Geochemical and isotopic characteristics of fluids in the Niutuozhen geothermal field,North China

Niutuozhen geothermal field is located in the Jizhong graben, belonging to the northern part of Bohai Bay Basin in North China. Chemical and isotopic analyses were carried out on 14 samples of the geothermal fluids discharged from Neogene Minghuazhen (Nm), Guantao (Ng), and Jixianian Wumishan (Jxw) formations. The δ²H and δ¹⁸O in water, δ¹³C in CH₄, δ¹³C in CO₂, and ³He/⁴He ratio in the gases were analyzed in combination with chemical analyses on the fluids in the Niutuozhen geothermal field. The chemical and isotopic compositions indicate a meteoric origin of the thermal waters. The reservoir temperatures estimated by chemical geothermometry are in the range between 60 and 108 °C. The results show that the gases are made up mainly by N₂ (18.20–97.42 vol%), CH₄ (0.02–60.95 vol%), and CO₂ (0.17–25.14 vol%), with relatively high He composition (up to 0.52 vol%). The chemical and isotopic compositions of the gas samples suggest the meteoric origin of N₂, predominant crustal origins of CH₄, CO₂, and He. The mantle-derived He contributions are calculated to be from 5 to 8% based on a crust–mantle binary mixing model. The deep temperatures in the Jxw reservoir were evaluated based on gas isotope geothermometry to be in the range from 141 to 165 °C. The mantle-derived heat fraction in the surface heat flow is estimated to be in the range of 48–51% based on ³He/⁴He ratios.     

Helium isotopes in geothermal systems: Iceland,The Geysers,Raft River and Steamboat Springs

Helium isotope ratios have been measured in geothermal fluids from Iceland, The Geysers, Raft River, Steamboat Springs and Hawaii. These ratios have been interpreted in terms of the processes which supply He in distinct isotopic ratios (i.e. magmatic He, ~10 R_a; atmospheric He, R_a; and crustal He, ~0.1 R_a) and in terms of the processes which can alter the isotopic ratio (hydrologic mixing, U-Th series alpha production and weathering release of crustal He, magma aging and tritiugenic addition of ³He). Using this interpretational scheme, Iceland is found to be an area of hot-spot magmatic He implying an active volcanic source although the data are suggestive of high-temperature weathering release of crustal He incorporated in the geothermal fluids. By comparison to fumarolic gases from Hawaii and Juan De Fuca and Cayman Trench basaltic glass samples, The Geysers contains MOR type magmatic He again implying an active volcanic source possibly a “leaky” transform related to the San Andreas Fault System. Raft River contains only crustal He indicating no active volcanic sources. Steamboat Springs He isotope ratios are distinctly less than typical plate margin volcanics but must still have a magmatic source. A preliminary assessment of the cause for this low ratio is made assuming an “aging” magma source.     

Origin and evolution of geothermal fluids from Las Tres Vírgenes and Cerro Prieto fields,Mexico – Co-genetic volcanic activity and paleoclimatic constraints

