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GRC Transactions, Vol. 35, 2011

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KeywordsGeothermal exploration, magnetotelluric, Menderes Massif, graphite, pelitic schist

ABSTRACT

Geothermal resource exploration is enjoying increased activ-ity in Western Turkey, following recent openings in geothermal permit licensing. As part of an assessment exercise of several license areas over the Menderes Massif, BM Holding has under-taken surface investigations including geology, geochemistry, and geophysics - gravity and broadband Magnetotelluric soundings (MT) - in addition to both shallow and deep drilling programs. While the Bouguer gravity anomalies respond mostly to graben structure (the dominant density contrast is between the basement and sedimentary cover) the MT soundings are sensitive to resistiv-ity variations from the near-surface, including graben fill, through the deep structures within the basement. The deep, 3D resistivity distribution and drilling results are compared from two areas of the western Menderes Massif. The interpretation of the resistivity response in this setting differs from classical applications of MT surveys over conventional hydrothermal resources of volcanic set-tings. As the basement composition varies from coherent gneisses through pelitic schists, with significant bands of graphite related to inter-nappe shear zones, the successful integration of the MT resistivity results requires the building of a structural geological / conceptual model as the basis to geothermal exploration rather than simple “anomaly hunting”. The resistivity distribution can be used to predict deep basement structure, e.g. through the mapping of this regionally continuous graphite schist marker horizon, and this deep structural information used to update the conceptual geo-structural model.

1. Introduction

The Tire and Gümüşköy geothermal areas are located in the Aydın region, Western Turkey, within the general geotectonic domain of the Menderes Massif. Various exploration disciplines

were applied in both of areas; lineaments extracted from satellite images, geological investigations, detailed structural geology including mapping, geochemical analysis and geophysics - grav-ity (legacy data) and new MT surveys. MT data was collected from 393 stations for Gümüşköy and 374 stations for Tire region. Low resistivity basement zones are evident from both areas, both in the MT impedance data and the later inversion models, whether 1D, 2D or 3D. The relationship of these anomalies to the lithology rather than geothermal fluid properties is the subject of this paper.

The Menderes Massif is a major metamorphic complex in Western Turkey bearing imprints of Precambrian and Eocene metamorphic and deformational events (Şengör et al. 1984; Boz-kurt & Oberhansli 2001). The Menderes Massif has a complex nappe-pile structure developed during the closing of Neotethys consisting of different tectonic slices including metamorphic and metaophiolitic rocks (Ring et al. 1999, 2003). Pan-African intense deformation and eclogitic metamorphism affected only the core of the Massif, followed by regional amphibolite-facies metamorphism with local anatexis (Rimmelé et al.2004; Şengör et al., 1984; Candan et al. 2001). Southwestern Turkey, including the Menderes Massif, has been in an extensional tectonic regime since Oligocene (Bozkurt and Satir 2000; Bozkurt and Oberhansli, 2001; Gessner et al., 2001); the main structures are the horst and graben systems, and their associated boundary normal faults.

Comparing the Pan African basement and Paleozoic cover units in different regions, common features are observed, despite some differences in the metamorphism stages (Konak, 2007). The cover unit is of overlying nappe sheets, observed in different sections of the Massif. These nappes contains schist and gneiss units of differing metamorphic grade - schist complex, gneiss complex, green schist, blue schist, pelitic schist and various other litho-formation names. But when considering the content of this complex, it seems that they contain same minerals like almandine garnet, muscovite, biotite, chlorite oligoclase, albite, quartz, clay minerals, and some iron minerals. According to some studies, these units are up to 1500-2000m thick, however it is cut 40-50 m thickness in the wells drilled by BM. (Konak, 2007). The units were called pelitic schist and gneiss in the wells drilled by BM.

