the Bjorko energy project.

HERBERT HEN KEL NGU-BUL L439,2002 - PA GE 45

The Bjorko geothermal energy project

HERBERT HENKEL

Henkel,H.2002:TheBj orko geothermalenergyproject.NorgesgeologiskeundersekelseBulletin439,45-50.

Impact craterformationresultsin changesin thephysicalpropertiesofrocksthatcanbe geophysicallydeterm ined andthereforeareof importanceforthemappingof impact structuresat depth .Especially elect ricalproperti eshave adirect bearingon theremaining porosityfound inimpactst ructu res incrystalline targetrocks.Severalst udies madein the Dellen,5iljanandLocknecraters showtypicalchangesin rock physicalproperties.Althou gh a signifi- cantly increasedporosit yinthe craterbasement seems to occur, thehydrauliccond uct ivity appearsto berather small.The totalvolume ofbrecciatedrocksinimpactst ruct uresmaybe verylarge andthatvolume could,after reac- tivationof thefractur es,beused as aheat exchangest ruct ure forgeoth ermal energyret rieval,exploit ing thenorm al geot hermalgradientat greaterdept hs.

TheBjo rkostr uctu reis a c.10 km-diameterimpact craterlocatedjust westofStockho lmin lakeMalaren.St udies madeso faronislands show thecharacteristicfeatures ofintensebrecciationand increasedporosit yknown from ot hercrate rs.TheBjor ko EnergyProjectisdesignedto assessthepotentialforgeoth ermalenergyretrievalbymap- ping thest ruct ure atdept hwith geophysicalmet hods and bydrilling . Theproject isfinancedby the Swedish NationalEnergyAdministrat ionand runsfor2years, start ing inOctober 2000.

HerbertHenkel,Dept.ofLand and WaterResourcesEngin eering , Royal InstituteofTechn ol ogy,SE-lOO44Stockh ol m, Sweden;Em ail:[email protected].

Fig.1.Impact struct uresin Fennoscandia(from Henkel&Pesonen1992).

Since 1992,afewmore impactcrate rshavebeen confirmedinFin land.

Fracturing related to impact craters in crystalline environments

The concept of fracturing has not previousl y attracted impact researchers asmuch asthe shock-metamorphictran- sit ions prop er,the craterform at ion processor the composi- tion of impact-gen erated rock types.Thus, thereare so far onlya few stu dies dealing wit h thissubje ct.The deepdrilling at thePuchezhKatunkistructure(Masait is et al. 1994)isone example- where impact-inducedfracturing is observed to considerable dept halong the enti re c.5 km depth of the

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Introduction

Impact cratersare caused by bodies in the solar system whose orbits have beendisturb ed, eit her bythe gravity of Jupit er,namely asteroids,or by thegravity ofstellar encoun- ters,namelycomets.Whentheir new orbit s crossthat of the Earth, they mayevent ually collide with our planet, result ing in a catastrophic explosion where thekineticenergy of the projectile istransferred to the Earthastherm aland mechan- ical energy.The cratering eventtransforms terrestrialmateri- als into various shock-metamorphic states.Theoretically,it is envisaged that the shock-inducedtransformation soccur in a hemisphericalregionaroundthepointof impact, wit hout- ward decreasing inte nsit y.The shock-induced transforma- tionsof thetargetmater ialthusgradefrom theformation of vapou r through melti ng to brecciati on. An exte nsive account ofimpactcrate ring hasbeen presented byMelosh (1989).Largeimpact s result in impact crate rs,which in the active geolo gical syst em at the Earth's surface are soon eroded or buriedbeneath sediments.Craterslargerthanc.4 kmin diamet er evolve as complex str uct ureswit h an ele- vated central rise.Thetransition to thiscomplexfinalcrater st ruct ure involvesmassive radia l flow of material,whichwill add to theshock-ind uced fracturin g of thetargetmaterial.In Fennoscandia, some20 large (Cretaceousand older)and 4 small (postglacial)impact cratersare present ly know n. An evenlarger number ofsuch structure sis suspected tooccur but remain to beconfirmed asimpact craters.In themap (Fig. 1).from Henkel&Pesonen (1992).anoverview is given of thesit uat ion 10 yearsago.

