Anhydrite Precipitation Research Articles

Links between anhydrite precipitation and dolomitization during cycles of sea level change: Insights from the Late Jurassic Arab Formation, Abu Dhabi, United Arab Emirates

Despite extensive research in the field of evaporative dolomitization, the close link between anhydrite precipitation and dolomitization is still poorly explored within the context of sequence stratigraphy. This comprehensive diagenetic and sequence stratigraphic study of the Late Jurassic Arab Formation, Abu Dhabi, United Arab Emirates, provides important insights into the relationship between anhydrite precipitation and dolomitization within the 2nd, 3rd, and 4th orders of relative sea-level cycles. Initial good connectivity between the inner ramp and open sea during 2nd order late transgressive-early regression cycles accounts for the limestone-dominated lithology with limited anhydrite formation. Conversely, the following progressive 2nd order fall in the relative periodic restriction of the inner platform during 2nd order early regressive cycles (early highstand systems tracts, HST) caused seepage reflux dolomitization and the precipitation of scattered to pervasive anhydrite cement in the dolostones. Variations in the extent of dolomitization were controlled by the permeability and reactivity of the precursor limestones, resulting in the formation of what is known as dolomite fingers. The most laterally extensive dolomitization during late 2nd order regression resulted in the formation of microcrystalline dolostones with nodular and chickenwire anhydrite by a combination of evaporative sabkha pumping and the seepage reflux of lagoon brines. The δ1³C and δ1⁸O of calcite and dolomite reveal the influence of degree of restriction of the inner ramp, and related extent of seawater evaporation. The lower limestone-dominated interval (3rd order HST) is characterized by lower δ1³C values (+2.0‰ to +2.5‰) owing to periodic restriction of seawater circulation, which resulted in oxidation of organic matter during aging of the seawater. This study approach provides important insights into the genetic links between interbedded platform limestones, dolostones, and anhydrites, and hence a better understanding and prediction of reservoir quality distribution and compartmentalization.

Kinetics of In-Situ Calcium Magnesium Carbonate Precipitation and the Need for Desulfation in Seawater-Flooded Carbonate Reservoirs

Summary Mixing of incompatible injection and formation brines leads to the deposition of inorganic sulfate scales such as barite, celestite, and anhydrite in and around production wells. This process is well documented in seawater-flooded clastic reservoirs. One technique to avoid the resulting formation damage is to remove sulfate from seawater before injection using nanofiltration; however, this process is costly. We identify in this paper that it may not always be necessary in higher-temperature carbonate reservoirs. In this paper, we describe the use of reactive transport reservoir simulation to investigate the impact of carbon dioxide (CO2) partitioning and changes in pH, ionic concentrations, and temperature on carbonate reactivity and the sulfate scaling risk in waterflooded carbonate reservoirs. Dissolution and precipitation of calcite, dolomite, gypsum, anhydrite, barite, and celestite are all modeled and found to be coupled through (various) common ion effects. The produced brine compositions are used to calculate the saturation ratios (SRs) and mass of precipitate that may form in the production system. Sensitivity to mineral reaction kinetics, particularly for the dolomite reactions, is accounted for. Results identify that there is a strong relationship between calcite dissolution and dolomite (or other calcium/magnesium carbonate mineral) precipitation reactions, which drive each other and are affected by the availability of CO2 in the residual oil phase. This evolves over time, and as the thermal front propagates, impacts the concentration of calcium and magnesium in the brines traversing the reservoir. Temperature changes around the injection wellbore impact CO2 and mineral solubilities. The concentration of calcium in the displaced brine mix is thus determined more by contact with rock and temperature than by mixing between injection and formation brines. Depending on location relative to the thermal front, this may lead to gypsum or anhydrite precipitation, thereby stripping sulfate out of the injection brine. Thus, the sulfate scaling risk at the production wells is significantly reduced by this sulfate depletion process: The sulfate is stripped out of the seawater as it warms up in the reservoir before it mixes extensively with the formation water and significantly before any mixture of the two brines reaches the production zone. Thus, any loss of permeability is restricted to deep within the reservoir, where the pore volume (PV) that can accommodate mineral precipitation is very large. In this work, we identify that for carbonate reservoirs above 90–100°C, stripping of sulfate due to coupled mineral reactions may reduce or eliminate the need for use of a sulfate reduction plant (SRP). The process is modeled for the first time, accounting for the impact of CO2 partitioning and thermal front propagation. Knowledge of the kinetics of calcium/magnesium carbonate precipitation is shown to be critical in predicting the extent of sulfate depletion.

