首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 15 毫秒
1.
The paper presents data on the composition of biomarkers from bitumen extracts and the chemical structure of kerogen from Corg-rich sedimentary rocks before and after hydrothermal treatment in an autoclave at 300°C. Samples selected for this study are kukersite and Ordovician Dictyonema shale from the Baltics, Domanik oil shale from the Ukhta region, Upper Permian brown coal from the Pre-Ural foredeep, carbonaceous shale from the Oxfordian horizon of the Russian plate, and Upper Jurassic oil shales from the Sysola oil shale bearing region. The rocks contain type I, II, III, and II-S kerogens. The highest yield of extractable bitumen is achieved for Type II-S kerogen, whereas Type III kerogen produces the lowest amount of bitumen. The stages of organic matter thermal maturation achieved during the experiments correspond to a transition from PC2–3 to MC1–2. The 13C NMR data on kerogen indicate that the aromatic structures of geopolymers underwent significant changes.  相似文献   

2.
The Upper Jurassic Madbi Formation, located in the Masila Basin, eastern Yemen, represents the major source rock in this basin. Organic rich shales from two oilfields (Kharir and Wadi Taribah) were analysed to evaluate the type and origin of the organic matter and to determine the factors controlling its deposition. This study is based on geochemical analyses of whole rock (total organic carbon content, Rock-Eval pyrolysis and carbon isotope data) and petrographic analyses on organic matter (kerogen maceral composition and palynofacies) by optical and scanning electron microscopy. Organic petrographic composition of kerogen shows that the Madbi shale is characterized by high amounts of organic matter, consisting predominantly of yellow fluorescing amorphous organic matter and alginite of marine origin. Terrigenous organic materials such as vitrinite, spores and pollen are present in low quantities. The predominance of marine plankton, as indicated by visual kerogen analysis, is consistent with reported carbon isotopic values. It appears that the high amounts of organic matter in the Madbi shale succession might be mainly due to good preservation under suboxic–anoxic conditions. Consequently the Madbi shales possess very good petroleum generative potential, owing to high content of hydrogen rich Type II and I oil prone kerogen.  相似文献   

3.
This paper presents new geological and geochemical data from the Shuanghu area in northern Tibet, which recorded the Early Toarcian Oceanic Anoxic Event. The stratigraphic succession in the Shuanghu area consists mostly of grey to dark-colored alternating oil shales, marls and mudstones. Ammonite beds are found at the top of the Shuanghu oil shale section, which are principally of early Toarcian age, roughly within the Harplocearasfalciferrum Zone. Therefore,the oil shale strata at Shuanghu can be correlated with early Toarcian black shales distributing extensively in the European epicontinental seas that contain the records of an Oceanic Anoxic Event. Sedimentary organic matter of laminated shale anomalously rich in organic carbon across the Shuanghu area is characterized by high organic carbon contents, ranging from 1.8% to 26.1%. The carbon isotope curve displays the δ^13C values of the kerogen (δ^13Ckerogen) fluctuating from -26.22 to -23.53‰ PDB with a positive excursion close to 2.17‰, which, albeit significantly smaller, may also have been associated with other Early Toarcian Oceanic Anoxic Events (OAEs) in Europe. The organic atomic C/N ratios range between 6 and 43, and the curve of C/N ratios is consistent with that of the δ^13Ckerogen values. The biological assemblage,characterized by scarcity of benthic organisms and bloom of calcareous nannofossils (coccoliths), reveals high biological productivity in the surface water and an unfavorable environment for the benthic fauna in the bottom water during the Oceanic Anoxic Event. On the basis of organic geochemistry and characteristics of the biological assemblage, this study suggests that the carbon-isotope excursion is caused by the changes of sea level and productivity, and that the black shale deposition, especially oil shales, is related to the bloom and high productivity of coccoliths.  相似文献   