Interpretation of 3D Magnetotelluric (MT) Surveys: Basement Conductors of the Menderes Massif, Western Turkey

Ö. C. Kuyumcu1, U. Z. Destegül Solaroglu1, S. Hallinan2, B. Çolpan1, E. Turkoglu2, and W. Soyer2

1BM Engineering & Construction Ltd., Ankara2Geosystem - WesternGeco Land EM, Milan

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2. Tire Study Area

Tire is located along the Southern margin of the Küçük Menderes Graben. Detailed structural geology and map-ping studies were completed (Koçyiğit, 2009). Basement rocks in the region are, from oldest to youngest, the Bozdağ Nappe, the Çaldağ Nappe, the Kaymakçı Nappe, the Ceviza-lan Metagranite-granodiorite, the Keskinler Mylonite and the Çakaldoğan Metagranite. Both Bozdağ and Çaldağ Nappes have 1,500m total observable

thickness. Both are very widespread units throughout the whole Menderes Massif and deformed by folding, divided into several tectonic slices, at times overlapping. The Bozdağ Nappes are high-grade metamorphic rocks comprising the lowermost part of the Menderes Massif. The Çaldağ Nappe, in general, is represented by the low-grade metamorphics comprising the upper part of the Massif. One of the other widespread units of the Menderes Massif is the Kaymakçı Nappe, dominated by the orthogneisses (augen gneisses) formed by the regional dynamometamorphism of its granitic protolith intruding both the Bozdağ and Çaldağ Nappes.

Cover rocks exposed in and adjacent to the Küçük Menderes graben are subdivided into two categories: volcanic rocks and sedimentary graben infill. The intermediate to felsic volcanics could be potentially related to a heat source, but are generally believed to be older. These volcanic rocks were named as the Başova Volcanics that occur in domes, dikes, lavas and pyro-clatites. Based on the lithofacies, sedimentary graben infills age’s and deformation patterns, the graben infill is subdivided into two categories; older and deformed graben infill, represented by a relatively thick fluvio-lacustrine sedimentary sequence and younger graben infill or modern graben infill, the youngest and widespread rock-stratigraphic unit in region: Plio-Quaternary basal to terrace deposits that forms lower and basal part and the most recent, neotectonic graben infill above.

Water samples were collected for detailed geochemical stud-ies. Temperature, PH, electrical conductivity, salinity and TDS were measured in situ using portable tools. Element analyses, Deuterium, Oxygen-18 and tritium analysis for groundwater age estimation were analyzed at mobile laboratory. Most of the ground and hot waters belong to HCO3 type and /or HCO3-SO4 type. Generally, HCO3 type and/or HCO3-SO4 type hot water are re-lated to a source reservoir origin in sedimentary rock and relatively lower temperature (less than 100°C). The Tritium, oxygen-18, deuterium isotopes and chloride concentration of all discharges in the region show that the origin of the hot waters discharging from district is a mixture of meteoric water from different altitudes

Figure 1. General litho-stratigraphy of the Kucuk Menderes Area, including approximate unit thicknesses.

Figure 2. Kucuk Menderes MT survey location over the central Menderes Massif, east of Tire. The main geological units are from Gessner (2000), tick marks are 5km, and the red NS line denotes the example cross-section in Figure 2.

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and latitudes. According to chemical character, stable isotope and tritium contents, the sampled shallow discharges in the field do not seem to be derived from a circulation in a deeper reservoir, possibly because of sealed fractures.

The 2008 and 2009 MT surveys in this area are largely located East of Tire, and inside the KMG (Figure 2). The MT data were recorded using broadband MT systems, deployed in full tensor, remote reference mode, and delivering a 7-decade frequency range covering 0.001 - 10,000 Hz. The processed recorded and processed data were analyzed for possible influence from both active and passive EM noise sources, and no obvious correlation is seen with the known power lines etc. Example MT soundings are shown in Figure 3; despite the cultural EM noise, and a slightly weaker responses in the two MT dead bands (around 2,000 and 0.1 Hz), the overall data quality is good.