Thisvast fracturing in connectionwit h a large impact craterin a crystallin e environment isthe researchtargetfor

the Bjorko energy project.

NGU -BULL 439,2002 - PAGE46 HERBERT HENKEL

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tivity attainsa minimum over the ringof the crater and arel- ative maximum over the 9 km-diameterregionwit h mag- netic impa ct melt. The resistivitytraverseisshown in Fig.2 (fromHenkel1992).

The central rise of the then suspected Bjorkoimpact structure was alsostudiedwit h respectto the distrib ut ionof electricresistivi ty.It was found thatthe resistivitysystemati- cally is below 10' ohm m,which isnon-typica lfor normal low -porosit y crystalline rocks such asgranite (Fig.3,from Henkel 1992).

In the cent ral uplift of the Siljan impact structure,the spacin gofa one-directionalset oflinear airborneVLF anom- alies was st udied and comparedwit h the spacingof sim ilar exterior anomalies.Such anomalieshave previously been foun d to be caused by water-bearingfracturezones.In the

20

Fig.3.Theresistivit ydistributionofthe central rise ofthe Bjorkostruc- ture(afterHenkel 1992).The cumulative frequency is markedcf.The broken lineindicates the approximatelower resistivitylimitof normal crystalline rocks.

10

drillhol e in the central rise.The deep drilling at the Silj an str uct ure has not explic itly been devoted to map ping the extentof fractur ing,but shock-induc ed planar deformation feat ures in quartz are reported to occur dow nto 2.8km depth(Juhlin 1991).

Fracturing in crystalline rocks normally result s in increased porosity,and the additi onal pore water content decreases the elect ric resistivity of the material. It would thereforebe possibleto mapthe extentof im pact-induced fractur ing with geophysical methods that rely on electric conductivity,acon ceptthat hasaparallel applica tioninfrac- turezonemapping.The relati onshi pbetweenfracturezones and changes in physical propert ies has been explored in several context s,forexample in conn ect ion wit h radwaste storage.It isa wellesta bl ished fact thatthemagnetisation (Henkel& Guzrnan 1977) and the elect ric resistiv ity both showa decreasein fract urezones(Henkel 1988).

Thisknowl edge hasbeenappliedtoimpact craterst ruc- turesin fourways:Thein-sit u measurem entof electricresis- tivityusingVLF-Rtechn ique;the st udyoffracture frequency usingairborne VLF data;a studyof the relationsh ipbetwe en in-sit uelect ricresisti vity and porosit y;andfinally a study of therelationshipbetw een in-situresistivityand fracturefre- quencyon a detailedscale.

In the Del/en im pact structure,a traversewith in-situ electric resist ivity measurementswith the VLF-Rtech niq ue was performedin connect ion wit hregular mapping activi- ties in the area.Theimpactoccurred ina relativelyhomoge- neousgneissicgranite(Lj usd algranite)with numerou s out- crops onwhich themeasurem ent array was located.Itwas found that the elect ric resistivity begins todecreasealready outside the present erosion al (topographic) crater edge.

Towards the crater interio r, outcrops becom e scarcerand event ually there isonly a glacialtill cover.Theelect ricresis-

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HERBERTHENKEL ^{NGU-BUL L} 439, 2002 - PAGE47

Fig.4. Correlat ion between surfacefrac- turefreq uency andin-situsurfaceelectric resistivity (from Backstrorn 2001). The impact brecciated rocks have a c. 100 timeshiqhersurfacefract ure frequencyas compared tonormal crystallinerocks.

event is likely to causesignificant hydroth ermal alterations, resulting in cementation of the created frac- tures.The heat exchange capacity may thusbe achieved first after a re-op ening of the fract ure system by hydraulic methods.

