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.     

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.     

Hydrothermal alteration minerals in Aluto Langano geothermal wells, Ethiopia

Aluto Langano geothermal field is characterized by alteration mineral assemblages of calcite, quartz, chlorite, undifferentiated clays, hematite, biotite and epidote. The presence of garnet and sphene is also reported for one of the wells. Measured temperature for the reservoir is above 300°C. Permeability of the reservoir is highly influenced by the deposition of hydrothermal minerals.     

Thermal and petrologic study of the CH-A well from the Chipilapa-Ahuachapán geothermal area, el salvador

Systematic petrological studies were performed at 10-m intervals along the 2700-m-deep Ch-A well. Results show mineralogical variations that define four zones which, in turn, represent different thermal zones. The shallowest zone (Zone l) is characterized by the presence of chalcedony + zeolites + amorphous silica + saponite + montmorillonite + minor amounts of pyrite and calcite; Zone 2 by chlorite + quartz + smectite + zeolites; Zone 3by chlorite + quartz + calcite + epidote + abundant pyrite and hematite + mixed-layeredillite-smectite + chalcopyrite; andZone4 byepidote + clinozoisite + gypsum + sericite + mixed-layered chlorite-illitesmectite + anyhdrite. Fluid inclusion analyses performed at 100-m intervals indicate that a low-to-moderate salinity fluid with ice-melting temperatures of -0.7 to -2.2°C was involved in the hydrothermal alteration of the rocks. At shallow depths, positive values of + 1.6°C were found, which probably indicate an increase in volatile components. Minimum homogenization temperatures gradually increased with depth. They range from 110°C at very shallow depths (153 m) to 244°C at total depth (2700 m); however, peak or maximum temperatures of 265–286°C are found at various depths between 1400 and 2500 m. Bottom-hole fluid inclusion temperatures agree well with static temperatures derived from the Homer (1951) and the Ascencio et al. (1994) methods. Comparisons at other depths show that, in general, Homer temperatures are the lowest and that fluid inclusion temperatures are the highest, except at about 1200 m depth where the Ascencio et al. (1994) method gives the largest values. It is believed that well Ch-A encountered a mineral paragenesis that does not correspond with present thermodynamic conditions of the reservoir and that the geothermal system has undergone natural cooling.     

Geochemical monitoring of the Krafla and Námafjall geothermal areas, N-Iceland

The Krafla and Námafjall high-temperature geothermal areas in N-Iceland have been exploited for steam production since the late and early 1970s, respectively. Power generation at Krafla was 30 MW until 1998, when it was increased to 60 MW. At Námafjall the steam has been utilized for operating a 3 MW back-pressure turbine unit, drying of diatomaceous earth and heating of fresh water for space heating. A total of 34 wells have been drilled at Krafla, of which 18 are producing at present. At Námafjall 12 wells have been drilled but only three are productive. The highest temperatures recorded downhole are 320 and 350 °C at Námafjall and Krafla, respectively. Geochemical monitoring in the two fields during the last 20–25 years has revealed decreases in the Cl concentrations in the water discharged from most of the wells that have been producing for more than 10 years. The cause is enhanced colder water recharge into the producing aquifers of these wells due to depressurization by fluid withdrawal from the geothermal reservoir. Such recharge is particularly pronounced in the central part of the Leirbotnar wellfield at Krafla but it is also extensive in the only producing well in the Hvíthólar wellfield. At Námafjall incursion of cold groundwater into the reservoir was particularly intense subsequent to the volcanic-rifting event in the area in 1977. Solute (quartz, Na/K, Na/K/Ca) geothermometry temperatures have decreased significantly in those wells where Cl concentrations have decreased but only to a limited extent in those wells which have remained constant in Cl. This indicates that the changes in the concentrations of the reactive components, on which these geothermometers are based, is largely the consequence of colder water recharge and not partial re-equilibration in the depressurization zone around wells where cooling of the fluid occurs in response to extensive boiling. Aqueous SO₄ concentrations increase as Cl concentrations decrease. Except for the hottest wells, which are low in SO₄, sulphate concentrations are controlled by anhydrite solubility. Increase in SO₄ concentrations is a reflection of cooling as anhydrite has retrograde solubility with respect to temperature. H₂S-temperatures are similar to the solute geothermometry temperatures for wells with a single feed. They are, on the other hand, higher, for wells with multiple feeds, if the feed zones have significantly different temperatures. H₂-temperatures are anomalously high for most wells due to the presence of equilibrium steam in the producing aquifers. The equilibrium steam fraction amounts to 0–2.2% by wt. of the aquifer fluid (0–47% by volume). CO₂ temperatures are anomalously high for some Krafla wells due to high flux of CO₂ from the magma intruded into the roots of the geothermal system during the 1975–1984 volcanic-rifting episode. During the early phase of this episode the Leirbotnar wells were the ones most affected. The new magma gas flux has migrated eastwards with time. Today some wells in the Sudurhlídar wellfield are the ones most affected whereas the Leirbotnar wells have recovered partly or fully. The depth level of producing aquifers in individual wells at Krafla and Námafjall has been evaluated by combining data on temperature and pressure logging and geothermometry results. The majority of wells at Krafla receive fluid from a single aquifer, or from 2–3 aquifers having similar temperature. The same applies to two of the three productive wells at Námafjall.     

