Aug 10, 1997 - complex history and has been repeatedly affected by major compressional and ... Continent (Laurentia + Baltica + East Avalonia) with Armorica.
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 102, NO. B8, PAGES 18,179-18,201, AUGUST 10, 1997
The German Continental Deep Drilling Program KTB: Overview and major results Rolf EmmermannandJ6m Lauterjung GeoForschungsZentrum Potsdam, Potsdam, Germany
Abstract. The GermanContinentalDeepDrilling Program(KTB) wasdesignedto studythe propertiesandprocesses of thedeepercontinentalcrustby meansof a superdeep borehole.Major researchthemeswere (1) the natureof geophysicalstructuresandphenomena,(2) the crustal stressfield and the brittle-ductiletransition,(3) the thermalstructureof the crust,(4) crustal fluids andtransportprocesses, and(5) structureandevolutionof the centralEuropeanVariscan basement.The projectwasconducted in distinctphases:a preparatory phase(1982-1984),a phaseof site selection(1985-1986), and a pilot phase(1987-1990), whichincluded sinkingof a pilot boreholeto 4000 m anda 1-yearexperimentation program.The mainphase(1990-1994) compriseddrilling of a superdeep boreholewhichreacheda final depthof 9101 m anda temperatureof ~265øC, and threesubsequent large-scaleexperimentsin the uncased-bottom hole section.Amongthe outstanding resultsare the following(1) A continuous profile of the completestresstensorwas obtained.(2) Severallinesof evidenceindicatethat KTB reachedthe present-daybrittle-ductiletransition.(3) The drilledcrustalsegmentis distinguished by large amountsof free fluidsdownto midcrustallevels.(4) The role of postorogenic brittle deformation hadbeengrosslyunderestimated. (5) Steep-angle seismicreflectionsurveysdepictthe deformationpatternof the uppercrust.(6) High-resolutionseismicimagesof the crustcanbe obtainedwith a newly developedtechniqueof true-amplitude, prestackdepthmigration.(7) The electricalbehaviorof the crustis determinedby secondarygraphite(+sulfides)in shearzones.
Introduction
requireclosecooperationbetweengeoscientists and engineers,a prerequisite whichdevelopedinto a fruitful symbiosis.Like any In October1994, after 1468 daysof drilling, the superdeep expedition to uncharted regions, the KTB project was boreholeof the GermanContinentalDeep Drilling Program meticulouslyplannedto foreseeand minimize potentialrisks. KTB (KontinentalesTiefbohrprogrammder Bundesrepublik Thus the projectproceeded in distinctphases,after each of Deutschland) reachedits final depthof 9101 m at a temperature whicha fundamental teevaluation wasmadeandstrategies were of ~265øC. The main phaseof the KTB was then concluded redefined. Aftera preparatory phase(1982-1984)anda phaseof and site selection(1985-1986),the KTB with threemajor experiments in the uncasedbottomsectionof presiteinvestigations the hole: a dipole-dipoleexperiment,a draw-downtest and a pilotphasebeganin 1987.Thisinvolvedsinkinga pilotholeto KTB-VB)anda subsequent 1-year combinedhydrofracturing andfluid injectionexperiment.With 4000m (the"Vorbohrung", program(until April this climax to the scientificprogram,the main phaseof the logging,testingand experimentation KTB was terminated as scheduledon December 31, 1994.
The KTB was the largest and most expensiveresearch programin the geosciences ever undertakenin Germany.The Federal Ministry for Researchand Technologycommitteda total of 528 million DM (~$350,000,000U.S.) to the project, fromtheplanningphasein 1982throughto completionin 1994.
1990).The resultsof the pilot phasehad a majorimpacton scientificand technicalplanningof the superdeep hole CHauptbohrung", KTB-HB), and providedthe basis for the
officialgo-ahead for themainphase. Approvalwasgivento the designand construction of a specializeddrill rig with a maximumcapacityof 12 km, the targetdepthwasset at the A KTB projectgroup,established at theGeological Surveyof temperature levelof 300øC(expected at about10km), a budget and a time frame for the main phasewas Lower Saxony,Hannover,wasresponsible for the technicaland was determined, operational realization of the program. The Deutsche Forschungsgemeinschaft (DFG) oversaw and coordinatedall KTB-relatedscientificactivities.At one time or anotherduring this period, more than 700 scientistsand technicianswere employedin KTB-relatedwork. KTB fully deservesthe term "superdeep adventure"because it advancedthe frontiersof many geoscientificand technical fields. It was clear from the very beginningthat successwould
limited to December31, 1994. The year 1995 was committed
for siteshut-down andfinal dataevaluation, andon January1, 1996, the GeoForschungsZentrum Potsdam(GFZ) took over responsibility for the final phase.In this phase,the two drill holes,whicharesome200 m apart,will be usedovera 5-year periodas a deepcrustallaboratoryfor in situ scientificand technicalexperiments.
Realization of an Idea
Copyfight1997 by the AmericanGeophysicalUnion Papernumber96JB03945. 0148-0227/97/96JB-03945509.00
In 1977 the Senate Commission on Geoscientific
Research of
the DeutscheForschungsgemeinschaft first discussedthe idea of investigatingthe continentalcrust by means of a superdeep 18,179
18,180
EMMERMANN AND LAUTEPOUNG: KTB DEEP DRILL HOLE
•oo
EMMERMANN AND LAUTERJUNG: KTB DEEP DRILL HOLE
18,181
borehole.At that time, conceptualmodelsof the makeupand Collapse of the Variscan orogen started in the late (~300 Ma) and wasenhancedby mantle-induced evolution of the continental crust were primarily based on Carboniferous interpretation of the geological record, interpretationof crustalextensionand magmaticactivity which heraldedthe
Pangea. Thisepisode createda geophysicalimages obtained from various deep-soundingbreak-upof thenewlyassembled methods,and applicationof laboratorydata from experimental multitude of Permian (300-250 Ma) bimodal volcanic suites, petrologyandgeochemistry to real rocks.The commission felt intramontanebasinsand grabenstructures,many of which are During the Alpine orogeny(~100-30 Ma) the that furtherprogress required"groundtruth"that couldonly be wrench-related. and transpressional obtained by direct observationthroughdrilling. From the region was affected by compressional beginning,therefore,two fundamental goalsof globalrelevance tectonicswhich were followedby yet anotherstageof rifting in theTertiary(since~25 Ma). Manyof weresingledout: (1) calibrationof crustalgeophysics, and(2) andgrabenformation studyof thecrustalstressfield andthe rheological behaviorof theseeventswere accompaniedby magmatismand enhanced activity,causingthe formationof widespread the crust.In the ensuingyears,discussions within the whole hydrothermal geoscience communityled to a muchbroaderdefinitionof the mineralization. This multiply reworkedcentralEuropeancrustis relatively conceptof continentalsuperdeep drilling. In 1984 the official both laterally and proposal to establish the KTB wassubmitted andfive priority thin (~30 km) and extremelyheterogeneous,
areaswere defined:(1) the natureof geophysicalstructures and vertically. It is distinguishedby regionally variable but
phenomena: seismicreflectorsand electrical,magneticand generally high heat flow values, complex gravimetric and gravimetricanomalies;(2) the crustalstressfield and the magnetic patterns, pronounced seismic reflectivity, the of high- and low-velocitylayers,and zonesof high brittle-ductile transition:orientationand magnitudeof stresses occurrence as a function of depth; (3) the thermal structureof the electricalconductivityat differentdepths. continental crust: temperaturedistribution, heat flow, heat
production, andheattransport; (4) crustalfluidsand transport Site Selection: A Difficult Decision processes: fluid systems,fluid sources, and fluid movements; and(5) structure andevolutionof thecentralEuropeanVailscan Altogether,more than 40 potentialdrill sites were initially basement: properties, deformation mechanisms, and suggested, but only four survivedthe final definitionof project geodynamics of a multiply reactivatedcontinentalcrustal objectives,and the first main selectionconferencein 1983
environment.
