Microcracks in granite cores from the EPS-1 geothermal drill hole, Soultz-sous-Forêts (France): paleostress directions, paleofluids and crack-related Vp-anisotropies
For the modelling of Hot Dry Rock systems the interactions between macrofractures, prominent microcrack sets and the in situ stress field is of fundamental importance. In this study complete 3D analyses of microcrack orientations were carried out on granitoid samples from the Soultz geothermal borehole (EPS-1). The crack population is dominated by healed cracks in quartz forming three orthogonal sets (I, II, III) with strong preferred orientation, which probably result from superposition of internal thermal stresses during cooling and external tectonic stresses. The inferred orientation of paleo-σH is NE-SW. Based on the microthermometry of secondary fluid inclusions it can be assumed that cyclic crack/healing events occurred at P/T conditions roughly ranging between 1 and 2 kbar and 200-400°C. A younger generation of open cracks in quartz which coincide with macroscopic fractures can be correlated with the Tertiary N-S direction of σH. The distribution of P-wave velocities (Vp) was measured by using the pulse transmission technique in 132 propagation directions at confining pressures up to 200 MPa. These Vp data and their variation with confining pressure are a suitable tool for detecting prominent crack patterns and to estimate the crack-controlled in situ properties. The observed bulk Vp anisotropy roughly reflects the composite microcrack fabric and the relative importance of individual sets. It can be assumed that the prominent crack sets represent planes of weakness and hence may cause a mechanically anisotropic behaviour dependent on their orientation with respect to the recent in situ stresses.
International Journal of Earth Sciences
- Pub Date: