Thermoluminescence as a new research tool for the evaluation of geothermal activity of the Kakkonda geothermal system, northeast Japan

The thermoluminescence glow-curve of quartz in volcanic and pyroclastic rocks of the Miocene and Quaternary in the Kakkonda geothermal field was divided into L (low), M (medium) and H (high) peaks in order of increasing temperature. Thermoluminescence emission is independent of stratigraphic boundaries but it is closely related to surface geothermal manifestations. Thermally stimulated processes of thermoluminescence caused by natural annealing occurred in the Quaternary after the eruption of the Tamagawa Welded Tuffs; radiation storage processes then began, as a consequence of the temperature drop. Thermoluminescence behavior indicates natural temperature manifestations, together with the paleo-temperature history.The H peak was thermally stable compared to the L and M peaks, and the area within which the relative intensity of the H peak is less than 5% coincides with the surface zone where dominant fluid flow is convective. In addition, L and M peaks indicate that a relatively low-temperature fluid mixes with the hot upflow around the western margin of the ascending flow zone.Thermoluminescence characteristics reflect paleo-temperature history and are related to geothermal fluid flow. Thermoluminescence is an effective exploration technique for evaluating natural temperature manifestations and subterranean heat flow in geothermal systems.     

Geophysical exploration of the Boku geothermal area, Central Ethiopian Rift

The Boku central volcano is located within the axial zone of the Central Ethiopian Rift near the town of Nazareth, Ethiopia. An integrated geophysical survey involving thermal, magnetic, electrical and gravimetric methods has been carried out over the Boku geothermal area in order to understand the circulation of fluids in the subsurface, and to localize the “hot spot” providing heat to the downward migrating groundwaters before they return to the surface. The aim of the investigations was to reconstruct the geometry of the aquifers and the fluid flow paths in the Boku geothermal system, the country's least studied. Geological studies show that it taps heat from the shallow acidic Quaternary volcanic rocks of the Rift floor. The aquifer system is hosted in Quaternary Rift floor ignimbrites that are intensively fractured and receive regional meteoric water recharge from the adjacent escarpment and locally from precipitation and the Awash River. Geophysical surveys have mapped Quaternary faults that are the major geologic structures that allow the ascent of the hotter fluids towards the surface, as well as the cold-water recharge of the geothermal system. The shallow aquifers are mapped, preferred borehole sites for the extraction of thermal fluids are delineated and the depths to deeper thermal aquifers are estimated.     

Geothermal systems in Iceland: Structure and conceptual models—II. Low-temperature areas

A review and assessment of data pertaining to the origin and nature of low-temperature geothermal activity in Iceland are presented. This activity is widely distributed in Quaternary and Tertiary formations on the American plate in western Iceland west of the active belts of volcanism and rifting but it is very sparse on the European plate east of these belts. Low-temperature systems occur in a few places within the active volcanic belts. Temperatures range from just above ambient to a little over 150°C. Generally speaking, resevoir temperatures decrease with increasing distance from the active volcanic belts. The distribution of the low-temperature areas can be correlated to a large extent with active tectonism. In Iceland the European plate is tectonically stable but in the American plate the shear stress field is complicated, leading to complex fracturing and faulting of the crust at present. No single generalized conceptual model describes the basic features of all low-temperature areas in Iceland. Low-temperature geothermal activity is considered to develop by one of the following four processes, or any combination of them: (1) deep flow of groundwater from highland to lowland areas through permeable structures driven by the hydraulic gradient; (2) convection in young fractures formed by tectonic movements in old and relatively impermeable bedrock; (3) drift of high-temperature geothermal systems out of the active volcanic belts in conjunction with their cooling and extinction of the magma heat source; and (4) magma intrusion into Quaternary or Tertiary formations adjacent to the active volcanic belts. Formation of permeable fractures by recent earth movements is probably the most common process responsible for the development of low-temperature activity through convection in these fractures. Convection in low-temperature systems with temperatures above some 60°C is probably mostly driven by pressure differences created by a relatively light hot water column within the system and a denser cold water column outside it. In systems of lower temperature the convection is driven by hydrostatic head in the recharge areas. The source of the low-temperature waters is largely meteoric. However, in some coastal areas a significant seawater-groundwater component is present, up to 10%. Waters not containing a seawater component are low in dissolved solids, or in the range 150–500 ppm. The reason is the low content of anions, particularly Cl, in the basaltic rock forming soluble salts with the major aqueous cations. Geothermal waters from the low-temperature areas in Iceland typically possess lower δD-values (more negative) than the local precipitation. This difference is variable; most often it lies in the range of 10–30% δD, but it may be as large as 70‰. This difference has been considered to indicate that the recharge areas to the low-temperature areas lie inland on higher ground, the distance being as much as 150 km. The interpretation favoured here is that at least some of the low-temperature waters contain a component of “ice-age water”, i.e. water that is older than about 10, 000 years. The “ice-age water” is depleted in deuterium relative to today's precipitation. When “ice-age water” is present in the geothermal water, deuterium cannot be used as a tracer to locate the recharge areas to the geothermal areas and in this way to deduce about regional groundwater flow.     

