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 th e pore fluid pressure as a function of depth in metamorphic continenta l crust to 9101 m depth and to ambient temperatures of ca. 265 degrees C. Open fissures identified in cuttings and cores along the entire pr ofile are partially coated by minerals crystallized from the pore flui d. The fluid inclusions in these minerals reflect pore fluid compositi on and pressure. A NaCl-dominated aqueous fluid with low salinity is e ncountered to ca. 4 km depth, whereas the fluid is CaCl2-dominated wit h a higher salinity of ca. 4 to 24 wt.-% NaCl equiv. between 4 and 9 k m depth, consistent with analyses of the free fluid (available to 6 km depth) and hydraulic tests. The densities of the fluid inclusions inv ariably indicate a hydrostatic pore fluid pressure to 9 km depth and t hus confirm the predictions based on rock strength.