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1.
The Kangankunde Carbonatite Complex from the Cretaceous Chilwa Alkaline Province in southern Malawi contains ankeritic and siderite carbonatite that are affected by late stage remobilisation by a carbothermal or hydrothermal fluid. The coarse pegmatitic siderite carbonatite that hosts exotic minerals like monazite, synchysite, bastnasite, strontianite and apatite in vugs and cavities constitutes some of the richest rare earth deposits in the world. Besides these minerals, our studies reveal the presence of collinsite and aragonite from the siderite carbonatite. Fine drusy monazites are seen as overgrowths on thin veinlets of siderite within the rare earth mineralised zones. We present unambiguous SEM-based surface textural evidence such as presence of dissolution-corrosion features like etching along cleavage, solution channels, solution pits, sinstered scaly surface, etc. along with rare earth mineralisation that suggests the exotic minerals in the siderite carbonatite did not crystallise from carbonate magma and are a result of sub-solidus processes involving carbonatite-derived fluids. We believe that the monazite-synchysitebastnasite-strontianite-collinsite assemblages were formed by juvenile post magmatic hydrothermal alteration of pre-existing carbonatite by a complex CO2-rich and alkali chloride-carbonate-bearing fluid at ～250 to 400°C in an open system. This late ‘magmatic’ to ‘hydrothermal’ activity was responsible for considerable changes in rock texture and mineralogy leading to mobility of rare earth elements during fluid-rock interaction. These aspects need to be properly understood and addressed before using trace and rare earth element (REE) geochemistry in interpreting carbonatite genesis.  相似文献   

2.
《Chemical Geology》1992,94(4):321-329
Light hydrocarbon and isotope compositions of methane were analyzed in well steam samples from the Matsukawa vapour-dominated type geothermal system. Alkanes (C1-C4) and alkene (C2) were detected in all samples. Light hydrocarbon contents of CO2-type steam are slightly higher than those of CO2-H2S-type steam. The isotope composition of methane and the relationship between methane/ethane ratio and δ13C-value of methane suggest that these light hydrocarbon gases are mixtures of thermogenic and abiogenic components. The abiogenic hydrocarbon may be attributed to magmatic hydrocarbon gases equilibrated with carbon dioxide at fo2 defined by the fayalite-magnetite-quartz buffer (FMQ).  相似文献   

3.
松辽盆地庆深气田异常氢同位素组成成因研究   总被引：2，自引：0，他引：2  
对松辽盆地徐家围子断陷庆深气田天然气组分、碳氢同位素和稀有气体同位素的分析表明,天然气以烷烃气为主,烷烃气碳同位素组成随着碳数增加呈变轻趋势,且δ13C1＆gt;-30‰, R/Ra一般大于1.0,δ13CCO2值介于-16.5‰~-5.1‰之间;氢同位素组成δD1=-205‰~-197‰,平均值为-203‰,δD2=-247‰~-160‰,平均值为-195‰,δD3=-237‰~-126‰,平均值为-163‰,且存在氢同位素组成倒转现象,即δD1＆gt;δD2＆lt;δD3。根据对庆深气田天然气不同地球化学特征分析,认为该气田烷烃气中重烃主要为有机成因,而 CH4有相当无机成因混入。庆深气田烷烃气氢同位素组成具有 CH4变化小,而重烃（δD2,δD3）变化大的特点。根据与朝阳沟地区天然气烷烃气氢同位素组成对比分析,认为 CH4主要表现为无机成因,而重烃气（δD2,δD3）主要为有机成因,且无机成因CH4氢同位素组成重于有机成因CH4。  相似文献   

