(jcoLhcrlllal ~ySICIllS 1

clIAlyrER I

GEOTHERMAL SYSTEMS

Geothermalfields arefound throughoutthe world in a rangeof geological

settings,and are increasinglyheingdevelopedas an energysource. Each of the

differenttypesofgeothermalsystemhasdistinctcharacteristicswhicharereflectedin

thechemistryof thegeothermalfluidsandtheirpotentialapplications.However,they

all haveincommonaheatsourceata fewkilometresdepth,andit is this which sets

water,presentin the uppersectionsof the Earth'scrust, into convection. Most

geothermalresourcescanbe usedfor spaceheatingapplications(eg. urbandistrict

heatingschemes,fish farming,greenhouseheating),hutit is only thehottersystems

(>-1swq whichareusedtogenerateelectricitythroughtheproductionof steam(see

Rowley,19SZ,for a reviewof systemsworldwide).Sinceaqueousgeochemistryis

involvedin all stagesof theexploration,evaluation,andproductionof a geothermal

field,anunderstandingof thechemistryof thefluids is essentialfor thedevelopment

of a resource.The chemistryof thegeothermalwatersandgasescontainsimportant

informationaboutthehydrologyof thefieldandconditionsin thereservoir.These

aspectsof geothermalfluid chemistryare discussedin the following chapters.

However,beforelookingatspecificaspectsof geothermalchemistry,it will he useful

to first placethe fluids in contextby brielly describingthe differenttypes of

geothermalsystem.

1.1 SYSTEM TYPES ANI>CHARACTERISTICS

Geothermalfieldsarecommonlyclassifiedor dividedhy a seriesof descriptive

terms.Theyarereferredto asliquidor vapourdominated,low or hightemperature,

sedimentaryor volcanichostedetc. This sectionoutlinesthemeaningof theseand

otherdescriptiveclassificationtermsfoundin theliterature.

Reservoirequilihriwnstate:This is the fundamentaldivision betweengeothermal

systemsandis basedon thecirculationof thereservoirfluid andthemechanismof

heattransfer. Systemsin dynamicequilihriumarecontinuallyrechargedby water

enteringthereservoir.Thewateris heatedandthendischargedoutof thereservoir,

2 GeothermalFluids lJl;()UICIIII'JI .')>lLI'" .J

eitherto thesurfaceor to undergroundpermeablehorizons. IIeat is transferred

throughthesystemby convectionandcirculationof the fluid. Systemsin static

equilibrium(also known as stagnantor storagesystems)haveonly minor or no

rechargetothereservoirandheatis transferredonlybyconduction,

Heatsource:TIle heatsourcefor the systemis a functionof the geologicalor

tectonicsetting.If thedrivingheatflux is providedby a magma,thensuchsystems

aretermedvolcanogenic.Theyareinvariablyhigh-temperaturesystems. IIeatdoes

not,however,haveto be suppliedby a magma,anda geothermalsystemcan be

generatedin areasof tectonicactivity. For example,heatmaybe suppliedby the

tectonicuplift of hot basementrocks,or watercan be heatedby unusuallydeep

circulationcreatedby foldingof a permeablehorizonor faulting,Thesearetermed

non-volcanogenicsystemsand includeexamplesof bothhigh andlow-temperaturereservoirs.

Fluidtype:The reservoirfluid can be composedmainlyof liquid water(liquid-

dominated)or steam(vapour-dom;,rated).In mostreservoirs,hothsteamandliquid

waterexistinvaryingproportionsastwo-phasezones.Liquid-dominatedsystemsare

mostcommon,and may containa steamcap which can expandor developon

exploitationashappenedatWairakei,New Zealand.Systemswhichdischargeonly

steamarerare- thebestknownareL'lrderello,ItalyandThe Geysers,USA. Notethatliquid-dominatedsystemsare sometimescalledwater-dominated;this is not a

goodtermsinceall hydrothermalfieldsarecomposedof waterin eithertheliquidor

vapourphase. Vapour-dominatedsystemsarealsoreferredto assteamfields. The

sourceof thewaterinageothermalsystemis discussedhelow.

