Nitrogen Isotope Analysis Method of Organic-enriched Shale
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摘要:
页岩氮同位素是重建古环境中生物地球化学循环的重要工具，也为判断原油的沉积环境、油源对比等提供地球化学指标，但页岩氮同位素比值分析在研究中面临着含量较低、前处理对分析的影响以及标准物质选用等问题，从而影响了页岩中氮同位素比值的准确分析，制约了该技术在相关研究中的深入和发展。本文以富有机质上扬子龙马溪组页岩样品为对象，采用元素分析仪-同位素质谱（EA-IRMS）进行分析，并结合该类页岩的性状特点以及当前页岩氮同位素分析中测试条件、前处理方法、标准物质的选用等方面的影响进行研究。结果表明：EA-IRMS分析时，适当地增加注氧量提高燃烧效能，采用碳、氮分别测定的方式，以及添加碳吸附剂有助于提高页岩中的氮同位素比值分析的精度和准确性。盐酸-酸洗法处理样品过程中，采用超声方式促进了酸/水和样品的反应，有助于提高水洗效果并减少水洗次数从而降低对δ15N的影响。页岩分析时采用国际标准USGS40、USGS41a以及IAEA-600为分析标准，适用并满足氮以及全碳和有机碳同位素比值分析的需要，国家一级海洋沉积物碳氮稳定同位素标准物质GBW04701、GBW04702、GBW04703适用于页岩中碳氮同位素比值分析以及酸处理过程的监控。尽管本次实验采用的盐酸-酸洗法对氮同位素比值的影响较小，但是在对样品分析中仍观察到明显含氮成分的损失和δ15N的变化，因而为了氮含量和δ15N分析的准确，分析该类样品全岩氮同位素比值建议采用直接分析的方法。本研究有助于提高富有机质页岩中氮同位素比值分析的精度和准确性，从而促进氮同位素在页岩分析和研究中获得更广泛的应用。
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关键词:
- 页岩 /
- 氮同位素比值 /
- 元素分析仪-同位素质谱 /
- 前处理方法 /
- 标准物质
Abstract:BACKGROUND Shale, as an important component of sedimentary rocks, is also a valuable source rock and “reservoir” of shale gas. The study of nitrogen isotope distribution characteristics can provide geochemical indexes for judging the sedimentary environment and oil-source correlation of crude oil. It is helpful to understand the organic matter enrichment mechanism of black shale, shale oil, and gas exploration[14-17]. Nitrogen isotope ratio analysis in sedimentary rocks is faced with problems such as low content, and the influence of pretreatment on analysis and the reference materials, which affect the accurate analysis of nitrogen isotope ratio in shale and restrict the development of this technology in related studies. First, the low content in rocks will affect the analysis results by elemental analyzer-isotope ratio mass spectrometry (EA-IRMS)[16,18,20]. Second, the method of direct sample analysis is often used to analyze nitrogen isotope ratio of whole rock, to improve the relative content of nitrogen and remove carbonate to meet the requirement of organic carbon isotope analysis; samples treated with acid can also be used for analysis. There are differences in treatment methods in different laboratories, which can also increase the variation in content and isotope ratios, as well as the difficulty of data comparison[9-13,21,25]. Finally, there are relatively few international standard materials based on rocks and minerals for nitrogen isotope and there are still some problems and limitations in these standards[7,21,24,30]. Therefore, the accurate analysis of nitrogen isotope ratio in shale is affected, thus restricting the development of this technology in related studies.
OBJECTIVES To improve the precision and accuracy of nitrogen isotope analysis in shales.
