Vanadium Isotope Composition of Rock Reference Materials by MC-ICP-MS
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摘要:
随着分析方法的发展和分析精度的提升，钒同位素已经越来越多地被用于各种地质过程研究。为了确保钒同位素分析测试过程中，可以有效地监控数据的准确度和精度，方便国际各实验室之间的数据对比，同时考虑到早期国际上常用美国地质调查局(USGS)的标准物质面临着库存不足等问题，本文采用多接收电感耦合等离子体质谱仪(MC-ICP-MS)测定了一系列国际地质标准物质和国家地质标准物质的钒同位素组成，δ51V值的测试精度优于0.08‰。本文选取的标准物质主要来自日本地质调查局(GSJ)和中国地质科学院地球物理地球化学勘查研究所(IGGE)，包括一个安山岩(JA-1)，三个玄武岩(JB-3、JB-1b和GBW07105)，一个辉长岩(JGb-1)，一个辉绿岩(GBW07123)以及一个土壤(GBW07454)，钒含量范围为77～635µg/g，涵盖了目前大部分火成岩和部分土壤等天然样品含量的范围。这些标准物质除了土壤GBW07454，其他标准物质的钒同位素组成未曾被报道。经测量表明，辉长岩标准物质JGb-1具有最高δ51V值，为−1.05‰±0.08‰，安山岩标准物质JA-1具有最低δ51V值，为−0.34‰±0.06‰，其余标准物质的δ51V值变化范围为−0.72‰～−0.81‰，均落在MORB范围内。本文对这些标准物质钒同位素组成的报道，丰富了钒同位素研究的标准物质数据库，有助于未来在更多领域开展钒同位素研究。
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关键词:
- 钒同位素 /
- 火成岩；MC-ICP-MS /
- 标准物质 /
- 高精度分析方法
要点(1)采用MC-ICP-MS对国际岩石和土壤标准物质进行了高精度的钒同位素比值测量。
(2)报道了目前具有最低δ51V值的火成岩钒同位素标准物质。
(3)扩大了火成岩标准物质的钒同位素数据库，补充了钒同位素标准物质数据。
HIGHLIGHTS(1) The vanadium isotopes of seven reference materials were analyzed using MC-ICP-MS with high-precision and accuracy.
(2) The igneous rock vanadium isotope reference material with the lowest δ51V value at present was reported.
(3) The range of vanadium isotopic composition of igneous rock standards was broadened, and the data of vanadium isotope standard materials was supplemented.
Abstract:In order to ensure the accuracy and precision of data during the analysis of vanadium isotopes, and facilitate the comparison of data among laboratories internationally, considering the shortage of inventory for the commonly used reference materials from the United States Geological Survey (USGS), seven reference materials (JA-1, JB-3, JB-1b, JGb-1, GBW07105, GBW07123, and GBW07454) with unreported vanadium isotope composition were selected from the Geological Survey of Japan (GSJ) and the Institute of Geophysical and Geochemical Exploration, Chinese Academy of Geological Sciences (IGGE) and their vanadium isotopes were measured using MC-ICP-MS. Among these reference materials, the gabbro reference material JGb-1 has the highest δ51V value of −1.05‰±0.08‰, and the andesite reference material JA-1 has the lowest δ51V (−0.34‰±0.06‰). The δ51V values of the other reference materials range from −0.72‰ to −0.81‰, all falling within the MORB range. The reporting of the vanadium isotopic composition of these reference materials in this article will enrich the database of vanadium isotopic research and contribute to the future study of vanadium isotopes in more fields. The BRIEF REPORT is available for this paper at http://www.ykcs.ac.cn/en/article/doi/10.15898/j.ykcs.202405280123.
