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

Published online by Cambridge University Press:  28 February 2024

Roland Schegg  and
Werner Leu
Roland Schegg
Affiliation:
Département de Géologie et Paléontologie, 13 rue des Maraîchers, 1211 Geneva 4, Switzerland Geoform Ltd, Anton-Graff-Str. 6, 8401 Winterthur, Switzerland
Werner Leu
Affiliation:
Geoform Ltd, Anton-Graff-Str. 6, 8401 Winterthur, Switzerland
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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 Thônex 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 Thônex well suggests maximum paleotemperatures of 80 to 115°C and an average paleogeothermal gradient of 27 °C/km during Late Miocene maximum burial conditions.

Keywords

Chlorite/SmectiteCorrensiteDiagenesisErosion estimateIllite/SmectitePaleotemperaturesThermal modelingVitrinite reflectanceXRD
Type
Research Article
Information
Clays and Clay Minerals , Volume 44 , Issue 5 , October 1996 , pp. 693 - 705
Copyright
Copyright © 1996, The Clay Minerals Society

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