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Hydrogeological, hydrogeochemical and isotope geochemical
features of the geothermal waters in Tekkehamam (Denizli) and
environs
Elif Ece
a
Yılmaz ,
Nevzat
b
Özgür
a Süleyman
Demirel University, Graduate School of Applied and Natural Sciences, Isparta, Turkey
b Süleyman Demirel University, Faculty of Engineering, Department of Geological Engineering,
Isparta, Turkey
ABSTRACT
From Early to Middle Miocene, the continental rift zones of the Büyük Menderes, the Küçük Menderes and the Gediz were formed by extensional tectonic features, which generally strike E-W and are represented by a great number of geothermal waters, epithermal Hg,
Sb and Au mineralizations, and volcanic rocks of Middle Miocene to recent age. The geothermal waters and epithermal mineralizations are related to faults, which strike preferentially NW-SE and NE-SW and are located transversely to the general strike of the rift zones.
These faults are probably generated by compressional tectonic stress, which leads to the deformation of uplift between two extensional rift zones. One of these continental rift zones is the rıft zone of the Büyük Menderes which is ascribed to a great number of geothermal
waters such as those issuing in very important locations of Kızıldere, Tekkehamam, Salavatlı, Germencik and others with a geothermal capacity of 860 MWe in the next future. The geothermal waters of Tekkehamam and surroundings are identified to belong to the
Na+K>Ca>Na and HCO3>SO4>CI facies. According to the Cl-SO4-HCO3 diagram the geothermal waters might be heated by a magmatic source due to the high content of sulfate and boron in geothermal waters. Geochemical thermometers were applied to the collected
samples in the region. According to the Na-K-Mg diagram (1), part of the geothermal waters can be considered as equilibrated geothermal waters. According to the results of geochemical thermometers, the reservoir temperatures of geothermal waters range from 160 to
250°C. The δ2H values of geothermal waters are between -61.9 to -51.8, while δ18O values range from -9.23 to -5.84. The tritium contents of geothermal waters are between 0.7 to 3.3 TU. These results show that there is no mixing with cold groundwaters.
1. INTRODUCTION
The Tekkehamam geothermal field is located in the southern part of the continental rift zone of the Büyük Menderes within the
Menderes Massive of Western Anatolia, and forms the one of the important geothermal areas (Figure 1 and 2). The aim of this
study is (i) to update the geological setting of Tekkehamam anf surroundings, (ii) to describe fluid-rock interaction in the study
area, (iii) to investigate the formation and development of the geothermal waters by hydrogeological, hydrogeochemical and
isotopic methods, and (iv) to develop an hydrogeological modelling of the geothermal waters in the investigated area.
Figure 1. Continental rift zones of the rift zones of the Gediz, Küçük Menderes ve Büyük Menderes in the Menderes
Massif1.
3.2.2 Hydrogeochemistry
To understand the hydrogeochemical features of the study area, 4 samples of geothermal waters were taken from different
localities representing the total area. The parameters measured in-situ were temperature, pH, Eh, dissolved O2, electrical
conductivity, and alkalinity. In summary, the temperature ranges from 56.3 - 95 °C, pH is between 6.55 and 9.00 and electrical
conductivity ranges from 3510 - 4410 μS/cm2. The cations, Na+, K+, Ca2+, Mg2+, SiO2 and B3+ were analyzed using ICP-OES,
while the anions F-, Cl-, SO42-, and NO3- were analyzed using Ion-chromatography. The HCO3- and CO32- were calculated from
the alkalinity measurements in the field. For the geochemical analysis of the results we have used the Aquachem v.3.7
software2,6. The geothermal waters in Tekkehamam and surroundings can be considered as Na-SO4-HCO3 type waters (Figure
4)2. The Cl-SO4-HCO3 ternary diagram was used to classify the geothermal fluids on the basis of the major anion
concentrations6 and shows that the waters of the study area plot on the bordering water region2, and are bicarbonate waters. It
shows that the water is most likely related to groundwater heated by steam in the deeper reservoir. Hence, it may not give the
best predictive result of the reservoir temperature using geothermometers. The Na+K-Mg-Ca ternary diagram2 of the study area
shows that Na+K are the predominant catoins (Figure 5). This is expected because Na+ contents of water increase with
temperatures while Ca2+ and Mg2+ contents decrease, explaining the low values of Ca2+ and Mg2+ in the geothermal waters of
the study area. The saturation index of some carbonates (commonly aragonite, calcite, and dolomite) and chalcedony help us
to estimate which one of these minerals may precipitate during the extraction and use of the geothermal fluids. These
calculations are useful in predicting the presence of reactive minerals and estimating mineral reactivity in a groundwater
system. Saturation index also help us to evaluate the chemical equilibrium between fluid and rock in a geothermal system. This
is accomplished by gathering information about the solubility of minerals in rocks that have undergone hydrothermal alteration
and about the activity of the mineral type in the solution5. Because of the large number of ions, ion-pairs and complexes in the
solution, generating the saturation index for each type as well as activities requires the use of a software program5. The
saturation index of the geothermal waters, for a given mineral, were calculated at the discharge temperature as well as
considering the simulation with increase temperature and measured pH values. Aragonite, calcite, dolomite and chalcedony are
oversaturated at discharge temperatures2. According to this saturation index, scaling of carbonate minerals is expected for the
geothermal waters and this agrees with field observation as waters from deep wells cause scaling during extraction. Inhibitors
are employed in the prevention of scaling in the drill holes.The hydrogeochemical results of the geothermal waters from the
study area were evaluated using cation geothermometers (Na-K, Na-K-Ca, and Na-K-Ca with Mg correction2 in order to
understand the reservoir temperature of the geothermal field. According to the above mentioned cation geothermometers, the
geothermal waters of Tekkehamam and surroundings have reservoir temperatures between 160 and 250 °C.
