Massive dolomitization of a late Miocene carbonate platform: a case of mixed evaporative brines with meteoric water,Nijar, Spain

Late Miocene platform carbonates from Nijar, Spain, have been extensively dolomitized. Limestones are present in the most landward parts of the platform, in stratigraphically lower units and topographically highest outcrops, suggesting that dolomitizing fluids were derived from the adjacent Nijar Basin. The dolomite crystals range from <10 to ≈100 μm existing as both replacements and cements. Na, Cl and SO₄ concentrations in the dolomites range from 200 to 1700 p.p.m., 250–650 p.p.m., and 600–7000 p.p.m., respectively, comparable with other Tertiary and modern brine dolomite values, and also overlapping values from mixing-zone dolomites. Sr concentrations range between 50 and 300 p.p.m., and the molar Sr/Ca ratios of dolomitizing fluids are estimated to range between 7× seawater brine to freshwater ratios. The δ¹⁸O and δ¹³C of the dolomites range from ?1·0 to +4·2‰ PDB, and ?4·0 to +2·0‰ PDB, respectively. ⁸⁷Sr/⁸⁶Sr values (0·70899–0·70928) of the dolomites range from late Miocene seawater to values greater than modern seawater. Mixtures of freshwater with seawater and evaporative brines probably precipitated the Nijar dolomites. Modelled covariations of molar Sr/Ca vs. δ¹⁸O and Na/Ca vs. δ¹⁸O from these mixtures are consistent with those of the proposed Nijar dolomitizing fluids. Complete or partial dolomite recrystallization is ruled out by well preserved CL zoning, nonstoichiometry and quantitative water–rock interaction modelling of covariations of Na vs. Sr and δ¹⁸O vs. δ¹³C. The possibility of multiple dolomitization events induced by evaporative brines, seawater and freshwater, respectively, is consistent with mineral-mineral mixing modelling. The basin-derived dolomitizing brines probably mixed with freshwater in the Nijar Basin or mixed with fresh groundwater in the platform, and were genetically related either to deposition of the Yesares gypsum or the Feos gypsum. Dolomitization occurred during either the middle Messinian or the early upper Messinian. Nijar dolomitization models may be applicable to dolomitization of other late Miocene platform carbonates of the western Mediterranean. Moreover, the Nijar models may offer an analogue for more ancient evaporite-absent platform carbonates fringing evaporite basins.     

Chemical evolution of a gas-capped deep aquifer,southwest of Iran

The Kangan Aquifer (KA) is located below a gas reservoir in the crest of the Kangan Anticline, southwest of Iran. This aquifer is composed of Permo-Triassic limestone, dolomite, sandstone, anhydrite and shale. It is characterized by a total dissolved solid of about 332,000 mg/L and Na–Ca–Cl-type water. A previous study showed that the source of the KA waters is evaporated seawater. Chemical evolution of the KA is the main objective of this study. The major, minor and trace element concentrations of the KA waters were measured. The chemical evolution of KA waters occurred by three different processes: evaporation of seawater, water–rock and water–gas interactions. Due to the seawater evaporation process, the concentration of all ions in the KA waters increased up to saturation levels. In comparison to the evaporated seawater, the higher concentrations of Ca, Li, Sr, I, Mn and B and lower concentrations of Mg, SO₄ and Na and no changes in concentrations of Cl and K ions are observed in the KA waters. Based on the chemical evolution after seawater evaporation, the KA waters are classified into four groups: (1) no evolution (Cl, K ions), (2) water–rock interaction (Na, Ca, Mg, Li and Sr ions), (3) water–gas interaction (SO₄ and I ions) and (4) both water–rock and water–gas interactions (Mn and B ions). The chemical evolution processes of the KA waters include dolomitization, precipitation, ion exchange and recrystallization in water–rock interaction. Bacterial reduction and diagenesis of organic material in water–gas interaction also occur. A new type of chart, Ca_excess versus Mg_deficit, is proposed to evaluate the dolomitization process.     

Geochemistry of deep formation waters in the Canning Basin,Western Australia,and their relationship to Zn‐Pb mineralization

The Canning Basin contains several Mississippi Valley‐type Zn‐Pb sulphide prospects and deposits in Devonian carbonate reef complexes on the northern edge of the Fitzroy Trough, and in Ordovician and Silurian marine sequences on the northern margin of the Willara Sub‐basin. This study uses the ionic composition and 5D, δ¹⁸O, δ³⁴S, ⁸⁷Sr/⁸⁶Sr isotopic data on present‐day deep formation waters to determine their origin and possible relationship to the Zn‐Pb mineralizing palaeofluids.