Major and trace elements, noble gases, and stable (δD, δ¹⁸O) and cosmogenic (³H, ¹⁴C) isotopes were measured from geothermal fluids in two adjacent geothermal areas in NW-Mexico, Las Tres Vírgenes (LTV) and Cerro Prieto (CP). The goal is to trace the origin of reservoir fluids and to place paleoclimate and structural-volcanic constraints in the region. Measured ³He/⁴He (R) ratios normalized to the atmospheric value (R_a = 1.386 × 10⁻⁶) vary between 2.73 and 4.77 and are compatible with mixing between a mantle component varying between 42 and 77% of mantle helium and a crustal, radiogenic He component with contributions varying between 23% and 58%. Apparent U–Th/⁴He ages for CP fluids (0.7–7 Ma) suggest the presence of a sustained ⁴He flux from a granitic basement or from mixing with connate brines, deposited during the Colorado River delta formation (1.5–3 Ma). Radiogenic in situ ⁴He production age modeling at LTV, combined with the presence of radiogenic carbon (1.89 ± 0.11 pmC – 35.61 ± 0.28 pmC) and the absence of tritium strongly suggest the Quaternary infiltration of meteoric water into the LTV geothermal reservoir, ranging between 4 and 31 ka BP. The present geochemical heterogeneity of LTV fluids can be reconstructed by mixing Late Pleistocene – Early Holocene meteoric water (58–75%) with a fossil seawater component (25–42%), as evidenced by Br/Cl and stable isotope trends. CP geothermal water is composed of infiltrated Colorado River water with a minor impact by halite dissolution, whereas a vapor-dominated sample is composed of Colorado River water and vapor from deeper levels. δD values for the LTV meteoric end-member, which are 20‰–44‰ depleted with respect to present-day precipitation, as well as calculated annual paleotemperatures 6.9–13.6 °C lower than present average temperatures in Baja California point to the presence of humid and cooler climatic conditions in the Baja California peninsula during the final stage of the Last Glacial Pluvial period. Quaternary recharge of the LTV geothermal reservoir is related to elevated precipitation rates during cooler-humid climate intervals in the Late Pleistocene and Early Holocene. The probable replacement of connate water or pore fluids by infiltrating surface water might have been triggered by enhanced fracture and fault permeability through contemporaneous tectonic–volcanic activity in the Las Tres Vírgenes region. Fast hydrothermal alteration processes caused a secondary, positive δ¹⁸O-shift from 4‰ to 6‰ for LTV and from 2‰ to 4‰ for CP geothermal fluids since the Late Glacial infiltration.     

Flux of4He from Carnmenellis granite: modelling of an HDR geothermal reservoir

The diffusion coefficient (D) of He in the Carnmenellis granite, recovered from the Rosemanowes hot dry rock (HDR) geothermal reservoir, is experimentally determined in the laboratory in a temperature range of 100–300°C. Temperature variation of D fits an Arrhenius plot, but yields an activation energy for He release from rock significantly lower than the value for feldspar or quartz, suggesting that most of the He in the reservoir granite resides within grain boundaries and jointing cement and may be easily released.The reservoir surface area (S) is estimated based on the laboratory determined value of D from granite and the measured⁴He contents of circulation fluids from RH 15 well. In the first year of reservoir circulation,⁴He-based reservoir surface area was about twice that based on²²²Rn. It increased and stabilized at about 6 times after 2 a. The excess He release from newly opened up fracture surfaces within the reservoir during its expansion is shown to be responsible for the temporal increase in the He-based reservoir surface area over that of Rn.     

Isotopic Geochemistry of Non—hydrocarbons in Natural Gases from the Sanshui Basin,Guangdong Province,China

This study is focused on geothermal heat flow and the origin of non-hydrocarbons in natural gases in terms of the isotope geochemical characteristics of Ar, He, CO₂ and N₂ in natural gases from the Sanshui Basin, Guangdong Province. China.³He/⁴He ratios are of (1.60-6.39) × 10^-6, and⁴⁰Ar/³⁶Ar ratios of 450–841. The carbon isotopic composition (δ_l3C PDB) of carbon dioxide ranges from -20‰ to -2‰. δ^l5N(air) ratios have a wider range of-57 ‰- +95 ‰. The isotope geochemical characteristics of non-hydrocarbons indicate that He, Ar and N₂ in the gas reservoirs enriched in non-hydrocarbons were derived largely from the upper mantle. Non-hydrocarbons in gaseous hydrocarbon reservoirs consist mainly of crustal radiogenic He and⁴⁰Ar and some mantle-derived He and Ar, as well as of¹³C-depleted carbon dioxide and nitrogen generated as a result of thermal decomposition of organic matter in strata. Carbon dioxide enriched in¹³C was derived largely from carbonate rocks and partially from the lower crust and upper mantle. Based on the relationship between geothermal heat flow (Q) and³He/⁴ He ratio in natural gases, the Q values for the area studied have been calculated. Similar Q values are reported from the upper mantle uplift area (77 mWm^-2) in Huabei and the Tancheng-Lujiang Rift Zone (88 mWm^-2). More than 60 percent of geothermal heat flow in the Sanshui Basin may have been derived from the upper mantle. The project is financially supported by the National Natural Science Foundation of China.     