Following preliminary 1D and 2D MT resistivity inversion modeling, detailed 3D inversion modeling was run to fully account for the observed multi-dimensional MT impedance variation, including 90m DEM topography, producing a coherent resistivity model covering the entire 374-station data set. The final 3D model used a 750x750m horizontal cell dimension, and had a minimum

vertical dimension of 25m throughout the range of topography. The final total model dimension was 127x72km laterally by 57km depth, with a total of 119x46x211 cells in the strike, dip, and verti-cal directions respectively. The 3D inversions were run “blind”, starting from a 20 ohm.m halfspace, and included residual “Static Shift” as an inversion parameter.

A north-south section in Figure 4 illustrates the main features of the 3D resistivity model and includes the Tire-1 well drilled during the MT survey period.

The Kucuk Menderes graben, mapped at surface over the northern part of the profile, is imaged as a shallow, relatively conductive feature overlying a resistive basement corresponding to the metamorphic nappes mapped at outcrop (Figure 2). The Çine nappe is observed as high resistivity structure southeast of Kiraz. The major resistivity feature, however, as predicted from

a quick examination of the MT soundings (e.g. Figure 3) is a widespread intra-basement conductor dipping down to the north, and to the west. The shallowest part of the deep conductive anomaly is south of Ödemiş and within the Bozdağ or underlying Bayındır nappe. The con-ductor does not outcrop at surface in the MT survey area, but follows the general trend of the inter-nappe thrust planes, or shearing, predicted from the structural studies of the area (e.g. Gessner et al., 2000). When examined in detail there are consistent lateral discontinuities in the deep conductor, which we interpret as major basement faults post-dating the nappe emplacement.

The 3D inversion reveals eastward continuation of the bowed (syncline-like) deep conductor. Taken independently, a striking conductive anomaly like this might look appealing as a “hot fluid” geothermal drill target. However, this region was known to have shear zones, some with graphitic units, and the likely non-exclusive nature of the deep conduc-tors (e.g. Geosystem report, 2009).

2.1 Drilling and ResultsThe first deep exploration well, Tire-

1, was drilled to a depth of 2,325m in the second half of 2009, and resulted with a temperature of (only) 85°C and 4 lt/min flow rate. The nearby Mescitli well was later drilled to 585 m depth, also in 2009. A shallow gradient well was drilled to 92 m on February 2010. Tire–1 drilled

into the Bozdağ and Bayındır nappes, within metapelites and schists as well as gneiss and marbles. Examining the blind 3D MT inversion model section closer (Figure 5 and 6) the resistiv-ity reduction observed at ~-500m remained constant till -1400m, and then rapidly decreased to very low resistivity values (1–5 Ώm) at around -1400m.

Figure 3. Example MT soundings from Tire survey sites 247 (LHS) and 205 (RHS), located on Figure 4. Data are rotated to Principal axes. Bold colours are Zxy and Zyx, whereas faint data are the weaker Zxx and Zyy impedances.

Figure 4. North-South cross-section through 3D MT resistivity inversion model, located in Figure 2.

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Samples were col-lected from the well at variable depths, and sent to the General Directorate of Mineral Research and Exploration(MTA) for XRD analyses. According to the XRD results, quartz, illite, mica minerals, feld-spar, chlorite, kaolinite minerals, siderites, mag-netite at different rates were observed. The well log contains quartz-schist, gneiss, pelitic gneiss, pelit-ic schist were seen similar to XRD results. The pelitic schists contain almandine garnet, muscovite, bio-tite, chlorite oligoclas, albite, quartz and opaque minerals. In other words, the minerals observed in XRD results are likely to be the pelitic schists identified during drilling. Graphite rich mud returns were recovered from the lower sections, but neither high porosity nor obvi-ously conductive fluids were noted. The returned graphite is consistent with graphite bands mapped at outcropping metamorphic

basement elsewhere (e.g. Kurt and Eren, 2000), drilled in the deep KTB Borehole of Germany (Bram et al., 1995, Hauk et al., 1997), and discussed in several studies as a likely cause for deep crustal conductors, particularly in tectonic regimes (e.g. Simpson, 1999; Ádám, 2005). Although the pelitic schists are more common in the deeper zones of Tire-1, where the MT resistivity decreases, the significant graphite returns in the drilling mud herald the most likely cause of the basement conductor.

