Distribution of hydrothermal mineral zones in the Cerro Prieto geothermal field of Baja California, Mexico

Our ongoing studies of water-rock interaction at Cerro Prieto have now been extended to include samples from 40 wells. We have confirmed the regular sequence of progressive hydrothermal alteration zones previously described, and have mapped these alteration zones across the geothermal field. Our earlier work showed the relationships between hydrothermal mineralogy, temperature and permeability, in that alteration occurs at lower temperature in sandstone than in the less permeable shales. The effects of chemical parameters such as silica activity and differences in CO₂ fugacity have also become apparent when mineral assemblages are compared in sandstones from different wells at the same temperature. A rather complete picture of the shape of the reservoir and the nature of its boundaries is developing, and we have begun to identify patterns in the observed hydrothermal mineral zonations which are characteristic of different temperature gradients. We infer such different gradients to be indicative of different parts of the hydrothermal flow regime. In certain wells mineral zones are closely spaced, indicating steep thermal gradients, while in others they are much farther apart. We believe that patterns characteristic of recharge, discharge and upwelling zones as well as areas of primarily horizontal flow can be recognized.The geothermal circulation system at Cerro Prieto appears to be rather young and shows no indication of retrograde reactions due to cooling. The pattern of fluid flow does not seem to be significantly affected by faults, stratigraphic horizons or by the presence of a cap-rock. The mineral zones define a thermal dome which is apparently fed from the east and spreads westward.     

Hg anomalies in soils: A geochemical exploration method for geothermal areas

Hg contents of soils in geothermal areas in the western U.S. were measured and a three-fold distribution was observed: peak, aureole and background. Peak values (up to several 100 ppm Hg) occur in fumaroles of vapour-dominated systems, around hot springs, and in zones overlying steeply dipping, hot-water aquifers. Aureoic values (up to several 100 ppb Hg) are found in zones surrounding the peak areas and delineate areas with shallow geothermal convection. Background values vary between 7 and 40 ppb Hg (geometric mean). Usually, Hg is present in a form that can be easily re-volatilized and released to the atmosphere. Altered areas related to fossil hydrothermal systems can be distinguished from alteration related to active systems by their Hg contents. In the rare cases of Hg enrichments as cinnabar or as traces in other sulphides (pyrite, sphalerite) the Hg is not easily released from its host phase, and distinction between active and fossil systems is not possible. Hg anomaly patterns yield information on the presence as well as the geometry of shallow geothermal circulation patterns. In conjunction with structural geologic data, Hg patterns can be helpful in defining reservoir boundaries and can aid in the selection of drill site location.     

Hydrothermal activity in the Tulancingo–Acoculco Caldera Complex,central Mexico: Exploratory studies

Mineral alteration and fluid inclusion studies of drill cuttings and core samples indicate that the sedimentary basement rocks and the volcanic rocks associated with Tulancingo–Acoculco Caldera Complex have been the site of two distinct and major hydrothermal events. The complex, located in the eastern portion of the Trans-Mexican Volcanic Belt, is formed by the Pliocene Tulancingo Caldera and the younger (Pleistocene) Acoculco Caldera, which developed within the older depression. The volcanic rocks are underlain by Cretaceous sedimentary rocks of the Sierra Madre Oriental. The earliest important hydrothermal event occurred during the emplacement of Mid-Tertiary granitic intrusions that metamorphosed the sedimentary rocks; these intrusives are not exposed at the surface. However, granitic rocks were encountered at the bottom of exploratory borehole EAC-1, drilled within the Caldera Complex. The second main event occurred during the formation of the Tulancingo and Acoculco Calderas. Both episodes lead to secondary mineralization that reduced the permeability of the reservoir rocks. A possible third hydrothermal event may be associated with the recent magmatic activity within the Acoculco Caldera.Thermal logs from well EAC-1 display a conductive thermal gradient with maximum temperatures exceeding 300 °C at 2000 m depth. Although there are no active thermal springs in the area, there is extensive fossil surface hydrothermal alteration and cold gas discharges with high He³/He⁴ ratios.     

