Geochemistry of metal-rich brines from central Mississippi Salt Dome basin, U.S.A.

Oil-field brines are the most favored ore-forming solutions for the sediment-hosted Mississippi Valley-type ore deposits. Detailed inorganic and organic chemical and isotope analyses of water and gas samples from six oil fields in central Mississippi, one of the very few areas with high metal brines, were conducted to study the inorganic and organic complexes responsible for the high concentrations of these metals. The samples were obtained from production zones consisting of sandstone and limestone that range in depth from 1900 to 4000 m (70–120°C) and in age from Late Cretaceous to Late Jurassic. Results show that the waters are dominantly bittern brines related to the Louann Salt. The brines have extremely high salinities that range from 160,000 to 320,000 mg/l total dissolved solids and are NaCaCl-type waters with very high concentrations of Ca (up to 48,000 mg/l) and other alkaline-earth metals, but with low concentrations of aliphatic acid anions. The concentrations of metals in many water samples are very high, reaching values of 70 mg/l for Pb, 245 mg/l for Zn, 465 mg/l for Fe and 210 mg/l for Mn. The samples with high metal contents have extremely low concentrations (<0.02 mg/l) of H₂S. Samples obtained from the Smackover Formation (limestone) have low metal contents that are more typical of oil-field waters, but have very high concentrations (up to 85 mg/l) of H₂S. Computations with the geochemical code SOLMINEQ.87 give the following results: (1) both Pb and Zn are present predominantly as aqueous chloride complexes (mainly as PbCl₄²⁻ and ZnCl₄²⁻, respectively); (2) the concentrations of metals complexed with short-chained aliphatic acid anions and reduced S species are minor; (3) organic acid anions are important in controlling the concentrations of metals because they affect the pH and buffer capacity of the waters at subsurface conditions; and (4) galena and sphalerite solubilities control the concentrations of Pb and Zn in these waters.     

The geochemistry of Ca,Sr, Ba and Ra sulfates in some deep brines from the Palo Duro Basin,Texas

The geochemistry of Ca, Sr, Ba and Ra sulfates in some deep brines from the Palo Duro Basin of north Texas, was studied to define geochemical controls on radionuclides such as ⁹⁰Sr and ²²⁶Ra. Published solubility data for gypsum, anhydrite, celestite, barite and RaSO₄ were first reevaluated, in most cases using the ion interaction approach of Pitzer, to determine solubility products of the sulfates as a function of temperature and pressure. Ionic strengths of the brines were from 2.9 to 4.8 m, their temperatures and pressures up to 40°C and 130 bars. Saturation indices of the sulfates were computed with the ion-interaction approach in one brine from the arkosic granite wash fades and four from the carbonate Wolfcamp Formation. All five brines are saturated with respect to gypsum, anhydrite and celestite, and three of the five with respect to barite. All are undersaturated by from 5 to 6 orders of magnitude with respect to pure RaSO₄. ²²⁶Ra concentrations in the brines, which ranged from 10^?11.3 to 10^?12.7 m, are not controlled by RaSO₄ solubility or adsorption, but possibly by the solubility of trace Ra solid solutions in sulfates including celestite and barite.     

Saline groundwaters in the hercynian granites (Chardon Mine,France): geochemical evidence for the salinity origin

In this study, the chemical evolution of high Cl⁻ Chardon mine groundwaters is modelled as a mixing between an oxidising recharge and an old marine component on which the water–rock interaction is superimposed. Chemical and isotopic similarities with saline Carnmenellis mine groundwaters are emphasised and a general comparison with other brines is discussed.The cation content of deep granitic groundwaters is indicative of the water–rock interaction. In the case of Chardon and Carnmenellis groundwaters, the high Na/Cl ratio can still be related to the contribution of a brine of sedimentary origin to the water salinity. The differences in the hydrochemistry related to their geological context only appears at the trace metals level. On the contrary, brines in plutonic rocks which exhibit a low Na/Cl ratio represent groundwaters having a residence time in the host rock, long enough to equilibrate with secondary aluminosilicates. In that case, the brine origin is difficult to assess if only based on the water cation content.     

