NORTHEAST-SOUTHWEST COMPRESSIVE STRESS IN ALBERTA - EVIDENCE FROM OIL-WELLS

The introduction of the four-arm dipmeter well-logging tool has permitted hole asymmetry to be recognised and hole ellipticity to be measured and oriented. Many wells in Alberta, western Canada, have been shown to exhibit depth ranges over which they are non-circular as a result of caving of their walls. These break-outs occur so as to elongate the holes in a northwest-southeast direction. This direction of elongation is consistent between the break-outs in a given well, and between wells distributed over an area of more than 3 × 105 km2. Asymmetric hole elongation occurs in siltstones, sandstones, limestones, dolomites and one shale formation, and through the stratigraphic column from Devonian to Cretaceous. It is unrelated to dip of the strata. The hypothesis is advanced that the break-outs are caused by concentration of stress at the walls of the wells, in a stress field with large, unequal horizontal principal stresses, the larger oriented NE-SW. It is shown that a normal stress field (with the largest principal stress, σ1, vertical) is unlikely to produce large enough horizontal stresses to produce the break-outs. Consequently our hypothesis requires σ1 to be horizontal and oriented NE-SW. The vertical principal stress could be either σ2 (strike-slip stress field) or σ3 (thrust stress field). The orientation of σ1 at right angles to the Rocky Mountains fold axes suggests the possibility that the stress field responsible for the thrust faulting in the mountains is still present. A strike-slip stress field is also possible. The most prominent system of surface joints outcropping in Alberta, shown by Babcock to have sets oriented NE-SW and NW-SE, could have been formed in the proposed stress field. Direct measurements of the stress tensor at two or three points, if they verified this interpretation, would combine with the oil well break-outs to demonstrate a remarkably uniform stress field over a substantial part of the North American plate. The stresses in the Alberta crust here proposed may be related to tractions now acting on the edges and underside of the North American plate, including those producing postglacial uplift of Hudson's Bay; or may be residual stresses from past tectonic events such as the Laramide compression of the Rocky Mountains. © 1979.