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centralrise of the Siljancrater,the spacing wasfoun dto be on averageabout1/5ofthatin theexte riorareas,indicati ng a significant lyincreasedfract ure frequ ency (Henkel1992).

From drillholesmade in connectionwit h the Siljan Deep GasProject (Juhlin1991),observed porosityfrom drillcores hasbeen correlatedwith electri cresistivity measurements.

The result shows a linear relat ion ship between the 1010g resistivityand the porosityoverarange ofseveralorders of magnit ude and up to 5% porosity,respectiv ely,indicat in g thatnear surface(wit hinca 200m) porositycanbe assessed wit h surface measurements of elect ric resist ivity (Henkel 1992).

At theLockneimpactstr ucture,a st udywasperformed correlatingthefract urefrequency seenonoutcrops wit h the electr icresist ivit y measured at the same location.A linear relat ionshipbetween the1°logof resistivity and the"loqof the areal fracture frequ ency could be esta blished (Fig. 4, from Backstrom 2001).

In summary,theobtained results clearly show that an increased fract ure frequency istypi calforimpactst ruct ures bothonanint ermediateandadetailed scale,and thatthisis reflectedin the electr icresistivitymeasuredovera relati vely largearea.Italsoindicatesthat fractu ring seemsto increase gradually from the erosional crater edge towardsa maxi- mum in thecent ralrise area.It hasnot yet beenpossible, however,to document thedetails ofthisvariation due tothe lack of basementout cropsundertheringsynform incom- plexcraters.Also,the vertical variationof impact-generated fracturing ofthebasementis still poorly know n.Theresults obtained so far,however, pointtowards the optionto use deep-penetrati ng, electromagnetic soundingtechniquesfor the mappi ngofthe fracturefrequency/ porosit y wit hinan enti recraterst ructure.

The assumpt ionofahemi sphericalfractured volumeof the crate r basement combined with the relativelylow geot- hermalgradientofc.15Kkm'typ ical fortheBalticShield, requires a rather largeim pactstruct urein order to represent alargeheat exchangevolumeata depthwithelevatedtem- perature.Thethermal energy generated by a large impact

Geophysical methods relevant for modelling of structures in glaciated crystalline environments

In largepart s of the BalticShield,onlya thin cover ofglacial deposits occurs directlyupon thecrystallinebasement,and a complete sedimenta rycoveris absent.This specific geo- logical sit uat io nisfavorable wit hrespect tothe applicatio n ofavariety of geophysicalmethods for a modelling of the subsurface.Furtherm ore,relatively commonoutcropsofthe crystalline rocks make it possible to sampletheir rock physi- cal propert ies at the surface. The applicable methods include measurement s of gravity,magnetics and elect ro- magnet ics.These methods are mutu ally interrelated and result in constraining option s when used in com bination.

Some of thesefeatu res and relat ions are out lined briefly below :

Gravityanomaliesareproport ion al to the density con- trast of rockvolumes,whichinturnisafunct ion ofrock com- posit ion and porosity/brecciation.

Magnetic anomaliesareproport ionaltothemagneti za- tion contrast, which depends on the contentofferrimag- net ic minerals.This contentisdecreased byoxidatio nin frac- turedvolum es.

The electric conductivity can be mapp ed with several electric or elect romagnet ic meth od s and isdependent on rock poro sity/brecciation and elect rolyte conte nt of the pore water,aswell asthe content of conducti ve minerals (sulphides and graphite). Relevant meth ods are Vertical Electrical Soundi ng (VES). Very Low Frequency Resistivity (VLF-R)measurementsand Magneto-Telluric(MT) measure- ment s.

Electromagnetic (VLF)anomalies are proportiona l to thecontrastinelectr icresist ivityand the (near-surface)vol- ume of conduct ivematerial.