4.
Previous studies of methane and higher hydrocarbon gases in Precambrian Shield rocks in Canada and the Witwatersrand Basin of South Africa identified two major gas types. Paleometeoric waters were dominated by hydrocarbon gases with compositional and isotopic characteristics consistent with production by methanogens utilizing the CO2 reduction pathway. In contrast the deepest, most saline fracture waters contained gases that did not resemble the products of microbial methanogenesis and were dominated by both high concentrations of H2 gas, and CH4 and higher hydrocarbon gases with isotopic signatures attributed to abiogenic processes of water-rock reaction in these high rock/water ratio, hydrogeologically-isolated fracture waters. Based on new data obtained for the higher hydrocarbon gases in particular, a model is proposed to account for carbon isotope variation between CH4 and the higher hydrocarbon gases (specifically ethane, propane, butane, and pentane) consistent with abiogenic polymerization. Values of δ13C for CH4 and the higher hydrocarbon gases predicted by the model are shown to match proposed abiogenic hydrocarbon gas end-members identified at five field sites (two in Canada and three in South Africa) suggesting that the carbon isotope patterns between the hydrocarbon homologs reflect the reaction mechanism. In addition, the δ2H isotope data for these gases are shown to be out of isotopic equilibrium, suggesting the consistent apparent fractionation observed between the hydrocarbon homologs may also reflect reaction mechanisms involved in the formation of the gases. Recent experimental and field studies of proposed abiogenic hydrocarbons such as those found at mid-ocean spreading centers and off-axis hydrothermal fields such as Lost City have begun to focus not only on the origin of CH4, but on the compositional and isotopic information contained in the higher hydrocarbon gases. The model explored in this paper suggests that while the extent of fractionation in the first step in the hydrocarbon synthesis reaction chain may vary as a function of different reaction parameters, δ13C values for the higher hydrocarbon gases may be predicted by a simple mass balance model from the δ13C values of the lower molecular weight precursors, consistent with abiogenic polymerization. Integration of isotopic data for the higher hydrocarbon gases in addition to CH4 may be critical for delineation of the origin of the hydrocarbons and investigation of formation mechanisms.  相似文献   

5.
Relationships between methane and its homologues mainly contained in fluid microinclusions have been studied in 332 monomineralic fractions from the Khibiny and Lovozero alkaline plutons. Hydrocarbon gases (HCG) were extracted for subsequent chromatographic analysis using the bulk method of sample comminution. The molecular weight distribution (MWD) of gaseous alkanes in the same and associated minerals is different depending on geological setting of the samples. The molecular mass of HCG increases (i) with decrease in temperature and capture of fluid inclusions in the course of transformation of primary magmatic minerals and the formation of late minerals as products of intensified postmagmatic processes; (ii) in the direction from khibinite at the margin and foyaite in the core of the Khibiny pluton to the central ring structure; and (iii) from the bottom to top of the differentiated complex in the Lovozero pluton. The results obtained coupled with other geochemical data suggest multistage generation and transformation of hydrocarbons from the magmatic to the final low-temperature hydrothermal stage. The MWD of hydrocarbon components in gases occluded by minerals can serve as an indicator of conditions characteristic of rock and ore formation, as well as of the consecutive formation and transformation of associated minerals revealing ambiguous and controversial relationships.  相似文献   

6.
Although hydrocarbon-bearing fluids have been known from the alkaline igneous rocks of the Khibiny intrusion for many years, their origin remains enigmatic. A recently proposed model of post-magmatic hydrocarbon (HC) generation through Fischer-Tropsch (FT) type reactions suggests the hydration of Fe-bearing phases and release of H2 which reacts with magmatically derived CO2 to form CH4 and higher HCs. However, new petrographic, microthermometric, laser Raman, bulk gas and isotope data are presented and discussed in the context of previously published work in order to reassess models of HC generation. The gas phase is dominated by CH4 with only minor proportions of higher hydrocarbons. No remnants of the proposed primary CO2-rich fluid are found in the complex. The majority of the fluid inclusions are of secondary nature and trapped in healed microfractures. This indicates a high fluid flux after magma crystallisation. Entrapment conditions for fluid inclusions are 450–550 °C at 2.8–4.5 kbar. These temperatures are too high for hydrocarbon gas generation through the FT reaction. Chemical analyses of rims of Fe-rich phases suggest that they are not the result of alteration but instead represent changes in magma composition during crystallisation. Furthermore, there is no clear relationship between the presence of Fe-rich minerals and the abundance of fluid inclusion planes (FIPs) as reported elsewhere. δ13C values for methane range from − 22.4‰ to − 5.4‰, confirming a largely abiogenic origin for the gas. The presence of primary CH4-dominated fluid inclusions and melt inclusions, which contain a methane-rich gas phase, indicates a magmatic origin of the HCs. An increase in methane content, together with a decrease in δ13C isotope values towards the intrusion margin suggests that magmatically derived abiogenic hydrocarbons may have mixed with biogenic hydrocarbons derived from the surrounding country rocks.  相似文献   

