Secondary changes in the chemical and isotopic composition of the geothermal fluids in Larderello field

Processes such as condensation of rising steam in colder horizons, mixing of deep steam with shallower waters and non-geothermal gas inflow are known to exist in the vapour-dominated geothermal system of Larderello. Deep steam drawn up by geothermal wells modifies its original chemico-physical characteristics in relation to the presence of one or more ‘secondary’ phenomena occurring in each point of the field. ¹³C/¹²C ratios in CO₂ and CH₄, chloride-ammonia-boric acid contents, H₂/H₂S and gas/steam ratios are used as useful parameters in delimiting the different zones in which this kind of phenomena predominate. Six main areas seem to be characterized geochemically, showing marked variations that are essentially due to differences in the geohydrological situations and thermal conditions.     

Comparison of carbon dioxide emissions with fluid upflow, chemistry, and geologic structures at the Rotorua geothermal system, New Zealand

During 2002 and 2003, carbon dioxide fluxes were measured across the Rotorua geothermal system in the Taupo Volcanic Zone (TVZ), New Zealand. The results of a 956-measurement survey and of modeling studies show that CO₂ fluxes could be used to determine the main hot fluid upflow areas in Rotorua, and perhaps in undeveloped geothermal regions. Elevated degassing was observed along inferred fault traces and structures, lending confidence to their existence at depth. Degassing was also observed along lineaments that were consistent with the alignment of basement faulting in the TVZ. Areas where elevated degassing was spatially extensive typically overlapped with known regions of hot ground; however, elevated CO₂ fluxes were also observed in isolated patches of non-thermal ground. The total emission rate calculated from sequential Gaussian simulation modeling of CO₂ fluxes across the geothermal system was 620 t d⁻¹ from an 8.9-km² area. However, because approximately one-third of the geothermal system is known to extend beneath Lake Rotorua, we expect the emissions could be minimally on the order of 1000 t d⁻¹. Comparing the emission rate with geochemical analyses of geothermal fluids and estimated upflows suggests that the majority of deep carbon reaches the surface in the form of carbon dioxide gas, and that less than one tenth of the CO₂ emissions is dissolved in, or released from, the fluids at depth. Thus, the geothermal reservoir exerts very little control on deep degassing of CO₂. Carbon isotopic analyses of soil gases suggest a primarily magmatic source for the origin of the CO₂. The total Rotorua emission rate is comparable to those from active volcanoes such as at White Island, New Zealand, and, when normalized by geothermal area, is comparable to other volcanic and hydrothermal regions worldwide.     

Net N2O and CH4 soil fluxes of annual and perennial bioenergy crops in two central German regions

The area used for bioenergy feedstock production is increasing because substitution of fossil fuels by bioenergy is promoted as an option to reduce greenhouse gas (GHG) emissions. However, agriculture itself contributes to rising atmospheric nitrous oxide (N₂O) and methane (CH₄) concentrations. In this study we tested whether the net exchanges of N₂O and CH₄ between soil and atmosphere differ between annual fertilized and perennial unfertilized bioenergy crops. We measured N₂O and CH₄ soil fluxes from poplar short rotation coppice (SRC), perennial grass-clover and annual bioenergy crops (silage maize, oilseed rape, winter wheat) in two central German regions for two years. In the second year after establishment, the N₂O emissions were significantly lower in SRC (<0.1 kg N₂O–N ha⁻¹ yr⁻¹) than grassland (0.8 kg N₂O–N ha⁻¹ yr⁻¹) and the annual crop (winter wheat; 1.5 kg N₂O–N ha⁻¹ yr⁻¹) at one regional site (Reiffenhausen). However, a different trend was observed in the first year when contents of mineral nitrogen were still higher in SRC due to former cropland use. At the other regional site (Gierstädt), N₂O emissions were generally low (<0.5 kg N₂O–N ha⁻¹ yr⁻¹) and no crop-type effects were detected. Net uptake of atmospheric CH₄ varied between 0.4 and 1.2 kg CH₄–C ha⁻¹ yr⁻¹ with no consistent crop-type effect. The N₂O emissions related to gross energy in the harvested biomass ranged from 0.07 to 6.22 kg CO₂ equ GJ⁻¹. In both regions, Gierstädt (low N₂O emissions) and more distinct Reiffenhausen (medium N₂O emissions), this energy yield-related N₂O emission was the lowest for SRC.     

