Dioctahedral clays from an active continental geothermal system have been studied to assess their usefulness as proxies of paleo-hydrological and thermal conditions in the subsurface. Drill cuttings from Well WK244 in the Te Mihi area of the Wairakei Geothermal Field, New Zealand, were analyzed to determine the mineralogical, morphological, and isotopic characteristics of hydrothermal clays in these samples. Mixed-layer illite-dioctahedral smectite (I-S) and R0 chlorite-trioctahedral smectite are the main clay minerals, with I-S clays varying downward from R1 to R3 ordering and 50 to >90% illite over 160 m. The proportion of illite in I-S correlates positively with downhole temperature (r = 0.98) and I-S morphology changes from high aspect ratio ribbons, laths, and hairy fibers to pseudo-hexagonal plates with depth. Swelling clay percentages determined using the methylene blue method show a strong positive correlation with %S in I-S (r = 0.91), validating use of methylene blue as a rapid field tool for characterizing the smectite to illite transition in this active geothermal environment. The oxygen isotopic composition of I-S (dd18OI-S) decreases systematically with depth, and mostly reflects a progressive increase in subsurface temperature during clay formation. Estimates of water/rock ratios calculated using δ18OI-S values display stratigraphic variability that corresponds to variations in permeability. Oxygen isotopic measurements of I-S are a useful tool for understanding reservoir and permeability evolution in such geothermal systems and their related fossil analogs.

The zeolites and coexisting minerals of the silicic vitric tuffs in the Alaçatı (Çeşme) area, west of İzmir (Turkey), were studied. Mordenite is the most abundant zeolite in tuffs of the Alaçatı area and usually coexists with clinoptilolite-heulandite, smectite and calcite. Opal-CT was identified by means of its crystal morphology and EDX spectrum. Scanning electron microscopy (SEM) revealed the relative age relationships between the zeolites and coexisting minerals, namely mordenite, clinoptilolite-heulandite, smectite, calcite, and, in addition, opal-CT. Smectite consistently crystallized earlier than any of the zeolites, and it occasionally coats the outer walls of some of the vitric material. The zeolites are commonly located on the smectite, although some mordenites were observed to be in direct contact with glass shards that lacked a smectite coating. Clinoptilolite-heulandite formed after smectite and before mordenite. Opal-CT is seen to postdate both smectite and needle-shaped mordenite. Calcite was probably the latest mineral to crystallize in the Alaçatı tuffs. The zeolites in the tuffs of the Alaçatı area formed by dissolution of silicic vitric tuffs by Na- and Ca-rich thermal waters which passed through the fracture zone. The appearance of zeolites together with smectite along this zone may be attributed to a semi-open system which subdivided into smaller closed systems. Small changes in the pH and chemical composition of the thermal waters during alteration produced the corrosion effects observed by SEM. Small amounts of clinoptilolite-heulandite were corroded prior to crystallization of coexisting mordenite. The different compositions of the thermal waters were probably inherited from water that resulted from mixing of thermal and groundwaters.

Faulted and fractured systems form a critical component of fluid flow, especially within low-permeable reservoirs. Therefore, developing suitable methodologies for acquiring structural data and simulating flow through fractured media is vital to improve efficiency and reduce uncertainties in modelling the subsurface. Outcrop analogues provide excellent areas for the analysis and characterization of fractures within the reservoir rocks where subsurface data are limited. Variation in fracture arrangement, distribution and connectivity can be obtained from 2D fractured cliff sections and pavements. These sections can then be used for efficient discretization and homogenization techniques to obtain reliable predictions on permeability distributions in the geothermal reservoirs. Fracture network anisotropy in the Malm reservoir unit is assessed using detailed structural analysis and numerical homogenization of outcrop analogues from an open pit quarry within the Franconian Basin, Germany. Several events are recorded in the fracture networks from the Late Jurassic the Alpine Orogeny and are observed to be influenced by the Kulmbach Fault nearby with a reverse throw of 800 m. The fractured outcrops are digitized for fluid flow simulations and homogenization to determine the permeability tensors of the networks. The tensors show differences in fluid transport direction where fracture permeability is controlled by orientation compared to a constant value. As a result, it is observed that the orientation of the tensor is influenced by the Kulmbach Fault, and therefore faults within the reservoirs at depth should be considered as important controls on the fracture flow of the geothermal system.

The decline of domestic natural gas production, increasing dependency on gas imports and lagging development of renewable energy production may pose serious challenges to the current high standards of secure energy supply in the Netherlands. This paper examines synergy between hydrocarbon- and geothermal exploitation as a means to reinforce energy security. The Roden gas field is used as an example to demonstrate potential delay of water breakthrough in the gas well and a resulting increase of recovered gas (up to 19%), by positioning of a geothermal doublet in the water leg of the gas field. The reservoir simulations show that the total increase of gas production primarily depends on the amount of aquifer support. An optimal configuration of gas- and geothermal wells is key to maximise gas recovery and strongly depends on the distribution of reservoir properties. The study also reveals that this option can still be beneficial for gas fields in a late stage of production.

Net Present Value calculations show that the added value from the geothermal doublet on total gas production could lead to an early repayment of initial investments in the geothermal project, thereby reducing the overall financial risk. If no subsidies are taken into account, the additional profits can also be used to finance the geothermal project up to break-even level within 15 years. However, this comes with a cost as the additional profits from improved gas recovery are significantly reduced.

