Present-day stress field analysis in the COSC-2 borehole, Sweden

The Collisional Orogeny in the Scandinavian Caledonides (COSC) scientific drilling project studies mountain building processes in a major mid-Paleozoic orogen in western Scandinavia and its comparison with modern analogues (i.e. Alpine-Himalaya mountain belt) by two boreholes (COSC-1 and COSC-2) in Jämtland, central Sweden. The COSC-2 borehole was drilled from mid-April to early August 2020 with nearly 100% core recovery and reached a total depth of 2276m. COSC-2 drilling encountered, from top to bottom, 780m of turbiditic greywackes, about 50m of a sheared black shale unit followed by sandstones and conglomerates in a turbiditic background sedimentation to about 1250m. Ignimbrites and volcanic porphyries with sporadic intervals of doleritic intrusions dominate the deeper stratigraphic sequence (from 1250 m to the bottom depth). To acquire the petrophysical properties of the rocks, three downhole logging campaigns were carried out by Lund University and the ICDP Operational Support Group from 2020 to 2022. In this study, high-resolution acoustic images of the open borehole below 100m were analysed to identify and interpret past and present tectonic features. Two main categories were detected on the image log: geological structures (i.e. foliation, fractures) and stress-induced alteration of the borehole (i.e. breakout). The latter allows the orientation of the present-day stress field to be constrained. For breakout identification, both manual and automatic peak-detection was deployed. In the manual interpretation, the breakout azimuth is assumed to be the center of each breakout, whereas in the automatic selection, the breakout azimuth is set to the average location of the peak when the minimum location in the filtered amplitude and the maximum location in the filtered radius image logs are close (difference less than 25°), based on the assumption that the breakout shape is symmetric. In the COSC-2 borehole, the breakouts were mainly concentrated between 1600m and 1897m. Only a few and poorly-developed breakouts were manually identified outside of dolerite intrusions and gabbroid rocks. Based on the manual approach, about 104 borehole breakouts were identified for a total length of 93m with an average orientation of the maximum horizontal principal stress (SH) of 160°. Automatic peaking detected 216 breakouts for a total length of 43m with an average SH-orientation of 161°. A high correlation was found between these two methods, and the SH-orientation remains fairly constant among the borehole. We also compared the results of COSC-2 with those of the 2496m deep COSC-1 borehole, located about 20 km to the northwest of COSC-2: 1. the orientation in the two boreholes diverges by about 33° (SH orientation of COSC-1 is 127°), 2. in COSC-2 the breakouts are well developed in width and length, and 3. they show a much greater cumulative length (93m compared to 22m in COSC-1). The paucity of breakouts in the COSC-1 well has been attributed to the type of rocks (metamorphic and crystalline) that are generally elastically stiff and have high mechanical strength, which inhibits the formation of breakouts. In contrast, in COSC-2, the dolerite and gabbroid rocks seem more prone to stress-induced enlargements.