A single flash integrated gas turbine-geothermalpower plant with non condensable gas combustion

In this work the possibility of improving the performance of the 20 MW standardgeothermal power plant of ENEL (Italian Electric Power Company) has been studied Theconventional geothermal cycle was modified by adding a gas turbine an organic Rankine cycle (ORC)and a flash separator Exhaust heat from the gas turbine is recovered in a geothermal steamsuperheater and by the ORC The results of a thermodynamic analysis have been applied to a testcase based on the Mt Amiata geothermal field where geothermal fluid has a water mass fractionranging from 30 to 50% by wt and about 8% by wt of the steam fraction is non condensablegases (NCG) The non condensable gases are a mixture of CO₂ (about 95% byweight)H₂S, H₂Hg,NH₃ and CH₄ Legislation onthe emission of some of these substances into the atmosphere means that geothermal powerplants are now equipped with gas clean-up devices but these increase the plant capital costs andreduce performance Some of the components contained in the NCG however such as H₂S H₂ and CH₄ have an acceptable lower heating value (LHV) andcan be considered a source of energy Hence they could be burned to reduce their environmentalimpact while recovering energy that would otherwise be lost In the power plant presented in thispaper NCG are mixed with inlet air in the gas turbine and burnt in the combustion chamber thushelping to increase hybrid cycle performance © 1999 CNR Published by Elsevier Science LtdAll rights reserved     

Biochar-mediated reductions in greenhouse gas emissions from soil amended with anaerobic digestates

This investigation examines nitrous oxide (N₂O) fluxes from soil with simultaneous amendments of anaerobic digestates and biochar. The main source of anthropogenic emissions of N₂O is agriculture and in particular, manure and slurry application to fields. Anaerobic digestates are increasingly used as a fertiliser and interest is growing in their potential as sources of N₂O via nitrification and denitrification. Biochar is a stable product of pyrolysis and may affect soil properties such as cation exchange capacity and water holding capacity. Whilst work has been conducted on the effects of biochar amendment on N₂O emissions in soils fertilised with mineral fertilisers and raw animal manures, little work to date has focused on the effects of biochar on nitrogen transformations within soil amended with anaerobic digestates. The aim of the current investigation was to quantify the effects of biochar application on ammonification, nitrification and N₂O fluxes within soil amended with three anaerobic digestates derived from different feedstocks. A factorial experiment was undertaken in which a sandy loam soil (Dunnington Heath series) was either left untreated, or amended with three different anaerobic digestates and one of three biochar treatments; 0%, 1% or 3%. Nitrous oxide emissions were greatest from soil amended with anaerobic digestate originating from a maize feedstock. Biochar amendment reduced N₂O emissions from all treatments, with the greatest effect observed in treatments with maximum emissions. The degree of N₂O production and efficacy of biochar amelioration of gas emissions is discussed in context of soil microbial biomass and soil available carbon.     

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.     

Gases in steam from Cerro Prieto geothermal wells with a discussion of steam/gas ratio measurements

As part of a joint USGS-CFE geochemical study of Cerro Prieto, steam samples were collected for gas analyses in April, 1977. Analyses of the major gas components of the steam were made by wet chemistry (for H₂O,CO₂,H₂S and NH₃) and by gas chromatography (He,H₂,Ar,O₂,N₂ and hydrocarbons).The hydrocarbon gases in Cerro Prieto steam closely resemble hydrocarbons in steam from Larderello, Italy and The Geysers, California which, although they are vapor-dominated rather than hot-water geothermal systems, also have sedimentary aquifer rocks. These sedimentary geothermal hydrocarbons are characterized by the presence of branched C_4–6 compounds and a lack of unsaturated compounds other than benzene. Relatively large amounts of benzene may be characteristic of high-temperature geothermal systems. All hydrocarbons in these gases other than methane most probably originate from the thermal metamorphosis of organic matter contained in the sediments.     