Geochemical study of fluids on Lesbos island, Greece

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

The thermal and geochemical structure of the broadlands-ohaaki geothermal system, new zealand

Fifty two wells have been drilled into the Broadlands-Ohaaki geothermal system, New Zealand, in the course of its development. Fluid samples collected from these wells and measured temperatures indicate that boiling is common within the East and West Bank production zones, separated at the surface by the Waikato River. Steam-heated waters form over the top of the system, above zones of boiling, and are also present on the margins of the system. They are C0₂-rich, and are responsible for dilution of the deep chloride fluids, particularly on the margins of the system. Thermal inversions are common on the margins of the system, associated with the steam-heated waters. The eastern portion of the East Bank and margins of the West Bank have cooled since peak thermal conditions, possibly due to dilution, as indicated by comparing fluid inclusion data with temperatures now present. However, fluid inclusion Th and Tm data indicate that boiling and dilution patterns similar to those now present have existedsince inclusion formation. The hydrothermal alteration of the silicic volcanics comprises an assemblage of quartz—albite—illite—adularia—calcite—chlorite—pyrite; epitode and wairakite are rare, and pyrrhotite, sphalerite and galena are generally confined to the margins of the system. Kaolin, Camontmorillonite, cristobalite and siderite are also present on the margins of the system to depths of 600–1200 m, and are related to the presence of the C0₂-rich, steam-heated waters. The deep production fluids originate from a parent (preboiled) fluid with a temperature of 300°C and CO₂ content of 0.6 mol. Excess enthalpy (i.e. two phase feed zone) discharges are not suitable for the calculation of activity ratios in the reservoir liquid and assessment of mineral—fluid equilibria; this is probably due to non-equilibrium distribution of gas species between liquid and vapor. However, an assessment of mineral—fluid equilibria is possible from the compositions of liquid feed wells. Based on these data, the reservoir fluids are now slightly undersaturated with respect to calcite and are in equilibrium with K-mica, pyrite and chlorite. The common presence of adularia and calcite in veins and open spaces may be due to a shift in mineral—fluid equilibria caused by extensive boiling and gas loss in fractures as compared to formation fluid. In contrast, the marginal steam-heated waters are in equilibrium with pyrite-pyrrhotite. Their lower pH values make them more undersaturated with respect to calcite and K-feldspar than the chloride fluids, due mainly to the lower temperatures and concentration of CO₂, resulting in interstratified illite-smectite and even kaolinite ± siderite stability. Dilution and cooling of the boiling fluids by the steam-heated waters has caused their shift to K-mica stability; the resulting deposition of illite in fractures of the East Bank may be responsible for the lower permeabilities here, causing excess enthalpy conditions.Steam-heated waters are common in geothermal systems throughout the world; recognition of dilution patterns helps in deducing the overall geochemical structure of each system. Knowledge of the distribution of steam-heated waters will also assist in locating upflow zones, and also allows their potential for casing corrosion and production-induced incursion to be assessed.     

Formation of geothermal resources at lithospheric subduction zones

This paper examines in a very broad fashion the formation of geothermal resources at lithospheric subduction zones. Regions of highly silicic calc-alkaline Quaternary volcanoes and/or plutons have been identified as prime candidates for having high-temperature hydrothermal systems. Regions of large tholeiitic Quaternary volcanoes have been identified as prime candidates for having large moderate-temperature hydrothermal systems. In addition, active magmatic, phreatomagnetic, and/or tectonic fracturing must be occurring in order to keep any moderate to high temperature hydrothermal system from chemically sealing. Connate, meteoric and/or oceanic water sources must also be present. Owing to tectonic and magmatic processes, volcanic arcs of subduction zones represent regions of the crust that have anomalously high mechanical and heat energy. Such arc regions are expected to contain significantly more moderate to high temperature hydrothermal systems than what is presently known. Many of these arcs are briefly discussed with respect to their potential for containing such resources.     