Classification

A simpleclassificationhasedon reservoirequilibriumstate,fluid type and

temperatureis usedin thistext(seebox). Geothermalsystemsareprimarilydivided

into dynamic(convective)systems,and slatic (conductive)systems. These are

suhdividedon thebasisof reservoirtemperature.The topographycanalso influence

thestructureof thesystem,andthiscreatesafurthersubdivisionforhigh-temperature,

liquid-dominatedreservoirs,asdiscussedinSection1.3.

Reservoirtemperature:The temperature(orenthalpy)of geothermalreservoirsis an

importantdiscriminatorin termsof fluid chemistryand potentialresourceusage.

Systemsarecommonlydescrihedas low-temperature-15WC) orhigh-temperature

(>-150C).TIle discriminatorytemperaturesarenotrigid.andsomeworkersalsouse

theterm"intem1ediate"to indicatereservoirtemperaturesin the 120-180.Cnlnge.

Low-temperaturesystemscanonly heusedfor "direct-use"applications(eg.healing),

while high-temperaturesystemscan be used for electricitygenerationas well as

direct-useapplications.

A classilicationof geothermalsystems

Hostrock: TIle rockswhichcontainthegeothermalreservoir(the"hostrocks")react

withthegeothermalfluid, As rock-fluidreactionsdeterminethefinalcompositionof

the geothermalwatersand gases,a knowledgeof thehost rocks is importantfor

confidentapplicationof geothermometersand predictionson potentialscaling

problemsif the field is developed. Only broaddistinctionssuch as volcanicor

sedimentary(clastic/carbonate)arenecessary.Metamorphosedequivalentsof these

lithologiescan be indicatedby addingthe prefix "meta-"to the aboveterms.

Volcanic,clastic-sedimentaryandcarbonate-sedimentaryrocks(andthemetamorphic

equivalentsof theselithologies)all yield geothermalfluids with contrastingand

distinctchemistries.If thegeologyis poorlyknown,it mayhepossihletoprediclthe

sub-surfacelithologiesfromthewaterchemistry,

Dynamicsystems(convective)

Hightemperature

liquid-dominatedlow-relief

high-relief

vapour-dominated

Low-temperature

Staticsystems(conductive)

Low-temperature

Geopressurised

Moredetailedcategoriescanhedevised,hulthisis adequateforgeneraldiscussion.

4 Geo(hennalFluids GeothermalSyslcms5

Dynamic(convective).'i)'...tems

High - temperaturesy...tems:These are found in geologicalsettingswhere the

geothermalgradientis severaltimesahovethecrustalaverageof -30C/km,and

whererocktemperaturesof severalhundreddegreesCelciusexistatdepthsof onlya

few kilometres. The locationsof geothermalfields is invariahlytectonically

controlled,andtheyareoftenfoundinareasof hlockfaulting,grabensor riftingand

in collapsedcalderastructures,with reservoirdepthsof around1-3km. Typical

settingsarearoundactiveplatemarginssuchassuhductionzones(eg.PacificRim),

spreadingridges (eg. Mid-Atlantic) and rift zones(eg. East Africa) and within

orogenichelts(eg.Mediterranean,Himalayas).

demonstratedthatLarderelloshowsmanycharacteristicsmore typical of a static

(conductive)systeminsteadystate.

High-temperaturesystemsareoftenvolcanogenic,with theheatprovidedby

intrusivemassescommonlyof rhyolitic-andesiticcomposition.ExamplesincludeEI

Tatio,Chile; fieldsin theTaupoVolcanicZone,New Zealand;CerroPrieto,Mexico;

ImperialValley,The Geysers,RooseveltHot Springs,USA. Hot or hoilingsprings

typicallydischargechloridewaters(with theexceptionof The Geyserswhich is a

vapour-dominatedsystemanddischargessteam)with a totaldissolvedsolids(TOS)

concentrationof -3000-5000mg/kg.Silicasinteris oftendepositedaroundboilingornear-hoilingsprings. Saline,or brinefieldsformwhereseawateris involvedin the

systemor wherethe chloridefluids passthroughevaporitesequences(eg. Cerro

PrietoTOS =-40,000mgikg;SaltonSeaTOS=-300,000mg/kg).