Fumarolic gas chemistry at Wairakei,New Zealand, 1936–1998

The Wairakei geothermal field fumarolic discharges are at their greatest intensity from the Karapiti Thermal Area. This part of the geothermal field contains numerous steam-dominated features in an area of approximately 1 km². Since 1952 there have been many changes to the surface features and thermal activity at Karapiti related to the development-induced pressure drawdown over most of the field. A greatly expanded steam cap fed by a large low-pressure steam zone in the Wairakei reservoir has replaced the hot chloride water originally underlying Karapiti. There have been intermittent chemical surveys of the steam vents at Karapiti since 1951, with major chemical surveys undertaken in 1961 and 1990. In 1990, the concentrations of CO₂ were found to be about 200 mmol/100 mol H₂O, double the 1961 values. Of particular interest is the change between 1936 and 1987 in gas chemistry of the main Karapiti feature, the Karapiti Blowhole (F712), which follows the change in heatflow from the Karapiti Thermal Area. Since 1990, gas concentrations appear to be dropping to low, pre-development levels, most likely due to the decreasing pressures in the lower pressure steam zone. Underground processes leading to a high gas content in a large fumarole formed in 1967 and other fumaroles in different parts of the Wairakei field are discussed.     

Gas chemistry and helium isotopes at Cerro Prieto

Seven producing wells and seven hot springs in the Cerro Prieto geothermal field were sampled during April and September 1977, for determination of gas chemistry and helium isotope ratios. Well gases are remarkably uniform in gas chemistry and helium isotope ratio, showing high ³He/⁴He ratios characteristic of mantle-derived helium, and higher than expected N₂/Ar ratios. Comparison with the hot spring data suggests that the deep gas component observed in wells is modified during transit to the hot springs by addition of crustal helium, dissolved air and possibly organic methane; alternatively, chemical re-equilibration at lower temperatures may be responsible for increasing the CH₄/H₂ ratio.     

Bioenergy driven land use change impacts on soil greenhouse gas regulation under Short Rotation Forestry

Second-generation bioenergy crops, including Short Rotation Forestry (SRF), have the potential to contribute to greenhouse gas (GHG) emissions savings through reduced soil GHG fluxes and greater soil C sequestration. If we are to predict the magnitude of any such GHG benefits a better understanding is needed of the effect of land use change (LUC) on the underlying factors which regulate GHG fluxes. Under controlled conditions we measured soil GHG flux potentials, and associated soil physico-chemical and microbial community characteristics for a range of LUC transitions from grassland land uses to SRF. These involved ten broadleaved and seven coniferous transitions. Differences in GHGs and microbial community composition assessed by phospholipid fatty acids (PLFA) profiles were detected between land uses, with distinctions between broadleaved and coniferous tree species. Compared to grassland controls, CO₂ flux, total PLFAs and fungal PLFAs (on a mass of C basis), were lower under coniferous species but unaffected under broadleaved tree species. There were no significant differences in N₂O and CH₄ flux rates between grassland, broadleaved and coniferous land uses, though both CH₄ and N₂O tended to have greater uptake under broadleaved species in the upper soil layer. Effect sizes of CO₂ flux across LUC transitions were positively related with effect sizes of soil pH, total PLFA and fungal PLFA. These relationships between fluxes and microbial community suggest that LUC to SRF may drive change in soil respiration by altering the composition of the soil microbial community. These findings support that LUC to SRF for bioenergy can contribute towards C savings and GHG mitigation.     

