Magnetic properties related to hydrothermal alteration processes at the Escondida porphyry copper deposit,northern Chile

Fluid–rock interaction related to the circulation of hydrothermal fluids can strongly modify the physicochemical properties of wall rocks in porphyry Cu deposits. These processes can also produce compositional and textural changes in ferromagnetic minerals, which can be quantified using magnetic methods. In the Escondida porphyry Cu deposit of northern Chile, each hydrothermally altered lithology is characterized by a discrete assemblage of Fe–Ti oxide minerals. These minerals have distinctive bulk magnetic susceptibility (K _bulk), temperature-dependent magnetic susceptibility, and magnetic hysteresis parameters. Selectively altered rocks (i.e., potassic and chloritic alteration types) exhibit the highest K _bulk values (>3.93?×?10^?3 SI units), and their hysteresis parameters indicate multidomain magnetic mineral behavior. This suggests that these rocks are composed of the coarsest magnetic grain sizes within the deposit. Optical analyses and susceptibility–temperature curves confirm that the magnetic signals in selectively altered rocks are mainly carried by secondary magnetite. In contrast, pervasively altered rocks (i.e., quartz-sericite and argillic alteration types) exhibit low K _bulk values (<1.93?×?10^?4 SI units) and contain smaller pseudo-single domain magnetic grain assemblages. This is consistent with the destruction and/or reduction in size of magnetite under acidic conditions. The results therefore demonstrate a genetic relationship between the hydrothermal alteration processes, Fe–Ti oxide minerals, and magnetic properties of the wall rock in the Escondida deposit. These magnetic methods can be considered a sensitive and efficient petrophysical tool for the identification and semi-quantification of alteration assemblages, and facilitating the recognition and mapping of discrete hydrothermal zones during exploration and operation of porphyry Cu deposits.     

Geo-microbial prospecting studies of surface sediments from petroliferous region of the Mehsana Block,North Cambay Basin

Surface adsorbed gas surveys and geo-microbiological surveys are well known techniques of petroleum exploration and aim towards risk reduction in exploration by way of identifying the areas warm with hydrocarbons and to establish inter-se exploration priorities amongst the identified warm areas. The thermogenic surface adsorbed gaseous hydrocarbons distribution patterns in petroliferous areas are considered to be a credible evidence for the upward migration of hydrocarbons. The present investigation aims to explore correlation between the adsorbed gas distribution pattern and microbial oxidizers in identifying the upward migration of hydrocarbons especially in the tropical black soil terrain of known petroliferous Mehsana Block of North Cambay Basin, India. A set of 135 sub-soil samples collected, were analyzed for indicator hydrocarbon oxidizing bacteria, adsorbed light gaseous hydrocarbons and carbon isotope ratios (¹³C_methane and δ¹³C_ethane). The microbial prospecting studies showed the presence of high bacterial population for methane (5.4 × 10⁶ cfu/gm), ethane (5.5 × 10⁶ cfu/gm), propane (4.6 × 10⁶ cfu/gm) and butane oxidizing bacteria (4.6 × 10⁶ cfu/gm) in soil samples. The light gaseous hydrocarbon analysis showed that the concentration ranges of C₁, C₂, C₃, iC₄ and nC₄ are 402 ppb, 135 ppb, 70 ppb, 9 ppb and 18 ppb, respectively, and the value of carbon isotope ranges of methane ?29.5 to ?43.0‰ (V-PDB) and ethane ?19.1 to ?20.9‰ (V-PDB). The existence of un-altered petroliferous microseep (δ¹³C, ?43‰) of catagenetic origin is observed in the study area. Geo-microbial prospecting method and adsorbed soil gas and carbon isotope studies have shown good correlation with existing oil/gas fields of Mehsana. Microbial surveys can independently precede other geochemical and geophysical surveys to delineate area warm with hydrocarbons, and mapped microbiological anomalies may provide focus for locales of hydrocarbon accumulation in the Mehsana Block of Cambay Basin.     