Geochemistry of the high-salinity geothermal field of Asal,Republic of Djibouti,Africa

In 1987–1988 deep exploratory geothermal wells were drilled in the Asal Rift in the Republic of Djibouti. The salinity of the brines produced in the Asal field exceeded 100 000mg/kg, with a low gas content of the order of 0.6 mmol/mol. Measured temperatures in the producing zones and at wellbottom vary from about 260 to 359°C. The sphalerite and galena scaling observed in producing wells starts at the flash level located at about 850 m depth. The calculated geochemical temperatures are close to 260°C, which is the average temperature of the main reservoir located at depths between 1000 and 1300 m. As a result of sulphide precipitation, about 90% of H₂S is removed from the original fluid before discharge. The computed composition of the brine at reservoir conditions is compatible with the presence of stable phases of epidote, (MgFe)-chlorite, albite, K-feldspar, K-mica, pyrite and anhydrite. The main recharge water is seawater.     

Geochemical study of fluid inclusions from the western upflow zone of the Matsukawa geothermal system,Japan

Prior to development, the Matsukawa geothermal field was partially vapor-dominated. The youngest mineral assemblage consists of early pyrophyllite, diaspore and pyrite, and later anhydrite and quartz, implying deposition from an acidic, high-temperature fluid. Fluid inclusions in anhydrite and quartz from core and cutting samples collected in wells drilled in the western upflow zone of the field were studied to characterize the temperatures and compositions of these late fluids.The results of fluid inclusion studies indicate that the temperatures during the deposition of anhydrite and quartz were up to several tens of degrees lower than the reservoir temperatures at the time of exploitation. Fluids trapped in anhydrite had temperatures of up to 257 °C, CO₂ concentrations in the 0.4–2.6 mol% range and salinities of 1.9–11.3 wt.% NaCl. This compositional variation is related to vapor loss occurring during boiling. The data suggest that the geothermal reservoir is currently being reheated by subvolcanic intrusions.     

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.     