IORSim: A Mathematical Workflow for Field-Scale Geochemistry Simulations in Porous Media

Reservoir modeling consists of two key components: the reproduction of the historical performance and the prediction of the future reservoir performance. Industry-standard reservoir simulators must run fast on enormous and possibly unstructured grids while yet guaranteeing a reasonable representation of physical and chemical processes. However, computational demands limit simulators in capturing involved physical and geochemical mechanisms, especially when chemical reactions interfere with reservoir flow. This paper presents a mathematical workflow, implemented in IORSim, that makes it possible to add geochemical calculations to porous media flow simulators without access to the source code of the original host simulator. An industry-standard reservoir simulator calculates velocity fields of the fluid phases (e.g., water, oil, and gas), while IORSim calculates the transport and reaction of geochemical components. Depending on the simulation mode, the geochemical solver estimates updated relative and/or capillary pressure curves to modify the global fluid flow. As one of the key innovations of the coupling mechanism, IORSim uses a sorting algorithm to permute the grid cells along flow directions. Instead of solving an over-dimensionalized global matrix calling a Newton–Raphson solver, the geochemical software tool treats the species balance as a set of local nonlinear problems. Moreover, IORSim applies basis swapping and splay tree techniques to accelerate geochemical computations in complex full-field reservoir models. The presented work introduces the mathematical IORSim concept, verifies the chemical species advection, and demonstrates the IORSim computation efficiency. After validating the geochemical solver against reference software, IORSim is used to investigate the impact of seawater injection on the NCS Ekofisk reservoir chemistry.Article HighlightsThe IORSim sorting algorithm decouples the nonlinear geochemical reaction calculations into recurring one-dimensional problems to assure numerical stability and computation efficiency. To the best of our knowledge, this work presents the mathematical concept, implementation, and application of topological sorting for the first time on (industry) field-scale problems.IORSim combines topological sorting with basis swapping and splay trees to significantly reduce computation times. Moreover, a high-speed forward simulation mode was developed to allow the post-advection of chemical components to visualize species distribution, water chemistry, and mineral interactions. If the geochemical reactions interfere with the fluid flow, the IORSim backward mode uses relative permeability curves to update the global fluid flow at each time step.We validate the implemented topological scheme on a reservoir grid, show the computation efficiency, and compare the impact of explicit, implicit, and grid refinement on numerical dispersion.The decoupled flow simulator and geochemical reaction calculations allow seamless integration of full-field reservoir models that contain complex geological structures, a large number of wells, and long production histories.The computation capabilities of IORSim are demonstrated by simulating and reproducing the impact of seawater injection in the southern segment of the giant Ekofisk field (more than 50 years of injection and production history). IORSim shows that seawater injection changed the Ekofisk mineralogy and impacted the produced water chemistry. In the investigated Ekofisk case, seawater promoted calcite dissolution and led to the precipitation of magnesite and anhydrite. Moreover, surface complexation modeling revealed that sulfate is adsorbed on the calcite surface.

Rock fluid interactions during seawater injection in carbonates: An experimental evaluation on dolomitization and anhydrite precipitation

Rock fluid interactions during seawater injection in carbonates: An experimental evaluation on dolomitization and anhydrite precipitation

Rare earth element systematics of chimney anhydrite from seafloor hydrothermal vents

Comparative studies of sulphide chemistry have been conducted over the past few decades to characterise seafloor hydrothermal mineralisation, but few studies have been carried out on sulphate minerals. Here we report on a systematic study of the S isotopic composition and rare earth element (REE) chemistry of chimney anhydrite from various types of seafloor hydrothermal vent fields in the Central Indian Ridge, North Fiji Basin, and Tonga intra-oceanic arc. Our results show that sulphate δ34S values of anhydrite cluster around that of seawater sulphate, indicating a significant role for fluid–seawater mixing during anhydrite precipitation. However, some anhydrite has δ34S values lower than that of seawater. This likely reflects sulphate derived from magmatic SO2 disproportionation. Chondrite-normalised REE patterns of the anhydrite are not uniform on the scale of individual grains and/or between each type of hydrothermal vent field. The REE compositions of the anhydrite do not have a clear relationship with the REE compositions of the primary host rocks with which the hydrothermal fluids interacted. This indicates that the REEs are not sensitive indicators of the host rock compositions, even though the host rocks are the dominant source of REEs in the seafloor hydrothermal fluids. In contrast, anhydrite REE concentrations and Eu anomalies are correlated with δ34S values and the redox state of the hydrothermal fluids, respectively. We suggest that relatively REE-enriched anhydrite is formed by efficient leaching of REEs from host rocks by acidic magmatic fluids (i.e., a low pH), whereas a low redox potential of the hydrothermal fluids is the most important factor controlling Eu mobility. The original concentrations of REEs supplied to the hydrothermal fluids can also be modified by fluid boiling and fluid–seawater mixing. The former produces light REE-depleted anhydrite because the availability of chloride ligands decreases in low-density vapor-dominated boiled fluids. In situ geochemical analysis of anhydrite on the intra-grain scale shows that high degrees of fluid–seawater mixing contribute to the relatively low REE concentrations and positive Eu anomalies. As such, our results indicate that the REE systematics of anhydrite are mainly controlled by the combined effects of fluid conditions (i.e., redox state, pH, and Cl concentrations) and sub-seafloor geochemical processes (i.e., magmatic input, fluid boiling, and seawater mixing) rather than the nature of the source materials. In situ analysis of anhydrite grains is a useful tool for constraining variable hydrothermal mineralisation conditions during chimney growth, which cannot be solely inferred from geochemical studies of sulphide minerals, particularly in inactive and relict systems where the hydrothermal fluids cannot be sampled.

