Technical Committee Earth Sciences

Predicting stress re-orientations associated with major geological structures in sedimentary sequences

The project goal was to create an improved stress database for the North German Basin and to investigate how the regional pattern of maximum horizontal principal stress SH (SH orientation) is perturbed locally. In the sub-Zechstein formations the regional SH orientation exhibits a fan-shaped pattern whilst above the Zechstein the SH orientations are more scattered. Locally, faults and halokinetic structures, e.g. diapirs and salt walls, cause deviations up to 70º from the regional SH orientation.
Finite element modelling suggests that the influence of faults on the stress field concentrates around the fault tips, with the zone of influence depending on the fault width and the material contrast between fault zone and surrounding rock mass. Halokinetic structures perturb the stress field out to a distance approximately equal to the horizontal extent of the structure.
Models of a chosen test area resulted in a good match to the observed SH orientation even when faults were not incorporated. However the best fit to the in-situ stress data was obtained when faults were incorporated as zones of weakness. The stress pattern is critically dependent on the differential gravitational loading due to the shape of the salt pillow.