4.
Complex biogeochemical studies including the determination of isotopic composition of Corg in both suspended particulate matter and surface horizon (0–1 cm) of sediments (more than 260 determinations of δ13C-Corg) were carried out for five Arctic shelf seas: White, Barents, Kara, East Siberian, and Chukchi. The aim of this study is to elucidate causes that change the isotopic composition of particulate organic carbon at the water-sediment boundary. It is shown that the isotopic composition of Corg in sediments from seas with a high river runoff (White, Kara, and East Siberian) does not inherit the isotopic composition of Corg in particles precipitating from the water column, but is enriched in heavy 13C. Seas with a low river runoff (Barents and Chukchi) show insignificant difference between the value of δ13C-Corg in both suspended load and sediment because of a low content of the isotopically light allochthonous organic matter (OM) in particulates. Complex biogeochemical studies with radioisotope tracers (14CO2, 35S, and 14CH4) revealed the existence of specific microbial filter formed from heterotrophic and autotrophic organisms at the water-sediment boundary. This filter prevents the mass influx of products of OM decomposition into water column, as well as reduces the influx of a part of OM contained in the suspended particulate matter from water into sediment.  相似文献   

5.
The relationship between pyritic sulfur content (Spyr) and organic carbon content (Corg) of shales analyzed from the New Albany Group depends upon Corg. For samples of <6 wt.% Corg, Spyt, and Corg are strongly correlated (r = 0.85). For Corg-“rich” shales (>6 wt.%), no Spty-Corg, correlation is apparent. The degree of Fe pyritization (DOP) shows similar relationships to Corg. These C-S-Fe relationships suggest that pyrite formation was limited by the availability of metabolizable organic carbon in samples where Corg < 6 wt.% and by the availability of reactive Fe for samples where Corg > 6 wt.%. Apparent sulfur isotope fractionations relative to contemporaneous seawater sulfate (Δ34S) for pyrite formation average −40% for non-calcareous shales and −25%. for calcareous shales. Δ34S values become smaller with increasing Corg, Spyt, and DOP for all Corg-“poar” (<6 wt%) and some Corg-“nch” (<6 wt.%) shales. These trends suggest that pyrite formation occurred in a closed system or that instantaneous bacterial fractionation for sulfate reduction decreased in magnitude with increasing organic carbon content. The isotopic trends observed in the New Albany Group are not necessarily representative of other shales having a comparable range of organic carbon content, e.g. Cretaceous shales and mudstones from the western interior of North America (GAUTIER, 1986). Δ34S values in the remainder of the Corg-rich New Albany Group shales are relatively large (−38 to −47%.) and independent of Corg, Spyr, and DOP, which suggests that pyrite in these shales formed mostly at or above the sediment-water interface by precipitation from an isotopically uniform reservoir of dissolved H2S.  相似文献   

6.
This study presents data on the composition of organic matter from the Late Silurian sediments of the Chernov uplift. These sediments are characterized by low Corg contents, which may reach 1–3% in individual layers. A relatively high thermal maturity of organic matter is confirmed by polycyclic biomarker distributions and Rock-Eval pyrolyisis data. Despite its higher thermal maturity level (T max = 456°C), kerogen in carbonaceous shales from the Padymeityvis River exhibits good preservation of long-chain n-alkyl structures, which are readily identified in the 13C NMR spectra and by the molecular analysis of the kerogen pyrolysis products.  相似文献   

7.
松辽盆地农安地区上白垩统油页岩含油率影响因素   总被引：3，自引：0，他引：3  
农安油页岩含矿区地处松辽盆地东南隆起区,主要发育于上白垩统青山口组一段、嫩江组一、二段。油页岩有机质的干酪根类型为Ⅰ-Ⅱ1型,质量较好,处于成岩的未成熟阶段。油页岩含油率与有机碳和氢元素含量间存在明显的线性正相关关系,当有机碳质量分数＞6％或氢质量分数＞1％时构成油页岩。此外,有机质干酪根的显微组分可以反映有机质的类型,与含油率之间也存在一定的相关性。青山口-嫩江组沉积时期,该区温暖湿润的古气候条件、大规模的海侵和缺氧事件影响了油页岩中有机质的丰度和类型,从而影响了油页岩的含油率。  相似文献   

