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

Abstract

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.

Correlation of core and downhole seismic velocities in high-pressure metamorphic rocks: A case study for the COSC-1 borehole, Sweden

“…At about 1750 and 1880 m, core and downhole velocities agree very well and the sample velocities (569-2 and 593-4) at atmospheric pressure are close to that at lithostatic pressure. Fracture mapping indicates a higher amount of low-angle fractures at these depths (Wenning et al, 2017). Moreover, these samples show the highest anisotropy values of all samples.…”

“…But the in situ stress field can be more complicated due to tectonic processes such as ridge push, postglacial relief, or mantel-driven stress. For the COSC-1 area, the in situ stress anisotropy is low (Wenning et al, 2017). Thus, we assume that lithostatic pressure is a good approximation for the in situ pressure conditions.…”

“…Beside some naturally occurring fractures (see fracture column in Fig. 10; Wenning et al, 2017), most fractures are due to core handling and there are of course those that occur inevitably between each core section. In total, natural fractures and core section transition zones account for about 10 % of the raw data set suggesting that they are not the reason for the general mismatch.…”

Correlation of core and downhole seismic velocities in high-pressure metamorphic rocks: A case study for the COSC-1 borehole, Sweden

“…Wenning et al (2016) used a slightly different relation for their six samples, proposed by Wepfer and Christensen (1991). For most applications aiming to determine the linear high-pressure part and intrinsic anisotropy, both relations give consistent results.…”

“…Another source for such misfits is the different pressure relation used for the samples investigated by Wenning et al (2016). This was based on the velocity-pressure relationship proposed by Wepfer and Christensen (1991). This empirical relationship is adequate at higher pressures but not for zero confining pressure.…”