The self-sealing geothermal field

A geothermal field producing dry steam or high temperature water is a trap for convection currents generated in an aquifer of high permeability and of sufficient thickness by a deep heat source. A basic implication of this concept is, that a geothermal field requires a cap-rock of more or less impermeable rocks above the producing aquifer. In Larderello, Monte Amiata, and Salton Sea geothermal fields, a clearly reconnaissable tight formation overlies the producing zone and limits the upward movements of the convection currents. In other fields,i.e. The Geysers (California), Wairakei and Waiotapu (New Zealand) we do not know a geologically well defined cap-rock formation, presenting a large difference in permeability in comparison with the reservoir formation. The hot water circulating in a hydrothermal system without a cap-rock can produce deposits and rock alteration in proper places along the flow paths. The tracture and pore filling and any other permeability reducing factors increase resistance to the water circulation: those processes can originate an effective cap-rock. By such processes a hydrothermal system can become a self-sealed geothermal field. The silica deposition is probably the main self-sealing process. In fact, 1) silica is very common. 2) it is available with almost no limitation, 3) its deposition is strictly related to temperature changes, and 4) it is likely to produce very effective patterns of deposition. Where an unlimited CO₂ supply is available at depth, the calcium carbonate deposition appears to be a noticeable sealing process, which is controlled by pressure, at relatively shallow depth. In other cases CaCO₃ precipitation should not be an important factor in the self-sealing of geothermal fields. Argillization appears to be an important shallow process. It is especially effective in the acid environment of many thermal shows, thus determining their migration and/or extinction. According to our analysis and to present evidence those three self-sealing processes are the most important ones. In The Geysers Field, the wells penetrated the same formation, the Franciscan graywackes, from top to bottom. The Franciscan Formation has a very low primary permeability; secondary or fissure permeability is at the contrary very high. It is evident that there is no recognizable cap-rock in the accepted sense of petroleum geology. The wells produce superheated steam; the producing zone begins at 300 m depth or so; the quantity of steam increases with the thickness of the producing zone penetrated by the holes. Beginning in 1964, the wells have been drilled with air as circulating medium. No steam or water has been observed in the top few hundreds meters drilled: we can safely conclude that the graywackes are impervious in the upper section of the holes. Cores and cuttings show frequent fissures filled with silica in different mineral forms and hydrothermally altered rocks are common. In the Geysers area, hot springs, steam vents, carbon dioxide and hydrogen sulphide fumaroles are numerous, and wide zones of rocks, altered by past hydrothermal activity, are prominent features. As usual in many hyperthermal areas, also in The Geysers the manifestations of surface heat change frequently in place, in size, and in fluids discharge. The filling of rock fissures by mineral deposition seems the simplest and most natural explanation of the place changes of the individual springs. The active faults continually generate new fissures, limit the sealing action, and account for the persistent surface thermal activity of the area. The composition of the waters from the hot springs at The Geysers has been re-considered, in comparison with both surface waters and natural steam. The hot springs mainly originate by natural steam condensation, as Allen and Day stated in 1927. This conclusion is now strenghtened and extended: the perched water table producing hot springs at The Geysers is purely condensed steam. Practically all its characteristics can be explained by this condition alone. Separation from other shallow water bodies is extremely sharp. Let us sumarize: the impermeability of the upper section of the holes is demonstrated by the lack of fluids in the Sulphur Bank area, whereas the geochemistry of the hot springs compared with shallow waters indicates that similar conditions occur in the Geysers and Little Geysers areas. Furthermore, silica and other fissure-filling processes occur all over the region, as well as argillization of graywackes. We conclude that:

1. a)
   a cap-rock exists in The Geysers Field; this fact readily explains the production of dry steam;     
   
   