Low-temperaturesystems:Low-temperature(alsocalledlow-enthalpy)systemscanoccurin avarietyof geologicalsettingsof bothelevatedandnormalheatflow. Deep

fluid circulationthroughfaultsor foldedpermeahlestrata,tectonicuplift of hotter

rocks from depthand theresidualheatfrom intrudedplutonscan all yield low-

temperaturefields. ThesearefoundthroughoutEuropeandAsia, and alongsome

areasof Tertiaryvulcanismin thePacific. The structureof low-temperaturesystems

is not illustratedasno idealisedmodelcanhedevelopedgiventhevariedoriginsof

thesetypesof system. They usuallydischargedilutewaters,with total dissolved

solidsconcentrationsof aroundIOOOmg/kgor less,throughwarmspringsat -30-

6SOC.Manysuchspringsdepositmineralsof retrogradesolubility(calcite,gypsum)

withonly minoror nosilicadeposition.Thecompositionof thewaterswill depend

ontherelativecontributionsof formationwatersandmeteoricwatersto thedischarge

features.Watersheldin limestoneandothercarhonate-richreservoirlithologieswill

hebicarbonate-rich,possihlywithlowchlorideconcentration.Watersfromreservoirs

composedof marine clastic sediments,especially shales, often have highconcentrationsof chloride,horonandcarhondioxide.

Geothermalsystemsalso developon the flanks of young volcanoes(eg.

Ahuachapan,EI Salvador;Kawah Kamojang,Indonesia;Puna,Ilawaii). In coastal

locationsthesemayincorporateseawaterintothecirculatingwaters(eg.Reykjanes,

Iceland).The deepchloridefluidsareoftennotdischargedatthesurfacebecauseof

thehighrelief. If theydoemergeit is usuallyasdilutewarmspringsmanykilometresalongtheflanksof thevolcanoatlowerelevations.Fumarolesaremorecommonand

anyspringsneartheupflowzonedischargeacidicsulphateand/orbicarbonatewaters.

Suh-surfacemixingandhydrologycanbecomplexin thesefields.

Static(co/lductive)systems

Static systemsare characteristicallyfound in stratadeposited in deep

sedimentaryhasins.The fluidsarederivedfromtheformationwaterstrappedwithin

the thick sedimentarysequences.TIlese watersattainreservoirtemperaturesof

around70-15

b UColhermalHuids l,collIClllla, ~}'~IClII~ I

pressure,greatlyexceedingthatof the watercolumn(hydrostaticpressure).Such

systemscanalsocontainsubstantialamountsof methane.

- water-rock ratios

- thechloride contentof hostrocks

1.2GENESISOF GEOTHERMAL FLUIDS

-theconcentrationof chlorideinwatersfromhigh-temperaturesystems- theaerialextentofgeothermalsystems

-thedurationofgeothermalactivity

Origin of waterandsolutes

The waterconstitutingthegeothermalfluid canbe derivedfroma numberof

sources.It mayrepresentsurface(meteoric)waterwhichhasgaineddepthsof several

kilometresthroughfracturesandpermeablehorizons,or it canbe waterwhichwas

buriedalongwiththehostsediments(formationor connatewaters).Othersourcesof

waterin geothermalsystemshavebeensuggested;theseincludewatersevolvedin

metamorphism(metamorphicwaters)and from magmas(juvenilewaters),but the

importanceof thesesourcesof wateris uncertain.

