GEOCHEMISTRY IN GEOTHERMAL EXPLORATION

Argeo-C2 Conference24th-28th November 2008

Entebbe, Uganda

James Wambugu – KenGen

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Role of Geochemistry in Geothermal Exploration

Mapping, sampling & chemical analysis of surface dischargesChemical data interpretation Discharges include: water (hot/cold), fumarole steamNon- manifestation areas – Soil gas surveySoil gas survey include measuring chemical components associated with geothermal activity (CO2 gas, Hg, Rn-222 radioactivity)

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Objectives of Geochemical Work

To gather enough geochemical data which would be adequate to address the following questions:Availability and extent of a geothermal resourceLocate possible drilling targetsCharacterize geothermal fluids using the chemistryPredict prevailing deep fluid temp.

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Geothermal Manifestations

Geothermal indicators occur in form of the following:FumarolesHot (boiling) springsMudpoolsHot altered groundsGeysers

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Steam jets

Steam jets at L. Bogoria which sprout like geysersFound at the western edge of the lakeThey are at local boiling temp (98 oC)Famous tourist attractionBoiling steam jets

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Fumarole occurrences

Sometimes fumaroles occur in extremely difficult terrainStrong fumaroles are associated with mineral alteration productsDischarging fumaroles

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Altered grounds

Some areas are characterized by hot altered grounds with no visible dischargesSuch areas could be having buried fumarolic activity

Altered grounds

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Case example

Surface exploration in Menengai Area in Nakuru District – Kenya.Area characterised by very few surface geothermal manifestations- only a few fumaroles inside the main Menengai crater and to the NW close to caldera rimFew boreholes produce warm water located to the NW and SW of craterCold water producing boreholes in the eastern side

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Geochemical Data Collection

Field work started on 25th January 2004Work divided into three parts:Water sampling (B/holes, springs etc)Fumarole steam and condensate samplingSoil gas survey (for Rn-222, CO2, temp)

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Fumarole /borehore location

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S OLAI

B AN I TAS IS AL ES TA TE

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kabaragi

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A TH IN A I SI SA L ES TA TE

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OL' RONGAI HILLS

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KA MP IYA M OTO

M OGOTI O

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Railway line

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Lake, Pond

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pyroclastic issuecrates

Lava eruption center

Major (rift scarp) faultRift floorfault

inferred fault

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Soil gas survey sample points

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kabaragi

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Fumarole sampling

Gases collected in special gas sampling flasks after evacuation and charged with NaOH solution to absorb acidic gases

Evacuated gas sampling flask

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Fumarole discharges

Some manifestations are too weak and pumping is necessarySamples analysed for gases and condensate analysed for volatile components in the lab

Sampling pump

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Why work in three Parts ?

1. Fumaroles: results help in computing reservoir temp at depth where the steam is being formed

2. Water analysis: results used to evaluate origin of the fluids (use of isotopes), temp estimations at depth, predict scaling and corrosion problems

3. Soil gas survey : Rn-222 and CO2 in the soil gas are indicators of permeability and possible location of a reservoir. CO2 may also be used in locating buried fumarolic activities where other evidence is lacking

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Soil gas survey

CO2 gas sampled from the soil gas in the field using an Orsat apparatusCO2 absorbed in vessels containing KOH solution and measured in %Gas absorption vessels

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Chemical Analysis

Equipments used for chemical analysis include:AAS, ICPGC, ICUV-VISTitrpoprocessorsetc

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Chemical analysis

Collected samples are analyzed in the lab Water samples analyzed for all the major & minor componentsSteam condensate analyzed for volatile components

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Gas analysis

Fumarole gases analyzed using a Gas chromatographGases analyzed by GC: N2, H2, O2, CH4

CO2 and H2S analyzed by titration

GC

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Fumarole gas analysis (mmol/kg steam)Fum No.