3.3.3 Isotope geochemistry
2. MATERIAL AND METHODS
In the study area, sampling from the geothermal hot springs and
geothermal wells and in-situ measurements such as
coordinates, temperature, pH, Eh (mV), dissolved oxygen (mg/l),
electrical conductivity (S/cm) and alkalinity were realized.2,3.
In the field, the pH values of the water samples for cation
analyses were adjusted in an interval between 2 and 3 by
dropping of pure HNO3.
The samples were analysed for cations and anions in the
Laboratory of the Mineral Research and Exploaration Institute,
Ankara, Turkey and for stabile istopes (18O ve 2H) and tritium
(3H) analyses in the Isotech Laboratories, Inc. (Illionis, ABD).
Figure 2. Continental rift zones of the rift zones of the
Gediz, Küçük Menderes ve Büyük Menderes
Menderes1
.
Figure 4. The geothermal waters of the study
area in Piper diagram.
Samples of the geothermal waters in Tekkehamam and surroundings
were analyzed for their 18O, 2H and tritium contents2. The mixed
groundwater-geothermal waters systems lie along the meteoric water line
whereas the high temperature geothermal waters deviate from the
meteoric groundwater line showing intense water-rock interaction under
high temperature conditions2. These data are well correlated with the
results of hydrogeochemical analyses which also indicate high water-rock
interaction and reactions with silicates. The tritium data reveal that (i) the
geothermal waters of Tekkehamam and do not contain any measurable
tritium and (ii) that both mineralized groundwaters and geothermal waters,
ascribed to the sedimentary formations, with temperatures up to 55 °C
contain atmospheric and anthropogenic tritium7. Therefore, a mixing
process between the fresh groundwater and deep geothermal water is
evidenced for the geothermal water in Babacık Pınarı.
In the diiagram of 18O versus 2H,the geothermal waters of the study
area from MWL showins an intensive water-rock interaction under high
temperature conditions (Figure 6).
3. RESULTS
3.1 GEOLOGIC SETTING
The Tekkehamam geothermal field and surroundings consist of Precambrian to Cambrian metamorphic rocks and Pliocene
sedimentary rocks. As basement rocks, metamorphic rocks of the Menderes Massive are composed of gneiss, mica schists and
the Iğdecik formation with altered mica schists, quartzites and marbles (Figure 3)2,3. Neogene sediments consisting of Kızılburun,
Sazak, Kolonkaya and Tosunlar formations overlie the metamorphic rocks discordantly which were overlaid by alluvium and
travertines during Quaternary. In the İğdecik formation, the thick marble sequence is in the upper parts of mica schists and forms
an alternation with schists and quartzites. Marbles are dark grey with light in colors, largely crystallized, good developed joints
and thin to moderate clear layers4. The metamorphic rocks are also overlain by Pliocene sedimentary rocks considered as four
lithological rock members. The Kızılburun formation overlies the metamorphic rocks of the Menderes Massive discordantly. The
thickness of this formation is about 300 m in which there are different lithological features. In the upper part of this formation, the
grain size decreases as the carbonate contents increases. In the deeper part the Kızılburun formation consists of thick and redbrown gravels, and continues with sandstones, siltstones and clay stone rocks. The Kızılburun formation can be considered as a
good geological formation for geothermal waters/exploitation, due to high contents of clay minerals. At the bottom, the Sazak
formation is composed by the alternation of clay stones, sandstones and conglomerates, silicified marls, white and yellowish
marls and lacustrine limestones. An alternation of clay stones, sandstones and conglomerates outcrop in a narrow area. The
Sazak formation has been generated in a lacustrine area with high carbonate sediments and low energy. The Sazak formation is
ascribed to a shallow geothermal reservoir in the Kızıldere geothermal area, with a depth of 800 m and a temperature of 198°C
due to the tectonic structure with faults and fissures. An age of Late Miocene to Pliocene was indicated by4 for the Sazak
formation.. The Kolonkaya formation consists of marls, siltstones, sands with gravels and weak cemented sands which display
the features of a typical fan delta. There are a great number of soft-sediment deformation structures, mainly composed by
medium grained, weak cemented sands, silts and marls. In this formation, load prints, drop structures, fire structures, debris
intrusions, disrupted layers, slump structures and synsedimentary faults can be observed.. The Tosunlar formation is widespread
in the western part of the study area and consists of multicolored red conglomerates, sandstones and fossiliferous clay stones.