The present‐day Canning Basin formation waters have salinity ranging from typically less than 5000 mg/L up to 250 000 mg/L locally. The brines are mixtures of highly saline water, formed by seawater which evaporated beyond halite saturation (bittern water), with meteoric water ranging in salinity from low (<5000 mg/L) to hypersaline water (up to about 50 000 mg/L) formed by re‐solution of halite and calcium sulphate minerals. The original marine chemical composition of the bittern‐dominated brines was changed to that of a Na‐Ca‐Cl water by addition of Ca and removal of Mg and SO₄, initially by bacterial sulphate reduction and later by dolomitization of carbonate. Other reactions with terrigenous components of the sediment have provided additional Ca and Sr, including a small proportion of ⁸⁷Sr‐rich material. The δ³⁴S values of the bittern‐containing waters are within the range over which marine sulphate has fluctuated from the Ordovician to the Holocene, although one of the hypersaline waters has a value of +6.8%, indicating SO₄ of non‐marine origin. The pH of the bittern‐containing waters is low (about 5) and they contain significant concentrations of dissolved Fe (up to 120 mg/L).

The Canning Basin bitterns appear similar in origin and chemical composition to highly saline marine brines in the Mississippi Salt Dome Basin, USA, which are known to be either metal or sulphide‐rich depending on the organic content of the host rock. In the Canning Basin, mixing of the bittern water with the various types of meteoric water has resulted in decreases in salinity, Na, Ca, Mg, K, Sr, Li and Fe, and increases in HCO₃, SO₄ and pH.

Mixing of the bitterns with other types of metalliferous fluids and/or with sulphate‐containing hypersaline meteoric waters formed from the same marine evaporite sequence should produce ore‐precipitating fluids which are relatively hot and saline, and the resulting ore deposit should be of high grade and contain abundant sulphate minerals. In the southern Canning Basin, this type of mixing and the corresponding style of ore deposit is evident in the evaporite‐associated areas of Zn‐Pb mineralization near the Admiral Bay Fault. If the bitterns mix with low salinity HCO₃‐waters in near‐surface environments, then the ore‐precipitating fluids should have relatively low salinities and carbonate minerals would precipitate during later stages of mixing. In the Lennard Shelf, the present‐day formation waters, the style of the Zn‐Pb deposits, and range of salinity and temperature of the ore‐forming palaeofluids are consistent with this type of mixing.     

Provenance, Tectonic Setting and Geochemistry of Ahwaz Sandstone Member (Asmari Formation, Oligo-Miocene), Marun Oilfield, Zagros Basin, SW Iran

The Asmari Formation deposited in the Zagros foreland basin during the OligoceneMiocene. Lithologically, the Asmari Formation consists of limestone, dolomitic limestone, dolomite, argillaceous limestone, some anhydrite(Kalhur Member) and sandstones(Ahwaz Member). This study is based on the analysis of core samples from four subsurface sections(wells Mn-68, Mn-281, Mn-292 and Mn-312) in the Marun Oilfield in the Dezful embayment subzone in order to infer their provenance and tectonic setting of the Ahwaz Sandstone Member. Petrographical data reveal that the Ahwaz Sandstone comprises 97.5% quartz, 1.6% feldspar, and 0.9% rock fragments and all samples are classified as quartz arenites. The provenance and tectonic setting of the Ahwaz Sandstone have been assessed using integrated petrographic and geochemical studies. Petrographic analysis reveals that mono- and poly-crystalline quartz grains from metamorphic and igneous rocks of a craton interior setting were the dominant sources. Chemically, major and trace element concentrations in the rocks of the Ahwaz Sandstone indicate deposition in a passive continental margin setting. As indicated by the CIW′ index(chemical index of weathering) of the Ahwaz Sandstone(average value of 82) their source area underwent "intense" recycling but "moderate to high" degree of chemical weathering. The petrography and geochemistry results are consistent with a tropical, humid climate and low-relief highlands.     