Geothermal resources in the imperial valley of california

The Imperial Valley is a major rift valley characterized by unusually high heat flow and large quantities of water in storage in the thick fill of alluvium provided by the sediments of the delta of the Colorado River. The inventory of hot water appears to be sufliciently large that if used for water desalination it might add several million acreleet of new water to the resources of the lower Colorado River basin. This distilled water would serve to lower river salinity and provide extra water to help meet the U. S. — Mexico treaty commitments. A major fraction of water desalination costs lie in the cost of energy and are related to desalination technology which is directly related to water chemistry. The discovery of low salinity geothermal waters in the Imperial Valley opened th possibility for a major breakthrough in lowered water desalination costs. We have tried to develop a broad understanding of the origins of the waters of the Imperial Valley and how natural recharge occurs. The chemical composition of the waters of the central portion of Imperial Valley basin waters, while not that of present surface flow of the rivers, nevertheless does have a close affinity to Colorado River water. No sea water seems to be present in the valley although marine sediments appear to occur on basement on West Mesa and on basement to the east in Arizona south of Yuma. Low salinity waters dominate the basin hydrology and waters as saline or more saline than sea water appear to be restricted to the immediate area of the Salton Sea. The isotope work of T. Coplen makes it possible to determine the relative contribution of precipitation runoff from California watersheds and from Colorado River water. Both sources are significant. The Colorado River water in aquifers from 100–400 m appears to have been entrapped from a relatively homogeneous basin which was subject to substantial evaporation. Its original source was snow melt water from the Colorado River. Five types of waters, none of them sea water, were recognized by their salt geochemistry. Bromide/chloride data are particularly effective in resolving different types of water masses. The bromide/chloride data agree with the isotope data and identify rainfall and precipitation runoff from the high mountains to the west. Modern Colorado River water is easily recognized by its salts and two types of ancient Colorado River waters from previous lake stage are proposed on the basis of the bromide/chloride data. One old lake occurring during the pluvial stage associated with the last Ice Age is proposed to account for much of the water in artesian aquifers. Another younger lake stage, possibly with Lake Cahuilla affinities is also suggested. Mountain runoff waters can be distinguished in the subsurface by their relatively lower salinity and high bicarbonate concentration, and their heavy isotopic composition. Revised fluid reserve calculations based on additional porosity data continue to show that the low salinity water resources of the Imperial Valley may exceed two billion acre-feet. The oceanic plate tectonic model is modified in the Imperial Valley by the evidence of a series of complex blocks with the generation of both tensional and compressional features in the valley. Major strike slip faults dominate the tectonic fabric but conjugate features increase complexity by a large degree and a major amount of work will be needed before any geologically sound structural models can be generated. Xenoliths within the obsidians at the volcanoes at the south end of the Salton Sea provide samples of the basement under the Imperial Valley. These xenoliths include partially remelted granitic rocks, fragments of basalt, greenschist, and baked shale and sandstone. This is taken as evidence that the basement in the valley consists in part of partially remelted granite. This would render basement plastic and readily deformed. The source of the heat is suggested to be derived from basalt that comes into the basement and deeper sediments from below. This upward movement of basalt along a spreading zone is the continental equivalent to a sea floor spreading area. In the continental case the insulating blanket of wet sediment retains the heat and appears to produce a major geothermal resource. The geothermal resources of the Imperial Valley are the aggregate of the thermal energy of the large inventory of subsurface water heated by the complex mix of intrusive phenomena. The net result is to generate a polygenetic geothermal resource of very large dimensions.     