8.
Subaerial exposure and oxidation of organic carbon (Corg)-rich rocks is believed to be a key mechanism for the recycling of buried C and S back to Earth's surface. Importantly, processes coupled to microbial Corg oxidation are expected to shift new biomass δ13Corg composition towards more negative values relative to source. However, there is scarcity of information directly relating rock chemistry to oxidative weathering and shifting δ13Corg at the rock-atmosphere interface. This is particularly pertinent to the sulfidic, Corg-rich alum shale units of the Baltoscandian Basin believed to constitute a strong source of metal contaminants to the natural environment, following subaerial exposure and weathering. Consistent with independent support, we show that atmospheric oxidation of the sulfidic, Corg-rich alum shale sequence of the Cambrian-Devonian Baltoscandian Basin induces intense acid rock drainage at the expense of progressive oxidation of Fe sulfides. Sulfide oxidation takes priority over microbial organic matter decomposition, enabling quantitative massive erosion of Corg without producing a δ13C shift between acid rock drainage precipitates and shale. Moreover, 13C enrichment in inorganic carbon of precipitates does not support microbial Corg oxidation as the predominant mechanism of rock weathering upon exposure. Instead, a Δ34S = δ34Sshale − δ34Sprecipitates ≈ 0, accompanied by elevated S levels and the ubiquitous deposition of acid rock drainage sulfate minerals in deposited efflorescent precipitates relative to shales, provide strong evidence for quantitative mass oxidation of shale sulfide minerals as the source of acidity for chemical weathering. Slight δ15N depletion in the new surface precipitates relative to shale, coincides with dramatic loss of N from shales. Collectively, the results point to pyrite oxidation as a major driver of alum black shale weathering at the rock-atmosphere interface, indicating that quantitative mass release of Corg, N, S, and key metals to the environment is a response to intense sulfide oxidation. Consequently, large-scale acidic weathering of the sulfide-rich alum shale units is suggested to influence the fate and redistribution of the isotopes of C, N, and S from shale to the immediate environment.  相似文献   

9.
林俊峰  胡海燕  黎祺 《地球科学》2017,42(7):1124-1133
焦石坝地区五峰组-龙马溪组页岩富集有大量的天然气,但针对页岩气地球化学特征研究还较薄弱,其蕴含的地质意义不甚明确.通过页岩气组分及其碳同位素特征和页岩干酪根的碳同位素特征分析,讨论了页岩气的来源、成因类型和完全倒转的碳同位素分布特征.五峰组-龙马溪组页岩具有很好的生烃能力、有机质丰度与含气量的关系有明显的正相关性、甲烷与干酪根相似的碳同位素特征、地层的超压特征等,综合表明研究区天然气应为页岩自生自储的页岩气;页岩气的甲烷含量均超过98%,其碳同位素平均为-29.93‰,反映了成熟度已经达到过成熟干气阶段;相对稳定的ln（C1/C2）和快速增大的ln（C2/C3）揭示其成因主要为二次裂解气;页岩气碳同位素完全倒转的分布特征主要受到在相对封闭环境中的原油裂解生气作用的影响,其完全倒转的碳同位素分布特征也反映了研究区良好的页岩气保存、富集条件.   相似文献   

