Migration and accumulation of hydrocarbons in the Swiss Molasse Basin: implications of a 2D basin modeling study

This 2D modelling study attempted to quantify the processes of hydrocarbon (HC) generation, expulsion and migration along a regional section (NW-SE) i n the North Alpine Foreland Basin of Western Switzerland. Modelled excess p ressures increase towards the Alpine front, and are mainly related to the l ithology distribution; excess pressure compartments are centred around shal y or evaporitic intervals of low permeability. By late Jurassic times, a fi rst major phase of HC generation is initiated in the deepest part of the Pe rmo-Carboniferous grabens in the external part (i.e. NW) of the Molasse bas in. A second more important generation phase starts in Oligocene-Miocene ti mes 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 p assive margin sequence. In contrast to many studies where vertical buoyancy driven migration dominates, the main driving mechanism for the migration a nd 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 zone s depends strongly on the subsidence rates, lithology and the occurrence of heterogeneities such as faults. Modelling results suggest that the presenc e of vertical fault zones have a dramatic influence on the pore pressure ev olution (pressure drain-off), and in consequence on the HC accumulation pat tern. (C) 1999 Elsevier Science Ltd. All rights reserved.