Nonconservative behavior of Br−/Cl− ratios during alteration of volcaniclastic sediments

Other than halite diagenesis and organic matter degradation, Cl⁻ and Br⁻ are considered to be conservative in marine pore fluids. Consequently, Br⁻/Cl⁻ ratios should remain constant during most diagenetic reactions. Nonetheless, Br⁻/Cl⁻ molar ratios decrease to 1.27 × 10⁻³ (≈18% less than seawater value) in pore fluids from Site 833 in the Aoba Basin of the New Hebrides convergent margin despite the lack of halite diagenesis and little organic matter. Sediment at this site is largely volcanic ash, which becomes hydrated with depth as it converts to clay and zeolite minerals. These hydration reactions remove sufficient water to increase the concentrations of most solutes including Cl⁻ and Br⁻. The resulting concentration gradients drive diffusion, but calculations indicate that diffusion does not decrease the Br⁻/Cl⁻ ratio. Some Cl⁻ may be leached from the ash, but insufficient amounts are available to cause the observed decrease in Br⁻/Cl⁻ ratio. The limited source of Cl⁻ suggests that proportionately more Br⁻ than Cl⁻ is lost from the fluids to the diagenetic solids. Similar nonconservative behavior of Cl⁻ and Br⁻ may occur during fluid circulation at ridge crests and flanks, thereby influencing the halide distribution in the crust.     

Iodine diagenesis in pelagic deep-sea sediments

Nineteen sediment cores from the Madeira, Seine, Tagus and Nares Abyssal Plains and the Alboran Sea have been used to evaluate the speciation, fluxes and diagenesis of iodine in the deep sea. The sediments have surficial molar I/C ratios of 10–30 × 10⁻⁴ in excess of previous reported values for planktonic material (~1 × 10⁻⁴). Solid phase I contents decrease exponentially with depth corresponding to decomposition rate constants of 5–260 × 10⁻⁶ yr⁻¹ which vary with the carbon accumulation rate.Iodine species in the pore waters follow a vertical sequence of four zones: 1. a zone of I⁻ production where total dissolved iodine (∑I) concentrations initially increase at the seawater-sediment interface; 2. a zone of I⁻ oxidation where interconversion of I⁻ to IO⁻₃ occurs; 3. a zone of IO⁻₃ reduction where interconversion of IO⁻₃ back to I⁻ occurs which corresponds to the suboxic part of the sediment column; and 4. a further zone of I⁻ production which is confined to the lower anoxic part of the sediment column. Benthic ∑I fluxes in the Madeira Abyssal Plain measured from shipboard incubation experiments and calculated from porewater gradients are similar, averaging 0.55 and 0.36 × 10⁻⁸ μmol cm⁻² sec⁻, respectively.In the surface sediment the observed I enrichment results from a quasi-closed cycle for iodine initially involving release of I⁻ from decomposing marine organic matter followed by rapid removal onto organic matter at the sediment-seawater interface where I/C regeneration ratios of up to 200 × 10⁻⁴ are found, lodate reduction occurs during suboxic diagenesis, after denitrification and before MnO₂ reduction, consistent with the sequence of reactions predicted from the free energy yields for organic matter oxidation. There is some further I⁻ production in the anoxic section of sediments but at much smaller rates than occur during the interfacial diagenetic cycling.     

Origin and age of saline waters in Busko Spa (Southern Poland) determined by isotope,noble gas and hydrochemical methods: evidence of interglacial and pre-Quaternary warm climate recharges