7.
松辽盆地徐家围子断陷无机成因天然气及其成藏模式   总被引：37，自引：0，他引：37  
杨玉峰 《地学前缘》2000,7(4):523-533
通过对徐家围子地区天然气组分、同位素资料以及气体包裹体资料的综合分析认为 ,该地区存在无机成因天然气。烃类气体中具有重碳同位素异常 ( >- 2 0‰ )和负碳同位素系列(δ13C1>δ13C2 >δ13C3)的同位素分布特征 ,表现出无机成因烃类气体的特点。高含量的无机CO2 气体和幔源氦的发现 ,指示着该区无机成因天然气主要来自于地球的深部。地幔热柱上升在地壳不同圈层中引起的岩浆火山活动是无机天然气的主要来源 ,而由此形成的地壳“网状”结构和基底大断裂是无机气上升运移至浅部聚集成藏的主要运移通道。构造活动间歇期和活动期分别以渗流方式和幕式涌流方式向上运移可能是深部流体运移的两种主要方式。古隆起带、岩浆侵入带以及基底大断裂带是无机成因天然气的有利聚集带。  相似文献   

8.
通过研究黄骅坳陷新生代的构造活动,对黄骅坳陷无机成因二氧化碳气藏的形成与分布特征进行分析,发现黄骅坳陷无机成因CO2成藏与断裂和岩浆活动密切相关。新生代的NE—NEE向的伸展断裂为幔源型二氧化碳提供了良好的通道,尤其是在这些NE—NEE向伸展断裂与NW向断裂的交汇处,有利于二氧化碳的运移和聚集;同时这些伸展断裂往往是地热较高的区域,有利于热变质成因二氧化碳的生成;岩浆作用不仅可以直接释放来自幔源的二氧化碳气体,而且岩浆作用带来的地热也可以促进碳酸盐岩的热变质作用,释放二氧化碳。同时,黄骅坳陷新生代断裂和岩浆活动的迁移性特点形成了无机成因二氧化碳气藏地域分布特征。  相似文献   

9.
This paper discusses the results of a light hydrocarbon gas (LHG) lithogeochemical survey carried out at Shannonbridge in central Ireland. Two methods of LHG extraction (dry grinding and sample solution in EDTA) are compared with a heat extraction method developed by Carter (1981). The results show that due to a more complete breakdown of the sample, LHG contents of up to an order of magnitude greater are released by grinding and sample solution, with no apparent decrease in precision. As a result, a potential problem related to the generation of additional gases by the thermal breakdown of organic matter during heating, can be avoided.Similar LHG component distribution patterns are produced by dry grinding and EDTA extraction suggesting that both methods could be useful alternatives to the heating technique. This is strengthened by the identification of a possible target zone in Shannonbridge based on the distribution of samples depleted in methane and enriched in butyl gases, C4 (butane plus butene plus their associated isomers). However, this empirical interpretation was not supported by the results of heating. Furthermore, subsequent exploration failed to establish the presence of a significant mineral occurrence at Shannonbridge. Evidently, the LHG patterns were either misinterpreted or they reflect hydrocarbon sources which are not related to mineralisation.Further evaluation of the data showed that LHG component distributions are significantly influenced by variations in lithology. Once this effect was reduced, it became apparent that the patterns are principally controlled by regional faulting. Since fault structures would be permeable to hydrocarbon gases formed from a variety of processes (such as maturation of organic matter in the host limestones), no relationship with buried mineralisation is necessary to explain the patterns observed.By using the Shannonbridge data as an example, this paper attempts to emphasise the equivocal nature of data collected using prospecting methods of this type. The aim is not to refute innovative and quality research already carried out in this field because the association of hydrocarbon gases with mineralisation is too frequent to be simply classified as coincidental. However, anomalous levels of LHG can be attributed to other processes which, in many cases, are too easily overlooked.  相似文献   

