Multiphase, decoupled faulting in the southern German Molasse Basin – evidence from 3-D seismic data

Abstract

Abstract. We use three-dimensional seismic reflection data from the southern German Molasse Basin to investigate the structural style and evolution of a geometrically decoupled fault network in close proximity to the Alpine deformation front. We recognise two fault arrays that are vertically separated by a clay-rich layer – lower normal faults and upper normal and reverse faults. A frontal thrust fault partially overprints the upper fault array. Analysis of seismic stratigraphy, syn-kinematic strata, throw distribution, and spatial relationships between faults suggest a multiphase fault evolution: (1) initiation of the lower normal faults in the Upper Jurassic carbonate platform during the early Oligocene, (2) development of the upper normal faults in the Cenozoic sediments during the late Oligocene, and (3) reverse reactivation of the upper normal faults and thrusting during the mid-Miocene. These distinct phases document the evolution of the stress field as the Alpine orogen propagated across the foreland. We postulate that interplay between the horizontal compression and vertical stresses due to the syn-sedimentary loading resulted in the intermittent normal faulting. The vertical stress gradients within the flexed foredeep defined the independent development of the upper faults above the lower faults, whereas mechanical behaviour of the clay-rich layer precluded the subsequent linkage of the fault arrays. The thrust fault must have been facilitated by the reverse reactivation of the upper normal faults, as its maximum displacement and extent correlate with the occurrence of these faults. We conclude that the evolving tectonic stresses were the primary mechanism of fault activation, whereas the mechanical stratigraphy and pre-existing structures locally governed the structural style.

3D thermo‐hydro‐mechanical simulation of the behaviour of a naturally fractured petrothermal reservoir in deep Upper Jurassic carbonates of the Bavarian Molasse Basin – Case study Geretsried

“…The overlying stratum is composed of cretaceous interbedded clay marls, marly limestones, and sandstones, as well as more than 4 km thick mainly clay marl of the Tertiary and a thin layer of quaternary sediments. The prominent tectonic structure in Upper Jurassic limestones and dolostones in the study area constitutes the E-W striking Gartenberg fault zone [4].…”

“…As the well Geretsried GEN-1ST-A1 is located south of Munich in the NAFB, strata is gently dipping with approx. 10° towards the Alpine thrust belt in the south [4].…”

“…Promising temperatures above 150 °C but low productivity were not sufficient for an economic development of a geothermal doublet, which led to temporarily closing the well. As a result, a new exploration concept was defined, targeting the crosscuttings of bounding faults of the Gartenberg graben-type fault zone beneath the well GEN-1 at a depth of 4.5 km [4,5]. Accordingly, the highly deviated sidetrack GEN-1ST-A1 was drilled in 2017 with a maximum measured depth of 5700 m MD (4735 m TVD).…”

“…Based on these combined analyses of data, in addition to the four primary faults, already interpreted by [4] from a 3D seismic survey, four interpreted major fractures are considered. These additional fractures cross the well path at depths of 4720, 5150, 5274 and 5400 m MD and are production (T prod = 7 ⋅ 10 -5 m 2 /s) than during injection (T inj = 3.5 ⋅ 10 -4 m 2 /s), see e.g.…”

“…A 3D reservoir model is built by integrating geophysical, geological and drilling data gained in the study area [4][5][6][7]9]. For the purpose of dynamic simulation, the 3D static geological model, derived from seismic interpretation by [4], is converted into the 3D structural architecture of the reservoir, capable of handling finite-element meshing. Using Skua-Gocad, the stratigraphic horizons and a large and intricate fault system are reprocessed in a geologically consistent way.…”