indicatethatunrealisticallylargevolumesof rockwouldhavetobe leachedoverthe

lifetimeof ageothermalsystem.A smallbutsignificantmagmaticcontributiontothe

geothermalfluid is thereforethoughtto be likely. Density differenceswould,

however,precludeany intimatemixingbetweenmeteoricwatersanda magmatic

brine. If smallpulsesof magmaticbrinedid enterthegeothermalconvectioncell

then,whilenotdetectableisotopically,theywouldmakea majorcontributionto the

solutecomposition.Suchbrineswouldbeattemperaturesin excessof 400.C andbe

rich in solutessuchasCI, SOz andCOz. Althoughtheextentto whichmixing may

occuris uncertain,recentanalyticaladvancesnow makeit possibleto distinguish

betweentheisotopesof chlorineandboron. Informationfrom thesemay isotopes

mayenableamodelof magmaticbrine-meteoricwatermixingtobederived.

Magmaswereinitiallythoughttobethesourceof theheat,waterandsolutesof

geothermalsystems.However,thisneatmodelwasradicallychangedin theearly

1960'swhenit wasdemonstratedthatthefluidswereof dominantlymeteoricorigin,

andsolutescould be derivedfrom rock-waterreactions. Work on the isotopic

signatureof thefluids by Craig (1963)showedthattheyhadthesamedeuterium

signatureas thatof localmeteoricwaterandcouldnotbemagmatic.In a seriesof

now classicstudiesin aqueousgeochemistry,Ellis and Mahon(1964, 1967)and

Mahon(1967)demonstratedthatall thesolutesingeothermalfluidscouldbederived

fromreactionsbetweenthe meteoricgroundwaterandthe host lithologies. Later

experimentswithseawaterandbasalt(eg.BischoffetaI.,1981)producedsolutionsof

similarchemistrytoseawater-influencedgeothermalsystemssuchasthosein Iceland.

Rock-waterreactionis thereforethoughtto be themajorsourcefor manyof the

solutes,althoughtheymay also be contrihutedby mixingwith formationwaters,

seawaterormagmaticbrine.

Evolutionof geothermalnuids

Bearingin mindthepossibleadditionalsourcesof boththefluid and solutes

discussedabove,theevolutionof geothermalfluids in dynamic,liquid-dominated

systemscanbesummarisedasfollows. Meteoricwaterspenetratethecrustthrough

permeablezonesandcirculateto depthsof up to around5-7km. As theydescend,

theyareheated,reactwiththehostrocksandrisebyconvection.Thesedeepwaters

aretheprimarygeothermalchloridefluid andall othertypesof geothermalwaterare

deriveddirectlyor indirectlyfromthesechloridewaters.At depth,thefluidstypically

contain1000-10,000mglkgCI attemperaturesof about350.C. The"soluble-group"elementsarethefirst to be leachedfromthehostrocksby thewaters,followedby

otherelementswhicharecontrolledby temperature-dependentreactions(seeChapter

2). TIlesereactionschangetheprimarymineralogyof thehostrockstoa distinctive

alterationassemblagecharacteristicof thefluid andits temperature.TIle fluids are

retainedwithin a permeablehorizonforminga reservoirin which mineral-fluid

equilibria,and a suiteof secondaryalterationminerals,are established.As the

chloridefluidsleavethereservoirandascendtothesurfacetheymayboil to createa

two-phase(steam+liquid)boilingzone.Theresidualchloridewatercandischargeat

thesurfacein hotspringsor travellaterallyto finallyemergemanykilometresfrom

theupflowzone. The vapoursfromthis boilingzonemaymigrateto the surface

While thereis no doubtthatthe geothermalfluids are of a predominantly

meteoricorigin,thereis sufficientlatitudein theisotopedatato permit 5-10%of the

fluidtobefromanalternativesource,possiblyamagmaticbrine.Mixingwithevena

smallamountof magmaticbrinewouldsignificantlyaffectthechemistryof thefinal

geothermalfluid,andisotopedeterminationscannotdiscountamagmaticcontributionsubsequentlydilutedby meteoricwaters. Ilowever,massbalanceconsiderations

usingtypicalvaluesfor:

independentlyof the liquid phase and discharge as fumaroles. Alternatively, the

vapours may dissolve in groundwatersor condense in the cooler ground to form

steam-heated,acid sulphateand/or bicarhonatewaters. The conceptualstructure of

geothermal systems, and the inter-relationship of the fluid types, is discussed in

