On the origin of saline fluids in the KTB (Continental Deep Drilling Project of Germany)

• Published in: Applied geochemistry 1998-07, Vol.13 (5), p.653-671
• Main Authors: LODEMANN, M, FRITZT, P, WOLF, M, IVANOVICH, M, HANSEN, B. T, NOLTE, E
• Format: Article
• Language: English
• Subjects: Applied geophysics; Crystalline rocks; Earth sciences; Earth, ocean, space; Exact sciences and technology; Igneous and metamorphic rocks petrology, volcanic processes, magmas; Internal geophysics
• ISSN: 0883-2927; 1872-9134

Highly saline fluids were encountered during the German Continental Deep Drilling Project (KTB) from depths ranging between 2 and 3 km to about 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. Some 460 m super(3) Ca-Na-Cl brines with about 68 g l super(-1) total dissolved solids (TDS) and some 270 m super(3) associated gases, mainly N sub(2) and CH sub(4) were pumped to the surface from the main fracture system situated near the bottom of the pilot hole. Geochemical and isotopic data support the hydraulic tests which suggest the presence of an open and large fluid reservoir at depth. The pumped fluids from this main fracture system were released from a deep reservoir situated at more than 5500 m depth which is hydraulically connected with the 9101 m deep KTB main hole, drilled some 250 m to the northeast of the pilot hole. While Ca and Sr contents of the extracted brines many be the result of water-rock interaction, Cl is most likely of external origin. The Cl is hypothesized to derive from geotectonic processes rather than to descending infiltration of paleo-seawater (evaporitic brines). The sampled fluids have probably migrated from a deeper reservoir to their present position since the Cretaceous-Tertiary period due to tectonic activity. However, several isotopic studies have identified an admixture of descending paleowaters down to more than 4000 m depth. The high super(36)Cl/Cl ratio of the fluids sampled during the long-term pumping test point to a host rock highly enriched in U-Th, unlike the sampled KTB country rocks. The fluid reservoir is believed to be in contact with the Falkenberg granite massif situated about 2 km to the E of the KTB holes, capable of supplying sufficient neutron flux for considerable subsurface production of super(36)Cl. The Na-Cl-(K-, SO sub(4)) 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.