Hydrostatic pore fluid pressure to 9 km depth ‐ Fluid inclusion evidence from the KTB deep drill hole

The KTB drill hole (Germany) provides a unique opportunity to study the pore fluid pressure as a function of depth in metamorphic continental crust to 9101 m depth and to ambient temperatures of ca. 265°C. Open fissures identified in cuttings and cores along the entire profile are partially coated by minerals crystallized from the pore fluid. The fluid inclusions in these minerals reflect pore fluid composition and pressure. A NaCl‐dominated aqueous fluid with low salinity is encountered to ca. 4 km depth, whereas the fluid is CaCl2‐dominated with a higher salinity of ca. 4 to 24 wt.‐% NaCl equiv. between 4 and 9 km depth, consistent with analyses of the free fluid (available to 6 km depth) and hydraulic tests. The densities of the fluid inclusions invariably indicate a hydrostatic pore fluid pressure to 9 km depth and thus confirm the predictions based on rock strength.