Fluid mixing induced by hydrothermal activity in the ordovician carbonates in Tarim Basin,China

Permian hydrothermal activity in the Tarim Basin may have been responsible for the invasion of hot brines into Ordovician carbonate reservoirs. Studies have been undertaken to explain the origin and geochemical characteristics of the diagenetic fluid present during this hydrothermal event although there is no consensus on it. We present a genetic model resulting from the study of δ¹³C, δ¹⁸O, δ³⁴S, and ⁸⁷Sr/⁸⁶Sr isotope values and fluid inclusions (FIs) from fracture‐ and vug‐filling calcite, saddle dolomite, fluorite, barite, quartz, and anhydrite from Ordovician outcrops in northwest (NW) Tarim Basin and subsurface cores in Central Tarim Basin. The presence of hydrothermal fluid was confirmed by minerals with fluid inclusion homogenization temperatures being >10°C higher than the paleo‐formation burial temperatures both in the NW Tarim and in the Central Tarim areas. The mixing of hot (>200°C), high‐salinity (>24 wt% NaCl), ⁸⁷Sr‐rich (up to 0.7104) hydrothermal fluid with cool (60–100°C), low‐salinity (0 to 3.5 wt% NaCl), also ⁸⁷Sr‐rich (up to 0.7010) meteoric water in the Ordovician unit was supported by the salinity of fluid inclusions, and δ¹³C, δ¹⁸O, and ⁸⁷Sr/⁸⁶Sr isotopic values of the diagenetic minerals. Up‐migrated hydrothermal fluids from the deeper Cambrian strata may have contributed to the hot brine with high sulfate concentrations which promoted thermochemical sulfate reduction (TSR) in the Ordovician, resulting in the formation of ¹²C‐rich (δ¹³C as low as ?13.8‰) calcite and ³⁴S‐rich (δ³⁴S values from 21.4‰ to 29.7‰) H₂S, pyrite, and elemental sulfur. Hydrothermal fluid mixing with fresh water in Ordovician strata in Tarim Basin was facilitated by deep‐seated faults and up‐reaching faults due to the pervasive Permian magmatic activity. Collectively, fluid mixing, hydrothermal dolomitization, TSR, and faulting may have locally dissolved the host carbonates and increased the reservoir porosity and permeability, which has significant implications for hydrocarbon exploration.     

Polyphasic hydrothermal and meteoric fluid regimes during the growth of a segmented fault involving crystalline and carbonate rocks (Barcelona Plain,NE Spain)

A polyphasic tectonic‐fluid system of a fault that involves crystalline and carbonate rocks (Hospital fault, Barcelona Plain) has been inferred from regional to thin section scale observations combined with geochemical analyses. Cathodoluminescence, microprobe analyses and stable isotopy in fracture‐related cements record the circulation of successive alternations of hydrothermal and low‐temperature meteoric fluids linked with three main regional tectonic events. The first event corresponds to the Mesozoic extension, which had two rifting stages, and it is characterized by the independent tectonic activity of two fault segments, namely southern and northern Hospital fault segments. During the Late Permian‐Middle Jurassic rifting, these segments controlled the thickness and distribution of the Triassic sediments. Also, dolomitization was produced in an early stage by Triassic seawater at shallow conditions. During increasing burial, formation of fractures and their dolomite‐related cements took place. Fault activity during the Middle Jurassic–Late Cretaceous rifting was localized in the southern segment, and it was characterized by hydrothermal brines, with temperatures over 180°C, which ascended through this fault segment precipitating quartz, chlorite, and calcite. The second event corresponds to the Paleogene compression (Chattian), which produced exhumation, folding and erosion, favouring the percolation of low‐temperature meteoric fluids which produced the calcitization of the dolostones and of the dolomite cements. The third event is linked with the Neogene extension, where three stages have been identified. During the syn‐rift stage, the southern segment of the Hospital fault grew by tip propagation. In the relay zone, hydrothermal brines with temperature around 140°C upflowed. During the late postrift, the Hospital fault acted as a unique segment and deformation occurred at shallow conditions and under a low‐temperature meteoric regime. Finally, and possibly during the Messinian compression, NW‐SE strike‐slip faults offset the Hospital fault to its current configuration.     

