Geochemical and isotopic variations in shallow groundwater in areas of the Fayetteville Shale development,north-central Arkansas

Exploration of unconventional natural gas reservoirs such as impermeable shale basins through the use of horizontal drilling and hydraulic fracturing has changed the energy landscape in the USA providing a vast new energy source. The accelerated production of natural gas has triggered a debate concerning the safety and possible environmental impacts of these operations. This study investigates one of the critical aspects of the environmental effects; the possible degradation of water quality in shallow aquifers overlying producing shale formations. The geochemistry of domestic groundwater wells was investigated in aquifers overlying the Fayetteville Shale in north-central Arkansas, where approximately 4000 wells have been drilled since 2004 to extract unconventional natural gas. Monitoring was performed on 127 drinking water wells and the geochemistry of major ions, trace metals, CH₄ gas content and its C isotopes (δ¹³C_CH4), and select isotope tracers (δ¹¹B, ⁸⁷Sr/⁸⁶Sr, δ²H, δ¹⁸O, δ¹³C_DIC) compared to the composition of flowback-water samples directly from Fayetteville Shale gas wells. Dissolved CH₄ was detected in 63% of the drinking-water wells (32 of 51 samples), but only six wells exceeded concentrations of 0.5 mg CH₄/L. The δ¹³C_CH4 of dissolved CH₄ ranged from −42.3‰ to −74.7‰, with the most negative values characteristic of a biogenic source also associated with the highest observed CH₄ concentrations, with a possible minor contribution of trace amounts of thermogenic CH₄. The majority of these values are distinct from the reported thermogenic composition of the Fayetteville Shale gas (δ¹³C_CH4 = −35.4‰ to −41.9‰). Based on major element chemistry, four shallow groundwater types were identified: (1) low (<100 mg/L) total dissolved solids (TDS), (2) TDS > 100 mg/L and Ca–HCO₃ dominated, (3) TDS > 100 mg/L and Na–HCO₃ dominated, and (4) slightly saline groundwater with TDS > 100 mg/L and Cl > 20 mg/L with elevated Br/Cl ratios (>0.001). The Sr (⁸⁷Sr/⁸⁶Sr = 0.7097–0.7166), C (δ¹³C_DIC = −21.3‰ to −4.7‰), and B (δ¹¹B = 3.9–32.9‰) isotopes clearly reflect water–rock interactions within the aquifer rocks, while the stable O and H isotopic composition mimics the local meteoric water composition. Overall, there was a geochemical gradient from low-mineralized recharge water to more evolved Ca–HCO₃, and higher-mineralized Na–HCO₃ composition generated by a combination of carbonate dissolution, silicate weathering, and reverse base-exchange reactions. The chemical and isotopic compositions of the bulk shallow groundwater samples were distinct from the Na–Cl type Fayetteville flowback/produced waters (TDS ∼10,000–20,000 mg/L). Yet, the high Br/Cl variations in a small subset of saline shallow groundwater suggest that they were derived from dilution of saline water similar to the brine in the Fayetteville Shale. Nonetheless, no spatial relationship was found between CH₄ and salinity occurrences in shallow drinking water wells with proximity to shale-gas drilling sites. The integration of multiple geochemical and isotopic proxies shows no direct evidence of contamination in shallow drinking-water aquifers associated with natural gas extraction from the Fayetteville Shale.     

Origin,distribution and hydrogeochemical controls on methane occurrences in shallow aquifers,southwestern Ontario,Canada