Low enthalpy geothermal fluids from the Paris sedimentary basin—1. Characteristics and origin of gases

Fifty-seven wells tapping low enthalpy geothermal waters from the Dogger limestone reservoir of the Paris Basin have been sampled and analysed for their gas composition. Methane/ethane ratios indicate that hydrocarbons originate from both biogenesis and thermogenesis. Relatively high and variable H₂ concentrations are likely to result from fluid interaction with geothermal installations. Regional trends among main species (N₂, CO₂, CH₄) reflect large-scale heterogeneities which were already revealed by the geochemistry of the fluids (Criaud et al., 1986). Northern sites are generally nitrogen-rich and southern sites methane-rich, a characteristic which may be linked to the occurrence of oil-bearing zones in the south. N₂, Ar and He absolute contents show clear positive correlations. In particular, N₂—Ar trends are best explained by the occurrence of a paleocomponent, likely to be an evolved seawater. Helium model ages are consistent with the geological age of the host formation but are in contradiction with hydrologic ages. Assuming an exotic flux of helium into the aquifer this discrepancy may be overcome, but its computed rate apparently exceeds the current estimates for the continental degassing flux.     

Methodology for calculating steam quality in geothermal reservoirs

A promising technique is presented for computing steam fraction in a two-phase reservoir, utilizing fluid composition and its variations. The molar fractions in the steam phase of the chemical species in the reservoir are first computed, starting with fluid composition at the wellhead. The gas species in the reservoir are partly in the vapour phase and partly dissolved in the liquid phase. We assume that the main gas species, such as CO₂, CH₄, H₂, H₂S and H₂O, are in chemical equilibrium in the reservoir. One other fundamental assumption is that the two-phase fluid is in phase and chemical equilibrium in the reservoir and fluid is transferred at the wellhead without any mass gain or loss, although phase changes may occur.     

Hot spring activity in the Dakongbeng geothermal area, China

The Dakongbeng geothermal area, whose hot springs reach a temperature of up to 96°C, has been considered one of the potential high-temperature hydrothermal systems in south-west China. The concentration of dominant cations and anions indicates an NaHCO₃ type of thermal water, whose major constituents in decreasing order are: Na>K>Ca>Mg, HCO₃>SiO₂>Cl>SO₄. On the basis of the silica geothermometer, cation geothermometers, gas geothermometer and activity diagram, the reservoir temperature is estimated at about 200°C. All the thermal waters have originated from meteoric water of a higher altitude that circulated as ground water at considerable depth along faults. The stability of their contents of Cl, SiO₂, δD, δ¹⁸O and of the Cl/B, Na/Li ratios suggests that the main heat loss process is through steam loss. The geochemistry of the initial liquid has been estimated by single and continuous steam loss. On the basis of its geologic and geographic setting, the Dakongbeng geothermal area appears to belong to the Himalayan geothermal belt and is thus regarded as an area of interest for further study.     

