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

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

The nestos delta geothermal field and its relation to the associated self potential (sp) field

Prospecting for geothermal reservoirs by geophysical methods has proved to be a challenge in recent years. In the case of Nestos geothermal field, considered to be a blind field (no surface manifestation), the geological and geophysical data were studied and intercorrelated. The geophysical results from SP, gravity, and VES data were compared with basement morphology and tectonics, as well as with the high temperatures measured in the area of main geothermal interest. As a result, the highly conductive subsurface zone and high temperatures observed on the ground surface were closely associated with a specific fracturing system. The latter was successfully mapped by the SP method. The SP method also defined certain fracture zones which, being highly electrically polarized, are hypothesized as future geothermal targets.All existing geophysical data have been re-evaluated, along with the SP data acquired over the geothermal field, and compared with the thermal contour maps and regional tectonics of the area.A deep borehole, drilled earlier in the area of the geothermal field, detected high temperatures in the basement (115°C), but very low flowrate of the geothermal fluids. According to the results of this study, this is due to the fact that the borehole was located outside the area of the main fracture zones of geothermal interest.     

Main aspects of geothermal energy in Mexico

With an installed geothermal electric capacity of 853 MW_e, Mexico is currently the third largest producer of geothermal power worldwide, after the USA and the Philippines. There are four geothermal fields now under exploitation: Cerro Prieto, Los Azufres, Los Humeros and Las Tres Vírgenes. Cerro Prieto is the second largest field in the world, with 720 MW_e and 138 production wells in operation; sedimentary (sandstone) rocks host its geothermal fluids. Los Azufres (88 MW_e), Los Humeros (35 MW_e) and Las Tres Vírgenes (10 MW_e) are volcanic fields, with fluids hosted by volcanic (andesites) and intrusive (granodiorite) rocks. Four additional units, 25 MW_e each, are under construction in Los Azufres and due to go into operation in April 2003. One small (300 kW) binary-cycle unit is operating in Maguarichi, a small village in an isolated area with no link to the national grid. The geothermal power installed in Mexico represents 2% of the total installed electric capacity, but the electricity generated from geothermal accounts for almost 3% of the national total.     

SolGeo: A new computer program for solute geothermometers and its application to Mexican geothermal fields

The freely available computer program Solute Geothermometers (SolGeo) was written and tested using geochemical data and reported geothermometric temperatures from several geothermal wells from around the world. Subsurface temperatures for the Mexican geothermal fields of Cerro Prieto, Las Tres Vírgenes, Los Azufres, and Los Humeros were estimated based on different solute geothermometers and found to be generally in close agreement with measured well temperatures when considering errors in the calculations and measurements. For Los Humeros wells it was concluded that a better agreement of chemical geothermometric temperatures is observed with static formation than with bottom-hole temperatures (BHTs). It was also found that the widely used Na–K geothermometric equations generally give more consistent and more reliable temperature estimates than the other geothermometers, which should therefore be applied with caution.     

Temperature field distribution from cooling of a magma chamber in La Primavera Caldera, Jalisco, Mexico

The temperature field distribution in La Primavera geothermal area, Jalisco, located in the western part of the Mexican Volcanic Belt (MVB), has been simulated from cooling of a shallow magma chamber (assumed as the primary heat source) during the entire volcanic history of the caldera. Similar to the other two geothermal fields of the MVB (Los Humeros and Los Azufres), it is considered that the evolution of the magma chamber is controlled by the processes of fractional crystallization as well as magma recharge. Besides these processes, heat contribution is also taken into account from decay of natural radioactive elements, U, Th, and K, present in all geological materials. In some models presented in this work, convection in the geothermal reservoir is simulated by assigning higher values of thermal conductivities (up to 20 times the rock conductivities) to respective geologic units. The heat transfer equation has been solved by a finite element implicit method. The results of temperature simulations from the magma chamber are compared with undisturbed formation temperatures in three drill wells. The subsurface depth of the top of the magma chamber is varied from 5 to 7 km. Similarly, the horizontal dimensions of the chamber are varied from 12 km (which is approximately the diameter of the La Primavera caldera) to 10 km. The thermal effects of this change in depth and horizontal dimensions of the magma chamber are readily seen in the predicted temperature distribution for this rather young caldera.     