Section 1.3and illustratedin Figures 1.4-1.6.

criticalpoint374.136'C --7

2099kJ/kg

Evolution of steam: boiling point-depth relations

As a geothermalfluid ascendstowards thesurface,thepressureimposedupon it

by the overlying column of water (hydrostatic pressure)will decrease. Eventually,

the pressurewill drop to a level which permits the dissolved gases and steam to

separatefrom the liquid phase. TIlis phase separation is commonly referred to as

"hoiling". It is one of themost importantprocessescontrolling thechemistryof liquid

andvapour (ie. waterandsteam)discharges.

Enthalpy-temperatureand enthaIpy-temperature-pressure-density relationships

are shown in Figs 1.1 and 1.2, which summarise thedata presentedin steamtahles

(Appendix 1). TIle relationshiphetweenhoiling point and depth has been described

by Haas (1971) and is illustrated in Fig. 1.3. The curve indicates the maximumtemperaturea fluid can attain at any given depth (or pressure),and thereforeshows

the depth at which a reservoir fluid at a given temperaturewill commence boiling.

From this depth the boiling, or two-phase zone, can extend upwards towards the

surface. The curve assumesthat only hydrostaticpressureacts upon the fluid. In

practice however, it has been found that hydrodynamic pressuresexist at depth in a

geothermalsystemat about 10% above hydrostatic. This excesspressureis necessary

to maintain flow through the system. It is createdby the huoyancy of hot water

relative to cold water rechargeand by a hydrostatichead in rechargewaters from

areas of greaterrelief (Grant et aI., 19H2; lien Icy, 19H5). This means that higher

temperaturescan exist at shallower depthsthan indicatedhy the curve, and therefore

thatboiling will occur atshallower depths.

00 50 100 150 200 250

Temperature (C)

300 350 400

li'igureLL Enthalpy-temperaturerelationship for saturatedwatervapour and liquidwater to the critical point (374.136C). Note the narrow range of enthalpy change

with increasingtemperaturefor watervapour ("steam")comparedwith that of liquidwater,and themaximumplateauof 2R04kl/kg at 227-242C(Appendix I).

The relationship shown in Fig. 1.3 is for purewater. An increasein thesalinity

of water lowers the vapour pressureof water, raises the curve and preventshoiling

until shallower depthsare attained(Sulton and McNabb, 1977). However, for most

geothermal systems, the fluids are dilute and small changes in salinity will not

significantly alter the boiling point-depth profile of the system. More significant

however, is thegas contentof the fluid. The presenceof severalwt% gas in the fluid

will depresstheisothermsin asystembelowtheusual'wiling point-depthcurve.

This means that boiling zones in high-gas systems will appearat far greater depths

than for gas-poor systems,which follow the relationship for pure water. This is

demonstratedin Fig. 1.3 for a fluid containing 4.4wt% CO2 (such as that found at

Ohaki-Sroadlands, New Zealand). TIlis depressionof the boiling point is caused by

an increase in the vapour pressureof the fluid, and this is createdby the additional

pressureof the dissolved gases. (A higher gas content requires greater confining

pressureto preventthe gasesfrom exsolving from solution.) Production from a gas-

rich field can rapidly depressurisethe systemas thegasesare removedfrom the fluid,

3000

2600

2200

2000

0;.:x:">

1600>-0.ro.cC 1200ill

800

400

10 GeothermalFluids l.COlllclllJa' ,,>YMCI"S 11

and this drop in pressuremay permit the entry of cold groundwaters into the field.