Natural gas reservoirs in organic-rich shales in the Appalachian and Michigan basins in the United States are currently being produced via hydraulic fracturing. Stratigraphically-equivalent shales occur in the Canadian portion of the basins in southwestern Ontario with anecdotal evidence of gas shows, yet there has been no commercial shale gas production to date. To provide baseline data in the case of future environmental issues related to hydraulic fracturing and shale gas production, such as leakage of natural gas, saline water, and/or hydraulic fracturing fluids, and to evaluate hydrogeochemical controls on natural gas accumulations in shallow groundwater in general, this study investigates the origin and distribution of natural gas and brine in shallow aquifers across southwestern Ontario. An extensive geochemical database of major ion and trace metal chemistry and methane concentrations of 1010 groundwater samples from shallow, domestic wells in bedrock and overburden aquifers throughout southwestern Ontario was utilized. In addition, select wells (n = 36) were resampled for detailed dissolved gas composition, δ¹³C of CH₄, C₂, C₃, and CO₂, and δD of CH₄. Dissolved gases in groundwater from bedrock and overburden wells were composed primarily of CH₄ (29.7–98.6 mol% of total gas volume), N₂ (0.8–66.2 mol%), Ar + O₂ (0.2–3.4 mol%), and CO₂ (0–1.2 mol%). Ethane was detected, but only in low concentrations (<0.041 mol%), and no other higher chain hydrocarbons were present, except for one well in overburden overlying the Dundee Formation, which contained 0.81 mol% ethane and 0.21 mol% propane. The highest methane concentrations (30 to >100 in situ % saturation) were found in bedrock wells completed in the Upper Devonian Kettle Point Formation, Middle Devonian Hamilton Group and Dundee Formation, and in surficial aquifers overlying these organic-rich shale-bearing formations, indicating that bedrock geology is the primary control on methane occurrences. A few (n = 40) samples showed Na–Cl–Br evidence of brine mixing with dilute groundwater, however only one of these samples contained high (>60 in situ % saturation) CH₄. The relatively low δ¹³C values of CH₄ (−89.9‰ to −57.3‰), covariance of δD values of CH₄ and H₂O, positive correlation between δ¹³C values of CH₄ and CO₂, and lack of higher chain hydrocarbons (C₃₊) in all but one dissolved gas sample indicates that the methane in groundwater throughout the study area is primarily microbial in origin. The presence or absence of alternative electron acceptors (e.g. dissolved oxygen, Fe, NO₃, SO₄), in addition to organic substrates, controls the occurrence of microbial CH₄ in shallow aquifers. Microbial methane has likely been accumulating in the study area, since at least the Late Pleistocene to the present, as indicated by the co-variance and range of δD values of CH₄ (−314‰ to −263‰) and associated groundwater (−19‰ to −6‰ δD-H₂O).     

Isotopic and geochemical identifications of groundwater salinisation processes in Salalah coastal plain,Sultanate of Oman

A detail investigation was carried out to improve the current knowledge of groundwater salinisation processes in coastal aquifers using hydrochemical and isotopic parameters. Data of major ions for 40 wells located in the Salalah plain aquifer, Sultanate of Oman, were collected during pre-monsoon 2004 and analysed. The groundwater changes along the general flow path towards the coast from fresh (EC < 1500 μS/cm), brackish (EC: 1500–3000 μS/cm) and saline (EC > 3000 μS/cm). Results of inverse modeling simulations using PHREEQC show that dissolution of halite may be the main source of Cl and Na in the study area. Ionic delta calculation indicates that the depletion of Na and K and enrichment of Ca and Mg in groundwater were probably attributed to reverse ion exchange reactions. During a sampling campaign conducted in October 2015, 11 groundwater samples were collected for Cl, Br and isotopic analysis (²H/¹⁸O). Molar Cl/Br ratios in fresh groundwater were higher than those of seawater, indicating the impact of halite dissolution on the groundwater quality. For saline groundwater, these ratios were less than those of seawater, showing the influence of anthropogenic input from agriculture on the same. Relatively depleted isotopic signature of all groundwater samples show that the monsoon precipitation is the main source of groundwater recharge in the study area.     

Statistical categorization geochemical modeling of groundwater in Ain Azel plain (Algeria)

Water analysis data of 54 groundwater samples from 18 uniformly distributed wells were collected during three campaigns (June, September and December 2004). Q-mode hierarchical cluster analysis (HCA) was employed for partitioning the water samples into hydrochemical facies. Interpretation of analytical data showed that the abundance of major ions was identified as follows: Ca ? Mg > Na > K and HCO₃ ? Cl > SO₄. Three major water facies are suggested by the HCA analysis. The samples from the area were classified as recharge area waters (Ca–Mg–HCO₃ water), transition area waters (Mg–Ca–HCO₃–Cl water), and discharge area waters (Mg–Ca–Cl–HCO₃ water). Inverse geochemical modeling suggests that relatively few phases are required to derive the water chemistry in the area. In a broad sense, the reactions responsible for the hydrochemical evolution in the area fall into two categories: (1) evaporite weathering reactions and (2) precipitation of carbonate minerals.     

The utilization of boron and strontium isotopes for the assessment of boron contamination of the Cecina River alluvial aquifer (central-western Tuscany,Italy)

The groundwater B concentration in the alluvial aquifer of the upper Cecina River basin in Tuscany, Italy, often exceeds the limit of 1 mg L⁻¹ set by the European Union for drinking water. On the basis of hydrogeological and geochemical observations, the main source of the B contamination of groundwater has been attributed to past releases into streams of exhausted, B-rich geothermal waters and/or mud derived from boric acid manufacturing in Larderello. The releases were discontinued 25–30 years ago.This study confirms that the B dissolved in groundwater is anthropogenic. In fact, the δ¹¹B values of groundwater B match the range −12.2‰ to −13.3‰ of the Turkish B mineral (colemanite) processed in boric acid manufacturing, in the course of which no significant isotopic effects have been observed. This isotopic tracing of the Cecina alluvial aquifer occurs just below the confluence of the Possera Creek, which carries the B releases from Larderello. Strontium isotope ratios support this conclusion.At about 18 km from the Possera Creek confluence, the groundwater δ¹¹B drops to much more negative values (−22‰ to −27‰), which are believed to be produced by adsorption–desorption interactions between dissolved B and the aquifer matrix. The δ¹¹B of B fixed in well bottom sediments shows a similar variation. At present, desorption is prevailing over adsorption because the releases of B-rich water have ceased. A theoretical model is suggested to explain the isotopic trends observed.Thus, B isotopes appear to be a powerful tool for identifying the origin of B contamination in natural waters, although isotopic effects associated with adsorption–desorption processes may complicate the picture, to some extent.     

