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.     

Evidence of a contact metamorphic aureole with high-temperature metasomatism in the deepest part of the active geothermal field of Larderello,Italy

The deep part (1.5–4.5 km) of the Larderello geothermal field (Tuscany, Italy) consists of rocks that were metamorphosed by the Hercynian and Alpine orogenies, and which were thermally metamorphosed in the same place during the emplacement of granite intrusions of 3.8–1.0 Ma age. These rocks are potential deep-seated reservoirs and could be the target of future exploitation. The petrographic, geochemical and fluid inclusion data indicate that thermally metamorphosed phyllite, micaschist, gneiss, amphibolite and carbonates underwent a recrystallisation at temperatures of 425–670 °C, under a lithostatic pressure regime of 95-130 MPa. Li–Na-rich fluids of magmatic origin, and aqueous-carbonic fluids with varying proportions of H₂O and CO₂ that formed during the contact metamorphism, were present during this stage. The fluids present during the contact metamorphic event were responsible for a widespread B-metasomatism and local F-metasomatism. In some cases, high-temperature metamorphism of graphitic schists can control the composition of the aqueous-carbonic fluids. A late-stage, lower temperature hydrothermal activity was responsible for both the propylitic and sericitic alterations and for the replacement of the high-temperature mineral assemblage. Stable isotope (δ¹⁸O and δD) data on the thermally metamorphosed rocks and granites indicate that these rocks underwent depletion in the heavy isotopes. Magma degassing and dehydration metamorphic reactions can explain the isotopic values of these rocks. Late-hydrothermal fluid of meteoric origin may also have contributed to the depletion of the heavy isotopes from the rocks. Under contact metamorphism conditions the rocks were plastic and impermeable. A transition from plastic to brittle conditions occurred as a consequence of the cooling of the system at depths of <4 km. The brittle-plastic transition at Larderello now occurs at a depth of 4.5–5 km, where present-day temperatures are in the range 400–450 °C. Deep-seated fluids probably occur in this zone, as suggested by the geophysical data.     

Oxygen isotopic ratios of sulfate ions-water pairs as a possible geothermometer

In this paper a brief review is given of the dependence of the oxygen isotopic fractionation of the sulfate ions-water system on temperature and the pH. From the available experimental data some relationships have been elaborated, which show that the isotopic exchange time is strongly temperature and pH dependent. The times for 97 per cent of isotopic exchange (near equilibrium conditions) at pH 7.0 are about 9 years at 200°C and 0.6 years at 330°C, while at pH 3.8 and at the same temperatures the times of exchange are 1.5 years and 0.08 years respectively. Thus, at the temperatures and pH of geothermal reservoirs the sulfate could be in isotopic equilibrium with environmental water, and the oxygen isotopic fractionation factors of sulfate-water geothermal pairs, being temperature dependent, can be used as geothermometers.Also reported here are some results on the O¹⁸ content of sulfate-water pairs from some wells on the edge of and outside the Larderello geothermal basin. The estimated isotopic temperatures are not very significant for the deep reservoir temperatures due to the geological features of the Larderello area which show important outcropping and deep anhydrite layers. Furthermore, as regards the wells outside the Larderello basin (Travale wells) some mixing of the geothermal water with colder underground water has been proved. However, the isotopic temperatures are generally higher than those measured at the well-head, and the highest ones are close to those estimated for the geothermal reservoir.In other geothermal areas more convenient from a geological point of view, the O¹⁸ content of the sulfate-water pair can be a useful and accurate thermometer.The O¹⁸/O¹⁶ ratios of several other sulfates (surface and deep anhydrite samples, sulfate ions in thermal springs) from the same area were also determined and differ substantially from borehole sulfate values.     

