTERN SWISS MOLASSE BASIN

Results are presented of a diagenetic study from the 1300 m thick Olig ocene Molasse deposits penetrated by the Thonex geothermal exploration well (Geneva, Switzerland). The x-ray diffraction (XRD) studies of fi ne-grained rocks indicate the following diagenetic changes: a decrease of illite/smectite (I/S) expandability from approximately 90% to 30% with depth, a decrease of the amount of I/S in the clay mineral fracti on, and the appearance of corrensite at depths >750 m. The transition from random I/S to ordered I/S occurs at the base of the Thonex well ( 1200 to 1300 m) and is associated with a coal rank of about 0.7% Rr (m ean random vitrinite reflectance) corresponding to paleotemperatures o f 110 to 115 degrees C. Corrensite appears at a vitrinite reflectance value of 0.6% Rr and a corresponding paleotemperature of 100 degrees C . The amount of post-Molasse erosion is estimated to be approximately 2 km. Thermal history modeling of the Thonex well suggests maximum pal eotemperatures of 80 to 115 degrees C and an average paleogeothermal g radient of 27 degrees C/km during Late Miocene maximum burial conditio ns.