The deposition of silica in hot springs

Natural hot spring waters ascending rapidly to the surface become supersaturated with respect to quartz because of rapid cooling, separation of steam and sluggish deposition of quartz and other crystallineSiO ₂ phases. Large amounts of silica are likely to be deposited in hot spring systems only after the solubility of amorphous silica has been exceeded. Cristobalite and chaleedony probably form in hot spring systems only by the crystallization of previously deposited silica gel rather than by direct deposition from solution. Experimental data indicate that the solubilit of quartz in water rises with increasing temperature along the vapor pressure curve to a maximum value of 725 ppm at 330°C. However, the maximum amount of silica likely, to occur in hot spring systems where quartz precipitates at depth is appreciably greater. Steam formation during adiabatic cooling of a water quickly brought to the surface from 330°C at depth might leave the silica in the remaining liquid concentrated to about 1150 to 1400 ppm. Under such conditions, amorphous silica might precipitate (probably as a colloidal suspension) after the water cooled below about 200°C to 250°C. Waters initially in equilibrium with quartz at a temperature less than 210°C probably will precipitate amorphous silica in channelways underground only when and where large quantities of steam separate from the waters as a result of sudden decreases in pressure or hydrostatic head. Above 150° to 200°C amorphous silica and volcanic glass can contribute very large quantities of silica to the solution. However, at these temperatures in natural systems they are eventually converted to crystalline phases. Thus, control of dissolved silica at depth is likely to be relatively short lived in respect to the ages of most hot spring systems. The dissolved-silica content of 90 hot spring waters from Yellowstone National Park was measured colorimetrically in the field immediately after collection. Comparison with laboratory studies on the solubility of amorphous silica indicates that many waters in «alkaline» springs are markedly undersaturated with silica with respect to amorphous silica at the temperatures of the pools. Thus, the dissolved silica content of these waters cannot be accounted for by equilibria with amorphous silica. Rather, silica appears to be controlled by the dissolution, deposition, or alteration of other silica-bearing phases at depth. Furthermore, many springs now have compositions essentially identical (with respect to all components) to those determined in 1888, indicating that either equilibrium or steady state conditions have prevailed at depth for a long time. Veins of fine-grained quartz were found in drill core from the Upper Basin, and it is reasonable to assume that quartz controlled the quantity of silica in solution in those places of deposition. Possibly the silica content of the surface waters might allow an estimate of the temperatures at which these waters were last in equilibrium with quartz at depth. Assuming adiabatic cooling along the vapor pressure curve and correcting for steam formation, quartz solubility data compared with natural water analyses suggests that underground temperatures approach 205°C in the Upper Geyser Basin of Yellowstone. In the Norris Geyser Basin, underground temperatures of 245°C are suggested.     

Phreatic eruption clouds: the activity of la Soufrière de Guadeloupe,F.W.I., August — October, 1976

From August to October, 1976, La Soufrière de Guadeloupe was observed, and recorded with an automated sequence camera and numerous handheld cameras. During the period of observation, the nature of volcanic activity ranged from mild steam emission to moderately energetic phreatic eruptions. Background fumarolic activity (steam emission) was characterized by the emission of generally tephra-free steam clouds 50 to 150 m above the summit. The clouds rose buoyantly above the vent and were blown downwind at prevailing wind velocities. Phreatic eruptions were well-documented on September 22, October 2, and October 4. In the latter two eruptions, small bursts of tephra-laden steam erupted at intervals of 30 to 45 min, and rose from 350 to 500 m above the summit. In the largest observed eruption, that of October 2, the steam and tephra cloud rose to a maximum height of 600 to 650 m in 20 min. A white vapor cloud and a medium gray, tephra-laden cloud were erupted simultaneously from the summit vent and both were surrounded by a vapor collar: the clouds were thoroughly mixed within 1 km downwind of the summit. The concurrent growth of clouds from separate vents (summit and flank) implies a common source. Simultaneous eruption of tephra-free and tephra-laden clouds from the same vent is puzzling and implies: (i) lateral changes in the degree of alteration of dome rocks along the elongate vent, hence erodability of the dome lavas, or (ii) differences in the gas velocities. These «mixed» clouds moved westward, downwind and downslope as a density current, along the watersheds of the R. Noire and R. des Pères with an approximate velocity of 10 to 25 m/sec. Upon reaching the sea the clouds continued to move forward, but at a decreased velocity, and spread laterally, having left behind the restrictions of valley walls. A thin gray veneer of moist tephra, ranging from several cm thick near the dome to less than 1 mm thick several km downwind, was deposited along a narrow corridor southwest of the summit. Tephra from the phreatic eruptions consisted mostly of hydrothermally altered lithic, mineral, and glass fragments derived from dome lavas; no fresh (juvenile) pyroclasts were present in the tephra. Absence of juvenile tephra at La Soufrière supports the view that activity was due to groundwater circulating in a vapor-dominated geothermal system, probably driven by a shallow heat source. At La Soufrière, most vapor-dominated systems are located in elevated areas of groundwater recharge where groundwater movement is downward and outward. The sporadic phreatic eruptions may be related to the rate of recharge of meteoric waters within the dome, the decrease in pore pressure during fortnightly tidal minimums or both. Whatever the triggering mechanism, vapor-dominated fluids eroded vent walls during phreatic eruptions and carried out fine-grained, hydrothermally altered, pre-existing dome material as tephra.     

