Drilling-induced fracturing processes and the evidence for the in situ stress field (KTB drilling site, NE Bavaria)

Fracture forming conditions (compression, tension, shearing, amount of stor ed energy etc.) are documented by joint surface markings as it is known fro m fractographic laboratory experiments of the Material Sciences for more th an 80 years. These markings can be applied for geological analyses. On join t surfaces in drill cores of the two KTB drilling sites (NE Bavaria, German y), fractographic markings could be recognized down to a depth of 9 km. The fracture propagation direction correlates with the maximum horizontal stre ss direction S-H By that changing stress directions in the borehole were de termined. Generally, S-H in the borehole runs NNW (165-180 degrees). Variat ions of S-H occur With increasing depth, mostly close to important fault zo nes. In that case, S-H rotates towards NE, nearly perpendicular to the faul t zone. In these sections the horizontal joints (separating disks) propagat e dominantly perpendicular towards the 60 degrees NE-dipping faults, or the y propagate parallel to the faults (135 degrees). The stress anisotropy was found to be not continuously decreasing with increasing depth. At a depth of 7000 m, a zone of high stress anisotropy exists (high deformation zone). Recently, brittle fracture conditions exist at the flanks of the Franconia n Lineament (KTB) down to a depth of 9 km. The brittle behaviour is current ly predominant under compressive stress conditions. More than 500 joint sur faces with fractographic markings, which were measured and studied along th e drilled cores of the KTB, are the statistical base for the results.