Magmatic-hydrothermal fluid evolution of the tin-polymetallic metallogenic systems from the Weilasituo ore district, Northeast China

The large Weilasituo Sn-polymetallic deposit is a recent exploration discovery in the southern Great Xing’an Range, northeast China. The ore cluster area shows horizontal mineralization zoning, from the inner granite body outward, consisting of high-T Sn–W–Li mineralization, middle-T Cu–Zn mineralization and peripheral low-T Pb–Zn–Ag mineralization. However, the intrinsic genetic relationship between Sn-W-Li mineralization and peripheral vein-type Pb–Zn–Ag–Cu mineralization, the formation mechanism and the deep geological background are still insufficiently understood. Here, we use fluid inclusions, trace elements concentrations in quartz and sphalerite, and H–O isotope studies to determine the genetic mechanism and establish a metallogenic model. Fluid inclusion microthermometry and Laser Raman spectroscopic analysis results demonstrates that the aqueous ore-forming fluids evolved from low-medium salinity, medium–high temperature to low salinity, low-medium temperature fluids. Laser Raman spectroscopic analysis shows that CH4 is ubiquitous in fluid inclusions of all ore stages. Early ore fluids have δ18OH2O (v–SMOW) values from + 5.5 to + 6.2‰ and δD values of approximately − 67‰, concordant with a magmatic origin. However, the late ore fluids shifted toward lower δ18OH2O (v–SMOW) (as low as 0.3‰) and δD values (~ − 136‰), suggesting mixing between external fluids derived from the wall rocks and a contribution from meteoric water. Ti-in-quartz thermometry indicates a magmatic crystallization temperature of around 700 °C at a pressure of 1.5 kbar for the magmatic ore stage. Cathodoluminescence (CL) imaging and trace element analysis of quartz from a hydrothermal vug highlight at least three growth episodes that relate to different fluid pulses; each episode begins with CL-bright, Al-Li-rich quartz, and ends with CL-dark quartz with low Al and Li contents. Quartz from Episode 1 formed from early Sn-(Zn)-rich fluids which were likely derived from the quartz porphyry. Quartz from episodes 2 and 3 formed from Zn-(Sn)-Cu-rich fluid. The early magmatic fluid is characterized by low fS2. The SO2 produced by magma degassing reacted with heated water to form SO42−, causing the shift from low fS2 to high fS2. The SO42− generated was converted to S2– by mixing with CH4-rich, Fe and Zn-bearing external fluid which led to late-stage alteration and dissolution of micas in vein walls, thus promoting crystallization of pyrrhotite, Fe-rich sphalerite and chalcopyrite and inhibiting the precipitation of anhydrite. This study shows that ore formation encompassed multiple episodes involving steadily evolved fluids, and that the addition of external fluids plays an important role in the formation of the later Cu–Zn and Ag–Pb–Zn mineralization in the Weilasituo ore district.

Calcium sulfates in planetary surface environments

Calcium sulfates play a key role in both industrial processes and the global sulfur cycle. In arid to hyper-arid planetary surface environments, they can be used to assess the availability of water to the environment. Yet, the processes through which calcium sulfate minerals (i.e. gypsum, bassanite, anhydrite) and the anhydrous γ-CaSO4 form and transform remain insufficiently understood. Especially the dissolution-reprecipitation reaction from gypsum to anhydrite has to this date been achieved in the laboratory only under very specific conditions.Recent evidence suggests the importance of solution flow as opposed to batch reactions in aiding the precipitation of anhydrite at room temperature. Our own results, however, clearly show, that solution flow alone cannot be the sole catalyst. In light of the recent abundance of research towards the nucleation and crystallization of calcium sulfates, we present this review in an attempt to summarize and contextualize these new results and to identify necessary directions for further research with focus on conditions of arid and hyper-arid planetary environments.

SEDIMENTOLOGY, PALAEOGEOGRAPHY AND DIAGENESIS OF THE UPPER PERMIAN (Z2) HAUPTDOLOMIT FORMATION ON THE SOUTHERN MARGIN OF THE MID NORTH SEA HIGH AND IMPLICATIONS FOR RESERVOIR PROSPECTIVITY