Controls on the Karaha–Telaga Bodas geothermal reservoir,Indonesia

Karaha–Telaga Bodas is a partially vapor-dominated, fracture-controlled geothermal system located adjacent to Galunggung Volcano in western Java, Indonesia. The geothermal system consists of: (1) a caprock, ranging from several hundred to 1600 m in thickness, and characterized by a steep, conductive temperature gradient and low permeability; (2) an underlying vapor-dominated zone that extends below sea level; and (3) a deep liquid-dominated zone with measured temperatures up to 353 °C. Heat is provided by a tabular granodiorite stock encountered at about 3 km depth. A structural analysis of the geothermal system shows that the effective base of the reservoir is controlled either by the boundary between brittle and ductile deformational regimes or by the closure and collapse of fractures within volcanic rocks located above the brittle/ductile transition. The base of the caprock is determined by the distribution of initially low-permeability lithologies above the reservoir; the extent of pervasive clay alteration that has significantly reduced primary rock permeabilities; the distribution of secondary minerals deposited by descending waters; and, locally, by a downward change from a strike-slip to an extensional stress regime. Fluid-producing zones are controlled by both matrix and fracture permeabilities. High matrix permeabilities are associated with lacustrine, pyroclastic, and epiclastic deposits. Productive fractures are those showing the greatest tendency to slip and dilate under the present-day stress conditions. Although the reservoir appears to be in pressure communication across its length, fluid, and gas chemistries vary laterally, suggesting the presence of isolated convection cells.     

The Bulalo geothermal field, Philippines: Reservoir characteristics and response to production

The Bulalo geothermal field has been operating since 1979, and currently has 330 MWe of installed capacity. The field is associated with a 0.5 Ma dacite dome on the southeastern flank of the Late Pliocene to Quaternary Mt. Makiling stratovolcano. The reservoir occurs within pre-Makiling andesite flows and pyroclastic rocks capped by the volcanic products of Mt. Makiling. Initially, the reservoir was liquid-dominated with a two-phase zone overlying the neutral-pH liquid. Exploitation has resulted in an enlargement of the two-phase zone, return to the reservoir of separated waste liquid that has been injected, scaling in the wellbores and rock formation, and influx of cooler groundwaters. Return of injected waters to the reservoir and scaling have been the major reservoir management concerns. These have been mitigated effectively by relocating injection wells farther away from the production area and by dissolving scale from wells with an acid treatment.     

辽河坳陷火山岩岩性特征及储层评价

辽河坳陷火山岩主要为玄武岩、粗面岩、响岩、安山岩、流纹岩和凝灰岩,以抗压强度较弱的少含暗色矿物的粗面岩和流纹岩为主.粗面岩主要分布在东部凹陷第三系,流纹岩主要分布在西部凹陷中生界.岩石化学分析表明,以浅色造岩矿物为主的火山岩抗压强度低,在应力作用下容易形成裂缝,而含暗色矿物较多的火山岩,抗压强度高,在相同应力下难以形成裂缝,并且暗色矿物蚀变后形成的绿泥石、方解石等,往往将早期形成的裂缝充填.运用岩石化学分析和测井响应特征,分别建立火山岩岩性和火山岩储层划分标准;同时,根据中子和电阻率测井响应特征,将火山岩划分为轻度蚀变、中度蚀变、重度蚀变和泥化火山岩4类.应用氧化物分析法,可将东部凹陷火山岩区分开;利用能谱测井,可将西部凹陷的火山岩划分出来:采用测井交会图法,可以划分辽河坳陷火山岩储层.     