Deposition of two anhydrous sulphate-carbonate minerals, burkeite and hanksite, around a vapour escape in the pipe of a geothermal well of the Copahue field in Argentina is described. These minerals usually occur as efflorescences in saline soils or in continental lacustrine evaporite deposits, but have not been reported as products from geothermal fluids. Both minerals have been studied by means of X-ray diffraction, scanning electron microscopy and electron dispersive energy, and thermo-gravimetric analysis, and have been determined as burkeite, Na6(CO3)(SO4)2, and hanksite, KNa22(SO4)9(CO3)2Cl.

Alunogen and meta-alunogen are the dominant phases present in transient sulphate efflorescences that are the latest products of the alteration of ignimbrite country rocks in the long-lived Te Kopia geothermal field. Meta-alunogen pseudomorphs alunogen and both species occur as white, fibrous, tangled masses, as prismatic, parallel growths, and as thin, platy, crystals, 8–15 μm across, that coalesce in an open cellular network. Small (<2 mm diam.) spherical aggregates of radiating, acicular halotrichite (Fe0.51Mg0.49Al2(SO4)4.22H2O), potash alum, mirabilite, melanterite and tschermigite are present locally. The cations needed to form these minerals derive from the host rocks with the exception of sulphur and ammonia that come from H2S and NH3 gases ascending with steam. The particular efflorescence assemblage reflects the prevailing conditions and ionic activities of a local micro-environment. Kaolinite formed by acid sulphate alteration is now being altered by steam to yield alunogen. In turn, alunogen can react with silica, or co-dissociate with silicic acid, to form kaolinite. The alternating dissolution and reprecipitation of kaolinite and alunogen moves aluminium in and through the surficial environment at Te Kopia.

Arsenic-rich scales are widely associated with geothermal fields and constitute a potential hazard to human health. Such arsenic has hitherto been reported to be almost exclusively hosted by sulphide or oxide phases or occurring as surface species. We report here, however, the occurrence of an arsenic-rich (1500 to 4000 mg kg—1 As) smectite from geothermal precipitates from a geothermal field in northwestern Japan and present evidence that the arsenic is predominantly hosted within this silicate mineral.

Consistently ∼80% of the total arsenic determined in these geothermal precipitates was found by selective chemical extractions to be associated with an operationally defined clay mineral fraction, with lesser proportions being associated with operationally defined amorphous silica, Fe oxide and sulphide fractions. Analysis by XRD, ATR IR and XRF showed the clay fraction to be dominated by Mg-rich trioctahedral smectite.

Arsenic K-edge XAS spectra of the smectite suggested the dominance of As(III)-O coordinated species with significant contributions from As(V)-O coordinated species. Both XPS and a magnesium chloride chemical extraction indicated minimal adsorption of arsenic on smectite surfaces suggesting that the arsenic was predominantly either dissolved within the smectite or occurred within mineral occlusions. No such occlusions greater than 1 μm in size were observed in the As-rich smectites.

The potential occurrence of arsenic-bearing clays should be considered when determining remediation strategies for geothermal environments or evaluating risks associated with the industrial usage of geothermal precipitates.

Following a time of expansion of the oil industry, the period 1987-2012 and especially the second half of it reflects a diversification of exploration activities in the Netherlands. In addition to the ongoing discovery of classical hydrocarbon reservoirs, successful attempts have been made to explore deep geothermal energy. Shale gas and Coal Bed Methane received interest in the Netherlands too but so far no wells have been drilled in order to test the potential. Storage of gas and liquids has been introduced and salt mining more or less continued without many changes. In terms of drilling activity, the last 25 years show a decreasing trend and the discovered volumes are smaller. Testing of new play concepts was either successful (shallow gas) or not (Dinantian) and sometimes an unexpected discovery was made (Triassic Fat Sands). New techniques proved older fields to be exploitable again, e.g. redevelopment of the Schoonebeek Field. Therefore, despite a decline in number of wells drilled and reserves added, the diversification of use of the subsurface will guarantee ongoing exploration.

Mathematical modelling of Wairakei geothermal field is reviewed, both lumped-parameter and distributed-parameter models. In both cases it is found that reliable predictions require five to ten years of history for calibration. With such calibration distributed-parameter models are now used for field management. A prudent model of Wairakei, constructed without such historical data, would underestimate field capacity and provide only general projections of the type of changes in surface activity and subsidence.

Deception Island in the South Shetland Islands is one of the most volcanically active sites south of 60°S. Between 1967 and 1970 three major eruptions devastated large expanses of the landscape and its vegetation. Since 1970 extensive recolonization has occurred on the more stable surfaces. Unheated ground supports several bryophyte and lichen communities typical of much of the Maritime Antarctic, but geothermal habitats possess remarkable associations of bryophytes, many of which are unknown or very rare elsewhere in the Antarctic. Nine geothermal sites and their vegetation are described. Communities associated with more transient sites have disappeared when the geothermal activity ceased. Mosses and liverworts occur to within a few centimetres of the vents where temperatures reach 90–95°C, while temperatures within adjacent moss turf can reach 35–50°C or more and remain consistently between 25 and 45°C. Most of the bryoflora has a Patagonian–Fuegian provenance. It is presumed that, unlike most species, the thermophiles are not pre-adapted to the Antarctic environment, being able to colonize only where the warm and humid conditions prevail. The floristic and ecological importance of these thermophilic communities, and their sensitivity to perturbation by the rapidly increasing annual summer influx of tourists, as well as scientists, has resulted in these unique sites being proposed as components of a new Antarctic Specially Protected Area under the Antarctic Treaty.