This paper provides an updated understanding of the reservoir stratigraphy, sedimentology, palaeogeography and diagenesis of the Upper Permian Hauptdolomit Formation of the Zechstein Supergroup (“Hauptdolomit”) in a study area on the southern margin of the Mid North Sea High. The paper is based on the examination and description of core and cuttings data from 25 wells which were integrated with observations based on existing and new 3D seismic.Based on thin‐section petrography of cuttings and core from the wells studied, it is evident that Hauptdolomit microfacies are distributed in a relatively predictable way, and well‐defined platform interior, platform margin, slope and basin settings can be distinguished. Platform margins are typically characterised by the development of ooid shoals and, to a lesser‐extent, by microbial build‐ups. High‐energy back‐shoal settings are characterised by a more complex combination of peloid grainstones, thrombolitic and microbial build‐ups, and fine crystalline dolomites. Lower energy lagoons which developed further behind the platform margin are characterised by a variety of microfacies types; fine crystalline dolomites are common in this setting as well as peloidal facies and local microbial build‐ups. Intertidal and supratidal settings are typified by increased proportions of anhydrite and the development of laminated microbial bindstones (stromatolites). Platform margins are in general relatively steep and pass into slope and basinal settings. Only a few wells have penetrated Hauptdolomit successions deposited in a slope setting, and these successions are characterised by a range of resedimented shallow‐water facies together with low‐energy laminated dolomicrites and fine crystalline dolomites. Slope zones in the study area are interpreted from seismic data to be typically 1‐1.5 km in width. Basinal Hauptdolomit deposits have been strongly affected by post‐depositional diagenesis and are dedolomitised to variable degrees. The original depositional facies are rarely preserved.Diagenetic studies show that dolomitisation has affected almost the entire Hauptdolomit Formation throughout the study area in both basinal and platform settings. The dolomite is considered to result from seepage‐reflux processes and is an early diagenetic phase. Mouldic porosity is present in many facies types as a result of dissolution, especially in ooid grainstones, thrombolitic build‐ups and peloidal facies. The dissolution cannot be associated with any one diagenetic phase but was most likely a result of the dolomitisation process itself. Stable isotope analyses indicate that all dolomites were precipitated from Permian marine‐derived pore fluids. Fluid inclusion analyses of dolomite cements indicate that cementation continued into the burial realm. Anhydrite cementation occurs in two phases: early anhydrite precipitation was associated with dolomitisation, and can be distinguished from a later, pore‐filling cement which is highly detrimental to reservoir quality.The Hauptdolomit succession in basinal wells (and in some slope wells) in the study area has undergone significant dedolomitisation. Dedolomitisation was a shallow burial process which affected precursor dolomites, whereby excess calcium from the transition of gypsum to anhydrite during burial combined with CO2 and organic acids derived from basinal sediments. The process was triggered by excess calcium reacting with excess carbonate ions from dissolution.3D seismic volumes supplemented by numerous 2D lines were available in the study area and allowed an interpretation to be made of Hauptdolomit gross depositional settings; platform margins and base‐of‐slope polygons were mapped, with the greatest confidence in areas of 3D seismic. The basin, slope and platform settings were distinguished using seismic data integrated with the results of micro‐facies analysis and incorporating seismic‐to‐well ties. The data shows that large parts of the study area are characterised by the presence of polyhalites within the overlying (Z2) Stassfurt Halite Formation, which may create particular seismic geometries at the Hauptdolomit slope. These are interpreted to be intra‐Stassfurt Halite features, providing an alternative model to the thickened, prograded Hauptdolomit which has been suggested in previous publications.Because few wells drilled in the study area had the Hauptdolomit as the primary target, cores were limited but significant data was obtained from cuttings analyses. More than 400 thin sections were evaluated, allowing depositional models based on microfacies observations to be developed, verifying the seismic‐scale observations.

Experimental Equilibrium and Fractional Crystallization of a H2O, CO2, Cl and S-Bearing Potassic Mafic Magma at 1.0 GPa, With Implications for the Origin of Porphyry Cu (Au, Mo)-Forming Potassic Magmas

ABSTRACT The origin of intermediate to felsic potassic magmas is debated, and not much is known about the volatile content of potassic magmas associated with porphyry Cu (Au, Mo) deposits. To better understand the liquid line of decent of mafic potassic magmas and the behavior of volatiles during magma differentiation, we performed 19 experiments at 1.0 GPa and 1150 °C to 850 °C using piston cylinder presses. We developed a new experimental technique that involves a capsule liner made of single-crystal zircon to prevent the loss of Fe and S in the starting material to the noble metal capsule. The starting material is a high-Mg, basaltic trachyandesite (52 wt% SiO2, 12 wt% MgO, 1.9 wt% Na2O and 5.3 wt% K2O) from the Sanjiang region in southwestern China, doped with geologically realistic amounts of volatiles (i.e. 4.0 wt% H2O, 0.34 wt% CO2, 0.27 wt% Cl and 0.25 wt% S). The addition of 0.25 wt% S in the form of anhydrite internally buffered the experiments at an oxygen fugacity of 2.0 ± 0.5 log units above the fayalite–magnetite–quartz buffer, which is similar to the redox state of the Sanjiang variously evolved potassic magmas. The experimentally produced silicate melts match well with the Sanjiang intermediate to felsic magmas in terms of major, minor and trace element compositions, and also with regard to the S and Cl contents. The sequence of crystallizing minerals (olivine + clinopyroxene –> biotite ± orthopyroxene –> apatite –> K-feldspar) also fits with the one observed in the Sanjiang mafic to intermediate magmas. These results suggest that the Sanjiang intermediate to felsic magmas, including the porphyry Cu (Au, Mo)-forming magmas, can form solely by differentiation of potassic mafic magmas without any involvement of old crustal material. During experimental differentiation at 1.0 GPa, the S content of the evolving silicate melt first increased until ~57 wt% melt SiO2, and then decreased in response to precipitation of sulfides, sulfate melt and/or anhydrite, whereas the H2O and Cl contents of the evolving silicate melt increased exponentially until saturation in a CO2-rich fluid was reached at 60 to 65 wt% melt SiO2 and ~ 8 wt% melt H2O. During further magma differentiation the H2O and Cl contents of the evolving silicate melt remained constant until ~70 wt% melt SiO2, after which point the Cl content of the silicate melt decreased due to increased partitioning of Cl into the fluid phase ± increased fluid/melt ratio. Based on these experimental results and petrographic and geochemical evidence from natural samples, the Sanjiang porphyry Cu (Au, Mo)-forming magmas (65–70 wt% SiO2) are interpreted to have formed through differentiation of primitive, mantle-derived, potassic magmas in the lower crust (≥1.0 GPa), and to have ascended ±directly from the lower crust to shallow crustal levels. They likely contained 8 to 13 wt% H2O, 0.37 to 0.90 wt% Cl and 0.07–0.29 wt% S. This case study on the magma evolution in the Sanjiang region may have implications for the origin and nature of intermediate to felsic potassic magmas in various tectonic settings.