Geochemical evaluation of flowback brine from Marcellus gas wells in Pennsylvania,USA

Large quantities of highly saline brine flow from gas wells in the Marcellus Formation after hydraulic stimulation (“fracking”). This study assesses the composition of these flowback waters from the Marcellus shale in Pennsylvania, USA. Concentrations of most inorganic components of flowback water (Cl, Br, Na, K, Ca, Mg, Sr, Ba, Ra, Fe, Mn, total dissolved solids, and others) increase with time from a well after hydraulic stimulation. Based on results in several datasets reported here, the greatest concentration of Cl⁻ in flowback water is 151,000 mg/L. For total Ra (combined ²²⁶Ra and ²²⁸Ra) in flowback, the highest level reported is 6540 pCi/L. Flowback waters from hydraulic fracturing of Marcellus wells resemble brines produced from conventional gas wells that tap into other Paleozoic formations in the region. The Br/Cl ratio and other parameters indicate that both types of brine formed by the evaporation of seawater followed by dolomitization, sulfate reduction and subsurface mixing with seawater and/or freshwater. Trends and relationships in brine composition indicate that (1) increased salt concentration in flowback is not mainly caused by dissolution of salt or other minerals in rock units, (2) the flowback waters represent a mixture of injection waters with highly concentrated in situ brines similar to those in the other formations, and (3) these waters contain concentrations of Ra and Ba that are commonly hundreds of times the US drinking water standards.     

Monitoring of fluid–rock interaction and CO2 storage through produced fluid sampling at the Weyburn CO2-injection enhanced oil recovery site,Saskatchewan, Canada

The Weyburn Oil Field is a carbonate reservoir in south central Saskatchewan, Canada and is the site of a large CO₂ injection project for purposes of enhanced oil recovery. The Weyburn Field, in the Mississippian Midale Formation, was discovered in 1954 and was under primary production until secondary recovery by water flood began in 1964. The reservoir comprises two units, the Vuggy and the Marly, and primary and secondary recovery are thought to only have significantly depleted the Vuggy zone, leaving the Marly with higher oil saturations. In 2000, PanCanadian Resources (now EnCana), the operator of the field, began tertiary recovery by injection of CO₂ and water, primarily into the Marly. The advent of this project was an opportunity to study the potential for geological storage of CO₂.Using 43 Baseline samples collected in August 2000, before CO₂ injection at Weyburn, and 44 monitoring samples collected in March 2001, changes in the fluid chemistry and isotope composition have been tracked. The initial fluid distribution showed water from discovery through water flood in the Midale Formation with Cl ranging from 25,000 to 60,000 mg/L, from the NW to the SE across the Phase 1A area. By the time of Baseline sampling the produced water had been diluted to Cl of 25,000–50,000 mg/L as a result of the addition of make up water from the low TDS Blairmore Formation, but the pattern of distribution was still present. The Cl distribution is mimicked by the distribution of other dissolved ions and variables, with Ca (1250–1500 mg/L) and NH_{3 (aq)} increasing from NW to SE, and alkalinity (700–300 mg/L), resistivity, and H₂S (300–100 mg/L) decreasing. Based on chemical and isotopic data, the H₂S is interpreted to result from bacterial SO₄ reduction. After 6 months of injection of CO₂, the general patterns are changed very little, except that the pH has decreased by 0.5 units and alkalinity has increased, with values over 1400 mg/L in the NW, decreasing to 500 mg/L in the SE. Calcium has increased to range from 1250 to 1750 mg/L, but the pattern of NW–SE distribution is altered. Chemical and isotopic data suggest this change in distribution is caused by the dissolution of calcite due to water–rock reactions driven by CO₂. The Baseline samples varied from −22 to −12‰ δ¹³C (V-PDB) for CO₂ gas. The injected CO₂ has an isotope ratio of −20‰. The Monitor-1 samples of produced CO₂ ranged from −18 to −13‰, requiring a heavy source of C, most easily attributed to dissolution of carbonate minerals. Field measured pH had increased and alkalinity had decreased by the second monitoring trip (July 2001) to near Baseline values, suggesting continued reaction with reservoir minerals.Addition of CO₂ to water–rock mixtures comprising carbonate minerals causes dissolution of carbonates and production of alkalinity. Geochemical modeling suggests dissolution is taking place, however more detail on water–oil–gas ratios needs to be gathered to obtain more accurate estimates of pH at the formation level. Geological storage of CO₂ relies on the potential that, over the longer term, silicate minerals will buffer the pH, causing any added CO₂ to be precipitated as calcite. Some initial modeling of water–rock reactions suggests that silica sources are available to the water resident in the Midale Formation, and that clay minerals may well be capable of acting as pH buffers, allowing injected CO₂ to be stored as carbonate minerals. Further work is underway to document the mineralogy of the Midale Formation and associated units so as to define more accurately the potential for geological storage.     