Oxygen-18 and carbon-13 contents of the sulfates and the carbonates associated in some oxidizing geothermal environments

Some theoretic calculations on the oxygen isotopic composition of sulfate formed by inorganical oxidation of sulfide compounds and the results obtained by measuring the O¹⁸/O¹⁶ and the C¹³/C¹² ratios of sulfates and carbonates associated in the salt incrustations around some fumarolic holes and in the calcareous marly flysch of an inactive geothermal manifestation (Larderello geothermal region) are reported here.Most of the analyzed samples show δO¹⁸ (SO₄^2?) values which are explainable with sulfur or sulfide oxidation processes in the soil around the geothermal manifestation. In the case of sulfate with δO¹⁸ negative value, a sulfur-oxidizing bacterial activity is probable. Only a few of the sulfates analyzed here have δO¹⁸ values compatible with an oxidative process of sulfurous gases in steam or in condensed water solution.The limestone in the calcareous marly flysch shows evident oxygen and carbon isotope exchanges caused by geothermal water and steam and by the dissolved oxy-carbonic compounds. The positive δO¹⁸-δC¹³ correlation found reflects a proportional isotope exchange for the oxygen and carbon isotopes.     

Oxygen and carbon isotope studies of calcite from the Cerro Prieto geothermal field

The inferences derived from oxygen and carbon isotope data for calcite samples from the Cerro Prieto geothermal field depend on the sample type. The δ¹⁸O values for calcite in sandstone provide a reliable basis for estimating stable reservoir temperatures and the δ¹⁸O values for calcite in shale can be related to the extent and spatial distribution of subsurface flow. The δ¹⁸O values for vein calcite record short-lived polythermal fracture-filling episodes at temperatures that may differ from those in the adjacent stable reservoir. The oxygen isotope data for shales indicate a minimum water - rock volume ratio of 2:1. Even this high flow was greatly exceeded in sandstones, with the result that the reservoir fluids are isotopically well-mixed and of relatively low salinity.     

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.     

Carbon isotope systematics and CO2 sources in The Geysers-Clear Lake region,northern California,USA

Carbon isotope analyses of calcite veins, organic carbon, CO₂ and CH₄ from 96 rock and 46 gas samples show that metamorphic calcite veins and disseminated, organically-derived carbon from Franciscan Complex and Great Valley Sequence rocks have provided a primary carbon source for geothermal fluids during past and present hydrothermal activity across The Geysers-Clear Lake region. The stable isotope compositions of calcite veins vary widely on a regional scale, but overall they document the presence of ¹³C-poor fluids in early subduction-related vein-precipitating events. δ¹³C values of calcite veins from the SB-15-D corehole within The Geysers steam field indicate that carbon-bearing fluids in the recent geothermal system have caused the original diverse δ¹³C values of the veins to be reset. Across The Geysers-Clear Lake region the carbon isotope composition of CO₂ gas associated with individual geothermal reservoirs shows a general increasing trend in δ¹³C values from west to east. In contrast, δ¹³C values of CH₄ do not exhibit any spatial trends. The results from this study indicate that regional variations in δ¹³C–CO₂ values result from differences in the underlying lithologies. Regional CO₂ contains significant amounts of carbon related to degradation of organic carbon and dissolution of calcite veins and is not related to equilibrium reactions involving CH₄. CO₂ from degassing of underlying magma chambers is not recognizable in this region.     

Correlations between steam saturation, fluid composition and well decline in vapor-dominated reservoirs

A large body of field data from Larderello and other vapor-dominated geothermal reservoirs shows striking temporal correlations between (1) decline of well flow rate, (2) gas/steam ratio, (3) chloride concentration, (4) degree of superheat and (5) vapor fraction. The latter is inferred from concentrations of non-condensible gases in samples of well fluid, using chemical phase equilibrium principles. Observed temporal changes in the vapor fraction can be interpreted in terms of a “multiple source” model, as suggested by D'Amore and Truesdell (1979, Models for steam chemistry at Larderello and The Geysers. Proc. 5th Workshop Geothermal Reserv. Engng, Stamford, California, pp. 283–297). This provides clues to the dynamics of reservoir depletion and to the evaluation of well productivity and longevity.     

