Geophysical logs collected from a deep borehole drilled into the North American Craton in Northeastern Alberta shed valuable light on the state of stress in stable cratons. Observed breakout (BO) azimuths rotate between three depth intervals, ranging from 100° at 1,650–2,000 m to 173° at 2,000–2,210 m, and finally to 145° at the bottom. No apparent fractures that might have disturbed the stress field were found. Two potential causes of these rotating BOs can be either a heterogeneous stress field or elastic and strength anisotropy of the formation. The latter interpretation is favored since observed BO azimuths are strongly controlled by rock metamorphic textures, as validated by their close correlations with dip directions of foliation planes and polarization directions from dipole sonic logs. Monte Carlo realizations further demonstrate that anisotropic metamorphic rocks subjected to a uniform horizontal stress direction could result in the observed azimuth‐rotating BOs. Stress magnitudes inferred from this analysis, which incorporates rock anisotropy and weak foliation failure planes, suggest a normal faulting regime and a maximum horizontal compressive direction consistent with that in the overlying Western Canadian Sedimentary Basin (NE‐SW) and the motion of the North American plate. The inferred stress magnitudes are low and Mohr‐Coulomb analyses demonstrate that the formation is not near the critical condition for slip on weak planes. However, more detailed investigations should be conducted since Monte Carlo calculations indicate that analyses from BO widths, particularly when a conventional Kirsch‐based formula is employed, are highly nonunique, allowing for large variations in potential stress states.
Geophysical logs collected from a deep borehole drilled to the Canadian Shield in Northeastern Alberta shed valuable lights on the state of stress in stable cratons. Observed breakout azimuths rotate between three depth intervals, from N100°E at 1650-2000 m to N173°E at 2000-2210 m, and finally to N145°E at the bottom. No obvious fractures that might disturb stresses were found; and these rotating breakouts can be interpreted either as being due to a heterogenous stress field or formation elastic and strength anisotropy. The latter interpretation is favored because the breakout azimuths are strongly controlled by rock metamorphic textures as validated by their close correlations with both dip directions of foliation planes and polarization directions from dipole sonic logs. Monte Carlo realizations further demonstrate that anisotropic metamorphic rocks subjected to a uniform horizontal stress direction could result in the observed azimuth-rotating breakouts. The stress magnitudes inferred from this analysis, which incorporates both the rock anisotropy and weak foliation failure planes, suggest a normal faulting regime and a maximum horizontal compression direction consistent with that in the overlying Western Canadian Sedimentary Basin (NE-SW) and the motion of the North American plate. The inferred stress magnitudes are low and Mohr-Coulomb analyses demonstrate that the formation is not near the critical loading for slip on weak planes. However, more detailed investigations should be conducted since Monte Carlo calculations indicate that analyses from breakout widths, particularly when a conventional Kirsch-based formula is employed, are highly nonunique, allowing for large variations in potential stress states.
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