Dawsonite formation in the Beier Sag,Hailar Basin,NE China tuff: A natural analog for mineral carbon storage

We determined the rock types, the authigenic minerals, the paragenetic sequence, and the origin of dawsonite in pyroclastic rocks from the Yimin Formation of Beier Sag in the Hailar Basin, China. Dawsonite, a diagenetic mineral, is thought to result from a large influx of CO₂ and, therefore, this system represents a natural analogue for in-situ mineral carbon storage. The studied host rocks are mainly tuffs/tuffaceous sandstones which now contain up to 70 vol% authigenic carbonates, including dawsonite, ankerite, and siderite. The initial alteration of the tuffs yielded minor siderite. Kaolinite, illite and mixed illite/smectite then formed as product phases. Dawsonite and quartz subsequently precipitated in response to CO₂ influx apparently coupled to feldspar and perhaps kaolinte dissolution. Dawsonite reaches a maximum 25 vol% of the bulk rock. Mass balance suggests that this CO₂ influx was coupled to the external import of sodium and export of SiO₂. Ankerite and additional siderite precipitated during the late-stage alkaline diagenesis. The carbon isotope values of the dawsonite are in the range −4.1‰ to −2.2‰, indicating the magmatic origin of the CO₂. Vitrinite reflectance and thermal gradient constraints suggest that the dawsonite at this location formed at a temperature of ∼75 °C.     

Monitoring groundwater flow and chemical and isotopic composition at a demonstration site for carbon dioxide storage in a depleted natural gas reservoir

Between March 2008 and August 2009, 65,445 tonnes of ∼75 mol% CO₂ gas were injected in a depleted natural gas reservoir approximately 2000 m below surface at the Otway project site in Victoria, Australia. Groundwater flow and composition were monitored biannually in two overlying aquifers between June 2006 and March 2011, spanning the pre-, syn- and post-injection periods. The shallower (∼0–100 m), unconfined, porous and karstic aquifer of the Port Campbell Limestone and the deeper (∼600–900 m), confined and porous aquifer of the Dilwyn Formation contain valuable fresh to brackish water resources. Groundwater levels in either aquifer have not been affected by the drilling, pumping and injection activities that were taking place, or by the rainfall increase during the project. In terms of groundwater composition, the Port Campbell Limestone groundwater is brackish (electrical conductivity = 801–3900 μS cm⁻¹), cool (temperature = 12.9–22.5 °C), and near-neutral (pH = 6.62–7.45), whilst the Dilwyn Aquifer groundwater is fresher (electrical conductivity = 505–1473 μS cm⁻¹), warmer (temperature = 42.5–48.5 °C), and more alkaline (pH = 7.43–9.35). Carbonate dissolution, evapotranspiration and cation exchange control the composition of the groundwaters. Comparing the chemical and isotopic composition of the groundwaters collected before, during and after injection shows no statistically significant changes; even if they were statistically significant, they are mostly not consistent with those expected if CO₂ addition had taken place. The monitoring program reveals no impact on the groundwater resources attributable to the C storage demonstration project.     

Mixing geochemistry of cold water around non-volcanic thermal springs in high-grade metamorphic terrain,Sri Lanka

A geochemical survey was conducted on thermal water and cold water around non-volcanic geothermal fields at Mahaoya and Marangala in Sri Lanka. One hundred forty-two samples were analyzed for fifteen selected irons to investigate geochemical relationships resulting from water-rock interactions and mixing. Based on measurements using a Na-K-Mg geothermometer, the maximum temperatures of thermal reservoirs were estimated to be 148 °C in Mahaoya and 191 °C in Marangala, which were higher values than those obtained using Na-Li and Li-Mg geothermometers. This suggests that the reservoirs extend from intermediate to deep levels. Hydrogeochemistry of thermal waters is distinct from that of cold water; higher contents of Na, K, Cl, F, SO_4, and TDS in thermal water are most likely due to the dissolution of feldspar, mica, and sulfide minerals in the granitic rocks. Conversely, lower values of Fe, Mg, Cu, Zn, and Pb imply less ferromagnesian minerals in the basement. Classification based on major ions reveals a Na-K-SO₄ type of thermal water for Mahaoya and Marangala. Cold water is dominated by the Na-K-HCO₃ type, which indicates deep groundwater influence by iron exchange. Non-mixing cold water indicates a Ca-HCO₃ type. In general, chemistry of cold water wells (<400 m) close to the thermal water changes significantly due to direct mixing of thermal water and cold water. In comparison, a contrasting action occurs with increasing distance from the geothermal field. Gradual decline of SO₄ with increasing distance from thermal water may indicate a trend of clear oxidation. However, the chemistry of more distant wells demarcates deep circulations through fractures and faults in the basement.     