Preliminary evaluation of soufriere geothermal field, St. Lucia (Lesser antilles)

Geological, geochemical and geophysical studies have been carried out in the Soufrière caldera, St. Lucia, Lesser Antilles. The results are in accordance with the data obtained from previously drilled wells. In particular, these studies have also been used to: (i) determine the extent of the heat anomaly; (ii) indicate the levels containing hot geothermal fluids for high enthalpy exploitation; (iii) estimate the nature and extent of the reservoir; (iv) construct a preliminary model of the geothermal system, with a fluid at 220°C and a deeper one at about 350°C, both originating from a concentrated brine. Heat flux is estimated to be 6–7 times the average terrestrial value (250 – 290 mW/m²); (v) determine the most favourable areas for deep drilling.     

Geologic structure and volcanic history of the Yanaizu-Nishiyama (Okuaizu) geothermal field, Northeast Japan

The Yanaizu-Nishiyama geothermal field, also known as Okuaizu, supports a 65 MWe geothermal power station. It is located in the western part of Fukushima Prefecture, northeast Japan. This field is characterised by rhyolitic volcanism of about 0.3–0.2 Ma that formed Sunagohara volcano. Drillcore geology indicates that volcanism began with a caldera-forming eruption in the center of this field, creating a 2-km-diameter funnel-shaped caldera. Subsequently, a fault-bounded block including this caldera subsided to form a 5-km-wide lake that accumulated lake sediments. Post-caldera volcanism formed lava domes and intrusions within the lake, and deposited ash-flow tuffs in and around the lake. The hydrothermal system of this field is strongly controlled by subvertical faults that have no relation to the volcanism. The principal production zone occurs at a depth of 1.0–2.6 km within fractured Neogene formations along two northwest-trending faults to the southeast of the caldera. These faults also formed fracture zones in the lake sediments, but there was no apparent offset of the sediments. Stratigraphic studies suggest that post-caldera activities of Sunagohara volcano have migrated southeastward to the present high-temperature zone. The source magma of Sunagohara volcano may contribute to the thermal potential of this field.     

An overview of the Awibengkok geothermal system,Indonesia

The Awibengkok (Salak) geothermal system is a liquid-dominated, fracture-controlled reservoir with benign chemistry and low-to-moderate non-condensable gas content. The geothermal system is hosted mainly by andesitic-to-rhyodacitic rocks, and floored by Miocene marine sedimentary rocks cut by igneous intrusions. The volcanic sequence is capped by an 8400-year-old phreatic explosion breccia, rhyolite fallout tuff (>8400 years and <40,000 years), rhyolite lavas, domes and related tuffs (≥40–120 ka), and dacite-to-rhyodate lavas and domes (185–280 ka) that were erupted across the eastern part of the field from NNE-trending vents controlled by a major fault. More regionally extensive basaltic–andesite to andesite volcanic centers are mostly between 180 and 1610 ka old.     

Stratigraphy, paleomagnetism, geochronology and structure of silicic volcanic rocks, Waiotapu/Paeroa range area, New Zealand