Origin of quartz geodes from Laño and Tubilla del Agua sections (middle–upper Campanian,Basque‐Cantabrian Basin,northern Spain): isotopic differences during diagenetic processes

Quartz geodes and nodular chert have been found within middle–upper Campanian carbonate sediments from the Laño and Tubilla del Agua sections of the Basque‐Cantabrian Basin, northern Spain. The morphology of geodes together with the presence of anhydrite laths included in megaquartz crystals and spherulitic fibrous quartz (quartzine‐lutecite), suggest an origin from previous anhydrite nodules. The anhydrite nodules at Laño were produced by the percolation of marine brines, during a period corresponding to a sedimentary gap, with δ³⁴S and δ¹⁸O mean values of 18.8‰ and 13.6‰ respectively, consistent with Upper Cretaceous seawater sulphate values. Higher δ³⁴S and δ¹⁸O mean values of 21.2‰ and 21.8‰ recorded in the Tubilla del Agua section are interpreted as being due to a partial bacterial sulphate reduction process in a more restricted marine environment. The idea that sulphates may have originated from the leaching of previously deposited Keuper sulphate evaporites with subsequent precipitation as anhydrite, is rejected because the δ³⁴S, δ¹⁸O and ⁸⁷Sr/⁸⁶Sr values of anhydrite laths observed at both the Tubilla del Agua and Laño sections suggest an origin from younger marine brines. Later calcite replacement and precipitation of geode‐filling calcite is recorded in both sections, with δ¹³C and δ¹⁸O values indicating the participation of meteoric waters. Synsedimentary activity of the Peñacerrada diapir, which lies close to the Laño section, played a significant role in the local shallowing of the basin and the formation of quartz geodes. In contrast, eustatic shallowing of the inner marine series of the Tubilla del Agua section led to the generation of morphologically similar quartz geodes. Copyright © 2002 John Wiley & Sons, Ltd.     

Evolution of saline waters and brines in the Benue-Trough,Nigeria

Hydrogeochemical assessment of 40 saline waters and brines from 20 locations within the lower (southern) and middle regions of the Benue-Trough, Nigeria are presented and discussed in terms of genesis of the primary salinity and subsequent hydrochemical evolution. The total dissolved ions range from 5263 to 88,800 mg/L and 5148 to 47,145 mg/L in the lower and middle region, respectively.The saline waters and brines are characteristically Na–Cl type enriched in Ca and Sr on the one hand and depleted in Mg and SO₄ on the other, relative to the seawater evaporation trend. Ionic ratios, Na–Cl–Br systematic and divalent cations suggest two likely sources of primary salinity: a fossil seawater source and dissolution of halite. However, water–rock interaction involving Mg uptake by clay minerals and possibly dolomitization during diagenesis appear to be responsible for further modification of the primary chemistry. A conceptualized hydrogeological/flow model for the brines is presented.     

Evaluating new fault-controlled hydrothermal dolomitization models: Insights from the Cambrian Dolomite,Western Canadian Sedimentary Basin

Fault-controlled hydrothermal dolomitization in tectonically complex basins can occur at any depth and from different fluid compositions, including ‘deep-seated’, ‘crustal’ or ‘basinal’ brines. Nevertheless, many studies have failed to identify the actual source of these fluids, resulting in a gap in our knowledge on the likely source of magnesium of hydrothermal dolomitization. With development of new concepts in hydrothermal dolomitization, the study aims in particular to test the hypothesis that dolomitizing fluids were sourced from either seawater, ultramafic carbonation or a mixture between the two by utilizing the Cambrian Mount Whyte Formation as an example. Here, the large-scale dolostone bodies are fabric-destructive with a range of crystal fabrics, including euhedral replacement (RD1) and anhedral replacement (RD2). Since dolomite is cross-cut by low amplitude stylolites, dolomitization is interpreted to have occurred shortly after deposition, at a very shallow depth (<1 km). At this time, there would have been sufficient porosity in the mudstones for extensive dolomitization to occur, and the necessary high heat flows and faulting associated with Cambrian rifting to transfer hot brines into the near surface. While the δ¹⁸O_water and ⁸⁷Sr/⁸⁶Sr ratios values of RD1 are comparable with Cambrian seawater, RD2 shows higher values in both parameters. Therefore, although aspects of the fluid geochemistry are consistent with dolomitization from seawater, very high fluid temperature and salinity could be suggestive of mixing with another, hydrothermal fluid. The very hot temperature, positive Eu anomaly, enriched metal concentrations, and cogenetic relation with quartz could indicate that hot brines were at least partially sourced from ultramafic rocks, potentially as a result of interaction between the underlying Proterozoic serpentinites and CO₂-rich fluids. This study highlights that large-scale hydrothermal dolostone bodies can form at shallow burial depths via mixing during fluid pulses, providing a potential explanation for the mass balance problem often associated with their genesis.     