narrowed the choice to two final candidates, the Schwarzwald
From technicalconsiderations, and basedon the expectation and Oberpfalzregions.As existingknowledgewas inadequate of importantchangesin rheologyand reactionkineticsabove to decide between the two, a 2-year program of geological, about250øC,the targetwas set at the 250ø-300øCtemperature petrological,andgeophysical studieswasstartedin eachregion. window. This temperaturetarget was combined with the The results were discussed at a final selection conference in objectiveof penetratingat least 8000 m into the continental September1986, attendedby over 200 geoscientists of all crust. disciplines. From its inception,closetieswereestablished betweenKTB At thatmeetingit wasacknowledged thateachregionoffered and the German Continental Seismic Reflection Program highly attractive targets for solving a range of pressing (DEKORP), which startedin 1983 and was also fundedby the geoscientificproblems and preferencefor one site over the FederalMinistry for Researchand Technology.Together,these other becamea partisanissue among the variousdisciplines. projectswere intendedto providea major Germancontribution The deadlock was broken by focusing on the expected to understanding the architecture,composition,andevolutionof geothermalgradient.Extrapolationof geothermaldata obtained the central Europeancrust. Perhapsbecauseof its relatively from shallow-drillingstudies carried out in both regions young age (typically < 500 Ma), this crustal type differs especiallyfor thispurposeindicatedthat the targettemperature fundamentallyfrom that sampledby the Russiansuperdeep of 250ø-300øCmight be encounteredat 7 km depth in the boreholeKola SG3. Nevertheless,centralEuropeancrusthas a Schwarzwald,whereasit could lie about 5 km deeperin the complex history and has been repeatedlyaffected by major Oberpfalz.Therefore,taking the original depth criterioninto compressional and tensionalprocesses. It was largelyformedor account, the Deutsche Forschungsgemeinschaft selectedthe reshapedduring the Variscan orogeny(~400-300 Ma), which Oberpfalz site for the superdeepborehole [Emmermannand was an importantsteptowardthe assemblyof Pangea(at ~300 Ma). The Variscan belt represents a collage of arcs and microcontinentsresulting from collision of the Old Red Continent (Laurentia + Baltica + East Avalonia) with Armorica and Gondwana.An important rifting phase in Cambrian and
Ordoviciantimes (~500 Ma) first separatedthe microplatesof East Avalonia
and Armorica
from
mainland
Gondwana
and
Behr, 1987]. The specificscientific attractionsof this site were seento be (1) its location in the suturezone between two firstorder units of the Variscan orogen; (2) the existenceof an
appealingand testablegeologicmodelwhich had far-reaching implicationsfor crustalarchitectureand geodynamics;(3) the occurrenceof marked gravity, magnetic, and electrical selfpotential anomalies; (4) the expected presence of seismic reflectors at drillable depths and of an electrical high-
createdlarge areasof thinnedcontinentalor even oceaniccrust. conductivity zoneat 10 __ 1 km; and(5) the chanceto testfor The crustalblocks involved began to convergein Ordovician thermal and geochemicalinfluencesfrom the nearby Tertiary time (by ~450 Ma) and were finally welded togetherduring a Eger rift. prolongedstageof collision in the Devonian and Carboniferous (400-300 Ma) to form the present-dayVariscides.Incorporation of magmaticarcs and back arc basins, long-distancenappe Two-Step Drilling Concept transport, a final stage of low-pressure, high-temperature Sincethegeologicalboundaryconditions andtheirimpacton metamorphism, and voluminous intrusion of late to for reachingthe envisageddepth postorogenic graniteshavedonemuchto obscurethe recordof the technicalrequirements targetwerenot knownsufficiently,the KTB conceptwasbased oceansopeningandclosing.
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EMMERMANN AND LAUTERJUNG: KTB DEEP DRILL HOLE
on drilling two holes,first a pilot hole as a sort of "fact-finding about1.5 wt % DEHYDRIL-HT,a synthetic, hectoritc-type, Limission"andthenthe superdeep holeitself. bearingNa-Mg silicatewhichyieldeda thixotropic,solid-free, Main objectives of the pilot phase were to acquire a highly lubricantmud system.Later, due to its electrolyte comprehensive set of geoscientific data from core sensitivity, corrosivebehavior,instabilityat hightemperatures, investigations,cuttings analyses,and boreholemeasurements and other factors,it was continuously modifiedby adding with which to develop the methodologyrequired for optimal HOSTADRILL,anorganic polymer, andNaOHplusNa•CO•to evaluation of the superdeephole and to test the geological fix a pH valueof 10 to 11. With increasing temperature in the prognoses.Moreover, the pilot hole would reducethe need for deeperpartsof the KTB-HB (below7100 m) and owing to samplingand loggingin the upper,large caliber sectionof the continuingsmallinfluxesof salineformationwaters,a steady superdeephole and would provide the information on rock deteriorationin rheologicalpropertiesand water-binding propertiesand drillability, boreholestability,potentialgain and capacityrequireda partialreplacement by addinga mixtureof loss zones, temperatureprofile, etc., critically neededfor the different commercial polymers (KEMSEAL, MILTEMP, PYROTROL). Furthermore, in order to reduce borehole technicalplanning. the mudweightwasraisedfrom 1.06kg/L to 1.40 The KTB pilot hole was spuddedon September27, 1987. instabilities The conceptdevelopedby the KTB engineers,which was to kg/L by addingbarite (seeTable 1 for a summaryof fluid modify a conventionaldrill rig from industryand to combine systemsusedat variousstages). After completionof the pilot hole, a 1-yearmeasuring and rotary drilling and wire line coringtechniques,turnedout to be very successful.With a high-speedtopdriverotatingsystemand experimentationprogram was conducted,which included a logging program,14 hydrofracmeasurements using an internal and external flush-jointed5 V2 inch mining comprehensive drill string with 6 inch thin-kerfeddiamondcorebits,560 days and a large-scale three-dimensional(3D) seismic reflection of drilling and logging were needed to achieve a basement survey,coveringan area of 19 x 19 km aroundthe drill site. In penetration of 4000 m, and a total of 3564 m of excellent April 1990 the hole was caseddown to 3850 m, leavingan quality cores was recovered. By applying straight vertical openholesectionof 150 m. The pilot phasewasthencompleted pumptest,whichyielded71 m3 of drilling capabilitiesthistechniquehasa depthpotentialof 5 to 6 with a first short-term brineswith highamounts of N2 andCH4 from the km and thusmay be of specialimportancefor futurecontinental basement uncased bottom zone. researchdrilling to intermediatedepths. The main resultsof the pilot phase[Emrnermann,1989], A prerequisitefor the successof the drilling techniquewas the technicalconceptfor the superdeep hole, the developmentof a new water-baseddrilling fluid system, whichdetermined which was undertaken in close cooperation between KTB were (1) the considerablyhigher than expectedgeothermal engineers and geochemists.This system met all technical gradient,whichled to a definitionof the depthtargetat 10 km to about300øC);(2) thelithologicheterogeneity requirements,was environmentallysafe and, for the first time, (corresponding alloweda quantitativegeoscientificmonitoringof the drilled andthe continuoussteepinclinationof rock units,whichcaused basement.The startingcompositionwasa mixtureof waterwith severe deviation of the hole out of the vertical and resulted in developmentof a vertical drilling strategy;and (3) the frequencyof cataclasticshear zones and fluid inflow zones which, in combinationwith the high horizontaldifferential Table 1. Drilling Fluid SystemsUsedin the KTB Main Hole stresses,led to the expectationthat boreholeinstabilitieswould becomea major problemat depth.This resultedin a slimBorehole DrillingFluid Properties clearancecasing strategy and developmentof water-based, section System high-temperature drillingmudsystems described above. Plate 1 shows the drill rig of the superdeep borehole KTB0 - 6760 m 0.7% Dehydril plasticviscosity 9-22 mPas HB, UTB 1, which,at a totalheightof 83 m, is the largestland(1, sidetrack) 1%Hostadrill density 1.06g/cm 3 6460- 7220 m (2, correction)
NaOH, NarCO,
pH 10-11
1.5%Dehydril
plasticviscosity29-53 mPas
1.5% Hostadrill
density 1.06g/cm 3
NaOH, Barite
pH 11
baseddrill rig in the world.The rig is fully electricallydriven, and incorporates a number of technical innovations and
improvements including,for example,an automaticpipehandling system and remotely controlled gear-driven drawworks.