In the Busko and Solec Spas, saline sulphide and sulphate waters are exploited at different depths. The shallow water system in Busko is related to an interglacial or interstadial period shown by the lack of tritium and¹⁴C and its δ¹⁸O and δD values equal to that of modern water. The noble gas temperatures (NGT) are somewhat lower than the present air temperatures, but the⁴He and⁴⁰Ar ages of about 90–480 ka confirm a Pleistocene recharge. The δ¹⁸O and δD values of the deep system are much heavier than those of local modern waters, which may mean the recharge of a warm pre-Quaternary climate. The⁴He,⁴⁰Ar and²¹Ne contents and the NGT values confirm the deep system to be recharged at the pre-Quaternary stages of the present hydrologic cycle, i.e. after the sea transgression in the Badenian. High SO₄²⁻ contents indicate that the Cl⁻ and SO₄²⁻ in both systems originated from simultaneous leaching of NaCl and gypsum. Other components can be explained by cation exchange between water and Badenian clay minerals in marls (e.g. Ca²⁺ for Mg²⁺ and Na⁺ for K⁺) and by the decay of marine organic matter supplying Br⁻, I⁻ and B.     

Dissolved noble gases and stable isotopes as tracers of preferential fluid flow along faults in the Lower Rhine Embayment,Germany

Groundwater in shallow unconsolidated sedimentary aquifers close to the Bornheim fault in the Lower Rhine Embayment (LRE), Germany, has relatively low δ²H and δ¹⁸O values in comparison to regional modern groundwater recharge, and ⁴He concentrations up to 1.7?×?10^?4 cm³ (STP) g^–1?±?2.2 % which is approximately four orders of magnitude higher than expected due to solubility equilibrium with the atmosphere. Groundwater age dating based on estimated in situ production and terrigenic flux of helium provides a groundwater residence time of ～10⁷ years. Although fluid exchange between the deep basal aquifer system and the upper aquifer layers is generally impeded by confining clay layers and lignite, this study’s geochemical data suggest, for the first time, that deep circulating fluids penetrate shallow aquifers in the locality of fault zones, implying  that sub-vertical fluid flow occurs along faults in the LRE. However, large hydraulic-head gradients observed across many faults suggest that they act as barriers to lateral groundwater flow. Therefore, the geochemical data reported here also substantiate a conduit-barrier model of fault-zone hydrogeology in unconsolidated sedimentary deposits, as well as corroborating the concept that faults in unconsolidated aquifer systems can act as loci for hydraulic connectivity between deep and shallow aquifers. The implications of fluid flow along faults in sedimentary basins worldwide are far reaching and of particular concern for carbon capture and storage (CCS) programmes, impacts of deep shale gas recovery for shallow groundwater aquifers, and nuclear waste storage sites where fault zones could act as potential leakage pathways for hazardous fluids.     

14C analysis of aliphatic hydrocarbon fractions from the hypersaline Lake Tyrrell,southeast Australia

Conventional biomarker studies typically interpret the distribution, structure and stable isotopic (e.g. ¹³C, D) composition of sedimentary hydrocarbons and polar compounds. However, compound and compound class specific ¹⁴C analysis (CSRA) is becoming increasingly relevant for characterising millennial scale residence and mobilisation of sedimentary organic carbon (OC). Here, the ¹⁴C content of the aliphatic and bulk fractions from shallow cores from the hypersaline playa, Lake Tyrrell, southeast Australia were compared. The aliphatic hydrocarbon fractions (predominantly n-alkanes) were substantially older than the corresponding bulk fractions, indicating the presence of active reservoirs of ancient carbon, likely derived from aeolian reworking of sediments. The ¹⁴C ages of the aliphatic hydrocarbons in the core revealed two noticeable shifts in age and source of ancient OC that were not apparent using biomarker composition and sedimentology alone. The study shows that aliphatic hydrocarbons are relatively simple to isolate, even from organically lean (ca. 0.05% TOC) terrestrial sediments, and their ¹⁴C ages yield information about carbon mobilisation and preservation not amenable to conventional analysis.     