Role of methane as a cushion gas for hydrogen storage in depleted gas reservoirs

Depleted natural gas reservoirs play an important role as a viable option for large-scale hydrogen storage and production. However, its deployment depends on the accurate knowledge of the cushion gas (such as CH₄, CO₂, and N₂) compositions, which are key components affecting the rock-fluid interfacial phenomenon. In addition, there are currently few reported studies on rock/brine/gas-mixture wettability and gas-mixture/brine surface tension representing this type of reservoir. Hence, we report the feasibility of using CH₄ as a cushion gas (in the presence of CO₂ and N₂) for H₂ storage at various pressures (500 up to 3000 psi), temperatures (30 up to 70) ^oC, and salinities (2 up to 20) wt.% using drop shape analyzer equipment. Contact angle (CA) and surface tension (ST) experiments were extensively conducted for the different gas mixtures (H₂–CH₄–CO₂–N₂) to establish relevant data for H₂ storage in depleted gas reservoirs.Our result indicates that unless when the rock's initial wetting state is altered, the studied gas-mixture compositions (Test case 1: 80% H₂ – 10% CH₄ – 5% CO₂ – 5% N₂; case 2: 70% H₂ – 20% CH₄ – 5% CO₂ – 5% N₂; case 3: 60% H₂ – 30% CH₄ – 5% CO₂ – 5% N₂; case 4: 50% H₂ – 40% CH₄ – 5% CO₂ – 5% N₂; case 5: 40% H₂ – 50% CH₄ – 5% CO₂ – 5% N₂; case 6: 30% H₂ – 60% CH₄ – 5% CO₂ – 5% N₂; and case 7: 20% H₂ – 70% CH₄ – 5% CO₂ – 5% N₂) will exhibit comparable wettability behavior as the CAs ranged between [20 to 41°] irrespective of the reservoir pressure, temperature, and salinity. ST decreases with increasing temperature and linearly with increasing pressure. ST for each gas mixture increased with salinity. ST decreases systematically with increasing CH₄ fraction (at any given salinity, temperature, and pressure) with the highest observed in Test case 1 and the lowest in Test case 7 compositions. Test cases 3 and 4 with H₂ (50–60%) and CH₄ (30–40%) fractions was selected as the optimal gas mixture based on CA and ST for H₂ storage and withdrawal. The study's findings offer precise and useful input data for the reservoir-scale simulation used in geo-storage optimization in depleted natural gas reservoirs.     

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.     

Geochemistry in geothermal exploration

Techniques based on the variations in composition of water, gas and stable isotopes in the liquid and gas phases of the geothermal fluids have been applied for some time now in the major geothermal fields and are now also used regularly in geothermal exploration. There are numerous processes capable of modifying isotopic composition after infiltration of water from the surface, such as water-rock exchanges, formation of secondary minerals and exchange with the gaseous phase (CO₂ and H₂S). During ascent to the surface, the two main processes are steam separation and dilution and mixing with shallower waters. This paper also deals with the chemical characteristics of the waters, their classification and the water-rock interaction producing hydrothermal alteration. During exploration the chemical and isotopic geothermometers represent a unique method for investigating the deep system. The choice of geothermometer and interpretation of geothermometric data are two crucial steps in geothermal exploration. Finally, the paper discusses the geochemistry of gas mixtures, especially the origin of the gas species and the main chemical reactions that produce semi-empirical geothermometers and some recent non-empirical geothermometers based on models of a two-phase system in the reservoir. Gas-geothermometers can be developed to calculate the reservoir temperature for natural manifestations.     

Conversion of carbon dioxide and methane in biomass synthesis gas for liquid fuels production

The premise of this research is to find whether methane (CH₄) and carbon dioxide (CO₂) produced during biomass gasification can be converted to carbon monoxide (CO) and hydrogen (H₂). Simultaneous steam and dry reforming was conducted by selecting three process parameters (temperature, CO₂:CH₄, and CH₄:steam ratios). Experiments were carried out at three levels of temperature (800 °C, 825 °C and 850 °C), CO₂:CH₄ ratio (2:1, 1:1 and 1:2), and CH₄:steam ratio (1:1, 1:2 and 1:3) at a residence time of 3.5 × 10³ g_cat min/cc using a custom mixed gas that resembles biomass synthesis gas, over a commercial catalyst. Experiments were conducted using a Box-Behnken approach to evaluate the effect of the process variables. The average CO and CO₂ selectivities were 68% and 18%, respectively, while the CH₄ and CO₂ conversions were about 65% and 48%, respectively. The results showed optimum conditions for maximum CH₄ conversion was at 800 °C, CO₂:CH₄ ratio and CH₄:steam ratios of 1:1.     