Conclusively, the highly conductive layer is not directly related to an active, prospective geothermal system. It appears that pelitic schist and gneiss formations, with graphite, may be resulting in low resistivity. This result also corresponds to alteration report results, which outlines the hydrothermal alterations observed in Tire area were not related with active geothermal system. As a conclusion, it was indicated that, the region was not promising in terms of any geothermal system.

3. Gümüsköy Study Area

The Gümüşköy & Ortaklar study area is located on the West side of the the Büyük Menderes Graben in Aydın, Turkey. The E-W-trending Menderes graben is divided into several sub-grabens and sub-horsts along its western tip around Ortaklar, Gümüşköy, Argavlı, and Kirazlı. According to heat flow map, these regions have high heat flow about 100-110 mW/m2 (Bal, 2004). Our studies here include; lineaments from remote sensing studies, structural geology and mapping studies, alteration, geochemis-try, hydrogeology, geophysics (gravity, MT, and VES), 2D GIS reservoir modeling and 3D reservoir modeling using existing well information.

The stratigraphy of the study area can be subdivided into three different groups (Figure 7). The oldest group, the Pan

African basement, which can only be seen to the south of the Büyük Menderes Graben is defined by augen gneisses. The second group, Cy-cladic Metamorphic Complex (CMC), mainly consist of metasedimentary rocks ranging in age from Palaeozoic to Tertiary (Bozkurt a n d O b e r h ä n s l i 2001; Okay 2001). The third group consists of Neo-gene and younger sedimentary rocks that unconformably overlie older meta-morphic units.

The geology of the study area and around, called as

Figure 5. 3D Section through Tire-1.

Figure 6. Lithology and XRD locations. Figure 7. General Stratigraphy of Gümüsköy Area.

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CMC commonly, is separated into four different nappes shown in Figure 7 in detail which are from base to top; a) Dipburun Nappe, b) Efes Nappe, c) Şirince Nappe, d) Bodrum Nappe (Çakmakoğlu, 2007).

Dipburun Nappe does not crop out in the study area. The Efes Nappe rests tectonically on the Dipburun Nappe in the Dilek Peninsula and comprises Late Paleozoic-Early Triassic (?) schist, gneiss and marble alternations (The Meryemana formation) at the base, and Triassic-Late Cretaceous (?) marbles (The Ayrıcadağ formation) at the top. Şirince Nappe comprises a chaotic meta-morphic unit (Şirince Metaflysch). The Bodrum Nappe consists of Early- Middle Triassic metaclastics and overlying Middle Triassic-Cenonian carbonates (Figure 8).

Geochemistry studies were conducted using available irriga-tion wells (<250m) and springs in the area and the database was extended with the wells drilled by BM. Initial geothermometer calculations for the Gümüşköy fields yielded temperatures of 130-174°C (Quartz), 136-160°C (Na/K) and 115-197°C (Na-K-Ca) (Yıldırım, 2008). Detailed geochemistry characteristics of the reservoir were identified upon completion of well testing. The reservoir fluid was found to be Na–Cl–HCO3 water, consistent with a moderate to high temperature liquid-dominated artesian geothermal reservoir within carbonate-bearing reservoir host rocks, with similar chemistry with other geothermal fields within the Menderes Graben. The most distinctive chemical feature of the resource is the pre-dominantly high concentration of carbon dioxide (CO2) dissolved in the reservoir fluid (99.8% by volume of total gas), based on analysis of dry gas samples. The ratio of NCGs relative to water by weight in the reservoir averages 0.021 in wells, within the range of the Non-Condensable Gases (NCG) concentrations observed in the Menderes Graben of 0.01 to 0.003 mg NCG: mg brine. The total dissolved solids of the reservoir fluid from wells averages approximately 8000 mg/kg or 0.8% dissolved solids by weight, which is relatively common for geothermal fluids world-wide. The TDS of shallow geothermal fluids sampled from surface manifestations and shallow wells

