Transtensional fault segments and fluid migration in the Upper Rhine Graben

The Upper Rhine Graben is utilized for geothermal- and hydrocarbon exploration, and considered for lithium mining from brines. Subsurface exploration requires an understanding of reservoir- and fault properties, the geomechanical response of faults, and associated fluid migration. Slip- and dilation tendency analysis is conducted to identify fault segments in the stratigraphy, which might potentially act as fluid migration pathways.

Fluid migration is reflected by upward hydrocarbon migration of different oil types sourced from the Posidonia shales along fault zones and locally from the Lower Pechelbronn Beds (eg. Bruss 2000). For example, the reservoir quality of the previously explored and hydrocarbon producing Cenozoic Meletta beds is highly variable, decreasing with depth, ranging from 4.5 to 26.3% porosity and 0.09 to 150 mD permeability at ambient conditions (Bruss 2000, Sauer 1981, Schad 1964).

Published stress field and structural data indicated the considered N-S trending fault section of the generally NNE-SSW oriented Leopoldshafen fault 7 km north of Karlsruhe to have a transtensional character enabeling fluid migration. High slip and moderate dilation tendencies indicate this fault segment to be favourably oriented within the present-day stress field to slip or creep, depending on rock strength and SGR fault properties.