geochemical mapping: an...

Sustainable Development Goals Short Course IIon Exploration and Development of Geothermal Resources

By Edwin Wafula, Geochemist, KenGen

GEOCHEMICAL MAPPING: AN INTRODUCTION

10th Nov. 2017, L. Bogoria Spa

SDG Short Course II on Exploration and Development of Geothermal ResourcesOrganzied by UNU-GTP, GDC and KenGen | Lake Bogoria and Lake Naivasha | Nov 9-29, 2017

INTRODUCTION

• Exploration of a geothermal system determines it’s energypotential. Assess the following

– the size of the system– the shape of the system– the structure of the system– The capacity of the system to produce energy

• Information can be obtained in two ways.– From surface scientific surveys– Through drilling exploration wells


OBJECTIVES OF GEOCHEMICAL WORK

• To gather enough geochemical data which would be adequate to address the following questions:

1. Availability and extent of a geothermal resource2. Locate possible drilling targets3. Characterize geothermal fluids using the chemistry4. Predict prevailing deep fluid temp.


PREDICTIONS ABOUT THE DEEPER SYSTEM

• Subsurface rock associated with the hot fluid– Mapping of faults and fractures- Radon and Carbon dioxide in soil gas

fluxes– The type of system present, steam heated or chloride water– Origin of the hot fluids, direction of flow through the area– Geothermal gradient and the depth to first boiling in the system.– Mineral deposition potential of the fluid– Natural heat flow– Zones of high up flow permeability– Possibility of underground(deep) acidity.– Fluid constituents which could have economic value.


PHASES OF THE FIELD WORK

• Spring and Borehole Sampling; results used to evaluate origin of the fluids (use of isotopes), temp. estimations at depth, predict scaling and corrosion problems

• Fumarole Sampling (Condensate and gas); results help in computing reservoir temperature at depth where the steam is being formed

• Radon (Rn‐222) and Carbon Dioxide(CO2) in Soil Gas; Rn-222 and CO2in the soil gas are indicators of permeability and possible location of areservoir. CO2 may also be used in locating buried fumarolic activities where other evidence is lacking


DESKTOP PREPARATION

• Review of historical data

• Equipment lists, sample bottles, PPE

• Base maps, aerial photos, GPS coordinates

• Arrange transport, lodging, emergency contacts


CONSIDER YOUR SAFETY

• PPEs• Complete first aid kit• Emergency response plan• Emergency evacuation route plan• Medical evacuation route plan• Security • Be aware of personal limitations


HAZARDS TO CONSIDER

•Wildlife burns, chemicals, toxic gas, unstable ground, volcanic eruption, earthquakes, sun exposure, steep terrain, local community, noise, abrupt weather changes


TOOLS AND EQUIPMENT

• Small equipment/tools• Field pH/EC kits• Chloride titration kit(Cl ion selective electrode, potentiometer, AgNO3)• Digital thermometer• Graduated bucket or cylinder & stopwatch • Camera • GPS

• Major equipment• Orsat apparatus• Gas flux meter• Radon Meter• Mercury vapour meter


WHILE IN THE FIELD

• Always safety first• Review possible contamination sources(sampling

equipment leaks, sampling location, steam condensate)

• Key field measurements(date/time,location,elevation,temp,EC,alteration,odors,geology,flowrate

• Notes on the area, no of features, sketch thermal area, areal extent, ground and fluid temp.


EXPLORATION TECHNIQUES

• Water surveys• Spring and well water samples are the main sampling medium of

most geothermal investigations• Surveys of freshwater courses (streams, lakes) may also be

undertaken to detect leakage of geothermal water into the drainage system.

• Gas surveys • These are undertaken on fumaroles, gas vents in pools, steaming

ground and wells.• Soil and soil-gas surveys’ principal use is to identify permeable

zones within the field, and to delineate the margins of the system.


GEOCHEMICAL SAMPLING AND ANALYSIS

• During surface exploration:– Sampling and chemical analysis of hot springs– Hot water seepages– Fumarole emissions– Gas discharges and cold surface waters– Soil gas fluxes (Radon & Carbon Dioxide)

• Aids predictions about the deeper systems.– Composition and homogeneity of hot fluids– Subsurface fluid temperatures and pressures


USEFUL THERMAL FEATURES TO SAMPLE

• Neutral chloride springs• Superheated fumaroles• Saturated fumaroles• Boiling springs( Cl>SO4>HCO3)• Warm springs (Cl,SO4,HCO3)• Bicarbonate springs(HCO3)• Acid-sulfate springs(SO4)• Acid-sulfate pools(SO4)


WHILE SAMPLING THERMAL SPRINGS

• Determine hottest and highest flow location in springs• Record all field parameters • Collect sample using peristaltic pump or hand vacuum

and water trap• Samples collected: 500 mL FU, 500 mL RU, 250mL

FAD, 250mL FA, • If chloride springs, collect multiple samples per

location


WHILE SAMPLING FUMAROLES

• Use hand vacuum• Keep end of sample line under water ( to avoid

contamination)• Fill bottle about 2/3 full with steam condensate• Do not overfill• Use water bucket to promote condensation


WHILE SAMPLING GASES

• Fill bottle to 2/3 full with condensate• If sampling gas in bubbling hot springs, slowly open/close

bottle to avoid sucking in water(bottles under vacuum)• Invert bottle so gases bubble through preservative solution• If water( not steam condensate) enters line, close bottle, re-

evacuate sample line with hand vacuum /water trap resume sampling

• Shake bottle during sampling to promote gas reaction with preservative

• Be careful with neck of bottle, do not over tighten stopcock


WHILE SAMPLING LOW SPRINGS

• Try to determine source and collect fresh sample• Record all field parameters determine level of

importance• Collect sample using peristaltic pump or hand vacuum

and water trap• Samples collected: 500 mL FU, 500 mL RU, 250mL

FAD, 250mL FA• If acidic, minimal geochemistry needed.