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Keywords:
- vanadium isotope /
- igneous rock; MC-ICP-MS /
- reference materials /
- high precision analysis method
BRIEF REPORTSignificance: With the development of analytical methods and the improvement of analytical accuracy, vanadium isotopes have been increasingly used in the study of various geological processes. In the analysis process of vanadium isotopes, using reference materials similar to the sample matrix can reduce experimental bias, which is conducive to obtaining more accurate and precise vanadium isotope data. Previous studies have reported the vanadium isotopic composition of some common rock reference materials provided by the United States Geological Survey (USGS), with δ51V values ranging from −1.65‰ to −0.61‰[12,15,28,42]. However, many USGS reference materials with reported vanadium isotopic composition are facing issues such as insufficient inventory (some are already sold out). In order to provide continuous support for research in related fields with high-precision vanadium isotope data, there is an urgency to calibrate the vanadium isotopes of more new geological reference materials, so as to better monitor the precise measurement of vanadium isotopes and enable data comparison between laboratories internationally. A series of international and national geological reference materials with unreported vanadium isotope composition have been selected from the Geological Survey of Japan (GSJ) and the Institute of Geophysical and Geochemical Exploration, Chinese Academy of Geological Sciences (IGGE) for measuring vanadium isotopic composition. Their composition are widely distributed, with vanadium content ranging from 77g/g to 635g/g, SiO2 from 43.44% to 64.43%, and TiO2 from 0.64% to 2.94%, which cover the components of most natural samples. The vanadium isotopic composition of these reference materials is intended to supplement the reference materials database of vanadium isotope research and also to provide more options for the comparison of vanadium isotope data between different laboratories.
Methods: The detailed information of the reference materials (GSP-2, BIR-1, GBW07454, JA-1, JB-3, JB-1b, GBW07105, JGb-1, and GBW07123) and isotope standard solutions (AA, USTC-V, BDH, and NIST-3165) used in the experiment is shown in Table 1. The samples are rock or soil standard materials. The rock standard materials were digested using the acid dissolution method on an electric heating plate at ordinary pressure, while the soil standard materials were digested using the dissolution method with bomb. Chemical purification used the four-column combined chemical separation method with AG50W-X12 cation exchange resin and AG1-X8 anion exchange resin described by Wu et al (2016)[15]. The vanadium isotopes were determined using a multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS), and the main working conditions of the instrument are shown in Table 2. The mass bias effect produced by the instrument during the test process was corrected using the sample standard bracket method. Although the majority of matrix elements Ti and Cr were removed during the resin purification, trace amounts of Ti and Cr can also affect the measurement of 50V during mass spectrometry analysis. Therefore, it is necessary to accurately correct the interference of residual 50Ti and 50Cr on 50V. Experimental condition testing shows that the sample solution must meet the conditions of 49Ti/51V<0.00004 and 53Cr/51V<0.00004. At this point, the interference of 50Ti and 50Cr on 50V can be corrected, and the test data is considered reliable[15].
Three methods were used to monitor the precision and accuracy of vanadium isotope measurements, including (1) internal laboratory standard monitoring; (2) monitoring of reference samples with recommended values; (3) monitoring of replicate samples. The vanadium isotopic composition of the laboratory internal standard solutions BDH and NIST-3165 during this test process is consistent with the long-term test results of this laboratory within the error range (Fig.2). The vanadium isotopic composition of the petrologic reference materials BIR-1 and GSP-2 is consistent with the data reported in previous literature within the error range. The vanadium isotopic composition of all standard materials in multiple replicate samples is consistent within the error range, and the measurement precision of δ51V is greater than 0.08‰ (2SD). The above results ensure the precision and accuracy of the test.
Date and Results: The δ51V value of the soil reference material GBW07454 is −0.78‰±0.06‰ (2SD, n=12), gabbro reference material JGb-1 is −1.05‰±0.08‰ (2SD, n=9), diabase reference material GBW07123 is −0.72‰±0.06‰ (2SD, n=15), basalt reference material JB-3 is −0.81‰±0.09‰ (2SD, n=9), basalt reference material JB-1b is −0.79‰±0.08‰ (2SD, n=9), basalt reference material GBW07105 is −0.80‰±0.06‰ (2SD, n=15), and andesite reference material JA-1 is −0.34‰±0.06‰ (2SD, n=9) (see Table 4 and Fig.3).