Analysis of reservoir pressure and decline curves in Serrazzano zone, Larderello geothermal field

An estimate of reserves in the Serrazzano reservoir was obtained from mass balance studies and production decline curve analyses.The Serrazzano reservoir consists of a geometrically well-defined structural high of permeable formations separated from the other productive regions of the Larderello field.Deep drilling began in the 1930s and was limited to a small area exhibiting natural manifestations. After the second World War the area of drilling was extended to about 20 km². Currently the drilling area is about the same. Even though the reservoir has been producing steam since the 1930s, a systematic collection of production data did not begin until after 1953.Data on average reservoir pressures were not available for the material balance calculations made in the study reported here. Calculated bottom hole pressures of shut-in wells were taken therefore to represent local static reservoir pressures. These pressures were used to calculate an “average reservoir pressure” which was graphed as a function of cumulative production. The reservoir pressure history corresponding to the first half of current cumulative production is not known. Data for the second half indicate a linear relationship between “reservoir pressure” and cumulative production.The conventional straight-line p/z vs cumulative production material balance relationship is known to be correct, of course, for closed single-phase gas reservoirs. The validity of this linearity for stream-producing systems with boiling water has not been proved. Regardless of this, the following observations were made: a line connecting the available data points extrapolated back to zero production indicates an initial reservoir pressure approximating at least 40 atm. Extrapolating the same data to zero reservoir pressure indicates the total initial steam in place to be about 170 × 10⁶ tons.An empirical type-curve matching technique was applied to the production decline curves of wells in the reservoir. The curve for each well was extrapolated to infinite production time to obtain an estimate of total past and future production. Summing these values for all producing wells in the reservoir, an estimated total production (past and future) of 200 × 10⁶ tons was obtained.The agreement between the estimated total production applying material balance principles and decline curve analyses is remarkably good. Although these results may be useful, further field and theoretical work are necessary to prove their validity.     

The Cerro Prieto IV (Mexico) geothermal reservoir: Pre-exploitation thermodynamic conditions and main processes related to exploitation (2000–2005)

The Cerro Prieto IV (CP IV) reservoir, located in the northeastern part of the Cerro Prieto (Mexico) geothermal field, was studied in order to define its pre-exploitation conditions and initial (2000–2005) response to exploitation. Bottomhole thermodynamic conditions were estimated by modeling heat and fluid flows using the WELLSIM program and well production data. Produced fluid chemical and isotopic data were also analyzed to investigate characteristic patterns of behavior over time, which were then compared against simulation results to obtain a conceptual model of the CP IV reservoir. According to the proposed model, two zones in the reservoir – separated by Fault H and producing fluids of different characteristics – were identified under pre-exploitation conditions. Wells in the area to the east-southeast (south block) produce very high-enthalpy fluids (≥2000 kJ/kg), with very low chloride (≤7000 mg/kg) and high CO₂ (>6‰ molar) and δD (<−94‰). In contrast, wells toward the west-northwest (north block) show moderate-enthalpy fluids (1400–1800 kJ/kg), with high chloride (∼12,000 mg/kg) and relatively low CO₂ (<6‰ molar) and δD (<−94‰). Dilution caused by cooler water entry, boiling due to steam gain, both occurring in the north block, and steam condensation in the south block were identified as the three main reservoir processes associated with exploitation. Also, it was found that the dynamics of the CP IV reservoir is controlled by the Fault H system.     

Isotope systematics of C-bearing gas compounds in the geothermal fluids of Larderello, Italy

The origin of carbon-bearing compounds (CO₂, CH₄, C₂–C₄ saturated hydrocarbons) and helium in the geothermal fluid of Larderello is investigated by means of the variations in concentration and isotopic composition. The CO₂ (δ¹³C from −1.4 to −7.1‰ versus V-PDB) is mainly of crustal origin. The carbon isotopes of methane (δ¹³C from −20.9 to −31.7‰) and other hydrocarbons indicate a complex thermogenic origin. The temperatures obtained with the CH₄–CO₂ isotope geothermometer are in rough agreement with those observed in deeper geothermal wells. The CH₄/C₂H₆ ratios show a tendency towards partial equilibrium with increasing temperature. He isotopes (R/R_A from 0.5 to 3) indicate that although the major part of helium derives from crustal sources, a significant fraction of mantle helium is also present. Helium contamination by air, deducted from He/Ne ratios, is generally negligible.     