7140 - 8330 m
Bentonitc
(2, side track)
Kemseal, Miltemp, density 1.25g/cm • Pyrotrol,NaOH
plasticviscosity35-91 mPas pH 10
Barite
To ensuresufficientreserves,the drill string layout was madefor a maximumdepthof 12,000m (diameterof 5 •A inch,
enhancedsteel quality). By using40 m standsof drill pipe insteadof the standard27 m stands,and in combination with the
7460 - 8730 m
Bentonite
(3, side track)
Kemseal, Miltemp, density 1.40g/cm • Pyrotrol,NaOH
plasticviscosity25-90 mPas pH 9-10
Barite
8630 - 9100 m
Bentonitc
plasticviscosity27-59 mPas
pipe-handlingsystem,roundtriptime was reducedby about 30%. In order to achievethe targeteddeptha verticaldrilling system(VDS) was developedand deployed.The VDS is an actively self-steeringsystem mounted directly above the downholemotor. It effectivelyminimizedthe friction between
drill string and boreholewall and allowed a 50% reductionof the drilled rock volume by realizationof the slim-clearance casingconcept. The VDS systemwas used to a depth of 7500 m at the All drillingfluid systems are water-based. Dehydrilis a synthetic maximumallowabletemperatureof 175øCfor the electronics, clay mineral (Hectoritc-type). Hostadrill,Kemseal,Milltemp and Pyrotrolare syntheticviscosifiersand organicadditives.NaOH and whosedata were pulsed through the mud to the surface.The
Kemseal, Miltemp, density 1.40g/cm •
Pyrotrol,NaOH Barite,Polyglycol
pH 9-10
Na:CO3 havebeenaddedto controlthe pH value.Baritecontrolsthe density.
horizontaldisplacementat this depth was only 12 m, which meantthat the boreholewas practicallyvertical.The hole then
EMMERMANN AND LAUTERJUNG' KTB DEEP DRILL HOLE VB
18,183
HB
- 0
1. Correction for Deviation 1000 rn
1000
1300 rn
2. Correction for Deviation 1800 rn 2000 m
track I 1710rnI
2000
1998 mI 3000
3. Correction for
Deviation 2600
m
2700
m
2.Side track 3767m
-4000
,.,3893 m 1. Correction for Deviation 5530
m
5600
rn
1.Side
13518" I 2. Correction for Deviation
Paragne•sses Metabasites
7140
m
7220
m
track I •o m I
track 7460m I
m,I
Variegated Sequences
track I 8630m
Major Faults
,, 8730m 10000
Figure1. An outlineof thefinalborehole configuration, showing thesidetracksandcorrections fordeviation, casing scheme andlithological profileof (left)thepilothole and(right)thesuperdeep hole.
deviatedtowardthe NE, almostperpendicular to the generaldip boreholeenlargements,preferentiallyrelated to shear zones. of the foliation, and at 9069 m, where the final deviationsurvey Below that depth, boreholeconvergence(i.e., reductionof the was conducted,it showeda horizontaldisplacementof about boreholediameter) first occurredand resultedin undergauge 300 m. Down to a depth of 8120 m the superdeephole was hole sections.Undergaugesections,with uniaxialnarrowingin drilledalmostexclusivelywith downholemotors,whichcould the direction of the maximum horizontal stress, developed be usedto maximumoperatingtemperatures of about 190øC. within hoursand becamethe major sourceof drilling problems The laststepto thefinal depthwasdonewith rotarydrilling. [Borm et aL, this issue].The drill string frequentlygot stuck, As coringis one of the mostexpensivedrilling operations, and side-trackingoperationsconsumedover 1 year of total
maximizingcore recoveryand core quality was of special importance. The experiencewith the thin-kerfeddiamondcore bits in the pilot hole justified the decisionto adapt this technologyto the superdeephole. A large diametercoring system(LDCS) wasdevelopedwhich,in the 12 •A inchphase, cut coresof 234 mm diameterandprovidedrockcolumnswith a lengthof up to 5 m. In comparison to rollerco.necorebits, which were also used, the averagecore recoverywith this systemwasincreased from 41% to about80%. Altogether,35 corerunswereperformed in thesuperdeep holewithan overall recoveryof 84 m. A large additionalcollectionof partly orientedrockfragments, upto 10cmin length,wasprovidedby a junk basket,a simple,but very effectivetool attachedabove the drill bit whichwasalwaysusedin noncoredsections. Figure 1 showsthe KTB-HB in its final conditionwith the differentsidetracksandthecasingscheme.Reasonsfor the side
operationtime.This typeof instability,whichcauseda creeplike structuraldisintegrationin the rock, resultedfrom the extremely unfavourablecombination of high differential stresses, a preferreddip of planesof weakness in the direction of the minimumhorizontalstresscomponent, the presenceof formationwatersand the elevatedtemperatures. The instability eventuallyformed a technicalbarrier which could not be overcomewithinthestrictlimitationsof budgetandtime.It was thereforedecidedto stopthedrillingoperations in October1994 at a depthof 9101 m anda temperature of ~265øCin orderto usethedrill rig andtheothertechnical facilitiesfor threelargescale scientificexperimentsin the uncasedopen-holebottom section.
Realizationof the ScientificProgram
The scientific program of KTB was carried out by an of dataandresults operations, and boreholeinstabilities.Down to about7500 m integratedevaluationandjoint interpretation theseinstabilitiesmainlyoccurredin the form of breakouts and obtainedfrom threesources'(1) the field laboratoryestablished drilling-inducedtensile fractures that produced localized at the drill site, (2) borehole measurementsand experiments, tracks were corrections for deviation, unsuccessfulfishing
18,184
EMMERMANN ANDLAUTERJUNG: KTBDEEPDRILLHOLE
and (3) individual specializedresearchprojectscarried out at
Field Laboratory
universities and other research institutions. All KTB-related scientific activities were coordinated and
reviewed by the Deutsche Forschungsgemeinschafi (DFG), whichestablisheda Priority Program"KTB" that was fundedat an annual level of ~7 million DM. During the lifetime of the KTB, from 1982 to 1995, the DFG administereda total of 75 million DM (~$ 50 million US) to over 300 different research
Solids
Fluids
Gases
Borehole measure-
Drillingfluid Fluidsampler
Cores Cuttings
Gas trap Sampler
ments
Rock flour ,,
,
projectsinvolving about500 scientists.In addition,some 100 i • I I foreignscientistsfrom 11 countriesparticipatedin KTB with a Petrology and Structure total of 70 researchprojectsfinancedby their nationalfunding Petrography Macrostructures agencies.Scientific communicationwithin the DFG Priority Ore microscopy Microstructures Program, periodic discussionof the resultsand definition of Texture C ore ode ntation major experimentswere ensuredby workshops,task group meetings,thematicsessions,and the annualKTB colloquium. As of 1995 a total of over 2000 publicationshavecome out of Petrophysics the variousresearchprojects. Density,Porosityand Permeability, Electricand magneticproperties, Establishmentof a field laboratoryat the drill site was a top Soundvelocities,Heat conductivity, priority from the time of the first discussionsabout a y-spectroscopy,Stress relaxation continental deep drilling program in Germany. The field laboratory became an indispensible component for the Geochemistry realization of the ambitious scientific program. Figure 2 Chemical& mineralogical composition summarizes the tasks of the field laboratory and its of rocks;Fluid-and Gasanalysis organizationalstructure.This lab, which was a modern and completelyequippedresearchinstitute,was supportedby nine KTB-Database university "mother institutes" (under the guidance of the Storage Documentation Instituteof Geosciences, Universityof Giessen),and staffedby Correlations Modelling up to 20 scientistsand 18 technicians.Its primary purposewas to collect extensivegeoscientificdata on cores,cuttings,rock flour, drilling fluids andgases.Propertiesandcharacteristics (1) were measuredwhich were necessaryfor operationaldecisions concerning drilling, sampling, and testing, (2) had to be Reports determinedon a quasi-continuous basisas a functionof depth, (3) are time-dependentand thereforehad to be recordedas soon and analysis as possibleafter sampling,and (4) were neededfor calibration Figure 2. A flow chartshowingthe sampling schemeof the KTB field laboratory. of boreholemeasurements and were requiredto guide sample ,
,
I Samplingparties •_•[ Universities KTB-
selection
and as basic information
for all individual