The geological framework of the Wairakei–Tauhara Geothermal System,New Zealand

The geology of the Wairakei–Tauhara geothermal system has been revealed in increments over more than 50 years of field development. Only two major reviews of geo-scientific information have been completed; the first was made more than 40 years ago, the second (unpublished) was completed more than 25 years ago. This paper is an overview and update of the stratigraphic and structural framework of the system and its controls on fluid flow and hydrothermal alteration. We provide information on new areas of drilling exploration in the west of the Wairakei Geothermal Field and on the first production-focused drilling in 40 years at the Tauhara Geothermal Field. The lithology, thickness and extent of several units have been refined, while new units have been discovered by recent deep wells; five new members of the Waiora Formation are proposed. Nomenclature of formations and members is also updated.     

Secondary mineral growth in fractures in the Miravalles geothermal system, Costa Rica

A mineralogical, fluid-chemical, and theoretical study of hydrothermal alteration in veins from drillcore from the Miravalles Geothermal Field, Costa Rica has revealed a complex history of mineral-fluid reaction which may be used to characterise changes in temperature and fluid composition with time. Mineralogical and mineral-chemical data are consistent with hydrothermal alteration in the temperature range 200°–270°C, with deeper portions of the system having undergone temperatures in excess of 300°C. Thermodynamic calculations suggest that the observed alteration assemblage is not in equilibrium with current well fluids, unless estimates of reservoir pH are incorrect. Fe-Al zoning of prehnite and epidote in veins is consistent with rapid, isothermal fluctuations in fluid composition at current reservoir temperatures, and may be due to changes in volatile content of the fluid due to tectonic activity.     

Hydrological disturbances of the conductive heat flow in the upper crust: Problems of correcting measured heat flow data

The upper crustal-scale convection of fluids is one of the main factors distorting the conductive geothermal field. Geothermal anomalies caused by water circulation, as a rule, exceed the contributions from other factors (relief, sedimentation, structural and climatic effects). The measured temperature gradient has to be corrected before being used for calculation of the heat flow. In the present work, certain 2-D hydrothermal convection models have been analyzed using various permeability distributions and spatial configurations of fluid systems. Permeability anisotropy has also been included in some of the models, and variations of this parameter in the range 10⁻¹⁰–10⁻¹⁶ m² have been considered. Numerical simulations reveal the possibility of considerable advective disturbances to the heat flow. These disturbances depend on the physical parameters, and strongly on the geometry of fluid circulation. The magnitude of the hydrologic correction has been assessed for a variety of geological environments.     

Magnetic anomalies associated with high temperature reservoirs in the taupo volcanic zone (New Zealand)

Distinct magnetic total force anomalies are associated with many, but not all, liquid-dominated geothermal reservoirs in the Taupo Volcanic Zone, which stand in thick, young (<0.7 Ma) volcanic host rocks. The anomalies are caused by demagnetised rocks (a result of hydrothermal alteration) in the upper 0.5–1 km section of the reservoirs; the phenomenon has been confirmed by core studies. Four different patterns of magnetic anomalies have been recognised. The most common type can be found over high-temperature systems where demagnetisation is confined to the main reservoir. Different anomaly patterns are associated with prospects characterised by subsurface outflows and reservoirs hosted in both normally and reversely magnetised rocks. A rather subdued and almost featureless pattern occurs over a few prospects where rocks lying outside the reservoir have also lost thei magnetisation by interaction with acidic, steam-heated water.     

The origin of the Cerro Prieto geothermal brine

The Cerro Prieto geothermal brine may have originated from mixing of Colorado River water with seawater evaporated to about six times its normal salinity. This mixture circulated deeply and was heated by magmatic processes. During deep circulation, Li, K, Ca, B, SiO₂ and rare alkalis were transferred from rock minerals to the water, and Mg, SO₄, and a minor quantity of Na were transferred to the rock. Similar alteration of seawater salt chemistry has been observed in coastal geothermal systems and produced in laboratory experiments. After heating and alteration the brine was further diluted to its present range of composition. Oxygen isotopes in the fluid are in equilibrium with reservoir calcite and have been affected by exploitation-induced boiling and dilution.     

