Abstract
Abstract. The stress field at depth is a relevant parameter for the design of subsurface constructions and reservoir management. Yet the distortion of the regional stress field due to local-scale features such as sedimentary and tectonic structures or topography is often poorly constrained. We conduct a stress sensitivity analysis using 3-D numerical geomechanical modelling with an elasto-plastic material law to explore the impact of such site-specific features on the stress field in a sedimentary sequence of the Swiss Alpine foreland. The model's dimensions are 14 × 14 × 3 km3 and it contains 10 units with different mechanical properties, intersected by two regional fault zones. An initial stress state is established involving a semi-empirical relationship between the ratio of horizontal to vertical stress and the overconsolidation ratio of argillaceous sediments. The model results indicate that local topography can affect the stress field significantly to depths greater than the relief contrasts at the surface, especially in conjunction with horizontal tectonic loading. The complexity and frictional properties of faults are also relevant. The greatest variability of the stress field arises across the different sedimentary units. Stress magnitudes and stress anisotropy are much larger in stiffer formations such as massive limestones than in softer argillaceous formations. The stiffer formations essentially carry the load of the far-field forces and are therefore more sensitive to changes of the boundary conditions. This general characteristic of stress distribution in the stiff and soft formations is broadly maintained also with progressive loading towards the plastic limit. The stress field in argillaceous sediments within a stack of formations with strongly contrasting mechanical properties like in the Alpine foreland appears to be relatively insensitive to changes in the tectonic boundary conditions and is largely controlled by the maximum stiffness contrast with respect to the load-bearing formations.
Highlights
Knowledge of the in situ stress in the subsurface and its local variability is a critical issue for academic questions and application in industry likewise (Fuchs and Müller, 2001; Hergert and Heidbach, 2011; Tingay et al, 2005)
The model results indicate that local topography can affect the stress field significantly to depths greater than the relief contrasts at the surface, especially in conjunction with horizontal tectonic loading
A stress sensitivity analysis using numerical geomechanical modelling was performed to assess the influence of topography, of faults and of mechanical properties on the stress state of a sedimentary sequence in northern Switzerland
Summary
Knowledge of the in situ stress in the subsurface and its local variability is a critical issue for academic questions and application in industry likewise (Fuchs and Müller, 2001; Hergert and Heidbach, 2011; Tingay et al, 2005). The stress field in the upper crust can vary strongly on a local scale due to topography, faults and variable properties of formations. Savage and Morin (2002) showed that topography can cause a highly variable stress field up to polarity reversals of the principal stresses. Examples for stress perturbations due to faults have been compiled by Barton and Zoback (1994) and Yale (2003). Examples for stress perturbations due to faults have been compiled by Barton and Zoback (1994) and Yale (2003). Warpinski (1989) showed on the basis of a large number of hydraulic fracturing data
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