10.
South Greenland has been the site of historic mining of cryolite, copper, graphite and gold, hosts mineral deposits with gold, uranium, zinc, niobium, tantalum, zirconium, hafnium, REE, iron, titanium, vanadium, fluorite and graphite, and has additional potential for lithium, beryllium, phosphorus, gallium and thorium. Data from stream sediment geochemical surveys document that South Greenland is enriched in a range of these elements relative to the rest of Greenland and to estimates of the upper crust composition. Distribution patterns for individual elements within south Greenland exhibit enriched regions that are spatially related to lithological units, crustal structure and known mineralisation.The Northern Domain of South Greenland includes the southernmost part of the orthogneiss-dominated North Atlantic craton. Orogenic gold mineralisation is hosted by quartz veins and hydrothermally altered rocks associated with shear zones intersecting the Mesoarchaean Tartoq Group of mafic metavolcanic rocks. Geochemical exploration indicates that additional potential for gold mineralisation exists within Palaeoproterozoic supracrustal rocks overlying the Archaean basement.Rocks formed during the Palaeoproterozoic Ketilidian orogeny occupy a major part of South Greenland and has been divided into two domains. The Central Domain is underlain by the Julianehåb igneous complex forming a 100 km wide ENE–WSW zone centrally across South Greenland. Intrusive and extrusive, mostly felsic magmatic rocks were emplaced in two main stages (1850–1830 and 1800–1780 Ma) in a continental arc setting. Positive anomalies in aeromagnetic data indicate that mafic plutons are common in the late igneous complex. Intra-arc mafic metavolcanic rocks contain syngenetic stratabound copper sulphide and epigenetic shear zone-hosted copper–silver–gold mineralisation at Kobberminebugt and Kangerluluk, whereas metasedimentary and metapyroclastic rocks contain stratabound uraninite mineralisation. Orthomagmatic iron–titanium–vanadium mineralisation is hosted by a gabbro. A potential for porphyry-type mineralisation related to the late intrusive stages of the Julianehåb igneous complex is suggested by showings with copper, molybdenum and gold together with stream sediment anomalies for these elements. Vein-type uranium mineralisation occurs in fault zones in the Julianehåb igneous complex related to Mesoproterozoic rifting.The Southern Domain contains an assemblage of Palaeoproterozoic metasedimentary and metavolcanic rocks that underwent moderate to strong deformation, peak HT–LP metamorphism and partial melting with subsequent retrograde exhumation at 1790–1765 Ma. The supracrustal rocks contain syngenetic Au, As, Sb, U, and Zn mineralisation in volcanic or graphite- and sulphide-rich sedimentary environments; graphite was mined historically at two sites. Many stream sediment gold anomalies are located in a NE-trending belt along the boundary between the early Julianehåb complex and the supracrustal rocks to the south. They reflect a number of auriferous quartz vein occurrences, including the Nalunaq gold deposit, hosted in a system of shear zones and probably generated as orogenic gold during Ketilidian accretion. The 1755–1730 Ma, A-type Ilua plutonic suite is the latest magmatic event in the Ketilidian orogen.The 1300–1140 Ma Gardar period involved continental rifting, sedimentation and alkaline magmatism. Numerous dykes and 10 ring-shaped intrusion complexes were formed across South Greenland. An orthomagmatic iron–titanium–vanadium deposit is hosted by troctolitic gabbro. Residual magmas and fluids resulting from extreme magmatic differentiation, possibly combined with assimilation of older crust, created mineral deposits including cryolite that was mined at Ivigtut, large low-grade deposits of uranium–rare earth elements–zinc at Kvanefjeld and tantalum–niobium–rare earth element–zirconium at Kringlerne, in the Ilímaussaq complex, as well as tantalum–niobium–rare earth elements at Motzfeldt Sø in the Igaliko complex.The South Greenland crustal evolution records effects of mantle processes, such as lithospheric extension, subduction and underplating, which resulted in recurrent magma emplacement in tectonically active environments. As such, the geology of South Greenland reflects events and circumstances that are favourable to the generation and preservation of hydrothermal ore-forming fluid systems during the Ketilidian orogeny as well as to the development of extreme rock compositions within the Gardar alkaline igneous province.  相似文献   

