Identification of structures inside basement at Soultz-sous-Foret (France) by triaxial drill-bit vertical seismic profiling

Seismic signals produced while drilling a Hot Dry Rock (HDR) geothermal well in Soultz-sous-Forêt (France) were detected using a 4-component seismic detector and two single-component detectors in the granite basement. The recorded signals had a signal-to-noise ratio of around 8 dB with respect to the background noise and show relatively high spectral whiteness. Results from three-dimensional hodogram analysis were successfully interpreted using a model in which the drill bit generates predominantly continuous shear waves.Reflected waves were detected using both the spatial and temporal cross-correlation of hodograms. The distribution of reflectivity from the tri-axial drill-bit vertical seismic profiling (TAD–VSP) suggests that the bottom of the artificial geothermal reservoir, created by hydraulic stimulation, has a high reflection coefficient. The pre-existing permeable zone inside the reservoir was also detected by the TAD–VSP. The reflection image of the TAD–VSP agrees well with that found by the AE (Acoustic Emission) reflection method. The TAD–VSP was successfully used to detect the internal structure of the deep basement.     

Geothermal exploration of kos island, Greece: Magnetotelluric and microseismicity studies

This paper reports the results of magnetotelluric (MT) and microseismicity studies, conducted as part of a multi-disciplinary project to explore the geothermal potential of the island of Kos, Greece. The MT survey, comprising 18 soundings, was carried out in the bandwidth 128 Hz–40 s, in order to determine the deep conductivity structure in the geothermally prospective western part of the island. Rigorous dimensionality analysis has indicated that the geoelectric structure could adequately be approximated with 1-D interpretation tools. Two significant and seemingly communicating conductive zones of potential geothermal interest were found within the first 2 km. The first is extensive and shallow, detected at depths of 400–600 m; the second is deeper (1000–1300 m), but of considerably smaller lateral dimensions. A very deep relative conductor (<25 Ωm) was also detected at depths of 7–10 km, which is thought to comprise part of an old magma chamber with brine-saturated rocks. The microseismicity studies revealed the partial or total attenuation of shear waves in many microearthquake records. The analysis of these observations determined the vertical and lateral extent of that attenuation zone, the greatest part of which is located underneath the marine area between western Kos and Nissyros island to the south, extending approximately from near the surface to about 1.5 km depth. The nature of this zone is discussed in terms of fluid concentration due to the geothermal system of the area.     

Investigation of the geothermal anomaly of Travale (Tuscany) by telluric, magnetotelluric and geomagnetic deep sounding measurements

As part of the European Community research programme telluric, magnetotelluric and geomagnetic deep sounding measurements were undertaken at 40 sites within the geothermal area of Travale. In the period range of 6–10,000 s the telluric field inside the Travale graben is strongly polarized and directed, independent of the period, about parallel to the graben strike. The lateral variation of the telluric field amplitude is determined mainly by the distribution of the rocks (e.g. the central part of the geothermal anomaly inside the graben is correlated with a horst structure of resistive rocks) and an influence of the geothermal anomaly on the telluric field distribution cannot be observed. The apparent resistivity, as well as the phase curves, are rather similar at all sites within the graben, exhibiting 4–40 ohm · m for periods of 10 s and 50–500 ohm · m for periods of 10,000 s in E-polarization. In the period range of 10–100 s the E- and B-polarization of magnetotelluric measurements can be interpreted by the 2-D effect of the Travale graben, while with increasing period the induced current system becomes more and more 3-D below all sites. This limits the determination of the sedimentary cover thickness (max. 2500 m) by 1-D and 2-D model calculations to periods of less than 100 s.     

四川地热流体水文地球化学及同位素特征简析

四川省范围内有沉积盆地型和隆起山地型两类地热资源,共划分五个地热区。从五个区采取的205组水样的水文地球化学特征及184组2H、18O和61组14C特征分析显示,四川省各地热区地热流体基本来自于大气降水补给,地热流体的水文地球化学和同位素特征与其所属的热储类型和热储开放性有关。盆地型热储主要为岩溶层状热储,山地型热储主要是变质岩为主的裂隙带状热储和层状带状复合型热储。盆地型热储开放性较山地型弱,地热流体矿化度和理疗元素含量均比山地型高,易形成深埋藏的卤水,地下水平均径流时间较山地型长。本研究可为四川省地热资源未来的开发利用规划提供参考。     

Volcano–tectonic structures, gravity and helium in geothermal areas of Tuscany and Latium (Vulsini volcanic district), Italy

Since the early 1980s, geological and structural mapping, gravity, and helium soil–gas studies have been performed in the eastern sector of the Vulsini Volcanic District (Roman Magmatic Province) in an attempt to locate potential geothermal reservoirs. This area is characterised by an anomalous geothermal gradient of >100°C/km, and by widespread hydrothermal mineralization, thermal springs, high gas fluxes, and fossil and current travertine deposits. The results of these surveys indicate the existence of a number of fault systems, with N–S and E–W structures that appear to be superimposed on older NW–SE and NE–SW features. Comparison of the results of the various studies also reveals differences in permeability and potential reservoir structures at depth.     