Thermal convection in faulted extensional sedimentary basins: theoretical results from finite-element modeling

Thermal convection has the potential to be a significant and widespread mechanism of fluid flow, mass transport, and heat transport in rift and other extensional basins. Based on numerical simulation results, large‐scale convection can occur on the scale of the basin thickness, depending on the Rayleigh number for the basin. Our analysis indicates that for syn‐rift and early post‐rift settings with a basin thickness of 5 km, thermal convection can occur for basal heat flows ranging from 80 to 150 mW m^?2, when the vertical hydraulic conductivity is on the order of 1.5 m year^?1 and lower. The convection cells have characteristic wavelengths and flow patterns depending on the thermal and hydraulic boundary conditions. Steeply dipping extensional faults can provide pathways for vertical fluid flow across large thicknesses of basin sediments and can modify the dynamics of thermal convection. The presence of faults perturbs the thermal convective flow pattern and can constrain the size and locations of convection cells. Depending on the spacing of the faults and the hydraulic properties of the faults and basin sediments, the convection cells can be spatially organized to align with adjacent faults. A fault‐bounded cell occurs when one convection cell is constrained to occupy a fault block so that the up‐flow zone converges into one fault zone and the down‐flow zone is centred on the adjacent fault. A fault‐bounded cell pair occurs when two convection cells occupy a fault block with the up‐flow zone located between the faults and the down‐flow zones centred on the adjacent faults or with the reverse pattern of flow. Fault‐bounded cells and cell pairs can be referred to collectively as fault‐bounded convective flow. The flow paths in fault‐bounded convective flow can be lengthened significantly with respect to those of convection cells unperturbed by the presence of faults. The cell pattern and sense of circulation depend on the fault spacing, sediment and fault permeabilities, lithologic heterogeneity, and the basal heat flow. The presence of fault zones also extends the range of conditions for which thermal convection can occur to basin settings with Rayleigh numbers below the critical value for large‐scale convection to occur in a basin without faults. The widespread potential for the occurrence of thermal convection suggests that it may play a role in controlling geological processes in rift basins including the acquisition and deposition of metals by basin fluids, the distribution of diagenetic processes, the temperature field and heat flow, petroleum generation and migration, and the geochemical evolution of basin fluids. Fault‐bounded cells and cell pairs can focus mass and heat transport from longer flow paths into fault zones, and their discharge zones are a particularly favourable setting for the formation of sediment‐hosted ore deposits near the sea floor.     

Heat transport at boiling,near‐critical conditions

Two‐phase flow and near‐critical phenomena are likely to enhance energy transport in high‐temperature hydrothermal systems. We present a series of two‐dimensional simulations of two‐phase flow of pure water at near‐critical conditions. The results show that at near‐critical conditions, two‐phase convection can be more efficient in transporting energy than single‐phase convection. The highest heat fluxes are attained when two‐phase heat‐pipes form near the bottom boundary, recharging the root of the upflow zone and thereby enabling the formation of broad upflow regions. When the system becomes more vapor‐dominated, it loses this ability, upflow zones become narrower and the energy efficiency drops to more moderate values.     

Long‐term chemical evolution and modification of continental basement brines – a field study from the Schwarzwald,SW Germany

Highly saline, deep‐seated basement brines are of major importance for ore‐forming processes, but their genesis is controversial. Based on studies of fluid inclusions from hydrothermal veins of various ages, we reconstruct the temporal evolution of continental basement fluids from the Variscan Schwarzwald (Germany). During the Carboniferous (vein type i), quartz–tourmaline veins precipitated from low‐salinity (<4.5wt% NaCl + CaCl₂), high‐temperature (≤390°C) H₂O‐NaCl‐(CO₂‐CH₄) fluids with Cl/Br mass ratios = 50–146. In the Permian (vein type ii), cooling of H₂O‐NaCl‐(KCl‐CaCl₂) metamorphic fluids (T ≤ 310°C, 2–4.5wt% NaCl + CaCl₂, Cl/Br mass ratios = 90) leads to the precipitation of quartz‐Sb‐Au veins. Around the Triassic–Jurassic boundary (vein type iii), quartz–haematite veins formed from two distinct fluids: a low‐salinity fluid (similar to (ii)) and a high‐salinity fluid (T = 100–320°C, >20wt% NaCl + CaCl₂, Cl/Br mass ratios = 60–110). Both fluids types were present during vein formation but did not mix with each other (because of hydrogeological reasons). Jurassic–Cretaceous veins (vein type iv) record fluid mixing between an older bittern brine (Cl/Br mass ratios ~80) and a younger halite dissolution brine (Cl/Br mass ratios >1000) of similar salinity, resulting in a mixed H₂O‐NaCl‐CaCl₂ brine (50–140°C, 23–26wt% NaCl + CaCl₂, Cl/Br mass ratios = 80–520). During post‐Cretaceous times (vein type v), the opening of the Upper Rhine Graben and the concomitant juxtaposition of various aquifers, which enabled mixing of high‐ and low‐salinity fluids and resulted in vein formation (multicomponent fluid H₂O‐NaCl‐CaCl₂‐(SO₄‐HCO₃), 70–190°C, 5–25wt% NaCl‐CaCl₂ and Cl/Br mass ratios = 2–140). The first occurrence of highly saline brines is recorded in veins that formed shortly after deposition of halite in the Muschelkalk Ocean above the basement, suggesting an external source of the brine's salinity. Hence, today's brines in the European basement probably developed from inherited evaporitic bittern brines. These were afterwards extensively modified by fluid–rock interaction on their migration paths through the crystalline basement and later by mixing with younger meteoric fluids and halite dissolution brines.     