Application of fluid inclusions to the study of Bagnore geothermal field (Tuscany, Italy)

A fluid inclusion study of the hydrothermal minerals in two breccias from two wells in the Bagnore geothermal field (Italy) has provided information on the evolution of the fluids, and has also demonstrated that fluid inclusions can be utilized as geothermometers in this geothermal field. Both breccias come from reservoir zones: one (Bagnore 3bis (Bg 3bis)) was cored at a depth of 3111 m below ground level (b.g.l.), whereas the other (Bagnore 22 (Bg 22)) was ejected during a blow-out, probably from a fractured zone present between 2200 and 2300 m b.g.l. The hydrothermal cement of the breccias is mostly made up of quartz, K-feldspar, Na-rich plagioclase, calcite, chlorite and illite. Fluid inclusion studies were carried out on quartz (Bg 3bis and Bg 22 breccias) and adularia (Bg 22 breccia). Three types of fluid inclusions were recognized in the Bg 3bis breccia. Type I (liquid-rich) inclusions trapped an aqueous fluid with a CO₂ concentration (1.7–2.7 mol/kg) that is significantly higher than present-day fluids (0.5 mol/kg). Type II (liquid-rich) inclusions formed after type I, and trapped a fluid with less CO₂ (0.6–1.0 mol/kg). Type III (vapor-rich) coexist with type I inclusions, and record an early fluid circulation under boiling conditions. The decrease of the CO₂ (and total gas) concentrations from type I inclusions to type II inclusions, and on to present-day conditions can be related to boiling with gas loss and/or mixing. Only one type of fluid inclusion (type II), with moderate CO₂ concentration (0.7–0.3 mol/kg), was found in the Bg 22 breccia. Boiling and/or mixing explain the variation of the CO₂ content in the Bg 22 reservoir fluid from inclusion formation to modern CO₂ concentration (0.3 mol/kg). The absence of any type I inclusions in Bg 22 breccia may be related to non-uniform CO₂ concentrations in different parts of the field. Present-day temperatures (295±10 °C for Bg 3bis and 320±10 °C for Bg 22) are close or equal to fluid inclusion average total homogenization temperatures (around 290 °C for Bg 3bis and 320 °C for Bg 22), suggesting that fluid inclusions can be useful for estimating local temperatures when direct measurements are not available or dubious.     

Isotopic (δO, δD) patterns in Los Azufres (Mexico) geothermal fluids related to reservoir exploitation

Isotopic patterns for the year 2000 in the Los Azufres geothermal reservoir were related to injection of a condensed steam–water–air mixture as well as to the occurrence of reservoir physical processes resulting from exploitation. Reservoir boiling and mixing of reservoir fluids with cooler fluids were the most important processes identified. Boiling takes place in two zones of the field. In the north, the boiling area includes wells AZ-13, AZ-28, AZ-48, AZ-43 and AZ-32, while, in the south, boiling affects wells AZ-16AD, AZ-22, AZ-18, AZ-26 and AZ-36. Mixing of reservoir fluids with cooler waters was identified in wells AZ-2, AZ-33, AZ-16 and AZ-46 located in the southern zone and in well AZ-4 in the north. The isotopic (oxygen-18 and deuterium) patterns of fluids collected in September 2000 show that the original convective process found in the unperturbed reservoir is still taking place, although mixing of reservoir and reinjected fluids is also indicated. According to N₂ data, the effects of reinjection on the physical and chemical characteristics of the reservoir fluids can be observed in the northern part of the field. Until now, however, only a steam phase, resulting from boiling and steam separation of the re-injected mixture, is evident in the fluids discharged by the northern wells.     

Predicting reservoir property trends under heat exploitation: interaction between flow, heat transfer, transport, and chemical reactions in a deep aquifer at Stralsund, Germany

