Effect of salinity on mass and energy transport by hydrothermal fluids based on the physical and thermodynamic properties of H2O‐NaCl

Various thermodynamic properties of H₂O that are defined as pressure or temperature derivatives of some other variable, such as isothermal compressibility (β, pressure derivative of density), isobaric thermal expansion (α, temperature derivative of density), and specific isobaric heat capacity (c_f, temperature derivative of enthalpy), all show large magnitudes near the critical point, reflecting large variations in fluid density and specific enthalpy with small changes in temperature and pressure. As a result, mass (related to fluid density) and energy (related to fluid enthalpy) transport in this PT region are sensitive to changing PT conditions. Addition of NaCl to H₂O causes the region of anomalous behavior, here defined as the critical region, to migrate to higher temperatures and pressures. The critical region is defined as that region of PT space in which the dimensionless reduced susceptibility  ≥ 0.5. When NaCl is added to H₂O, the critical region migrates to higher temperature and pressure. However, the absolute magnitudes of thermodynamic properties that are defined as temperature and/or pressure derivatives (α, β, and c_f) all decrease with increasing salinity. Thus, the mass and energy transporting capacities of hydrothermal fluids in the critical region become less sensitive to changing PT conditions as the salinity increases. For example, quartz solubility can be described as a function of fluid density, and because density becomes less sensitive to changing PT conditions as salinity increases, quartz solubility also becomes less sensitive to changing PT conditions as fluid salinity increases. Similarly, fluxibility describes the ability of a fluid to transport heat by buoyancy‐driven convection, and fluxibility decreases with increasing salinity. Results of this study show that the mass and energy transport capacity of fluids in the Earth's crust are maximized in the critical region and that the sensitivity to changing PT conditions decreases with increasing salinity.     

Landscapes at the Periphery of Glacierization – Retrospect and Prospect

This paper reviews the evolution of palaeoglaciological reconstructions in regions at the periphery of glacierization, where an improved understanding of the role of glacial thermal regime has helped refine the delimitation of ice sheet maxima. Also significant has been the recognition in numerical models that some areas, especially ice sheet marginal zones, are subject to short periods of ice occupancy and hence that glacial landsystem signatures can be extremely subtle. This is compounded wherever cold-based conditions dominate during early stages of ice sheet recession, giving rise to a landform imprint typical of peripheral regions and hence often misinterpreted as unglaciated or glaciated only by older glaciations. Subtle landform imprints include meltwater channels, thin glacigenic veneers or scattered erratics and modified tors. More substantial glacigenic landform-sediment assemblages (‘drift belts’) do not always represent ice sheet maxima but instead may record significant changes in thermal regime, possibly linked to periods of ice-marginal stabilization. Some upland areas that lie beyond the traditionally demarcated limits of glaciation (e.g. Dartmoor in SW England) may contain subtle evidence of satellite ice cap development which has been overlooked due to the strong periglacial landform signature.     

Massive dolomitization of a Messinian reef in the Great Bahama Bank: a numerical modelling evaluation of Kohout geothermal convection

The hypothesis that Kohout thermal convection may have induced the massive dolomitization of the 60 m thick lowest more reefal unit in well Unda [top of Great Bahama Bank (GBB)] is evaluated through numerical modelling. A two‐dimensional (2‐D) section, including lithological and petrophysical data, together with datings for the sediments of the GBB, was used in the basin model TEMISPACK to reconstruct the history of the whole platform, with a focus on the reef unit. Simulations showed that during high sea‐level periods, Kohout convection is a valid mechanism in the settings of the GBB, although the convection cell remains flat in most cases because of high permeability anisotropy. This mechanism induces rapid fluid flow in the superficial as well as in the deeper parts of the platform, with velocities of at least two orders of magnitude higher than with compaction alone. Lithology appears as a strong control of fluid circulations at the margin scale through the permeability anisotropy, for which a critical value lies between values of 10 and 100. The reefal unit in Unda is part of a larger area determined by the lithologic distribution, in which flow velocities are significantly higher than in the rest of the platform. These velocities are high enough to bring the magnesium necessary to precipitate the observed amounts of dolomite, within durations in agreement with the available time of post‐reef deposition high sea level(s). However, neither fluid flow pattern nor flow velocities are able to explain the preferential massive dolomitization of the lower reef unit and the complete absence of dolomite in the upper one.     