Oil families and their source rocks in the Weixinan Sub-basin, Beibuwan Basin, South China Sea

Thirty-one crude oils and 15 source rocks were selected for molecular geochemical and isotopic analyses in order to establish the genetic relationships between discovered oils and petroleum source rocks in the Weixinan Sub-basin, Beibuwan Basin, South China Sea. Three groups of oils were recognized. Group I oils are only found in the upper section of the Liusagang Formation, with a moderate abundance of C₃₀ 4-methylsteranes, low oleanane contents and lighter δ¹³C values, showing a close relation to the shale occurring in the upper section of the Liusagang Formation. Group II is represented by the majority of the discoveries and is distributed in multi-sets of reservoirs having different ages. The oils are characterized by a high abundance of C₃₀ 4-methylsteranes, low to moderate abundance of oleanane and heavy δ¹³C values, and shows a good correlation with the lacustrine shale and oil shale in the middle section of the Liusagang Formation. Group III oils occurred in the lower section of the Liusagang Formation. The oils have a lower concentration of C₃₀ 4-methylsteranes, relatively high abundance of oleananes and their δ¹³C values are intermediate. Oils of this group correlated well with the shallow lake-delta mudstone of the lower section of Liusagang Formation. These oil-source genetic relationships suggest a strong source facies control on the geographic distribution of oil groups within the Weixinan Sub-basin. The geochemical data indicate shale in the middle section of the Liusagang Formation has an excellent oil generation potential and the lower and upper sections contain dark shale and mudstone with good to fair oil potential. Future exploration or assessment of petroleum potential of the sub-basin could be improved by considering the proposed genetic relationship between the oil types and source rocks, as well as their distribution.     

Fossil brines preserved in the St-Lawrence Lowlands, Québec, Canada as revealed by their chemistry and noble gas isotopes

Brines in Cambrian sandstones and Ordovician dolostones of the St-Lawrence Lowlands at Bécancour, Québec, Canada were sampled for analysis of all stable noble gases in order to trace their origin and migration path, in addition to quantifying their residence time. Major ion chemistry indicates that the brines are of Na-Ca-Cl type, possibly derived from halite dissolution. ⁸⁷Sr/⁸⁶Sr ratios and Ca excess indicate prolonged interactions with silicate rocks of the Proterozoic Grenville basement or the Cambrian Potsdam sandstone. The brines constrain a 2-3% contribution of mantle ³He and large amounts of nucleogenic ²¹Ne^∗ and ³⁸Ar^∗ and radiogenic ⁴He and ⁴⁰Ar^∗. ⁴He/⁴⁰Ar^∗ and ²¹Ne^∗/⁴⁰Ar^∗ ratios, corrected for mass fractionation during incomplete brine degassing, are identical to their production ratios in rocks. The source of salinity (halite dissolution), plus the occurrence of large amounts of ⁴⁰Ar^∗ in brines constrain the residence time of Bécancour brines as being older than the Cretaceous. Evaporites in the St-Lawrence Lowlands likely existed only during Devonian-Silurian time. Brines might result from infiltration of Devonian water leaching halite, penetrating into or below the deeper Cambrian-Ordovician aquifers. During the Devonian, the basin reached temperatures higher than 250 °C, allowing for thermal maturation of local gas-prone source rocks (Utica shales) and possibly facilitating the release of radiogenic ⁴⁰Ar^∗ into the brines. The last thermal event that could have facilitated the liberation of ⁴⁰Ar^∗ into fluids and contributed to mantle ³He is the Cretaceous Monteregian Hills magmatic episode. For residence times younger than the Cretaceous, it is difficult to find an appropriate source of salinity and of nucleogenic/radiogenic gases to the Bécancour brines.     

Temporal trends of aliphatic and polyaromatic hydrocarbons in the Bohai Sea, China: Evidence from the sedimentary record