10.
Structures and carbon isotopic compositions of biomarkers and kerogen pyrolysis products of a dolomite, a bituminous shale and an oil shale of the Kimmeridge Clay Formation (KCF) in Dorset were studied in order to gain insight into (i) the type and extent of water column anoxia and (ii) changes in the concentration and isotopic composition of dissolved inorganic carbon (DIC) in the palaeowater column. The samples studied fit into the curve of increasing δ13C of the kerogen (δ13CTOC) with increasing TOC, reported by Huc et al. (1992). Their hypothesis, that the positive correlation between TOC and δ13CTOC is the result of differing degrees of organic matter (OM) mineralisation in the water column, was tested by measuring the δ13C values of primary production markers. These δ13C values were found to differ on average by only 1‰ among the samples, implying that differences in the extent of OM mineralisation cannot fully account for the 3‰ difference in δ13CTOC. The extractable OM in the oil shale differs from that in the other sediments due to both differences in maturity, and differences in the planktonic community. These differences, however, are not likely to have significantly influenced δ13CTOC either. All three sediments contain abundant derivatives of isorenieratene, indicating that periodically euxinia was extending into the photic zone. The sediments are rich in organic sulfur, as revealed by the abundant sulfur compounds in the pyrolysates. The prominence of C1-C3 alkylated thiophenes over n-alkanes and n-alkenes is most pronounced in the pyrolysate of the sediment richest in TOC. This suggests that sulfurisation of OM may have played an important role in determining the TOC-δ13CTOC relationship reported by Huc et al. (1992).  相似文献   

11.
Anhydrous non-isothermal heating experiments were conducted under controlled compressive stress on cylindrical plugs of six oil shales from Permian through Eocene age. The objective of this study was to compare the distribution of acyclic paraffins in initial, residual and expelled organic matter and to highlight causes of compositional differences resulting from expulsion. Pristane generation from kerogen is highest in the Eocene Messel shale and affects the pristane / phytane (pr / ph) ratio commonly used as a redox proxy. The isoprenoid to n-alkane ratios (pr / n-C17, ph / n-C18) decrease during generation and are lowest in the residual bitumen due to preferential generation and retention of n-alkanes. The n-alkane distribution shows that only lacustrine shales produce high wax oils. Evaporative fractionation leads to loss of n-alkanes up to n-C20 with boiling points below 350 °C. This demonstrates that lacustrine and marine shales may lead to accumulation of low wax oils due to evaporative fractionation after expulsion.  相似文献   

12.
The molecular and isotopic composition of an extract from the rocks of the Bazhenov Formation in the southeast part of Western Siberia, oils that are genetically related to this formation, and products of hydrous pyrolysis of the organic matter of oil shale (Middle Volgian) of the East European Platform were studied. Common features and differences in their molecular composition were shown. The carbon isotope composition for C14–C30n-alkanes was determined for the first time; a negative isotope anomaly was revealed for C26n-alkane in all samples studied. It was concluded that in spite of the geographical distance, specific biota was revealed in the Volga Basin. This biota conditioned an isotopic indicator, which is genetically significant.  相似文献   

13.
Bituminous rocks in the Ozankoey （Ankara） field are different from those of the Paleocene- Eocene Mengen and Giineytepe （Bolu） regions in metal enrichment levels. Organic carbon （Corg） content of organic material-rich rocks in the Ozankoey （Ankara） field is 3.66-40.72% wt averaging 14.34%. The dominant organic materials are algae/amorphous accompanied by minor amount of herbaceous material （The dominant kerogen type is Type-I with a limited amount of Type-Ⅱ kerogen.）. The bituminous rocks in the Ozankoey field are enriched in heavy metals such as Ni, Mn, As and Cr. In comparison with the average enrichment values of dements, Ni, Mn, As and Cr in bituminous shales of the Ozankoey field are as about 4.38, 14.93, 10.90 and 5.58 times as average values. The average concentrations of these heavy metals are also as high as 215× 10^-6, 828 × 10^-6, 58.54 × 10^-6, and 148 × 10^-6 respectively. In addition, sorption properties of day and organic materials are also important for metal enrichments in the bituminous shales.  相似文献   