Formation of the Dongmozhazhua Pb–Zn Deposit in the Thrust‐Fold Setting of the Tibetan Plateau,China: Evidence from Fluid Inclusion and Stable Isotope Data

The Dongmozhazhua deposit, the largest Pb–Zn deposit in south Qinghai, China, is stratabound, carbonate‐hosted and associated with epigenetic dolomitization and silicification of Lower–Middle Permian—Upper Triassic limestones in the hanging walls of a Cenozoic thrust fault system. The mineralization is localized in a Cenozoic thrust‐folded belt along the northeastern edge of the Tibetan plateau, which was formed due to the India–Asia plate collision during the early Tertiary. The deposit comprises 16 orebodies with variable thicknesses (1.5–26.3 m) and lengths (160–1820 m). The ores occur as dissemination, vein, and breccia cement. The main sulfide assemblage is sphalerite + galena + pyrite + marcasite ± chalcopyrite ± tetrahedrite, and gangue minerals consist mainly of calcite, dolomite, barite, and quartz. Samples of pre‐ to post‐ore stages calcite yielded δ¹³C and δ¹⁸O values that are, respectively, similar to and lower than those yielded by the host limestones, suggesting that the calcite formed from fluids derived from carbonate dissolution. Fluid inclusions in calcite and sphalerite in the polymetallic sulfidization stage mostly comprise liquid and gas phases at room temperature, with moderate homogenization temperatures (100–140°C) and high salinities (21–28 wt% NaCl eq.). Micro‐thermometric fluid inclusion data point to polysaline brines as ore‐forming fluids. The δD and δ¹⁸O values of ore fluids, cation compositions of fluid inclusions, and geological information suggest two main possible fluid sources, namely basinal brines and evaporated seawater. The fluid inclusion data and regional geology suggest that basinal brines derived from Tertiary basins located southeast of the Dongmozhazhua deposit migrated along deep detachment zones of the regional thrust system, leached substantial base metals from country rocks, and finally ascended along thrust faults at Dongmozhazhua. There, the base‐metal‐rich basinal brines mixed with bacterially‐reduced H₂S‐bearing fluids derived from evaporated seawater preserved in the Permo–Triassic carbonate strata. The mixing of the two fluids resulted in Pb–Zn mineralization. The Dongmozhazhua Pb–Zn deposit has many characteristics that are similar to MVT Pb–Zn deposits worldwide.     

Origin of brine in the Kangan gasfield: isotopic and hydrogeochemical approaches

The Kangan Permo-Triassic brine aquifer and the overlying gas reservoir in the southern Iran are located in Kangan and Dalan Formations, consisting dominantly of limestone, dolomite, and to a lesser extent, shale and anhydrite. The gasfield, 2,900 m in depth and is exploited by 36 wells, some of which produce high salinity water. The produced water gradually changed from fresh to saline, causing severe corrosion in the pipelines and well head facilities. The present research aims to identify the origin of this saline water (brine), as a vital step to manage saline water issues. The major and minor ions, as well as δ²H, δ¹⁸O and δ³⁷Cl isotopes were measured in the Kangan aquifer water and/or the saline produced waters. The potential processes causing salinity can be halite dissolution, membrane filtration, and evaporation of water. The potential sources of water may be meteoric, present or paleo-seawater. The Na/Cl and I/Cl ratios versus Cl^? concentration preclude halite dissolution. Concentrations of Cl, Na, and total dissolved solid were compared with Br concentration, indicating that the evaporated ancient seawater trapped in the structure is the cause of salinization. δ¹⁸O isotope enrichment in the Kangan aquifer water is due to both seawater evaporation and interaction with carbonate rocks. The δ³⁷Cl isotope content also supports the idea of evaporated ancient seawater as the origin of salinity. Membrane filtration is rejected as a possible source of salinity based on the hydrochemistry data, the δ¹⁸O value, and incapability of this process to dramatically enhance salinity up to the observed value of 330,000 mg/L. The overlaying impermeable formations, high pressure in the gas reservoir, and the presence of a cap rock above the Kangan gasfield, all prevent the downward flow of meteoric and Persian Gulf waters into the Kangan aquifer. The evaporated ancient seawater is autochthonous, because the Kangan brine aquifer was formed by entrapment of brine seawater during the deposition of carbonates, gypsum, and minor clastic rocks in a lagoon and sabkha environment. The reliability of determining the source of salinity in a deep complicated inaccessible high-pressure aquifer can be improved by combining various methods of hydrochemistry, isotope, hydrodynamics, hydrogeology and geological settings.     