The altered ignimbrite sequence underlying the Waiotapu Geothermal Field has long been correlated with unaltered ignimbrite sequences exposed in the Ngapouri and Paeroa fault-scarps to the west. Recent age-dating (fission-track and K-Ar) and magnetic polarity studies have indicated this correlation is invalid.The Paeroa Scarp section comprises three major ignimbrites (Paeroa, Te Weta and Te Kopia) erupted between 0.34 and 0.38 Ma ago from local caldera sources. The Caldera Boundary Fault (CBF) marking the eastern boundary of the Paeroa Caldera crosses the northern Paeroa Range between the Waiotapu and Waikite thermal areas. East of the CBF, the Ngapouri Scarp section comprises the Waiotapu Ignimbrite (0.58 ± 0.03 Ma) unconformably overlying a reversely magnetised sequence of late Matuyama age ignimbrite (Akatarewa A) and epiclastic tuff (Unit X), intruded by rhyolite (Ngapouri Rhyolite). Episodic caldera collapse has spalled exotic blocks of Waiotapu Ignimbrite and Ngapouri Rhyolite into the caldera, where they form a discontinuous layer between two sheets of Paeroa Ignimbrite.In the Waiotapu Geothermal Field - separated from the Ngapouri Ridge by a younger rhyolite dome - the Waiotapu Ignimbrite is underlain by Akatarewa ignimbrites (A,B) separated by epiclastic tuffs (Unit Y) overlying a truncated andesite cone (Ngakoro Andesite) and an older unnamed Ignimbrite C; all are of late Matuyama age, possibly extending back to the Jaramillo sub-chron.Younger sheets of Rangitaiki (0.35 ± 0.03 Ma), Matahina (0.28 ± 0.04 Ma), Ohakuri (0.27 ± 0.03 Ma) and Kaingaroa (0.22 ± 0.04 Ma) ignimbrite were emplaced across the area, the latter from the nearby Reporoa Basin (caldera). The progressive uplift of the Paeroa and Ngapouri tilted fault-blocks and further subsidence of the Reporoa Basin allowed the accumulation of thick lake beds (Huka Group) which capped geothermal reservoirs, originally established and later rejuvenated by episodic caldera formation. Present-day upflow zones are largely concentrated by active faults and fault intersections breaching the cap-rocks. Hydrothermal eruptions from self-sealed shallow reservoirs throughout the Waiotapu-Waikite area were commonly initiated by volcanic and/or tectonic events on major faults cutting and linking major calderas.     

Chemical variations in hydrothermal minerals of the Los Humeros geothermal system, Mexico

The Los Humeros geothermal system is composed of more than 2200 m of Quaternary altered volcanic rocks and an underlying Cretaceous sedimentary sequence. The low salinity of the fluids discharged at present (Na⁺ and Cl⁻ concentrations <500 ppm), and the excess steam, indicate that the reservoir contains a mixture of steam and dilute groundwater. Water-rock equilibrium is not attained. Hydrothermal minerals are present in veinlets, vugs, and replacing primary minerals. Three mineral zones are recognized: 1) a shallow argillic zone (<400 m depth), 2) a propylitic zone (ranging between 500 and 1800 m) and 3) a skarn zone (>1800 m). Petrographic examination of cuttings from five wells and temperature data indicate at least two stages of hydrothermal activity. Temperature is the main factor that affects the chemical composition of chlorite, epidote and biotite. Fe²⁺ and Al^IV increase in chlorite with temperature [from 1.4 formula position unit (fpu) to 2.8, and from 0.7 to 2.4 fpu, respectively]. The pistacite content of epidote varies from 18 to 33 mol% in high-temperature regions (>270 °C) and from 13 to 26 mol% in low-temperature regions (<250 °C). Biotite displays a slight increase in Al^IV contents (1.55–2.8) and octahedral occupancy (5.93–6.0 fpu) with temperature. Whole rock composition and variations in oxygen fugacity conditions are factors that also affect the concentrations of Fe, Al and Mg in the octahedral sites of chlorite, epidote, biotite and amphiboles. Chemical variations observed in alteration minerals at different depths in the Colapso Central-Xalapazco region could be used as indicators of relict physico-chemical conditions in the reservoir, before the present economic exploitation.     