Hydrothermal He and CO2 at Wudalianchi intra-plate volcano,NE China

Chemical and isotopic compositions have been measured for CO₂-rich bubbling gases discharging from cold springs in Wudalianchi intra-plate volcanic area, NE China. Observed ³He/⁴He ratios (2–3 R_A) and δ¹³C values of CO₂ (−5‰ to −3‰) indicate the occurrence of a mantle component released and transferred to the surface by the Cenozoic extension-related magmatic activities. The CO₂/³He ratios are in wide range of (0.4–97 × 10⁹). Based on the apparent mixing trend in a ³He/⁴He and δ¹³C of CO₂ diagram from all published data, the extracted magmatic end-member in the Wudalianchi Volcano has ³He/⁴He, δ¹³C and CO₂/³He value of ∼3.2 R_A, ∼−4.6‰ and ∼6 × 10¹⁰, respectively. These values suggest that the volatiles originate from the sub-continental lithospheric mantle (SCLM) in NE China and represent ancient fluids captured by prior metasomatic events, as revealed by geothermal He and CO₂ from the adjacent Changbaishan volcanic area.     

Geothermal potential of Egypt

One hundred and sixty samples of groundwater from nearly all parts of Egypt have been collected and chemically analyzed in order to assess the country's geothermal potential. The samples considered to be thermal include 20 wells (T > 35°C), 4 springs (T > 30°C) and 1 spring not included in the present inventory. The remaining samples, together with data from the literature, establish background chemistry. The hottest springs are located along the east shore of the Gulf of Suez: Uyun Musa (48°C) and 'Ain Hammam Faraoun (70°C). Additional warm springs are located along both shores of the Gulf of Suez and this region is the most promising for geothermal development. The Eastern Desert of Egypt, particularly the coastal area adjacent to the Red Sea has above normal heat flow ( ~ 72.0 < mWm⁻²) and therefore some geothermal potential although only one thermal well (Umm Kharga: 35.8°C) could be located, In the major oases of the Western Desert (Kharga, Dakhla, Farafra and Bahariya), the regional temperature gradient is low (< 20°C/km), but many of the wells tap deep artesian aquifers and produce large volumes of water in the 35–43°C range. Such wells constitute a low temperature geothermal resource. None of our samples in northern Egypt can be considered thermal including several reported “hot springs.” Application of the silica, NaKCa. and NaKCaMg geothermometers does not indicate the presence of a high temperature geothermal resource at any area we visited.     

Composition and origin of thermal waters in the Gulf of Suez area,Egypt

Thermal waters with discharge temperatures ranging from 32 to 70°C are being discharged along the Gulf of Suez (Egypt) from springs and shallow artesian wells. A comprehensive chemical and isotopic study of these waters supports previous suggestions that the waters are paleometeoric waters from the Nubian sandstone aquifer. The chemical and isotopic compositions of solutes indicate possible contributions from Tertiary sedimentary aquifer rocks and windblown deposits (marine aerosols and/or evaporite dust) in the recharge area. There is no chemical or isotopic evidence for mixing with Red Sea water. Gas effervescence from the Hammam Faraoun thermal water contains about 4% CH₄ (δ¹³C = −32.6‰) and 0.03% He having an isotopic ratio consistent with a mixture of crustal and magmatic He (³He/⁴He = 0.26 Re). Geothermometers for the thermal waters indicate maximum equilibration temperatures near 100°C. The waters could have been heated by percolation to a depth of several km along the regional geothermal gradient.     