Evolution of isotopic composition and deuterium excess of brines in the Sichuan Basin, China

The isotopic composition and parameters for deuterium excess of brines, which were sampled in the Si-chuan Basin, show obvious regularities of distribution. The brine isotopic composition shows distinct two systems of marine and terrestrial deposits, with the Middle Triassic strata as the boundary. Brine hydrogen isotopic composition of marine deposits is lower while oxygen isotopic composition is higher than that of the SMOW, respectively, indicating that the brines were derived from seawater with different evaporating degrees at different times. From the Sinian strata, up to the Cambrian, Permian Maokou Formation and the Triassic Jialingjiang Formation, the δD values of brines tend to become relatively positive with the strata becoming younger. Brines of terrestrial deposits are considered to have been derived from precipitation and their isotopic composition is close to the globe meteoric water line (GMWL). Brines of transitional deposits between marine and terrestrial ones (the Upper Triassic Xujiahe Formation) have δD and δ18O values falling between the two end members of marine deposit brines and precipitation, indicating that the brines are a mixture of precipitation and vaporing seawater. Water samples from the brine-bearing strata of different ages show various deuterium excesses (d) with an evident decreasing trend as the age of strata gets older and older. Brine-bearing strata of the Triassic Leikoupo-Jialingjiang Formation, the Permian Maokou Formation, the Cambrian and Sinian strata are all carbonate rocks which have experienced intensive water/rock reaction and the deuterium excess essentially changes with time. All brine-bearing-strata surrounding the basin or faults, as well as those brine wells exploited for resources, have been obviously influenced by the precipitation supply. Therefore, the deuterium excesses of their brines have increased to different extents, depending on the amount of involvement of meteoric water. The variation and distribution of d values of the brines from different Triassic strata are related to the embedded depth of the strata. The deuterium excesses of brines become lower with increasing burial depth of the strata.     

四川盆地三叠系飞仙关组气藏储层成岩作用研究拾零

近年来在四川盆地三叠系飞仙关组鲕粒白云岩储层中发现的高含硫天然气三级储量已近1×1012 m3。鲕粒白云岩储层集中分布在含蒸发岩的层序中,储层中有残余石膏、硬石膏及它们的模孔、方解石铸模（假象）等,表明白云石化流体与蒸发海水有关。白云石化由层间古暴露面向下增强的成岩层序说明这种白云石化是成岩早期发生的,并可能有大气降水与蒸发海水混合的影响。白云岩样品各结构组分氧、碳同位素微区测试数据差别显著,这说明采用岩石混合样做研究白云石化成因的地球化学分析可能导致误解。飞仙关组的白云岩中多数保存了原岩结构幻影、结构残余或原岩的全部结构说明白云石化过程是等体积交代。白云岩储层中的孔隙是各种溶蚀孔而非白云石化等摩尔交代的体积收缩孔。飞仙关组高含硫气藏储层中沥青和溶蚀孔的关系表明深埋晚期溶蚀孔最发育。在溶液中碳酸的电离常数大大高于氢硫酸,飞仙关组高含硫气藏天然气组分中的CO2相对于H2S的质量亏损和储层中有富轻碳的高温方解石充填晚期溶孔、裂缝表明储层中的晚期溶解作用是由硫酸盐热还原过程（TSR）中生成的CO2引起的。     

Brines in the Carboniferous Sydney Coalfield,Atlantic Canada

Formation waters within Upper Carboniferous sandstones in the sub-sea Prince and Phalen coal mines, Nova Scotia, originated as residual evaporative fluids, probably during the precipitation of Windsor Group (Lower Carboniferous) salts which underlie the coal measures. Salinity varies from 7800 to 176,000 mg/l, and the waters are Na–Ca–Cl brines enriched in Ca, Sr and Br and depleted in Na, K, Mg and SO₄ relative to the seawater evaporation curve. Br:Cl and Na:Cl ratios suggest that the brine composition corresponds to an evaporation ratio of as much as 30. The brines lie close to the meteoric line on H/O isotopic plots but with a compositional range of δ¹⁸O from −4.18 to −6.99 and of δD from −42.4 to −23.5, distant from modern meteoric or ocean water. Mine water composition contrasts with that of nearby salt-spring brines, which are inferred to have originated through dissolution of Windsor Group evaporites by modern meteoric waters. However, a contribution to the mine waters from halite dissolution and from Br in organic matter cannot be ruled out. Present concentrations of several elements in the brines can be explained by water–rock interaction. The original Windsor brines probably moved up into the overlying coal-measure sandstones along faults, prior to the Late Triassic. The high salinity and irregular salinity distribution in the Phalen sandstones suggests that the brines have undergone only modest dilution and are virtually immobile. In contrast, Prince waters show a progressive increase in salinity with depth and are inferred to have mixed with surface waters. Basinal brines from which these modern formation fluids were derived may have been important agents in base-metal and Ba mineralisation from the mid-Carboniferous onwards, as saline fluid inclusions are common in Zn–Pb sulphide deposits in the region.     