Hydrothermal alteration and fluid inclusion geothermometry of los humeros geothermal field, Mexico

The Los Humeros geothermal field, located in Puebla State, Mexico, occurs in a caldera; drillholes to 3000 m depth encountered a sequence of Quaternary lavas and pyroclastic rocks that range in composition from rhyolite to basalt but are dominantly andesitic. These rest upon the local basement comprising limestone and siltstone of Cretaceous age, which was encountered below 2500 m in the northern part of the field and 1000 m in its southern part.Examination of 29 cores, mostly from below 900 m depth, from 14 wells show that the hydrothermal minerals that occur in the volcanic host rocks include quartz, calcite, epidote, amphibole, sericite, smectite, illite, chlorite, biotite, pyrite and hematite. Their distribution mainly reflects the prevailing hydrological and thermal regime where temperatures locally exceed 300°C. The limestone basement rocks, however, have altered to an assemblage that includes calcite, quartz, wairakite, garnet, wollastonite, parawollastonite, sericite and fluorite.The homogenization temperatures of 356 fluid inclusions were measured and the freezing temperatures of 200 determined. All except two sets of inclusions homogenized into the liquid phase and neither daughter minerals nor a clathrate phase were seen. The homogenization temperatures mostly match measured bore temperatures that range from 250 to 360°C and the apparent salinities are from 0.2 to 2.7 weight per cent NaCl equivalent, but some contribution to freezing point depression by CO₂ is likely.A preliminary model for the hydrology of the field based upon the hydrothermal alteration mineralogy and fluid inclusion data suggests that dilute hot water ascends via faults in the Central Caldera collapse area of the field and moves laterally outward to elsewhere within the caldera.     

Simulation of water–rock interaction in the Yellowstone geothermal system using TOUGHREACT

The Yellowstone geothermal system provides an ideal opportunity to test the ability of reactive transport models to simulate the chemical and hydrological effects of water–rock interaction. Previous studies of the Yellowstone geothermal system have characterized water–rock interaction through analysis of rocks and fluids obtained from both surface and downhole samples. Fluid chemistry, rock mineralogy, permeability, porosity, and thermal data obtained from the Y-8 borehole in Upper Geyser Basin were used to constrain a series of reactive transport simulations of the Yellowstone geothermal system using TOUGHREACT. Three distinct stratigraphic units were encountered in the 153.4 m deep Y-8 drill core: volcaniclastic sandstone, perlitic rhyolitic lava, and nonwelded pumiceous tuff. The main alteration phases identified in the Y-8 core samples include clay minerals, zeolites, silica polymorphs, adularia, and calcite. Temperatures observed in the Y-8 borehole increase with depth from sub-boiling conditions at the surface to a maximum of 169.8 °C at a depth of 104.1 m, with near-isothermal conditions persisting down to the well bottom. 1-D models of the Y-8 core hole were constructed to simulate the observed alteration mineral assemblage given the initial rock mineralogy and observed fluid chemistry and temperatures. Preliminary simulations involving the perlitic rhyolitic lava unit are consistent with the observed alteration of rhyolitic glass to form celadonite.     

Geochemical study of fluids on Lesbos island, Greece

Sixty-five water samples and seven associated gas samples have been collected on Lesbos island. The lithology and structural setting have resulted in two main types of hydrological circulation: a shallow circulation hosting low-salinity cold waters and a deeper one, hosting high-salinity hot waters that often emerge in thermal springs near the coast. The cold waters are characterized by Ca(Mg)-HCO₃(SO₄) composition, while the thermal waters generally have an Na-Cl composition. The chemical features of the former can be explained by their circulation in the ophiolite-bearing phyllitic basement and volcanic rocks. Waters circulating in the ultramafic layers of the basement are richer in Mg than the waters whose circulation is mainly within marble levels or volcanic rocks. The Na-Cl thermal waters are characterized by salinities ranging from 1910 to 35,700 mg/kg. As indicated by previous hydrogeochemical and isotopic studies, the Na-Cl composition of the thermal waters on Lesbos is the result of mixing between shallow meteoric waters and marine waters. While interacting with the minerals of the geothermal reservoir, the saline waters retain the Na/Cl sea water ratio but become enriched in Ca²⁺ and depleted in Mg²⁺ with respect to sea water.Processes of hydrothermal alteration at depth are activated by a gas phase enriched in CO₂, which reaches the geothermal reservoir by rising along the deep fractures of the basement. Thermodynamic calculations based on hydrothermal alteration processes occurring at the estimated temperatures of the geothermal reservoir (about 120 °C) indicate that the thermal waters of Lesbos are in equilibrium with talc and dolomite.     