11.
Gold and base metals of the Mpanda Mineral Field (MMF) is the focus of this paper. Gold veins and gold-bearing base metal occurrences are structurally controlled by conjugate sets of NW–SE and E–W trending faults and/or shear zones that crosscut high-grade metamorphic rocks and post-kinematic intrusions. It was anticipated that Palaeoproterozic country rocks could have been potential host rocks for the gold mineralisation in this area. This argumentation was based on Pb model ages of various deposits from the MMF. Recent fieldwork and Pb isotopic results presented herein indicate that epigenetic gold and base metal vein-type mineralisation in the MMF is post-Palaeoproterozoic.Our Pb isotope study concentrates on constraining the sources of metals in gold-bearing quartz reefs and base metal occurrences. Pb isotopes of whole rocks and minerals indicate that mineralisation was emplaced during the Neoproterozoic, contemporaneous with the intrusion of alkaline granites and carbonatite complexes (e.g., Sangu–Ikola carbonatite complex) at 720 Ma. The source of Pb in the mineral occurrences is compatible with that characteristic of the Palaeoproterozoic host rocks. Aeromagnetic data suggest that the gold-bearing, NNW–SSE trending area continues to the north beyond Mpanda town. Pb isotope results and aeromagnetic data have significant implications for future exploration programs within the region, in that the search should potentially focus on the defined geophysical borders and trendlines, and on Neoproterozoic, rather than Palaeoproterozoic vein systems.  相似文献   

12.
Complex structure, poorly understood sedimentology and poor biostratigraphic control make the Upper Jurassic Humber Group of the South Central Graben one of the least understood and most complex hydrocarbon reservoirs of the North Sea. Detailed logging of available core from 19 exploration wells has been combined with an improved understanding of the relevance of trace fossils and a recognition of important base-level variations to provide a greatly enhanced understanding of the depositional system active within the area at that time. A new sedimentological model, based upon the distribution of facies and facies associations, illustrates that Upper Jurassic structure and consequent basin geometry were the principal controls upon the distribution of depositional environments. Rifting and second-order transgression controlled the back-stepping onlap patterns observed and higher frequency base-level fluctuations controlled the internal architecture of individual sandbodies. The model presented accounts for features of these deposits that were previously considered anomalous, such as the thickness of bioturbated sandstones, paucity of foreshore deposits and complex age relationships of sands.  相似文献   

13.
A review of the geochemistry of methane in natural gas hydrate   总被引：7，自引：0，他引：7  
The largest accumulations on Earth of natural gas are in the form of gas hydrate, found mainly offshore in outer continental margin sediment and, to a lesser extent, in polar regions commonly associated with permafrost. Measurements of hydrocarbon gas compositions and of carbon-isotopic compositions of methane from natural gas hydrate samples, collected in subaquatic settings from around the world, suggest that methane guest molecules in the water clathrate structures are mainly derived by the microbial reduction of CO2 from sedimentary organic matter. Typically, these hydrocarbon gases are composed of > 99% methane, with carbon-isotopic compositions (δ13CPDB) ranging from − 57 to − 73‰. In only two regions, the Gulf of Mexico and the Caspian Sea, has mainly thermogenic methane been found in gas hydrate. There, hydrocarbon gases have methane contents ranging from 21 to 97%, with δ13C values ranging from − 29 to − 57‰. At a few locations, where the gas hydrate contains a mixture of microbial and thermal methane, microbial methane is always dominant. Continental gas hydrate, identified in Alaska and Russia, also has hydrocarbon gases composed of > 99% methane, with carbon-isotopic compositions ranging from − 41 to − 49‰. These gas hydrate deposits also contain a mixture of microbial and thermal methane, with thermal methane likely to be dominant. Published by Elsevier Science Ltd  相似文献   

14.
The geological and mineralogical data on the Chailag-Khem F-Ba-Sr-REE occurrence in the Western Sayan Range, Russia, are discussed. The chemical compositions of rocks, ores, and minerals (ICP-MS, Link) are reported. The occurrence is localized in a tectonic crush zone composed of Cambrian quartz-sericite slates intruded by quartz syenite porphyry. Ore mineralization occurs as veins, cement of tectonic breccia, and metasomatic disseminations in host rocks. Massive ore consists of calcite, strontianite, and quartz; impregnations of euhedral fluorite, ankerite, and bastnaesite crystals; and fine-grained barite aggregate. Accessory minerals include parisite, synchysite, barytocelestine, sulfides, rutile, and uraninite. Late metasomatic calcite and strontianite segregations and veinlets are abundant. In genetic, mineralogical, and geochemical features, the Chailag-Khem occurrence is similar to the Late Mesozoic carbonatite deposits of Central Tuva, of which the Karasug Fe-F-Ba-Sr-REE deposit is the largest and best known. All carbonatite deposits and occurrences are located within a longitudinal zone transverse to the major tectonic elements of the region.  相似文献   