Ferric iron in sediments as a novel CO2 mineral trap: CO2–SO2 reaction with hematite

Thermodynamic simulations of reactions among SO₂-bearing CO₂-dominated gas, water and mineral phases predict that Fe^III in sediments should be converted almost entirely to dissolved Fe^II and siderite (FeCO₃), and that SO₂ should simultaneously be oxidized to dissolved sulfate. The reactions are however, subject to kinetic constraints which may result in deviation from equilibrium and the precipitation of other metastable mineral phases. To test the prediction, a laboratory experiment was carried out in a well stirred hydrothermal reactor at 150 °C and 300 bar with hematite, 1.0 m NaCl, 0.5 m NaOH, SO₂ in quantity sufficient to reduce much of the iron, and excess CO₂. The experiment produced stable siderite and metastable pyrite and elemental S. Changes in total dissolved Fe are consistent with nucleation of pyrite at ∼17 h, and nucleation of siderite at ∼600 h. Dissolution features present on elemental S at the conclusion of the experiment suggest nucleation early in the experiment. The experiment did not reach equilibrium after ∼1400 h, as indicated by coexistence of hematite with metastable pyrite and elemental sulfur. However, the results confirm that Fe^III can be used to trap CO₂ in siderite if partly oxidized S, as SO₂, is present to reduce the Fe with CO₂ in the gas phase.     

The impact of severe biodegradation on the molecular and stable (C,H, N,S) isotopic compositions of oils in the Alberta Basin,Canada

Although the effects of biodegradation on the composition and physical properties of crude oil have been well studied, effects of in-reservoir petroleum biodegradation on molecular and isotopic compositions of crude oils are not yet clearly understood. The Alberta Basin, in western Canada, is one of the world’s largest petroleum accumulations and constitutes an ideal example of a natural suite of sequentially biodegraded oils. The basin hosts moderately to severely biodegraded petroleum, regionally distributed and in single, more or less continuous, oil columns. In this study, a series of oil samples from the Alberta heavy oil and oil sands provinces, with varying degrees of biodegradation, were analyzed to assess the impact of progressive biodegradation on the molecular and C, H, N, and S isotopic compositions of oils. The results of the molecular characterization of the hydrocarbon fraction of the studied oils show that the oils have suffered biodegradation levels from 2 to 10⁺ (toward the Alberta–Saskatchewan border) on the Peters and Moldowan scale of biodegradation (abbreviated PM 2 to PM 10) and from tens to hundreds on the Manco scale. Within single reservoirs, increasing biodegradation was observed from top to bottom of the oil columns at all sites studied. The whole oil stable isotopic compositions of the samples varied in the ranges δ¹³C = −31.2‰ to −29.0‰, δ²H = −147‰ to −133‰, δ¹⁵N = 0.3–4.7‰ and δ³⁴S = 0.4–6.4‰. The maximum differences between δ values of samples (Δ) within single oil columns were Δ¹³C = 1.4‰, Δ²H = 7‰, Δ¹⁵N = 1.7‰ and Δ³⁴S = 1.0‰. Regional variations in the isotopic compositions of oil samples from different wells (averaged values from top to bottom) were 1.2‰ for δ¹³C, 12‰ for δ²H, 4.1‰ for δ¹⁵N and 5.5‰ for δ³⁴S and hence generally significantly larger variations were seen than variations observed within single oil columns, especially for N and S. It appears that even severe levels of biodegradation do not cause observable systematic variations in carbon, nitrogen or sulfur isotope composition of whole oils. This indicates that sulfur and nitrogen isotopic compositions may be used in very degraded oils as indicators for oil charge from different source rock facies.     