Spatial data analysis and integration for regional-scale geothermal potential mapping,West Java,Indonesia

Conceptual modeling and predictive mapping of potential for geothermal resources at the regional-scale in West Java are supported by analysis of the spatial distribution of geothermal prospects and thermal springs, and their spatial associations with geologic features derived from publicly available regional-scale spatial data sets. Fry analysis shows that geothermal occurrences have regional-scale spatial distributions that are related to Quaternary volcanic centers and shallow earthquake epicenters. Spatial frequency distribution analysis shows that geothermal occurrences have strong positive spatial associations with Quaternary volcanic centers, Quaternary volcanic rocks, quasi-gravity lows, and NE-, NNW-, WNW-trending faults. These geological features, with their strong positive spatial associations with geothermal occurrences, constitute spatial recognition criteria of regional-scale geothermal potential in a study area. Application of data-driven evidential belief functions in GIS-based predictive mapping of regional-scale geothermal potential resulted in delineation of high potential zones occupying 25% of West Java, which is a substantial reduction of the search area for further exploration of geothermal resources. The predicted high potential zones delineate about 53–58% of the training geothermal areas and 94% of the validated geothermal occurrences. The results of this study demonstrate the value of regional-scale geothermal potential mapping in: (a) data-poor situations, such as West Java, and (b) regions with geotectonic environments similar to the study area.     

徐家围子断陷兴城地区营城组火山岩旋回和期次的划分与对比

徐家围子断陷位于松辽盆地北部的东南断陷区,断陷形成于晚侏罗世到早白垩世早期,中生代火成岩较为发育,主要目的层段为营城组一段.在钻井取心研究基础上,根据火山岩的岩性、电性及地震反射特征,对研究区火山岩旋回和期次进行了划分与对比,建立了营城组火山岩旋回、期次模式,总结了旋回、期次的岩心标志、测井响应标志及地震反射标志.通过对研究区火山岩旋回、期次的对比发现,研究区营城组一段发育4个喷发旋回,旋回I为中基性岩浆喷发,规模小,旋回Ⅱ为流纹质酸性熔浆喷发,局部可见少量安山岩,火山岩发育厚度和范围比旋回I都有明显的增大,旋回Ⅲ为流纹质的酸性岩浆喷发,是本区火山喷发活动的高潮期,旋回Ⅳ以沉火山碎屑岩为主.通过对研究区火山岩地层的划分与对比,为下一步的火山岩勘探开发工作奠定了基础.     

Observations on the application of chemical geothermometers to some hydrothermal systems in Sardinia

There are many empirical geothermometers that utilize the composition of the elements dissolved in water, yet it is difficult to judge just how reliable they are when applied to low enthalpy thermal systems.The main objective of this paper is to show how a reasonable estimate of the temperature of some typical thermal systems in Sardinia can be obtained from the calculation of the saturation indices of water with some silicates in the local rock matrix. Despite some uncertainties, mainly regarding possible mixing phenomena, the applied methodology appears to give more reliable reservoir temperature values than those given by the empirical geothermometers.The method has been applied to thermal waters circulating in the crystalline basement rocks (granites) and in the sedimentary cover rocks (mainly volcanic effusive products, andesitic lava, ignimbrites, etc.). The temperatures calculated for the hydrothermal systems range between 50 and 110°C.     

Geothermal reservoir potential of volcaniclastic settings: The Valley of Mexico,Central Mexico

The geothermal potential of the Valley of Mexico has not been addressed in the past, although volcaniclastic settings in other parts of the world contain promising target reservoir formations. An outcrop analogue study of the thermophysical rock properties of the Neogene rocks within the Valley of Mexico was conducted to assess the geothermal potential of this area. Permeability and thermal conductivity are key parameters in geothermal reservoir characterization and the values gained from outcrop samples serve as a sufficient database for further assessment. The mainly low permeable lithofacies types may be operated as stimulated systems, depending on the fracture porosity in the deeper subsurface. In some areas also auto-convective thermal water circulation might be expected and direct heat use without artificial stimulation becomes reasonable. Thermophysical properties of tuffs and siliciclastic rocks qualify them as target horizons for future utilization of deep geothermal reservoirs.     