research
projects. Apart from establishedmethods,a numberof new techniques were developedand continuouslyimproved. Among them were a computerizedreorientationtechniquefor cores,a new tool for high-qualitydensityimagingof coresby gammaray absorption and a four-componentdilatometerfor the measurementof the stressrelaxion of cores. Becauseof the new drilling fluid systemand a speciallydesignedautomaticsamplingsystem,a real break-throughwas achievedin the evaluationof cuttings, rock flour, drilling fluid, and gasesdissolvedin the drilling fluid. The quantitativechemicaland mineralogicalcomposition of cuttings and rock flour was determined using a newly developed combination of X ray fluorescenceand X ray diffraction, which allowed a reliable reconstruction of the
drilled basementwithin 1 hour after sampling[Emrnermannand Lauterjung, 1990]. Because the pilot hole was almost completelycored, it was possibleto comparethe lithological profile obtained from direct study of cores with that reconstructedfrom cuttings and rock flour analyses. It was demonstratedthat the agreeementwas excellent and that every rock type, even minor lithological changes,fault zones, and alteredintervals,hadbeenunequivocallyidentified. In addition, quasi-onlineanalysesof the drilling fluid, and the gasesreleasedfrom drilling fluid, were performed.These fluid logs turned out to be very sensitive indicators of cataclastic shear zones and fluid inflow zones. Even minor fluid
influxes could be identified and localized, facilitating quick operational decisions concerning positioning of drill stemtests anddownholefluid sampling. Togetherwith the work in the field laboratory,downhole measurements playeda centralrole in the reconstruction of the drilled basementand providedimportantinformationon the in
situ propertiesof the rocks. Prior to KTB there was little experiencein the integratedinterpretationof logging data obtained from crystalline rocks [Pechnig et al., this issue]. Thereforean extensiveloggingtest programwas conductedin the pilot hole usingall availabletypesof tools,and the borehole log responseswere calibratedagainstcore, cuttingsand other dataprovidedby the field laboratory. During the drilling phaseof the pilot hole, sevenlogging campaignswere performedat variousintervals,mainly for data acquisitionand technical purposes.After completion of the hole,24 differentexperimentswereconducted, anda total of 55 toolsweredeployed.On the basisof the resultsandexperiences from thesemeasurements, the combinationof loggingtools for the KTB superdeephole was defined and, altogether,266 logging runs were performed. Although there were some standard high-temperature logging tools available from industry, several tools were upgradedespeciallyfor KTB. Among thesewas a high-temperature formationmicroscanner (FMS) whichcould be usedup to 260øC.Figure3 presentsan
EMMERMANN ANDLAUTERJUNG: KTBDEEPDRILLHOLE
[
18,185
KTB-Oberpfalz HB BHTV
VM..•..•__• GLT SV-R.•o•
14.02.9515:17 / KTB-GP-BM Kiick
Figure 3. Summary ofthemajor logging tools and their respective measuring interval inthesuperdeep hole. Abbreviations areNGS,natural gamma spectroscopy; DLL,electrical resistivity; sonic, seismic velocity; BGT,
borehole geometry' TEMP, temperature log'MAG, magnetometer; GLT,geochemical logging tool; SP,self potential' IP,induction log;FMS/FMI, formation microscanner/formation microimager; BHTV, borehole televiewer; BHGM,borehole gravimeter; VSP,vertical seismic profiling.
Apartfromarchiving, themaintaskof dataprocessing was tointegrate all dataintoa single,accessible information system, A newandveryprecise method of lithologic evaluation of which was achievedusing the metadataconcept.Metadata logging datawithmultivariate statistics wasdeveloped by the characterize data sets and include information on data sources experimental boundary conditions, experimental use of a broad spectrumof Schlumberger tools and the andstructure, comparison andcalibration withcore,cuttings, andmudsample methods,literature,and,mostimportant,the actuallocationof conceptis a basicrequirement for data. This method, which relies on discriminationof thedatasets.The metadata distributed database whichcan "electrofacies", is discussed by Pechniget al. [thisissue],andit therealizationof an integrated, by anycomputernetwork. contributed greatlyto therefinement of thelithologic profileof be accessed
overviewof the majortoolsdeployed andthe measured depth sections.
the superdeep borehole.
The KTB database "KTBase" is a relational database: that is, it contains material such as tables of measured data and
The Information SystemKTBase'.
observational descriptions (e.g., thin sectionpetrography) from the field laboratory,the downholelogginggroup,andthe mud
Access to KTB Data
andcanberetrieved KTBhasproduced anenormous amount of dataof various logginggroup,whicharestoredseparately and connected in any way by the user. KTBase is embedded in kinds(descriptive versusquantitative numerical data)from
layercomprising interfaces for thepresentation differentsources,includingthe field laboratory,borehole an application of KTBdatain differentways.Theapplications measurements, mudlogging, technical monitoring, geophysicalandapplication
experiments, field investigations, and externalscientificcover investigations.
administration tasks, telecommunication facilities,
databasemanagement,and software modules developed
18,186
EMMERMANN AND LAUTERJUNG: KTB DEEP DRILL HOLE
especially for the scientific technical treatmentof data from drilling projects,includingnumericaland graphicalsoftware packages. KTBase is now available at the GeoForschungsZentrum Potsdam.A hypertextlink is installed on the home page of the GFZ WWW server(http://www.gfzpotsdam.de). Scientific
Results
The metabasicunits comprisecoarse-and fine-grained garnet-bearing amphibolites(plagioclase,hornblende,garnet andilmenite),massivemetagabbros with relictophitictextures, and thin layers (up to 6 m thick) of mafic and ultramafic metacumulaterocks. Most rock types display chemical characteristics of enrichedmid-oceanridge basalts(MORB); however, normal MORB compositionsalso occur. These metabasicunitsperhapsrepresentslicesof formeroceanfloor
formedin a backarc environment or RedSeatypeoceanic Crustal Structure
and Evolution
basin.
The variegatedseriesconsistsof an alternationof massive
fine-grainedbandedamphibolites with The Oberpfalzis situatedat the westernmargin of the garnetamphibolites, andmarbles, hornblende-biotite gneisses, BohemianMassif, the largestcoherentsurfaceexposureof layersof calcsilicates Among the metabasicrocks,alkali basaltic basement rocksin centralEurope,andit encompasses partsof and paragneisses. threefirst-ordertectonometamorphic unitsof the Variscanfold compositionspredominatewith gradual transitionsinto belt: the Saxothuringian, the Moldanubian,and the Tepla- trachyandesiticrocks (hornblendegneisses).These units a metamorphosed volcano-sedimentary association Barrandian(Plate 2). This basementblock is separatedfrom constitute material,thin lava flows, and pelites Pertoo-Mesozoic forelandsediments (up to 3000 m thick) by with volcaniclastic the FranconianLineament(FL), a NW-SE trending,deep- interbeddedwith minor marls and limestones.The combination reachingandmultiplyreactivated systemof reversefaults.The of turbiditic material with volcanicsand limestoneindicatesa environment in a tectonically activesetting. KTB locationis about4 km eastof theFL andjust southof the marinedepositional All threeof theselithologicunitshavesuffered a pervasive boundarybetweenthe Saxothuringian and Moldanubianunits. metamorphism at upper amphibolitefacies This boundaryhasbeenregardedas a suturezoneformedby Barrovian-type closureof an early Paleozoicoceanbasinduringthe Variscan conditions(6-8 kbar and 720øC at peak conditionsin the paragneisses),connectedwith an intense ductile deformation The drill site was located in a small, isolated thatproduced a penetrative foliation.Whereas theparagneisses tectonometamorphic unit called the ZEV (Zone of Erbendorf- seem to record progrademetamorphicevolutionalong the
collisionin Devonian/Carboniferous times(~400-330Ma).