Magnetic fabric variations in Mesozoic black shales, Northern Siberia, Russia: Possible paleomagnetic implications

A 28-m-long section situated on the coast of the Arctic Ocean, Russia (74°N, 113°E) was extensively sampled primarily for the purpose of magnetostratigraphic investigations across the Jurassic/Cretaceous boundary. The section consists predominantly of marine black shales with abundant siderite concretions and several distinct siderite cemented layers. Low-field magnetic susceptibility (k) ranges from 8 × 10^− 5 to 2 × 10^− 3 SI and is predominantly controlled by the paramagnetic minerals, i.e. iron-bearing chlorites, micas, and siderite. The siderite-bearing samples possess the highest magnetic susceptibility, usually one order of magnitude higher than the neighboring rock. The intensity of the natural remanent magnetization (M₀) varies between 1 × 10^− 5 and 6 × 10^− 3 A/m. Several samples possessing extremely high values of M₀ were found. There is no apparent correlation between the high k and high M₀ values; on the contrary, the samples with relatively high M₀ values possess average magnetic susceptibility and vice versa. According to the low-field anisotropy of magnetic susceptibility (AMS), three different groups of samples can be distinguished. In the siderite-bearing samples (i), an inverse magnetic fabric is observed, i.e., the maximum and minimum principal susceptibility directions are interchanged and the magnetic fabric has a distinctly prolate shape. Triaxial-fabric samples (ii), showing an intermediate magnetic fabric, are always characterized by high M₀ values. It seems probable that the magnetic fabric is controlled by the preferred orientation of paramagnetic phyllosilicates, e.g., chlorite and mica, and by some ferromagnetic mineral with anomalous orientation in relation to the bedding plane. Oblate-fabric samples (iii) are characterized by a bedding-controlled magnetic fabric, and by moderate magnetic susceptibility and M₀ values. The magnetic fabric is controlled by the preferred orientation of phyllosilicate minerals and, to a minor extent, by a ferrimagnetic fraction, most probably detrital magnetite. Considering the magnetic fabric together with paleomagnetic component analyses, the siderite-bearing, and the high-NRM samples (about 15% of samples) were excluded from further magnetostratigraphic research.     

Time dependence of magnetic susceptibility of soil chronosequences on the California coast

As the age of the soils in a chronosequence on the California coast increases, the difference between the magnetic susceptibility of eluvial and illuvial horizons increases, and the residual susceptibility after extraction with citrate-bicarbonate-dithionite (CBD) decreases. Enhanced susceptibility results from the conversion of nonferrimagnetic minerals to secondary ferrimagnetic forms (most likely maghemite) and the preferential accumulation of inherited and pedogenic magnetic minerals. Little enhancement occurs for pedons younger than 40,000 yr. By 124,000 yr, most of the magnetic susceptibility can be attributed to forms soluble in CBD. Magnetic susceptibility appears to vary systematically over time for three chronosequences from areas with mean annual precipitation ranging from 650 to 1500 mm yr⁻¹. Magnetic susceptibility enhancement may be a useful parameter for estimating soil age in certain climates.     

Environmental magnetic and geochemical characteristics of Chennai coastal sediments,Bay of Bengal,India

In this study, environmental magnetic, heavy metal and statistical analyses were conducted on 21 surface sediments collected from Chennai coast, India, to examine the feasibility of heavy metal pollution using magnetic susceptibility. The Chennai coastal sediment samples are dominated by ferrimagnetic minerals corresponding to magnetite-like minerals. The percentage of frequency dependent magnetic susceptibility reflects the presence of super-paramagnetic/single domain magnetic minerals in Chennai harbour, Cooum and Adayar rivers sediments. High pollution load index in sample E1, E2, CH7, C11, C12 and A16 is mainly due to anthropogenic activities such as, harbour activities, Cooum and Adayar rivers input and industrial effluent. Factor analysis shows that the magnetic concentration dependent parameters (χ, χ _ARM and SIRM) covary with the heavy metal concentrations, suggesting that the input of magnetic minerals and heavy metals in Chennai coastal sediments are derived from the same anthropogenic sources. Strong correlation obtained between pollution load index (PLI) and concentration dependent parameters (χ, χ _ARM and SIRM) for the polluted samples with magnetic susceptibility excess of 50×10^− 8 m³kg^− 1. Significant correlations between heavy metals and magnetic susceptibility point out the potential of magnetic screening/monitoring for simple and rapid proxy indicator of heavy metal pollution in marine sediments.     