This work reports the historical trends and sources of aliphatic hydrocarbons (AHs) and polycyclic aromatic hydrocarbons (PAHs) in two ²¹⁰Pb dated sediment cores extracted from the central mud areas of Bohai Sea (Bohai). The TOC/TN ratio of the bulk organic matter (OM) and the composition of the n-alkanes suggest that the sedimentary organic matter was of mixed marine and terrigenous sources. The coarser sediment grain size and decreasing C/N ratios since ∼1976 could be attributed to the relocation of the Yellow River mouth causing a decreased influence of Yellow River derived sediments and associated OM into the central Bohai. The concentration of total 16 PAHs in the two cores ranged from 34.2-202 ng/g (mean, 91.5) for BC1, and from 53.3-186 ng/g (mean, 103) for BC2, with a high abundance of 2-3 ring PAHs. Perylene in the two cores mainly originated from terrigenous sources via riverine discharge and thus could be potentially related to changes in the sediment load from the Yellow River into the Bohai over time. Petroleum inputs could be revealed by ratios of methylphenanthrenes to phenanthrene (MP/P) and the patterns of more stable geochemical biomarkers (hopanes and steranes) along the two cores, in addition to the presence of unresolved complex mixtures (UCM) in the surface layers. Source diagnostic ratios of PAHs indicated a pyrogenic origin from biomass and coal combustion with a minor petroleum contribution. Downcore trends of compositional PAHs profiles were in agreement with the socio-economic development in China in the past decades. This temporal variation of sedimentary PAHs can also reflect a different evolution stage of energy structure in China as compared with those of the western developed countries.     

Fluid-rock interactions in a geothermal Rotliegend/Permo-Carboniferous reservoir (North German Basin)

Comprehensive data on the chemical composition of reservoir rocks and geothermal brines from the geothermal well doublet Groβ Schönebeck (North German Basin) drilled into a Rotliegend sedimentary and Permo-Carboniferous volcanic rock reservoir were sampled over the past years. They were characterized with respect to their major and minor elemental composition including various isotope ratios. The study considered the impact of drilling and reservoir operations on fluid composition and aimed at determining fluid–rock interactions to gain information on fluid origin and hydraulic pathways.The highly saline fluids (up to 265 g/L TDS) show δ ¹⁸O and δD of water (2.7–5.6 and −3.1–15, respectively) as well as δ ³⁴S of sulfate (3.6–5), and ⁸⁷Sr/⁸⁶Sr ratios (0.715–0.716) that resemble Rotliegend brines from an area located around 200 km in the west (the Altmark). Halogen ratios indicated that brines developed predominantly by evaporation of meteoric water (primary brine) together with halite dissolution brine (secondary brine). Indication for mixing with Zechstein brine or with younger meteoric water was not found.No geochemical distinction was possible between fluids deriving from different rock formations (dacites or sedimentary rocks, respectively). This is due to the evolution of the sediments from the effusive rocks resulting in a similar mineralogical and chemical composition and due to a hydraulic connectivity between the two types of rock. This connection existed probably already before reservoir stimulation as indicated by a set of faults identified in the area that could connect the Rotliegend formation with both, the volcanic rocks and the lower units of the Zechstein. Additional geochemical indication for a hydraulic connectivity is given by (1) the very high heavy metal contents (mainly Cu and Pb) in fluids and scaling that derive from the volcanic rocks and were that were also found in increased amounts up at the Zechstein border (Kupferschiefer formation). (2) The ⁸⁷Sr/⁸⁶Sr isotope ratios of fluid samples correspond to the ratios determined for the sedimentary rocks indicating that initially the fluids developed in the sedimentary rocks and circulated later, when faults structures were created by tectonic events into the volcanic rocks.     

Hydrochemical and isotopic characterisation of the Bathonian and Bajocian coastal aquifer of the Caen area (northern France)

This paper describes the geochemical evolution of groundwater in the Bathonian and Bajocian aquifer along its flowpath. Since this aquifer represents one of the main sources of fresh water supply in the Caen area and has been subjected to a Holocene marine intrusion, its management requires a sound knowledge of (1) the primary conditions and (2) the potential influence of either natural or anthropogenic pressures. Groundwater vertical sampling validity is discussed with the contribution of high resolution temperature logging. The main processes of geochemical evolution along a groundwater flow line and the sea-water intrusion characteristics are discussed using ionic concentrations (Br⁻, F⁻ and major elements) and isotopes (water δ²H and δ¹⁸O, TDIC δ¹³C and A¹⁴C, sulphate δ¹⁸O and δ³⁴S). As the ¹³C content of TDIC is used as a tracer of water-rock interaction, it shows evidence of specific chemical and isotopic evolutions of groundwater within the aquifer, both related to water-rock interaction and mineral equilibria in groundwater. All the above-mentioned tracers evolve downflow: cation concentrations are modified by exchange with clay minerals allowing a high F⁻ concentration in groundwater, whereas Br⁻ and SO²⁻₄ concentrations appear to be redox condition dependant. Superimposed on these geochemical patterns, δ¹⁸O and δ²H compositions indicate that aquifer recharge has varied significantly through time. The chemical evolution of groundwater is locally affected by a salty water intrusion that is characterised by mixing between Flandrian fresh water and sea-water which has interacted with peat as evidenced by a high Br⁻/Cl⁻ ratio and SO²⁻₄ reduction.     