Lithological correlations of the cerro prieto wells, based on well - log interpretation

This paper presents the results of correlations of the geophysical well logs obtained from wells drilled at the Cerro Prieto geothermal field. From this correlation, a structural interpretation of the ‘receptacle’ holding the geothermal fluids has been obtained.Based on the temperature and electrical resistivity logs, the presence of two main geothermal reservoirs has been established. It was also found that most of the wells were completed such that production is obtained simultaneously from several sandy layers. In all probability this has created some mechanical problems in the wells in addition to a certain confusion in the interpretations based on fluid production data.An hypothesis is postulated about the deposition environment and the genesis of the reservoir, on the basis of which it is concluded that the reservoir is a paleochannel. It is also postulated that thinning of the basement rocks is the fundamental cause of the heat transmission to the reservoirs.Tables listing the correlating marker horizons for each well and maps showing the top of the two main reservoirs and the isopacks for the shallow one are presented. From these data have been inferred the areas showing the best possibilities for immediate geothermal development.     

Joint 1D inversion of TEM and MT data and 3D inversion of MT data in the Hengill area,SW Iceland

An extensive study of the resistivity structure of the Hengill area in SW Iceland was carried out by the combined use of TEM and MT soundings. Joint inversion of the collected data can correct for static shifts in the MT data, which can be severe due to large near-surface resistivity contrasts. Joint 1D inversion of 148 TEM/MT sounding pairs and a 3D inversion of a 60 sounding subset of the MT data were performed. The 3D inversion was based on full MT impedance tensors previously corrected for static shift. Both inversion approaches gave qualitatively similar results, and revealed a shallow resistivity layer reflecting conductive alteration minerals at temperatures of 100–240 °C. They also delineated a deep conductor at 3–10 km depth. The reason for this deep-seated high conductivity is not fully understood. The distribution of the deep conductors correlates with a positive residual Bouguer gravity anomaly, and with transform tectonics inferred from seismicity. One model of the Hengill that is consistent with the well temperature data and the deep conductor that does not attenuate S-waves, is a group of hot, solidified, but still ductile magmatic intrusions that are closely associated with the heat source for the geothermal system.     

Electrical conductivity studies in the Travale geothermal field, Italy

Magnetotelluric and geomagnetic depth soundings have been carried out in the area of the Travale high enthalpy geothermal field (central Tuscany, Italy) in 1980 and 1981 to study the distribution of electrical conductivity in the geothermal anomaly and the crust beneath. Within this project the possible contributions of electromagnetic investigations to geothermal research were to be tested and a geothermal model of the Travale area was to be developed. The time-varying electric and magnetic fields have been recorded in a broad period range from 6–10,000 s, mainly on two profiles, the one parallel, the other perpendicular to the Travale graben. Strong lateral variations of apparent resistivities have been observed. Up to periods of 50–100 s the Travale graben is the dominating 2-D structure, but for longer periods of investigation the three-dimensionality of electrical conductivity structures has to be considered. The apparent resistivities inside the geothermal anomaly are extremely low, reaching not more than 50 ohm · m, even in the lower crust, but they increase to 100–300 ohm · m north of the geothermal field. Total conductance also indicates the geothermal field as a local conductivity anomaly, whereas further to the north the poorly conducting “barrier” has been confirmed. The cause of the high conductivity structures inside the geothermal area is to be seen in a highly fractured basement within this zone, allowing upward movement of hydrothermal fluids.     

Structure and hydrology of the Ogiri field, West Kirishima geothermal area, Kyushu, Japan

The geothermal system in the West Kirishima area is controlled by a system of faults and fractures oriented along two main directions, northwest to southeast and east–northeast to west–southwest. The Ginyu fault extends through the Ogiri field in the Ginyu area, which is one of the east–northeast to west–southwest striking faults in this area. This fault is the reservoir target for developing the geothermal resources in the Ogiri field. The Ginyu fault is a near planar fracture with a uniform temperature of 232°C and has near-neutral pH, chloride fluids. Based on the results of a detailed analysis of the Ginyu fault, all production wells drilled in the Ogiri field intersected the Ginyu fault reservoir successfully, securing steam production for a 30 MW_e power plant. A typical fracture-type geothermal model for the Ogiri field was developed on the basis of the geology, electric and geophysical logs, fluid chemistry, and well test data.     

