ON THE ORIGIN OF SALINE FLUIDS IN THE KTB (CONTINENTAL DEEP DRILLING PROJECT OF GERMANY) (VOL 12, PG 831, 1997)

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. So me 460 m(3) Ca-Na-Cl brines with about 68 g 1(-1) total dissolved soli ds (TDS) and some 270 m(3) associated gases, mainly N-2 and CH4 were p umped to the surface from the main fracture system situated near the b ottom of the pilot hole. Geochemical and isotopic data support the hyd raulic tests which suggest the presence of an open and large fluid res ervoir at depth. The pumped fluids from this main fracture system were released from a deep reservoir situated at more than 5500 m depth whi ch is hydraulically connected with the 9101 m deep KTB main hole, dril led some 250 m to the northeast of the pilot hole. While Ca and Sr con tents of the extracted brines may be the result of water-rock interact ion, Cl is most likely of external origin. The C1 is hypothesized to d erive from geotectonic processes rather than to descending infiltratio n of paleo-seawater (evaporitic brines). The sampled fluids have proba bly migrated from a deeper reservoir to their present position since t he Cretaceous-Tertiary period due to tectonic activity. However, sever al isotopic studies have identified an admixture of descending paleo-w aters down to more than 4000 m depth. The high Cl-36/Cl ratio of the f luids sampled during the longterm pumping test point to a host rock hi ghly enriched in U-Th, unlike the sampled KTB country rocks. The fluid reservoir is believed to be in contact with the Falkenberg granite ma ssif situated about 2 km to the E of the KTB holes, capable of supplyi ng sufficient neutron flux for considerable subsurface production of C l-36. Th, Na-Cl-(K-, SO4) precursor fluids of the Ca-Na-Cl brines were produced in the course of extensive tectonic processes since the Late Caledonian within the Bohemian Massif. (C) 1998 Elsevier Science Ltd. All rights reserved.