Boiling leads to a drop in the temperatureof the residual liquid as steamseparation

involves hoth massand enthalpyloss. Dilution with ground waterandconductionare

the othertwo main processeswhich lower fluid temperature.Fluids which arediluted

may neverattainhoiling point heforedischargeat thesurface. As noneof theoriginal

gas hasheenlost, thesewaterscan he recognisedfrom theirchemistry.

discharged. Steam reservoir temperatures are consistently around 236C, the

maximum enthalpy of dry steam, pressures are almost constant throughout the

reservoir (White, 1970; White et aI., 1971)and gas contentswithin the vapour are

around0.5-2.0%.Thiscompareswith-0.01-0.5%in liquid-dominatedsystems wherethevapour represents10-30%of the fluid.

0

400

~ ~~

~

enL-

12 Gcolhermal Fluids

Age of geothermalfluidsandgeothermalsystemsThe timethemeteoricwaterspendsin thegeothermalsystembeingheated,

reactingwith thehost rocks,storedin the reservoirandfinally dischargedat the

surface(the"residencetime")is difficultto determine.Omtaminationwith carhonfromseveralsourcesmakes14C-datingineffectiveandtritiumhastooshorta half-life

toheusefulexceptfor veryhriefresidencetimes.Circulationmodelsfor geothermalfluidsestimateresidencetimescanhearound10,000years,although100-1000years

mayhe moretypical(Ryhach,19HI). The residencetimeof thehrinein theSalton

Sea geothermalsystemhasheenestimatedto he 100-1000yearsfrom uranium-

thoriumdecayisotopes,with theageof thesystemitself placedat 20,000-40,000

years(ZukinetaI.,19H7).

:;f::;a:wwzE2

I fu.'l

15~~~"'h8i1 ~~ E u= I

~

~Io~N'15~t~~::;,;;;o~

'"c~.~8",:e1Oy c"'.8~ro'i5Jj

"The lifetimeof a geothermalsystemis difficult to determineandwill vary

hetweensystems. Often limitson theage of a systemare determinedindirectly

throughgeologicalrelationships.For example,Browne(1979)determinedfromthe

presenceof alterationmineralsin a clastwithina hydrothermaleruptionbrecciaof

known age thatgeothermalactivityat Kawerau,New Zealandcommencedover

200,000yearsago. A literaturereviewhy P.R.L. Browne(unpuhlished)showsthat

activegeothermalsystemsaroundtheworldareestimatedtohebetween-2,000years

(Nesjave\lir,Iceland;KristmannsdOllirandTomasson1974;StefanssonetaI., 19H3)and - 3 million yearsold (Larderello,Italy; Del Moro et aI., 19H2; Steamboat

Springs,Nevada,USA; White,1979).Althougheachgeothermalsystemwill followan individualevolutionpath,typicallifetimesmaybe around500,000years. Note

thatoverthisperiodof timethesystemneednotbecontinuouslydischargingfluids,

andit is mostlikely thatactivitywill heepisodicfollowingfluctuationsin heatflux,

sealingof fluid pathwaysand tectonicfracturingcreatingnew permeahlezones.

Duringsuchcyclesof flow, sealingandfracturethecentreof activitymaymoveas

theundergroundplumbing,andthereforehydrology,of thesystemchanges.

"'-"'"

[

~.~- g>g>W "'E5~ .8 ~N:;:3;:E J!!g~ ~~~, .['5::;';s ;,jE

,,~:.!.~

;;;roo>a2' ~~

1.3THERMAL, HYDROLOGICAL ANDCHEMICAL STRUCTURE :;0~ :'~SE "'-"'1\!+ 0~1'!:c.~

r;;.;-~' wz.Q z2~ 2~H ~H!;;?6~w=

~~rncS~Q."~13~eH

The emphasisof this textis on dynamic,high-temperature,liquid-dominated

systems.Suchsystemscommonlyhaveamagmaticheatsourceatdepthsof -Hkmor

more. The magmaitselfmustheconvectingto keeptheuppersectionsmolten,and

therehyprovidea continuoussourceof heatto drive the geothermalconvection

8-5. (/J

~ t::,...; 'OJ -'"::I '0OD.-.- ;::3~tH

14 OcothennalI'luids

descent,geothermalfluid formation,(replenishedby meteoricwatersdescending

from the rechargezone)and surfacedischargeof geothermalwatersandvapours

throughspringsand fumaroles.