14.
In the Upper Cretaceous sequence of the Negev (southern Israel) the organic matter in phosphorites and cherts differs from that associated with oil shales in its higher content of humic substances and lower kerogen content, and in its more intensive microbial alteration. The n-alkane distribution pattern of the oil shales, phosphorites and cherts indicates that marine biota, probably algae, are the main organic precursors of their organic matter. In some of the oil shales, however, some contribution of terrestrial organic matter is also evident. Similar high phytane/pristane ratios indicate that the organic matter in the oil shales as well as in the phosphorites and cherts accumulated under reducing conditions. The main differences in the organic matter composition are attributed to early diagenetic processes rather than to different biotic precursors or to late modifications due to temperature-induced maturation. The depositional model suggested for the sequence involves upwelling conditions at the boundary between the deep Tethys and the shallow shelf, which induced high organic productivity deep into the inner shelf. Bottom water circulation enabled intensive microbial alteration of the organic matter, followed by a winnowing process leading to phosphorite formation. Since humification is considered an oxygen-consuming reaction, these processes favoured the formation of oxygen-enriched humic substances and the oxidation of humic substances already present. These humic substances are relatively resistant to further alteration and their conversion into kerogen is thus retarded. Subsequently, syndepositional tectonic activity resulted in the introduction of less saline water, restriction of bottom-water circulation and the establishment of a density stratification in the water body. Consequently, aeration of the bottom layer and the sediments was inhibited, microbial alteration was reduced and later winnowing processes were prevented. Such conditions favoured the formation of kerogen directly, rather than through humic substances, and also favoured the preservation of most of the organic matter in the form of oil shale deposits instead of phosphorites.  相似文献   

15.
Ocean Drilling Program Leg 207 recovered thick sequences of Albian to Santonian organic-carbon-rich claystones at five drill-sites on the Demerara Rise in the western equatorial Atlantic Ocean. Dark-colored, finely laminated, Cenomanian–Santonian black shale sequences contain between 2% and 15% organic carbon and encompass Oceanic Anoxic Events 2 and 3. High Rock-Eval hydrogen indices signify that the bulk of the organic matter in these sequences is marine in origin. However, δ13Corg values lie mostly between −30‰ and −27‰, and TOC/TN ratios range from 15 to 42, which both mimic the source signatures of modern C3 land plants. The contradictions in organic matter source indicators provide important implications about the depositional conditions leading to the black shale accumulations. The low δ13Corg values, which are actually common in mid-Cretaceous marine organic matter, are consequences of the greenhouse climate prevailing at that time and an associated accelerated hydrologic cycle. The elevated C/N ratios, which are also typical of black shales, indicate depressed organic matter degradation associated with low-oxygen conditions in the water column that favored preservation of carbon-rich forms of marine organic matter over nitrogen-rich components. Underlying the laminated Cenomanian–Santonian sequences are homogeneous, dark-colored, lower to middle Albian siltstones that contain between 0.2% and 9% organic carbon. The organic matter in these rocks is mostly marine in origin, but it occasionally includes large proportions of land-derived material.  相似文献   

16.
Abstract

Small- and medium-sized basins are widely distributed, and some contain commercial gas reservoirs demonstrating their gas-generation potential. The Xuanhua Basin, which is a small-sized coal-bearing basin in north China, includes a promising target for shale-gas exploration in the Xiahuayuan Formation. In this study, we used this basin as a case study to assess the critical geochemical features for small or medium-sized basins to form commercial gas reservoirs. Total organic carbon (TOC) analysis, Rock-Eval pyrolysis, microscopic observation of macerals, vitrinite reflectance measurement and kerogen stable carbon isotope analysis were performed to characterise the organic geochemistry of the Xiahuayuan shales. The original total organic carbon (TOCo) content and hydrocarbon-generative potential (S2o) were reconstructed to further evaluate the gas-generation potential of these shales. In addition, geochemical data of shales from other similar-sized basins with gas discoveries were compared. The results showed that the kerogen from the Xiahuayuan Formation is Type III (gas-prone), and macerals are dominated by vitrinite. TOC values showed a strong heterogeneity in the vertical profiles, with most higher than 1.5?wt%. The measured Ro values ranged from 1.4 to 2.0%. However, thermal maturity was not correlated with the present-day burial depth with higher maturity in the wells closest to the diabase intrusion centre. The remaining generation potential (S2) averaged 0.91?mg HC/g rock, equal to 1.4?cm3 CH4/g rock, and the average amount of hydrocarbon generated was 4.33?cm3 CH4/g rock. In small and medium-sized basins, the TOC content of commercially developed gas shales ranged from 0.5 to 2.5?wt%, organic matter was mainly humic (gas-prone), and the burial depth was generally shallow. Biogenic gas reservoirs for commercial exploitation tend to have larger shale thicknesses (120–800?m) than thermogenic gas reservoirs (60–90?m).