The δ51V value of the gabbro reference material JGb-1 is currently the lowest in reported igneous rock samples, lower than the average vanadium isotope composition of MORB (−0.84‰±0.10‰). Its lighter V vanadium isotope composition may be influenced by high-temperature hydrothermal alteration, but the specific genesis requires further research[27]. The vanadium isotopes of the diabase standard material and three basalt standard materials GBW07105, JB-3, and JB-1b are relatively homogeneous, all falling within the MORB range. The δ51V value of the andesite reference material JA-1 is the highest, indicating a significant enrichment of vanadium isotopes compared to basalt, indicating the possible presence of vanadium isotope fractionation during magma evolution[26-28,32-33]. The δ51V value of the soil reference material GBW07454 is consistent with previous reports (−0.74‰±0.08‰) and is close to the composition of MORB. It also indicates that continental weathering does not cause significant vanadium isotope fractionation, which is consistent with previous research findings on the weathering product of basalt, Zhanjiang laterite[34].
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得益于海相页岩层系油气资源的钻完井和压裂技术革新，美国成为世界上最早实现页岩气商业化开采的国家, 其页岩气资源主要来自海相或海陆过渡相地层[1]。美国页岩气的成功商业化为中国页岩油气勘探开发关键技术探索提供了一定经验和借鉴[2]。我国广泛发育有海相、陆相和海陆过渡相三类富有机质页岩[3]，页岩气可采资源量排名世界第二，其中陆相页岩气可采资源潜力为7.9×1012m3[4]。然而，与海相富有机质页岩相较，陆相页岩在沉积环境、干酪根类型、有机质成熟度和矿物组分等方面存在较大差异[5-7]。当前，我国海相页岩气储量评估方法与压裂技术基本达成共识，然而关于陆相页岩气(油)的研究在广度与深度上远远不足。鄂尔多斯盆地作为我国中新生代大型内陆坳陷沉积盆地，发育有石炭系、二叠系和三叠系等系列页岩沉积。其中三叠系延长组7段(C7)烃源岩埋藏浅，有机质丰度高，热演化程度低，具有良好的页岩气(油)成藏条件[8-9]，有必要对其展开深入研究。