However, fluid flow from depthis unlikelyto follow the idealisedvertical

pathshownin Figures1.4and1.5,andsomedegreeof lateralflow is probable.Once

thegeothermalfluid is a few hundredmetresfromthesurface,localtopographicandstructuralinfluenceswill exertanimportantcontrolonthedirectionof flow andarea

of discharge.

The thermal,hydrologicalandchemicalstructureof high-temperaturedynamic

systemsareillustratedin theconceptualmodelsof Figures1.4-1.6.TIle hydrologyand distributionof dischargefeaturesis controlledby topographyand permeable

structures,egofaultsandtheconduitsproducedby hydrothermaleruptions.Vapour-

fed dischargefeaturesoccupy highergroundthanthechloride-watersprings,and

includefumaroles,hotspringsdischargingsteam-heatedwatersandsteamingground.

Thesecan form aboveboiling zoneswherecarbondioxideandhydrogensulphide

dissolveinto groundwatersor steamcondensates.Bicarbonatewaters(CO2-rich

steamheatedwaters)are also commonon the marginsof many fields.

Condensationof volcanicgasesat elevatedlevels in andesiticterraincreateacidic

chloride-sulphatewaters.Thesecanfilterbackdownintothesystemas is seenin

manyfieldsin thePhilippines.

fE

w~t'i~N1'1;:..

~Io~

!i;!~

[2~;:~3!~.~05.

Liquid-dominatedsystems

Many systemsdisplay lateralflow structurescreatedby stronghydraulic

gradients.Theseis turnareformeddueto highrelief,oftenwitha near-surfacelow-

permeabilityhorizon. Coolingby conductionandgroundwatermixingarereflected

in thechemistryof thedischarges.Evenin low-relief-250m) settings,including

thosetypicalof silicic volcanicterrain(eg.Taupo VolcanicZone,New Zealand),near-surfacelateralflowscanextendfor severalkilometres.This is greatlyextended

in terrainof high relief(>-100001),typicalof andesiticvolcanoes,wherelowsare

to-SOkOlin length.

Low-relief(Fig. 1.4)