loc date Temp oC

CO2 H2S H2 CH4 N2 O2

MF1 E1724N9975

29.1.04 60 28.28 0.1 0.3 - 1.97 0.722

MF2 E1753N9977

6.2.04 91.2 3590 0.42 0 0.34 4.69 1.39

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Steam condensate chemistry

Fum T oC pH/20oC

CO2 H2S Cl SO4 B F TDS Cond

MF1 60 6.0 33 0.17 19.5 3.45 0.17 0.19 11 20

MF2 94 6.0 38 0.17 24.1 5.1 0.07 0.96 20 50

MF3 77 7.5 88 0.03 31.4 6.7 0.12 0.13 6 10

MF4 81 5.8 143 0.17 15.6 7.0 0.14 <0.1 4 10

MF5 88 6.7 77 0.17 16 5.43 0.05 0.214 6 10

MF6 84 6.7 33 0.7 16.3 6.74 0.2 <0.1 5.6 10

MF7 73 6.9 88 0.07 33 29 0.38 0.26 12 25

MF8 90 6.9 220 0.03 34.1 8.6 0.14 1.64 8 15

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Gas ratios

Fuma CO2/H2S CO2/H2 H2/H2S H2/CO2 CO2/N2

MF1 282.8 94.3 3 0.011 14.4

MF2 8547.6 - - - 764.8

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Gas geo temp (oC)

Fuma TCO2/H2 TH2 TH2S TCO2

MF1 285.3 266.2 251.2 298.5

MF2 - - 265.5 371.6

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Carbon dioxide distribution

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K IAMUNY

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KABARAK ESTATE

MENENGAI CALDERA

ENGOSHURA FARM

KA B ATI N ICrat er s tream

BA H ATI

S OLAI

BA N IT ASI S AL ES TA TE

OLB AN I TA SW AM P & PO ND

Olbanita stream

kabaragi

K IS AN A N A

M AJ AN I M IN GI ES TA TE

AT HI N AI S I SA L E ST AT E

EL BO N WA L A

OL' RONGAI HILLS

M EN E GA I H ILL FA RM

K AM PIY A MOT O

MOGO TIO

0 10KMLEGEND

2 000Contours in meters above sea level

Roads

Railway line

Rivers

Lake, Pond

Shoppingcenter

pyroclastic issuecrates

Lava eruption center

Major (rift scarp) faultRift floorfault

inferred fault

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Radon-222 distribution

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Roads

Railway line

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Lake Pond

Shoppingcenter

pyroclastic issuecrates

Lava eruption center

Major (rift scarp) faultRift floorfault

inferred fault

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Ground temp (oC)

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K AB AT IN IC rater s tream

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S OLA I

B AN I TAS IS AL ES TA TE

OLB A NI TA S W AM P & P ON D

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kabaragi

KI SA N AN A

MA JA N I MI N GI E STA TE

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0 10KMLEGEND

200 0 Contours in meters above sea level

Roads

Railway line

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Shoppingcenter

pyroclastic issuecrates

Lava eruption center

Major (rift scarp) faultRift floorfault

inferred

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Conclusions & Recommendations

A geothermal resource exists in Menengai caldera with reservoir temp in the range of 250ºCThe high values of CO2 measured to the NW part of the caldera could suggest a possible resource area.From the geochemical data obtained during this study, the area to the south and SE of caldera do not seem to be attractive at all.Deep drilling is recommended to proof the resource

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Proposed drill sites

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kabaragi

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EL B O NW AL A

OL' RONGAI HILLS

M EN E GAI H ILL F AR M

K AM PIY A MOT O

M OGOT IO

0 10KMLEGEND

2 000 Contours in meters above sea level

Roads

Railway line

Rivers

Lake, Pond

Shoppingcenter

pyroclastic issuecrates

Lava eruption center

Major (rift scarp) faultRift floorfault

inferred fault

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Proposed Site 2

Proposed Site 1

Proposed Site 3 ?

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Menegai Caldera floor