Fluid flow processes in the BEPPU geothermal system, Japan

The Beppu geothermal system is centred beneath the late Quaternary volcanoes of Tsurumi and Garandake at the northern end of the Ryukyu volcanic arc. The deep fluid has a temperature of at least 250–300°C, and an inferred chloride concentration of 1400–1600 mg/kg. Apart from fumarolic areas near the summits of the two volcanoes, most thermal activity occurs at low elevation along the two main outflow paths towards the coast. The hot spring waters of downtown Beppu have originated from outflow along the Asamigawa Fault, with their chemistry indicating predominantly dilution of the deep fluid by groundwater. The second outflow zone towards the hot spring area of downtown Kamegawa coincides with a ridge of lavas. Here boiling, steam loss, and subsequent mixing with steam-heated groundwaters have significantly modified both the deep fluid and host rocks. The area of the geothermal system above 200°C is at least 15 km² at sea level, and the total natural heat output is inferred to be at least 250 MW. Most of this heat output occurs as subsurface hot water outflows towards the coast due to the 1300 m of topographic relief across the system.     

Temperature and chemical changes in the fluids of the Obama geothermal field (SW Japan) in response to field utilization

Thermal waters from Quaternary volcanic rocks (predominantly andesites) discharge along faults in the Obama geothermal field of southwestern Japan. The chemistry of more than 100 thermal and ground water samples collected between 1936 and 2005 indicate that the Na–Cl hot spring waters are a mixture of “andesitic” magmatic, sea and meteoric waters. Mixing models and silica and cation geothermometry were used to estimate the SiO₂ and Cl composition and the temperature (∼200 °C) of the reservoir fluids deep in the geothermal system. The isotopic data (¹⁸O and D) are consistent with a mixed origin interpretation of the waters feeding the Obama hot springs, i.e. a large proportion of meteoric and sea waters, and a small magmatic component. Temperatures and chemical concentrations of the thermal waters were affected by the 1944–1959 salt production operations, but have recovered after closure of the salt factories; now they are similar to their pre-1940 values. In the future, the Obama geothermal field may be suitable for electric power generation, although heat and fluid extraction will require careful management to prevent or minimize reservoir cooling.     

Temporal and spatial distribution of local seismicity in the Chipilapa-Ahuachapán geothermal area, El Salvador

Microseismic monitoring of the Chipilapa-Ahuachapán area was carried out during August-November 1988 and October 1991–April 1992. The objective was to use the study of microearthquakes as an exploration tool to invvestigate the geothermal potential of the Chipilapa area and to evaluate the main characteristics of the seismic activity, prior to and during the exploitation tests. Since 1989, seven wells have been drilled in the area, two of which have encountered three geothermal aquifers that could be exploited for electricity generation by means of binary-cycle technology. The 1988 survey detected important, shallow and low magnitude seismic activity, located mainly south and southwest of the explored area. This activity is possibly related to the recharge zone of the Chipilapa-Ahuachapán geothermal system, located further south, beneath the Pleistocene Pacific Volcanic Chain. The 1991–1992 survey confirmed the existence of seismicity beneath the southern volcanic axis, but other important clusters of activity were recorded northward, related to the deeper structures of the Central Graben, and southwest of the Ahuachapán geothermal field, close to the 1990 hydrothermal eruption of Agua Shuca. Shallow microseismic activity also appeared along the faults limiting the Chipilapa geothermal field to the east. Although it is probable that this seismicity is due to fluid circulation in fractures, no geothermal reservoirs were intercepted by wells CHA and CH8. Moreover, no significant induced seismicity was recorded during production and injection tests.     

Geology, mineralogy and stable isotope geochemistry of the cesano geothermal field (Sabatini Mts. volcanic system, Northern Latium, Italy)

Some deep wells (1400–3000 m) have been drilled into the volcanic system of the Sabatini Mts., north of Rome, in search of high enthalpy geothermal fluids.Baccano caldera has formed over a carbonate structural high with a Quaternary volcanic cover, on the inside of a large graben that developed from the Upper Miocene and is characterized by a strong positive thermal anomaly. A spatially irregular Neogenic neoautochthon cycle, an allochthonous flysch complex (‘Sicilidi’ s.l.) and a Meso-Cenozoic ‘basal’ carbonate sequence were recognized in the sediments crossed by these wells below the volcanic cover. The observed structural setting can be correlated to that of analogous outcropping structures (Mt. Soratte) and shows a tensional tectonic phase overlying an older, mainly compressional phase.The stratigraphic and mineralogic analyses revealed important differences in the Trias, which is mainly calcareous in the western sector of the area and anhydritic in the eastern: the two facies at present seem to be tectonically brought closer to one another.The isotopic analyses all show values comparable to those of the sedimentary carbonate host rocks, suggesting limited circulation of the hydrothermal fluids. ¹⁸O/¹⁶O analyses on specimens in the most restrictive locations next to interfaces show a relatively larger extent of isotopic exchange with aqueous fluids.Fluid circulation, which is mainly extinct, seems to belong to a closed system. Two types of circulation have been recognized: the first of these is connected to the stratigraphic discontinuities (especially at the contact between the allochthonous flysch complex and the basal carbonate sequence) and is characterized by mainly K-feldspar and Sulphide-Sulphate mineralizations; the second circulation is tied to the fracture systems and is particularly widespread in the pyroclastic cover where it appears in the form of sulphate- and carbonate-rich dykes. This system seems to have undergone a mixing with surface waters.     