Components of conglomerates are gneiss, several schists, quartzites, marbles, Mesozoic limestones and gravels and blocks of
the Sazak formation and Kolonkaya formation5.
Figure 3. . Geological map of Kızıldere and Tekkehamam..
1-Allluvium; 2-Travertine; 3-Plio-Quaternaryr; 4-Pliocene;
5-Late Miocene; 6-Miocene; 7-Metamorphic Series; 8Sampling Locations3.
3.2 Hydrogeology, hiydrogeochemistry and
isotope geochemistry
3.2.1 Hydrogeology
In the study area, Pliocene limestones and Paleozoic
marbles and quartzites obtained secondary porosity and
permeability due to faults and fractures caused by tectonic
forces2. The shallow reservoir rock consists of Sazak
formation in the Pliocene sequence. However, the lateral
facies change of Sazak formation restricts the continuity of
reservoir quality.
According to drilling data of the Kızıldere geothermal field,
the production wells KD-1, KD-1A, KD-2, KD-3, KD-4, KD-12
and KD-8 are fed from Pliocene limestones. In the
production well KD-1, the highest reservoir temperature of
198°C has been measured. The thickness of the Sazak
formation changes between 100 and 250 m where an
average temperature of 170 °C was estimated.
A second deep reservoir rock is composed by an alternation
of marbles, quartzites and schists of the İğdecik formation of
metamorphic rocks from the Menderes Massive. These
rocks show more porosity and permeability in comparison to
the previous reservoir rocks. These second reservoir rocks
have continuities for more large area and give higher
temperatures due to the reservoir depth. In the production
well of KD-12, the temperature is about 212°C. Through the
wells KD-6, KD-7, KD-9, KD-13, KD-14, KD-15, KD-16 and
KD-111, the second reservoir rocks have been reached
which thickness changing between 100 to 300 m.
A third reservoir rock consists of gneisses of Precambrian to
Cambrian age, due to the strong development of fracture
systems in the research area. The first cap rocks consist of
Kolonkaya and Tosunlar formations which overlie the Sazak
formation1. Shallow reservoir rocks are composed by an
alternation of clay stones, marls and sandstones and are a
very excellent cap rock for the first shallow reservoir. The
thickness of the first cap rocks is about 350 to 600 m. The
second cap rocks are the Kızılburun formation under the first
reservoir rocks which are composed of an alternation of
hard cemented conglomerates, sandstones and clay stones,
and form the best cap rocks. The thickness of these rocks is
between 100 and 250 m.
Figure 5. Na-K-Mg diagram of the geothermal waters
of the study area7,
4. CONCLUSIONS
Concerning the hydrogeological conceptual model of
the studied geothermal system, the meteoric waters in
the drainage area of Tekkehamam and surroundings
percolate along deep fault zones and impermeable
clastic sediments into the reaction zone of the “roof”
area of a magma chamber situated probably up to 5 km
depth, where meteoric waters are heated by the
cooling magmatic melt and ascend to the surface due
to their lower density caused by convection cells
(Figure 7)2. The volatile components such as CO2,
SO2, HCl, H2S, HB, HF, and He out of magma reach
the geothermal reservoir where equilibrium between
altered rocks, geothermal waters, and gas components
is reached. Thus, the geothermal waters ascend in the
weakness tectonic zones at the rift zone of the Büyük
Menderes as hot springs, steams, and gases8. These
geothermal waters are exploited for various uses, such
as: geothermal energy production, balneology, and
greenhouses heating
Şeki 6. 18O versus 2H diagram of the geothermal
waters of Tekkehamam and environs..
Şekil 8. Simplified hydrogeological modeling of the geothermal
waters of Tekkehamam and environs.
5. ACKNOWLEDGEMENTS
This study has been funded by the Scientific Research Coordination Office within Suleyman Demirel University, under contract
number 4137-YL1-13.
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