Fault leakage and reservoir charging in the Upper Rhine Graben, Germany – Assessment of the Leopoldshafen fault bend

Abstract

Faults control subsurface fluid flow in sedimentary basins worldwide, often affecting hydrocarbon migration as well as geothermal fluid circulation. Since the Upper Rhine Graben hosts geothermal, lithium brine, and hydrocarbon resources, the role of faults on fluid flow in the basin is of great interest. To better understand structurally controlled fluid flow in the central Upper Rhine Graben, this study investigates the hydrocarbon migration of the fault-related Leopoldshafen oil field. As fault geometry can change along-strike, fault conductivity might also vary spatially as it depends on, among others, the fault orientation relative to the active stress field. Therefore, slip and dilation tendencies are used as proxies for stress-controlled fault conductivity, computed for a strike-slip faulting regime with NW to NNW oriented σHmax. Fault Response Modelling is applied to derive subsurface strain distributions, associated with fault activity during the Miocene phase of basin development. The Leopoldshafen fault, as a fundamental element of the fault-related three-way dip closure trap structure, is characterized by a changing along-strike geometry with NNE-SSW trending fault segments linked by a NNW-SSE trending left-stepping fault bend. Slip and dilation tendencies are highest around the fault bend segment, with a range of considered σHmax orientations. As Expansion Index analysis indicates across-fault thickening and thus fault activity of the Leopoldshafen fault during the main phase of graben rifting (Late Eocene to Oligocene), subsurface strain distribution pattern associated with Miocene fault activity highlight strain concentrations around the fault bend segment, which has acted as a releasing bend. Thus, based on slip and dilation tendency analyses, as well as Fault Response Modelling, the fault bend segment is identified as the up-fault fluid migration zone along the Leopoldshafen fault. This underscores the importance of comprehending the structural inventory of the Upper Rhine Graben for the prospect of future subsurface exploration and alternative post-oil utilization.