Anisotropy of magnetic susceptibility of eclogites: mineralogical origin and correlation with the tectonic fabric (Cabo Ortegal,Spain)

Abstract

The fabric and the anisotropy of magnetic susceptibility of the Cabo Ortegal eclogite (NW Spain) are studied. These mafic rocks were metamorphosed and deformed under high pressures and temperatures between 390 and 370 Ma in a subduction/collision tectonic setting. Massive eclogite slices and deformed eclogite in shear zones have bulk magnetic susceptibilities of 31 to 82·10^?5 S.I. and 28 to 75·10^?5 S.I., respectively. The paramagnetic mineral fraction is the principal magnetic susceptibility carrier. This fraction includes notably garnet and clinopyroxene as matrix minerals, and ilmenite and rutile as accessory constituents. Though magnetic anisotropy degree varies between 3.1 % and 6.6 %, variations of this parameter in each rock type are marked. In the deformed eclogite, magnetic lineation (K_max) and the pole to the magnetic foliation (K_min) are coaxial and coincident with macroscopic petrofabric elements (foliation and lineation). In the massive eclogite, the magnetic fabric is dispersed along the principal structural planes and inversions are associated with samples with small degrees of anisotropy. The anisotropy of magnetic susceptibility is interpreted as being due to the crystallographic preferred orientation and spatial organisation of the polymineralic aggregate. Relating the evolution of the symmetry of magnetic fabric to the symmetry of petrofabric or deformation is rather precluded since susceptibility has multiple origins and bulk magnetic fabric is due to minerals of different symmetry. © Elsevier, Paris     

Magnetic properties and heavy metal content of sanitary leachate sludge in two landfill sites near Bandung,Indonesia

Magnetic properties and heavy metal content of landfill leachate sludge samples from two municipal solid waste disposal sites near Bandung, West Java, Indonesia, and their correlation with heavy metals are studied in the present work. Leachate was found to be sufficiently magnetic with mass-specific magnetic susceptibility that varies from 64.8 to 349.0 × 10⁻⁸ m³ kg⁻¹. It is, however, less magnetic than the soils around the landfill sites. The magnetic minerals are predominantly pseudo-single domain and multidomain magnetite. Leachate samples from the older but inactive disposal site, Jelekong, are found to be more magnetic than that from Sarimukti, the younger and active site. The enhancement of leachate due to the soil-derived ferrimagnetic particles is possibly the same for both Sarimukti and Jelekong. The fact that strong correlation between magnetic parameters and heavy metals is observed in Jelekong but is absent in Sarimukti suggests that the use of magnetic measurement as a proxy measurement for heavy metal content in leachate is plausible provided that the magnetic susceptibility exceeds certain threshold value. Moreover, the accumulation of magnetic minerals and heavy metals in leachate might depend on the activity and the age of landfill site.     

Magnetic susceptibility of ultrahigh pressure eclogite: The role of retrogression

Retrograde metamorphism played the dominant role in changing the low-field rock magnetic properties and density of 198 specimens of variably retrograded eclogites from the main borehole of the Chinese Continental Scientific Drilling Project (CCSD) and from surface outcrops in the Donghai area in the southern part of the Sulu UHP belt, China. Bulk magnetic susceptibility (κ) of unretrogressed UHP eclogite is controlled by whole-rock chemical composition and ranges from 397 to 2312 μSI with principal magnetic susceptibility carrying minerals paramagnetic garnet, omphacite, rutile and phengite. Partially retrograded eclogites show large variations in magnetic susceptibility between 804 and 24,277 μSI, with high mean magnetic susceptibility values of 4372 ± 4149 μSI caused by appreciable amounts of Fe-Ti oxide minerals such as magnetite, ilmenite and/or titanohematite produced by retrograde metamorphic reactions. Completely retrograded eclogites have lower susceptibilities of 1094 ± 600 μSI and amphibolite facies mineral assemblages lacking high magnetic susceptibility minerals. Jelínek's corrected anisotropy (P_j) of eclogites ranges from 1.001 to 1.540, and shows a positive correlation with low-field magnetic susceptibility (κ). Arithmetic mean bulk density (ρ) shows a steady decrease from 3.54 ± 0.11 g/cm³ (fresh eclogite) to 2.98 ± 0.06 g/cm³ (completely retrograded eclogite). Retrograde metamorphic changes in mineral composition during exhumation appear to be the major factor causing variations in low field magnetic susceptibility and anisotropy. Retrograde processes must be taken into account when interpreting magnetic surveys and geophysical well logs in UHP metamorphic terranes, and petrophysical properties such as density and low-field magnetic susceptibility could provide a means for semi-quantifying the degree of retrogression of eclogite during exhumation.     

