Search Results (58)

Geothermal energy is a locally available, low-carbon resource that may support heat supply, building decarbonisation and regional energy diversification in non-volcanic crystalline settings. This study proposes an exploratory GIS-based approach for geothermal favourability mapping in the Guarda District, Central Portugal, where Hercynian granites, major fault systems and thermal and mineral water occurrences define a structurally controlled hydrogeothermal framework. Hydrogeochemical data from 54 groundwater abstraction points were integrated through silica-derived apparent geothermometric indicators, classical hydrothermal-parameter estimation and Empirical Bayesian Kriging Regression Prediction (EBKRP). Apparent silica-derived temperature indicators, circulation depth, geothermal gradient and theoretical thermal power were estimated, with log₁₀ transformed thermal power used as the dependent variable and distance to major mapped faults as the structural covariate. Apparent silica-derived temperature indicators range from 21.3 °C to 121.2 °C, with a mean of 64.6 °C, while estimated geothermal gradients range from 20.3 °C/km to 92.1 °C/km. Higher estimated values occur preferentially near NE–SW and NNW–SSE fault systems, suggesting that structural permeability may influence deep groundwater circulation. The interpretation explicitly acknowledges that, in low-temperature systems, dissolved silica may be influenced by chalcedony or amorphous silica control, as well as by cooling, mixing and incomplete re-equilibration during fluid ascent. The resulting map is interpreted as a screening-level favourability product, not as a definitive assessment of exploitable geothermal resources, and supports the prioritisation of future structural mapping, geophysical surveys, exploratory drilling, borehole temperature logging and applied geothermal assessment in fractured granitic terrains. Full article

The widespread abundance of silicified wood and fossil leaves in southwestern Georgia is associated with the upper Miocene-lower Pliocene volcanic deposits of the Goderdzi Formation. Neogene volcanic terrains frequently preserve exceptionally detailed fossil records, providing valuable insights into ancient environments, climate regimes, and vegetational dynamics. Extensive upper Miocene volcanic activity produced thick pyroclastic deposits, lahar flows, and localized sedimentary basins that facilitated the rapid burial and preservation of diverse plant remains, including silicified wood and well-preserved fossil leaves. The mineralogy of Goderdzi Formation fossil woods is surprisingly complex, with compositions that include opal-A, opal-Ct, chalcedony, and microcrystalline quartz. These minerals are evidence of variations in hydrothermal fluid circulation that led to episodes of mineral precipitation that typically occurred in several discrete steps. Full article

This study explores the hydrochemical and thermal characteristics of a fault-controlled geothermal field within the Northern Qinghai Lake Coalfield Area on the northeastern Qinghai–Tibetan Plateau (QTP). This research integrates hydrochemical analyses, isotopic tracers, and the regional geological framework to define hydrochemical signatures, identify recharge sources and flow paths, assess cold–hot water mixing, estimate reservoir temperatures, determine circulation depths and residence times, and explain the geothermal system’s formation. Systematic sampling included geothermal waters, cold springs, and surface waters, followed by laboratory analysis of major ions, stable isotopes (δ²H, δ¹⁸O), radiocarbon (¹⁴C), and tritium (³H). The geothermal water is categorized as a low-temperature, weakly acidic to near-neutral HCO₃-Ca•Mg type, exhibiting temperatures from 35.6 to 46.2 °C. Isotopic analyses indicate that cold spring and river waters align with the local meteoric water line, while geothermal waters display distinct isotopic signatures, suggesting deeper circulation. A silica–enthalpy mixing model reveals substantial cold-water mixing during upwelling, with mixing ratios between 74.5% and 85.6%. The corrected recharge elevation is estimated to be 4378–4456 amsl, implying a primary recharge zone in the branch of the Qilian mountains—the middle section of Datong Mountain to the northeast. Geothermometry, employing quartz and chalcedony temperature scales and accounting for mixing, estimates reservoir temperatures of 150–202 °C. The calculated circulation depth spans 3211–4291 amsl. Low tritium levels and carbon dating suggest a deep-cycling system predating 1952, characterized by deeply circulating “ancient water”. The geothermal system’s development is associated with regional tectonics, fault systems, and the Kesuer Formation (J_xk) acting as the reservoir. This study provides a scientific foundation for the development and sustainable use of geothermal resources in the northern Qinghai Lake region and offers insights applicable to comparable fault-controlled geothermal systems across the QTP. Full article