超临界地热资源的地球物理分析——以鲜水河高温水热系统为例

鲜水河高温水热系统位于川西地热区, 是我国重要的"水热型"地热资源区和高温地热发电工程建设选区, 其深层超临界地热资源可以大幅提高热-功转换效率和地热发电能力, 具有极大的应用价值.超临界地热资源是地热学的全新研究方向, 研究深层超临界地热资源促进地热学拓宽研究领域, 具有重要的科学意义.本文利用地球物理资料、地热地质资料, 对鲜水河高温水热系统超临界水热流体构造条件、有利赋存部位、热结构与热状态开展计算分析.结果表明: 鲜水河断裂带浅源小震群与深层超压热流体层相关, 热泉群串珠状负重力异常圈闭之下超厚沉积地层是赋存深层超临界地热流体储层的有利构造部位; 康定中谷—二道桥—榆林宫热水区的水源补给、热源补给、水热循环通道等地热地质条件优良, 其下方存在酸性岩浆活动, 是形成高温水热系统浅层热储、深层超临界热液区的重要热源条件.我们认为: 鲜水河断裂带是深部热流体上升至浅表产生强烈水热活动的通道, 沿此通道, 在160~250 ℃温度区间, 是鲜水河水热系统地热储层的赋存区域; 在350~400 ℃温度区间, 是深层超临界流体(H2O、CO2)的赋存区域; 随温泉水大量溢出的高浓度CO2地热气体, 伴随出现pH值略小于7的弱酸性热水, 其下方是形成深层超临界地热资源的有利区域.     

Hydrothermal melting of some New Zealand greywackes and argillites

The temperatures at which melting begins of three New Zealand greywackes and two argillites were determined as a function of water pressure up to 3000 atmospheres. The purpose of these experiments was to provide data possibly relevant to the genesis of the North Island ignimbrites and for comparison with the experiments ofBowen andTuttle (1958) on the melting temperatures of granites and the ternary minimum system (NaAlSi₃O₈ — KAlSi₃O₈ — SiO₂ — H₂O). Powdered samples of the rocks were heated in unsealed silver tubes in cold seal bombs, the water pressure being measured on a Bourdon gauge and applied during heating. Temperature control was ± 5°C. Twelve days was the longest heating period, most being for 24 hours, which was sufficient for apparent reaction. At the end of an experiment, the pressure was released, and the sample quickly cooled by removal from the bomb. The samples were crushed and examined by X-ray diffraction and the petrographic microscope. The PT curve for the beginning of melting of the greywackes and argillites is very close toBowen andTuttle’s for granites and the ternary minimum to 1000 atmospheres water pressure. Above this pressure the greywackes melt at slightly higher temperatures than on the granite curve with the argillites a little higher still. These observations are similar to those reported for shales byWyllie andTuttle 1960, 1961, for greywackes byWinkler andvon Platen 1961, and pelitic sediments byWyart andSabatier, 1959. The amount of anatectic melt increases rapidly above the temperature of initial melting and is inversely related to the quartz present in the greywackes. The partially melted products were often notably vesicular. Cordierite, mullite, hypersthene, scapolite and mica were identified in the X-ray diffractograms as coexisting with the melt. These experiments are in agreement with published work in showing that the quartz and alkali feldspars of granites, shales and arkosic sediments can in the presence of water react and begin to melt at 20–25 kilometres depth in the earth assuming a geothermal gradient of 30°C/km. The melt is granitic or granodioritic in composition.     