An experimental comparison of gas generation from three oil fractions: Implications for the chemical and stable carbon isotopic signatures of oil cracking gas

Although oil cracking has been documented as one of the important sources of gas in many overmature marine sedimentary basins, the chemical and carbon isotopic signatures of gases of this origin are still open to question. In this study a Cambrian crude oil from the central Tarim basin, along with its main separated fractions (saturates, aromatics and asphaltenes), were pyrolyzed in sealed gold tubes to investigate how generated gases vary in chemical and carbon isotopic composition and how this variation would influence the genetic interpretation of oil cracking gas. The results indicate that the gases from cracking of aromatics and asphaltenes are much drier and more enriched in ¹³C than the gases from the cracking of saturates and crude oil at the same level of thermal maturity. In the experimental run of 20 °C/h, the dryness index of the gases (defined as the volume percentage of C₁ in C_1–5) from the cracking of saturates ranges from 26.2–90.6% with the methane carbon isotope change ranging from −54.8‰ to −35.5‰, whereas the dryness index is never lower than 60.6% for the gases from the cracking of aromatics with methane carbon isotope ranging from −39.9‰ to −32.2‰. Correspondingly, experimental data for the four samples plot in different areas in diagrams designed to distinguish oil cracking gas from kerogen cracking gas, such as ln(C₂/C₃) vs. δ¹³C₂–δ¹³C₃ and δ¹³C₁ vs. δ¹³C₂–δ¹³C₃, indicating compositional variability of crude oil could assert an important influence in these diagrams. Therefore it is prudent to bring other geological constraints into consideration to avoid misinterpretation.The kinetic parameters for the bulk generation of C_1–5 gas and the methane carbon isotope fractionation extrapolated to geological conditions of 2 °C/Ma and an initial temperature of 50 °C show that the temperatures of C_1–5 gas generation from the aromatics and asphaltenes are lower than those from the saturates and crude oil due to their lower activation energies and frequency factors. Generation of C_1–5 gases from the aromatics is modeled to be initiated about 122 °C whereas the initiation temperature for the saturates sample is 176 °C. Below 189 °C (EasyRo = 1.8%), the yields of C_1–5 gases follow the order: aromatics > asphaltenes > crude oil > saturates. At similar thermal maturity levels, the methane carbon isotopic compositions are significantly different for the four samples, with an order of ¹³C enrichment: aromatics > asphaltenes > crude oil > saturates, however the difference in methane carbon isotopes becomes smaller with increasing temperature. This indicates that methane carbon isotopic values can be significantly different for gases cracked from oils that are compositionally diverse, especially in the early stage of methane generation.     

Chloride imbalance in a catchment undergoing hydrological change: Upper Barwon River,southeast Australia

Documenting whether surface water catchments are in net chemical mass balance is important to understanding hydrological systems. Catchments that export significantly greater volumes of solutes than are delivered via rainfall are not in hydrologic equilibrium and indicate a changing hydrological system. Here an assessment is made of whether a saline catchment in southeast Australia is in chemical mass balance based on Cl. The upper reaches of the Barwon River, southeast Australia, has total dissolved solids, TDS, concentrations of up to 5860 mg/L and Cl concentrations of up to 3370 mg/L. The high river TDS concentrations are due to the influxes of groundwater with TDS concentrations of up to 68,000 mg/L. Between 1989 and 2011, the median annual Cl flux from the upper Barwon catchment was 17.8 × 10⁶ kg (∼140 kg/a/ha). This represents 340–2230% of the annual Cl input by rainfall to the catchment. Major ion and stable isotope geochemistry indicate that the dominant source of solutes in the catchment is evapotranspiration of rainfall, precluding mineral dissolution as a source of excess Cl. The upper Barwon catchment is not in chemical mass balance and is a net exporter of solutes. The chemical imbalance may reflect the transition within the last 100 ka from an endorheic lake system where solutes were recycled producing shallow groundwater with high TDS concentrations to a better drained catchment. Alternatively, a rise in the regional water table following land clearing may have increased the input of groundwater with high TDS concentrations to the river system.     

Carbon isotope stratigraphy using carbonate cements in the Triassic Sherwood Sandstone Group: Corrib Field,west of Ireland

Carbon stable isotopes from carbonate minerals (mainly dolomite) from six wells from the Lower Triassic Sherwood Sandstones of the Corrib Gas Field, Slyne Basin, west of Ireland, allow stratigraphic correlation. The results also provide information on palaeoenvironmental change during the deposition of these continental redbed sedimentary rocks. The Triassic reservoir rocks have been buried to > 4000 m and heated to > 165 °C and now contain methane-rich gas. Although the oxygen isotopic signal has been at least partially reset during burial and heating, a primary carbon isotopic signal appears to have survived diagenesis. The carbon isotope ratio varies from − 3.2‰ to + 2.1‰. All six wells show similar stratigraphic changes when all the carbon isotope data are plotted relative to a major playa horizon. δ¹³C increases from about − 3‰ at the base of the Sherwood to about + 2‰ 170 m above the base. δ¹³C then decreases to about − 2‰ for the next 70 m and remains steady for the following 50 m. The top 20 m of the Sherwood contains carbonate with a δ¹³C values decreasing to about − 3‰. The occurrence of a stratigraphically-correlatable carbon isotope pattern implies that the primary evolution signal has been preserved. The change in δ¹³C correlates with indicators of aridity and biological stress such that the highest δ¹³C values are in sedimentary rocks deposited in a playa lake (arid times); these rocks contain the greatest quantity of dolomite cement. Conversely, the lowest δ¹³C values correspond to sedimentary rocks deposited from well-developed rivers (relatively humid times) from the lowest quantity of dolomite cement. The same carbon isotope evolution has been found in another well in the Slyne basin and in Belgium, suggesting that the palaeoenvironmental isotope signal in the Triassic sedimentary rocks of the Corrib Field may have a regional significance.     