Hydrogeochemical exploration of geothermal prospects in the Tecuamburro Volcano region, Guatemala

Chemical and isotopic analyses of thermal and nonthermal waters and of gases from springs and fumaroles are used to evaluate the geothermal potential of the Tecuamburro Volcano region, Guatemala. Chemically distinct geothermal surface manifestations generally occur in separate hydrogeologic areas within this 400 km² region: low-pressure fumaroles with temperatures near local boiling occur at 1470 m elevation in a sulfur mine near the summit of Tecuamburro Volcano; non-boiling acid-sulfate hot springs and mud pots are restricted to the Laguna Ixpaco area, about 5 km NNW of the sulfur mine and 350–400 m lower in elevation; steam-heated and thermal-meteoric waters are found on the flanks of Tecuamburro Volcano and several kilometers to the north in the andesitic highland, where the Infernitos fumarole (97°C at 1180 m) is the primary feature; neutral-chloride hot springs discharge along Rio Los Esclavos, principally near Colmenares at 490 m elevation, about 8–10 km SE of Infernitos. Maximum geothermometer temperatures calculated from Colmenares neutral-chloride spring compositions are 180°C, whereas maximum subsurface temperatures based on Laguna Ixpaco gas compositions are 310°C. An exploration core hole drilled to a depth of 808 m about 0.3 km south of Laguna Ixpaco had a bottom-hole temperature of 238°C but did not produce sufficient fluids to confirm or chemically characterize a geothermal reservoir. Hydrogeochemical data combined with regional geologic interpretations indicate that there are probably two hydrothermal-convection systems, which are separated by a major NW-trending structural boundary, the Ixpaco fault. One system with reservoir temperatures near 300°C lies beneath Tecuamburro Volcano and consists of a large vapor zone that feeds steam to the Laguna Ixpaco area, with underlying hot water that flows laterally to feed a small group of warm, chloriderich springs SE of Tecuamburro Volcano. The other system is located beneath the Infernitos area in the andesitic highland and consists of a lower-temperature (150–190°C) reservoir with a large natural discharge that feeds the Colmenares hot springs.     

Fluid geochemistry of the San Vicente geothermal field (El Salvador)

The volcano Chichontepeque (San Vicente) is one of the nine recent volcanoes making up the El Salvador sector of the WNW-ESE-trending active Central American volcanic belt. Thermal activity is at present reduced to a few thermal springs and fumaroles. The most important manifestations (Agua Agria and Los Infernillos Ciegos) are boiling springs and fumaroles located on the northern slope of the volcano (850 m a.s.l.) along two radial faults. The chloride acid waters of the Los Infernillos area are partly fed by a deep hydrothermal aquifer (crossed at 1100–1300 m by a geothermal exploration well), which finds a preferential path to the surface through the radial fault system. C0₂ is the most important gas (>90%) of the Los Infernillos Ciegos and Agua Agria fumaroles. Part of the Los Infernillos gases may also come from a deeper, hotter source, given their high HCl/S_tot. ratio and their more reducing conditions. The application of geothermometric and geobarometric methods to the gases and thermal waters suggests that both thermal areas are linked to the identified 1100–1300 m reservoir, whose temperature (250°C), lateral extension and chemical composition, as resulting from this study, are of interest for industrial development.     

Geothermal exploration of Roccamonfina volcano, Italy

The Quaternary volcano of Roccamonfina, located within the Roman high-potassic province, developed in the time interval 1.5-0.25 Ma, initially as a stratocone which later was partially destroyed by phreatomagmatic explosions. The last evolutionary stage is represented by intra-crater latite domes and trachybasalt flows (ca. 15 Ma). Geophysical data, primarily from gravity and MT/EM surveys, demonstrate that the volcano formed at a complex structural intersection above a NE-trending Pliocene graben. The same data, confirmed by the result of an 887 m test well, indicate that the volcano has not undergone caldera subsidence of the type exemplified by the Valles Caldera of New Mexico, but more probably had a series of post-stratocone lateral blasts akin to the 1980 Mount St. Helens eruptions. Hot springs near the western foot of the volcano, and the presence of young highly-evolved rocks suggested that the area could have geothermal potential. However, geochemical and isotopic data provide no indication of the present existence of a hydrothermal system, whilst the lack of alteration (both at the surface and in the test well) suggest that no such system existed in the past. Geophysical data show no anomalous features which might be associated with a geothermal reservoir or with hydrothermal alteration. These negative conclusions, strongly supported by the low temperature of the test well (35°C at 886 m), demonstrate fairly conclusively that Roccamonfina is non-prospective for high-enthalpy geothermal resources.     