boundary and do not showany signsof Vohenstrau13), which representsa variegatedassociationof sillimanite-kyanite events,the metabasicrockscontainrelics paragneisses and orthogneisses and metabasicrockswith minor earlier high-pressure from an early metapegmatites. Accordingto the resultsof presitestudies,in whichclearlydisplaya multi stageevolution metamorphism undereclogitefaciesconditions particular lithologic-metamorphiccomparisonswith the high-pressure
MiJnchberg Massifin thenorthandpreliminary interpretation of (P > 14kbar,T > ~700øC) followed bya garnet granulite facies conventionallymigratedDEKORP seismicprofiles,the ZEV overprint(P=10-13kbar, T=620-720øC)prior to the dominant faciesmetamorphism [O'Brienet al., thisissue]. had been interpretedas a fiat, bowl-shapedremnant of a amphibolite New age determinations using a broad spectrumof supracrustal nappe complex straddling the methods confirmthatthepreVariscan andearly Saxothuringian/Moldanubian boundary. In targeting the geochronologic superdeepborehole,it was thereforeexpectedto penetrate Variscanhistoryof the ZEV is much more complexthan througha 3-5 km thick nappecomplexand to drill into the previously thought andis distinguished by at leasttwoseparate postulatedsuturezone beneathit. metamorphiccycles[O'Brienet al., this issue].The formation
In fact,however,the KTB-HB encountered steeplyinclined agesof the metabasicrocksare about485 Ma and nearly unitsbelonging to the ZEV overtheentiredrilledsection,and contemporaneous withtheearlyOrdovician depositional ageof no evidencesupporting the nappeconceptwasfound.Figure4 the paragneissprotoliths.The earliest metamorphicevent showsa schematicSW-NE profile throughthe ZEV down to recordedin boththe metabasic rocksandparagneisses hasbeen about 10 km which summarizesall available geological dated at .•475 Ma, using U/Pb chronologyon zircon and information. The drilled crustal segmentconsistsof an monazite,and it appearsthat the relict high-pressure mineral preservedin the metagabbros can be attributedto alternating sequence of three main lithologic units: parageneses paragneisses, metabasites anda "variegated" seriesof gneisses this Early Ordovician high-pressuremetamorphism. and amphibolites.Most rocks show a penetrativefoliation Furthermore, the dataimply thattherewasonlya veryshort whichdipssteeplybetween50ø and80ø to the SW or NE andis time spanbetweenformationof the rocks,their subductionto at
least40 km, and their subsequent rapid uplift into shallow foldedintolarge-scale openfoldswithNW-SEtrending axes. Theparagneisses havea ratheruniformcomposition andare (cool)crustallevels. madeup essentiallyof plagioclase(oligoclase),quartz,biotite, The second metamorphiccycle, that occurred under muscovite, garnet,sillimanite,and/orkyanite;theycommonly Barrovian conditions, can be bracketed between about 405 Ma contain flakes of graphite. The protoliths of these rocks representa turbidite sequenceof graywackesand pelitic graywackesinterlayeredat a centimeterto meter scalewhich has preservedits original compositionalfeatures. Former graywackesshow an equigranularquartz-feldsparfabric,
(U-Pbagesof zirconsandRb-Sr,K-Ar and4øAr?Aragesof muscovites) and 375 Ma (Rb-Sr agesof muscovite from shear zones,probablydatingthe regionaldeformation).NumerousKAr, 4øAr?Arand Rb-Sr dateson mineralsindicatethat the drilled basementsegment,after coolingto 350-300øCin the
whereas peliticlayersare coarser andricherin micaand LateDevonian (-360 Ma), stayed in theupper crustand,even typicallydisplaya biotiteflasertexture.According to their moreimportant, remained therefora longtimeperiod withina chemical characteristics theprotoliths wererather immature and narrowdepthandtemperature interval.Onlymarginal and containa significant amountof basaltic material. Theywere deeperpartsof theZEV wereaffected by theCarboniferous probably deposited atanactivecontinental margin. deformation andlow-pressure, high-temperature metamorphism
EMMERMANN ANDLAUTER]UNG: KTBDEEPDRILLHOLE
Ei
18,187
18,188
EMMERMANN AND LAUTERJUNG: KTB DEEP DRILL HOLE
$w
KTB-HB Franconian
:::::::::
.'
NE
Lineament
.w•'•:'""'"':•'•'•••.'"••ii:•"i•'.??•' ..•:•ii•" .:.:•
..:.
Figure4. Schematic SW-NEprofilethrough the ZEV in the surroundings of the KTB site(no vertical exaggeration). The metamorphic sequence of the ZEV hasa polyphase history,with earlyhigh-pressure metamorphism atabout 475Ma,andlater,Barrovian-type metamorphism atabout400-375Ma. TheFalkenberg granite,to the NE, intruded at about310 Ma. To the SW are foreland sedimentary sequences of Upper Cretaceous, Triassic,andPermo-Carboniferous ages.
(335-325 Ma) which strongly overprinted the neighboring rocksof the variegatedsequenceand are connectedwith a local greenschist facies retrograde overprint. These faults are The first evidencefor a commonhistoryof the ZEV and the displacedby reversefaults of possibleCretaceousage, which Moldanubian/Saxothuringian zonesis the emplacementinto all formed under prehnite-actinolite facies conditions. The theseunitsof late Carboniferousgraniteswhich occurredin two youngeststructuralelementsare steepnormal faults probably major "pulses",a late tectonic phase at 335-325 Ma and a relatedto a phaseof grabenformation(Eger Rift) during the posttectonicphase at 315-305 Ma. The Falkenberg granite, late Oligocene/Miocene(-25-20 Ma). Figure 4 depicts the major faults encounteredby the whichborderstheZEV on the NE, hasan ageof 311 _+8 Ma (Figure 4). The intrusionof dikes of aplites, calcalkaline superdeepborehole. The most prominent fault system was lamprophyres (dated by *øAr?Ar at -306 Ma) and penetratedbetween6850 m to 7260 m and consistsof a broad monzodiorites, whichcrosscutthe metamorphic rocksdownto bundleof individualfault planes.Fissiontrack data on sphene depthsof about7800 m, is closelyrelatedto the granific indicatethat a vertical displacementof more than 3 km took magmatism. place along this fault system in Cretaceoustime. A second The post-Variscan historyof the ZEV is distinguished by major systemoccursbetween7820 m and 7950 m, and vertical unexpectedlyintense, polyphase deformation under brittle displacement therewasat least500 m. Both systemsdip steeply conditions [Wagneret al., thisissue].Majordeformation phases to the NE and can be directly correlatedwith the Franconian basement units.
includeintensereversefaultingduringthe late Variscan(.--300 Lineament at the surface. Ma) andCretaceous (.--130-65Ma) followedby normalfaulting Fission track studies on apatite, zircon and sphene, in duringthe Neogene( 4000 m), lower Triassic (>1500 m), and during the Cretaceous(> 3000 m). All are connectedto prominent stagesof crustalshortening. A highly unexpectedresult, confirmedby a number of independent observations, is the lack of any P-T gradientsdown to at least 8000 m and the uniformity of radiometricages.This
18,189
gaseousinclusionswith admixturesof CH4 and N•, and often containinggraphite,formed in connectionwith the late Variscan cataclastic deformation (-300 Ma) and are associatedwith the graphite-containing shearzones. 'I-'hedominanttype of fluid inclusionsare highly ........ Na(e K, Mg)-CI aqueoussolutionswhoseabundance,as well as
salinity(upto 48 wt % NaCI-CaCI• •.) andCa-content, increase
significantlywith depth. These inclusionsrepresenta young (8.5 to 12 'km. This latter groupmarksa zone of high seismicreflectivity which, basedon wide-anglereflectionseismics,is immediately
seismograms.Comparison of these with the measured seismograms showsa fairly goodagreement.On the otherhand, syntheticseismogramscalculatedusing seismic impedance valuesderivedfrom compositional datameasuredon cuttingsin underlain bya high-velocity zone(V•> 7 km/s).Thisprominent the field laboratoryyielded only very small amplitudes.This mid crustal phenomenon, which combines high seismic resultprovesthat the seismicreflectivity of the SEI is not due reflectivity and high P wave velocity, is called the Erbendorf to lithologiccontrastbut is mainly causedby the effects of
body(darkshading onFigure5).
cataclastic deformation.