The somma-vesuvius magma chamber: a petrological and volcanological approach

The volcanic history of Somma-Vesuvius indicates that salic products compatible with an origin by fractionation within a shallow magma chamber have been repeatedly erupted («Plinian» pumice deposits). The last two of these eruptions, (79 A.D. and 3500 B.P.) were carefully studied. Interaction with calcareous country rocks had limited importance, and all data indicate that differentiated magmas were produced by crystal-liquid fractionation within the undersaturated part of petrogeny’s residua system at about 1 kb water pressure. The solid-liquid trend indicates that the derivative magmas originated by fractionation of slightly but significantly different parental liquids. Some lavas of appropriate composition were selected as parental liquids to compute the entity of the fractionation. Results suggest that in both bases a fractionation of about 70 weight % was needed to produce liquids with the composition of the pumice. The combination of all data indicates that the two Plinian eruptions were fed by a magma chamber (3–4 km deep) having a volume of approx. 2.0–2.5 km³. The temperature of the magma that initially entered the chamber was about 1100°C, whereas the temperature of the residual liquids erupted was Plinian pumice was 800° and 850°C respectively. There is no evidence that such a magma chamber existed at Vesuvius after the 79 A.D. eruption. These results have relevant practical implications for volcanic hazard and monitoring and for geothermal energy.     

Geothermal data analysis at the high-temperature hydrothermal area in Western Sichuan

The western Sichuan hydrothermal area is located at the northeastern margin of the eastern syntaxis of the Qinghai-Tibet Plateau, which is also the eastern end of the Mediterranean-Himalayan geothermal activity zone. There are 248 warm or hot springs in this area, and 11 have temperatures beyond the local boiling temperature. Most of these hot springs are distributed along the Jinshajiang, Dege-Xiangcheng, Ganzi-Litang, and Xianshuihe faults, forming a NW-SE hydrothermal belt. A geothermal analysis of this high-temperature hydrothermal area is an important basis for understanding the deep geodynamic process of the eastern syntaxis of the Qinghai-Tibet Plateau. In addition, this study offers an a priori view to utilize geothermal resources, which is important in both scientific research and application. We use gravity, magnetic, seismic, and helium isotope data to analyze the crust-mantle heat flow ratio and deep geothermal structure. The results show that the background terrestrial heat flow descends from southwest to northeast. The crustal heat ratio is not more than 60%. The high temperature hydrothermal active is related to crustal dynamics processes. Along the Batang-Litang-Kangding line, the Moho depth increases eastward, which is consistent with the changing Qc/Qm(crustal/mantle heat flow) ratio trend. The geoid in the hydrothermal zone is 4–6 km higher than the surroundings, forming a local "platform". The NW-SE striking local tensile stress zone and uplift structure in the upper and middle crust corresponds with the surface hydrothermal active zone. There is an average Curie Point Depth(CPD) of 19.5–22.5 km in Batang, Litang, and Kangding. The local shear-wave(S-wave) velocity is relatively low in the middle and lower crust. The S-wave shows a low velocity trap(Vs3.2 km s.1) at 15–30 km, which is considered a high-temperature partial melting magma, the crustal source of the hydrothermal active zone. We conclude that the hydrothermal system in this area can be divided into Batang-type and Kangding-type, both of which rely on a crustal heating cycle of atmospheric precipitation and surface water along the fracture zone. The heat is derived from the middle and lower crust: groundwater penetrates the deep faults bringing geothermal energy back to the surface and forming high-temperature springs.     

