Clay Mineral Diagenesis and Thermal History of the Thonex Well, Western Swiss Molasse Basin

Roland Schegg1, 2 and Werner Leu2
1 Département de Géologie et Paléontologie, 13 rue des Maraîchers, 1211 Geneva 4, Switzerland
2 Geoform Ltd, Anton-Graff-Str. 6, 8401 Winterthur, Switzerland

Abstract: Results are presented of a diagenetic study from the 1300 m thick Oligocene Molasse deposits penetrated by the Thônex geothermal exploration well (Geneva, Switzerland). The x-ray diffraction (XRD) studies of fine-grained rocks indicate the following diagenetic changes: a decrease of illite/smectite (US) expandability from approximately 90% to 30% with depth, a decrease of the amount of US in the clay mineral fraction, and the appearance of corrensite at depths >750 m. The transition from random US to ordered I/S occurs at the base of the Thónex well (1200 to 1300 m) and is associated with a coal rank of about 0.7% Rr (mean random vitrinite reflectance) corresponding to paleotemperatures of 110 to 115 °C Corrensite appears at a vitrinite reflectance value of 0.6% Rr and a corresponding paleotemperature of 100 °C. The amount of post-Molasse erosion is estimated to be approximately 2 km. Thermal history modeling of the Thónex well suggests maximum paleotemperatures of 80 to 115°C and an average paleogeothermal gradient of 27 °C/km during Late Miocene maximum burial conditions.

Key Words: Chlorite/Smectite • Corrensite • Diagenesis • Erosion estimate • Illite/Smectite • Paleotemperatures • Thermal modeling • Vitrinite reflectance • XRD

Clays and Clay Minerals; October 1996 v. 44; no. 5; p. 693-705; DOI: 10.1346/CCMN.1996.0440513
© 1996, The Clay Minerals Society
Clay Minerals Society (www.clays.org)