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Basin Inversion in the Alpine Foreland of Austria: The Impact of Tectonic Wedge Mechanics in Petroleum Systems

Abstract

The complex kinematics of fold and thrust belts (FTB) are known to control hydrocarbon generation and migration, trap formation/breaching, seal and charge. Such complexity has resulted in the common acceptance of FTB as complicated and risky, but with high potential rewards (Goffey et al. 2010; Roeder, 2010). Traditionally, FTB have been considered either thin- or thick-skinned, but it is well recognised (Copper, 2007) that a mixed mode of deformation can take place at different locations and times throughout the same FTB. This is why the oversimplified approach of thin- vs. thick-skinned FTBs should be taken comparatively rather than in absolute terms (Butler and Mazzoli, 2006). Such complexity has been mathematically described by the critically Previous HittaperedTop tectonic wedge model (Davis et al., 1983). As shortening thickens the wedge and erosion/sedimentation changes the surface slope, the wedge can lose its critical state. To regain the critical taper, the system reacts by adding material by thrusting at the toe and the base of the wedge, by out-of-sequence thrusting and internal deformation, and/or by the extensional collapse of the hinterland summits. These processes have been shown by the integrated study of high-quality 2D and 3D seismic data coupled with sandbox analogue models, providing key insights into the evolution of tectonic wedges. Existing models, tough, rarely include the reactivation and inversion of the lower plate rift fault array, a process that would lower the surface slope of the wedge by adding material to the deformation front. A case study from the foreland of the Alpine-Carpathian junction shows the impact of tectonic wedge mechanics in petroleum systems. Recent successful drilling and source rock studies in the sub-thrust region of the Alpine foreland of Austria has shown that mildly inverted foreland structures can host large amounts of hydrocarbons (i.e., Höflein field). These structural traps were formed by the inversion of the former European Jurassic continental margin of the Alpine Tethys, and remain preserved underneath the thin-skinned system. Additionally, hydrocarbon generation was provided by the burial associated with the opening of the Vienna Basin in the hinterland, and by the flexural bending of the lower plate in the foreland. All these processes can be linked to the overall mechanical evolution of tectonic wedges.