Most of the steeplyinclined reflectorscan be traced to the Altogether,the geophysicalresultscombinedwith "ground surfaceand related to known major fault systems.The strongest truth" from the boreholehave shownthat steep-angleseismic of these reflectors is the so-called SE1 reflector, which strikes reflectionprofiling, at least in this basementregion, mainly SE-NW and dips 55ø to the NE, has a considerablelateral depictsthe effects of brittle faulting and images the young extent, and crosscutsall lithologic boundaries.It can be deformationpattern of the upper crust. Faulting is also of the correlateddirectly to the FranconianLineamentat the surface obviouslyresponsiblefor a 2-3 km verticaldisplacement and clearly representsits depthcontinuation.The SE1 reflector subhorizontalreflectors belonging to the Erbendorf body was predicted to 'occur between 6600 and 7100 m in the (Figure 5). This body may well be a metabasicrelict of a borehole,and in fact, the most prominentcataclasticfault paleosubductionzone, or a sliver of dense rock emplaced bundle of the KTB, consistingof at least four major fault tectonicallyduringthe Variscancollision.It is thereforea key planes,wasdrilledin thedepthintervalbetween6850 and7260 elementin the geodynamicinterpretationof the entirebasement m. Calculationssuggestthat the contrastof seismicimpedance region,but moreinterpretivework, andperhapsevenadditional fully its nature. betweenthe wallrocksand the permeable,fluid-bearing,and experiments,will be neededto understand In thecourseof theinterpretive worksummarized above•the mineralizedfault zonecanproducetheobservedreflectivity. On the basisof thesefindings,a'specialseismicexperiment KTB seismic group developeda promising new processing was designed whose major goal was to quantitatively methodof true-amplitude,prestackmigrationwhich represents understand seismicreflectorsin the crystallinebasement an importantgeneraladvancein seismicdata processing.This [Harjes et al., this issue]. The spatial orientationof the methodis basedon a generaldiffraction conceptinsteadof a reflectingelementSE1 wasdefinedaspreciselyas possible,and reflectionconceptandprovidesa quantitativeandgeometrically of the reflectivitydistributionwith depth. the geometryof the seismicexperimentwasoptimizedin order correctreconstruction to achievemaximumresolution.Then the seismicimpedanceof Plate 3 showsa sectionof the KTB 8502 profile which was the rock section between 6500 and 7500 m was calculated from reprocessed with thismethod.Comparisonwith Figure5 shows borehole measurementsand transformed into synthetic the interpretivepowerof the new method.
18,192
EMMERMANN AND LAUTERJUNG:KTB DEEPDRILL HOLE
3 CH4q' Geoelectrics. In the last decade many electromagnetic favorablefreeenergyvalueat therelevanttemperature: surveysof continentalbasementregionshave revealeddeep- 2FeS+ 2H•SO4 • 3Cq'2FeS:+8H20. In summary, theresultsobtained on corematerialandfrom seatedzonesof high electricalconductivitywhosenatureis still enigmatic.Suggested causesof thesefeaturesare the presence various borehole measurements indicate that the basement despitetheubiquitous occurrence of salinebrines,has, of saline fluids, interconnectedfilms of graphite or sulfide section, mineralizations,or some combinationof these. The European in general, a relativelyhighelectricresistivity whichincreases from 103k• m at surfaceto 10•k•m at 9 km depth. GeotraverseProject (EGT) proved the existence and large downward extent of such layers at midcrustal levels in the central Intermediatediscretezonesof low to very low resistivity(1 to EuropeanVariscancrust [ERCEUGT Group, 1992], and they 100 k• m) are in most casesclearly related to graphitewere also found during KTB site selectionstudiesin both the containing shear zones. Graphite (__+sulfides) therefore determinesthe electricalbehaviorof this crustalsegment,andit SchwarzwaldandOberpfalzregions. Magnetotelluric surveys and long-period magnetic field appears thatgeoelectrical methods canimagetectonic processes variations indicated a zone of high electrical conductivity and can be usedto tracepaleoshearzones. Gravimetry and Magnetics.The KTB superdeep borehole underlyingthe KTB drill site at about 10 km, which has a anomalyandgravityhigh,andit pronouncedanisotropywith maximumconductivityvaluesin wassitedona strongmagnetic theN-S direction.After completionof the superdeep borehole,a was expected that borehole geophysicsand laboratory of rock samplesobtainedduringdrilling would large-scaledipole-dipoleexperiment(Plate 4), the first of its measurements of such kind, wascarriedout to investigatethe extentand propertiesof give importantnew insightsinto interpretation Plate5 showsa new, high-resolution 3-D gravity this high-conductivityzone [ELEKTB group, this issue], The anomalies. resultconfirmedthe expectationthat the boreholereachedthis map of the KTB surroundingswhich portrays the density gravitylow in the NE corresponds zone and that the electrodeat 9065 m depthwas indeedlocated distribution.The pronounced to the Falkenberg granite, and the strongpositive anomaliesat within a layer of high conductivity.The positionof this layer at 9 kin roughly coincideswith the postulatedbasal detachment the KTB site are due to metabasicbodies.However,despitea horizon which is related to the post-Variscancrustal thrust relativelylargedensitycontrastbetweenthe majorrock typesof stack.
Extensive sufacemeasurementswith a variety of electrical methodscarriedout in the KTB surroundings duringthe presite surveyand the pilot phaserevealedthe existenceof shallowlow electricalresistivityanomaliesand confirmedthat the drill site wascloseto the centreof an unusuallylarge surfaceanomalyof the electric self-potential(about -600 mV) extendingNW-SE. That is, the drill site is situated on a "geobattery"which probablydraws its energy from redox potentialdifferencesin the subsurface,with graphite-coatedcataclasticshear zones actingas electronconductingbridges.The model presentedby Stoll et al., [1995] providesa generalexplanationof the nature and source of the observed
anomalous
electric
fields.
It is
supportedby downholemeasurements of the self-potentialand the redox potentialobtainedfrom speciallydevelopedlogging tools.
Hence it appearsthat graphiteis of special importancein producing the observedgeoelectricphenomena.Among the ZEV rocks, the paragneisses contain primary graphite whose crystallinity indicatestemperaturesof about 700øC, in accord with the temperaturesof peak metamorphism.This graphite representsoriginal organic material in the protoliths and is finely dispersedso that it does not contribute to the overall electrical conductivity. More important for electrical conductivityis the secondarygraphite,which is ubiquitousin cataclasticshearzones, where it is always associatedwith iron sulfides and ohiorite. This graphite often forms a quasicontinuous coating along shear planes and is locally concentratedin millimeter-thick layers which constitute good electrical conductors over hundreds of meters.
All the evidencesuggeststhat this graphitewas precipitated from hydrocarbon-bearing fluids at about400øC and 2 kbar. A possiblemodeof formationis described by the reactionCHn + CO2 • 2C + 2H20. This reactionrequiresa relativelyhigh activation energy, which could be provided by mechanical, tribochemical
effects in connection
with
brittle
deformation.
Alternatively, since graphite is intimately associatedwith Fe sulfides,a reactioninvolvingmethaneand sulfatein the system C-O-H-Fe-S could also be considered, which has a lower, more
the ZEV (paragneisses 2740 kg/m3 and metabasites 2890 kg/m3),theirsteepdips,interlayering andcomplexstructure make lithologic interpretation of the gravity pattern very difficult. A partialsolutionto thisproblemcan be achievedby a combination of gravity data with results of geomagnetics [Bosumet al., this issue]. The magnetic data give independentinformation about
lithology (inventory of ferri-magneticminerals) and can also portray tectonic features, like mineralizedshear zones. The KTB superdeephole penetratedmany magnetic anomalies, which have a vertical extent of some meters up to about hundredmeters.One of the mostimportantof theseanomalies, with a strongdecreaseof the magneticfield intensity, occurs near the surface.Below about 1200 m the total magneticfield intensityincreasessystematicallywith depthwith a gradientof up to 200 nT/km, which is much higher than the undisturbed Earth'smagneticfield would produce(about22 nT/km). This result requiresa magneticbody at depth, the exact nature of which, however, remains unknown. Unfortunately,the deep section of the superdeep hole was cased before a hightemperature modification of the magnetometer tool was developed.Therefore the interval between6000 m and 8600 m was only measuredby a SchlumbergerGPIT inclinometertool, whose resolution is about 100 times poorer than the magnetometer tool. Nevertheless, the deep magnetic data obtained with this tool documentthat the strongestmagnetic anomaly encounteredin the boreholeoccursbetween7300 m and 7900 m.