Technology of Medieval Mortars: An Investigation into the Use of Organic Additives

This work proposes a multi‐analytical approach to determine the additives in historical mortars, the use of which is widely described in bibliographical sources, but has rarely been reported in the literature. A protocol to thoroughly analyse mortars was created (optical microscopy, X‐ray diffraction, infrared spectroscopy, thermal analyses and gas chromatography – mass spectrometry). These techniques, which had already been carried out on samples from various sites from the Roman to the modern era, determined that additives had only been used in the mortars from the internal masonry at our sampling site: the medieval military shipyard of Amalfi (Italy). The investigations yielded information on the production technology, and FT–IR and GC–MS revealed a saccharide material‐based additive in the mortars, of plant origin. The FT–IR spectra suggested the presence of a natural gum, which has been used since ancient times to strengthen the cohesion properties of mortars and their resistance to tensile stress.     

The salt chimney effect: delay of thermal evolution of deep hydrocarbon source rocks due to high thermal conductivity of evaporites

Half of the topseals to the world's largest oilfields are evaporites. Rock salt has a thermal conductivity two to four times greater than that of other sedimentary rocks found in oil‐ and gas‐bearing basins. Strong heat conduction through evaporites can increase the geothermal gradient above evaporite deposits, resulting in a positive thermal anomaly and above‐average temperature while simultaneously decreasing the geothermal gradient below evaporites, resulting in a negative thermal anomaly. Most Triassic–Jurassic hydrocarbon source rocks in the Kuqa Basin, western China, are overlain by ~1500‐m‐thick Tertiary evaporites with underlying Cretaceous sandstones and mudstones. Directly measured strata temperatures indicate an obvious break in the steepness of the geothermal gradient above and below Paleogene evaporites, with a significantly steeper geothermal gradient above the evaporites. Simulations of the thermal evolution of source rocks based on data collected from well Kela‐2 indicate that if the thickness of evaporites (mainly rock salt and anhydrite rock) in overlying rocks above source rocks increases compared with the thickness of siliciclastic rocks in the overlying rocks, then strata temperatures and vitrinite reflectance in Jurassic source rocks will decrease accordingly. Our thermal simulations based on the thickness and thermal conductivity of evaporites accurately coincide with previous studies based on homogenization temperatures, hydrocarbon–water contact retrospection, and carbon isotope results from natural gases. The gas generation center located in the Kalasu Tectonic Belt today is also sealed in an evaporite‐related structural trap that formed at this time. Therefore, the speculated natural gas generation times not only correlate with the evaporite‐related structural trap formation, but the calculated maturity of deep source rocks below the evaporites also coincides with current gas reserves. And our studies can help to find the deep oils and gases under thick evaporites.     

Linked thermal convection of the basement and basinal fluids in formation of the unconformity‐related uranium deposits in the Athabasca Basin,Saskatchewan, Canada

World‐class unconformity‐related U deposits in the Athabasca Basin (Saskatchewan, Canada) are generally located within or near fault zones that intersect the unconformity between the Athabasca Group sedimentary basin rocks and underlying metamorphic basement rocks. Two distinct subtypes of unconformity‐related uranium deposits have been identified: those hosted primarily in the Athabasca Group sandstones (sediment‐hosted) and those hosted primarily in the underlying basement rocks (basement‐hosted). Although significant research on these deposits has been carried out, certain aspects of their formation are still under discussion, one of the main issues being the fluid flow mechanisms responsible for uranium mineralization. The intriguing feature of this problem is that sediment‐hosted and basement‐hosted deposits are characterized by oppositely directed vectors of fluid flow via associated fault zones. Sediment‐hosted deposits formed via upward flow of basement fluids, basement‐hosted deposits via downward flow of basinal fluids. We have hypothesized that such flow patterns are indicative of the fluid flow self‐organization in fault‐bounded thermal convection (Transport in Porous Media, 110, 2015, 25). To explore this hypothesis, we constructed a simplified hydrogeologic model with fault‐bounded thermal convection of fluids in the faulted basement linked with fluid circulation in the overlying fault‐free sandstone horizon. Based on this model, a series of numerical experiments was carried out to simulate the hypothesized fluid flow patterns. The results obtained are in reasonable agreement with the concept of fault‐bounded convection cells as an explanation of focused upflow and downflow across the basement/sandstone unconformity. We then discuss application of the model to another debated problem, the uranium source for the ore‐forming basinal brines.     