页岩孔隙结构是决定储层储集与运移能力的关键，对构建页岩气渗流模型[10]、完善压裂技术[11]具有重要意义。Loucks等[12]研究表明页岩中孔隙类型丰富，包括粒间孔、粒内孔、微裂缝和有机孔等，不同类型孔隙结构成因不同，对页岩气储集能力的贡献亦不相同。Wang等[13]证实页岩矿物基质与有机质分布模式在一定程度上控制和反映页岩的孔隙结构特征。王跃鹏等[14]提出延长组页岩中普遍发育的纹理结构会形成平行层理的孔缝结构，严重时造成井壁坍塌。前人工作表明，研究页岩孔隙结构有必要同时考虑有机质与矿物基质的空间分布特征。研究者围绕鄂尔多斯C7页岩孔隙结构已开展了大量研究工作，但是受测试方法的限制，对其微观结构的认识仍较为薄弱。吴银辉等[15]、杨维磊等[16]、庞铭等[7]、Jiang等[17]通过气体吸附实验、高压压汞实验和核磁共振实验对C7页岩孔隙结构特征展开研究，结果表明C7页岩中3~30nm的孔隙对总孔隙度贡献最大，为构建页岩孔隙模型提供了重要数据参考，但是上述成果对闭孔表征不足且缺乏对孔隙空间结构特征的直观认识。徐红卫等[18]采用扫描电镜对C7页岩孔隙结构特征展开研究，其研究成果表明C7页岩中主要发育纳米级狭缝型黏土矿物层间孔，然而其采用机械抛光制样对页岩表面孔隙结构造成了一定破坏，同时缺乏对页岩不同组分三维空间分布特征的认识。
目前，油气领域多采用氩离子抛光-扫描电镜方法对页岩孔隙结构进行直观观测[13, 19-20]。该方法有助于快速认识页岩孔隙类型、分布模式及孔径范围等，然而不足以提供精确数据用以构建页岩气三维渗流模型。对页岩三维结构的认识主要是利用微米X射线显微镜(micro X-ray microscope)[13, 21]、纳米X射线显微镜(nano-transmission X-ray microscope, TXM)[21-24]与聚焦离子束-扫描电子显微镜(focused ion beam-scanning electron microscope, FIB-SEM)[25-27]。与TXM和FIB-SEM方法相较，微米X射线显微镜方法的扫描范围大且成本较低，是研究陆相页岩纹理结构特征的重要技术手段。基于此，本文选取鄂尔多斯盆地南缘代表性C7页岩，采用氩离子抛光-扫描电镜与同步辐射微米X射线显微镜方法，对C7页岩的孔隙结构特征进行详细表征，探究陆相页岩孔隙结构特征影响因素，以期为完善陆相页岩气产能评估与压裂技术提供参考。
1. 实验部分
1.1 实验样品
C7页岩实验样品采自鄂尔多斯盆地遥科一(YK1)井。YK1井位于陕西省铜川市，构造位置属于鄂尔多斯盆地南缘渭北隆起构造单元(图 1)。长7油层组位于延长组中下部，代表湖盆最大扩张期。该时期随着盆地强烈沉陷，湖盆从长8期开始至长7期达到鼎盛时期，环绕湖盆退积型三角洲广泛发育并在开阔的浅湖-深湖区形成延长组的最主要的生油岩系[16]。岩心观测发现，C7页岩以黑色油页岩、凝灰岩与含油砂岩为主，夹杂碳酸盐岩薄层，如图 2所示。
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图 1 鄂尔多斯盆地遥科1井构造位置示意图1—逆冲推覆带；2—古板块碰撞对接带；3—河流。Figure 1. Geotectonic position of Yaoke-1 Well, Ordos Basin![]()
图 2 遥科1井延长组7段岩性柱状图与岩心样品Figure 2. Lithological column and core sample of Member 7, Yanchang Formation shale from Yaoke-1 WellX射线粉晶衍射测试表明，C7页岩样品中的石英与长石平均含量较高(分别为32%和24%)，碳酸盐岩和黄铁矿含量相对较低，分别为9%和3%。脆性矿物平均含量大于50%，有利于天然裂隙和人工诱导裂缝的发育。黏土矿物含量为33%，几乎全部由伊蒙混层组成。伊蒙混层中伊利石含量为80%，主要为有序混层矿物(R=1型)，表明延长组7段页岩处于成岩阶段中期[28]。实验样品有机碳平均含量为4.79%，属于富有机质页岩，其平均镜质体反射率为0.72%，对应有机质未成熟或低成熟阶段。
1.2 样品测试和数据分析方法