  1. The Xiahuayuan Formation is a good gas-source rock with gas-prone kerogen type, relatively high TOC values and moderate thermal maturity.

  2. The average amount of hydrocarbon generated from the Xiahuayuan shales is about 4.33?cm3 CH4/g rock, indicating a potential to form a shale gas reservoir.

  3. Owing to the influence of diabase intrusions, the Xiahuayuan shales have entered the dry gas window at relatively shallow-buried depths.

  4. Small- and medium-sized basins have the potential to generate commercial gas reservoirs with the generated volume mainly a product of the thickness and maturity of black shales.

  相似文献   

17.
Carbon isotope (δ13Corg) analyses of non-marine clastic rocks and neritic carbonates and black shales spanning the Silurian/Devonian transition are compared from two richly fossiliferous sequences in Qujing of East Yunnan and Zoige of Sichuan, South China. The two sections, Xishancun and Putonggou sections in South China, reveal positive δ13Corg shifts happening in the Upper Pridoli and Lower Devonian and reaching peak values as heavy as ?25.2‰ (Xishancun) and ?19.9‰ (Putonggou) in the lowermost Lochkovian following the first occurrence of the thelodont Parathelodus and the conodont Icriodus woschmidti woschmidti (only in Putonggou Section and together with Protathyris-Lanceomyonia brachiopod fauna). These results replicate a globally known positive shift in δ13Corg from the uppermost Silurian to the lowermost Devonian. The δ13Corg variations across the Silurian/Devonian Boundary (SDB) at the two sections in South China exhibit a shift in carbon isotopic composition similar to the detailed SDB curves from the borehole Klonk-1 drilled at top of the Klonk Global Standard Stratotype-Section and Point (GSSP) in the Prague Basin, Czech Republic. In addition, four microvertebrate assemblages, including the Liaojiaoshan, Xishancun, Yanglugou and Xiaputonggou assemblages, are recognized from the Silurian/Devonian transition exposed in the Xishancun and Putonggou sections, respectively. The results from both carbon isotope stratigraphy and microvertebrate assemblage sequences suggest that the SDB in South China is located at the base of the Xishancun Formation (between sample QX-20 and sample QX-21) in the Xishancun Section and the lower part of the Xiaputonggou Formation (between sample ZP-09 and sample ZP-10) in the Putonggou Section. The isotopic trend for organic carbon together with the changes of microvertebrate remains across the SDB can offer an approach to a potential correlation of the SDB from different sedimentary facies, which help to correlate the marine with non-marine deposits.  相似文献   

18.
A maturity indexing procedure based on the isotopic difference between the total accumulated methane produced by exhaustive pyrolysis and the kerogen (Δ13C) and the mole ratio of methane to kerogen carbon (CMR), has been tested by applying a standardized technique, i.e. exhaustive pyrolysis (600°C for 120 hr) of extracted-powdered samples and measurement of the amounts and isotopic composition of the methane and kerogen carbon, on a suite of 15 Bakken shale samples.A linear relation was found between the carbon mole ratio of pyrolysis-derived methane and total organic carbon and the δ13C difference between the pyrolysis-derived methane and total organic carbon (r = −0.79); and between the amount of CH4 generated from exhaustive pyrolysis and H/C atomic ratios (r = +0.91).  相似文献   