Simulation of heat extraction from crystalline rocks: The influence of coupled processes on differential reservoir cooling

Processes operating during the extraction of heat from fractured rocks influence dynamically their fluid flow and heat transport characteristics. The incorporation of pressure- and temperature-dependent rock parameters, coupled with geomechanical deformation, is particularly important for predictive modelling of geothermal reservoirs hosted in crystalline rock masses. Changes in flow and transport parameters of fractures caused by variations in local effective stress are computed using an experimentally validated geomechanical model [McDermott, C.I., Kolditz, O., 2006. Geomechanical model for fracture deformation under hydraulic, mechanical and thermal loads. Hydrogeol. J. 14, 487–498]. Local effective stress changes are linked to alterations in reservoir fluid pressures, and to in situ stress conditions, including the build-up of thermal stresses resulting from the cooling of the rock mass. These processes are simulated using a finite-element model in order to study the behaviour of the Spa Urach (southwestern Germany) potential geothermal reservoir. The model couples mechanical deformation and alteration of fracture parameters with pressure-, temperature- and salinity-dependent fluid parameter functions. The effects of potential reservoir damage on reservoir productivity are investigated to help identify optimal heat recovery schemes for the long-term economical exploitation of geothermal systems. Simulation results indicate that preferential fluid flow paths and shortcuts may develop, depending on the mechanical and thermal stress releases that occur during intense exploitation of these systems.     

Takigami geothermal system, northeastern Kyushu, Japan

The Takigami geothermal reservoir is bounded by a system of faults and fractures oriented along two main directions, north to south and east to west. The Noine fault has a large vertical displacement and trends north to south, dividing the subsurface characteristics of resistivity, permeability, temperature and reservoir depth. The Takigami geothermal fluid has a near neutral pH and is of the Na–Cl type, with a chloride content ranging from 400 to 600 ppm. The southwestern part of the area has the highest subsurface temperature, up to 250°C. The deep fluid originates from the southwest, and flow is mainly to the north and partly to the east along faults and fractures, decreasing in temperature with increasing lateral flow.     

The oxygen isotope exchange between carbon dioxide and water in the Larderello geothermal field (Italy) during fluid reinjection

The oxygen isotope compositions of CO₂ and water vapor samples collected from Larderello geothermal wells after the start of the fluid reinjection program suggest that if the oxygen isotope exchange in the vapor phase does, in fact, exist, it is a very slow process when compared with the residence time of the fluids in the geothermal reservoir. This is because carbon dioxide and water vapor phases could not have equilibrated significantly in the vapor-dominated reservoir. This conclusion implies that the oxygen isotope composition of carbon dioxide may possibly be used as a tool in geothermal exploration for revealing the presence of liquid water in deep geothermal systems. Based on the interpretation of the oxygen isotope data of the CO₂, I propose that the origin of the low oxygen isotope ratios of carbon dioxide at Larderello is the high-temperature exchange with liquid water in the lower reservoir. In Larderello, the liquid water–rock interaction in the lower reservoir may have increased the ¹⁸O/¹⁶O ratio of the recharge meteoric component. By contrast, lack of high-temperature liquid water in the upper reservoir suggests that the large “δ¹⁸O shift” described for the upper-reservoir steam during the last decades reflects varying degrees of dilution of the lower-reservoir fluid by the low-¹⁸O vaporized liquid water of meteoric origin that recharges the field at shallow depth, with local contribution from still deeper high-¹⁸O water vapor of magmatic origin. The low oxygen isotope composition of the Mesozoic carbonaceous rocks that form the upper reservoir, consequently, likely represents a “fossil” record of the past hot-water geothermal stage.