ON THE ORIGIN OF SALINE FLUIDS IN THE KTB (CONTINENTAL DEEP DRILLING PROJECT OF GERMANY)

Highly saline fluids were encountered during the German Continental De ep Drilling Project (KTB) from depths ranging between 2 and 3 km to ab out 9 km. The most reliable data were obtained from samples extracted during a long-term pumping test in the 4000-m deep KTB Pilot hole. Som e 460 m(3) Ca-Na-Cl brines with about 68 g l(-1) total dissolved solid s (TDS) and some 270 m(3) associated gases, mainly N-2 and CH4 were pu mped to the surface from the main fracture system situated near the bo ttom of the pilot hole. Geochemical and isotopic data support the hydr aulic tests which suggest the presence of an open and large fluid rese rvoir at depth. The pumped fluids from this main fracture system were released from a deep reservoir situated at more than 5500 m depth whic h is hydraulically connected with the 9101 m deep KTB main hole, drill ed some 250 m to the northeast of the pilot hole. While Ca and Sr cont ents of the extracted brines may be the result of water-rock interacti on, Cl is most likely of external origin. The Cl is hypothesized to de rive from geotectonic processes rather than to descending infiltration of paleo-seawater (evaporitic brines). The sampled fluids have probab ly migrated from a deeper reservoir to their present position since th e Cretaceous-Tertiary period due to tectonic activity. However, severa l isotopic studies have identified an admixture of descending paleowat ers down to more than 4000 m depth. The high Cl-36/Cl ratio of the flu ids sampled during the long-term pumping test point to a host rock hig hly enriched in U-Th, unlike the sampled KTB country rocks. The fluid reservoir is believed to be in contact with the Falkenberg granite mas sif situated about 2 km to the E of the KTB holes capable of supplying sufficient neutron flux for considerable subsurface production of Cl- 36. Th, Na-Cl-(K-, SO4) precursor fluids of the Ca-Na-Cl brines were p roduced in the course of extensive tectonic processes since the Late C aledonian within the Bohemian Massif. (C) 1997 Elsevier Science Ltd. A ll rights reserved.