1.2.1 扫描电镜测试
选取新鲜页岩样品薄片(~1cm2)，对其进行初步机械抛光后(2400目)，放入离子减薄仪(LJB-1A，沈阳华业公司)，利用氩离子束轰击预抛光的表面，得到品质较高的平面进行扫描电镜观察。制样过程中，氩离子减薄仪的工作电压为5kV，电流为100μA，抛光时间为10~12h[19, 25]。扫描电镜观测利用Merlin Compact LE0 1530 VP电镜(卡尔·蔡司公司)完成，矿物元素组成定性定量分析使用AZtec X-Max能谱仪(牛津仪器)进行。实验时扫描电镜加速电压为5kV，工作距离为4~6mm；能谱仪的工作电压为15kV，工作距离为8~10mm。
1.2.2 同步辐射X射线显微镜测试
利用砂纸将C7页岩样品打磨成圆柱形，直径大约5mm。微米X射线显微镜扫描实验在上海同步辐射装置X射线成像及生物医学应用光束线(BL13W1)[29]开展。X射线在磁场强度1.9T、磁周期14cm的十六极摆动器中激发，通过Si(111)双晶单色器进行单色化后穿过样品，然后被带有闪烁体的电荷耦合元件系统接收[11]。本文实验像素尺寸为3.25μm，能量为25keV, 样品到探头的距离为10cm。扫描过程中将样品的一端固定在旋转样品台上。整个实验中样品台共旋转180°，每隔0.2°采集一张投影图像。通过样品台的旋转，采集不同视角的投影图像，利用PITRE (phase-sensitive X-ray image processing and tomography recons-truction)软件进行图像重构[30]。首先，利用5张暗场图像与20张明场图像进行背景强度校正；其次，将采集的投影图像转换成正弦图像并进行归一化处理；最后，利用滤波反投影方法对正弦化图像进行重构[31]。
1.2.3 数据分析方法
微米X射线显微镜重构数据利用Avizo软件进行分析。首先，选择页岩代表性体积块1625μm × 1625μm × 1625μm，利用非局部均值滤波算法对所选CT图像进行平滑。其次，依据孔隙、有机质、碳酸盐岩和硅酸盐岩等对X射线的吸收系数差异, 采用阈值分割方法进行图像分割，实现延长7段页岩三维结构的重建与可视化。
2. 结果与讨论
2.1 孔隙二维结构特征
本文在参考Loucks等[12]页岩孔隙分类方案基础上，依据孔隙成因与其赋存特征将C7页岩气储集空间划分为无机孔、有机孔和微裂缝三类，如表 1所示。
表 1 遥科1井延长7段页岩孔隙类型及其特征Table 1. Pore types and characteristics of C7 Member shale from Yaoke-1 Well孔隙类型 孔隙示意图 孔隙形态 孔径范围 分布特征 无机孔 粒间孔 
三角形或狭缝形 30nm~1μm 发育在脆性矿物周缘及粉砂级黏土矿物碎屑之间，普遍发育，连通性较好 黏土矿物层间孔 
平直狭缝状 长1~3μm，孔宽数十纳米 分布于黏土矿物层间，不甚发育 溶蚀孔 
凹坑状 50~300nm 多见于石英、长石等矿物内部，彼此孤立 晶内孔、生物孔、晶间孔等 
圆形、椭圆形或方形等 百纳米~数十微米 与矿物相关，如生物遗体被黄铁矿充填，彼此孤立; 或形成于矿物晶间 有机质与有机孔 致密有机质 
连续且不规则状 N/A 有机质最主要的赋存方式，即粒间孔被有机质完全充填，且有机质内部无孔隙发育 
集合体形式 N/A 与黄铁矿呈现出包裹关系，与微晶之间残余少许孔缝 
有机质分散状，发育锯齿状孔隙 数百纳米至数微米 致密有机质与基质矿物接触面之间发育孔隙，较为普遍 有机孔 
狭缝状、三角状 50~300nm 受控于黏土矿物层间孔结构，有机孔最主要的存在形式 
凹坑状或椭圆状 30~200nm 受生烃作用控制，发育较少 微裂缝 
狭缝状 长数微米，宽几百纳米 发育于脆性矿物的边缘或机械不稳定部位，较平直，延伸长 注：N/A表示not applicable(不适用)。 2.1.1 无机孔和微裂缝结构特征