Shallow subsurface temperature surveys in the basin and range province, U.S.A.—I. Review and evaluation

Temperature surveys at depths of 1–2 m have had varying success in geothermal exploration in the Basin and Range province. The most successful surveys have identified patterns of near-surface thermal-fluid flow within areas of less than 2 km². Results have been less consistent in larger areas where zones of hydrothermal upflow are less well known, nongeothermal perturbing factors are significant and lateral variations in shallow subsurface temperature are small. Nongeothermal perturbations can be minimized by use of mean annual temperatures instead of synoptic temperatures, by physically based simulation of ground temperatures or by statistical modeling.     

High-temperature fluid flows in the dachny field of the Mutnovsky hydrothermal system, Russia

High-temperature hydrothermal reservoirs typically have complex structures that are difficult to characterize even after a number of wells have been drilled. The most effective methods for characterizing the flow regime within a reservoir are: (1) three-dimensional mapping of the geological structure, temperature, pressure and permeability; (2) interpretation of tracer tests and reservoir fluid chemistry; and (3) flow test data analysis. (It is assumed that the petrophysical parameters of the various lithologic units have been determined on the basis of core and geophysical log data.) When these methods are applied to the Dachny reservoir of the Mutnovsky geothermal system, they yield the distribution in the field of lithologies, temperatures, phases and pressures, as well as the characteristics of the high-temperature fluid circulation (natural state initial and boundary conditions for the associated heat transfer problem).     

Dynamic performance of a natural circulation loop with end heat exchangers under different excitations

In the present work the dynamic performance of a natural circulation loop (NCL) has been studied under step, ramp, exponential and sinusoidal excitations. The loop is equipped with two heat exchangers at its lower and upper end for the heating and cooling of the loop fluid. For the analysis, transient one-dimensional conservation equations have been constructed for the loop fluid as well as for the two fluid streams of hot and cold end heat exchangers. The solution of a set of differential equations and one integro-differential equation has been obtained through a finite element method (FEM). For different excitations imposed to the inlet temperature of the hot fluid responses have been studied for the outlet temperature of the two fluid streams and the mass flow rate of the coupling fluid. It has been observed that all these quantities experience some initial transients before reaching the steady state. Time needed for the attainment of steady state varies with the type of excitation. A finite time delay is observed before the cold fluid stream temperature starts responding to the excitation. This delay is related to the time required for the advection of a fluid particle.     

Deep fluid circulation and isotopic alteration in The Geysers geothermal system: profile models

Rock alteration patterns related to the large felsic intrusive complex (felsite) beneath The Geysers steam field (California, USA) are important indicators of the origins of the modern geothermal system. Metagreywacke host rocks for the system show widespread moderate oxygen isotope alteration in the permeable steam reservoir above the felsite, with concentrated alteration low on the flanks of the intrusion. Numerical models of fluid, heat and oxygen isotope transport in the pre-development (natural state) Geysers system demonstrate that an unbroken caprock is required throughout the liquid-dominated lifetime of the system to produce this pattern. The widespread moderate alteration throughout the steam reservoir suggests a long-lived liquid system, and probably required upwardly increasing permeability in the steam reservoir. The models indicate that the maximum hydrothermal lifetime for the system is 0.5 million years (Myr), whereas the youngest dated large intrusive is 1.2 Myr. Combined, these factors indicate repeated igneous intrusions at The Geysers, up to at least 0.5–0.6 Myr ago, and development of a stable liquid-dominated system after that, the evolution of which was truncated by a relatively recent transition to vapor-dominated conditions. Observed chemical compartmentalization of fluids in The Geysers steam reservoir is inconsistent with the lateral extensiveness of alteration at depth, since the latter requires good horizontal connectivity of deep permeable zones to allow penetration of ¹⁸O-depleted fluids. This compartmentalization is probably recent, developing as a consequence of vapor-dominated conditions introducing relative permeability effects. Petrologic evidence for high paleo-fluid temperatures (300°C) within 1 km of the surface is difficult to reconcile with subdued ¹⁸O alteration beneath these locations. These peak paleotemperatures are likely to indicate small, short-lived penetrations of the caprock. Natural-state models allow the combined influence of these factors on the evolution of The Geysers to be analyzed quantitatively. Internet-accessible graphical animations of these results are available at http://www.utdallas.edu/∼brikowi/Research/Geysers.     