METHODS (1) The analysis was carried out by EA-IRMS, with the use of MAT253 Plus gas stable isotope mass spectrometer, Flash 2000 HT element analyzer and ConFlo Ⅳ interface. The furnace tube was filled with the recommended scheme of the instrument, and the water removal trap was adjusted according to the measured object, that is, only magnesium perchlorate was used or carbon adsorbent was added on this basis (the main component was sodium hydroxide), which accounted for 50% each. Carbon and nitrogen isotope ratios were measured separately. (2) Carbon and nitrogen isotope standard materials included: USGS40, USGS41a, IAEA-600, Urea, GBW04701, GBW04702, GBW04703, GBW07424 and GBW07107 as calibration, monitoring and experimental study (Table 1). Typical shale samples of the upper Yangtze Longmaxi Formation were selected as the research object. Both samples are rich in organic carbon. Combined with hand samples and XRD analysis, their mineral composition is mainly as follows: Quartz, potassium feldspar, illite and pyrite in SAMPLE-1, SAMPLE-2 are similar in mineral composition in general, but there is obvious dolomite in SAMPLE-2. (3) The experiment consisted of three parts. First, the impact of increasing oxygen injection time and using double tin cup encapsulation on nitrogen isotope ratio analysis was studied. Urea was applied to determine δ15N under the same conditions but with different oxygen injection time (3s or 5s) and the differences were compared. Urea and GBW07424 were used to encapsulate the sample with double tin cup (two layers of tin cup for the same sample) and conventional single tin cup for comparison under the same conditions. The composition of total nitrogen and total carbon isotope ratios in GBW04701, GBW04702 and GBW04703 was determined by single-tin cup encapsulation. Second, the influence of acid rinse method on the analysis was conducted. Standards were weighed and gradually added with an excess of 6mol/L HCl to remove carbonate minerals. During the reaction, the centrifuge tube was placed in an ultrasonic cleaner for ultrasonic treatment for 3 to 5 times, 10min each time. Centrifuge was used, acid was discarded and deionized water was added. After ultrasonic treatment in ultrasonic cleaner, centrifuge treatment was continued. This process was repeated until it reached neutral. The samples were then compared and analyzed with unprocessed samples and recommended values. Finally, the shale samples SAMPLE-1 and SAMPLE-2 from Longmaxi Formation were analyzed. The isotopic ratios of total nitrogen and total carbon were determined respectively. The samples were treated with the above acid rinse method to analyze the ratio of organic carbon and nitrogen isotopes, and the mineral composition before and after treatment was analyzed by XRD. Nitrogen isotope difference and total carbon and organic carbon isotope ratio were analyzed before and after treatment.
RESULTS The results of single/double tin cup method showed that the accuracy of δ15N value determined by double tin cup method were obviously worse than that by single tin cup method at the same oxygen injection time (Table 2). GBW07424 analysis showed that nitrogen amounts had more obvious influence on the analysis. The results of total nitrogen analysis in GBW04701, GBW04702 and GBW04703 measured under the condition of oxygen injection time 5s were consistent with the recommended values (Table 3), and the standard deviation of the samples was improved (SD≈0.1‰). After acid rinse, the nitrogen values of GBW04701 and GBW04702 were consistent with the recommended values (Table 3), and the relative deviation was better (SD<0.1‰). The overall carbon isotope ratio analysis values were consistent with the recommended values (Table 4). The difference of δ13CVPDB between total carbon and organic carbon determined by GBW07107 was small and consistent with its carbon composition. The δ15N values of shale samples before and after acid treatment were basically the same in SAMPLE-1, while the δ15N values of shale samples after acid treatment were obviously changed in SAMPLE-2, which was related to the composition loss in the wash acid process of SAMPLE-2.
DISCUSSION (1) The optimized measurement conditions of EA-IRMS. Attempts were made to improve combustion efficiency by using double tin cup wrapping and increasing oxygen injection time. The relative deviation of double tin cup method was greater than 0.2‰ and obviously higher than that of single tin cup method, indicating that this method is not suitable. The reason may be related to the difference in combustion efficiency caused by the gap between double tin cups and the introduction of air when the sample is wrapped[19]. Appropriate increase of oxygen injection time in the analysis of GBW04701, GBW04702 and GBW04703 obtained satisfactory results, indicating that this method can effectively improve the accuracy of the technique, while this method did not significantly improve the standard deviation of δ15N value of Urea, which may be related to the matrix of the sample[20]. It may be the abundant carbon in shale. Therefore, it is necessary for nitrogen isotope ratio analysis in shale to be determined by carbon and nitrogen separately and adding carbon adsorbent. (2) The acid rinse method. The acid rinse method adopted has no significant effect on GBW04701 and GBW04702 on δ15N analysis and can effectively remove carbonate components. Ultrasonic was used to promote the reaction between acid/water and sample in the process of acid reaction and acid washing in the experiment, and the centrifugation time was appropriately increased in the centrifugation stage, which was conducive to the preservation of fine particle components. Compared with static reaction, the addition of ultrasonic reaction could improve the washing effect. The less washing times were thus contributing to the accuracy of δ15N analysis[47]. (3) The shale sample analysis. Although the acid treatment method adopted in the experiment has no obvious effect on the δ15N analysis of standards, significant composition loss and δ15N change were still observed in the analysis of shale samples. Moreover, the nitrogen content of shale samples of Longmaxi Formation was relatively high. Therefore, in order to accurately analyze the nitrogen content and δ15N analysis, direct analysis of samples is recommended for the whole-rock nitrogen isotope analysis of such samples. (4) The standard and reference materials. Carbon and nitrogen isotope reference materials with different characters and compositions were selected as calibration, monitoring and experimental study. USGS40, USGS41a and IAEA-600 standard samples were used for calibration and quality monitoring, and no obvious influence of matrix difference was found. Although they are Marine sediments, GBW04701, GBW04702, GBW04703 provide isotopic compositions of total carbon, organic carbon, total nitrogen and organic nitrogen. They are suitable for the analysis of carbon and nitrogen isotope ratios and the monitoring of acid treatment processes, especially the analysis of EA-IRMS technology[50]. In shale analysis, GBW04701, GBW04702 and GBW04703 were selected as monitoring to meet the requirements of nitrogen and total carbon and organic carbon isotope ratio analysis.