Comparison of stable isotopes,ratios of Cl/Cl and I/I in brine and deep groundwater from the Pacific coastal region and the eastern margin of the Japan Sea

Fifty-three samples, including brines associated with oil and natural gas reservoirs and groundwater samples from deep boreholes, were collected from the Pacific and Japan Sea coastal regions in Japan. The ¹²⁹I/¹²⁷I and ³⁶Cl/Cl ratios, and stable isotopes (δD and δ¹⁸O) are compared to investigate differences related to the geotectonic settings of the two regions. The δD and δ¹⁸O data indicate that brine and groundwater from the Pacific coastal region reflect mixing of meteoric water with connate seawater in the pores of sedimentary rocks. On the other hand, brine and groundwater from the Japan Sea coastal region have been hydrothermally altered. In particular, brines associated with petroleum accumulations at Niigata and Akita showed the same isotopic characteristics as fluids found in the Kuroko deposits of the Green Tuff region in northeastern Japan. There is little difference in the ³⁶Cl/Cl ratios in brine and groundwater from the Pacific and Japan Sea coasts. Most brine and some deep groundwater, except those from the Pleistocene Kazusa Group, have already reached the average secular equilibrium ratio of 9.9 ± 2.7 × 10⁻¹⁵ for their mudstone and sandstone reservoirs. There was no correlation between the ³⁶Cl/Cl ratios and differences in geotectonic setting between the Pacific and the Japan Sea coast. The molar I/Br ratio suggests that the I in all of water samples was of biogenic origin. The average ¹²⁹I/¹²⁷I ratio was 290 ± 130 × 10⁻¹⁵ to 294 ± 105 × 10⁻¹⁵ in both regions, showing no relationship to the different geotectonic settings. The uncontaminated brine and groundwater samples are likely to have retained the original ¹²⁹I/¹²⁷I ratios of marine I released from the old organic matter stored in sedimentary rock.     

Deep reaching fluid flow in the North East German Basin: origin and processes of groundwater salinisation

Major element chemistry, rare-earth element distribution, and H and O isotopes are conjointly used to study the sources of salinisation and interaquifer flow of saline groundwater in the North East German Basin. Chemical analyses from hydrocarbon exploration campaigns showed evidence of the existence of two different groups of brines: halite and halite Ca–Cl brines. Residual brines and leachates are identified by Br^?/Cl^? ratios. Most of the brines are dissolution brines of Permian evaporites. New analyses show that the pattern of rare-earth elements and yttrium (REY) are closely linked to H and O isotope distribution. Thermal brines from deep wells and artesian wells indicate isotopically evaporated brines, which chemically interacted with their aquifer environment. Isotopes and rare-earth element patterns prove that cross flow exists, especially in the post-Rupelian aquifer. However, even at depths exceeding 2,000 m, interaquifer flow takes place. The rare-earth element pattern and H and O isotopes identify locally ascending brines. A large-scale lateral groundwater flow has to be assumed because all pre-Rupelian aquifer systems to a depth of at least 500 m are isotopically characterised by Recent or Pleistocene recharge conditions.     