Dissolved gases in brackish thermal waters: an improved analytical method

An improved method based on equilibrium partitioning between water samples and an inert host gas, introduced after sampling, is proposed for determining multiple species of dissolved gases in brackish water. The method itself, and the most convenient equations for describing gas solubilities in brackish waters, is described in detail. The method allows the rapid characterization of several sites and represents a useful tool for geochemical surveys. A comparison between replicate samples analyzed using different procedures demonstrates the efficiency of the method and indicates that the abundances of the main dissolved gases can be obtained, which can then be used to determine underlying geochemical processes. A Microsoft Excel worksheet is provided to easily calculate the concentration of dissolved gas species.     

Boundary making in anti-corruption policy: behaviour,responses and institutions

Boundaries are an increasing focus for academics and practitioners seeking to understand public policy dynamics. In anti-corruption policy how these boundaries are established and by whom has far-reaching implications for public trust. Drawing on interview data, this paper explores the enduring tensions and administrative conflicts at play in three key aspects of the anti-corruption boundary making process: behaviour, responses and institutions. The evidence presented demonstrates that these boundaries are unstable and unresolved. While some tensions, such as those between morality and the law, are enduring, many of the administrative conflicts indicate a sensible and purposeful balance between the important considerations of accountability and responsiveness on the one hand, and efficiency and effectiveness on the other, is yet to be achieved.     

Analysis of the role of fluids in causing fractures in the Spraberry Trend,Midland Basin

The Spraberry Formation in west‐central Texas is a highly fractured formation with both extension and shear fractures. At least two sets of natural fractures exist in two reservoir intervals. We have considered two possible origins for the fracturing: (i) high fluid pressure plus tectonic stress and (ii) tectonic stress at near‐hydrostatic fluid pressure. Reconstruction of geologic, thermal and hydrodynamic histories suggests that high fluid pressures probably did not occur during the basin’s history. To explore the second hypothesis, we developed and applied a calibrated, discrete‐element model of Spraberry strata to investigate whether weak Laramide compressional forces could cause fractures in the absence of high fluid pressures. Simulation results suggest that a mild compressional episode of geologically short duration may indeed have induced conjugate shear fractures.     

Modeling thermal convection in supradetachment basins: example from western Norway

The juxtaposition of fault‐bounded sedimentary basins, above crustal‐scale detachments, with warmer exhumed footwalls can lead to thermal convection of the fluids in the sediments. The Devonian basins of western Norway are examples of supradetachment basins that formed in the hanging wall of the Nordfjord‐Sogn Detachment Zone. In the central part of the Hornelen and Kvamshesten basins, the basin‐fill is chiefly represented by fluvial sandstones and minor lacustrine siltstones, whereas the fault margins are dominated by fanglomerates along the detachment contact. Prominent alteration and low‐greenschist facies metamorphic conditions are associated with the peak temperature estimates of the sediments close to the detachment shear zone. Fluid circulation may have been active during the burial of the sediments, and we quantify the potential role played by thermal convection in redistributing heat within the basins. Different models are tested with homogeneous and layered basin‐fill and with material transport properties corresponding to sandstones and siltstones. We found that thermally driven fluid flow is expected in supradetachment basins as a transient process during the exhumation of warmer footwalls. We demonstrate that the fluid flow may have significantly affected the temperature distribution in the upper five kilometers of the Devonian basins of western Norway. The temperature anomaly induced by the flow may locally reach about 80°C. The sedimentary layering formed by sand‐ and siltstones strata does not inhibit fluid circulation at the scale of the basin. The presence of fluid pathways along the detachment has an important impact on the flow and allows an efficient drainage of the basin by channelizing fluids upward along the detachment.