The understanding of gas geochemical model to reduce the exploration uncertainty

Proceedings Indonesia International Geothermal Convention & Exhibition 2015

Jakarta Convention Center, Indonesia – August 19th – 21st, 2015

The Understanding of Gas Geochemical Model to Reduce the Exploration Risk; A Case

Study in Rantau Dedap

Rizal Abiyudo1, Julfi Hadi

1, Alfiady

1, Dayinta Adi Dyaksa

2 and Tom Powell

2

1Supreme Energy, Jl. Jend. Sudirman Kav. 52-53, Jakarta 12190 - Indonesia

[email protected]

Keywords: Exploration, Geochemistry modelling, Gas

Geothermometer, Fumaroles, Rantau Dedap

ABSTRACT

The variation in gas chemistry between the various

fumaroles areas in the prospect are due to boiling depletion.

Geothermal waters would boil to create a first set of

fumaroles and then the boiled, gas-depleted water outflows to the site of a second set of fumaroles where it boils again,

creating fumaroles with gas-depleted chemistry

The models are in the form of relating the gas chemistry

between two fumarole areas. It is assumed that the gas chemistry of one of these areas represents steam boiled

from an underlying high temperature liquid aquifer. The

gas chemistry of the other fumarole represents steam from a

second boiling of the residual geothermal water from the first boiling, after it has travelled some distance in the

subsurface to the second fumarole area.

The various fumaroles areas in the prospect are can be

modelled by boiling depletion model and the relation between each of fumaroles. Geothermal waters would boil

to create a first set of fumaroles and then the boiled, gas-

depleted water outflows to the site of a second set of

fumaroles where it boils again, creating fumaroles with gas-depleted chemistry.

1. INTRODUCTION

Rantau dedap is located in Lahat Regency, South Sumatra.

This place is a geothermal working area which is being explored by Supreme Energy. Thermal features occur over

an area of about 40 km2 and include the full range of

features found with high-temperature geothermal systems,

including fumaroles, steaming ground and hot springs. The location of thermal features mapped is shown in Figure 1.

Gas contents in surface steam at Rantau Dedap cover a

wide range from 0.05 to 20% by weight. Much of the

variation is due to boiling and condensation processes. The fumaroles at ‘A’ have about 1.2 wt% NCG (consistent for

three samples) and are probably most representative of

steam coming from the deep high-temperature reservoir.

Another fumarole NE of fumarole ‘A’ is generally lower than at ‘A’ (ignoring a single high value) ranging from 0.56

to 1.1%. The CO2/H2S ratio is also lower, suggesting this

steam may have undergone some earlier boiling. The lowest

gas contents (and lowest CO2/H2S ratio) are found in steam associated with the northern outflow at ‘C’. Fumaroles in

the NW of ‘A’ fumarole also have low gas contents

indicating second stage boiling, but much less H2S and H2

than ‘C’, suggesting equilibration to cooler temperatures in this part of the field. High gas contents can be produced by

condensation of steam, which is clearly occurring at

fumaroles in ESE of ‘A’ fumarole (13-24 wt %). The

condensation of steam is probably as result of passing through groundwater on passage to the surface, as indicated

by the sulphur-laden cold springs that issue at Asahan River

and Sumber Rejeki (to the north). The gas chemistry of a

single sample at fumarole in WSW of ‘A’ fumarole is

distinct from ‘A’. It has a gas content of 0.74 wt% (but

much more H2S (lower CO2/H2S) and higher H2, indicating higher reservoir temperatures here. The lower gas content

(compared to ‘A’ fumarole) may simply be the result of

increased boiling here.

2. GAS GEOTHERMOMETER FUMAROLES

Gas geothermometry temperatures for the principal

fumaroles are given in an earlier SE memo on geochemical

modelling of Rentau Dedap fumarolesi. These temperatures

are biased toward high values due to two factors:

1. The gas content in fumarole steam is often higher than

at the source due to near-surface condensation. The gas

geothermometry temperatures of the three principal

geothermometers (CO2, H2S & H2) increase with gas concentration in fumarole steam. In essence, the higher t

gas concentrations give higher the geothermometer

temperature. As a consequence, condensation of

fumarole steam will bias these geothermometers to higher temperatures.

