Stress orientation to 5 km depth in the basement below Basel (Switzerland) from borehole failure analysis

摘要: A vertical profile of maximum horizontal principal stress, SHmax, orientation to 5 km depth was obtained beneath the Swiss city Basel from observations wellbore failure derived ultrasonic televiewer images in two 1 km distant near-vertical boreholes: a 2755 m exploration well (OT2) imaged 2550 2753 across granitic basement-sediment interface at 2649 m; and deep borehole (BS1) entirely within granite 2569 4992 m. Stress-related form breakouts or drilling-induced tension fractures (DITFs) occurs throughout range logs with predominant. Within granite, DITFs are intermittently present, more less continuously present over all but uppermost 100 where they sparse. The mean SHmax orientations is 151 ± 13° whereas yield 143 ± 14°, combined value weighted for frequency occurrence being N144°E ± 14°. No marked dependence averaged several hundred meters intervals evident. This consistent results inversion populations focal mechanism solutions earthquakes occurring between depths 10–15 km regions immediately north south Basel, T-axis events reservoir (Deichmann Ernst, this volume). identified OT2 above below sediment-basement suggest that change N115°E ± 12° Rotliegendes sandstone near its basement. origin 20–30° uncertain, as lateral extent. do not extend higher than 80 interface, so data define whether further stress evaporites. Near-surface measurements taken 50 km N–S, albeit large variability, hydrofractures up 850 evaporite layers an active salt diapir, also Basel. Thus, available evidence supports notion evaporites on average N–S oriented thus differs NW–SE inferred basement BS1/OS2 earthquake data. Changes can have significant practical consequences development EGS reservoir, serve emphasise importance obtaining estimates target rock mass.