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Image log analysis of in situ stress orientation, breakout growth, and natural geologic structures to 2.5 km depth in central Scandinavian Caledonides: results from the COSC-1 borehole
Department of Earth Sciences, Geological Institute, ETH Zürich.
Department of Earth Sciences, Geophysics, Uppsala University.
Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.ORCID iD: 0000-0003-1998-0769
Swiss Seismological Service and Department of Mechanical and Process Engineering, ETH Zürich.
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2017 (English)In: Journal of Geophysical Research - Solid Earth, ISSN 2169-9313, E-ISSN 2169-9356, Vol. 122, no 5, 3999-4019 p.Article in journal (Refereed) Published
Abstract [en]

Stress-induced borehole deformation analysis in the Collisional Orogeny in the Scandinavian Caledonide deep scientific borehole establishes in situ stress orientation in a poorly characterized region in central Sweden. Two acoustic televiewer logging campaigns, with more than 1 year between campaigns, provide detailed images along the full length of the 2.5 km deep borehole for breakout, drilling-induced tensile fracture (DITF), and natural occurring structural analysis. Borehole breakouts occur in 13 distinct zones along total length of 22 m, indicating an average maximum horizontal stress, SHmax, orientation of 127° ± 12°. Infrequent DITFs are constrained within one zone from 786 to 787 m depth (SHmax orientation: 121° ± 07°). These SHmax orientations are in agreement with the general trend in Scandinavia and are in accordance with many mechanisms that generate crustal stress (e.g., ridge push, topographic loading, and mantel driven stresses). The unique acquisition of image logs in two successions allows for analysis of time-dependent borehole deformation, indicating that six breakout zones have crept, both along the borehole axis and radially around the borehole. Strong dynamic moduli measured on core samples and an inferred weak in situ stress anisotropy inhibit the formation of breakouts and DITFs. Natural fracture orientation below 800 m is congruent to extensional or hybrid brittle shear failure along the same trend as the current SHmax. Analysis of foliation in the image logs reinforces the interpretation that the discontinuous seismic reflectors with fluctuating dip observed in seismic profiles are due to recumbent folding and boudinage.

Abstract [en]

Knowledge of the in situ stress state gives information on dynamic processes, for example, earthquakes within the Earth, and is also applied to various geotechnical drilling and excavation projects. This paper analyzes stress-induced deformation features in image logs that span the length of the 2.5 km deep borehole in Central Scandinavia, to address the subsurface stresses in the region. We identified a stress orientation in the borehole that is oriented parallel to the general trend in Scandinavia. This orientation is compatible with many drivers of stresses including pushing and pulling forces on the sides from the crust, gravitational loading from mountain ranges, and driving below due to flow of the mantle. A unique observation from this study is that some breakouts have grown both along and radially around the borehole wall. Such radial breakout growth around the borehole will affect how the magnitude of subsurface stresses is estimated.

Place, publisher, year, edition, pages
American Geophysical Union (AGU), 2017. Vol. 122, no 5, 3999-4019 p.
Keyword [en]
in situ stress, borehole image log analysis, central Scandinavian Caledonides, INternational Continental Drilling Program
National Category
Natural Sciences Geochemistry
Research subject
Applied Geochemistry
Identifiers
URN: urn:nbn:se:ltu:diva-63415DOI: 10.1002/2016JB013776ISI: 000403465600041OAI: oai:DiVA.org:ltu-63415DiVA: diva2:1096436
Note

Validerad;2017;Nivå 2;2017-06-14 (rokbeg)

Available from: 2017-05-17 Created: 2017-05-17 Last updated: 2017-07-07Bibliographically approved

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Publisher's full texthttp://onlinelibrary.wiley.com/store/10.1002/2016JB013776/asset/jgrb52070.pdf?v=1&t=j2t47rwr&s=868fb9f2c9a9617d8fd51d37726e0761310f1b2e

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