C7页岩中无机孔广泛发育，包括粒间孔、黏土矿物层间孔和溶蚀孔等。粒间孔主要发育在脆性矿物周缘和黏土碎屑之间。脆性矿物周缘孔以狭缝形为主，孔长约1μm，孔宽约100nm (图 3a)，在C7页岩中普遍发育；粉砂级黏土岩屑彼此连接，形成原生粒间孔，是C7页岩中发育最多的孔隙类型，以三角形为主，孔径集中于30~250nm，连通性较好(图 3b)。黏土矿物层间孔发育在黏土矿物层间，以平直狭缝状为主，孔长1~3μm，孔宽数十纳米(图 3c)，发育较少。溶蚀孔是烃源岩生烃过程中产生大量有机酸将矿物溶解所形成的次生孔隙。C7页岩中溶蚀孔数量较少且孤立存在，以凹坑状为主，孔径50~300nm(图 3d)。
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C7页岩中同时发育黄铁矿晶内孔和锐钛矿晶间孔等特殊的孔隙类型。黄铁矿晶内孔见图 3e。能谱图(图 3f)表明图 3e中十字标记区域元素组成主要为Fe、S、O、Si。该类孔隙基本呈圆形，黄铁矿质孔隙壁厚3~5μm，孔径数十微米，孔隙部分或全部被矿物碎屑或有机质充填，彼此之间并无连通性。锐钛矿晶间孔如图 3h和图 3i所示，孔径集中于数百纳米，连通性较好。此前在海相沉积威远九老洞组页岩[19]、辽河凹陷沙河街组页岩[32]中均观测到该类型孔隙，表明锐钛矿晶间孔的主要形成机理并非受控于沉积环境。C7页岩中亦发育少许生物孔，如图 3g所示，孔径30~50nm，推测为生物遗体被矿物质充填所形成。
微裂缝在C7页岩中较为发育，以构造裂缝为主。构造裂缝一般沿机械不稳定面发育，如脆性矿物与片状黏土矿物的外部或颗粒之间(图 3j，3k)，或沿黏土矿物层理发育(图 3l)，较平直，延伸尺度较大，长数微米，宽度几百纳米。普遍发育的微裂缝是沟通各类微观孔隙的桥梁，为页岩气运移提供了重要的渗流通道。
2.1.2 有机质与有机孔发育特征
C7页岩中有机孔发育较少，依据发育形态和成因可分成两类。第一类有机孔发育在离散状有机质内部，其形成受控于有机质生烃作用，发育极少，呈椭圆状或凹坑状(图 4a)等，孔径30~200nm。另外一类有机孔与有机黏土矿物共生，是C7页岩中有机孔最主要的存在形式。该类有机孔具有继承性结构，受黏土矿物层间孔的形貌控制，以狭缝状或层状为主，如图 4b和4c所示。黏土矿物强烈的吸附能力促使有机质在烃源岩中富集，与黏土矿物以结合态存在。在有机质生烃过程中，黏土矿物降低了生烃反应活化能，同时为其提供了电子(OH-)和质子(H+)来源。类似结构在四川盆地页岩气勘探有利储层龙马溪组海相页岩中发现较多[6]，不仅佐证了黏土矿物的催化生烃作用，而且说明陆相页岩有机孔的形成机理与海相页岩具有一定相似性。
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图 4 遥科1井延长7段页岩中有机孔和有机质的赋存特征a, b, c—有机孔, 主要发育在有机质之间; d, e, f, g, h, i—有机质赋存特征。Figure 4. Organic pores and organic matter in C7 Member shale from Yaoke-1 WellC7页岩中存在大量致密有机质，大小数百纳米至数微米，形态复杂，主要受控于无机孔缝的形状。图 4d中有机质彼此连接，填充于不规则状矿物碎屑粒间孔中，与粒间孔边缘完全接触；图 4e中条带状有机质充填于粒间孔中，彼此孤立存在，单个有机质长达数十微米；图 4f中条带状有机质则充填于黏土矿物层间，取向与黏土矿物一致，宽50~200nm。部分有机质与黄铁矿共生，表现为包裹关系，如图 4g所示，黄铁矿晶间孔几乎完全被有机质充填，仅在有机质与微晶之间残余少许孔缝。致密有机质与矿物基质接触面之间发育部分孔隙，该类孔隙往往与有机质取向一致，呈锯齿状、平直状和三角状等，孔径集中于数微米，如图 4h和图 4i所示。
2.2 页岩三维结构特征