Surprisingly,pyrrhotiteturnedout to be the main carrier of rock magnetismin the drilled sequenceand is responsiblefor mostof the observeddownholemagneticanomalies.Magnetite plays only a very subordinaterole and is restrictedto a few distinctintervals.It is particularlyenrichedin somepartsof the variegatedseries(marbleand calcsilicate-bearing amphibolites) between 7320 and 7800 m, where it producesthe magnetic anomaly mentioned above, which is about 2 orders of magnitudehigherthanany of thosegeneratedby pyrrhotite. The pyrrhotitecontentin paragneisses and metabasicrocksis very similar and mostly below 1 wt %. Enrichmentsup to
EMMERMANN AND LAUTERJUNG:KTB DEEPDRILL HOLE
18,193
18,194
EMMERMANN AND LAUTERJUNG: KTB DEEP DRILL HOLE
v r
n
¾ I
Plate 4. Schematicsetupof the dipole-dipoleexperimentcarriedout at the final depthof 9101 m. Electric currentwas injectedat variabledistancesfrom the drill site on two perpendicularprofiles.The electricalfield was measuredby two probesat the bottonsof the superdeepholeand the pilot hole.The first resultsconfirm that the superdeephole penetratedinto the high-conductivitylayer predictedto lie at 10ñ1 km (shadedband).
KTB 0'-.
-15-
--o0• -25.
-30. SW -35
-
s -40 •
-45 -
-50 -.
"•
448O• 4490
/
4500•
/•i/otoetere 4510 • •/•/
5510
ssoo
4520
453O
/ 5540 / 5530 5520
549O
Plate5. Three-dimensional gravitymapof the KTB surroundings, showing thegravityhighat the drill site causedby metabasites of theZEV, andthepronounced gravitylowsrelatedto theFalkenberg granite(F) to the NE and the forelandsediments(S) to the SW:
EMMERMANNAND LAUTERJUNG: KTB DEEPDRILL HOLE
iii
ml i
i i
iii
i i
18,195
i
De•oth m) 0................ Temperature ............. 300 •:.'. •
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Plate 6. Diagramshowingthe predictedand measured temperatures at depthin the KTB site. The curves outlinedin yellowshowthepredictedthermalgradientfrompresitestudies (with lo and20 envelopes). This prediction turnedoutto bemuchtoolow.The bluelinefromsurface to 4000m showsthemeasured temperature profilein thepilothole.Bottomholetemperatures (BHT) at differentstages of drillingthe superdeep holeare shownby redsquares andthesecorrespond to a constant gradient of 28 K/km.The temperature at final depth (red dot) was measured48 hoursafter drilling ceased.The red shadedfield showsthe temperatureprofile predictedfrom the 6000 m BHT and basedon 31 K/km and28 K/kin.
18,196
EMMERMANN AND LAUTERJUNG:KTB DEEPDRILL HOLE
several weight percent are typical for hornblendegneissesand all major shear zones. Since it was known from laboratory experiments that pyrrhotite occurs in at least two lowtemperaturepolymorphs, a monoclinic ferrimagnetic and a hexagonalantiferromagneticmodification, and that the Curie temperatureof this mineral is around 300øC the superdeep borehole offered an ideal opportunity to study the in situ magnetomineralogical properties of this hitherto poorly investigatedmineral [Kontnyet al., this issue]. Among the most important results of this study is that frequently, both monoclinic and hexagonal pyrrhotite occur together,mostly in intimate intergrowths,with the monoclinic polymorph being dominant in metabasic units. Hexagonal pyrrhotite is the stable phase at higher temperatures(above about 220øC) and it predominates below 8000 m. The observations of monoclinic pyrrhotite grains with relict hexagonalcoresindicate low-temperaturetransformationfrom an originally hexagonal phase during uplift and cooling. Accordingto experimentalfindingsthe Curie temperatureof hexagonalpyrrhotitemightbe as low as 260øC.It is well known from laboratorymeasurements that the magneticsusceptibility
wasalsodesigned,whichprovideda wealthof new information
aboutthe thermalstructureof the drilled segmentand led to
improved methodsof handling downholetemperature measurements.Table 3 presentsa summary of values for thermalconductivityand heat production.Rock foliation and
microcracksare responsiblefor significantanisotropyin thermalconductivity (up to 20%), with highestvaluesparallel to foliation.The distribution of heat production with depth reflectslocal lithologies. The data,in general,showa slight decrease butdefinitelydo notconfirmthe widelycitedlaw of an exponential decrease of heatproduction with depth(Figure 6).
Geothermal datafrom the superdeep boreholeare consistent with the resultsandpredictions from thepilot hole.Withinthe upper 1500 m the temperaturegradientincreasedfrom 20 K/km to about 28 K/km at 1500 m and remained at that value down to
the final depth.On that basis,the undisturbed equilibrium temperatureat 9100 m is about265øC.Within uncertainty,the
verticalheatflow density,whichis theproductof temperature gradientand vertical thermal conductivity,is also constant below 1500 m, with a mean value of around 85 mW/m'.
increases strongly at temperaturesjust below the Curie Estimation of thecontribution of thecumulative heatproduction temperature(the Hopkinson effect), and one of the open rateto the verticalheatflow densityshowsthatthe upper6 km questionsrelevantto the boreholemagneticstudiesis whether of the penetratedbasementonly provideabout 10% of the heatflow densityvalueof 85 mW/m' at 1.5 km. If the Hopkinsoneffect contributessignificantlyto the observed calculated anomalousdepth gradientin the magneticfield intensity, or this value is typical for the crust, then a shift in the thermal to lowervaluesmustoccurwellbelow10kmdepth. whetherthe magnetite-containing layer is sufficientto produce gradient The results of thermal studies in the two boreholes raise a thisphenomenon. The geopotential fields, electric, gravity, and magnetic, number of basic questionsabout the heat budgetin the provide complementaryinformation. Whereas geoelectrics continental crust.Oneof theproblemsis howto explainthelow mainly imagetectonicfeaturesandgravityprovideslithological thermalgradientin the uppermost1500 m, andhow to reconcile information, geomagneticscarry informationabout both. The the--25mW/m'deficitin heatflowdensity between theupper combined evaluation of data from the three sources, now and lower boreholesections.Three alternatives are currently underway, will contribute to a detailed 3-D geologic discussed, whichmay work in concert:(1) topography-enhanced reconstruction of the KTB surroundings. heat advection by groundwater flow; (2) paleoclimatic Geothermal studies.Evaluationof the geothermaldatafrom perturbationof the steadystatetemperaturefield due to changes the KTB pilot hole providedthe highly unexpectedresult that of the mean surfacetemperature(e.g., in connectionwith the the thermal gradient(21 K/km) and vertical heat flow density last ice age); and (3) disturbancesof the steady state
(55 mW/m')metthepredicted values onlyin theupper1000m. temperaturefield due to lateral refraction of heat flow in a subsurface Both parametersthen increasedrapidly to about 1500 m, after compositionallyand structurallyvery heterogeneous which almost constant gradients of 28 K/km and heat flow [Clauser et al., this issue]. Modeling of available data prove that both groundwater valuesof 85 mW/m'prevailed. The temperature in the pilot hole at 4000 m was 119øC and lay well above even the upper advection and paleoclimatic effects are able to producethe error limit calculated from the data obtained by the shallow- measurednear-surfacethermal gradient and heat flow, but the drilling geothermalstudies(Plate 6). exact contributionof each processcannotyet be quantified.In The desireto explainthe failed prognosisled to an ambitious any case, the observationsclearly demonstratethat external research effort [Clauser et al., this issue]. The KTB field lab factorsinfluencethe near-surfacetemperaturefield in the crust, carried out a large number of measurements on core material and this suggeststhat values of geothermal gradient and heat and cuttingsto determinestatisticallysoundvaluesfor thermal flow from shallow drill holes in crystallineterranestend to be conductivity,heat diffusion,and heat productionto be usedin systematicallytoo low. thermal modeling. An extensive geothermal logging program Another set of problems is raised by the surprisinglyhigh
Table
3. Parameters Used for Geothermal
Calculations
Paragneisses
HornblendeGneisses
Amphibolites
Thermalconductivity, W/m K Heatproduction, laW/ms Naturalgammaactivity,c/skg
1.50 + 0.19 51 ñ 5 3.4 ñ 0.2
1.15 ñ 0.11 39 ñ 5 2.8 ñ 0.2
0.53 ñ 0.28 23 ñ 7 2.6 ñ 0.2
Potassium,wt %
2.24 ñ 0.25
1.9 ñ 0.22
0.88 ñ 0.42
Uranium,ppm Thorium,ppm
2.8 ñ 0.4 7.9 ñ 1.0
2.0 ñ 0.2 5.7 ñ 0.8
1.0 ñ 0.6 2.5 ñ 1.5
EMMERMANNAND LAUTERJUNG' KTBDEEPDRILLHOLE
18,197
at the KTB site well correspondsto the averagebackground
valueof about80mW/m2in southern Germany. ß.:..:.•",'.