15.
The c. 2060 Ma Phalaborwa Igneous Complex forms an elongate intrusion into Archean granitic gneiss. The carbonatite within the central pyroxenite core of the complex (Loolekop) is well-mineralized in copper. Open pit mining operations started in 1965, followed by underground block caving in 2003. Although little attention has been paid to large-scale structures associated with intrusive phases and mineralization, ongoing infrastructure development and block caving, as part of the new Lift II Project, require far greater resolution of structural discontinuities. 3D modelling of these structures, from over 50 years of data, reveals that Loolekop occurs at the confluence of several major shears or fault zones. Of these, five major structures were pivotal in the emplacement of banded carbonatite, transgressive carbonatite and very late-stage, narrow, E-W trending, sulphide veinlets with short down-dip and along-strike extensions, which form the bulk of mineralization. Modelled structures typically have two or more segments, which are rotated with respect to one another, in turn suggesting repeated rotation or torsion of the entire intrusive volume, aided by cross-cutting structures. The oldest structure is the N-S trending Mica Fault Zone, which shows the same trend as the entire carbonatite complex and the nearby eastern edge of the Kaapvaal Craton and the Lebombo Lineament. The youngest structure is the Central Fault, which shows an E-W inflection that is co-incident with the carbonatite and the E-W, vein-hosted Cu mineralization trend. Based on cross-cutting relationships, sinistral movement along the Central Fault Zone and its localized E-W dilational jog is invoked as a mechanism for transgressive carbonatite emplacement and the introduction of late-stage Cu-rich fluids into numerous tensional veinlets. This shearing would have been caused by an E-W trending maximum principal stress orientation. In turn, this corresponds with the orientation of near-field, eastward-directed stress along the eastern lobe of the Bushveld Complex during its emplacement and subsequent deformation.  相似文献   

16.
In this study, compositions and δ13C and δ2H isotopic values of hydrocarbon gases from 5 mines in the Witwatersrand basin, South Africa, support the widespread occurrence of microbially produced methane in millions of years-old fissure waters. The presence of microbial methane is, to a large extent, controlled by the geologic formations in which the gases are found. Samples from the Witwatersand Supergroup have the largest microbial component based on δ13C and δ2H signatures and CH4/C2+ values. Based on mixing between a microbial CH4 component and a more 13C-enriched and 2H-depleted C2+-rich end member, conservative estimates of the % contribution of microbial CH4 to the gas samples range from >90% microbial CH4 at Beatrix, Masimong, and Merriespruit, to between 5 and 80% microbial CH4 at Evander, and <18% microbial CH4 at Kloof. The Witwatersrand basin’s history of thermal alteration of organic-rich ancient sedimentary units suggests a thermogenic origin for this 13C-enriched end member. Alternatively, the potential for an abiogenic origin similar to hydrocarbon gases produced by water-rock interaction at other Precambrian Shield mines is discussed. Microbial methane is predominantly found in paleo-meteoric fissure waters with δ18O and δ2H values that fall on the meteoric waterline, and have temperatures between 30 to 40°C. In contrast, fissure waters with a larger component of nonmicrobial hydrocarbon gases show a trend towards more enriched δ18O and δ2H values that fall well above the meteoric waterline, and temperatures of 45 to 60°C. The enrichment in 18O and 2H in these samples, and their high salinity, are similar to the isotopic and compositional characteristics of saline groundwaters and brines produced by water-rock interaction at Precambrian Shield sites elsewhere. The reported 100 Ma ages of fissure waters from the Witwatersrand and Ventersdorp formations suggest that these microbial hydrocarbon gases are the product of in situ methanogenic communities in the deep subsurface of the Witswaterand basin. Small subunit ribosomal RNA genes were amplified using archaeal-specific primer sets from DNA extracts derived from several of these waters. Fissure waters with a high proportion of microbial methane also contained sequences resembling those of known methanogens.  相似文献   

17.
Ordovician natural gases in the Tahe Oilfield are composed predominantly of hydrocarbon gases dominated by methane with a significant amount of heavy hydrocarbon gas component. The non-hydrocarbon gases include N2, CO2 and minor H2S. The Ordovician natural gases are believed to have originated from the same source rocks, and are composite of gases differing in thermal maturity. Carbon dioxide was derived from thermal metamorphism of Ordovician carbonate rocks. The generation of natural gases involves multiple stages from mature normal oil and condensate-associated gas to thermally cracked gas at the maturity to over-maturity stages. In the main part of the Tahe Oilfield, the Ordovician natural gases appear to be filled in two major phases with a typical petroleum-associated gas from southeast to northwest and from east to west in the early stage; and a thermally cracked gas from east to west in the late stage. At the same time, the oil/gas filling boundary has been primarily established between the two stages.  相似文献   