Geoelectric study of some indian geothermal areas

The electrical resistivity technique has been used extensively in the Indian sub-continent for the exploration of geothermal areas. The first systematic application of the resistivity method for locating the geothermal reservoir was made in the Puga area, which is situated very close to the collision junction of the Indian and the Asian plates and has numerous hot springs with temperatures varying from 30 to 84°C (boiling point at that altitude). The resistivity depth probes indicated the presence of a conductive zone, with a value of 10–25 ohm·m and a thickness varying from 50 to 300 m over an area of 3 km², which was inferred to correspond to a shallow thermal reservoir. Thermal surveys also revealed a significant anomaly corresponding to this zone, which, when drilled, encountered a reservoir of wet steam with a temperature of up to 135°C, thus confirming the results of the resistivity surveys. Somewhat similar results have been obtained in the adjoining area, where much thicker zones with moderate electrical conductivity have been mapped.Another significant application of the electrical resistivity method has been made in the NNW-SSE extending West Coast geothermal belt of India, which is covered by Traps (Basalts) of the Cretaceous-Eocene. The area is characterized by the existence of a number of hot springs, with temperature up to 70°C, along a 400 km long alignment, associated with steep gravity gradients and an isolated occurrence of native mercury in the zone of a gravity “high”. The enigmatic geology of this area has been mapped, giving quantitative estimates of the thickness of the Traps and inferring the structural features. In addition, the electrical resistivity depth probes have also been used to identify the pre-Trappean geology, thereby locating the probable areas which could act as geothermal reservoirs.This paper presents the results of the electrical resistivity surveys in the form of geoelectric sections for some of the geothemal fields in the Indian sub-continent.     

The shallow structure of Los Humeros and Las Derrumbadas geothermal fields, Mexico

Los Humeros caldera and Las Derrumbadas rhyolitic domes located in the State of Puebla (Mexico) are being extensively studied by Comisión Federal de Electricidad (CFE) to assess the geothermal potential of that area. Both geothermal sites have been locally studied with geological, geochemical and geophysical methods. In order to get a geological picture of the sub-surface of the area comprising both sites, a regional structural survey was conducted together with gravity and magnetics studies. These studies showed a regional weakness zone (the Libres-Oriental depression), where the volcanic activity related to the geothermal manifestations took place.The tectonic events in the area are represented by two systems of structures. The first one (N140°–N170°) gave rise to the folds and overthrusts affecting the sequence of calcareous rocks resting upon the basement. This system is affected by a second system (N40°–70°E). Regionally these tectonic events remodelled the sedimentary sequence giving rise to a series of topographic highs that constitute the core of several ranges, and to depressions that were filled with volcano-sediments during the ensuing volcanic activity.The rhyolitic domes of Las Derrumbadas are emplaced in a regional NW-SE lineament, which reflects clearly on the Bouguer anomaly, and is interpreted as a front of overthrust affecting the marine sedimentary sequence. Estimations of depths to the basement, based on magnetic data, showed that the basement under Los Humeros caldera is shallower than in Las Derrumbadas. The gravity data were interpreted in terms of tabular bodies associated to calcareous rocks, of igneous intrusions, and of depressions filled with volcano-sediments. A joint interpretation of the geological and geophysical data led to a model of the structure of the shallow crust in the area.     

Hengill geothermal volcanic complex (Iceland) characterized by integrated geophysical observations

Structural features of volcanic and hydrothermal systems can be used to infer the location of magma chambers or productive geothermal areas. The Hengill volcanic triple-junction complex has a well-developed geothermal system, which is being exploited to extract hot fluids that are used for electrical power and heat production. In the framework of the I-GET project, a 4-month temporary seismological network including seven high-dynamic broadband instruments was deployed and 1D transient electromagnetic soundings (TEM) and 3D magnetotelluric (MT) surveys were performed to improve the understanding of the relationships between structural features, seismic activity and fluid production at the Hengill geothermal system. The MT and TEM data set are analysed elsewhere. The analysis of the seismological data set allowed the detection and classification of more than 600 earthquakes, among which long-period (LP) earthquakes were observed for the first time in this area. This work focuses first on a joint inversion for the 3D velocity structure and determination of the locations of the hypocentres from about 250 local volcano-tectonic earthquakes with clear P- and S-wave arrival times. The results confirm those from earlier tomography studies in this area. Integrating the seismic velocity and resistivity models in a semi-quantitative approach by cross-plotting the resistivity model with the velocity ratio V_P/V_S delineates a structural body with a high seismic velocity ratio and low resistivity that is interpreted as the main heat source of the geothermal system.     

