DGMK/ÖGEW Frühjahrstagung

Celle

25.04.-26.04.2019

Merz L, Baier U, Hilgers C

Merz L

Fracture distribution is of prime importance in URG carbonate reservoirs and reflects the
orientation of stress during deformation. Since subsurface data are limited, numerous studies
derive data from field analogs. They include fault-fracture scaling relationships (e.g. Meier et
al., 2015), fold-fracture relationships (e.g. Lamarche et al., 2012), fracture-bedding
relationships (e.g. Ladeira and Prince, 1981) and connectivity of fracture networks (e.g.
Sanderson et al., 2018). We studied outcrops on the eastern shoulder of the URG with dronebased
photogrammetry in order to conduct fracture geometries of a reservoir analogue and
evaluate the data with an automated workflow.
W-E opening of the URG commenced in the late Eocene, with sinistral strike slip caused by NS
alpine compression and continued with W-E extension (Schuhmacher 2002). However, insitu
stress variations from the generally described regional stress were observed in the URG,
e.g. depending on lithology (Meixner 2016; Valley and Evans, 2009) and structures.
Additionally, fracture geometry depends on the rock mechanical properties, which determine
clustering of fractures (Olson et al., 1993). Outcrop analogs themselves might influence bulk
fracture orientation as they are over-consolidated due to their uplift to the surface where they
reached zero horizontal stress (Laubach and Gale, 2018). By applying the graph theory based
on distinction of I-, X-, and Y-nodes (Sanderson et al., 2018) on a quarry floor we quantify the
connectivity of the fracture network for later flow simulation.
Comparison of LiDAR and UAV data shows similar quality of both datasets, and we can
underline the value of UAV based photogrammetry in outcrop characterization. From our digital
outcrop analogue data on a hundred-meter-scale we can deduce that distribution, orientation
and persistency of fracturing is related to: (1) decameter-scale buckling as kink-bandmechanism,
(2) faulting in tri-shear-mechanism, with veins in fault-bend folds rather than a
significant fractured damage zone, (3) bedding thickness and (4) uplift. We observe fracture
clusters on a 30 m scale. Additionally, fracture networks show a high connectivity with a high
ratio of Y- and X-nodes. Our results implicate compression tectonics and inversion on the URG
shoulders manifested by ten- to hundred-meter-scale kinking and tri-shear zones with faulting
and fracturing. This may be related to late Cretaceous, pre-alpine compression tectonics as
described by Kley and Voigt (2008) and coincides with abutting relationships revealing NNESSW
fracture sets to be the oldest.
We conclude that the Muschelkalk in southwestern Germany was subject to compression,
resulting in kink-folds, tri-shear zones, reverse faults and associated fractures. Valuable
outcrop analog data can be derived by UAV photogrammetry and digital processing.
Combination with quantified connectivity of the fracture network allows for construction of a 3D
fracture model and fluid flow simulation. Absolute timing of faulting and vein formation will
further constrain our understanding of fracture history to reservoir charging.