Hydrogeochemistry and groundwater circulation in the Xi’an geothermal field, China

Geothermal waters from the Tertiary aquifers located at 1000–3000 m beneath Xi’an city, Shaanxi Province, China, show unique isotopic composition as compared to local groundwaters from shallower Quaternary aquifers. Positive oxygen shifts of as much as 8‰ VSMOW are observed, while the corresponding δ²H values remain essentially constant at about −80‰ VSMOW, which is significantly different from those of waters in the Quaternary aquifers with a mean δ²H value of −60‰ VSMOW. The strong ¹⁸O shift is a result of isotope exchange between geothermal water and carbonate minerals such as calcite over a residence time of several thousand years up to 30,000 years, based on ¹⁴C dating. A comparison of the isotopic composition of geothermal waters with neighbouring groundwater units on both sides of the Guanzhong Basin indicates that the geothermal reservoirs are recharged by rain that falls on the northern slope of the Qinling Mountains, south of the Xi’an geothermal field, but not from the North Mountains to the north of the field. Based on chemical geothermometers the highest temperature estimated for the Tertiary aquifers of the Xi’an area is around 130 °C.     

An oxygen isotope study of silicates in the larderello geothermal field, Italy

Stable-isotope analyses were carried out on hydrothermal minerals sampled from the deep metamorphic units at Larderello, Italy. The ∂¹⁸O values obtained for the most retentive minerals, quartz and tourmaline, are from + 12.0‰ to + 14.7‰ and 9.9‰, respectively, and indicate deposition from an ¹⁸O-rich fluid. Calculated ∂¹⁸O values for these fluids range from + 5.3‰ to + 13.4‰. These values, combined with available fluid inclusion and petrographic data, are consistent with the proposed existence of an early thermal fluid of probable magmatic origin and a late meteoric water. Mixing between these two fluids occurred locally.     

Isotope geothermometry in the larderello geothermal field

The isotope geothermometers based on the ¹³C/¹²C fractionation between carbon dioxide and methane and on the ¹⁸O/¹⁶O fractionation between carbon dioxide and water vapour have been applied in Larderello geothermal field. The CO₂ - CH₄ thermometer gives temperatures which are 50–200°C higher than those measured at the well head. The distribution of the isotopic temperatures within the field follows more or less similar patterns to those given by the well-head temperatures. They are believed to reflect the temperatures of formation of CO₂ and CH₄.The CO₂ - H₂O thermometer gives the temperature of the geothermal reservoirs tapped, and the difference between the isotopic temperature and the temperature measured at the well head is a measure of the cooling undergone by geothermal fluid on its way up to the surface.     

Oxygen and hydrogen isotopes in deep thermal waters from the south meager creek geothermal area, british columbia, canada

Deuterium and oxygen-18 (¹⁸O) have been measured in deep thermal, shallow thermal and non-thermal water samples collected at various times between 1982 and 1989 from the Meager Creek area, with the aim of assessing the origin of the thermal waters. The isotopic composition of the reservoir waters (δ¹⁸O = −13‰ and δD = −114.8‰) was calculated from data on post-flash deep thermal waters, using a two-stage steam loss model. The reservoir composition shows an oxygen shift of 2.4‰ relative to the local meteoric water line. The composition of the recharge, obtained by removing the oxygen shift, is isotopically heavier than the average local meteoric waters, suggesting that the recharge may be from an area to the west of Mt Meager where isotopically heavier ground-waters are likely to be found. The small δ¹⁸O shift of the deep high-temperature waters is indicative of dominance of fracture-related permeability in the reservoir. Analysis of the chemistry and the temperature of the waters from hot springs and shallow thermal wells suggests that these waters have evolved from the deep geothermal waters through dilution by meteoric waters and about 40°C adiabatic cooling (steam loss).     

