Estimation of the complete stress tensor to 8 km depth in the KTB scientific drill holes: Implications for crustal strength

TLDR

In situ stress in the brittle crust has long been measured at shallow depths and linked to crustal mechanical behavior inferred from laboratory studies and faulting theory. The authors estimated a continuous stress tensor profile to 8.6 km depth using hydraulic fracturing tests at 1–3 km, modified tests at 6–9 km, and analysis of borehole wall breakouts and tensile fractures throughout the KTB borehole. The stress profile shows a uniform maximum horizontal principal stress orientation of N160°±10°E, consistent with western Europe, with only a change below a major fault, and indicates that up to 8 km the brittle crust is in frictional equilibrium, as the shear‑to‑normal stress ratio on preexisting faults matches Coulomb predictions for a friction coefficient of ~0.7 and near‑hydrostatic pore pressure.

Abstract

For many years, in situ stress in the brittle crust has been measured at relatively shallow depth and related to the mechanical behavior of the crust as inferred from laboratory studies and faulting theory. A continuous profile of the magnitudes and orientations of the three principal stresses has been estimated to depths of 7.7 km and 8.6 km in the German Continental Deep Drilling Program (KTB). This was achieved by hydraulic fracturing tests at relatively shallow depth (1–3 km), estimates of the magnitude of the least horizontal principal stress provided by modified hydraulic fracturing experiments at 6 km and 9 km depths, and analysis of compressional (breakouts) and tensile (drilling‐induced tensile wall fractures) failures of the borehole wall over nearly the entire depth of the KTB borehole. The orientation of the maximum horizontal principal stress was found to be uniform with depth with an orientation of N160°±10°E, which is consistent with the average orientation found throughout western Europe. The only significant change in stress orientation was observed directly below a major fault zone crosscutting the borehole. The profile of stress magnitudes we have obtained demonstrates that to a depth of 8 km, the state of stress in the brittle crust in southern Germany is in frictional equilibrium. That is, the ratio of shear to normal stress as resolved on preexisting faults which are well‐oriented to the in situ stress field is comparable to their frictional strength based on predictions of Coulomb faulting theory for a coefficient of friction of about 0.7 and near‐hydrostatic pore pressure.