Such systemsarecharacterisedby springsandpoolsof chloridewater. The

deepgeothermalfluidcanattainthesurface,oftencloseto theupflowarea,because

of thegentletopography.Lateralflowis possiblebutis notasextensiveasinareasof

z0

~~~wa:!i;!z--- 1C:E2 I

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16 GeolhcnnalFluids

high relief. Two-phase or steamzones arepresentbut are not asthick as in high-relief

systems(describedbelow). However, thesesteamzones can increasein depthwhen

fluid removal on exploitation of the system exceeds natural fluid recharge,as has

happenedatWairakei,New Zealand. Oxidationof hydrogensulphidegas in thesteam, together with condensation or mixing of the steam with ground waters,

produced acidic sulphate waters. Condensation of carbon dioxide, which is less

soluble than hydrogen sulphide, produces bicarbonate-rich waters which are often

found on the margins on the field. Becauseof the low relief over these systems,

chloride waterhot-springs, sulphatewaterhot-springs,bicarhonatewater hot-springs,

fumaroles and steaming ground often occur in relatively close proximity to one

another. These types of system are found in New Zealand, USA, East Africa and

Iceland, andmany occur in tectonicrift sellings.

IliKh-relief(FiK' 1.5)Common in island-arcsellings with characteristicandesiticvulcanism, thesteep

topographyover thesesystemspreventsthe chloride fluid from reachingthesurface.

Large lateral flows, often over IOkm, are not unusual. Over this distancethechloride

fluid can be diluted with groundwater or mix with descendingsulphate waters and

steamcondensates.TIlese acid watersareproducedin a two-phasezone oftenseveral

hundred metres deep, where steam condenses and/or mixes with groundwaters.

Fumaroles, steaming ground and acidic sulphate-water 1101-springsare common

surface discharge features near Ihe upflow zone. The springs are fcd from thc

condensatelayers which lie nearthe surface,perchedabove thechloride water flows.

These acid waters can alsoflow laterallyto emergedown-slope as IlOt-springs,or

descendinlo the system through fracturesto mix wilh theascendingchloride waters.

Examples of thesesyslems are found in Indonesia,Taiwan, Japan ami thePhilippines.

w-

[Z150",N",;q'g2C"-.c;s

Vapour-dominated systems(Fig. 1.6)

Fumaroles, steaming ground and acid sulphate-water IlOt-springs are the

characteristicdischarge features of these systems. TIle reservoir is composed of

steam(with gases), although saline, hoiling water feeding steam into the reservoir

probably occurs at depth. Vapour-dominated reservoirs show a relatively constant

temperaturewith depth of about 236C,(Ihe temperatureof maximum enthalpy ofsaturatedsteam; Appendix I). The pressureprofile of the reservoir is controlled hy

steam(steam-static)and is similarly relatively constantwith depth. The system is...,n ,;,.;"" I'm", d..nlh nile!l1owinl'laterallyalongthe

",'"N~

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baseof thecappinglow-permeabilityrockhorizon.The steamcoolsas it flows and

eventuallycondensesanddescendsintothedeepreservoirfor recirculation.As less-

solublegasesremainconcentratedin thesteamphasemorereadilythanthemore-

soluble gases,the chemistryof the steamchangeswith the lateral flow and

condensation(Chapter3). Oxidationof hydrogensulphidein thestearnwill produceacid condensateswhich dissolvethehost rock, therebyincreasingthesize of the

steamreservoiras the systemmatures.Vapour-dominatedsystemsare much less

commonthanliquid-dominatedsystemsandonly threehavebeenwell characterised:

TheGeysers,California,USA; Larderello,ItalyandKawahKamojang,Indonesia.

CIIAIYI'ER 2

WATER CHEMISTRY

2.1WATER TYPES

The mostcommontypeof fluid foundatdepthin high-temperaturegeothermal

systemsis of near-neutralpH, with chlorideas thedominantanion. Otherwaters

encounteredwithintheprofileof a geothermalfieldarecommonlyderivedfromthis

deepfluid as a consequenceof chemicalor physicalprocesses.Thesewaters,the

characteristicsof whicharedescribedhelow,areclassifiedaccordingto thedominant

anions. Althoughnota formalgeneticscheme,thisdescriptiveclassificationdoes

permitsomegeneralisationstobemadeonthelikelyoriginsof thewaters.Examples

of thecompositionof thedifferentwatertypesaregiveninTable2.1.

Chloride

Occurrence.This watertype,alsotermed"alkali-chloride"or "neutral-chloride",is

typicalof thedeepgeothermalfluid foundin mosthigh-temperaturesystems.Areas

whichcontainhot,large-flowspringswiththegreatestCI concentrationarefedmore

directlyfrom the deepreservoir,and identifypermeablezoneswithin the field.

However,theseareasmaynotnecessarilyoverliethemajorupflowzone sincethe

local topographycan exerta significantcontrolon the hydrology,as shown in

Chapter1.

Surfacefeatures.Chloridefluid is commonlydischargedfromhotspringsandpools

of good flow, andfrommostgeysers.The waterin deeppoolsappearsclearand

blue-greenincolour- adistinctivefeaturesof chloridewaters.

Chemistry.Chlorideis thedominantanion,andusuallyattainsconcentrationsin the

thousandsof mg/kgrange,up to ahout10,000mg/kg. Less commonly,CI levels

exceedtOO,()(){)mg/kgin moresalinesystems(eg.SaltonSea,California,USA). In

suchsystemsformationwatersorseawatermayhavemixedwiththeoriginalchloride

fluid. Othermain constituentsincludesodiumand potassium(often in a -10:1

concentrationratio),astheprincipalcations,withsignificantconcentrationsof silica

(higherconcentrationswithincreasingtemperatureatdepth)andboron. Sulphateand

bicarbonateconcentrationsare variable, but are commonlyseveral orders of