Anisotropy of magnetic susceptibility of eclogites: Mineralogical origin and correlation with the tectonic fabric (Cabo Ortegal,Spain)

The fabric and the anisotropy of magnetic susceptibility of the Cabo Ortegal eclogite (NW Spain) are studied. These mafic rocks were metamorphosed and deformed under high pressures and temperatures between 390 and 370 Ma in a subduction/collision tectonic setting. Massive eclogite slices and deformed eclogite in shear zones have bulk magnetic susceptibilities of 31 to 82 · 10⁻⁵ S.I. and 28 to 75 · 10⁻⁵ S.I., respectively. The paramagnetic mineral fraction is the principal magnetic susceptibility carrier. This fraction includes notably garnet and clinopyroxene as matrix minerals, and ilmenite and rutile as accessory constituents. Though magnetic anisotropy degree varies between 3.1 % and 6.6%, variations of this parameter in each rock type are marked. In the deformed eclogite, magnetic lineation (K_max) and the pole to the magnetic foliation (K_min) are coaxial and coincident with macroscopic petrofabric elements (foliation and lineation). In the massive eclogite, the magnetic fabric is dispersed along the principal structural planes and inversions are associated with samples with small degrees of anisotropy. The anisotropy of magnetic susceptibility is interpreted as being due to the crystallographic preferred orientation and spatial organisation of the polymineralic aggregate. Relating the evolution of the symmetry of magnetic fabric to the symmetry of petrofabric or deformation is rather precluded since susceptibility has multiple origins and bulk magnetic fabric is due to minerals of different symmetry.     

Relationship between chemical composition and magnetic susceptibility in the alkaline volcanics from the Isparta area,SW Turkey

Potassium-rich volcanic rocks in the Isparta area (SW Turkey) consist mainly of older (Pliocene) volcanic rock suites (e.g., lamprophyre, basaltic trachyandesite, trachyandesite, trachyte) and younger (Quaternary) caldera forming lava dome/flows (e.g., tephriphonolite, trachyte) and pyroclastics (ash/pumice fall deposits and ignimbritic flows). The magnetic susceptibility (K) was performed for both groups. The magnetic susceptibility value of the less evolved rocks characterized by SiO₂ < 57 wt% (e.g., basaltic trachyandesite, tephriphonolite, lamprophyric rocks) and having mostly mafic phenocrysts such as pyroxene, amphibole, and biotite-phlogopite is over 10 (10⁻³ [SI]). Fine to medium-grained and subhedral to anhedral opaque minerals are scattered especially in the matrix phase of the less evolved volcanic rocks. However, the K value of the more evolved rocks (e.g., trachyte and trachyandesites) with SiO₂ over 57 wt% vary between 0.1 and 28, but most of them below 10. SI values are negatively correlated with SiO₂, Na₂O, but positively correlated with Fe₂O₃, CaO, MnO, P₂O₅ and MgO contents, suggesting inverse variation of SI with fractionation of potassic magma. That is to say that less evolved volcanic rocks have relatively higher magnetic susceptibility values in the volcanic suite. Fine to medium-grained and subhedral to anhedral Fe-Ti oxides are scattered mainly in the matrix phase of the less evolved volcanics, presumably cause the pronounced relatively higher magnetic susceptibility.     