Sandstone diagenesis in the Pattani Basin (Gulf of Thailand): history of water-rock interaction and comparison with the Gulf of Mexico

In the Pattani Basin, a failed-rift basin, extensive water-rock interaction has occurred between subquartzose alluvial sandstones of Miocene age and their pore fluids. Diagenetic rates and pathways have been strongly influenced by high geothermal gradients, high CO₂ fugacities, and low pore water salinities. Depositional pore water was fresh to brackish, depending on the depositional environment of the sediments. Chloride concentrations in modern formation water are believed primarily to reflect the proportions of river and sea water in the depositional environment. However, the concentration of other important solutes and the isotopic composition of the formation waters can not be explained by roportional mixing of these two end-member waters. Dissolution of detrital plagioclase (An = 3) and K- feldspar are reactions of major significance that are reflected chemically in the Na/Cl and K/Cl ratios of the formation water. Despite the high temperature of the sandstones (120–200°C), diagenetic albite does not occur. Geochemical calculations indicate the formation water is undersaturated with respect to both orthoclase and albite. This style of feldspar diagenesis differs significantly from that of sandstones of similar composition in other basins, and has probably influenced other aspects of silicate diagenesis.Important authigenic minerals are: 1. locally abundant calcite cement (δ¹³C= −12.8, δ¹⁸O= −17.3 PDB), an early diagenetic phase that formed at about 60°C; 2. pore-filling kaolinite (δ¹⁸O= 9.9, δD= −83.5SMOW) that was closely associated with feldspar dissolution and formed over a range of temperatures; and 3. fibrous pore-lining and pore-bridging illite (δ¹⁸O = 9.8, δD = − 86.7 SMOW, the last significant cement, formed at temperatures of 120 to 150°C. Potassium/argon dates on illite indicate that sandstone diagenesis took place during a period of rapid sedimentation in the first two-thirds of the burial history.Comparison of Pattani Basin diagenesis with diagenesis of sandstones of similar age in other sedimentary basins demonstrates that chemical diagenesis, relative to mechanical compaction, has been especially rapid in the Pattani Basin. This reflects the effect of high temperatures on reaction rates. The net effect is a high average rate of porosity loss with burial (11% km).     

Mantle-derived CO_2 in Hot Springs of the Rehai Geothermal Field,Tengchong,China

Gas concentrations and isotopic compositions of He and CO2 were determined on free gas samples from ten hot springs of the Rehai geothermal field, Tengchong, China. The results showed that hot-spring CO2 gas, together with He,was derived mainly from the mantle, indicating the accumulation of mantle-derived volatiles beneath the survey area. The δ^13C values of CO2, higher than those of the typical mantle-derived carbon and the isotopic composition of hot-spring-free CO2 in unequilibrium with dissolved CO2, are recognized only in the Rehai geothermal field, suggesting that there seems to be a still-degassing magmatic intrusion at depths, which provides mantle-derived volatiles to the hydrothermal system above. The accumulation of those volatiles has probably played an important role in triggering earthquakes in this region.In addition, the isotopic characteristics of He and C also indicate that the magmatic intrusion seems to have been derived from the MORB source, and could be contaminated by crustal materials during its upwelling through the continental crust.     

Regional and temporal variations in CO2/He,He/He and δC along the North Anatolian Fault Zone,Turkey