Siliceous rocks of various colors and types are extensively developed within the Middle–Lower Ordovician carbonate along the Northwest Tarim Basin. Their genesis provides important insights into the evolution of basinal fluids and the associated diagenetic alterations of the carbonates. Based on petrographic, geochemical, fluid inclusion, and petrophysical analyses, this study investigates the origin of siliceous rocks within the Middle–Lower Ordovician carbonate formations (Penglaiba, Yingshan, and Dawangou formations) in the Kalpin area, Tarim Basin, and investigates the impact on hydrothermal reservoirs. The results reveal two distinct episodes of siliceous diagenetic fluids: The first during the Late Ordovician involved mixed hydrothermal fluids derived from deep magmatic–metamorphic sources, formation brines, and seawater. Characterized by high temperature and moderate salinity, it generated black chert dominated by cryptocrystalline to microcrystalline quartz through replacement processes. The second episode developed in the Middle–Late Devonian as a mixture of silicon-rich fluids from deep heat sources and basinal brines. In conditions of low temperature and high salinity, it generated gray-white siliceous rocks composed of micro- to fine crystalline quartz, spherulitic-fibrous chalcedony, and quartz cements via a combination of hydrothermal replacement and precipitation. A reservoir analysis reveals that the multi-layered black siliceous rocks possess significant reservoir potential amplified by the syndiagenetic tectonic fracturing. In contrast, the white siliceous rocks, despite superior petrophysical properties, are limited in scale as they predominantly infill late-stage fractures and vugs, mainly enhancing local flow conduits. Hydrothermal alteration in black siliceous rocks is more intense in dolostone host rocks than in limestone. Thus, thick (10–20 m), continuous black siliceous layers in dolostone and the surrounding medium-crystalline dolostone alteration zones, are promising exploration targets. This study elucidates the origins of Ordovician siliceous rocks and their implications for carbonate reservoir properties. The findings may offer valuable clues for deciphering the evolution and predicting the distribution of hydrothermal reservoirs, both within the basin and in other analogous regions worldwide. Full article

The utilization of geothermal resources as renewable energy is a subject of interest for the regions that possess these resources. The exploitation of geothermal energy must consider local geological conditions and an integrated approach, which should include practical studies on the chemistry of geothermal waters and their effect on thermal installations. Geothermal waters from Bihor County, Romania, have a variable composition, depending on the crossed geological layers, but also on pressure and temperature. Obviously, water transport and heat transfer are involved in all applications of geothermal waters. This article aims to characterize certain geothermal waters from the point of view of composition and corrosion if used as a thermal agent. Atomic absorption spectroscopy (AAS) and UV–Vis spectroscopy were employed to analyze water specimens. Chemical composition includes calcite (CaCO₃), chalcedony (SiO₂), goethite (FeO(OH)), and magnetite (Fe₃O₄), which confirms the corrosion and scale potential of these waters. Corrosion resistance of mild carbon steel, commonly used as pipe material, was studied by the gravimetric method and through electrochemical methodologies, including chronoamperometry, electrochemical impedance spectroscopy (EIS), potentiodynamic polarization method, and open circuit potential measurement (OCP). Statistical analysis shows that the medium corrosion rate of S235 steel, expressed as penetration rate, is between 0.136 mm/year to 0.615 mm/year. The OCP, EIS, and chronoamperometry experiments explain corrosion resistance through the formation of a passive layer on the surface of the metal. This study proposes an innovative methodology and a systematic algorithm for analyzing chemical processes and corrosion phenomena in geothermal installations, emphasizing the necessity of individualized assessments for each aquifer to optimize operational parameters and ensure sustainable resource utilization. Full article

Ensuring long-term wellbore integrity is critical for CO₂ injection and storage operations. Conventional cement degrades in CO₂-rich environments, compromising zonal isolation and increasing leakage risks. This study presents a novel self-healing cement formulation incorporating Barite, Pozzolan, and Chalcedony, optimized using a Design of Experiment (DOE) approach. Geochemical simulations were conducted using PHREEQC and Python to evaluate porosity evolution, mineral stability, and self-sealing efficiency under CO₂ exposure. The results demonstrate that the optimized formulations significantly reduce porosity (within 7–14 days) through the formation of calcium silicate hydrate (C-S-H) gels, enhancing crack sealing and mechanical resilience. Saturation index and phase volume analyses confirm the long-term stability of ECSH2 and Calcite, reinforcing the cement matrix. Compared to conventional cement, the self-healing formulations exhibit improved durability, lower permeability, and superior resistance to CO₂-induced degradation. These findings support the use of self-healing cement in carbon capture and storage (CCS), geothermal energy, and deep-well applications, offering a cost-effective and durable solution for long-term wellbore integrity. However, further experimental validation and field-scale evaluation are needed to confirm the practical performance of these formulations under real-world reservoir conditions. Full article

