Tatsuya Numakura Kiyotoshi Sakaguchi Hanae Saishu Atsushi Okamoto Steven E. Ingebritsen Noriyoshi Tsuchiya Noriaki Watanabe 2017 <p><span>The hypothesis that the brittle–ductile transition (BDT) drastically reduces permeability implies that potentially exploitable geothermal resources (permeability &gt;10</span>⁻¹⁶<span class="mb"><span class="mb"> </span></span><span>m</span>²<span>) consisting of supercritical water could occur only in rocks with unusually high transition temperatures such as basalt. However, tensile fracturing is possible even in ductile rocks, and some permeability–depth relations proposed for the continental crust show no drastic permeability reduction at the BDT. Here we present experimental results suggesting that the BDT is not the first-order control on rock permeability, and that potentially exploitable resources may occur in rocks with much lower BDT temperatures, such as the granitic rocks that comprise the bulk of the continental crust. We find that permeability behaviour for fractured granite samples at 350–500</span><span class="mb"><span class="mb"> </span></span><span>°C under effective confining stress is characterized by a transition from a weakly stress-dependent and reversible behaviour to a strongly stress-dependent and irreversible behaviour at a specific, temperature-dependent effective confining stress level. This transition is induced by onset of plastic normal deformation of the fracture surface (elastic–plastic transition) and, importantly, causes no ‘jump’ in the permeability. Empirical equations for this permeability behaviour suggest that potentially exploitable resources exceeding 450</span><span class="mb"><span class="mb"> </span></span><span>°C may form at depths of 2–6</span><span class="mb"><span class="mb"> </span></span><span>km even in the nominally ductile&nbsp;crust.</span></p> application/pdf 10.1038/NGEO2879 en Macmillan Potentially exploitable supercritical geothermal resources in the ductile crust article