2. The modelling procedure was biased toward fitting the

gas content of fumarole steam to the highest

temperature reservoir liquid that could be accommodated. In the modelling procedure, the initial

CO2 gas content of reservoir liquid was chosen to boil

steam that would match the CO2 concentration of the

fumarole, assuming a reasonable boiling fraction. The boiled fraction was constrained by the CO2

concentration of the fumarole steam and by the

temperature of the residual liquid after boiling. If the

temperature of the initial model liquid were too high, there would be too much CO2 in the model fumarole

steam to match the actual fumarole. It the boiled

fraction were too high (lowering the CO2 concentration

of model fumarole steam) then the temperature of the residual liquid would be too low for secondary boiling

at peripheral fumaroles, which was the original

objective of the modelling. Although these two factors

do limit the model geothermometer temperature, there still was a bias toward higher temperature, perhaps on

the order to 5-10°C.

Results of modelling yield geothermometer temperatures

for the principal prospect fumaroles given in Table 1. The hydrogen geothermometer is perhaps the least reliable of

these. Hydrogen can be enhanced above source

temperature by residence in a steam cap, and it can be

depleted by re-equilibration in highly condensed fumaroles. ‘A’fumarole show hydrogen to have the highest

temperature, suggesting these fumaroles may originate from

steam caps.

The interpreted geothermometer fumaroles are based on gas geochemical modeling to match the initial and the output of

fumarole steam chemistry. Based upon this analysis the

geothermometry at ‘A’ is about 30°C too high when

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compared to the temperature of the shallow thermal

aquifers. Further methodology to obtain the interpreted

geothermometer will be discussed in the next section.

Table 1: Geothermometer temperatures from

geochemical modelling of fumarole steam boiled from

reservoir liquid. Temperatures are in degrees

Centigrade.

Geothermometer CO2 H2S H2 Interpreted

‘A’ Fumaroles 240 244 261 210

3. REVIEW OF GAS GEOCHEMICAL MODELING

3.1 Gas Modelling Procedure

The modelling follows three steps. First, initial geothermal

liquid gas chemistry is modelled to match the fumaroles

steam chemistry, with the starting CO2 concentration

dictated by the CO2 geothermometer. The gas chemistry of this liquid is adjusted by trial and error to find reasonable

starting values for initial temperature and degree of boiling,

giving regard to gas geothermometers for the other gases.

Second, the gas chemistry and temperature of the geothermal liquid remaining after first boiling is calculated

and mixed with a fraction of unboiled, initial liquid, as

might be expected due to diffusive mixing within the

geothermal aquifer. Finally, the geothermal liquid is boiled again, at the temperature of the mixed geothermal liquid.

The degree of boiling in this final step is constrained to

prevent the remaining liquid from dropping below the

temperature of hot springs at the surface.

The starting chemistry for the model is geothermal liquid at

equilibrium with the CO2 gas geothermometer of

Giggenbach & Goguel (1989) at a specified temperature. Other gas geothermometers for this liquid have been

calculated as a guide and reference. The H2 and H2S

concentrations in the initial liquid are calculated from the

single constituent geothermometers for these gases assuming ideal gas behaviour (Giggenbach & Goguel, 1989

and Giggenbach, 1997, respectively). N2 and Ar

concentrations are assumed from air saturated groundwater

(10 ppm & 0.28 ppm, respectively). NH3 and CH4 concentrations are calculated from the ammonia breakdown

and Fischer-Tropsch geothermometers, respectively,

(Giggenbach, 1980) assuming concentrations of CO2, H2

and N2 from the other geothermometers and solubilities. The CO2 and H2 geothermometers and Ar concentration are

the same used in the Giggenbach CO2/Ar – H2/Ar

geothermometer grid.

This initial liquid is boiled to produce fumarole steam whose chemistry matches closely with that of the first

fumaroles set. In that the CO2 concentration of the

reservoir water is constrained by geothermometer

temperature, reasonable boiled steam fractions place a close constraint on possible resource temperature. The modelling

procedure selects the highest initial liquid temperature

whose CO2 concentration could be matched to the

fumaroles steam. Since most geothermometers, including CO2, predict higher gas concentrations with increasing

temperature (see Figure 7) the gas concentration of

fumarole steam limits aquifer temperature. For the

fumaroles at Rantau Dedap, this boiled fraction is assumed to range from about 1% to 15%. Steam fractions less than

10% produce gas concentrations which are still high at

expected geothermometer temperatures, which require

lowering aquifer temperature. Steam fractions greater than

15% leave behind residual liquid which is too cool to create

significant secondary boiling.

Geochemical models for fumarole pairs are presented in the

following sections of the report, ordered by “goodness” of fit. It is assumed that a good fit between model and

fumaroles gas is no more and a factor of two difference.