Hydrothermal activity associated with the Ries impact event, Germany

Combined field studies, optical and scanning electron microscopy, and electron microprobe studies of impactites from the Ries impact structure, Germany, have allowed a clearer picture of the hydrothermal system associated with the Ries impact event to be made. Hydrothermal alteration is concentrated within impact‐generated suevites in the interior of the crater (crater suevites) and around the periphery (surficial suevites), with minor alteration in the overlying sedimentary crater‐fill deposits. The major heat source for the Ries hydrothermal system was the suevite units themselves. Hydrothermal alteration of crater‐fill suevites is pervasive in nature and comprises several distinct alteration phases that vary with depth. An early phase of K‐metasomatism accompanied by minor albitization of crystalline basement clasts and minor chloritization, was followed by pervasive intermediate argillic alteration (predominantly montmorillonite, saponite, and illite) and zeolitization (predominantly analcite, erionite, and clinoptilolite). Hydrothermal fluids were typically weakly alkaline during the main stage of alteration. In contrast to the crater‐fill suevites, alteration within surficial suevites was typically restricted to montmorillonite and phillipsite deposition within cavities and fractures. The pervasive nature of the alteration within the crater‐fill suevites was likely due to the presence of an overlying crater lake; whereas alteration within surficial suevites typically occurred under undersaturated conditions with the main source of water being from precipitation. There are exceptional outcrops of more pervasively altered surficial suevites, which can be explained as locations where water pooled for longer periods of time. Hydrothermal fluids were likely a combination of meteoric waters that percolated down from the overlying crater lake and groundwaters that flowed in from the surrounding country rocks.     

The potential for abiotic organic synthesis and biosynthesis at seafloor hydrothermal systems

Calculations are presented of the extent to which chemical disequilibria are generated when submarine hydrothermal fluids mix with sea water. These calculations involve quantifying the chemical affinity for individual reactions by comparing equilibrium compositions with the compositions of mixtures in which oxidation–reduction reactions are inhibited. The oxidation–reduction reactions that depart from equilibrium in these systems provide energy for chemotrophic microbial metabolism. Methanogenesis is an example of this phenomenon, in which the combination of carbon dioxide, hydrogen and methane induced by fluid mixing is far from equilibrium, which can be approached if more methane is generated. Similarly, the production of other organic compounds is also favorable under the same conditions that permit methanogenesis. Alkanes, alkenes, alcohols, aldehydes, carboxylic acids and amino acids are among the compounds that, if formed, would lower the energetic state of the chemical composition generated in mixed fluids. The resulting positive values of chemical affinity correspond to the thermodynamic drive required for abiotic organic synthesis. It is also possible that energy release accompanies biosynthesis by chemotrophic organisms. In this way, hydrothermal ecosystems differ radically from familiar ecosystems at Earth's surface. If captured, the energy released may be sufficient to drive biosynthesis of carbohydrates, purines, pyrimidines and other compounds which require energy inputs.     

