MICROCRACKS IN GRANITE CORES FROM THE EPS-1 GEOTHERMAL DRILL HOLE, SOULTZ-SOUS-FORETS (FRANCE) - PALEOSTRESS DIRECTIONS, PALEOFLUIDS AND CRACK-RELATED VP-ANISOTROPIES

For the modelling of Hot Dry Rock systems the interactions between mac rofractures, prominent microcrack sets and the in situ stress field is of fundamental importance. In this study complete 3D analyses of micr ocrack orientations were carried out on granitoid samples from the Sou ltz geothermal borehole (EPS-1). The crack population is dominated by healed cracks in quartz forming three orthogonal sets (I, II, III) wit h strong preferred orientation, which probably result from superpositi on of internal thermal stresses during cooling and external tectonic s tresses. The inferred orientation of paleo-sigma(H) 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 ra nging between 1 and 2 kbar and 200-400 degrees C. A younger generation of open cracks in quartz which coincide with macroscopic fractures ca n be correlated with the Tertiary N-S direction of sigma(H). The distr ibution of P-wave velocities (Vp) was measured by using the pulse tran smission technique in 132 propagation directions at confining pressure s up to 200 MPa. These Vp data and their variation with confining pres sure are a suitable tool for detecting prominent crack patterns and to estimate the crack-controlled in situ properties. The observed bulk V p anisotropy roughly reflects the composite microcrack fabric and the relative importance of individual sets. It can be assumed that the pro minent crack sets represent planes of weakness and hence may cause a m echanically anisotropic behaviour dependent on their orientation with respect to the recent in situ stresses.