This study investigates the impact of widely used mineral fillers in self-compacting concrete compositions applied in vibration-free reinforced concrete production technology, as a means of enhancing rheological characteristics and cost-effectiveness. Three distinct types of mineral fillers, including the well-studied fillers microsilica and metakaolin, as well as the lesser-explored filler Kazakhstani natural opal-chalcedony opoka, are examined in this research. In addition to the evaluation of conventional rheological and performance properties of concretes containing these fillers, the internal processes within the cement–filler matrix are analyzed. This includes X-ray phase analysis and microstructural examination of cement hydration products in combination with a superplasticizer and each of the three minerals. The findings confirm the potential for optimizing the rheological parameters of the concrete mixture by substituting up to 15% of the cement with mineral fillers, achieving optimal viscosity and workability. It is established that compositions with the addition of microsilica and metakaolin have a more homogeneous structure, mainly represented by low-basicity calcium hydrosilicates of the CSH(B) type, along with an increase in compressive strength of up to 10%. The addition of these mineral fillers to C30/35 strength class self-compacting concrete resulted in improved frost resistance up to F300, a reduction in volumetric water absorption by up to 30%, and a decrease in shrinkage deformations by 32%. The developed SCC compositions have successfully passed production testing and are recommended for implementation in the operational processes of reinforced concrete product manufacturing plants. Full article

The discovery of an Aurignacian lithic assemblage along the northern coast of the Island of Lemnos in the northeastern Aegean Sea has opened new perspectives on the study of the beginning of the Upper Paleolithic in this region. The site is located some 93 m from the present seashore. It was discovered in the summer of 2020, ca. 2 km west of the Pournias Bay. The lithics were exposed in a well-defined oval concentration, ca. 25 × 10 m wide, buried by a Holocene sand dune. They were uncovered following sand removal by a bulldozer for the construction of a parking lot. The knapped stones are made almost exclusively from hydrothermal siliceous rocks, a raw material available on the island. Raman spectroscopy and optical observations confirmed that this raw material is chalcedony. The surfaces of most artefacts are weathered due to deposition in an environment rich in marine salt, which does not preserve any organic material suitable for radiocarbon dating. The knapped stone assemblage consists of diagnostic artefacts, among which are different types of carinated end scrapers, cores, and a few bladelets. The discovery of an Aurignacian site plays an important role in the study of the Paleolithic peopling of Lemnos and the Northeast Aegean in general, a period that was previously known only through Epipaleolithic sites discovered and excavated mainly along the eastern coast of the island. Full article

Optical microscopy and SEM imaging are powerful tools for evaluating the origins of quartz in fossil wood. Silicification is the most common mechanism for wood petrifaction, but this silica is commonly in cryptocrystalline or microcrystalline form (e.g., chalcedony or agate). Two essential requirements for the formation of macrocrystalline quartz is a concentration of dissolved silica low enough to allow the development of well-ordered lattices, and sufficient open space to allow euhedral or subhedral crystals to grow. Macrocrystalline quartz commonly occurs as a late-stage precipitate in open spaces that remained after initial mineralization had occurred. These spaces include vessels in angiosperm wood, and vugs and fractures in all types of wood. Exterior surfaces may also be suitable sites for quartz crystal growth. In some occurrences, crystalline quartz has directly encrusted or replaced wood cells. Diagenetic transformation of opal can produce cryptocrystalline or microcrystalline forms of quartz, but this process is not likely to produce macrocrystals. Full article

