--> Abstract: Burial History and Thermal Evolution of Wells Weiach and Entlebuch 1, Swiss Molasse Basin, by W. Leu; #90990 (1993).

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LEU, WERNER, Geoform, Geological Consulting and Studies Ltd., Winterthur, Switzerland

ABSTRACT: Burial History and Thermal Evolution of Wells Weiach and Entlebuch 1, Swiss Molasse Basin

Only 30-50% of the stratigraphic records are preserved in the Swiss foreland basin as a consequence of at least three major erosional events in the Permian, the Late Cretaceous/Paleocene, and the late Neogene. The detailed set of geological and petrophysical data of well Weiach (exploration well for radioactive waste disposal sites) is used to assess the influence of burial and thermal history scenarios on porosity development, organic maturity evolution, and hydrocarbon generation/expulsion. Results of this calibration are taken as constraints for the analysis of the hydrocarbon exploration well Entlebuch l, the only gas-producing well in Switzerland.

Modeling results from well Weiach indicate that recently proposed, large amounts of up to 2.5 km of removed overburden at the level of the late Neogene unconformity, stay in conflict with observed thermal maturities, unless contemporaneously heat-flow variations of substantial magnitude are assumed. Additional erosion of up to 700 m of missing Jurassic and Cretaceous sediments (Tertiary unconformity) is compatible with the observed maturity gradients in the Mesozoic section. Today, only the proven Paleozoic source rocks (Autunian shale and Carboniferous coal) are in the main oil and gas windows. However, hydrocarbon generation rates are low because of ongoing uplift and cooling in this external position in the foreland basin.

In the more proximal position of the Entlebuch 1 well (only a limited data set is publicly available) major thrust wedges of Tertiary clastics were emplaced during the late Alpine orogenic phase (Miocene/Pliocene). Modeling demonstrates that this influences the thermal regime up to the present day, resulting in the generation of mainly gas from the hypothetical Mesozoic, and oil and gas from Paleozoic source rocks.

AAPG Search and Discovery Article #90990©1993 AAPG International Conference and Exhibition, The Hague, Netherlands, October 17-20, 1993.