Rock-water interactions in the Fenton Hill, new Mexico, hot dry rock geothermal systems I. fluid mixing and chemical geothermometry

The chemistry of fluids circulated through an artificially-stimulated, hot dry rock (HDR) fractured geothermal reservoir system in granitic rock is described in terms of mixing phenomena, geothermometry, and approach to saturation with reservoir rock minerals. Based on the similar dynamic behavior of Na⁺, K⁺, Li⁺, CI⁻, and B species and other isotopic evidence, the presence of a concentrated in-situ pore fluid was identified. Mixing and displacement of this in-situ fluid with meteoric make-up water is responsible for the observed behavior of the major dissolved species in the circulated fluid of this HDR system.     

Hydrothermal alteration of Hercynian granites,its significance to the evolution of geothermal systems in granitic rocks

We discuss geochemical and isotopic (¹⁸O/¹⁶O, ²H/¹H and ⁸⁷Sr/⁸⁶Sr) data recording the hydrothermal alteration of northern Portuguese Hercynian granites by Na–HCO₃–CO₂-rich mineral waters. Whole-rock samples from drill cores of Vilarelho da Raia granite have δ¹⁸O values in the +11.47 to +10.10‰ range. The lower values correspond to highly fractured granite samples displaying vein and pervasive alteration. In the pervasive alteration stage, which probably results from a convective hydrothermal system set up by the intrusion of the granites, the metamorphic waters are in equilibrium with hydrous minerals. In contrast, the vein alteration of these granitic rocks was caused by water of meteoric origin. The oxygen ratios between water (W) and rock (R), the so-called W/R ratios, obtained for the open system (where the heated water is lost from the system by escape to the surface) range between 0.05 and 0.11, suggesting that the recrystallization of the veins was influenced by a small flux of meteoric water. Stable isotope analyses performed on the cores show that the vein alteration stage relates to post-emplacement tectonic stresses acting on the granite, probably of late Hercynian age. Our results are consistent with the existence of two separate alteration events (pervasive and vein) caused by hydrothermal waters of different isotopic characteristics. The studies presented in this paper should be viewed as a natural analogue that uses the alteration features observed in a “fossil” geothermal system at Vilarelho da Raia to assess possible water–rock reactions presently occurring at depth in granitic rocks of the nearby Chaves area.     

Oil and Gas Occurrences in Southwest Turkey

Abstract

In SW Turkey, on both sides of the Antalya basin, two continental micro-plates can be distinguished: the Anatolian micro-plate to the NE and Bey Da?lar[idot] micro-plate to the W. These micro-plates are composed of a Paleozoic basement of Gondwana type, surrounded by Mesozoic carbonate platforms. From south to north the Bey Da?lar[idot] platforms are made of shallow marine carbonates from Triassic time up to Cenomanian. The southern part of this carbonate platform remained under shallow marine conditions up to the Eocene. On the contrary, the northern part subsided during Late Cretaceous time. In the Bey Da?lar[idot] platform, occurrences of organic materials are known in the Triassic and in the Early Cretaceous-Cenomanian carbonates. Rock-eval analysis indicate 0.25 to 1.46% total organic carbon (TOC). These facies could have represented a source rock for oil and gas. In the same area the uncomformable Early Miocene carbonates are made of bioclastic calcarenites. In some areas these calcarenites are dark colored and contain oil. These carbonates represent a reservoir rock The Anatolian micro- plate is also composed of shallow marine carbonates from Triassic time up to Eocene. Organic materials are known in the Akkuyu formation, Late Jurassic in age. According to organic geochemical characteristics, this unit is a good source rock. The total organic carbon (TOC) content of this unit is up to 12%. In the same area, other occurrences of oil and gas are known in two locations, Çirali to the SW of Antalya Bay and the Kizildag to the NE of Antalya. The Çirali location is known since the ancient civilizations as Chimera. Gas of this area is pouring out from a complex tectonic imbricate structure including ultrabasic rocks. The source rock and the reservoir are poorly known. The Kizildag is a large klippen of peridotites in the Antalya nappes complex. Liquid petroleum seepage is observed in this ophiolitic unit.     