3D numerical modeling of hydrothermal processes during the lifetime of a deep geothermal reservoir

Understanding hydrothermal processes during production is critical to optimal geothermal reservoir management and sustainable utilization. This study addresses the hydrothermal (HT) processes in a geothermal research doublet consisting of the injection well E GrSk3/90 and production well Gt GrSk4/05 at the deep geothermal reservoir of Groß Schönebeck (north of Berlin, Germany) during geothermal power production. The reservoir is located between ?4050 to ?4250 m depth in the Lower Permian of the Northeast German Basin. Operational activities such as hydraulic stimulation, production (T = 150°C; Q = ?75 m³ h^?1; C = 265 g l^?1) and injection (T = 70°C; Q = 75 m³ h^?1; C = 265 g l^?1) change the HT conditions of the geothermal reservoir. The most significant changes affect temperature, mass concentration and pore pressure. These changes influence fluid density and viscosity as well as rock properties such as porosity, permeability, thermal conductivity and heat capacity. In addition, the geometry and hydraulic properties of hydraulically induced fractures vary during the lifetime of the reservoir. A three‐dimensional reservoir model was developed based on a structural geological model to simulate and understand the complex interaction of such processes. This model includes a full HT coupling of various petrophysical parameters. Specifically, temperature‐dependent thermal conductivity and heat capacity as well as the pressure‐, temperature‐ and mass concentration‐dependent fluid density and viscosity are considered. These parameters were determined by laboratory and field experiments. The effective pressure dependence of matrix permeability is less than 2.3% at our reservoir conditions and therefore can be neglected. The results of a three‐dimensional thermohaline finite‐element simulation of the life cycle performance of this geothermal well doublet indicate the beginning of thermal breakthrough after 3.6 years of utilization. This result is crucial for optimizing reservoir management. Geofluids (2010) 10 , 406–421     

Chemical modification of pyroclastic rock by hot water: an experimental investigation of mass transport at the fluid–solid interface

Hydrothermal water–(pyroclastic) rock interactions were examined using flow-through experiments to deduce the effect of mass transport phenomena on the reaction process. A series of experiments were conducted over the temperature range 75–250°C, with a constant temperature for each experiment, and at saturated vapour pressure, to estimate the apparent rate constants as a function of temperature.
Based on the chemistry of analysed solutions, the water–rock interaction in the experiments was controlled by diffusion from the reaction surface and by the existence of a surface layer at the rock–fluid interface, which regulated the chemical reaction rate. The reaction progress depended to a high degree on flow velocity and temperature conditions, with element abundances in the fluid significantly affected by these factors. Mass transport coefficients for diffusion from the rock surface to the bulk solution have been estimated. Ca is selectively depleted under lower temperature conditions ( T  < 150°C), whereas Na is greatly depleted under higher temperature conditions ( T  > 150°C), and K reaction rates are increased when flow velocity increases. Using these conditions, specific alkali and alkali earth cations were selectively leached from mineral surfaces. The 'surface layer' comprised a 0.5–1.8 mm boundary film on the solution side (the thickness of this layer has no dependence on chemical character) and a reaction layer. The reaction layer was composed of a Si, Al-rich cation-leached layer, whose thickness was dependent on temperature, flow velocity and reaction length. The reaction layer varied in thickness from about 10⁻⁴ to 10⁻⁷ mm under high temperature/low fluid velocity and low temperature/high fluid velocity conditions, respectively.     

Modeling fluid flow in a heterogeneous, fault-controlled hydrothermal system

Previous studies have shown that most hydrothermal systems discharging at the land surface are associated with faulting, and that the location, temperature and rate of discharge of these systems are controlled by the geometry and style of the controlling fault(s). Unfortunately, the transport of heat and fluid in fault-controlled hydrothermal systems is difficult to model realistically; although heterogeneity and anisotropy are assumed to place important controls on flow in faults, few data or observations are available to constrain the distribution of hydraulic properties within active faults. Here, analytical and numerical models are combined with geostatistical models of spatially varying hydraulic properties to model the flow of heat and fluid in the Borax Lake fault of south-east Oregon, USA. A geometric mean permeability within the fault of 7 × 10⁻¹⁴ m² with 2× vertical/horizontal anisotropy in correlation length scale is shown to give the closest match to field observations. Furthermore, the simulations demonstrate that continuity of flow paths is an important factor in reproducing the observed behavior. In addition to providing some insight into possible spatial distributions of hydraulic properties at the Borax Lake site, the study highlights one potential avenue for integrating field observations with simulation results in order to gain greater understanding of fluid flow in faults and fault-controlled hydrothermal and petroleum reservoirs.