New He and C relative abundance, isotope and concentration results from nine geothermal locations situated along an 800-km transect of the North Anatolian Fault Zone (NAFZ), Turkey, that were monitored during the period November 2001–November 2004, are reported. The geothermal waters were collected every 3–6 months to study possible links between temporal geochemical variations and seismic activity along the NAFZ. At the nine sample locations, the He isotope ratios range from 0.24 to 2.3R_A, δ¹³C values range from −4.5 to +5.8‰, and CO₂/³He ratios range from 5 × 10⁹ to 5 × 10¹⁴. The following geochemical observations are noted: (1) the highest ³He/⁴He ratios are found near the Galatean volcanic region, in the central section of the NAFZ, (2) at each of the nine sample locations, the ³He/⁴He ratios are generally constant; however, CO₂/³He ratios and He contents both show one order of magnitude variability, and δ¹³C values show up to ∼4‰ variability, and (3) at all locations (except Re?adiye), δ¹³C values show positive correlations with CO₂ contents. The results indicate that at least three processes are necessary to account for the geochemical variations: (1) binary mixing between crustal and mantle-derived volatiles can explain the general characteristics of ³He/⁴He ratios, δ¹³C values, and CO₂/³He ratios at the nine sample locations; (2) preferential degassing of He from the geothermal waters is responsible for variations in CO₂/³He values and He contents at each sample location; and (3) CO₂ dissolution followed by calcite precipitation is responsible for variations in CO₂ contents and δ¹³C values at most locations. For each of the geochemical parameters, anomalies are defined in the temporal record by values that fall outside two standard deviations of average values at each specific location. Geochemical anomalies that may be related to seismic activity are recorded on June 28, 2004 at Yalova, where a M = 4.2 earthquake occurred 43 days earlier at 15 km distance from the sample location, and on April 7, 2003 at Efteni, where a M = 4.0 earthquake occurred 44 days later at a distance of 12 km. At both locations, the sampling periods containing geochemical anomalies were preceded by an increase in M ? 3 earthquakes occurring within 60 days and less than 40 km distance.     

Helium-3 anomalies and crust-mantle interaction in Italy

The distribution of primordial ³He in the crust of Italy has been investigated through a survey of groundwaters, hot-springs, exploration and production geothermal wells. The mantle-derived component varies from 0% to > 60% and reaches a maximum at Vulcano in the Eolian Islands. ³He anomalies occur where the conductive heat flow is relatively high, but the relationships between heat flow and anomaly are uncertain.The distribution of ³He in the main reservoir and cap rock to the Larderello geothermal system shows that the mantle-derived ³He varies from ~5% to ~40% of total He and reaches a maximum in fluids extracted from a depth of ca. 3 km within the basement. Bomb-produced tritiogenic ³He appears to make a near-negligible contribution. Well 107 at Larderello has been monitored over a period of 17 months and the ${}^3\text{He}{}^4\text{He}$ ratio (R) varies from 1.4 to 1.6 times the atmospheric ratio (Ra) whilst He/Ne varies from 9 to 160. A model is proposed for the Larderello geothermal system whereby mantle-derived heat and volatiles are advected to shallow (6–8 km) depths in association with melts and released into the main geothermal reservoir at Larderello in a non-uniform manner possibly associated with fracturing. Isotopic and elemental equilibrium of gaseous species (C, O, H) appears to have taken place in the basement to the main reservoir from where fluid with R/Ra ≥ 3.2 has been extracted.     

Noble-gas evidence for geothermal activity in a karstic terrain: Rocky Mountains,Canada.

Geothermally heated fluids are identified as a component in warm (up to 54°C) springs in the southern Canadian Rocky Mountains. High concentrations of radiogenic He (10^?3 ccSTP/cc gas) and atmospheric Ne, Ar, Kr and Xe in the gases that vigorously bubble at the Fairmont Hot Spring, assign the latter to be a “drowned” fumarole, fed by geothermal steam. Up to 75% depletions in the atmospheric noble gases in several warm springs indicate contributions of residual geothermal water. On the other hand, in a few cases noble gases were found in excess (W) over the expected concentrations in airequilibrated recharge water. The observed “reversed” pattern of w_nc >W_Ar >W_kr >W_xe is interpreted as excess air, incorporated during recharge through karstic conduits. The mixing ratios of geothermal and karstic waters can be deduced.     