Adsorption and heterogeneous oxidation of arsenite on modified granular natural siderite: Characterization and behaviors

Although natural siderite has been investigated to remove both As(III) and As(V), it has relatively low adsorption rate and capacity. It is crucial to enhance its adsorption characteristics for As removal prior to being used in practical application. Modified granular natural siderite (MGNS) was fabricated through addition of organic binder, extrusion granulation and calcination, and evaluated for adsorption characteristics by means of batch and column tests. Results showed that MGNS had higher adsorption rate and capacity for As(III) in comparison with natural siderite. Arsenic(III) adsorption achieved equilibrium at 24 h, with adsorption capacity of 9.43 mg/g estimated from Langmuir isotherm at 25 °C. Column tests showed that there was less difference in total As loads in MGNS-packed filters for As(III)-spiked deionized water, As(III)-spiked tap water, and real-world high-As groundwater. The coexistence of anions had no significant effect on As adsorption in both batch and column experiments. Results of XRD, SEM and BET analysis indicated that MGNS, as an Fe(II)/(III) hybrid system, had a much larger specific surface area relative to the pristine natural siderite due to massive spherical aggregates attaching to the siderite matrix. XANES spectra showed that As(V) was the major species in the adsorbent after As(III) adsorption. Its proportion in total As slightly increased with the increase in contact time. Adsorption and heterogeneous oxidation of As(III) were believed to be the main mechanisms of As(III) removal by MGNS. This study suggested that MGNS is a potential adsorbent for effectively removing As from As-contaminated groundwater in filter application.     

A conceptual framework of groundwater flow in some crystalline aquifers in Southeastern Ghana

A conceptual groundwater flow model was developed for the crystalline aquifers in southeastern part of the Eastern region, Ghana. The objective was to determine approximate levels of groundwater recharge, estimate aquifer hydraulic parameters, and then test various scenarios of groundwater extraction under the current conditions of recharge. A steady state groundwater flow model has been calibrated against measured water levels of 19 wells in the area. The resulting recharge is estimated to range from 8.97 × 10^?5 m/d to 7.14 × 10^?4 m/d resulting in a basin wide average recharge of about 9.6% of total annual precipitation, which results in a basin wide quantitative recharge of about 2.4 million m³/d in the area. This compares to recharge estimated from the chloride mass balance of 7.6% of precipitation determined in this study. The general groundwater flow in the area has also been determined to conform to the general northeast–southwest structural grain of the country. The implication is that the general hydrogeology is controlled by post genetic structural entities imposed on the rocks to create ingresses for sufficient groundwater storage and transport. Calibrated aquifer hydraulic conductivities range between 0.99 m/d and over 19.4 m/d. There is a significant contribution of groundwater discharge to stream flow in the study area. Increasing groundwater extraction will have an effect on stream flow. This study finds that the current groundwater extraction levels represent only 0.17% of the annual recharge from precipitation, and that groundwater can sustain future increased groundwater demands from population growth and industrialization.     

Carbonate dissolution in Mesozoic sand- and claystones as a response to CO2 exposure at 70 °C and 20 MPa

The response to CO₂ exposure of a variety of carbonate cemented rocks has been investigated using pressurised batch experiments conducted under simulated reservoir conditions, 70 °C and 20 MPa, and with a durations of up to14 months. Calcite, dolomite, ankerite and siderite cement were present in the unreacted reservoir rocks and caprocks. Core plugs of the reservoir rocks were used in order to investigate the alterations in situ. Crushing of the caprock samples was necessary to maximise reactions within the relatively short duration of the laboratory experiments. Synthetic brines were constructed for each batch experiment to match the specific formation water composition known from the reservoir and caprock formations in each well. Chemical matched synthetic brines proved crucial in order to avoid reactions due to non-equilibra of the fluids with the rock samples, for example observations of the dissolution of anhydrite, which were not associated with the CO₂ injection, but rather caused by mismatched brines.Carbonate dissolution as a response to CO₂ injection was confirmed in all batch experiments by both petrographical observations and geochemical changes in the brines. Increased Ca and Mg concentrations after 1 month reaction with CO₂ and crushed caprocks are ascribed to calcite and dolomite dissolution, respectively, though not verified petrographically. Ankerite and possible siderite dissolution in the sandstone plugs are observed petrographically after 7 months reaction with CO₂; and are accompanied by increased Fe and Mn contents in the reacted fluids. Clear evidence for calcite dissolution in sandstone plugs is observed petrographically after 14 months of reaction with CO₂, and is associated with increased amounts of Ca (and Mg) in the reacted fluid. Dolomite in sandstones shows only minor dissolution features, which are not clearly supported by increased Mg content in the reacted fluid.Silicate dissolution cannot be demonstrated, either by chemical changes in the fluids, as Si and Al concentrations remain below the analytical detection limits, nor by petrographical changes, as partly dissolved feldspar grains and authigenic analcime are present in the sediments prior to the experiments. It is noteworthy, that authigenic K-feldspar and authigenic albite in sandstones show no signs of dissolution and consequently seem to be stable under the experimental conditions.     