Comparison of the Sr isotopic signatures in brines of the Canadian and Fennoscandian shields

A synthesis of Sr isotope data from shallow and deep groundwaters, and brines from the Fennoscandian and Canadian Shields is presented. A salinity gradient is evident in the water with concentrations varying from approximately 1–75 g L⁻¹ below 1500 m depth in the Fennoscandian Shield and from 10 up to 300 g L⁻¹ below 650 m depth in the Canadian Shield. Strontium isotope ratios were measured to assess the origin of the salinity and evaluate the degree of water–rock interaction in the systems. In both shields, the Sr concentrations are enriched relative to Cl, defining a positive trend parallel to the seawater dilution line and indicative of Sr addition through weathering processes. The depth distribution for Sr concentration increases strongly with increasing depth in both shields although the variation in Sr-isotope composition does not mirror that of Sr concentrations. Strontium-isotope compositions are presented for surface waters, and groundwaters in several sites in the Fennoscandian and Canadian Shields. Numerous mixing lines can be drawn reflecting water–rock interaction. A series of calculated lines links the surface end-members (surface water and shallow groundwater) and the deep brines; these mixing lines define a range of ⁸⁷Sr/⁸⁶Sr ratios for the deep brines in different selected sites. All sites show a specific ⁸⁷Sr/⁸⁶Sr signature and the occurrence of large ⁸⁷Sr/⁸⁶Sr variations is site specific in both shields. In Canadian Shield brines, the Sr isotope ratios clearly highlight large water rock interaction that increases the ⁸⁷Sr/⁸⁶Sr ratio from water that could have been of marine origin. In contrast to the Canadian Shield, groundwater does not occur in closed pockets in the Fennoscandian, and the well-constrained ⁸⁷Sr/⁸⁶Sr signatures in deep brines should correspond to a large, well-mixed and homogeneous water reservoir, whose Sr isotope signature results from water–rock interaction.     

元坝地区长兴组储层溶蚀作用期次与机制研究

溶蚀作用是元坝地区长兴组碳酸盐岩储层形成的关键,本文通过区内长兴组岩芯及岩石薄片观察,岩石学特征与地球化学特征研究充分结合,认为元坝地区长兴组储层存在着三期溶蚀作用,不同期次溶蚀作用产物的岩石学特征和地球化学特征明显有别。第Ⅰ期溶蚀作用发生于同生期-成岩早期,成岩流体为海岸带大气降水,遵循高频层序界面、层序不整合界面、礁滩控制的大气水溶蚀作用机制;第Ⅱ期溶蚀作用发生于晚三叠世-中侏罗世的埋藏过程中,相当于中成岩早期阶段,成岩流体为上二叠统地层内部流体,溶蚀作用与烃源岩系有机质热演化释放的有机酸进入成岩流体有关;第Ⅲ期溶蚀作用发生于晚白垩世-古近纪的埋藏过程中,相当于成岩晚期阶段,成岩流体为长兴组地层内部来源或中下三叠统来源流体,或为深部热液流体,溶蚀作用遵循深埋藏条件下构造作用驱动的多类型溶蚀作用机制。第Ⅰ期、第Ⅱ期溶蚀作用形成的储渗空间对于液态原油运移进入长兴组聚集成藏(古油藏)具有重要意义,第Ⅲ期溶蚀作用形成的储渗空间对于现今天然气成藏具有重要意义。     

Influences of the basin brines on extractable bitumen of Kupferschiefer from southwestern Poland

A study of the influences of the basin brines on hydrocarbon generation of the Kupferschiefer in southwestern Poland has been carried out.The samples from the Konrad and Polkowics mines were analyzed by orgainc geochemical,microscopic and FTIR methods.The results indicate that organic matter of Kupferschiefer tends to decrease with the ascending,oxidizing brines,In the Konrad profile,the Kupferschiefer was strongly oxidized.The extract yields were depleted up to 50mg Ext/g Corg.Gas chromatography(GC) and gas chromatography-mass spctrometry(GC/MS) data indicate that the depletion occureed predominantly in saturated hydrocarbon compounds.The identified n-alkanes in smpale KD1 were depleted at least to 5000μg/g Corg.The aromatic compounds show a fidderent trend of variation.The concentrations of phenanthrene alkylphenanthrenes(Ph-PAH) and naphthalene alkylnaphthalenes(Na-PAH) show a decrease,whereas sulfur polyaromatic hydrocarbons(S-PAH)and oxygen polyaromatic hydrocarbons(O-PAH) show an incrase under the influences of oxidizing brines,In the Polkowice profile,organic matter under the influences of oxidizing fluids shows a simlar trend of varation as in the Konrad mine.Analyses of polar compounds shed light on the oxidation processes at the molecule level.The dominant products of oxidation are aliphatic acid.alcohol and ester.FTIR results indicate that the oxidation of organic matter led to a decrease in aliphatic CH3 and an increase in C-O,C=O bands.     

