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Abstract
Abstract. Hydrothermally active and altered fault/shear zones in crystalline rocks are of practical importance because of their potential similarities with petrothermal reservoirs and exploitable natural hydrothermal systems. The petrophysical and hydraulic characterization of such structures is therefore of significant interest. Here, we report the results of corresponding investigations on a prominent shear zone of this type located in the crystalline Aar massif of the central Swiss Alps. A shallow borehole was drilled, which acutely intersects the core of the shear zone and is entirely situated in its surrounding damage zone. The focus of this study is a detailed characterization of this damage zone based on geophysical borehole measurements. For this purpose, a comprehensive suite of borehole logs, comprising passive and active nuclear, full-waveform sonic, resistivity, self-potential, optical televiewer, and borehole radar data, was collected. The migrated images of the borehole radar reflection data together with the optical televiewer data reveal a complicated network of intersecting fractures in the damage zone. Consequently, the associated petrophysical properties, notably the sonic velocities and porosities, are distinctly different from intact granitic formations. Cluster analyses of the borehole logs in combination with the structural interpretations of the optical televiewer data illustrate that the variations in the petrophysical properties are predominantly governed by the intense brittle deformation. The imaged fracture network and the high-porosity zones associated with brittle deformation represent the main flow pathways. This interpretation is consistent with the available geophysical measurements as well as the analyses of the retrieved core material. Furthermore, the interpretation of the self-potential and fluid resistivity log data suggests a compartmentalized hydraulic behavior, as evidenced by inflows of water into the borehole from different sources, which is likely to be governed by the steeply dipping structures.
Highlights
As opposed to their sedimentary counterparts, crystalline rocks tend be characterized by very small matrix porosities, and fluid pathways are mostly associated with brittle deformation structures at scales ranging from micrometers to kilometers, such as fractures and fault/shear zones as well as their associated damage zones (e.g., Brace, 1980; Barton et al, 1995; Evans et al, 1997; Caine et al, 1996; Faulkner and Armitage, 2013)
With the objective to characterize the fracture network of the damage zone surrounding the Grimsel Breccia Fault (GBF) and its petrophysical properties, we have performed an integrated analysis of the geophysical borehole log measurements
The log data are affected by challenging borehole conditions, notably numerous and large enlargements, the dataset contains a multitude of valuable information, which is in agreement with previous studies and adds to their findings
Summary
As opposed to their sedimentary counterparts, crystalline rocks tend be characterized by very small matrix porosities, and fluid pathways are mostly associated with brittle deformation structures at scales ranging from micrometers to kilometers, such as fractures and fault/shear zones as well as their associated damage zones (e.g., Brace, 1980; Barton et al, 1995; Evans et al, 1997; Caine et al, 1996; Faulkner and Armitage, 2013). Building on the results of these previous works, the focus of this study is a detailed characterization of the fracture network in the damage zone of the main fault core from geophysical borehole log data with a particular focus on the geometrical and petrophysical properties of the network as well as the links of the latter to brittle deformation. An integrated workflow utilizing a variety of geophysical borehole log measurements is necessary to mitigate these ambiguities Such an integrated analysis, in combination with evidence from OTV and core data, is used in this study to constrain geometrical and petrophysical properties of the GBF damage zone. The paper starts with a brief description of the geological setting, the challenges associated with the acquisition of the geophysical borehole logs in the intensely fractured crystalline environment, and their resulting impact on the quality and reliability of the data
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