Groundwater quality evaluation based on water quality indices (WQI) using GIS: Maadher plain of Hodna, Northern Algeria.

In a semi-arid region of Maadher, central Hodna (Algeria), groundwater is the main source for agricultural and domestic purposes. Anthropogenic activities and the presence of climate change's effects have a significant impact on the region's groundwater quality. This study's goals were to use water quality indices to evaluate the groundwater's quality and its suitability for drinking and irrigation, as well as to identify contaminated wells using a geographic information system (GIS) and the spatial interpolation techniques of ordinary kriging and inverse distance weighting (IDW). The results reveal that all water samples exceeded the World Health Organization's standards for nitrate ions and had alarming concentrations of calcium, chlorine, and sulfate (WHO). According to Piper's diagram, the groundwater hydrochemical facies is composed of the elements sulfate-chloride-nitrate-calcium (SO42--Cl-NO3--Ca2+ water type). The majority of samples fall into the poor water category, slightly more than 10% fall into the very poor water category, and less than 10% fall into the good to the excellent quality category, per the water quality indices, which classify samples in a similar manner. According to irrigation water indices, every sample is suitable for irrigation. Depending on the direction of groundwater flow, the spatial distributions of Ca2+, Na+, Mg2+, SO42-, and Cl- show that their concentrations are high north of the area and relatively low south of Maadher village (Fig.3). Nitrate concentrations are high in the majority of samples, particularly those close to the Bousaada wadi. In most samples, particularly those close to the Bousaada wadi, nitrate levels are high. Various water quality models were described, and GIS spatial distribution maps were created using standard kriging and inverse distance weighting (IDW) techniques through selected semi-variograms predicted against measurements. To determine the origin of mineralization and the chemical processes that take place in the aquifer-which include the precipitation and dissolution of dolomite, calcite, aragonite, gypsum, anhydrite, and halite-the groundwater saturation index was calculated.

The role of nanofiltration modelling tools in the design of sustainable valorisation of metal-influenced acidic mine waters: The Aznalcóllar open-pit case

Acid Mine Waters (AMWs) are considered the most challenging environmental problem in the Iberian Pyrite Belt (South of Spain) so far, being characterised by an acidic pH (H2SO4-based), high content of heavy metals (e.g. Co, Pb, rare earth elements (REEs)) and non-metals (e.g. As). Nowadays, AMWs are being contemplated as a source of valuable metals (e.g. Zn, Cu, REEs), and different recovery routes have been studied (e.g. ion-exchange, membrane technologies). Among them, nanofiltration (NF) provides the possibility of water (or H2SO4) recovery and concentration of metals, which can be recovered in a subsequent stage. This work evaluates the performance of the Desal DK NF membrane to treat AMW from Aznalcóllar open-pit, which presents a low pH (2.6) and high contents of Zn (920 mg/L), Mg (2540 mg/L) and REEs (17 mg/L). The effect of solution pH on membrane performance was evaluated. Results showed that the equilibrium SO42-/HSO4- controlled the transport of species. Rejections of sulphate were around 84 % at pH 2.1 (prevalence of HSO4-) and increased to 92 % at pH 3.1 (prevalence of SO42-). Metal rejections remained higher than 74 % (except for monovalent species). Additionally, membrane permeances were determined using the Solution-Electro-Diffusion model coupled with reactive transport, and later used to assess the design of the process treatment of the AMW. Results demonstrated the possibility of concentrating metals and recovering H2SO4 as permeate, but also showed that gypsum and anhydrite precipitation can occur. Additionally, CAPEX and OPEX were estimated for a NF plant treating 2 Hm3/year.

Theoretical estimates of sulfoxyanion triple-oxygen equilibrium isotope effects and their implications