Groundwater circulation in the Nea Kessani low-temperature geothermal field (NE Greece)

The low-temperature geothermal field of Nea Kessani, located in NE Greece, is characterized by a thermal reservoir made up of arkosic sandstones. The temperature distribution at depth, inferred from exploratory and productive wells, indicates that hot fluids rising from depth enter the arkosic reservoir in a restricted area of the field and flow towards local thermal springs. Well production tests have revealed the presence of hydrogeological boundaries within the arkosic reservoir.The geochemical characteristics of the thermal waters, which have an NaCl/HCO₃ composition and salinity varying between 5 and 6 g/L, indicate that these waters undergo conductive cooling within the reservoir. No admixture of waters from the aquifers in the cover has been observed. The slight chemical differences existing between the thermal waters are probably caused by CO₂, which represents about two thirds by volume of the discharged fluid. This CO₂, as indicated by its isotopic composition, could originate from decomposition of marbles of the Paleozoic basement underlying the arkosic reservoir and may also affect the isotopic composition of the thermal waters, which exhibit an interesting positive oxygen shift. However, such a shift could also be the result of water-rock exchange processes at low temperatures, since the water feeding the field comes from a regional circulation which, as indicated by its deuterium content, has recharge areas on the Rhodope Chain. Alternatively, the shift could be attributed to the contribution of a deep-seated high-temperature geothermal reservoir, but a present there is no evidence of high-temperature resources in the region. A maximum temperature of 110°C has been estimated by quartz geothermometry.The physical, chemical and hydrogeological data available so far have permitted us to formulate a fluid circulation model for the Nea Kessani geothermal field.     

Characterization of geothermal reservoirs with electrical surveys: Beowawe geothermal field

The work reported here was undertaken to test the utility of electrical surveys for geothermal reservoir characterization using existing exploration and well data sets from the operating Beowawe geothermal field located in the Basin and Range Province of western USA. The STAR geothermal reservoir simulator was used to model the natural state of the system, and to compute the subsurface distributions of temperature and salinity, which were in turn utilized to calculate pore-fluid resistivity. Archie's law, which relates formation resistivity to porosity and pore-fluid resistivity, was adopted to infer the formation resistivity distribution. Subsequently, direct current (DC) resistivity, magnetotelluric (MT) and self-potential (SP) postprocessors were used to compute the expected response corresponding to available survey data. The measured apparent resistivity distribution from a dipole–dipole DC resistivity survey is in good agreement with the computed values. The calculated self-potential distribution agrees with the main features of an available SP survey. Although the computed MT apparent resistivity sounding curves reproduce the shapes of the measured MT sounding curves, an overall scale factor exists between the measured and calculated MT responses, and similarly with the computed dipole–dipole resistivity model. Possible reasons are static shifts in the coarsely sampled MT stations, and resistivity anisotropy due to the stratigraphy. Taken as a whole, the results of this study support the view that a suite of carefully designed electrical surveys (DC, MT, and SP) may be employed to infer favorable subsurface geothermal reservoir characteristics.     

Chemical and isotope characteristics of the Chachimbiro geothermal fluids (Ecuador)

The parent geothermal water proposed for the Chachimbiro geothermal area has calculated values of 2250 mg/L Cl and approximately 5 bar P_CO2. It comes from a reservoir having an estimated temperature of 225–235 °C, although temperatures somewhat higher than 260 °C may be present at the roots of the system. The geothermal reservoir at Chachimbiro is recharged mainly by meteoric water (about 92%) and secondarily by arc-type magmatic water. Carbon and sulfur isotope data support a magmatic origin for the C and S species entering the geothermal system from below, consistent with indications provided by He isotopes.The thermal springs of Na–Cl to Na–Cl–HCO₃ type located in the Chachimbiro area originate through dilution of the parent geothermal water and have reached different degrees of re-equilibration with country rocks at lower temperatures.     

