Kontinentales Tiefbohrprogramm Der Bundesrepublik Research Articles

Upper crustal structure at the KTB drilling site from ambient noise tomography

Summary In this study, we show results from ambient noise tomography around the KTB (Kontinentales Tiefbohrprogramm der Bundesrepublik Deutschland), a continental deep drilling site located at the western edge of the Bohemian Massif, within the Variscan belt of Europe. At the KTB site, crustal rocks have been drilled down to 9 km depth. Before the drilling activity started, several active seismic surveys had been performed to explore its surroundings during the 1980s and early 1990s, in the frame of an extensive exploration of the area aimed at unravelling the characteristics of the continental lower crust that is exposed at surface in this location. Despite the exploration campaigns held at and around the KTB drilling site, there are important targets that are worth further investigation; these are related in particular to the obduction of lower crustal units to the surface, and to the mechanism of orogenic processes in general. Here we present a new 3-D shear wave velocity model of the area from cross-correlations of ambient seismic noise. The model is obtained by a unique data set composed of 2 yr of continuous data recorded at nine 3-component temporary stations (installed from July 2012 to July 2014) located on top and around the drilling site, and together with the data from 19 permanent stations throughout the region. This paper is focusing on the upper crustal layers, and we show velocity variations at short scales that correlate well with known geological structures in the region of the KTB site, at the surface and at depth. These are used to discuss features that are less well-resolved at present.

Drilling-induced tensile wall-fractures: implications for determination of in-situ stress orientation and magnitude

Abstract Detailed investigation of failure of the borehole wall in two scientific drilling projects, the German KTB (Kontinentales Tiefbohrprogramm der Bundesrepublik Deutschland) and the European geothermal research project at Soultz-sous-Forets, France, has lead to new insight in the phenomena of tensile fractures induced in the wellbore wall during drilling. Comparison of the orientation of the fractures with the orientation of the horizontal principal stress known from breakout and hydraulic fracturing analysis demonstrates that these fractures are reliable indicators of the orientation of the maximum horizontal principal stress S H . A model for the initiation of the fractures is presented which points out the important influences of (a) the tectonic stress state, (b) increased mud pressures during drilling operation and (c) thermal stresses induced by circulation of relatively cold drilling mud. Analysis of drilling-induced fractures in the GPK1 borehole at Soultz-sous-Forets (where the magnitude of S H is known from hydraulic fracturing experiments) demonstrates the validity of this model for the initiation of the fractures. Further, a new method is proposed to estimate the magnitude of S H from the occurrence of drilling-induced fractures and knowledge of thermally induced stress and pumping pressure during drilling. The method is successfully applied to both KTB boreholes. An independent method to estimate the magnitude of S H based on the analytical calculation of the stress intensity factor for drilling-induced fractures taking into consideration both, increased mud pressure and thermal stress, is also presented. Application of this method confirms the results derived with the analysis described above. Additionally, the evaluation of the orientation of the fractures with respect to the wellbore axis indicates that over major depth sections of the investigated wells the vertical stress is a principal stress.

Supracrustal intraplate thickening of Variscan basement due to Alpine foreland compression: Results from the superdeep well KTB (Bohemian Massif, Germany)

Until its final depth of 9101 m the superdeep well KTB (Kontinentales Tiefbohrprogramm der Bundesrepublik Deutschland) drilled through Variscan basement rocks which suffered strong brittle‐ductile and brittle deformation during late‐ and post‐Variscan times. Investigations of faults and mineralized tension gashes revealed the following deformation stages: (1) Upper Carboniferous subvertical tension gashes, (2) Upper Carboniferous reverse faults, (3) Cretaceous subhorizontal tension gashes and reverse faults, (4) ?Neogene normal faults. Clear depth‐dependent variations of metamorphic conditions of the post‐Variscan deformation structures are obvious, whereas the Upper Carboniferous structures show almost no variations with depth. This refers to the metamorphic index minerals as well as to the deformation fabrics in quartz. We explain this considerable lack of gradients as being the result of reactivative reverse faulting that led to considerable vertical thickening of the upper crust. The fault geometry indicates an antiformal stack, the frontal ramp of which was drilled by KTB at 7000 m depth. There is strong evidence that this frontal ramp, referred to as the Franconian Lineament at the surface, rises from a subhorizontal detachment at about 10 km depth which corresponds to the brittle‐ductile boundary layer of quartz‐bearing rocks. The considerable amount of supracrustal vertical thickening above the Franconian Lineament results from repeated movements along this detachment since Upper Carboniferous times. It seems reasonable to suppose that in intraplate tectonic settings the brittle‐ductile boundary layer is prone to form a long‐lived decoupling horizon along which the upper brittle crust is detached from the viscous middle and lower crust.

Chemistry and petrography of calcite in the KTB pilot borehole, Bavarian Oberpfalz, Germany

The KTB (Kontinentales Tiefbohrprogramm der Bundesrepublik Deutschland) pilot borehole in northeast Bavaria, Germany, penetrates 4000 m of gneiss, amphibolite, and subordinate calc-silicate, lamprophyre and metagabbro. There are three types of calcite in the drilled section: (1) metamorphic calcite in calc-silicate and marble; (2) crack-filling calcite in all lithologies; and (3) replacement calcite in altered minerals. Crack-filling and replacement calcite postdate metamorphic calcite. Multiple calcite generations in individual cracks suggest that different generations of water repeatedly flowed through the same cracks. Cracks that eventually filled with calcite carried water to sites where replacement calcite formed by plagioclase hydrolysis or other decarbonation reactions. Whole-rock contents of all three calcite types range from ∼ 35 wt% in near-surface calc-silicate to < 0.1 wt% in amphibolite with calcite-filled cracks at 3872 m. Calcite contains average Sr concentrations of 180 ppm and Mn concentrations of 3980 ppm. Crack-filling calcite microdrilled from thin sections has average δ 13 C = −9.0±2.2‰ and δ 18 O = +10.5±1.5‰ (± values are 1 standard deviation). Crack-filling mineral assemblages that include calcite originally formed at temperatures of 150–350°C. Presently, crack-filling calcite is in chemical and isotopic equilibrium with saline to brackish water in the borehole at temperatures of ⩽120°C. The saline to brackish water contains a significant proportion of meteoric water. Re-equilibration of crack-filling calcite to lower temperatures means that calcite chemistry tells us little about water-rock interactions in the crustal section at temperatures higher than ∼ 120°C.

The Determination of 222Rn in Drilling Fluids of the KTB Pilot Borehole Using SSNTD Techniques

222Rn dissolved in drilling fluids of the KTB (Kontinentales Tiefbohrprogramm der Bundesrepublik Deutschland) pilot hole has been determined using solid state nuclear track detector (SSNTD) technique:The α-sensitive SSNTD CR-39 was used to measure the a-activity of 222Rn. Well-defined conditions for Rn-measurements could be established by employing a mica nuclear track microfilter to separate the detector containing air volume from the volume containing the water. This technique has been applied to determine quantitatively the 222 Rn-concentration as a function of depth of the borehole down to 4000 m.