Triple-oxygen isotope (δ18O and Δ′17O) analysis of sulfate is becoming a common tool to assess several biotic and abiotic sulfur-cycle processes, both today and in the geologic past. Multi-step sulfur redox reactions often involve intermediate sulfoxyanions such as sulfite, sulfoxylate, and thiosulfate, which may rapidly exchange oxygen atoms with surrounding water. Process-based reconstructions therefore require knowledge of equilibrium oxygen-isotope fractionation factors (18α and 17α) between water and each individual sulfoxyanion. Despite this importance, there currently exist only limited experimental 18α data and no 17α estimates due to the difficulty of isolating and analyzing short-lived intermediate species. To address this, we theoretically estimate 18α and 17α for a suite of sulfoxyanions—including several sulfate, sulfite, sulfoxylate, and thiosulfate species—using quantum computational chemistry. We determine fractionation factors for sulfoxyanion “water droplets” using the B3LYP/6-31G+(d,p) method; we additionally calculate higher-order method (CCSD/aug-cc-pVTZ and MP2/aug-cc-pVTZ) scaling factors, and we qualitatively estimate the importance of anharmonic zero-point energy (ZPE) corrections using a suite of gaseous sulfoxy compounds. Methodological scaling factors greatly impact 18α predictions, whereas ZPE corrections are likely small (i.e., ⩽1‰) at Earth-surface temperatures; existing experimental data best agree with 18α predictions when including redox state-specific CCSD/aug-cc-pVTZ scaling factors. Theoretical pH- and temperature-specific bulk-solution (i.e., abundance-weighted average of all species) 18α values yield root-mean-square errors for sulfate/water, sulfite/water, and thiosulfate/water equilibrium of 4.5‰ (n=18 experimental conditions), 3.7‰ (n=27), and 2.2‰ (n=3), respectively. However, sulfate- and sulfite-system agreement improves considerably when comparing experimental results only to SO3(OH)−/H2O (RMSE = 1.6‰) and SO2(OH)−/H2O (RMSE = 2.2‰) predictions, rather than bulk solutions. This is particularly true for the sulfite system at high and low pH, when SO2(OH)− is not the dominant species. We discuss potential experimental and theoretical biases that may lead to this apparent improvement. By combining 18α and 17α predictions, we additionally estimate that sulfate, sulfite, sulfoxylate, and thiosulfate species can exhibit Δ′17O values as much as 0.199‰, 0.205‰, 0.101‰, and 0.186‰ more negative than equilibrated water at Earth-surface temperatures (reference line slope = 0.5305). This theoretical framework provides a foundation to interpret experimental and observational triple-oxygen isotope results of several sulfur-cycle processes including pyrite oxidation, microbial metabolisms (e.g., sulfate reduction, thiosulfate disproportionation), and hydrothermal anhydrite precipitation. We highlight this with several examples.

Copper mobilization via seawater-volcanic rock interactions: New experimental constraints for the formation of the iron oxide Cu-Au (IOCG) mineralization

The involvement of modified seawater and/or basinal brine in IOCG mineralization was documented by numerous geologic studies. Still, this process remains poorly understood. In this study, we reacted seawater with andesite and basalt, the most common arc volcanic rocks, at 150 and 240 °C and water vapor-saturated pressure to assess the Cu leaching capacity and fluid evolution due to the seawater-volcanic rock interactions. The experimental results show that Mg, Fe, Al, and SO42− in the modified seawater decreased significantly, accompanied by an increased Ca concentration. The fluid pH also decreased from 8.3 of the initial seawater to ∼7.5 and ∼7.0 after 150 and 240 °C experiments, respectively. The incongruent calcsilicate (e.g., plagioclase) dissolution and anhydrite precipitation were observed in the reacted rock samples. Thermodynamic modeling confirmed the trends of those concentration changes but predicted several alteration minerals that were not observed in the experiments. Our experiments show that Cu-leaching efficiency is much higher from the andesite than from the basalt, also higher at 240 °C than at 150 °C. The andesite contains more Cu than the basalt samples, and abundant Cu-rich (average 106 ppm) plagioclase-hosted silicate melt inclusions and Cu-bearing (average 59 ppm) groundmass are identified in the original andesite sample but not in the original basalt sample, which is considered to be the cause of the Cu-leaching differences between the two rock samples. The δ34S value of resultant fluids is dominated by seawater sulfate with a minor contribution of magmatic sulfur from the rock. Thermodynamic modeling combined with recent experimental studies show that deposition of Cu sulfides is the most effective when Cu-bearing fluids react with early-stage pyrite. Our results suggest that the seawater/(brine)-volcanic rock reactions at 150–250 °C can produce Cu-bearing fluids, which in turn can form IOCG deposits, and that Cu mineralization with sulfides zonation can result from replacement reactions between the Cu-bearing fluids and Fe-rich minerals in precursor iron ore bodies.

Geochemical Assessment of Gypsum Scale Formation in the Hydrated Lime Neutralization Facility of the Daedeok Mine, South Korea

Scale is widely observed in the hydrated lime mine drainage treatment plant of the Daedeok Mine in South Korea. In order to understand the environment in terms of the formation of scale minerals, scale and water were collected from the AMD treatment facility and analyzed. In addition, the saturation index was calculated based on geochemical modeling to predict the minerals that could be produced in the AMD treatment facility, and the results were then compared with an analysis of onsite scale minerals. Furthermore, the onsite mine drainage was neutralized from pH 3 to pH 9 in the laboratory, and the precipitates produced were identified. The changes in the Ca2+ and SO42− concentrations were also identified over time for each pH. The results of geochemical modeling predicted the possible precipitation of gypsum, anhydrite, and bassanite after AMD neutralization. Scanning electron microscope/energy dispersive X-ray spectroscopy (SEM/EDS) analysis results showed that the main mineral in scale formed at the AMD treatment facility was gypsum, produced by the reaction of SO42− and Ca2+ from lime during AMD. The laboratory neutralization experiment showed that gypsum was produced in all neutralization ranges from pH 3 to pH 9, and the higher the neutralization pH, the greater the amount of gypsum produced. It was demonstrated that simulated amounts of 2 g/L and 7 g/L gypsum at pH 5 and 9 were well matched with the experimental results. Iron (Fe), a major pollutant in the mine drainage system, was rapidly precipitated in the form of iron hydroxides after neutralization. As gypsum is produced slowly and continuously for a long period of time, it results in the growth of scale throughout the flow path. As a method of minimizing gypsum production in the AMD treatment facility using hydrated lime, it is recommended that the facility should be operated at the lowest pH possible, which will also enable the removal of major pollutants, such as iron and aluminum.

