Abstract: The Role of Overpressure for the Migration and Accumulation of Hydrocarbons in the Molasse Basin of Western Switzerland: Implications of a 2-D Modeling Study

SCHEGG, R, W. LEU, and C. CORNFORD

This 2-D modeling study attempted to quantify the processes of hydrocarbon (HC) generation, expulsion and migration along a regional section (NW-SE) in the North Alpine Foreland Basin of Western Switzerland.

Modeled excess pressures increase towards the Alpine front and are mainly related to the lithology distribution; excess pressure compartments are centered around shaly or evaporitic intervals of low permeability. By late Jurassic times, a first major phase of HC generation was initiated in the deepest part of the Permo-Carboniferous grabens in the external part (i.e., NW) of the Molasse basin. A second more important generation phase started in Oligocene-Miocene times in the internal parts close to the Alpine front. Migration of HC seems to be controlled predominantly by the layer-cake geometry of the Mesozoic passive margin sequence. The main driving mechanism for the migration and accumulation of HC is the excess pressure evolution. Large overpressure zones in the frontal part of the orogen (i.e. Subalpine Molasse) can drive deep fluids far updip into the foreland. The build-up of overpressured zones depends strongly on the subsidence rates, lithology and the occurrence of heterogeneities, such as faults. Modeling results suggest that the presence of vertical fault zones have a dramatic influence on the pore pressure evolution (pressure drain-off), and in consequence on the HC accumulation pattern.

AAPG Search and Discovery Article #90942©1997 AAPG International Conference and Exhibition, Vienna, Austria