18.
Examination of mineral deposits in the Central Anatolian Crystalline Complex provides broad new insights regarding their genesis. Collision and postcollision-related magmatic processes during closure of the northern branch of the Neotethyan Ocean, caused by northward subduction of the oceanic crust beneath the Sakarya Microcontinent in the Late Cretaceous-Eocene, led to the formation of several types of mineral deposits. These include: (1) skarn-type deposits (Pb-Zn, Fe, and Fe-W skarns); (2) vein-type deposits (molybdenum, fluo-rite, stibnite-cinnabar, and stibnite-cinnabar-scheelite vein deposits); (3) sedimentary diatomite, kaolinite, salt, and uranium deposits; and (4) volcanogenic perlite, pumice, and sulfur deposits. Considering their regional distribution and relationship to the geologic evolution of the region, the skarn and vein deposits constitute an important part of the metallogeny of the Central Anatolian Crystalline Complex.  相似文献   

19.
Shale gas exploration and development carry the risk of causing groundwater contamination and enhancing the greenhouse effect through methane leakage. Identifying the source of abnormal methane in groundwater of shale gas development areas is becoming a research hotspot in the fields of groundwater and climate change. This paper reviews the traditional methodology in identifying sources of methane and its deficiency in groundwater application. Then potential and advantages of using noble gases were discussed on how to overcome these limitations of the traditional method. Finally, based on noble gas, the current application status and future challenges of methane source identification in groundwater were analyzed. It can be summarized as:(1) due to chemical and/or microbial processes in the aquifer system, the traditional methodology for methane source identification, which utilizes molecular and isotopic compositions of hydrocarbon gas, has multiple interpretationsand large uncertainties;(2) the non-reactive nature and well-characterized isotopic compositions of noble gases in the atmosphere, hydrosphere, and crust, make noble gases ideal indicators of the sources of methane in groundwater. Moreover, the mechanism of formation and release of crustal noble gas prevent shale gas signatures from being interfered with by natural gas;(3) the key scientific tasks surrounding the use of noble gases for methane source identification include quantitatively separating the components of atmosphere-derived, mantle-derived, and crust-derived noble gases from the bulk noble gases in groundwater. It quantifies the solubility fractionation of noble gases induced by water-gas interaction during methane migration to the aquifer. The application of noble gases can bring a new perspective to tracing the source of methane in groundwater and is of great significance to the protection of groundwater quality in shale gas development areas and mitigation of climate change.  相似文献   

20.
A large suite of natural gases (93) from the North West Shelf and Gippsland and Otway Basins in Australia have been characterised chemically and isotopically resulting in the elucidation of two types of gases. About 26% of these gases have anomalous stable carbon isotope compositions in the C1–C4 hydrocarbons and CO2 components, and are interpreted to have a secondary biogenic history. The characteristics include unusually large isotopic separations between successive n-alkane homologues (up to +29‰ PDB) and isotopically heavy CO2 (up to +19.5‰ PDB). Irrespective of geographic location, these anomalous gases are from the shallower accumulations (600–1700 m) where temperatures are lower than 75°C. The secondary biogenic gases are readily distinguishable from thermogenic gases (74% of this sample suite), which should assist in the appraisal of hydrocarbons during exploration where hydrocarbon accumulations are under 2000 m. While dissolution effects may have contributed to the high 13C enrichment of the CO2 component in the secondary biogenic gases, the primary signature of this CO2 is attributed to biochemical fractionation associated with anaerobic degradation and methanogenesis. Correlation between biodegraded oils and biodegraded “dry” gas supports the concept that gas is formed from the bacterial destruction of oil, resulting in “secondary biogenic gas”. Furthermore, the prominence of methanogenic CO2 in these types of accumulations along with some isotopically-depleted methane provides evidence that the processes of methanogenesis and oil biodegradation are linked. It is further proposed that biodegradation of oil proceeds via a complex anaerobic coupling that is integral to and supports methanogenesis.  相似文献   

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