High-resolution stable carbon isotope monitoring indicates variable flow dynamic patterns in a deep saline aquifer at the Ketzin pilot site (Germany)

Stable isotopes of injected CO₂ act as useful tracers in carbon capture and storage (CCS) because the CO₂ itself is the carrier of the tracer signal and remains unaffected by sorption or partitioning effects. At the Ketzin pilot site (Germany), carbon stable isotope composition (δ¹³C) of injected CO₂ at the injection well was analyzed over a time period of 4 months. Occurring isotope variances resulted from the injection of CO₂ from two different sources (an oil refinery and a natural gas-reservoir). The two gases differed in their carbon isotope composition by more than 27‰. In order to find identifiable patterns of these variances in the reservoir, more than 250 CO₂-samples were collected and analyzed for their carbon isotope ratios at an observation well 100 m distant from the injection well. An isotope ratio mass spectrometer connected to a modified Thermo Gasbench system allowed quick and cost effective isotope analyses of a high number of CO₂ gas specimens. CO₂ gas from the oil refinery (δ¹³C = −30.9‰, source A) was most frequently injected and dominated the reservoir δ¹³C values at the injection site. Sporadic injection of the CO₂ from the natural gas-reservoir (δ¹³C = −3.5‰, source B) caused isotope shifts of up to +5‰ at the injection well. These variances provided a potential ideal tracer for CO₂ migration behavior. Based on these findings, tracer input signals that were injected during the last 2 years of injection could be reconstructed with the aid of an isotope mixing model and CO₂ delivery schedules. However, in contrast to the injection well, δ¹³C values at the observation well showed no variances and a constant value of −28.5‰ was measured at 600 m depth. This is in disagreement with signals that would be expected if the input signals from the injection would arrive at the observation well. The lack of isotope signals at the observation well suggests that parts of the reservoir are filled with CO₂ that is immobilized.     

Geological,geochronological, and H–O isotopic constraints on the genesis of the Tongjing Cu–Au deposit in the Ningwu basin,east China

The Tongjing Cu–Au deposit is a medium-sized deposit within the Ningwu volcanic basin, east China, and is hosted by Cretaceous volcanic rocks of the Dawangshan and Niangniangshan Formations. The veined and lenticular Cu–Au orebodies are spatially and temporally related to the volcanic and subvolcanic rocks of the Niangniangshan Formation in the ore district. The wall-rock alteration is dominated by silicification, siderite alteration, carbonation, sericitization, chloritization, and kaolinization. On the basis of field evidence and petrographic observations, two stages of mineralization are recognized: (1) a siderite–quartz–sulfide stage (Stage 1) associated with the formation of chalcopyrite and pyrite in a quartz and siderite gangue; and (2) a quartz–bornite stage (Stage 2) cutting the Stage 1 phases. Stage 1 is the main mineralization stage. Quartz that formed in Stage 1 has δ¹⁸O_H2O values of − 4.3‰ to 3.5‰ with δD values of fluid inclusion waters of − 97.1‰ to − 49.9‰, indicating that the ore-forming fluids were derived from early magmatic fluids and may have experienced oxygen isotopic exchange with meteoric water during Stage 1 mineralization.LA–MC–ICP–MS zircon U–Pb dating of the mineralization-related nosean-bearing phonolite and nosean-bearing phonolitic brecciated tuff at Tongjing yields ages of 129.8 ± 0.5 Ma and 128.9 ± 1.1 Ma, respectively. These results are interpreted as the crystallization age of the volcanic rocks of the Niangniangshan Formation. A hydrothermal sericite sample associated with Cu–Au mineralization at Tongjing yields a plateau ⁴⁰Ar–³⁹Ar age of 131.3 ± 1.3 Ma. These results confirm a genetic link between the volcanism and associated Cu–Au mineralization. The Tongjing Cu–Au deposit in the Ningwu basin is genetically and possibly tectonically similar to alkaline intrusion-related gold deposits elsewhere in the world.