Faultzonescan thusbeidentifiedby bothmagnetic and electromagnetic method sdirectly,and indirectly from their distorti onofmarker st ruct ures. Inadd it ion, data on terrain morpholog y, suchas elevation data and radar data,reflect the occurrenceof faultscarps/lineament s.

NGU-BULL 439,2002 - PAGE48 HERBERT HENKEL

Sim ilarly,impact crater structuresresult ingravity,mag- netic and electromagnet ic anomalies,which can be mod- elledonce the physicalprop ert ycontrasts of therockshave been constrainedby measurements.

The Bjorko structure

Bjorkoislocatedin the easte rn part ofLake Malaren, and its anomalouscharacter is related to the occurrenceof non- metamorphosed sand stone of supposed Jotnian age (Gorbatschev & Kint 1961). InHoden et al.(1993),the struc- ture was suggest ed to be caused by ameteorit e impact. The structureis ca 10 km in diameter, with a centrallylocated hill c.2km in diameter.Based onseismic mappin g,the sand- stone was found toencircle the central risein a ca 170⁰sec- tortothe sout heastof it,wit hanestima ted thickness vary- ing between40 and 280 m based on refractionseismic inter- pretation s.Drilling on the island of Midsommar, however, revealed a sandstone thickness of over 900 m. The erosion levelof thestr uct ure is unknown anditsageisestimatedto about 1.2 Ga based on K-Ar data from a c1aystone bed immediat ely above the granit ic basement (Floden et al.

1993).The stru cture is also tilted down to the southeast.

Most of it iscovered wit hwaterexcept for the cent ral por- tion,some islandswit hin thering,and parts of the edge.The natur e of the target at the time of impac t is unknown - some sandstone may alreadyhave existed.To the south,the crystalline basementis dominated by gneissicrocks(parag- neissesand gneissosegranites-tonalites)wit h distinct E-W str ikeand steep dip.Tothe nort h, theserocks are intr udedby younger granites(Stockholmg ranite)and have a more irreg-

ular orientation of their foliati on.An ENE-WSW- strikingdyke wit h magnet ic signat ure can be seen cutting through the northern part of theBjor ko stru cture .It is offset in several segmentswit hin the str ucture,and providesa usefulmarker feature for st ruct ural mod elling.Inspection ofthin-sect ions from thecent ral rise region reveal numerousimprints that are typical for impact-ex posed rocks,like sets of planar deformation features inquartz,kinkbandingin biotiteand feldspars,andageneralfragment ed appearanceon acrystal scale.Similarfeatur es were found in sedimen t c1asts over- lying thefract ured basement.These have therefore been int erpret edasre-depos it ed ejecta .

The entire centralpartof the stru ct ureisintensely frac- tured on the cm scale.Thisbrecciation is alsoobserved at several localitie snearthe edge of thestructure.In the cen- tralrise,VLF-Rdata indi cate abu ndant low resistivity down to a depth ofafewhundred metres.VESmeasuremen tsindi- cate similarprope rt iesdown to ca 600 m depth,and MT measurements indicate decreased electri cresitivitydown to adepth ofc.7 km.

Geothermal energy and the energy situation in Sweden

On asmall scale,geothermalenergy is frequently usedfor heating ofsingle familyhousesinmany placesinSweden.

Onamedium scale,onlyone prod uctionfacility is operating.

Thisis locatedinLundinsout hern Sweden and isbased on the exploitationofwarmwaterat moderatedept h.The heat exchangevolum e isasandstoneformati on atc.1kmdept h.

The geothe rm algradient in crystalline rocksisc.15Kkm',

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Fig.5.Cartoon show ing theimp ortantfeaturesrepresentingthepotenti al forgeot hermalenergyincrystallinerocks.Thegeothe rmalgradientacross sedimentary cover rocksdependson theirthermalconductivity.The cited norm algeothermalgradientof 15K krn-l was obtainedfrom thedeep drilli ngat5iljan(Juhlin 1991l.