average 2,400 mg/kg, indicating that these fluids are mixtures of groundwater and the deep fluids of exploration wells drilled by BM. Geochemical analysis results from this area show that water comes from deep reservoir rocks and the water has high temperature.

Magnetotelluric (MT) and Time Domain Electromagnetics (TDEM) were measured by Geosystem at 393 stations with an interval of around 1 km. Data collected in this study was inverted in 3D using 0.5x0.5 km horizontal and 25 to 90m vertical resolution Furthermore, magnetic and gravity data from MTA were analyzed.

The 3D MT inversion results show that resistivity generally decreases with depth for in the Gümüşköy area. Surface and shallow basement resistivities (10-100Ωm) are significantly above values found for the conductive Quaternary sediments in the Menderes graben. Shallow resistivity decreases towards the area of low elevation in the South-east, towards the northern limit. The highest resistivity at intermediate depth is observed in the centre-north of the survey area. With increasing depth (≥ ~2000m), a transition to a low resistivity and deep basement zone of sub-horizontal to concave geometry is observed and modeled; resistivities decrease to 1-10Ωm, the resistive cap extending deepest in the central Gümüşköy survey area. The conductivity increase (resistivity decrease) with depth occurs shallowest in the south-west and south-east of the area at about -1000m asl. Lowest resistivity values are reached below around -3000m asl, as shown Figure 9. These low resistivity zones could be related to saline fluids, water related to geothermal reservoir or conductive minerals like graphite.

3.1 Drilling and ResultsThere are two exploration wells. The first exploration well

GK-1 has been drilled up to 1,402 meters depth in 2008 and further deepened to 2,100 meters in May 2009. GK-1 has a flow rate of 64 lt/s and a maximum reservoir temperature of 181°C. The second exploration well GK-3 has been drilled up to 2,056 meters depth in 2010 yielding a 165°C reservoir temperature and 42 lt/s flow rate. In addition; there are several gradient wells in the region. One of them, the deepest cored gradient well in Turkey, was drilled to 1,692 meters depth in 2011, yielding a 152°C temperature (well located to the west of the GK-1 well)

Figure 8. Geology Map of Gümüsköy Area (Genç and Tüysüz, 2010).

Figure 9. MT Profiles of the wells.

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The deep MT conductive anomaly below 2000m might at first sight suggest “a large reservoir” located underneath Gümüşköy, given the wells drilled in Gümüşköy, but it is likely the very deep conductor is again reflecting conductive geological structure; the structure in turn may indeed be related to enhanced vertical permeability and hence the encouraging exploration and reservoir environment. Gravity data, specifically horizontal derivative maps also show significant vertical structure here; faulted and fractured structures that have possibilities of carrying thermal water.

4. Discussion

Magnetotelluric (MT) 3D model results and their applications are presented from two geothermal study areas of the metamorphic Menderes Massif of Western Turkey. Deep electrically conductive zones characterize both regions, clearly within the metamorphic basement rather than the shallow graben cover, and exploration wells were drilled above these zones. Positive geothermal results were obtained from Gümüşköy geothermal area only; in Tire region the low resistivity zones were related to drilled pelitic schists and graphite.

The presence of a conductive, low resistivity zone cannot be used to blindly indicate the productive reservoir zone of geother-mal systems. Rather the conductive zones should be interpreted within the overall geological structure model at the core of the geo-thermal concept models used in modern exploration workflows.

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