Multiple fluid components of salt diapirs and salt dome cap rocks, Gulf Coast, U.S.A.

Salt diapirs contain a few percent of anhydrite that accumulated as residue to form anhydrite cap rocks during salt dissolutions. Reported ⁸⁷Sr/⁸⁶Sr ratios of these salt-hosted and cap rock anhydrites in the Gulf Coast, U.S.A., indicate their derivation from Middle Jurassic seawater. However, a much wider range of ⁸⁷Sr/⁸⁶Sr ratios, incorporating a highly radiogenic component in addition to the Middle Jurassic component, has been found in several Gulf Coast salt domes. This wide range of ⁸⁷Sr/⁸⁶Sr ratios of anhydrite within the salt stocks records Sr contributions from both marine water and formation water that has equilibrated with siliciclastics. During cap rock formation this anhydrite either recrystallized in the presence of, or was cemented by, a low-Sr fluid with a Late Cretaceous seawter-type Sr isotope ratio or simply lost Sr during recrystallization. Later, the cap rock was invaded by warm saline brines with high Sr isotope ratios from which barite and metal sulfides were precipitated. Subsequently, low-salinity water hydrated part of the anhydrite bringing to six the total number of fluids that interacted througout the history of salt dome and cap rock growth. The progenitor of these salt diapirs, the Louann Formation, is generally thought to have formed from marine water evaporated to halite and, rarely, higher evaporite facies. Salt domes in the East Texas, North Louisiana, and Mississippi Salt Basins have ⁸⁷Sr/⁸⁶Sr and δ³⁴S values that corroborate a Mid-Jurassic age for the mother salt. However, salt domes in the Houston and Rio Grande Embayments of the Gulf Coast Basin have ⁸⁷Sr/⁸⁶Sr ration ranging to values higher than both Middle Jurassic seawater and all Rb-free marine Phanerozoic rocks. These anomalous ⁸⁷Sr/⁸⁶Sr ratios are probably derived from radiogenic Sr-bearing fluids that equilibrated with siliciclastic rocks and invaded the salt either prior to, or during, diapirism. Potential sources of the radiogenic ⁸⁷Sr component include clay and/or feldspar (located either in older units beneath the Louann Formation or younger units flanking the salt diapirs) and K-salts within the Louann evaporites. Because partial Sr exchange in anhydrite had to take place in a fluid medium, admittance of radiogenic ⁸⁷Sr-bearing fluids into the salt may have led to diapirism by lowering the shear strength of the crystalline salt. The slight number of anomalous ⁸⁷Sr/⁸⁶Sr values in the interior basins indicates that anomalous values are related to areally discrete structural or stratigraphic controls that affected only the Gulf Coast Basin.     

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.     

Rise in seawater sulphate concentration associated with the Paleoproterozoic positive carbon isotope excursion: evidence from sulphate evaporites in the ∼2.2–2.1 Gyr shallow‐marine Lucknow Formation,South Africa

Abstract Past oceanic sulphate concentration is important for understanding how the oceans’ redox state responded to atmospheric oxygen levels. The absence of extensive marine sulphate evaporites before ～1.2 Gyr probably reflects low seawater sulphate and/or higher carbonate concentrations. Sulphate evaporites formed locally during the 2.22–2.06 Gyr Lomagundi positive δ¹³C excursion. However, the ～2.2–2.1 Gyr Lucknow Formation, South Africa, provides the first direct evidence for seawater sulphate precipitation on a carbonate platform with open ocean access and limited terrestrial input. These marginal marine deposits contain evidence for evaporite molds, pseudomorphs after selenite gypsum, and solid inclusions of Ca‐sulphate in quartz. Carbon and sulphur isotope data match the global record and indicate a marine source of the evaporitic brines. The apparent precipitation of gypsum before halite requires ≥2.5 mm L^?1 sulphate concentration, higher than current estimates for the Paleoproterozoic. During the Lomagundi event, which postdates the 2.32 Gyr initial rise in atmospheric oxygen, seawater sulphate concentration rose from Archean values of ≤200 μm L^?1, but dropped subsequently because of higher pyrite burial rates and a lower oceanic redox state.     