Magnetic parameters reflecting pedogenesis in Pleistocene loess deposits of Argentina

Quaternary loess sequences of Argentina, with interbedded loess and buried soils (palaeosols), provide terrestrial records of past climates and environmental conditions. Study of rock magnetic parameters measured over a large area of the Pampean loess seems to indicate that the existing magnetoclimatological models cannot adequately account for the complexities of the Pampean loess.The Chinese loess has been considered as typical, where magnetic properties are largely controlled by pedogenesis. On the other hand, the Siberian loess is an alternative magnetoclimatological model in which palaeosols appear as magnetic lows and the intercalated loess as magnetic highs. Argentine loess is apparently closer to the Siberian model. However, considering the data obtained in Argentina, the situation seems to be more complicated. The higest magnetic values (SIRM and susceptibiliy values) in silty and sandy loess indicate a more efficient entrainment of dense iron oxides particles during stormy dry (glacial) intervals.The parent material shows the highest susceptibility values (>100 × 10⁻⁸ m³/kg) while the waterlogged horizons show the lowest ones (below 20 × 10⁻⁸ m³/kg). Pedogenesis resulting in the development of BC and B soil horizons of palaeosols in the parent loess produced decreases in susceptibility values and increases in the F factor. This phenomenon occurs at some degree of humidity in which the process of gleying caused the total depletion of both susceptibility and frequency factor. The magnentic data allows consideration of the relevance of major cycles (arid/humid) separated by discontinuities as the main factor favoring one particular behavior of the magnetic parameters. The B horizons of palaeosols developed during an arid cycle will not show a notable difference in the magnetic records from the parent material. In contrast, during humid climate condition the pristine loess can be progresively obliterated by pedogensis through to the extreme situation of gleying.     

Crystalline basement relief beneath the Baltic Sea aquatorium,from magnetometric data

The compilation of a magnetic map for the Baltic Sea floor shows it to be an extremely anomalous region whose magnetic anomalies differ markedly in intensity, area and form, trending northwest to meridionally as observed in the adjacent land areas. The trend of the majority of gravity and magnetic anomalies coincide. The magnetic susceptibility of the basement rocks ranges from 500 x 10^?6 to 10,000 x 10^?6 CGSM. The basement plunges north to south (2 km and more). Data suggest that the structure of the Baltic Sea floor does not differ from the adjacent land area and may be considered as a subsided segment of the continental surface. — IGR Staff.     

Granitoid Series in Terms of Magnetic Susceptibility: A Case Study from the Barberton Region,South Africa

Magnetic susceptibilities were measured on a representative collection of Archaean granitoids of the Barberton region using a portable KT5 magnetic susceptibility meter. The studied granitoids comprise, (1) syn-tectonic tonalite-trondhjemite-granodiorite (TTG) granitoids (132 samples), (2) late-tectonic calc-alkaline granitoids (402 samples) and (3) post-tectonic low-Ca and high-Ca granitoids (12 samples). Most of the early-stage syn-tectonic granitoids (∼3450 Ma) have low magnetic susceptibilities, less than 3 × 10⁻³ SI units, and correspond to ilmenite-series granitoids. The late-stage Kaap Valley tonalite pluton (∼3230 Ma) contains sporadically distributed higher magnetic susceptibility values (greater than 3 × 10⁻³ SI units), which are less than one-third in magnetic susceptibility of typical magnetite-series TTG of the Japanese Island Arc and thus strictly belong to an intermediate series. The Barberton TTG suite is essentially derived from reduced amphibolitic lower crust that reflects the anoxic nature of the Earth surface during the Archaean Eon. The more oxidized nature of the Kaap Valley tonalite may be generated in an oxidized lower crust by fluids squeezed out of the subducting plate.Late-tectonic granodiorite - adamellite batholithic complexes (∼3105 Ma) belong mostly to the magnetite series, and seem to suggest that relatively oxidized continental crust, reflecting oxic atmosphere and subduction mechanism operating, had evolved it by this time. Post-tectonic granitic plutons formed largely between circa 2900 Ma and 2700 Ma can be subdivided into low-Ca ilmenite series and high-Ca magnetite series.     