Magnetostratigraphy of the Ahuachapán-Chipilapa geothermal field, El Salvador

The volcanic stratigraphy for the Ahuachapán-Chipilapa geothermal field is defined on the basis of the magnetostratigraphic results on 156 oriented samples from 33 sites. The magnetostratigraphic sequence shows that the major volcanism associated with the Concepción de Ataco caldera and the Cuyanausul volcano took place during the middle Brunhes chron (Quaternary). Pre-caldera activity of small centers such as Empalizada and Apaneca in the southern sector of the field occurred during the early Brunhes (0.77±0.07 Ma). Basaltic-andesitec activity associated with the Cuyanausul volcano took place earlier, i.e. during the Matuyama chron, possibly around 1.3±0.15 and 1.7±0.3 Ma.The local igneous basement is composed of Late Miocene-Pliocene andesites, ignimbrites and volcano-sedimentary deposits. Normal polarities and a K---Ar date of 7.37±0.73 Ma indicate that the volcanic activity in the study area extends beyond the Gauss chron. The polarity of some of the units in the post-caldera sequence and in the Concepción de Ataco and Cuyanausul sequences suggests that they may have recorded short polarity subchrons.     

Results of a 3D seismic survey at the Travale (Italy) test site

In the last 15 years geothermal exploration in Tuscany, Italy, has addressed deep reservoirs (depth ≥ 3000 m), hosted within complex geological systems, such as metamorphic formations and/or intrusive bodies. Reservoir productivity is linked to fractured and permeable zones that are rather confined and not uniformly distributed. In this context, the seismic methods represent one of the most reliable geophysical techniques for locating potential drilling targets. A 3D seismic survey has been acquired at the Travale test site, and its results have been used to develop a geological and structural model of the site, and to identify and characterize fractured zones inside the deep geothermal reservoir. A correlation between a high-amplitude reflector (H marker) and fractured contact-metamorphic rocks has been highlighted. More than 70% of the total geothermal fluid production at the Travale area comes from this seismic marker.     

Chilean geothermal resources and their possible utilization

Most of the hot spring areas in Chile are located along the Andean Cordillera, associated with Quaternary volcanism. The volcanic—geothermal activity is mainly controlled by the subduction processes of the Nazca and Antarctic oceanic plates under the South America continental plate, and occurs at three well-defined zones of the Chilean Andes: the northern zone (17°30′–28°S), the central—south zone (33φ–46°S) and the southern-most or Austral zone (48°–56°S).Some tested high temperature geothermal fields, and geological and geochemical surveys of many other hot spring areas, evidence a great potential of geothermal resources in this country. Both electrical and non-electrical applications of this potential are considered in this paper.Taking into account the potentially available geothermal resources, the development of natural resources, the geographic and social—economic conditions existing in the different regions of Chile, it is concluded that power generation, desalination of geothermal waters, recovery of chemicals from evaporite deposits and brines and sulfur-refining are the main possible applications of geothermal energy in northern Chile; in central—south Chile geothermal energy is suitable for agribusiness such as greenhouses, aquaculture and animal husbandry.     

