Geomechanical forward modeling of the stress field, pore pressure and compaction in the North Alpine Thrust Front, SE Germany

The North Alpine Thrust Front (NATF) is an example of the classical onshore fold-and-thrust belt and foreland system [1]. There are ongoing heat production projects in this area. However, complex compaction and stress fields require detailed investigation for safe and economical drilling activities. Previous field investigation of the wedge and foredeep shed light on the possible driving mechanisms for overpressure generation in the wedge, foredeep and footwall in the SE of Germany. To do this, 20 deep wellbores are selected in this area and their geophysical and drilling data are investigated [2]. This study is a complementary work to find possible explanations for observations through numerical modeling. Examining the mechanics behind these complex deformations is beyond the capabilities of the critical taper theory. However, a large strain geomechanical numerical simulator coupled with critical state soil mechanics constitutive model can provide useful insights in this regard. Geomechanical forward modeling requires boundary conditions at far distances. Also, except some basic geometrical features, other deformations are not predefined and they are developing during the simulation. Therefore, it is not only insightful regarding the final shape of the system, but also progressive development of the deformations is trackable [3].  A plane-strain framework is implemented to simulate the interested processes through the Elfen software [4]. A quasistatic criterion is assumed throughout the simulation to decrease the possible boundary effects of the loading. Adaptive-remeshing helps to capture the large-strain behavior of the system in a reasonable computational time. Data from different sources of the drilling, geophysical tools and field observation is used to tune the model and test the capability of the model to estimate the required properties. Numerical simulations result in a similar geometry which is observed in the field works. Obtained stress values and pore pressure are comparable to the field data.  The differences between the simulation results and field observations can be attributed to the assumptions which were made during the simulation. For example, thermal impacts and possible diagenetic processes are neglected during the simulation. Also, a homogeneous material is assumed for the different layers, while in the real case, there are heterogeneities inside the layers.

1. Pfiffner, O, A. (1986) “Evolution of the North alpine Foreland Basinn in the central Alps”, Foreland Basins, 219-228.

2. Drews, M., Duschl, F. (2022) “Overpessure, vertical stress, compaction and horizontal loading along the North Alpine Thrust Front, SE Germany”, Marine and Petroleum Geology, 143, 105806.

3. Albertz, M., Sanz, P, F. (2012) “Critical state finite element models of contractional fault-related folding: Part 2. Mechanical analysis”, Tectonophysics, 150-170

4. Rockfield (2017) “Elfen explicit manual (Version 4.10)”, Swansea, UK, Rockfield Software.