Permeability of the rocks from the Kola superdeep borehole at high temperature and pressure: implication to fluid dynamics in the continental crust

Permeability of the samples collected from the surface and from the depths of 8–11 km in the Kola SG-3 and from the depth of 3.8 in the KTB boreholes was studied at temperatures up to 600 °C and pressures up to 150 MPa. These PT correspond to in situ conditions of the deep parts of the superdeep boreholes and to the conditions of progressive and regressive metamorphism of the Kola series rocks. The experiments were carried out with fluid filtration parallel and normal to rock foliation and parallel to core axis. The temperature–permeability trend behavior depends on effective pressure and depth of sample collection. At low effective pressure, a temperature increase leads first to a permeability decrease and then to its increase. At higher effective pressure, inversions appear on all the temperature trends of the samples collected from great depths. In contrast, permeability of the samples selected at shallow depth (3.8 km) and on the surface decreases within the entire temperature range. As a rule, with flow parallel to foliation, the values of permeability are higher than with flow normal to foliation. The results of microstructure studies allow to conclude that microcrack initiation and closure, due to a competitive influence of temperature and pressure cause such permeability behavior. In the samples, there are two families of microcracks: with low aspect ratio and those with high aspect ratio. Their effect on rock permeability changes with temperature. On sample heating, the low aspect ratio microcracks close and, on the contrary, high aspect ratio ones open. The total effect is expressed by minima in the temperature–permeability trends. Permeability anisotropy increases with temperature, reaches a maximum at 200 °C and then decreases. Sample permeability decreases with different gradients at simultaneous increase of temperature and pressure, simulating in situ depth increase. Hence, the deep seat rocks can vary greatly in permeability and against the common background of permeability decrease with depth, local deep aquifers may occur. At PT of progressive metamorphism the permeability values were high enough to permit the fluid flow to penetrate the whole volume of rock massif. At PT of regressive metamorphism, the permeability values were a few decimal orders lower, so fluid flow could be concentrated in large disjunctive zones only.