The linear relation between temperatures based on the silica content of groundwater and regional heat flow: A new heat flow map of the United States

Application of the silica geothermometer to over 70,000 non-thermal groundwaters from the United States has shown that there is a correlation between the average silica geotemperatures for a region (T SiO₂ in °C) and the known regional heat flow (q in mW m^?2) of the form: 1 $$TSiO_2 = mq + b$$ wherem andb are constants determined to be 0.67°C m² mW^?1 and 13.2°C respectively. The physical significance of ‘b’ is the mean annual air temperature. The slope ‘m’ is related to the minimum average depth to which the groundwaters may circulate. This minimum depth is estimated to be between 1.4 and 2.0 km depending on the rock type. A preliminary heat flow map based on equation (1) is presented using theT SiO₂ for new estimates of regional heat flow where conventional data are lacking. Anomalously high localT SiO₂ values indicate potential geothermal areas.     

Calc-alkaline volcanic rocks from NW Sardinia: Evaluation of a fractional crystallization model

The volcanic centre of Monte Seda Oro, N. W. Sardinia, representative of a Cenozoic calc-alkaline andesitic suite of rocks is composed of a variety of rocks ranging from high alumina basalts to dacites. The minerals of basaltic, andesitic and dacitic rocks show only limited variation in chemical composition. The geochemical data suggest that the various rock-types are related by a crystal-liquid fractionation. Least-squate numerical calculations, using major element data, support the derivation of andesites with SiO₃ content ranging from 53.8 to 59.0% from basalts having about 48.7% of SiO₂ by low pressure crystal fractionation of the phenocryst phases present in these rocks. However, the origin of dacites cannot be readily explained by this mechanism.     

Soufrière of guadeloupe 1976–1977 eruption — mass and energy transfer and volcanic health hazards

The Soufrière volcano in Guadeloupe island delivered a phreatic eruption that commenced on July 8th, 1976 and lasted until March 1st, 1977. This eruption was similar to the 1797, 1798, 1809 and 1956 outbreaks. Phreatic activity ejected blocks derived from the fissure walls and fine pyroclasts produced by hydrothermal alteration of the old dome’s rocks. Field observations and measurements allowed the present authors to calculate the mass and energy transfer of steam and ashes: 10⁷ tons of water (very low considering that on the mountain summit the annual precipitation is 10 tons m)²,10⁶ m³ of ashes. The most important energy transfers was thermal: about 5 × 10²⁰ ergs for each phreatic eruption. The total kinetic energy output was 2 × 10¹⁹ ergs for a total thermal energy output of 64 × 10²⁰ ergs. The gases and fine pyroclasts did pollute the atmosphere, waters and soils and consequently affected the population living on the slopes of the volcano.     

Some remarks on contact anatexis

The chemical variability of the products of contact-anatexis, completely different from the normal trend of magmatic differentiation, may be explained by the quantitative variation of gaseous transfer, according to the state of the basaltic magma which may be pyromagma or hypomagma at the contact with the surrounding sialic rocks. Therefore, two types of contact-anatexis must be distinguished: 1st.Anatexis at the contact with pyromagma. If the tectonical conditions are favourable, then the basaltic magma rises so high in the sialic crust that the gas tension overcomes the hydrostatic pressure. A gas phase will separate and cause a considerable gas transfer by which pneumatophilic substances (Na, Fe, Ti etc.) are supplied to the overlying anatectic magma. 2nd.Anatexis at the contact with hypomagma. If the rising basaltic magma cannot reach very high levels in the sialic crust, then the gas tension remains lower than the hydrostatic pressure, and the gases are molecularly dispersed within the melt. The gas transfer will be insignificant, and the anatexis is merely due to the supply of heat without any appreciable change of the chemical composition of the anatectic magma.     