C7页岩微米X射线重构图像如图 5a所示。图像中灰度值由白(255)到黑(0)代表物质的密度由大到小。结合扫描电镜数据判断，白色部分主要为黄铁矿，灰白色部分主要是钙质或铝铁质矿物；黑色部分代表低密度物质，理论上包括微孔隙、微裂缝和有机质。以下将低密度物质界定为有机质，主要原因如下：扫描电镜观测证实C7页岩中主要发育纳米孔，本文微米X射线扫描实验(分辨率3.25μm)不足以识别；从重构图像中可以看出，低密度物质大小集中于亚微米至微米，主要呈带状或星点状等，与扫描电镜下有机质的形貌大小一致；剖面线(图 5b)经过低密度物质时，灰度值均在90左右(孔隙灰度值接近0)。
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图 5 基于X射线重构图像的物质识别a—页岩微米X射线重构图像:有机质(黑色)，黄铁矿(白色)；b—L1、L2为a图中剖面线所示物质的灰度值剖面图。纵坐标:灰度值。Figure 5. Segmentations based on X-ray microscopy images经过物相分割，通过三维数值模拟得到C7页岩三维结构，如图 6所示。有机质、钙质(包括铝铁质)矿物和黄铁矿分别以红色，绿色以及黄色标记，空白区域代表硅铝质矿物(石英和长石等)。图 6表明C7页岩中，有机质与钙质、铝铁质矿物排列有序，在微米尺度上具有明显的纹层结构，黄铁矿则以无序状分散在矿物基质中。
图 6b展示了C7页岩有机质三维空间分布特征。有机质体积含量为3.4%，整体呈纹层结构。大部分有机质体积较小，均匀地分散于矿物基质中；少数有机质体积较大，取向与纹层一致，如图 7a所示。图 7a是图 6b中部分有机质的放大图，从图中能够看出该有机质颗粒长约700μm，宽约80μm，呈不完全连续分布。钙质和铝铁质矿物的三维空间结构如图 6c所示，其体积百分数达7.5%，同样具有纹层结构，取向与有机质分布模式一致。与有机质的均匀分布不同，钙质与铝铁质矿物在某些纹层内出现富集。此外，少量铝铁质矿物呈球状，体积较大, 如图 7b所示，随机分布于矿物基质中。黄铁矿体积含量较低，约0.7%，在三维空间中随机分布，如图 6d所示。黄铁矿颗粒大小集中于10~20μm，相对均匀地分散于矿物基质中；少数黄铁矿聚集在一起，形成团块儿状集合体。部分黄铁矿可能充填于生物遗体中，其三维结构放大图如图 7c所示。从图中可以看出，该类孔呈不封闭的球状，孔径约45μm。
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图 7 延长7段页岩中不同物相三维结构的放大图a—有机质; b—铝铁质矿物; c—黄铁矿晶内孔。Figure 7. Enlarged images showing three dimensional structures of different phases in C7 Member shale2.3 孔隙结构特征与其控制因素
扫描电镜观测结果表明C7页岩中无机孔和微裂缝最为发育。无机孔以发育在黏土碎屑颗粒之间的原生粒间孔为主，黏土矿物层间孔和脆性矿物周缘孔等海相页岩中普遍发育的孔隙类型发现较少。一方面，页岩中大量存在的碎屑状黏土矿物为孔隙发育提供了物质基础，是控制主要孔隙类型发育的重要因素之一；另一方面受成岩作用控制，原生层间孔和脆性矿物周缘孔难以保存[33]。与四川盆地广泛发育的海相页岩相较，C7页岩中有机孔鲜有发育[5-7]。有机孔的形成、形貌及大小主要与有机质含量、类型和热成熟度密切相关。鄂尔多斯盆地三叠系C7页岩热演化程度较低，大部分处于低成熟阶段，因此不具备大量发育有机孔的条件。有机质主要呈致密态，不均匀地充填于无机孔与微裂缝中，在一定程度上进一步降低了孔隙度。
扫描电镜测试表明C7页岩中介孔数量占绝对优势，对孔隙体积贡献最大，提供了主要的储集空间。黏土矿物颗粒细小，分选好，是介孔大量发育的关键因素。鄂尔多斯盆地长7期深湖细粒沉积背景导致原始的粒间孔隙相对较小[34-35]；强烈的压实作用使颗粒间紧密接触，同时胶结作用使大的粒间孔隙消失，残留了小尺度的粒间孔隙[36]。C7页岩中亦发育少量宏孔，孔径主体数百纳米，主要以粒间孔和微裂缝形态存在。脆性矿物含量较高是宏孔发育的重要原因。但是细粒黏土矿物与有机质的充填作用，在一定程度上导致宏孔数量降低。