Basement structure, lithology and permeability at Kawerau and Ohaaki geothermal fields, New Zealand

Poorly permeable basement rocks commonly occur in geothermal regions around the world, and the Quaternary Taupo Volcanic Zone (TVZ) of New Zealand is no exception. Production from basement terrane requires detailed knowledge of its geological and geophysical parameters, as shown by the history of Kawerau and Ohaaki, the only geothermal fields in the TVZ where Mesozoic Torlesse terrane greywacke (litharenite) basement is commonly penetrated at drilled depths of 1–2.5 km. In both fields the basement is step-faulted down into the TVZ. Although hot and hydrothermally altered, the greywackes have little permeability. Some production wells feed from elusive basement faults at Kawerau, but rarely at Ohaaki. Greywackes at Ohaaki are of “granite-rhyolite” provenance, and have more interbedded argillite than the “andesite-dacite” derived Kawerau greywackes. In consequence, the Kawerau basement may sustain brittle fracture at higher temperatures and depths than the more ductile Ohaaki basement, allowing convective circulation of higher enthalpy fluids into permeable Quaternary aquifers.     

Geothermal energy potential related to active volcanism in Indonesia

About 90 thermal areas in Indonesia are indicated, most of which could be grouped into hyperthermal areas located in active volcanic belts. The thermal manifestations are fumaroles, geysers, hot springs and hot mud-pools with surface temperatures generally at boiling point or more than 70°C. A tentative evaluation has been made of the potential of 54 thermal areas with a view to their further development for electrical power. The successful results of these studies in several thermal areas suggest that these volcanic geothermal systems have a high energy potential of about 13,000 – 14,000 MW.The Kawah Kamojang geothermal field in West Jawa is the first promising attempt at utilizing this geothermal energy for electrical power; a 30 MW geothermal power plant has already been installed, and a further 3 units totalling 165 MW are planned.     

The Geysers-Clear Lake geothermal area, California—An updated geophysical perspective of heat sources

The Geysers-Clear Lake geothermal area encompasses a large dry-steam production area in The Geysers field and a documented high-temperature, high-pressure, water-dominated system in the area largely south of Clear Lake, which has not been developed. Both systems have been extensively studied with geophysical techniques, drilling, and geological mapping during the past 20 years. An updated view is presented of the geological/geophysical complexities of the crust in The Geysers-Clear Lake region in order to address key unanswered questions about the heat source and tectonics. Early geophysical interpretations used a gravity low centered in the area between Clear Lake and The Geysers to suggest that a large magma chamber existed at depths starting at about 7 km. This first-order assumption of a large magma chamber expressed in the gravity data was used as a guide in subsequent geophysical and geological interpretations. Drill-hole temperature evidence is strongly suggestive of a shallow, hot-intrusive body, but in this paper the complexities are documented of the geological and geophysical data sets that make it difficult to pinpoint the location of “magma” or hot, solidified intrusive material. Forward modeling, multidimensional inversions, and ideal body analysis of the gravity data, new electromagnetic sounding models, and arguments made from other geophysical data sets suggest that many of the geophysical anomalies have significant contributions from rock property and physical state variations in the upper 7 km and not from ”magma“ at greater depths. Regional tectonic and magmatic processes are analyzed to develop an updated scenario for pluton emplacement that differs substantially from earlier interpretations. In addition, a rationale is outlined for future exploration for geothermal resources in The Geysers-Clear Lake area.     

Structure and hydrology of the Ogiri field, West Kirishima geothermal area, Kyushu, Japan

The geothermal system in the West Kirishima area is controlled by a system of faults and fractures oriented along two main directions, northwest to southeast and east–northeast to west–southwest. The Ginyu fault extends through the Ogiri field in the Ginyu area, which is one of the east–northeast to west–southwest striking faults in this area. This fault is the reservoir target for developing the geothermal resources in the Ogiri field. The Ginyu fault is a near planar fracture with a uniform temperature of 232°C and has near-neutral pH, chloride fluids. Based on the results of a detailed analysis of the Ginyu fault, all production wells drilled in the Ogiri field intersected the Ginyu fault reservoir successfully, securing steam production for a 30 MW_e power plant. A typical fracture-type geothermal model for the Ogiri field was developed on the basis of the geology, electric and geophysical logs, fluid chemistry, and well test data.