Solid bitumens: an assessment of their characteristics, genesis, and role in geological processes

Familiar since antiquity, and subject in contemporary times to various characterization schemes, the exact nature of solid bitumen is not yet fully known. Bitumens have ‘random polymer-like’ molecular structures, are mobile as highly viscous fluids or were once fluids but have since turned into solids. Solid bitumens consist mainly of large moieties, of polyclyclic aromatic hydrocarbons, occasionally with finely admixed, fine-grained cryptocrystalline graphite. Solid bitumens are distinguished from kerogen, which is the syngenetic and generally finely dispersed particulate organic matter in sedimentary rock that virtually does not migrate following its deposition. Occurrences of solid bitumens are relevant to petroleum exploration as well as the search for, and evaluation of, a variety of metallic mineral deposits. Genesis of bitumen is in many cases linked to the thermal and hydrothermal history of organic matter in sedimentary rock. Apparently bitumen, or more specifically organic acids generated along with bitumen during diagenesis, may alter porosity of reservoir rocks or otherwise prepare the ground for ore deposition. Bitumen is also relatively sensitive to alteration processes, some of which, such as oxidative weathering, water leaching, biodegradation (contact) metamorphism and ionizing radiation may likewise affect its nature. Elemental composition of bitumen commonly reflects the nature of mineral deposits. Is is possible that in petroleum exploration, trace metal abundances of bitumen may eventually allow prediction of crude oil types and volumes anticipated from a given source rock? Beside transition elements, notably Ni and V, highly anomalous concentrations of U, Pt and Au occur in some solid bitumens. During the generation of petroleum from kerogen, the trend in δ¹³C is toward lighter values. The opposite seems to occur when liquid petroleum is subjected to thermal cracking (and /or related processes) yielding solid bitumen enriched in ¹³C, and isotopically light methane. In fact, except for deasphalting and possibly some irradiation processes, the result of thermal cracking, oxidation, water leaching, inspissation (drying) and bacterial degradation of crude oil is that lower molecular weight hydrocarbons are removed leaving bitumen residues enriched in aromatic hydrocarbons, heteroatomic compounds (NSO) and ¹³C. Such phenomena are relevant to bitumen paragenesis in petroleum reservoir rocks, to certain Phanerozoic occurrences of multiple generations of bitumens, and to bitumens in mineral deposits.     

Hydrogen,oxygen, helium and strontium isotopic constraints on the formation of oilfield waters in the western Qaidam Basin,China

Surveys and exploration for oil and gas have revealed many oil-field brines in the Tertiary strata of the western Qaidam Basin, China. The source and formation of these brines are unclear. Brine samples collected from petroleum wells in the oilfield area were analyzed for their general chemical composition and for hydrogen, oxygen, strontium and helium isotopes in order to trace their origin, formation, and resource distribution. Results show that the concentrations of resources such as K, B, Sr, Br and Li are unusually enriched in oil-field brines and have excellent potential for development and utilization in future. The geology, H, O, Sr and He isotopic composition, the chemical composition, and the geophysical measurements all show that the tertiary oil-field brines predominantly originated from the deep mixture of formation water and magmatic residual fluids in the western Qaidam Basin. The tectonic geology, intense and frequent magmatic-hydrothermal activity as well as high ³He/⁴He ratio indicate that the magmatic fluids possibly originate from the upper mantle-derived magmatic fluids. Thus, according to our interpretation of the formation and evolution of the oilfield brine, this source is adequate for the volume and composition to be explored and utilized latterly.