Applications of steranes,terpanes and monoaromatics to the maturation,migration and source of crude oils

Novel biological marker parameters are applied to problems of geochemical correlation of crude oils in the McKittrick Field, California. An attempt is described to distinguish four diagenetic parameters; namely, source input, source maturation, migration and ‘in reservoir’ maturation. The tools include the absolute concentration of steranes, terpanes and paraffins (_{n + iso}) in combination with internal ratios of individual biomarkers such as primary/secondary terpanes, 17α(H)-trisnorhopane/18α(H)-trisnorhopane II (both maturation specific), 5β/5α-steranes, 5β-steranes/17α(H)-hopanes and rearranged steranes/5α-steranes (all migration oriented), 5α/5α-steranes and a number of terpane ratios of partially unknown chemical structure (source input specific).Among other new correlation parameters are: two series of mass chromatograms (m/e 253 and 239), signaling monoaromatized steranes, a series of presumably rearranged steranes (m/e 259), and a series of methylhopanes (m/e 205).The results obtained on the molecular level exceed the degree of information obtainable from organic geochemical ‘bulk’ parameters such as yields of saturates, aromatics, sulfur compounds and C¹³/C¹² ratios by far; however, both types of parameters are mutually supporting. All conclusions are consistent with subtle stratigraphie and overall geologic prerequisites.     

Thermally induced alterations of minerals during measurements of the temperature dependence of magnetic susceptibility: a case study from the hydrothermally altered Soultz-sous-Forêts granite,France

Identification of magnetic minerals using the temperature dependence of magnetic susceptibility in low field (κ–T) combined with optical microscopy, microprobe, X-ray diffraction, and chemical analysis provided new constraints on the alteration of Fe-bearing minerals of the magnetite-bearing Soultz-sous-Forêts granite from the EPS-1 borehole (upper Rhine Graben, France). While relatively fresh granite shows largely reversible κ–T curves typical of magnetite, the altered granite revealed a wide variety of irreversible heating and cooling curves, which allowed an assignment to different alteration stages under specific geochemical conditions. Though paramagnetic minerals like Fe-bearing carbonates, pyrite, or antiferromagnetic hematite could not be detected according to their Curie or Neél temperature, they were identified due to reactions to new ferrimagnetic phases during the heating/cooling experiments at specific temperatures. Mineral reactions were proved by measurements of the single mineral phases hematite, Fe-carbonates, and illite. Our mineralogical results combined with the thermomagnetic measurements imply that first faulting of the granite occurred already during cooling of the magma, which caused a first magnetite oxidation event. During uplift of the granitic body and exposure to a paleo-erosion surface, strongly acidic fluids, emerged from pyrite oxidation, caused a decomposition of Fe-bearing minerals like martite (hematite derived from magnetite oxidation) and Fe-carbonates and an ongoing transformation of magnetite to martite. Subsequently, precipitation of fine-grained hematite was restricted only to the upper part of the pluton. In the deeper part of the borehole, pyrite was preserved from oxidation. In an active fault, zone martite was reduced back to magnetite, which can be explained with the occurrence of organic matter transported by fluids.     

Magnetic measurements and pollutants of sediments from Cauvery and Palaru River,India

Rock-magnetic techniques have become a useful tool in environmental issues; in particular, magnetic studies constitute an alternative way to study pollution in different media. The present contribution focuses on magnetic parameters as pollution indicators, especially from their relationship with contents of heavy metals. The work was carried out in two Indian rivers located in Tamil Nadu, southern India. Several sediment samples were collected and studied in the laboratory using magnetic techniques, magnetic susceptibility, anhysteric remanent magnetization, isothermal remanent magnetization, and chemical techniques to determine contents of heavy metals. Magnetic mineralogy indicates the predominance of ferrimagnetic minerals; although magnetite-like minerals are the main magnetic carriers, antiferromagnetic minerals can be present as subordinate carriers. Concentration-dependent magnetic parameters revealed noticeable differences between both rivers, e.g. magnetic susceptibility is four times higher in Cauvery than in Palaru River. Moreover, such increase can be interpreted as “magnetic enhancement” and therefore related to the pollution status. This magnetic enhancement indicated a different pollutant contribution in both rivers, and also, a different spatial distribution along these rivers, where critical (or more polluted) sites were identified. On the other hand, univariate and multivariate statistical analyses—e.g. PCoordA, Multifactorial Analysis of distance, PCA and RDA—were examined, revealing a link between magnetic and chemical variables. Among magnetic parameters, the concentration-dependent magnetic parameters (e.g. magnetic susceptibility) seem to be the most relevant for this study.     