A bluish green chalcedony (a micro to crypto polycrystalline form of silica) from Africa has been marketed with the trademark AQUAPRASE^TM. A multimethodological approach, combining gemological analyses, thin section examination, scanning electron microscopy, X-ray powder diffraction, Raman spectroscopy, and trace elements chemical analyses by LA–ICP–MS, was carried out to characterize this material from a gemological and mineralogical point of view. The chalcedony samples consist of a mixture of quartz and moganite, as shown by the X-ray powder diffraction analysis and Raman spectroscopy. “Aquaprase” showed a strong microstructural zoning in terms of grain size, from macrocrystalline to micro and crypto, and morphology. Trace element variations correlated well with the different colored areas of the samples. In particular, the main chromophore ion present in the bluish green areas of the “aquaprase” chalcedony was chromium, followed by iron and nickel, so this chalcedony could be included in the group of chromium-bearing chalcedony. Rayleigh light scattering contributed to the blue hue of the gems. Full article

Nanhong agate, esteemed for its vivid color and natural shine, is experiencing a scarcity in supply despite its high demand. The primary deposits of agate, typically found near the surface, have not been extensively explored due to the predominance of traditional manual excavation methods. This research examined the Nanhong agate deposits in the Zhaojue–Meigu region of Liangshan, China, employing the integration of geological and geophysical surveys. Field geological surveys allowed us to outline the general areas where agate is found. Following this, using magnetic surveys, vertical electrical sounding, and controlled-source audio magnetotellurics, agate deposits were located within the conglomerate layer of the second member of the Feixianguan Formation from the Lower Triassic period at depths of less than 100 m. Our results identify mineralized layers, Xuanwei Formation mudstone, and the underlying bedrock, thus supporting the creation of a mineral prediction map. This research provides essential insights and guidance for agate exploration and the development of associated mineral resources. Full article

Agate veins and nodules occur in the Lece Volcanic Complex (Oligocene-Miocene) situated in the south of Serbia and occupying an area of 700 km². This volcanic complex is composed predominantly of andesites, with sporadic occurrences of andesite-basalts, dacites and latites, and features agate formations that have been very little investigated. This study focuses on five selected agate occurrences within the Lece Volcanic Complex, employing optical microscopy, scanning electron microscopy (SEM), X-ray powder diffraction analysis, inductively coupled plasma mass spectrometry (ICP-MS), and Fourier transform infrared spectroscopy (FTIR). In three localities (Rasovača, Mehane, and Ždraljevići), agate mineralization is directly related to distinct fault zones with strong local brecciation. In the other two localities (Vlasovo and Sokolov Vis), the agate is found in nodular form and does not show any connection with fracture zones. The silica phases of the Lece volcanic agates consist of cristobalite and tridymite, length-fast chalcedony, quartzine (length-slow chalcedony), and macrocrystalline quartz. Vein agates show a frequent alternation between length-fast chalcedony and quartz bands. Nodular agates consist primarily of length-fast chalcedony, occasionally containing notable quantities of opal-CT, absent in vein agates. Microtextures present in vein agates include crustiform, colloform, comb, mosaic, flamboyant, and pseudo-bladed. Jigsaw puzzle quartz microtexture supports the recrystallization of previously deposited silica in the form of opal or chalcedony from hydrothermal fluids. Growth lines in euhedral quartz (Bambauer quartz) point to agate formations in varying physicochemical conditions. These features indicate epithermal conditions during the formation of hydrothermal vein agates. Due to intense hydrothermal activity, vein agate host rocks are intensively silicified. Vein agates are also enriched with typical ore metallic elements (especially Pb, Co, As, Sb, and W), indicating genetic relation with the formation of polymetallic ore deposits of the Lece Volcanic Complex. In contrast, nodular agates have a higher content of major elements of host rocks (Al₂O₃, MgO, CaO, Na₂O, and K₂O), most probably mobilized from volcanic host rocks. Organic matter, present in both vein and nodular agate with filamentous forms found only in nodular agate, suggests formation in near-surface conditions. Full article