HERBERT HEN KEL

knownfrom thedeepdrilling at Siljan.There are no anom- alous thermal struct ures known in the Balt ic Shield, and therefore geothermal energyhasfor longbeendisregarded as an important energy resour ce.The clim at icsituat ion in Sweden causes alargedemandfor energy for heat ing dur- ingmorethan 6 monthsofthe year.Thisdemand hasbeen covered by elect ricenergy,oiland gascombu stion, and to a lesser extentby the burningofwasteandbiomass.ln many places,alarge-scale energy infrast ructurewit hdistrict heat- inghas beenestablis hed.

Presently,the political decisions to reduce andphaseout nuclear power and to reduceCO,emissio nsarenotviabl e as no sufficientlylarge-scale alterna tives havebeendevelop ed.

Geothermal low-temperat ure energy,however, is present everyw hereandwould be anobvi ous resour ceifaneff icient heatexchangest ructure could befound or createdat depth in the crystallinebasemen t.It sho uld alsobelocated closeto anexistingenergy infrastructure.

The extraction of geothermal energy in ordi nary crys- talline rockshas been tested at Fj allbacka (Wallrothet al.

1999),andincluded hydraulic fracturing inorde rtocreate a heat exchange volume at depth.The exchange eff iciency, however, couldnot bebroug ht toaccept ablelevels andthe project wasnotdevelop edfurth er.Ittherefore seems neces- sary tofind already brecciated rock volumesatdept handto explore orincreasetheirhydr aul icconductivity.Inessence, two such types of structure can be envisaged:Fracture zones,with anessentially planar extent of fractu redrock,or impact craters,wit han assumed, essentiallyhemispherical exte nt of thefracturedrock.Geotherm al energy prospecting sho uld thereforeaimat mapping the extentof brecciation of suchstructuresand to estimate their pot ent ial asheat exchange volumes.Inad dition to thefracturing ,whic heven- tuallymayprovideasufficient ly effic ient heat exchangevol- ume, the actual geothermal gradient,the radiogenic heat prod uction, and the effect of a shielding cover rock sequence are im por ta nt fact orsfor the evaluation of the potent ial for geothermalenergyincrystall ine environments (Fig.s).

The Bjorko energy project

With thisbackg round,theidea wasformulated totestthe Bjorkostruct urefor its pot enti al asaheatexchange volu me for geothermal energy retrieval. An applicati on was for- wardedtothe SwedishNatio nal EnergyAdmin istrationbya research team from theRoyal Inst it uteofTechn ology and the Stockholm University.The proje ct explicit ly aims at developing tools for energy prospecti ng by integ rated analysisof relevant surface data and their calib ration by drill in g.Themostimporta ntaspect to startwit histofindthe localrelat ionship betw eenelectrical properti esand porosit y downto several kilomet res depth.

Thegeot hermal energy potentialofthe 10 km-diameter Bjorko str uct ure isdramatic.The porosityvariationas esti-

mated from electric resis tivity data is

NGU -B ULL 439, 2002- PAGE49

Sodertalje

Fi9.6 .Locati onof theBjorko st ruct ure in relationtothe exist ing energy infrastructurein theStockhol mregion .Theapproximateedge of the Bjorko str uct ure(circle) and its c.2 km-di ameter cent ral uplift are marked.Thethreesmall ringsNW, SandSW ofStockholm marktheloca- tions of major dist rict heati ng plants.The study area is west of Stockholmwit hin the coordinate frame withmarkers every 5 km.

brecciatedvolumemay exceed250km'.Assuming a1%dif- ference in porosity in ahemispherical volume wit h 5 km radius and atemperature difference of40°C,the energy conten t ofthe st ruct ure isover4 000TWh,morethan 10 timesthe annualenergy useinSweden.Thestructu re also lieswit hi n therealms ofthe distri ct heat ing systemsof the Sto ckho lm regi on (Fig.6) and its energy potent ial cou ld cover70 %of the energy demand for heati ng on along- termbasis.