Effects of Deep Fluids on Hydrocarbon Generation and Accumulation in Chinese Petroliferous Basins

Deep fluids in a petroliferous basin generally come from the deep crust or mantle beneath the basin basement, and they transport deep substances(gases and aqueous solutions) as well as heat to sedimentary strata through deep faults. These deep fluids not only lead to large-scale accumulations of CO_2, CH_4, H_2, He and other gases, but also significantly impact hydrocarbon generation and accumulation through organic-inorganic interactions. With the development of deep faults and magmatic-volcanic activities in different periods, most Chinese petroliferous basins have experienced strong impacts associated with deep fluid activity. In the Songliao, Bohai Bay, Northern Jiangsu, Sanshui, Yinggehai and Pearl Mouth Basins in China, a series of CO_2 reservoirs have been discovered. The CO_2 content is up to 99%, with δ~(13)C_(CO2) values ranging from-4.1‰ to-0.37‰ and ~3He/~4He ratios of up to 5.5 Ra. The abiogenic hydrocarbon gas reservoirs with commercial reserves, such as the Changde, Wanjinta, Zhaozhou, and Chaoyanggou reservoirs, are mainly distributed in the Xujiaweizi faulted depression of the Songliao Basin. The δ~(13)CCH4 values of the abiogenic alkane gases are generally -30‰ and exhibit an inverse carbon isotope sequence of δ~(13)C_(CH4)δ~(13)C_(C2H6)δ~(13)C_(C3H8)δ~(13)C_(C4H10). According to laboratory experiments, introducing external H_2 can improve the rate of hydrocarbon generation by up to 147% through the kerogen hydrogenation process. During the migration from deep to shallow depth, CO_2 can significantly alter reservoir rocks. In clastic reservoirs, feldspar is easily altered by CO_2-rich fluids, leading to the formation of dawsonite, a typical mineral in high CO_2 partial pressure environments, as well as the creation of secondary porosity. In carbonate reservoirs, CO_2-rich fluids predominately cause dissolution or precipitation of carbonate minerals. The minerals, e.g., calcite and dolomite, show some typical features, such as higher homogenization temperatures than the burial temperature, relatively high concentrations of Fe and Mn, positive Eu anomalies, depletion of 18 O and enrichment of radiogenic ~(87)Sr. Due to CO_2-rich fluids, the development of high-quality carbonate reservoirs is extended to deep strata. For example, the Well TS1 in the northern Tarim Basin revealed a high-quality Cambrian dolomite reservoir with a porosity of 9.1% at 8408 m, and the Well ZS1 C in the central Tarim Basin revealed a large petroleum reserve in a Cambrian dolomite reservoir at ~6900 m. During the upward migration from deep to shallow basin strata, large volumes of supercritical CO_2 may extract petroleum components from hydrocarbon source rocks or deep reservoirs and facilitate their migration to shallow reservoirs, where the petroleum accumulates with the CO_2. Many reservoirs containing both supercritical CO_2 and petroleum have been discovered in the Songliao, Bohaiwan, Northern Jiangsu, Pearl River Mouth and Yinggehai Basins. The components of the petroleum trapped with CO_2 are dominated by low molecular weight saturated hydrocarbons.     