Modeling Solubility of Anhydrite and Gypsum in Aqueous Solutions: Implications for Swelling of Clay-Sulfate Rocks

The swelling of clay-sulfate rocks is a well-known phenomenon often causing threats to the success of various geotechnical projects, including tunneling, road and bridge construction, and geothermal drilling. The origin of clay-sulfate swelling is usually explained by physical swelling due to clay expansion combined with chemical swelling associated with the transformation of anhydrite (CaSO4) into gypsum (CaSO4∙2H2O). The latter occurs through anhydrite dissolution and subsequent gypsum precipitation. Numerical models that simulate rock swelling must consider hydraulic, mechanical, and chemical processes. The simulation of the chemical processes is performed by solving thermodynamic equations, which usually contribute a significant portion of the overall computation time. This paper employs feed-forward neural network (FFNN) and cascade-forward neural network (CFNN) models trained with a Bayesian regularization (BR) algorithm as an alternative approach to determine the solubility of anhydrite and gypsum in the aqueous phase. The network models are developed using calcium sulfate experimental data collected from the literature. Our results indicate that the FFNN-BR is the most accurate model for the regression task. The comparison analysis with the Pitzer ion interaction model as well as previously published data-driven models shows that the FFNN-BR model is highly accurate in determining the solubility of sulfate minerals in acid and salt-containing solutions. We conclude from our results that the FFNN-BR model can be used to determine the solubility of anhydrite and gypsum needed to address typical subsurface engineering problems such as swelling of clay-sulfate rocks.

Compositional Analysis of Conventional and Unconventional Permian Basin-Produced Waters: A Simple Tool for Predicting Major Ion Composition

Summary Treatment methods for produced water (PW) are significantly affected by a high concentration of total dissolved solids (TDS), a summation of dissolved organic and inorganic compositions. Understanding the constituents of TDS to eliminate or prevent chemical reactions is critical in the design optimization of the treatment processes. In this paper, two PW geochemical data sets generated from conventional and unconventional reservoirs in the Permian Basin were analyzed to correlate constituents with TDS. Compositional data sets from over 115,000 PW samples originally reported by the U.S. Geological Survey (USGS) and 45 oil and gas operations were analyzed. Data preprocessing, culling, systematized- and meta-analysis, and statistical techniques were adapted to associate the data. Subcompositional geochemical data were transformed into isometric log ratios and are presented in bivariate and multivariate plots. Results indicate that Na+ and Ca2+ were the dominant cations and Cl− was the dominant anion. No observable trend differences in the Na+, Cl−, Ca2+, Mg+, and SO42− concentrations between PW from conventional and unconventional wells were registered. Variations in the isometric log ratio of Na/Cl and Ca/SO4 with TDS revealed that Na/Cl was nearly constant over the range of TDS, suggesting mineral buffering or a lack of significant water/rock reactions involving Na and Cl, and that Ca/SO4 increased with TDS, indicating that low-salinity fluids may have dissolved anhydrite producing a value near zero, with Ca gain and/or SO4 loss with increasing salinity. In all 10 counties and 8 formations investigated in this work, the ln (Ca/SO4) denotes Ca gain/SO4 loss relative to their composition in anhydrite or Permian seawater. Likely mechanisms leading to elevated ln (Ca/SO4) include sulfate reduction, dolomitization of calcite, Na/Ca cation exchange, albitization, and anhydrite precipitation from Ca-rich fluids. Results from this work are important as it is revealed that Ca/SO4 and Na/Cl can potentially be predicted from PW TDS concentrations. This information was combined to create a reservoir or location-specific model to estimate Na, Cl, Ca, and SO4 concentrations in Permian Basin PW, a powerful tool to improve treatment and reuse options in areas with few direct data.

Influence of seawater intrusion on the hot springs in a coastal area: The case of the Anak-Sinchon Uplift, Korean Peninsula

Temperatures and chemical compositions of hot springs are decisive of their roles in heating, power-generation, therapeutic applications and recreational activities. However, the influence of seawater intrusion on hot springs are still not fully understood. Typical ten hot springs in and around the Anak-Sinchon Uplift in a coastal area of the northern Korean Peninsula were investigated. All the hot springs were near-neutral to weakly alkaline. High TDS value of 25300 mg/L was found at the hot spring closest to the coastline. TDS values, and concentrations of Na+, K+, Ca2+, Cl− and SO42− in the hot spring waters near the coast showed decreasing trends with the distance from the coastline. According to the calculation of seawater fraction, three hot springs near the coast were contaminated by seawater. Temperatures of the geothermal reservoir were estimated to range from 103 to 170 °C, which were interpolated by thin plate spline to predict the local trend of the reservoir temperature distribution. Mg2+ concentration was used to determine the time order between the heating process and seawater intrusion. Among the three seawater-contaminated hot springs, very low Mg2+ concentrations in two hot spring waters probably indicate that they were mixed with seawater before being heated in the geothermal reservoir and not affected by seawater intrusion again afterwards; relatively high Mg2+ concentration in the hot spring water closest to the coastline likely indicates that it was mixed with seawater again during the ascent from the geothermal reservoir. The geochemical modeling performed with PHREEQC indicates that dolomitization of calcite and precipitation of anhydrite were the dominant water-rock interactions for the seawater-contaminated hot spring waters.