Research and development of geothermal energy production in Hungary

The basement of the Pannonian (Carpathian) basin is represented by Paleozoic metamorphic and Mesozoic dolomite and limestone formations. The Tertiary basin gradually subsided during the Alpine orogeny down to 6000 m and was filled by elastic sediments with several water horizons.A heat flow of 2.0 to 3.4 μcal/cm²s gives temperature gradients between 45 and 70 °C/km in the basin. At 2000 m depth the virgin rock temperature is between 110 and 150°C. 80 geothermal wells about 2000 m deep have shown the great geothermal potential of the basin.The main hot water reservoir is the Upper Pliocene (Pannonian) sandstone formation. Hot water is produced by wells from the blanket or sheet sand and sandstone, intercalated frequently by siltstone. Between a 100–300 m interval, 3 to 8 permeable layers are exploited resulting in 1–3 m³/min hot water at 80–99°C temperature.Wells at present are overflowing with shut-in pressures of 3–5 atm.The Pannonian basin is a conduction-dominated reservoir. Convection systems are negligible, hot igneous systems do not exist. The assessment of geothermal resources revealed that the content of the water-bearing rocks down to 3000 m amounts to 12,600 × 10¹⁸cal. In the Tertiary sediments 10,560 × 10¹⁸cal and in the Upper Pannonian, 1938 × 10¹⁸cal are stored. In the Upper Pannonian geothermal reservoir, below 1000 m, where the virgin rock temperature is between 70 and 140°C, the stored heat is 768 × 10⁸cal. A 10¹⁸ cal is equivalent to the combustion heat of 100 million tons of oil. The amount of recoverable geothermal energy from 768 × 10⁸cal is 7.42 × 10¹⁸cal, i.e. about 10,000 MW century, not considering reinjection.At present the Pannonian geothermal reservoir stores the greatest amount of identified heat which can be mobilized and used. Hungary has 496 geothermal wells with a nominal capacity of 428 m³/min, producing 1342 MW heat. 147 wells have an outflow temperature of more than 60°C producing 190 m³/min, that is, 845 MW. In 1974 290 MWyear of geothermal energy was utilized in agriculture, district heating and industry.     

Geology and fluid chemistry of the Fushime geothermal field, Kyushu, Japan

The Fushime geothermal field is located in a depression close to the coast line. The system is characterized by very high reservoir temperature (>350°C), and a high salinity production fluid. Geological analysis shows that the main reservoir in this field occurs in a fractured zone developed around a dacite intrusion located in the center of the field. High permeability zones recognized by drilling data are found to be associated with fault zones. One of these zones is clearly associated with a NW–SE trending andesite dike swarm which was encountered in some wells.Alteration in the system can be divided into four zones, in order of increasing temperature, based on calcium–magnesium aluminosilicate mineral assemblages: i.e., the smectite, transition, chlorite and epidote zones. The feed zone is located in the chlorite and epidote zones, which can be further divided into three sub-zones according to their potassium or sodium aluminosilicate mineralogy, from the center of the discharge zone: K-feldspar–quartz, sericite–quartz, and albite–chlorite zones.Chloride concentration of the sea-water is 19,800 mg/l, and Br/Cl mole ratio is 1.55. Based on geochemical information, the reservoir chloride concentration of this field ranges from 11,600 to 22,000 mg/kg. The Clres (Cl in reservoir), Br/Cl ratios and stable isotope data indicate that the Fushime geothermal fluid originated from sea-water and is diluted by ground water during its ascent. Some fluids produced from geothermal wells show low pH (about 4). It is thought that sulfide mineral (PbS, ZnS) precipitation during production produces this acidic fluid.     

Numerical modeling of Basin and Range geothermal systems

Basic qualitative relationships for extensional geothermal systems that include structure, heat input, and permeability distribution have been established using numerical models. Extensional geothermal systems, as described in this paper, rely on deep circulation of groundwater rather than on cooling igneous bodies for heat, and rely on extensional fracture systems to provide permeable upflow paths. A series of steady-state, two-dimensional simulation models is used to evaluate the effect of permeability and structural variations on an idealized, generic Basin and Range geothermal system of the western U.S.Extensional geothermal systems can only exist in a relatively narrow range of basement (bulk) permeability (10⁻¹⁵ m² to 10⁻¹⁶ m²). Outside of this window, shallow subsurface fault zone temperatures decrease rapidly. Mineral self-sealing does not significantly affect the flow system until the flow path is almost completely sealed off. While topography gives an extra “kick” to convective circulation, it is not a requirement for geothermal system development. Flow from the ranges to the fault dominates the circulation, while secondary flow systems exist on the range front slopes. A permeable fault in one valley can also induce cross-range flow if there are no equally good upflow paths in the adjacent valleys. When bulk permeability is high enough, additional deep circulation cells develop in adjacent valleys, diverting heat and fluid from the fault and consequently reducing temperatures in the fault itself. Qualitative comparison between temperature–depth logs from actual geothermal systems and from the generic models is a significant aid to understanding real-world geothermal fluid flow, and suggests new or better interpretations of existing systems.