钻孔岩心红外光谱-便携式XRF-磁化率测试在攀西太和钒钛磁铁矿床勘查中的应用

近年来,红外光谱技术因其可以绿色、快速、无损、精确探测矿物和提高勘查效率而备受关注.攀西超大型太和钒钛磁铁矿床位于镁铁质-超镁铁质层状岩体中,该矿床的典型矿物的红外光谱特征研究相对缺乏,制约了勘查效率的提高.本文应用便携式傅里叶变换红外光谱仪对四川太和钒钛磁铁矿床钻孔ZK1307岩心开展热红外光谱测试工作,并辅以便携式...     

Distribution,sediment magnetism and geochemistry of the Saksunarvatn (10 180 ± 60 cal. yr BP) tephra in marine,lake, and terrestrial sediments,northwest Iceland

In 1997, seismic surveys in the troughs off northwest and north Iceland indicated the presence of a major, regional sub‐bottom reflector that can be traced over large areas of the shelf. Cores taken in 1997, and later in 1999 on the IMAGES V cruise, penetrated through the reflector. In core MD99‐2269 in Húnaflóaáll, this reflector is shown to be represented by a basaltic tephra with a geochemical signature and radiocarbon age correlative with the North Atlantic‐wide Saksunarvatn tephra. We trace this tephra throughout northwest Iceland in a series of marine and lake cores, as well as in terrestrial sediments; it forms a layer 1 to 25 cm thick of fine‐ to medium‐grained basaltic volcanic shards. The base of the tephra unit is always sharp but visual inspection and other measurements (carbonate and total organic carbon weight %) indicate a more diffuse upper boundary associated with bioturbation and with sediment reworking. Off northwest Iceland the Saksunarvatn tephra has distinct sediment magnetic properties. This is evident as a dramatic reduction in magnetic susceptibility, an increase in the frequency dependant magnetic susceptibility and ‘hard’ magnetisation in a −0.1T IRM backfield. Geochemical analyses from 11 sites indicate a tholeiitic basalt composition, similar to the geochemistry of a tephra found in the Greenland ice‐core that dates to 10 180 ± 60 cal. yr BP, and which was correlated with the 9000 ¹⁴C yr BP Saksunarvatn tephra. We present accelerator mass spectrometry ¹⁴C dates from the marine sites, which indicate that the ocean reservoir correction is close to ca. 400 yr at 9000 ¹⁴C yr BP off northwest Iceland. Copyright © 2002 John Wiley & Sons, Ltd.     

Hydrocarbon Accumulation Process in the Marine Strata in Jianghan Plain Area,Middle China

Marine strata in the Jianghan Plain area are widely distributed with a total depth of more than 8,000 m from the Upper Sinian to the Middle Triassic. Six reservoir caprock units, named Z–∈2, ∈2–O, S, D–C, P and T1, can be identified with each epoch. The geology, stratigraphy, drilling, oil testing and other basic data as well as the measured inclusion and strontium isotope data in the study area are used in the analysis of the formation and evolution process of marine petroliferous reservoirs in the Jianghan Plain area. This study aims to provide a scientific basis for the further exploration of hydrocarbons in the Jianghan Plain and reduce the risks by analyzing the key factors for hydrocarbon accumulation in the marine strata. Our findings show that in the Lower Palaeozoic hydrocarbon reservoir, oil/gas migration and accumulation chiefly occurred in the early period of the Early Yanshanian, and the hydrocarbon reservoir was destroyed in the middle–late period of the Early Yanshanian. In the Lower Triassic–Carboniferous hydrocarbon reservoir, oil/gas migration and accumulation chiefly occurred in the Early Yanshanian, and the hydrocarbon reservoir suffered destruction from the Late Yanshanian to the Early Himalayanian. The preservation conditions of the marine strata in the Jianghan Plain area have been improved since the Late Himalayanian. However, because all source beds have missed the oil/gas generation fastigium and lost the capacity to generate secondary hydrocarbon, no reaccumulation of hydrocarbons can be detected in the study area's marine strata. No industrially exploitable oil/gas reservoir has been discovered in the marine strata of Jianghan Plain area since exploration began in 1958. This study confirms that petroliferous reservoirs in the marine strata have been completely destroyed, and that poor preservation conditions are the primary factor leading to unsuccessful hydrocarbon exploration. It is safely concluded that hydrocarbon exploration in the marine strata of the study area is quite risky.