Geology of the platanares geothermal area, Departamento de Copán, Honduras

The Platanares geothermal area, Departamento de Copán, Honduras, is located within a graben that is complexly faulted. The graben is bounded on the north by a highland composed of Paleozoic (?) metamorphic rocks in contact with Cretaceous - Tertiary redbeds of unknown thickness. These are unconformably overlain by Tertiary andesitic lavas, rhyolitic ignimbrites, and associated sedimentary rocks. The volcanic rocks are mostly older than 14 Ma, and thus are too old to represent the surface expression of an active crustal magma body. Thermal fluids that discharge in the area are heated during deep circulation of meteoric water along faults in a region of somewhat elevated heat flow. Geothermometry based upon the chemical composition of thermal fluids from hot springs and from geothermal gradient coreholes suggests that the reservoir equilibrated at temperatures as high as 225 to 240°C, within the Cretaceous redbed sequence. Three continuously cored geothermal gradient holes have been drilled; fluids of about 165°C have been produced from two drilled along a NW-trending fault zone, from depths of 250 to 680 m. A conductive thermal gradient of 139°C/km, at a depth of 400 m, was determined from the third well, drilled 0.6 km west of that fault zone. These data indicate that the Platanares geothermal area holds considerable promise for electrical generation by moderate- to hightemperature geothermal fluids.     

Geochemistry and isotopes of fluids from sulphur springs, Valles Caldera, New Mexico

Detailed geochemistry supported by geologic mapping has been used to investigate Sulphur Springs, an acid-sulfate hot spring system that issues from the western flank of the resurgent dome inside Valles Caldera. The most intense activity occurs at the intersection of faults offsetting caldera-fill deposits and post-caldera rhyolites. Three geothermal wells in the area have encountered pressures <1 MPa and temperatures of 200°C at depths of 600 to 1000 m. Hot spring and fumarole fluids may discharge at boiling temperatures with pH 1.0 and SO₄ 8000 mg/l. These conditions cause argillic alterations throughout a large area.Non-condensible gases consist of roughly 99% CO₂ with minor amounts of H₂S, H₂, and CH₄. Empirical gas geothermometry suggests a deep reservoir temperature of 215 to 280°C. Comparison of ¹³C and ¹⁸O between CaCO₃ from well cuttings and CO₂ from fumarole steam indicates a fractionation temperature between 200 and 300°C by decarbonation of hydrothermally altered Paleozoic limestone and vein calcite in the reservoir rocks. Tritium concentrations obtained from steam condensed in a mudpot and deep reservoir fluids (Baca #13, 278°C) are 2.1 and 1.0 T.U. respectively, suggesting the steam originates from a reservoir whose water is mostly >50 yrs old. Deuterium contents of fumarole steam, deep reservoir fluid, and local meteoric water are practically identical even though ¹⁸O contents range through 4‰, thus, precipitation on the resurgent dome of the caldera could recharge the hydrothermal system by slow percolation. From analysis of D and ¹⁸O values between fumarol steam and deep reservoir fluid, steam reaches the surface either (1) by vaporizing relatively shallow groundwater at 200°C or (2) by means of a two-stage boiling process through an intermediate level reservoir at roughly 200°C.Although many characteristics of known vapor-dominated geothermal systems are found at Sulphur Springs, fundamental differences exist in temperature and pressure of our postulated vapor-zone. We propose that the reservoir beneath Sulphur Springs is too small or too poorly confined to sustain a “true” vapor-dominated system and that the Sulphur Springs system may be a “dying” vapor-dominated system that has practically boiled itself dry.     

Chemical composition of thermal springs, cold springs, streams, and gas vents in the Mt. Amiata geothermal region (Tuscany, Italy)

A geochemical study of thermal and cold springs, stream waters and gas emissions has been carried out in the Mt. Amiata geothermal region.The cold springs and stream waters do not seem to have received significant contribution from hot deep fluids. On the contrary, the thermal springs present complex and not clearly quantifiable interactions with the hot fluids of the main geothermal reservoir.The liquid-dominated systems in the Mt. Amiata area, like most of the high-enthalpy geothermal fields in the world, are characterized by saline, NaCl fluids. The nature of the reservoir rock (carbonatic and anhydritic), and its widespread occurrence in central Italy, favor a regional circulation of “Ca-sulfate” thermal waters, which discharge from its outcrop areas. Waters of this kind, which have been considered recharge waters of the known geothermal fields, dilute, disperse and react with the deep geothermal fluids in the Mt. Amiata area, preventing the use of the main chemical geothermometers for prospecting purposes. The temperatures obtained from the chemical geothermometers vary widely and are generally cooler than temperatures measured in producing wells.Other thermal anomalies in central Italy, apart from those already known, might be masked by the above-mentioned circulation. A better knowledge of deep-fluid chemistry could contribute to the calibration of specific geothermometers for waters from reservoirs in carbonatic rocks.     