Geochemistry of the surface and deep fluids of the Miravalles volcano geothermal system (Costa Rica)

The Miravalles high-temperature geothermal reservoir, located in the northwestern part of Costa Rica, is liquid-dominated. Reservoir temperatures generally range between 230 and 240 °C. The highest measured value is 255 °C. Bottom-hole measurements and solute geothermometry indicate that thermal conditions within the reservoir are very stable over time. The waters discharged from the wells have a neutral or slightly alkaline pH and are of the sodium-chloride type. Based on isotope data, the main recharge zone appears to be located on the northeastern side of the Guanacaste Cordillera. Several mixing trends have been identified between reservoir fluids and regional groundwaters. Gas discharges are dominated by CO₂, with minor amounts of H₂S and N₂. Relative N₂, Ar and He contents reveal a typical arc-type signature and significant inflow of meteoric-derived gases. Cl–SiO₂-enthalpy and δ¹⁸O–δ²H–Cl relationships suggest the existence of a maturation trend that is the result of both natural (i.e. direct drainage of deeper fluids) and anthropogenic causes (reinjection of Cl-rich waste waters). Acid fluids with SO₄-acidity (pH ranging between 2.4 and 3.7) have been encountered in three wells at the eastern border of the well field. Preliminary data assessment indicates two possible sources, either superficial H₂S oxidation or inflow of “immature” volcanic waters.     

Experimental study on high temperature catalytic conversion of tars and organic sulfur compounds

A 400 cpsi noble metal catalyst was used to test the conversion of tars and sulfur containing hydrocarbons in the presence of steam, hydrogen sulfide and ethene. In order to reproduce producer gas from biomass gasification, higher molecular hydrocarbons (toluene, naphthalene, phenanthrene, pyrene) and sulfur containing hydrocarbons (thiophene, benzothiophene, dibenzothiophene) were added to a syngas. The syngas consisted of H₂, CH₄, H₂O, CO, CO₂ and N₂. The catalyst was operated at temperatures between 620 °C and 750 °C and at gas hourly space velocity (GHSV) of 9000 h⁻¹ and 18,000 h⁻¹.     

Fluid inclusion study of the Kirishima geothermal system, Japan

Gases from fluid inclusions in quartz and anhydrite were analyzed with a quadrupole mass spectrometer and a capacitance manometer. The quartz and anhydrite occur in hydrothermal veins in volcanic and pelitic rocks collected from geothermal wells in the Kirishima area, southwest Japan. The geothermal wells are located in a graben made up of Quaternary volcanic rocks underlain by sedimentary rocks of the Shimanto Group.Results of individual fluid inclusion analyses show that the fluid inclusions comprise mainly H₂O and a variable but small amount of CO₂. CH₄ and other hydrocarbons are also detected in inclusions in a hydrothermal sample from the pelitic Shimanto Group. Peak ratios of CO₂/H₂0 in individual fluid inclusions are variable in some samples. This indicates that there is a difference in gas compositions of the fluid inclusions, and suggests that the inclusions were formed in multistages or trapped heterogeneous boiling fluids.Results of bulk analyses show that the inclusions are mainly composed of H₂O (98–99 mol%) with small amounts of non-condensable gases, mainly C0₂ and N₂, CH₄ and Ar. The proportion of N₂ is about one order of magnitude lower than C0₂, CH₄ is generally two orders of magnitude lower than C0₂ and Ar is just above the detection limit of the mass spectrometer. The gas concentration in the fluid inclusions is much higher than that in the present-day discharge fluids in this area. CO₂/N₂ and C0₂/CH₄ ratios of the fluid inclusions from the volcanic rocks are lower than those of the present-day discharge fluids. CO₂/N₂ and CO₂/CH₄ ratios in residual fluids increase with progressive degassing, because N₂ and CH₄ are released from the residual fluids more easily than CO₂. Thus, the difference in the CO₂/N₂ and CO₂/CH₄ ratios between the fluid inclusions and the present-day discharge fluids in the Kirishima area may be ascribed to the degree of degassing, and the fluid inclusions in the area were probably formed by trapping fluids that were weakly influenced by degassing. P_co2, values calculated from the gas compositions of the fluid inclusions are higher than that of buffer systems involving alteration minerals in the area. This suggests that the fluid inclusions might be trapped fluids which were not in equilibrium with the alteration mineral assemblages, that is, fluids prior to considerable degassing and alteration.