Energy crops in Ireland: Quantifying the potential life-cycle greenhouse gas reductions of energy-crop electricity

This study uses life-cycle assessment (LCA) to compare greenhouse gas (GHG) emissions from dominant agricultural land uses, and peat and coal electricity generation, with fuel-chains for Miscanthus and short-rotation-coppice willow (SRCW) electricity. A simple scenario was used as an example, where 30% of peat and 10% of coal electricity generation was substituted with co-fired Miscanthus and SRCW, respectively. Miscanthus and SRCW cultivation were assumed to replace sugar-beet, dairy, beef-cattle and sheep systems. GHG emissions of 1938 and 1346 kg CO₂ eq. ha⁻¹ a⁻¹ for Miscanthus and SRCW cultivation compared with between 3494 CO₂ eq. ha⁻¹ a⁻¹ for sugar-beet cultivation and 12,068 CO₂ eq. ha⁻¹ a⁻¹ for dairy systems. Miscanthus and SRCW fuel chains emitted 0.131 and 0.132 kg CO₂ eq. kWh⁻¹ electricity exported, respectively, compared with 1.150 and 0.990 kg CO₂ eq. kWh⁻¹ electricity exported for peat and coal fuel chains. 1.48 Mt CO₂ eq. a⁻¹ was saved from electricity production, and 0.42 Mt CO₂ eq. a⁻¹ was saved from displaced agriculture and soil C-sequestration. The total reduction of 1.9 Mt CO₂ eq. a⁻¹ represents 2.8% of Ireland's 2004 GHG emissions, but was calculated to require just 1.7% of agricultural land area and displace just 1.2% of the dairy herd (based on conservative Miscanthus and SRCW combustible-yield estimates of 11.7 and 8.81 t ha⁻¹ a⁻¹ dry matter, respectively). A 50% increase in cultivation emissions would still result in electricity being produced with an emission burden over 80% lower than peat and coal electricity. Lower yield assumptions had little impact on total GHG reductions for the scenario, but required substantially greater areas of land. It was concluded that energy-crop utilisation would be an efficient GHG reduction strategy for Ireland.     

Geochemistry of thermal waters on the island of Ischia (Campania, Italy)

The stratigraphic and structural situation on the island of Ischia (southern Italy), the recent volcanic activity and the presence of hot springs and fumaroles, suggest the existence of a geothermal field. The chemical and isotopic compositions of the waters from several springs and wells were examined to obtain information on deep temperatures and to formulate a geothermal model of the island. δD values range from −33.60 to −12.50‰ and δ¹⁸O from −7.10 to −1.71‰, relative to SMOW. These variations have mainly been attributed to the presence of seawater, as confirmed by the general shift to more positive values with the increase of Cl content. Water-rock reactions, evaporation and subsurface boiling also contribute to the δ¹⁸O−δD trend. The chemical analyses reveal the presence of alkaline sulphate chloride water (seawater), bicarbonate waters and waters interpreted as the result of mixing. The chemical and isotopic composition of the latter are dependent on water-rock interactions, water circulation rates and eventual evaporation and condensation phenomena. The silica geothermometer, which seems to be the most suitable for determining the deep temperatures of these waters, gave values of about 200°C, even for mixing models. Our data suggest the following geothermal model: the heat flow heats up a deep reservoir, causing steam to rise through faults and fractures and transfer heat to a shallower aquifer. The temperatures of 200°C obtained by the geothermometers are not the maximum reservoir temperatures, but are probably water-rock equilibrium temperatures for the shallower aquifers. The high boron contents and the isotopic data confirm the presence of steam in the system.