Modeling the thermal evolution of an active geothermal system

Temperature inversions at shallow to moderate depths have been observed commonly in boreholes drilled in geothermal areas. The inversions result from thermal disequilibria generated by steam and/or hydrothermal fluids invading shallow horizontal, or sub-horizontal fractures, or permeable horizons, from a deep vertical, or sub-vertical feeder-fracture.Subsurface distribution of temperatures in Momotombo geothermal area of Nicaragua, Central America, indicates that the anomaly is generated by steam and water, convecting in a narrow feeder-fracture-zone located at the western edge of the field. The north-trending zone of the feeder-fracture is bound on the west by the area of massive, impermeable andesitic rocks, and is capped by an impermeable, approximately 300 m. thick silica-cap, which seals if from the ground surface. The thermal fluids penetrate a system of horizontal, or sub-horizontal fractures, extending east of the feeder-fracture beneath the silica cap. The flow of thermal fluids eastward through the system of the horizontal, or sub-horizontal fractures is generating a plume-like geothermal anomaly, which is expressed by the temperature inversion zone pervasive in the boreholes to the east of the feeder-fracture.A time-dependant model for a semi-infinite half-space (z > 0) in contact with a hot, well stirred, isotropic fluid flowing through an aquifer overlain by a finite space of constant thickness is solved for the data collected from the Momotombo geothermal boreholes. Curve fitting between the simulated and observed temperature/depth profiles suggests that the thermo-tectonic events which caused the present-day Momotombo hydrothermal system occurred approximately 5,500 years ago, following development of vertical, or subvertical fractures along a N5°E trending faultline. Hot fluids emerging from these fractures move eastward through a system of horizontal, or sub-horizontal fractures, with a velocity of 11 to 20 m/yr.     

松辽盆地北部地热场深部控制因素研究: 基于大地电磁探测的结果

松辽盆地北部存在中低温地热场,地热场呈现中间高、四周环状降低的特征.松辽盆地内部形成高地热场的主要因素,一是深部热源供给;二是浅部热能储集.通过深部结构特征研究可揭示热源及热储的分布及相互联系,对松辽盆地北部地热场成因研究具有重要意义.为了揭示松辽盆地北部地热场的深部控制因素,本文基于古龙镇至依安县的246 km长大地电磁剖面,对71个宽频测点数据通过傅里叶变换、Robust估计以及相位张量分解等处理手段,在精细分析维性特征及电性主轴的基础上,利用非线性共轭梯度反演获得了剖面40 km深二维电性结构模型.研究发现：电阻率模型具有"纵向分层,横向分块"的特征,以水热性温泉富集的林甸地区为界,剖面南北两侧电性结构存在明显的差异,南侧呈现"低阻-高阻-低阻"的三元电性结构,北侧呈现"低阻"的一元电性结构,这两种结构与地温场分布具有良好的对应关系,林甸以南的三元电性结构区对应高地热异常,以北的一元电性结构区热异常下降明显;林甸地区位于这两种端元的分界区,地表温泉丰富,且发育有基底断裂,为水热型地热发育的"热点"地区;林甸南北两侧深部存在两个高导体C1、C2,这与普遍认识的松辽盆地存在软流圈隆起有关,说明盆地下方具有统一的热源,部分熔融热物质作为深部热源向上传递热量,不同之处在于林甸以南地区,中地壳存在巨厚高阻特征的前寒武纪结晶基底R1,为地热的保存提供了有利条件,而林甸以北地区深部缺少聚集热量的结晶基底,导致地热异常迅速降低.