SOIL GAS SURVEYS

• Soil gas surveys very useful where surface discharges are absent

• These surveys can identify permeable regions in a field and possible upflow or boiling zones. They can also delineate the margins of a geothermal system

• The anomalies are produced by vapour leaking from the underlying geothermal system

• Such leakage will be greatest along permeable regions. especially faults, and may indicate directions of subsurface flow or possible upflow zones


WHILE DOING SOIL GAS SURVEY

• CO2 by far the most abundant• Usually 85 to 97% of the geothermal gas• Other major gases in appreciable concentrations (>1%)

include:–H2S, H2, N2

• Trace gases include:– Ar, CH4, He, Ne, Xe• CO2 gas sampled from the soil gas in the field using an Orsat

apparatus• CO2 absorbed in vessels containing KOH solution and

measured in %


EMISSION OF GEOTHERMAL GAS

• Two main pathways–Point sources–Diffuse degassing

• Point sources include:–Steam vents–Gas bubbling through water

• Diffuse degassing–Takes place through surface


SAMPLING GRIDS• Samples collected in a grid pattern to enable interpretation of

any anomaly• The grid should be orientated in such a way that it crosses

major geological structures, such as faults and the regional strike, and does not parallel these features.

• It should be organized so that several samples are taken from soils overlying different lithologies and different topographies, and be sufficiently large to cover the entire prospective geothermal area.

• Regional reconnaissance grids 1km by 1km, in potential geothermal areas use 250 by 250m, detailed surveys are done on 100 by 100m.


GRIDS


CARBON DIOXIDE SAMPLING

• Direct measurement • Soil-gas is pumped through a steel tube from the base of a

hole, typically 1m in depth and 1O-25 mm in diameter. • Carbon dioxide is determined in situ with an Orsat

apparatus.• The instruments are calibrated in the field, either by

determination of air or by use of a reference standard.• Replicate determinations are made until consistent gas

compositions are attained in concurrent aliquot


CO2 MEASUREMENT


CARBON DIOXIDE SAMPLING

• Gas anomaly detected as gas flux through soil or gas concentrations in soil

• Automated gas flux meters are used.• CO2 flux most commonly measured but equipment available

for CH4 and H2S measurements.• Measurement completed in 3 minutes• Measured in g/m2/day


DIFFUSE MEASUREMENT


COMPARISON


RADON MEASUREMENT• 2 isotopes of Radon, 222 Rn(238U) and 220 Rn (232Thorium)

with half lifes of 3.8 days and 54.5 seconds respectively. With their solid daughter nuclides of Polonium- 218 and Polonium-216

• These isotopes decay by alpha particle emission,· and it is the density of these particles which is measured in radon surveys, not the concentration of radon gas.


RADON MEASUREMENT

• Measurement done using a Radon meter.• Measure background values first.• Take measurements in the 1M deep hole. Counts/min.• As radon is part of the uranium and thorium decay

series, the concentration of these elements in the underlying lithologies is important.

• Radon surveys have been successful in identifying fault zones and areas of high thermal gradients


RADON MEASUREMENT


MERCURY VAPOUR MEASUREMENTS

• Applied mostly to vapour dominated systems. Less commonly used

• A set volume of soil-gas is pumped at a 1m depth• Use a mercury vapour meter.• First determine background readings.• Read off concentrations in the vapour meter.


SOIL TEMPERATURE

• Measure soil temperature

• Measurement done using a thermocouple with a long steel wire.

• Insert thermocouple into the 1m hole


DATA REPORTING• A map of sample locations showing the local or assigned names of the

geothermal features from which the samples were taken. • A table summarizing the fluid geochemistry of the sampled

geothermal features, keyed to the map. • A table summarizing the gas geochemistry of the sampled geothermal

features, keyed to the map. • A table showing the results of the geothermometry calculations. • Graphs of the geochemical data should be provided


REPORTING

• Accompanying reports should explain the inferences and conclusions drawn from the data

• Estimated resource temperature at depth;• The genesis (origin) of the resource:• The locations of different aquifers or reservoirs in two and three

dimensions;• Mixing between aquifers;• Sources of recharge to the geothermal system;• Pathways of discharge from the geothermal system; and• The potential for corrosion and/or scaling of the geothermal

fluids.


AN INITIAL CONCEPTUAL MODEL

• Initial scientific assessment- Derive a tentative model of the system and sites for exploration wells.

• Allows for an initial assessment of the energy potential of the system

• Capacity of the system to produce energy


INITIAL MODEL


Thank you

geochemical mapping: an...

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