Effects of gypsum-salt rock on mineral transformations in a saline lacustrine basin: Significance to reservoir development

Gypsum-salt rock is typically developed in carbonate reservoirs, and this rock has both constructive and destructive effects on the reservoir. The ways in which gypsum rock controls reservoir development are closely related to the diagenetic conditions. In this study, a typical saline lacustrine basin, the Qaidam Basin in China, was selected to examine the influence of gypsum-salt rock on the development of the carbonate reservoir under different diagenetic conditions. Four main geological factors were assessed: formation condition (temperature), typical salt mineral (anhydrite), and fluid properties (Ca2+ and Mg2+), along with multi-group fluid–rock chemical reaction models devised using multiphase-flow solute-transport simulation technology. Mineral dissolution, precipitation, and transformation in the reservoir under various temperature, pressure, fluid, and mineral conditions were analyzed, and the change of reservoir porosity was calculated. The results showed that the concentration of Ca2+ in fluid controls the dissolution and precipitation of carbonate minerals in the reservoir; moreover, continuous and sufficient Mg2+ is a necessary condition for dolomitization. Precipitation of anhydrite decreases with increase of temperature, verifying that anhydrite precipitates more easily at low temperature. Dissolution of calcium-containing minerals in overlying gypsum-salt rock can lead to mineral precipitation in the subjacent reservoir and reduce its quality.

Investigation of Water Composition on Formation Damage and Related Energy Recovery from Geothermal Reservoirs: Geochemical and Geomechanics Insights

The main challenge in extracting geothermal energy is to overcome issues relating to geothermal reservoirs such as the formation damage and formation fracturing. The objective of this study is to develop an integrated framework that considers the geochemical and geomechanics aspects of a reservoir and characterizes various formation damages such as impairment of formation porosity and permeability, hydraulic fracturing, lowering of formation breakdown pressure, and the associated heat recovery. In this research study, various shallow, deep and high temperature geothermal reservoirs with different formation water compositions were simulated to predict the severity/challenges during water injection in hot geothermal reservoirs. The developed model solves various geochemical reactions and processes that take place during water injection in geothermal reservoirs. The results obtained were then used to investigate the geomechanics aspect of cold-water injection. Our findings presented that the formation temperature, injected water temperature, the concentration of sulfate in the injected water, and its dilution have a noticeable impact on rock dissolution and precipitation. In addition, anhydrite precipitation has a controlling effect on permeability impairment in the investigated case study. It was observed that the dilution of water could decrease formation of scale while the injection of sulfate rich water could intensify scale precipitation. Thus, the reservoir permeability could decrease to a critical level, where the production of hot water reduces and the generation of geothermal energy no longer remains economical. It evident that injection of incompatible water would decrease the formation porosity. Thus, the geomechanics investigation was performed to determine the effect of porosity decrease. It was found that for the 50% porosity reduction case, the initial formation breakdown pressure reduced from 2588 psi to 2586 psi, and for the 75% porosity reduction case it decreased to 2584 psi. Thus, geochemical based formation damage is significant but geomechanics based formation fracturing is insignificant in the selected case study. We propose that water composition should be designed to minimize damage and that high water injection pressures in shallow reservoirs should be avoided.

A model for the solubility of anhydrite in H2O-NaCl fluids from 25 to 800 °C, 0.1 to 1400 MPa, and 0 to 60 wt% NaCl: Applications to hydrothermal ore-forming systems

Anhydrite (CaSO4) is a common gangue mineral in hydrothermal ore-forming systems, but currently there is no comprehensive numerical model for the solubility of anhydrite in H2O-NaCl hydrothermal fluids over wide ranges of pressure, temperature and fluid salinity (P, T and X). Here, we present a new thermodynamic model for the solubility of anhydrite in saline aqueous fluids throughout a wide range of P-T-X conditions relevant to crustal hydrothermal settings. A database of experimentally determined solubility of anhydrite was used to calibrate the model, and the experimental data covers P-T-X ranges of 0.1–1400 MPa, 25–800 °C, and 0–60 wt% NaCl. The model is successful at reproducing solubility values and overall trends from the experimental dataset within error, and also reproduces results for experimental solubility data that were not included in the calibration. We demonstrate that the model can perform well over a wide-range of P-T-X conditions, including conditions of liquid + vapor coexistence. We also provide examples of applications of the solubility model to predict the precipitation and/or dissolution of anhydrite along fluid flow paths representative of hydrothermal systems.