Geochemical fingerprints of dolomitization in Bahamian carbonates: Evidence from sulphur,calcium, magnesium and clumped isotopes

In an effort to constrain the mechanism of dolomitization in Neogene dolomites in the Bahamas and improve understanding of the use of chemostratigraphic tracers in shallow‐water carbonate sediments the δ³⁴S, Δ₄₇, δ¹³C, δ¹⁸O, δ^44/40Ca and δ²⁶Mg values and Sr concentrations have been measured in dolomitized intervals from the Clino core, drilled on the margin of Great Bahama Bank and two other cores (Unda and San Salvador) in the Bahamas. The Unda and San Salvador cores have massively dolomitized intervals that have carbonate associated sulphate δ³⁴S values similar to those found in contemporaneous seawater and δ^44/40Ca, δ²⁶Mg values, Sr contents and Δ₄₇ temperatures (25 to 30°C) indicating relatively shallow dolomitization in a fluid‐buffered system. In contrast, dolomitized intervals in the Clino core have elevated values of carbonate associated sulphate δ³⁴S values indicating dolomitization in a more sediment‐buffered diagenetic system where bacterial sulphate reduction enriches the residual in ³⁴S, consistent with high sediment Sr concentrations and low δ^44/40Ca and high δ²⁶Mg values. Only dolomites associated with hardgrounds in the Clino core have carbonate associated δ³⁴S values similar to seawater, indicating continuous flushing of the upper layers of the sediment by seawater during sedimentary hiatuses. This interpretation is supported by changes to more positive δ^44/40Ca values at hardground surfaces. All dolomites, whether they formed in an open fluid‐buffered or closed sediment‐buffered diagenetic system have similar δ²⁶Mg values suggesting that the HMC transformed to dolomite. The clumped isotope derived temperatures in the dolomitized intervals in Clino yield temperatures that are higher than normal, possibly indicating a kinetic isotope effect on dolomite Δ₄₇ values associated with carbonate formation through bacterial sulphate reduction. The findings of this study highlight the utility of applying multiple geochemical proxies to disentangle the diagenetic history of shallow‐water carbonate sediments and caution against simple interpretations of stratigraphic variability in these geochemical proxies as indicating changes in the global geochemical cycling of these elements in seawater.     

Neogene evaporites in desert volcanic environments: Atacama Desert, northern Chile

The Upper Miocene and Pliocene evaporite deposits of the Atacama Desert of northern Chile (Hilaricos and Soledad Formations) are among the few non‐marine evaporites in which aridity not only formed the deposits, but has also preserved them almost unaltered under near‐surface conditions. These deposits are largely composed of displacive Ca sulphate and halite together with minor amounts of glauberite, thenardite and polyhalite. However, at the base and top of these deposits, there are also beds of gypsum crystal pseudomorphs that originally formed as free‐growth forms within shallow brine bodies, rather than as displacive sediments. The halite is present as interstitial cement, displacive cubes and shallow‐water, bottom‐growth chevron crusts. Most of the calcium sulphate is presently anhydrite, pseudomorphous after gypsum, that was the primary depositional sulphate mineral. The secondary anhydrite formed under early diagenetic conditions after slight burial (some metres) resulting from the effect of strongly evolved pore brines. The anhydrite has been preserved without rehydration during late diagenetic and exhumation stages on account of the arid environment of the Atacama Desert. Both the Hilaricos and the Soledad Formations contain geochemical markers indicating that these Neogene evaporites had a largely non‐marine origin. Bromine content in the halite is very low (few p.p.m.), indicating neither a sedimentological relation with sea water nor the likelihood of direct recycling of prior marine halites. Moreover, the δ³⁴S of sulphates (+4·5‰ to +9‰) also reflects a non‐marine origin, with a strong volcanic influence, although some recycling of Mesozoic marine sulphates cannot be ruled out. δ³⁴S of dissolved sulphate from hot springs and streams in the area commonly displays positive values (+2‰ to +10‰). Leaching of oxidized sulphur and chlorine compounds from volcanoes and epithermal ore bodies, very common in the associated drainage areas, have been the main contribution to the accumulation of evaporites. The sedimentary and diagenetic evolution of the Hilaricos and Soledad evaporites (based on lithofacies analysis) provides information about the palaeohydrological conditions in the Central Depression of northern Chile during the Neogene. In addition, the diagenesis and exhumation history of these evaporites confirms the persistence of strongly arid conditions from Late Miocene until the present. A final phase of tectonism took place permitting the internal drainage to change and open to the sea, resulting in dissolution and removal of a significant portion of these deposits. Despite the extensive dissolution, the remaining evaporites have undergone little late exhumational hydration.     