Fluid Flows in an Industry Hydrothermal Autoclave with Seed Crystals and Raw Lascas

The fluid flows in hydrothermal growth vessels have significant effects on growth quality. This article presents a numerical investigation on the fluid flow and heat transfer in an industry hydrothermal autoclave. The focus is on the modeling of the seed crystals in the growth chamber, the raw lascas in the dissolving chamber, and their effects on the turbulent flows. The fluid and heat transport processes in the autoclave at representative stages of a growth run are simulated. Results show that the growth chamber has one flow cell at the beginning and another flow cell is developed later in a growth run. The dissolving chamber has a cell flow in the fluid region. A wall layer and core flow structure is established in the raw material region which carries significant heat transfer resistance. The transport through the baffle opening is between two flow cells. The fluid transport processes varies, and the thermal environment in the growth chamber changes significantly as the raw material bed is dissolved and the seed crystals grow.     

Geology of the tengchong geothermal field and surrounding area, West Yunnan, China

This paper describes the regional tectonics, active structures and thermal springs of the Tengchong geothermal field and surrounding area. Regional tectonic evidence shows that Tengchong and the surrounding area was a microcontinent between the Gondwanaland and Eurasian plates. The distribution of the thermal springs is controlled by active faults in the “arched” and circular structures. The majority of the thermal areas have a deep circulation within the fault system. Only a few high-temperature hydrothermal systems issuing sodium chloride may be heated by a local magmatic source.     

CFD analysis and experimental investigations towards optimizing the parameters of Ranque-Hilsch vortex tube

Computational fluid dynamics (CFD) and experimental studies are conducted towards the optimization of the Ranque-Hilsch vortex tubes. Different types of nozzle profiles and number of nozzles are evaluated by CFD analysis. The swirl velocity, axial velocity and radial velocity components as well as the flow patterns including secondary circulation flow have been evaluated. The optimum cold end diameter (d_c) and the length to diameter (L/D) ratios and optimum parameters for obtaining the maximum hot gas temperature and minimum cold gas temperature are obtained through CFD analysis and validated through experiments. The coefficient of performance (COP) of the vortex tube as a heat engine and as a refrigerator has been calculated.     

Hydrothermal alteration in the Aluto-Langano geothermal field, Ethiopia

The hydrothermal mineral assemblages found in eight wells (with a depth range of 1320–2500 m) of the active geothermal field of Aluto-Langano (Ethiopia) indicate a complex evolution of water-rock interaction processes. The zone of upflow is characterized by high temperatures (up to 335°C) and the presence of a propylitic alteration (epidote, calcite, quartz and chlorite, as major phases) coexisting with calcite and clay minerals. The zone of lateral outflow is characterized by mixing of deep and shallow waters and the occurrence of a calcite-clay alteration that overprints a previous propylitic assemblage. Clay minerals have a mushroom-shaped zonal distribution consistent with the present thermal structure of the field. Microprobe analyses have been carried out on chlorite and illite in order to apply several geothermometers. Most of the chlorite is iron-rich chlorite. It is found that the temperatures calculated from the chlorite geothermometer (159–292°C) after Cathelineau and Nieva [Contrib. Mineral. Petrol. 91, 235–244 (1985)] are in good agreement with in-hole measured temperatures (155–300°C). In the upflow zone, temperatures calculated from this geothermometer (217–292°C), together with fluid inclusion data of Valori et al. [Eur. J. Mineral. 4, 907–919 (1992)], and computed saturation indices of alteration minerals, indicate thermal stability or slight heating. On the other hand, evidence of a significant cooling process (up to 171°C) in the outflow zone is provided by the comparison between fluid inclusion homogenization temperature (240–326°C) and in-hole temperature (155–250°C). The apparent salinities (0.8–2.3 wt% NaCl eq.) of the fluid inclusions are generally higher than the salinity of the present reservoir fluid (0.29–0.36 wt% NaCl eq.). Clay minerals (illite, smectite, Ill/S mixed layers, vermiculite and chloritic intergrades) generally occur at temperatures consistent with their stability fields.