The subsurface flow and hydrogeothermal simulation system SHEMAT (Bartels, J., Kuhn, M., Pape, H., Clauser, C., 2000. A new aquifer simulation tool for coupled flow, heat transfer, multi-species transport and chemical water-rock interactions. In: Proceedings World Geothermal Congress 2000, Kyushu – Tohuku, Japan, May 28 – June 10, pp. 3997–4002) is used to investigate a typical hydrothermal sandstone reservoir situated in the North German Basin. This study focuses on the prediction of long-term behavior of reservoir properties for the entire operation time with reinjection during heat exploitation for district heating. The Stralsund location in NE Germany and the Detfurth sandstone horizon (Buntsandstein) are chosen due to the combination of its already confirmed geothermal potential and the availability of a complete data set. An installation of two production wells and one well for reinjection implements heat exploitation. Reinjection is required due to high salinity of the water. In order to quantify injectivity changes and allow the separation of thermal from chemical effects, changes in the hydraulic parameters of the reservoir are at first studied without chemical reactions. Reinjection of cooled water of higher viscosity than the natural reservoir fluid leads to a continuous reduction of the injectivity. This effect is partially balanced by thermally induced mineral reactions. Dissolution of anhydrite in the vicinity of the injection well dominates the effect of anhydrite precipitation at the propagating thermal front leading to a net increase of injectivity. Observed calcite precipitation around the injection well and dissolution at the thermal front are too small to alter reservoir properties significantly. Coupled numerical simulation indicates that the injectivity of the reservoir is influenced primarily by the viscosity effect, but that mineral reactions weaken this negative trend. Operation of a geothermal heating plant at the Stralsund location would not be restricted by a long-term reduction in the injectivity of the reinjection well.     

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.     

Thermodynamic calculations of calcium carbonate scaling in geothermal wells, dixie valley geothermal field, U.S.A.

Wells in the Dixie Valley geothermal field of central Nevada intercept a fracture-dominated hydrothermal system at depths of 2.5 to 3 km. The reservoir water is a dilute sodium-bicarbonate-chloride type solution thought to be in equilibrium with quartz, calcite, chlorite, and albite. Fluid sampling and chemical analysis of production during an early flow test gave remarkably low calcium concentrations. Thermodynamic calculations of mineral stability in the presence of the reservoir water indicate that five times the amount of calcium measured in fluid reaching the surface is actually in solution in the reservoir fluid. Approximately 80 percent of the calcium is lost as calcium carbonate scale on the well casing before the fluid reaches the surface. The results of thermodynamic calculations compare well with the scale-volume measurements of Benoit (1989).     

Induced micro-seismicity during well discharge: OW-719, Olkaria, Kenya rift

A micro-seismic survey was carried out in concurrence with reservoir interference tests around Olkaria well 719, which has a mass output of 140–160 ton/h. The correlation between output and micro-seismicity shows that the seismic intensity peaks have a time lag of 6 to 11 days behind the mass output peaks. Seismicity spreads from the well bore in two major directions, a faster NE–SW trend at about 0.6 m/h and a slow NW–SE trend at about 0.4 m/h. These rates are comparable to those obtained by injecting organic tracers and monitoring in adjacent wells. Fluid feeder zones are at 1300–1400 m and 1600–1800 m depth and are coincident with contacts between different lithologies that provide capping and horizontal permeability. A possible NNE–SSW trending barrier to fluid flow has been observed between OW-719 and OW-718 wells.     

Temporal and spatial distribution of local seismicity in the Chipilapa-Ahuachapán geothermal area, El Salvador

Microseismic monitoring of the Chipilapa-Ahuachapán area was carried out during August-November 1988 and October 1991–April 1992. The objective was to use the study of microearthquakes as an exploration tool to invvestigate the geothermal potential of the Chipilapa area and to evaluate the main characteristics of the seismic activity, prior to and during the exploitation tests. Since 1989, seven wells have been drilled in the area, two of which have encountered three geothermal aquifers that could be exploited for electricity generation by means of binary-cycle technology. The 1988 survey detected important, shallow and low magnitude seismic activity, located mainly south and southwest of the explored area. This activity is possibly related to the recharge zone of the Chipilapa-Ahuachapán geothermal system, located further south, beneath the Pleistocene Pacific Volcanic Chain. The 1991–1992 survey confirmed the existence of seismicity beneath the southern volcanic axis, but other important clusters of activity were recorded northward, related to the deeper structures of the Central Graben, and southwest of the Ahuachapán geothermal field, close to the 1990 hydrothermal eruption of Agua Shuca. Shallow microseismic activity also appeared along the faults limiting the Chipilapa geothermal field to the east. Although it is probable that this seismicity is due to fluid circulation in fractures, no geothermal reservoirs were intercepted by wells CHA and CH8. Moreover, no significant induced seismicity was recorded during production and injection tests.     

