Crustal stress in the Michigan Basin

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Abstract
We conducted four successful hydrofracturing tests in a 5325‐m deep well in Gratiot County, near the center of the Michigan Basin. The top three tests were run through perforations and were limited by the availability of thoroughly cemented cased zones. The controlled manner of hydrofracturing and the repeatability of results during repressurizations suggest that the perforation tests were probably as reliable as the open hole test, at least with respect to the least compressive stress. Data analysis shows a steady increase with depth of both horizontal principal stresses between the depths of 1230 and 5105 m. However, the least horizontal compressive stress at 1230 m (295 bars) is considerably higher than was expected from extending the linear curve fitting the stress‐depth relationship for the three deeper values (−265 bars + 0.24 bars/m). This is in agreement with a regional trend for high horizontal stresses in the top kilometer of the crust. The vertical stress variation with depth, based on the gravity log, was estimated to be 0.26 bars/m, quite similar to the least horizontal stress in the 2800 to 5100‐m range. No fracture orientations were obtained, but on the basis of other measurements throughout the upper Midwest the direction of the largest horizontal compressive stress is expected to lie within the N45°E–N90°E range.