The overall quality of the model fit to fumaroles pair is

measured by the number of gases (excluding CO2) that make a good fit. For example, 6 of 6 is a good fit and

suggest a good correlation by boiling and gas depletion

between the fumarole pair; 3 of 6 would be a poor fit and

suggests that the fumaroles are not related to boiling of the same aquifer fluids.

3.2 Rantau Dedap Geochemical Model from Fumaroles

‘A’ to Fumaroles ‘C’

The ‘A’ fumaroles are located 2 km to the south and up the

flank of Bukit Besar from the ‘C’ fumaroles, which are

adjacent to chloride springs of the same name. The

geochemical model results are presented as a graph in

Figure 2 with model parameters shown in Table 1. The initial reservoir liquid is modelled at 210°C, yielding a CO2

gas/steam ratio close to the lowest gas ‘A’ sample, after

2.7% boiling. This temperature is significantly lower to

that predicted by the CO2/Ar-H2/Ar geothermometer for the ‘A’ fumaroles. The higher gas concentration observed at the

other ‘A’ fumaroles is assumed to be due to condensation,

the model shows that these compositions can be matched

reasonably well by 23.4% steam condensation (dashed lines in Figure 2). The model assumes that the lowest gas

fumarole is the most representative of steam boiled off the

aquifer since its composition is otherwise difficult to

explain, whereas the chemistry of the higher gas fumaroles can be explained by steam condensation of the low gas

fumarole steam. The low gas fumarole may have also

experienced some degree of condensation, but the amount

of condensation cannot be determined, so its composition is

assumed to be representative of uncondensed fumarole

steam. A lower CO2 concentration for the ‘A’ fumaroles

steam would require a lower geothermometer temperature

or higher boiling fraction.

3. CONCLUSION

The variation in gas chemistry between the various

fumaroles areas in the prospect are due to boiling depletion.

Based on this chemistry model assumes that the lowest gas fumarole is the most representative of steam boiled off the

aquifer whereas the chemistry of the higher gas fumaroles

can be explained by steam condensation of the low gas

fumarole steam. A lower CO2 concentration steam would require a lower geothermometer temperature or higher

boiling fraction.

ACKNOWLEDGEMENTS

The authors wish to thank the management of Supreme Energy Rantau Dedap (SERD) for the permission to publish

this work. The invaluable suggestions and the quality of the

operational work from Subsurface & Engineering

Department have been much appreciated.

REFERENCES

Geochemical Model of the Rentau Dedap Prospect, SE memo, dated 18 Oct 2013.

Giggenbach, W.F, 1997, The origin and evolution of fluids

in magmatic hydrothermal systems, In Geochemistry



of Hydrothermal Ore Deposits, 3rd edition, H.L.

Barnes ed., John Wiley & Sons, NY, June 1997.

SKM, 2012, Rantau Dedap Geothermal Field – Conceptual

Model, Internal draft report to Supreme Energy,

dated March 2012.

Figure 1. Map of thermal areas in the Rantau Dedap Prospect

Table 1. Model parameters for ‘A” to ‘C’ secondary boiling model

Temp (0C)

Mass Fraction

Gas / Steam Ratio - X10^6

CO2 H2S NH3 Ar N2 H2 CH4

Equilibrium Res Liquid 210 123 3.3 14.4 0.28 10 0.8 1.4

Initial Input Liquid 123 4.2 0.38 0.05 2.3 0.9 0.2

First Boil Vapour 210 2.7% 4056 144 2.1 1.8 84 33 7.2

Residual Liquid 199 14 1.2 0.33 0.0010 0.029 0.017 0.005

Mix Unboiled Liquid 185 20.0% 36 1.8 0.34 0.011 0.48 0.19 0.044

Second Boil Vapour 200 20.0% 177 8.5 1.1 0.05 2.4 1.0 0.22

Residual Liquid 99 0.47 0.07 0.16 0.00002 0.00059 0.00037 0.00011

Table 2. Model results for ‘A” to ‘C’ secondary boiling model

Sample

Name

Gas/Steam Ratio – X10^6 % air

contamination CO2 H2S NH3 Ar N2 H2 CH4

A 96 4987 127 2.2 148 30 8.7 NA

A 08 4000 105 2.3 1.6 80 37 7.1 1.24

A 08 5088 178 3.7 2.8 74 48 9.7 1.17

C 96 182 33 1.4 4.4 1.4 0.066 NA

C 08 365 30 1.5 0.14 2.1 0.52 12.0

C 08 329 46 2.4 0.12 2.9 1.04 40.0



4

Figure 2. Results of geochemical modelling of the ‘A’ and ‘C’ fumaroles

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The understanding of gas geochemical model to reduce the exploration uncertainty