In thefollowing,brief descript ions are given ofthe ongo- ing activities.

Compilation of existing dataand complement arymea- sureme nts areperformed within a20 km-di ameter region around the Bjorko st ructure.These inclu de:airborne geo- physics(magnetictot alintensit y,VLF,andgamm a radiation), data abo ut the terrai n shape (elevation and bath ym et ric data, to form a 50 grid),gravit y measurementsand petro- physicalmeasurement s onrocksam ples fromoutcro psand drillcores(density,magneti c susceptibility,remanent mag- netization,elect ricconductivity).

Measurementsofnewdata,especially ontheelect rical prop erti es of rocks and rock volumes.These include:VLF-R measurem ent stoest imate the near-surfaceelectric resist iv- ity variatio n,MTmeasurementsfor mapping oftheelectric resist ivity at depth, fractu re frequency st udies in type regions(cent ral part,ring- edge and exte rior) to provid e a basis for the correlation between elect rical and fractu re pro pert ies.

Drilling of2 or 3drillhol esdownto markers observed in MT measurem entsand accompanying tests.Theseinclud e:

chemicalanalysisofthe waterin drillhol es,estimation ofthe

NG U-BULL 439,2002 - PAG E50

hydraulic conductivity around drill holes,measurement of the local geothermalgradient and the componentsof the stress field.

Modelfingof the collected data to provideinformati on on the thermal and hydraulic potentialof thestr uct ure.This includes: esti mat ion of the radiogenic heat production, modelling of gravity,magnetic VES,MT and fracture fre- quencydata.

The expect ed results of this energy prospecting project are to describe the Bjorko st ruct ure in 3-d, estimate its potenti alforgeothermalenergy retr ieval, identi fy possible relations hip sbetweensurface data and hydraulic properties at depth, and suggest further approachesto verify the obtained modelling results.

Conclusions

Geothermalenergy resourcesmust be a targetfor renewed research, evenin regionswith lowgeoth ermalgradient.This ismoti vated bytwofactors:the need forheating at highlat- it udesand the localnatureof this energy resource (apart from thepoliticalgoals to reduceboth CO,emissionsand the use of nuclear energy).

Meteori t eimpact structuresaresuggestedto be suitable target sfor geothermalenergystudiesas theymayprovide a very large volumeof fracturedrock in comp arisontofrac- turezones.

Thephysicalpropertythatismost likely akey factorin mapping the extent of fracturing at depth is the electric resistivit y,and therefore effort saredirected towards estab - lishing and modelli ng the relation s bet ween the volume fracture frequ ency and the electrical properties that are measurablefrom the surface.

The physical conditi on that is necessary in order to establishaheat exchangesystem at depth isthe hydraulic condu ct ivit ythat can be createdand maintained with artifi- cial methods.How large volumes of fractured rock react involves issues that demand further experi ments and research.

Acknowledgements

Iwouldlike to thank Lars Kirkhusmoand Knut Ell ingsen fortheir helpful andcon structi vereview s ofthe manuscrip t.

References

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Hoden,T.,Sbde rberg,P.& Wickman,F-E.1993: Bjorko- a possibleMidd le Proterozoic impact structure west of Stockholm. Geol og iska F6reningen i Sto ckho lm F6rh andlin gar11S-I ,25-38.

Gorbatschev,R. &Kint ,O.1961:TheJot nian Malar sandsto ne of the Stockho lm region,Swede n.Bulletinof the Geolo gicalInstitution sof theUniversityof Uppsala40,51-68 .

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Henkel,H.1992: Geophysicalaspec tsof meteorite impact craters in

HERBERTHENKEL

eroded shie ld envi ronm ents,with special emphasis on electric resistivity.Tectonop hysics216,63-89.

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