19.
Surma Group is the most important geological unit of Bengal basin, Bangladesh, because petroleum resources occur within this group. It is mainly composed of alternation of shale and sandstone and the shale fraction has long been considered as source rocks and the sandstone fraction as reservoir. These source and reservoir rocks have been studied by different authors by different approach but none of them adopted organic geochemistry and organic petrology as a means of study of source rock and their possible depositional environment. A total of thirty shale core samples have been collected from eight different gas fields to fulfill the short coming. The collected samples have been subjected to Source Rock Analysis (SRA) and/or Rock-Eval (RE) followed by pyrolysis gas chromatography (PyGC), gas chromatography mass spectrometry (GCMS), elemental analysis (EA) and organic petrological study such as vitrinite reflectance measurement and maceral analysis. The analyzed organic matter extracted from the shales of Surma Group consists mainly of Type III along with some Type II kerogen. The studied shales are mostly organically lean (TOC ±1%) and the extracted organic matter is fair to moderate. Based on these results, the analyzed shales have been ranked as poor (mostly) to fair quality source rock. The organic matter of the analyzed shale samples is thermally immature to early mature for hydrocarbon generation considering their Tmax and measured mean vitrinite reflectance values. The hopane 22S/(22S + 22R), moretane/hopane ratio and sterane parameters are also in good agreement with these thermal maturity assessments. The predominance of odd carbons over even carbons (most common) and/or even carbons over odd carbon numbered n-alkanes, moderate Pr/Ph ratio, low to high Tm/Ts ratio, comparative abundance of sterane C29 (i.e., C29 >C27>C28), Pr/nC17 — Ph/nC18 values, C/S ratio and dominance of vitrinite macerals group with the presence of liptinite macerals demonstrate that the organic matter has derived mainly from terrestrial inputs with an insignificant contribution from the marine sources. The condition of deposition alternates from oxic to anoxic.  相似文献   

20.
Molecular transport (diffusion) of methane in water-saturated sedimentary rocks results in carbon isotope fractionation. In order to quantify the diffusive isotope fractionation effect and its dependence on total organic carbon (TOC) content, experimental measurements have been performed on three natural shale samples with TOC values ranging from 0.3 to 5.74%. The experiments were conducted at 90°C and fluid pressures of 9 MPa (90 bar). Based on the instantaneous and cumulative composition of the diffused methane, effective diffusion coefficients of the 12CH4 and 13CH4 species, respectively, have been calculated.Compared with the carbon isotopic composition of the source methane (δ13C1 = −39.1‰), a significant depletion of the heavier carbon isotope (13C) in the diffused methane was observed for all three shales. The degree of depletion is highest during the initial non-steady state of the diffusion process. It then gradually decreases and reaches a constant difference (Δ δ = δ13Cdiff −δ13Csource) when approaching the steady-state. The degree of the isotopic fractionation of methane due to molecular diffusion increases with the TOC content of the shales. The carbon isotope fractionation of methane during molecular migration results practically exclusively from differences in molecular mobility (effective diffusion coefficients) of the 12CH4 and 13CH4 entities. No measurable solubility fractionation was observed.The experimental isotope-specific diffusion data were used in two hypothetical scenarios to illustrate the extent of isotopic fractionation to be expected as a result of molecular transport in geological systems with shales of different TOC contents. The first scenario considers the progression of a diffusion front from a constant source (gas reservoir) into a homogeneous “semi-infinite” shale caprock over a period of 10 Ma.In the second example, gas diffusion across a 100 m caprock sequence is analyzed in terms of absolute quantities and isotope fractionation effects. The examples demonstrate that methane losses by molecular diffusion are small in comparison with the contents of commercial size gas accumulations. The degree of isotopic fractionation is related inversely to the quantity of diffused gas so that strong fractionation effects are only observed for relatively small portions of gas.The experimental data can be readily used in numerical basin analysis to examine the effects of diffusion-related isotopic fractionation on the composition of natural gas reservoirs.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号