The CSM HDR site is located at Rosemanowes Quarry on the Carnmenellis granite in Cornwall, UK. The Carnmenellis granite is roughly ellipsoidal in outline and forms part of a continuous granite batholith of early permian age extending from Dartmoor, in the east, to the Isles of Scilly, in the west (Figure). The base of the granite extends well below a depth of 9 km. At the Rosemanowes site the granite is porphyritic near to the surface tending to equigranular at about 2 km.

The Rosemanowes site was selected on the basis that it did not have any major geological features (such as a faults) at the surface and also that the absence of sedimentary or metamorphic cover rock made installation of the comprehensive microseismic network less costly and more effective. Heat flow at the exposed granite surface is around 120 mW/m2, which is approximately double the normal heat flow at sites away from the granite's influence. Three-dimensional heat flow models, based on extensive heat flow measurements and gravity surveys, have indicated an almost linear change in temperature with depth in the upper 7 km of crust over large portions of the Cornubian granite batholith. In the vicinity of the Rosemanowes site the geothermal gradient is close to 35°C/km.

The undisturbed hydraulic properties of the granite have been investigated to depths of 2 km. Low flow rate hydraulic tests at low injection pressures indicated permeabilities between 1 and 10 D at up to 0.7 MPa fluid overpressure. Then permeabilities rose to 60 D at a pressure of 3 MPa, prior to onset of significant discontinuous behaviour at over 5 MPa. Microseismicity was first observed during a low flow rate injection (0.5 l/s) at a wellhead injection pressure of 3.1 MPa.