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All available information relevant to in situ stress orientations and magnitudes in the Western Canadian Sedimentary Basin (WCSB) were examined to provide a better understanding of how regional stress fields may affect geothermal development. The smallest principal stress is horizontal over most of the Western Canadian Sedimentary Basin, and it varies in magnitude across the region. Horizontal stress trajectories show that SHmax axes are generally aligned SW–NE. A total of 1643 measurements of microfracture and minifracture closure pressures, leak‐off pressures and fracture breakdown pressures have been harnessed to map SHmin gradients across the basin at depths of 156–500, 500–1000, 1000–4185 and 2000–4185 m. Vertical stress magnitudes, calculated in 91 wells, showed that at constant depth, SV increases towards the Canadian Rocky Mountains. Resultant regional stress maps show consistent trends in orientation of stress axes. As a result, predictions can be made that propagation axes of subsurface hydraulic fractures will be dominantly SW–NE, except over the Peace River Arch area, where they will trend more towards SSW–NNE. Engineered geothermal systems in the WCSB can be optimised by drilling horizontal wells parallel to SHmin.

Azimuthally aligned breakouts in oil wells are explained as shear fractures in the zone of amplified stress difference near the borehole, in a stress field having unequal horizontal principal stresses. Brittle fracture theory, on the Mohr–Coulomb failure criterion, shows that the fractures will propagate from the wall and cause wide spalled zones observable with the four-arm dipmeter. In homogeneous rock, breakouts formed in this way should lengthen the diameter by no more than 8–10%. The breakouts occur near the ends of the diameter parallel to the smaller horizontal stress. Tensile fractures may occur in the orthogonal azimuth, but are unlikely to be seen by the four-arm dipmeter calipers.Three examples are given of principal stress orientations inferred from borehole breakouts. At Rangely, Colorado, breakout azimuths suggest an approximately east–west principal compression in agreement with results obtained previously from direct stress measurements, hydrofracture, and earthquake mechanism analysis. In the east Texas Basin, north-northwest to northwest aligned breakouts suggest maximum horizontal stresses oriented at right angles to these directions. This is consistent with inferences from recent extensional faulting and one hydraulic fracture determination. In the Norman Wells area of northern Canada, northwest–southeast aligned breakouts suggest a contemporary horizontal principal compression closely parallel to natural, probably hydraulic fractures of Laramide age in a subsurface limestone reservoir. The inferred principal stress axes are consistent with the structural evolution of this area, and extend the evidence for coherent stress orientation in western Canada from southern Alberta to Norman Wells, a distance of 1900 km.

Estimates of the in situ stress tensor in sedimentary basins can be inferred from information gathered during the drilling and logging of hydrocarbon exploration wells. Principal stress magnitudes can be obtained from information provided by density logs, drilling histories, leak-off test results and pore pressure measurements. Horizontal principal stress orientations are indicated by borehole breakouts recorded on four-arm dipmeter logs. The stress magnitude estimates are comparable with values obtained from hydraulic fracturing and the orientation data provide a level of detail hitherto unobtainable. The information obtained assists in designing hydraulic fractures, in secondary recovery programmes and in obtaining optimum drill bit performance. It also contributes to understanding lithospheric plate movements, crustal subsidence and tectonic deformation.

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