Mineral mapping from satellite images provides valuable insights into subsurface mineral alteration for geothermal exploration. In previous studies, eight fundamental algorithms were used for mineral mapping utilizing USGS spectra, a collection of reflectance spectra containing samples of minerals, rocks, and soils created by the USGS. We used an ASD FieldSpec 4 Hi-RES NG portable spectrometer to collect spectra for analyzing ASTER images of the Coso Geothermal Field. Then, we established the ground-truth information and the spectral library by analyzing 97 samples. Samples collected from the field were analyzed using the CSIRO TSG (The Spectral Geologist of the Commonwealth Scientific and Industrial Research Organization). Based on the mineralogy study, multiple high-purity spectra of geothermal alteration minerals were selected from collected data, including alunite, chalcedony, hematite, kaolinite, and opal. Eight mineral spectral target detection algorithms were applied to the preprocessed satellite data with a proposed local spectral library. We measured the highest overall accuracy of 87% for alunite, 95% for opal, 83% for chalcedony, 60% for hematite, and 96% for kaolinite out of these eight algorithms. Three, four, five, and eight algorithms were fused to extract mineral alteration with the obtained target detection results. The results prove that the fusion of algorithms gives better results than using individual ones. In conclusion, this paper discusses the significance of evaluating different mapping algorithms. It proposes a robust fusion approach to extract mineral maps as an indicator for geothermal exploration. Full article

Siliceous minerals of the Dengying Formation in the Gaoshiti–Moxi area in the central Sichuan Basin exhibit four types of quartz crystals (cryptocrystalline quartz, chalcedony, microcrystalline quartz, and megacrystalline quartz) and three structural types: cryptocrystalline, microcrystalline, and mosaic (laminated mosaic, window-hole interrupted mosaic, and arc-laminated mosaic). Siliceous minerals have a great influence on the storage performance of the reservoirs in the Dengying Formation. According to the petrophysical parameters of the Dengying Formation and porosity intersection diagrams, the siliceous dolomite and the reservoirs have low impedance characteristics, which makes it difficult to distinguish between them and leads to difficulties in the characterization and prediction of the reservoirs. The transverse wave velocity is favorable for reservoir characterization. Currently, the main method used to estimate the transverse wave velocity is petrophysical modeling, which establishes a relationship between the elastic and physical parameters of the reservoir. In this paper, the siliceous minerals in the dolomite in the study area are regarded as solid inclusions, and the calculation method of the rock matrix modulus is improved by using solid replacement. Then, an improved petrophysical model is constructed by combining the KT (Kuster–Toksöz) model, the DEM (Discrete Element Method) model, the Gassmann equation, and the Wood equation. The transverse wave velocity is estimated using the improved model under the constraint of the longitudinal wave velocity. The shapes of the transverse wave velocity curves obtained by the improved model and the deviations from the measured velocities are significantly better than those of the Xu–Payne model and other models. The results show that the improved model can effectively estimate the transverse wave velocity of the reservoir in this area, which provides a basis for future reservoir predictions in this area. Full article

The Jizhong Depression boasts rich geothermal resources with a lengthy history of geothermal exploitation. Buried hill geothermal reservoirs, which serve as primary thermal sources for hydrothermal resource exploitation, are prevalent in this region and have advantages such as extensive development potential, significant geothermal reservoir capacity, superior water quality, and straightforward recharge. This study investigates the formation and evolution of deep geothermal water in the Jizhong Depression by analyzing the hydrochemical and isotopic data of geothermal water samples collected from buried hill geothermal reservoirs in the northern part of the depression. The findings reveal that the subsurface hot water samples from the carbonate geothermal reservoirs in this region were predominantly weakly alkaline water with a pH ranging between 6.61 and 8.87. The hot water samples collected at the wellhead exhibited temperatures varying from 33.9 °C to 123.4 °C and total dissolved solids (TDS) lying between 473.9 mg/L and 3452 mg/L. Based on the δ²H-δ¹⁸O stable isotope analysis, the geothermal fluids in the Jizhong Depression are predominantly sourced from atmospheric precipitation and exist in a somewhat isolated hydrogeological environment, exhibiting pronounced water–rock interactions and deep water circulation (with depths ranging from 1324 m to 3455 m). Through a comparison of various methods, it is deduced that the most appropriate geothermometer for deep karst geothermal reservoirs in the Jizhong Depression is a chalcedony geothermometer, and when using it, the deep reservoir temperature was estimated at 63–137.6 °C. The precipitation in the adjacent mountainous areas enables the groundwater to infiltrate and descend deep into the earth along piedmont faults. Subsequently, lateral runoff over extended periods replenishes the groundwater into the depression. This process allows for the groundwater to fully absorb heat from deep heat sources, resulting in the formation of the deep geothermal reservoirs in the northern Jizhong Depression. The insights obtained from this study offer a theoretical and scientific foundation for the exploitation and utilization of regional geothermal resources and the transformation of the energy structure in China. Full article