Clay Mineral Diagenesis And Thermal History Of The Thonex Well, Western Swiss Molasse Basin

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Journal Article: Clay Mineral Diagenesis And Thermal History Of The Thonex Well, Western Swiss Molasse Basin

Abstract
Results are presented of a diagenetic study from the 1300 m thick Oligocene Molasse deposits penetrated by the Thonex 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 (I/S) expandability from approximately 90% to 30% with depth, a decrease of the amount of I/S in the clay mineral fraction, 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 (mean random vitrinite reflectance) corresponding to paleotemperatures of 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 paleotemperatures of 80 to 115 degrees C and an average paleogeothermal gradient of 27 degrees C/km during Late Miocene maximum burial conditions.

Authors 
R. Schegg and W. Leu








Published Journal 
Clays and Clay Minerals, 1996





DOI 
Not Provided
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Citation

R. Schegg,W. Leu. 1996. Clay Mineral Diagenesis And Thermal History Of The Thonex Well, Western Swiss Molasse Basin. Clays and Clay Minerals. (!) .