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 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.     

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.     

Intersecting faults and sandstone stratigraphy at the cerro prieto geothermal field

The NW - SE trending Cerro Prieto fault zone is part of a major regional lineament that extends into Sonora, Mexico, and has characteristics of both a wrench fault and an oceanic transform fault. The zone includes a number of separate identified faults and apparently penetrates deep into the basement and crustal rocks in the area. The zone serves as a conduit for both large and rapid heat flow. Near well M-103, where the Michoacán fault obliquely intersects a shorter NE - SW trending fault (i.e., the Pátzcuaro fault), large circulation losses during drilling indicate greater permeability and hence increased natural convective fluid flow. Temperature contour maps for the southern portion of the field suggest that a shear fault zone also exists in the vicinity of wells M-48, M-91 and M-101. This shear zone aids in rapidly distributing geothermal fluid away from the Cerro Prieto fault zone, thus enhancing recharge to the western part of the reservoir.We have studied the distribution of lithologies and temperature within the field by comparing data from well cuttings, cores, well logs and geochemical analyses. Across the earliest developed portion of the field, in particular along a 1.25 km NE - SW section from well M-9 to M-10, interesting correlations emerge that indicate a relationship among lithology, microfracturing and temperature distribution. In the upper portion of the reservoir of this section, between 1200 and 1400 m, the percentage of sandstones ranges from 20 to 55. Well logs, calcite isotope maxima, and the Na - K - Ca geothermometer indicate temperatures of 225–275°C. The isothermal high in this vicinity corresponds to the lowest total percentage of sandstones. Scanning electron microphotographs of well cores and cuttings from sandstone and shale units reveal open microfractures, mineral dissolution and mineral precipitation along microfractures and in pores between sand grains. Our working hypothesis is that these sandy shale and siltstone facies are most amenable to increased microfracturing and, in turn, such microfracturing allows for higher temperature fluid to rise to shallower depths in this part of the reservoir.Our ongoing research is aimed at achieving a coherent geological model that provides a basis for estimating reservoir capacity, and that illustrates our understanding of fluid flow along major faults, laterally through fault shear zones, and within predominantly silty and shaley deltaic clastics that have been microfractured.     

Occurrence and significance of anomalous chloride waters at the Orakei Korako geothermal field, Taupo Volcanic Zone, New Zealand

Water with Cl concentrations from 15 to almost 10,000 mg/kg, and molecular SO₄/Cl ratios ranging from 0.003 to 1.87, drips periodically from the roof of Ruatapu cave and a side chamber, Rahu Rahu, located in the Orakei Korako geothermal field, Taupo Volcanic Zone, New Zealand. Pools in the bottom of both Ruatapu and Rahu Rahu contain sulfate–chloride waters with pH values ranging from 2.5 to 3.0; their Cl contents have varied temporally from 120 to 240 mg/kg and their molecular SO₄/Cl ratios from 0.86 to 1.30. The Cl in the water dripping from the cave roof cannot come directly from the alkali chloride–bicarbonate water that circulates in the reservoir at Orakei Korako since the modern and historic piezometric surfaces are several meters below the cave roof. Nor does all the Cl in the cave pool waters derive from the reservoir fluid as the volume input required is incompatible with their Na and K contents. A more likely source for the Cl is one whereby rain water, percolating through the fractured rhyolitic country rock, dissolves Cl present either in glass shards or halite deposited in prehistoric times when trapped alkali chloride water boiled to dryness. Given that Cl in the cave pool waters is therefore supplied from a source above rather than, as previously assumed, below, the axiom that Cl present in acid sulfate–chloride ± bicarbonate waters is necessarily a signature of a deep water or magmatic input needs qualification.