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Key words:
- shale /
- nitrogen isotope ratio /
- EA-IRMS /
- pretreatment method /
- standard
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表 1 本次实验用于碳、氮质量和同位素比值分析的标准和参考物质
Table 1. Standard and reference materials for carbon and nitrogen content and isotope ratio analysis in this experiment.
标准物质
编号来源 性质 δ13CVPDB(‰) δ15NAIR-N2(‰) 碳氮含量和同位素
比值说明Urea Element Microanalysis Ltd 尿素 −37.02±0.06 −2.91±0.2 − USGS41a USGS 谷氨酸 36.55±0.08 47.55±0.15 − USGS40 USGS 谷氨酸 −26.39±0.04 −4.52±0.06 C: 40.8%; N: 9.52% IAEA-600 IAEA 咖啡因 −27.77±0.04 1.0±0.2 − GBW04701 国家标准物质 海洋沉积物 −8.22±0.17 3.99±0.22 总碳和总氮的同位素比值 −20.79±0.14 3.8±0.24 有机碳和有机氮的同位素比值 GBW04702 国家标准物质 海洋沉积物 −18.68±0.15 6.25±0.23 总碳和总氮的同位素比值 −23.63±0.11 6.48±0.28 有机碳和有机氮的同位素比值 GBW04703 国家标准物质 海洋沉积物 −10.64±0.15 4.68±0.23 总碳和总氮的同位素比值 −22.57±0.14 4.78±0.29 有机碳和有机氮的同位素比值 GBW07424 国家标准物质 土壤 − − 土壤成分标准，N: 0.126%±0.011% GBW07107 国家标准物质 页岩 − − 岩石成分标准，N: 540±60µg/g 
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表 2 不同注氧时间和单/双锡杯方式下氮含量和同位素分析结果
Table 2. The results of nitrogen content and isotope ratio under different oxygen injection time and single/double tin cup condition.
标准物质
编号测定条件 称样量
（mg）N含量(%) δ15NAIR-N2(‰) 推荐值 测定值（n≥5） 推荐值 测定值（n≥5） Urea O: 3s+单锡杯 − − − −2.91±0.2 −2.93 ±0.13 O: 5s+单锡杯 − − − −3.03 ±0.13 O: 5s+双锡杯 − − − −3.16 ±0.29 GBW07424
O: 5s+单锡杯 ＜7 0.126±0.011 0.106±0.009 − 5.47±0.16 O: 5s+双锡杯 ＜7 0.100±0.010 6.09±0.27 O: 5s+单锡杯 15~20 0.119±0.004 6.80±0.16 O: 5s+单锡杯 ≥20 − 6.85±0.12 注：“-”表示该项无参考值或未作分析。
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表 3 不同标准和样品的氮同位素分析结果
Table 3. The results of nitrogen isotopes for reference materials and samples.
标准物质和
样品编号样品前处理 δ15NAIR-N2 (‰) 样品前处理 δ15NAIR-N2 (‰) 推荐值 测定值（n≥5） 推荐值 测定值（n≥5） GBW04701 未前处理 3.99±0.22 3.82±0.08 6mol/L HCl 3.8±0.24 3.73±0.07 GBW04702 未前处理 6.25±0.23 6.30±0.07 6mol/L HCl 6.48±0.28 6.29±0.09 GBW04703 未前处理 4.68±0.23 4.65±0.1 − 4.78±0.29 − SAMPLE-1 未前处理 − −1.40±0.17 6mol/L HCl − −1.34±0.17 SAMPLE-2 未前处理 − −1.69±0.13 6mol/L HCl − 0.62±0.12 注：“-”表示该项无参考值或未作分析。
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表 4 不同标准和样品的碳同位素分析结果
Table 4. The results of carbon isotopes of reference materials and samples.
标准物质和
样品编号样品前处理 δ13CVPDB (‰) 样品前处理 δ13CVPDB (‰) 推荐值 测定值（n≥5） 推荐值 测定值（n≥5） GBW04701 未前处理 −8.22±0.17 −8.39±0.02 6mol/L HCl −20.79±0.14 −21.03±0.05 GBW04702 未前处理 −18.68±0.15 −18.71±0.1 6mol/L HCl −23.63±0.11 −23.65±0.12 GBW07107 未前处理 − −30.47±0.12 6mol/L HCl − −30.34±0.07 SAMPLE-1 未前处理 − −28.68±0.15 6mol/L HCl − −29.53±0.11 SAMPLE-2 未前处理 − −30.69±0.02 6mol/L HCl − −30.75±0.11
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