.:•;•'.•......
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Rheology and Stress Field. A key target of the KTB programfrom the very beginningwas the studyof the state of stressin the Earth's crustand the rheologicalbehaviorof rocks
.
. .
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understanding of the rheologyof the continentalcrustis based primarily on laboratorystudiesof rock strengthunder greatly simplified conditions.These studiesindicate that two major processes controlthe mechanicalbehaviorof crustalrocks:(1) In upper crustal levels, deformationcausesbrittle failure and rock strengthis limited by frictional strengthof preexisting faults. Because the frictional strength increaseslinearly with confining pressure,the strengthof the crust correspondingly increaseswith depth. (2) At greater depth, beyond a certain temperatureand at low strain rates, rock strengthis determined by flow laws and decreasesexponentiallywith further rise in temperature.
The change from brittle to ductile (plastic) behavior, commonlyreferred to as the brittle-ductiletransition,depends on factors such as, among others, rock mineralogy, fluid
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ß ,..
content, and strain rate. Therefore, in nature a more or less
broadtransitionzonecan be expected.Most modelsof rheology are basedon the mechanicalpropertiesof quartzbecauseof its abundance in continentalcrustalrocks.Accordingto the "quartz model"(Figure7), thereis a linearbuildupof differentialstress with depth in the crust to the brittle-ductiletransitionat about 300øC, after which rock strength,and thus the magnitudeof "stored"stress,falls exponentially. 0.0 1.0 2.0 3.0 Obviously,there are shortcomings in sucha simplemodel. Apart from uncertaintiesin the physical-mechanical properties of mineralsand rocks,a more fundamentalproblemis that the Figure 6. Diagram showing the changesin heat production thresholdtemperature for ductileflow is criticallydependent on values, as measuredby laboratoryexperimentsand downhole strainrate, and realisticstrainrates(10'•ns'• to 10'• s'•) are logging, with depth in the superdeepborehole.The commonly unattainablein the laboratory.On diagramslike Figure 7, the cited exponentialdecreasein heat productionwith depth is not expectednaturalsituationis thereforedepictedas a continuous confirmedby thesedata. Instead,one finds intervalsof constant average heat production(vertical lines) which correlate with transitionfrom the brittle to the ductileregime. With a basaltemperatureof about265øC,the KTB superdeep lithologicchanges. hole has penetratedinto depths in which the brittle-ductile transitioncan be expected,and thus a uniqueopportunityis available to study this fundamentalregion in situ. The most valueof 85 mW/m 2 for theheatflowdensityat 8 km depth. importantelementsof thesestudiesare the determinationof the Extrapolationof the temperaturegradient measuredin the stresstensor with depth and the structuralanalysisof rocks superdeephole using conventionalmodels for the lower crust from depthapproaching the transitionzone.From the beginning and assuming conductive heat flow leads to predicted of the KTB program, geoscientistsand engineers worked temperatureswell above 800øC and heat flow densitiesof 55 to closely together to develop an integrated stressmeasurement 80 mW/m 2at thecrust/mantle boundary in 30 kmdepth,There strategy that involved a number of different methods and is no evidencefor partial melting at this depth from seismic experimentswhosecommongoal was to establisha continuous data, and there are also other reasons to expect lower stressprofile from the surfaceto the final depth.Theseinclude temperatures for the crust/mantleboundary.Thereforeonemust modified hydraulic fracturing tests and analysis of borehole questionthe model assumptions.Is there an additional heat breakoutsand drilling-inducedtensile fractures [Brudy et al., . . ß-.' ..s.•&-•.: ::'
ß .....•.:;•.-•j¾.:..:.:. .
':'""' :-'"'
ß
HPR [laW/m3]
source in the crust below the drill site? Could the Erbendorf
this issue].
bodybe responsible for high heat productionor are thereeven significantheat inputsfrom exothermalhydrationreactionsin the middle and lower crust?Is the assumptionof conductive heat transportincorrect,and can fluid convectionrelatedto the FranconianLineamentexplainthedilemma? These and other ideas will be explorednumericallyand additionalconstraintsare expectedfrom plannedgeothermal experimentsin the KTB "Deep Crustal Laboratory."The proposedidea that the thermal anomaly around the Tertiary Eger Rift, to the northof the drill site, is the causeof the high heatflow densitycan be discountedbecausethe value measured
The magnitude of the least horizontal principal stress component (S•) was determined by conventionalhydraulic fracturing experiments in the pilot hole (14 experiments between 800 m and 3000 m), and by two modified hydraulic fracturing experimentsat 6018 and 9070 m depth in the superdeephole. Theseexperimentsyieldedinprecisevaluesfor
the maximumhorizontalstresscomponent (S•t),andthereforea new methodwas developedto estimatethe magnitudeof from a combinedanalysisof boreholebreakoutsand drillinginducedtensile fracturesof the boreholewall. Assumingthat the vertical stressS,., whose value was calculated from the
18,198
EMMERMANN AND LAUTERJUNG: KTB DEEP DRILL HOLE
differential
stress
,
',
criteriom for friction-
',,,••r ..•" controlled sliding \, ß
.•.
transitional I ,
•=10'•e
range /.•':'•,, "brittle plastic transition" ,,•,,"
--300 øC
,, .......
•= 10-14 KTB
/
depth (z)
T = f(z) p = f(z)
-
flow law for quartzite strainrate • as indicated[s-•] (dislocationcreep)
Figure 7. Schematicdiagramshowingthe dependence of crustalrheology(basedon the behaviorof quartzite) on pressure(depth)and temperatureand the magnitudeof differentialstress[Stoeckhert,1994]. The final depth of the KTB borehole(9101) reachedthe zone of transitionalbrittle-ductilebehavior.
lithology of the drilled section and density of the respective hundredsof metersfrom the injectionzone in the depthrange rock types,is a principalstresscomponent,a continuousprofile between8 and9 km [Zobackand Harjes, this issue]. The inducedseismicitywasrecordedby a surfacenetworkof of the complete stress tensor down to about 9 km was established. 70 stationsand with a three-component geophonetool deployed at 4 km depthin the pilot hole. Clusteranalysisshowedthat the The combined results of all stress measurements indicate that the directionof S, is remarkablyuniformat N160øE+10ø over events were concentratedat two different depth levels, both almosttheentireinvestigated depthrangefrom3 km to 9 km, above the injection zone and extendingabout 1 km from the andit corresponds well with the N146øEregionalorientationof borehole.Fault plane solutionsof the seismiceventsindicatea S, in centralEurope.The only significantchangeof Ss with strike-slipcharacter,and this documentsthat the crustat 8-9 km
depth is a shiftof about 60ø,toN220øW, at7200m whichis stillin a stateof frictional equilibrium. However, the coincides withthelowermost faultplaneof theSE1fault complete absence of induced earthquakes near9030m depth,
bundle. Values ofthemagnitude ofS,at6018 mand9070m coupled withtheobservation thattheseismic events clustering
at a depth of about 8.7 km, indicatesan abruptchangeof the depth in the superdeephole are 111 MPa and 183 MPa, stressstate,and markedlylower shearstressessuggestthat the respectively,and the estimateddifferential stressesare between bottomof the boreholemay be near the brittle-ductiletransition. 180 MPa and 147 MPa. Apart from the uppermostsectionof the A different approachto identifying directly the current drill hole down to about 1000 m, whereS. appearsto be the brittle-ductiletransitionzone comes from combiningdata on leastprincipalstress,thestressmagnitudes obtained fromS,, Ss the presentstateof stresswith the resultsof microscopicand andSv (-240 MPa at 9100 m) indicatestrike-slipconditions (S, submicroscopic textural studiesof minerals,especiallyquartz,