The interaction of tectonics,climate and eustasy in controlling dolomitization: A case study of Cenomanian–Turonian,shallow marine carbonates of the Iberian Basin

During the Cretaceous, high global sea-level and low latitudinal temperature variations led to the growth of epeiric carbonate platforms. Platform-scale dolomitization of these platforms is not common, reflecting the low Mg/Ca ratio of seawater and a humid climate. This study describes the processes governing pervasive dolomitization of a land-attached carbonate platform within the Iberian Basin. Dolomite is planar to sub-planar with a geochemical signature consistent with dolomitization from penesaline seawater. Dolomitization was most pervasive during a 1 Myr period in the middle Cenomanian, by repeated reflux of seawater from brine pools formed on the top of a southward-prograding carbonate platform. Tilting and structural reorganization in the Upper Cenomanian led to a reversal in polarity of the platform, and dolomitization was restarted by the northward reflux of seawater. Rising relative sea-level and oceanic acidification led to back-stepping of the platform such that the supply of dolomitizing fluids was cut off. In the Lower Turonian, pervasively dolomitized rudist rudstone facies in the south of the study area indicate that dolomitization restarted, either penecontemporaneously or later, from highly evaporated Campanian–Maastrichtian seawater. A systematic increase in dolomite crystal size up-section ties broadly, but not entirely, to stratigraphy. It is possible that these textural differences reflect changes in fluid chemistry, limestone permeability or precursor rock texture. However, the lack of stratigraphic conformance, and the preservation of the earliest-formed dolomite only in the oldest sediments, could indicate a progressive recrystallization of early-formed dolomite through repeated reflux of brines. As such, the succession appears to preserve a fossilized record of dolomite recrystallization through time during the Cenomanian–Turonian. The results of this study therefore provide a record of the progressive dolomitization of a carbonate platform and demonstrate the important interplay of climate and basin-scale tectonics on dolomite distribution and crystallinity.     

四川盆地川中地区震旦系灯影组碳酸盐沉积物成岩作用与孔隙流体演化

以四川盆地川中高石梯-磨溪地区钻井岩芯和水样资料为主,盆地周边地面剖面露头资料为辅,采用微量元素、阴极发光、包裹体、同位素等分析手段进行岩石学和地球化学分析,在此基础上讨论了灯影组碳酸盐岩的成岩作用变化和成岩作用过程中孔隙流体的演化。研究结果表明:灯影组碳酸盐岩沉积物在沉积作用和成岩作用过程中广泛地受到孔隙流体和成岩作用变化的影响,发生过海水的胶结作用、早期大气水成岩作用（溶解作用和沉淀作用）、表生喀斯特化作用、构造控制的热液白云石化和密西西比谷型矿化作用、碳氢产生时的溶解作用、油热裂解时焦沥青的充填孔洞作用等,其中喀斯特化和热液白云石化对储层的发育起着重要的作用。然而,灯影组孔隙流体广泛地被白云石化作用和喀斯特化作用的修饰。现今地层水资料分析表明这些成岩作用明显地影响蒸发海水的地层水流体。地层水分析资料在Ca-Br及Mg-Br关系图上显示出蒸发海水的特征,Ca的浓度比蒸发海水分布曲线要高,Mg的浓度比蒸发海水曲线要低,为灯影组沉积物在成岩作用过程中发生过白云石化作用的结果。     

Traces of brine and hydrocarbon migration in carbon,oxygen, and sulfur isotopic compositions of reservoir rocks in the Talakan petroleum field,Southwestern Yakutia

Dolomites from the productive Osa horizon (upper subformation of the Lower Cambrian Bilir Formation) in the Talakan petroleum field show a prominent 1–2‰ decrease in δ¹⁸O (from 23–24 to 21–22‰), which presumably marks a zone of relatively high water/rock ratios. Productive boreholes are characterized by moderate δ³⁴S values (from 25.1 to 30.6‰) and negative correlation between δ³⁴S in anhydrite and δ¹⁸O in associated dolomite, which points to a partial sulfate reduction during catagenesis. In nonproductive borehole, δ³⁴S values increase significantly (from 31.4 to 35.6‰) and show positive correlation with δ¹⁸O in dolomite. Rocks recovered by nonproductive borehole possibly recrystallized during early diagenesis, and, correspondingly lost their permeability and capacity to form pores. Limestones and dolomites of the Osa horizon have a carbon isotopic composition within the range of normal marine carbonates (δ¹³C = 0 ± 1 ‰), which does not indicate a significant role of organic matter in postsedimentary recrystallization of carbonate sediments. A positive δ¹³C excursion up to 4.5‰ recorded in the lower subformation of the Bilir Formation presumably occurred at the sedimentation stage under conditions of high rates of bioproductivity and organic matter burial in sediments.