Application of geochemical methods in the search for geothermal fields

From 1983 to 1985, an orientation study was carried out in about 20 geothermal fields in China. Semi-detailed and detailed surveys were conducted in an area of 90 km² in Yangbajing, Tibet. Hg, As, Sb and Bi were used as indicators to extend the promising area of known geothermal fields to 16 km². In the northern part of the extended promising area, three drill holes were sunk and high-temperature thermal water of 160°C was obtained. In the southern part, outside the geochemical anomaly, two holes were drilled but no thermal water was found. The inner zone of the Hg anomaly coincides closely with the distribution of drill holes that met thermal water with temperatures higher than 140°C. The distribution of Bi values may indicate that the thermal water in the southern part is at shallower depth. A semi-regional survey in an area of 200 km² in Tengchong, Yunnan Province delineated several new prospects, which will possibly extend the resources of the Rehai geothermal field. An element zoning pattern similar to that of hydrothermal mineral deposits was discovered in the Rehai geothermal field. A semi-regional survey conducted in Xiaotangshan near Beijing indicated that the possible extent of the Xiaotangshan geothermal field may reach 30 km². Drilling confirmed that the outer zone of the Hg anomaly is in accordance with the distribution of 40°C thermal water, and the intermediate and inner zones of the Hg anomaly are associated with higher temperature thermal water. Research during the three years 1983–1985 demonstrated that exploration geochemistry is an effective tool for finding geothermal fields.     

Helium measurements of pore fluids obtained from the San Andreas Fault Observatory at Depth (SAFOD, USA) drill cores

⁴He accumulated in fluids is a well established geochemical tracer used to study crustal fluid dynamics. Direct fluid samples are not always collectable; therefore, a method to extract rare gases from matrix fluids of whole rocks by diffusion has been adapted. Helium was measured on matrix fluids extracted from sandstones and mudstones recovered during the San Andreas Fault Observatory at Depth (SAFOD) drilling in California, USA. Samples were typically collected as subcores or from drillcore fragments. Helium concentration and isotope ratios were measured 4?C6 times on each sample, and indicate a bulk ⁴He diffusion coefficient of 3.5?±?1.3?×?10^?C8 cm²?s^?C1 at 21°C, compared to previously published diffusion coefficients of 1.2?×?10^?C18 cm²?s^?C1 (21°C) to 3.0?×?10^?C15 cm²?s^?C1 (150°C) in the sands and clays. Correcting the diffusion coefficient of ⁴He_water for matrix porosity (??3%) and tortuosity (??6?C13) produces effective diffusion coefficients of 1?×?10^?C8 cm^2?s^?C1 (21°C) and 1?×?10^?C7 (120°C), effectively isolating pore fluid ⁴He from the ⁴He contained in the rock matrix. Model calculations indicate that <6% of helium initially dissolved in pore fluids was lost during the sampling process. Complete and quantitative extraction of the pore fluids provide minimum in situ porosity values for sandstones 2.8?±?0.4% (SD, n?=?4) and mudstones 3.1?±?0.8% (SD, n?=?4).     

Hydrothermal deposition on the East Pacific rise at 21°N

In the spring of 1979, 350°C springs precipitating hydrothermal sulphides and sulphates directly on to the sea-floor were discovered on the crest of the East Pacific Rise (EPR) at 21°N by the astonished scientific party of the RISE submersible expedition. These hot springs are within a linear field of active and inactive hydrothermal vents extending 6 km along the rise axis. Typically the mineral deposits at EPR, 21°N consist of basal sulphide mounds surmounted by mineralized sulphide-sulphate edifices, or “chimneys”, reaching heights up to 13 m above the sea floor. The mounds rest directly on fresh basalt and cover areas up to 450 m². Chimneys atop mounds may be active or dead. The hottest active chimneys (350°C) spew forth fluids blackened by fine-grained sulphide precipitates, dominantly hexagonal pyrrhotite and iron-rich sphalerite. These “black smokers” are distinguished from cooler “white smoker” chimneys which are encrusted by worm tubes and emit milky fluids bearing amorphous silica, barite, and pyrite.     

He diffusion and (U–Th)/He thermochronometry of zircon: initial results from Fish Canyon Tuff and Gold Butte

To evaluate the potential of (U–Th)/He geochronometry and thermochronometry of zircon, we measured He diffusion characteristics in zircons from a range of quickly and slowly cooled samples, (U–Th)/He ages of zircons from the quickly cooled Fish Canyon Tuff, and age-paleodepth relationships for samples from 15 to 18 km thick crustal section of the Gold Butte block, Nevada. (U–Th)/He ages of zircons from the Fish Canyon Tuff are consistent with accepted ages for this tuff, indicating that the method can provide accurate ages for quickly cooled samples. Temperature-dependent He release from zircon is not consistent with thermally activated volume diffusion from a single domain. Instead, in most samples apparent He diffusivity decreases and activation energy (E_a) increases as cycled step-heating experiments proceed. This pattern may indicate a range of diffusion domains with distinct sizes and possibly other characteristics. Alternatively, it may be the result of ongoing annealing of radiation damage during the experiment. From these data, we tentatively suggest that the minimum E_a for He diffusion in zircon is about 44 kcal/mol, and the minimum closure temperature (T_c, for a cooling rate of 10 °C/myr) is about 190 °C. Age–paleodepth relationships from the Gold Butte block suggest that the base of the zircon He partial retention zone is at pre-exhumation depths of about 9.5–11 km. Together with constraints from other thermochronometers and a geothermal gradient derived from them in this location, the age–depth profile suggests a He T_c of about 200 °C for zircon, in reasonable agreement with our interpretation of the laboratory measurements. A major unresolved question is how and when radiation damage effects become significant for He loss from this mineral.     

Boron isotope composition of geothermal fluids and borate minerals from salar deposits (central Andes/NW Argentina)

We have measured the boron concentration and isotope composition of regionally expansive borate deposits and geothermal fluids from the Cenozoic geothermal system of the Argentine Puna Plateau in the central Andes. The borate minerals borax, colemanite, hydroboracite, inderite, inyoite, kernite, teruggite, tincalconite, and ulexite span a wide range of δ¹¹B values from −29.5 to −0.3‰, whereas fluids cover a range from −18.3 to 0.7‰. The data from recent coexisting borate minerals and fluids allow for the calculation of the isotope composition of the ancient mineralizing fluids and thus for the constraint of the isotope composition of the source rocks sampled by the fluids. The boron isotope composition of ancient mineralizing fluids appears uniform throughout the section of precipitates at a given locality and similar to values obtained from recent thermal fluids. These findings support models that suggest uniform and stable climatic, magmatic, and tectonic conditions during the past 8 million years in this part of the central Andes. Boron in fluids is derived from different sources, depending on the drainage system and local country rocks. One significant boron source is the Paleozoic basement, which has a whole-rock isotopic composition of δ¹¹B=−8.9±2.2‰ (1 SD); another important boron contribution comes from Neogene-Pleistocene ignimbrites (δ¹¹B=−3.8±2.8‰, 1 SD). Cenozoic andesites and Mesozoic limestones (δ¹¹B≤+8‰) provide a potential third boron source.     

孤岛油田馆陶组热储地热资源开发利用分析

孤岛油田蕴藏丰富的中、低温地热资源,对其进行合理的开发利用,对推进该区新、旧热能转换,促进地方经济发展具有重要意义。在总结以往勘探成果的基础上,查明了孤岛油田为大地热流高值异常区,平均值为72.62 mW/m²。重点研究馆陶组热储地热地质条件,查明了馆陶组下段热储厚度为106~145 m,平均孔隙度约为30%,热储温度为75.5~82 ℃,单位降深涌水量为3.71~10.55 m³/(h·m),是地热资源开发的有利目标热储。采用热储法估算区内馆陶组下段热储中蕴藏的地热资源量为3.745×10¹⁸ J,折合标准煤量1.28亿t,地热水储存量约为60.87×10⁸ m³; 采用开采强度法估算的该区地热水允许开采量约为253万m³/a,可支持供暖面积约100万m²。