2.4 对页岩气运移和压裂的启示
本文利用微米X射线显微镜证实C7页岩在微米尺度上具有明显的纹层结构，有机质纹层发育且连续性强，表明C7页岩具有较强塑性。李丽慧等[37]通过三轴压裂实验证实该类结构中，压裂缝以沿纹层扩展为主且易再次闭合，从而降低了储层的可压裂性。不过，随着有机质成熟度的增加，该结构沿层理方向易于形成相互连接的孔隙网络，从而有利于页岩气的横向运移[38]。扫描电镜测试表明各纹层中的微米级微构造裂缝发育较多，王跃鹏等[14]认为该类裂缝可能是由于水进入页岩内部发生水化作用，导致黏土矿物沉淀而形成的。该结构意味着水力压裂开采技术容易破坏C7页岩结构的完整性，引发井壁坍塌等严重问题。
3. 结论
本文利用氩离子抛光-扫描电镜和微米X射线显微镜方法，对鄂尔多斯陆相延长7段页岩孔隙结构特征进行研究。测试结果表明：①受黏土碎屑和成岩作用控制，延长7段页岩中主要发育纳米级粒间孔与微米级微裂缝，是页岩气的主要储运空间。有机质主要呈致密状，有机孔发育较少，一般与有机黏土矿物共存。②延长7段页岩在微米尺度上具有明显的纹层结构，有机质纹层发育且连续性强，不利于储层压裂。同时，大量存在的黏土矿物与微裂缝在水力压裂时容易引发井壁坍塌等严重问题。
微米X射线显微镜技术是深入研究陆相页岩纹层三维结构特征的有效方法，与氩离子抛光-扫描电镜技术结合，有利于实现页岩孔隙结构特征的多尺度多维度综合表征。本文研究成果可为后期页岩气水平井结构设计、提高井壁稳定性提供改进思路。
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图 2 金属稳定同位素实验内标(NIST-3165、BDH)长期分析测量精密度
Figure 2. Long-term precision of in-house standard solutions for NIST-3165 and BDH in metal stable isotope geochemistry laboratory
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图 3 地质标准物质的钒同位素组成(灰色圆圈代表前人研究数据，彩色符号代表本文数据)
Figure 3. Vanadium isotopic composition of geological reference materials (Grey circles represent data from previous studies and colored symbols represent the data in this study)
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图 4 火成岩标准物质的钒同位素组成与V、TiO2、SiO2、MgO含量关系图，其中钒同位素组成与SiO2含量呈正相关，与其他元素没有明显相关性
Figure 4. The diagrams of the relationship between vanadium isotope composition and V, TiO2, SiO2, MgO content of igneous rock reference materials. The vanadium isotope composition is positively correlated with SiO2 content, but not significantly correlated with other elements
表 1 地质标准物质和钒同位素标准溶液详细信息
Table 1 Detail information of reference materials and vanadium isotope reference solutions.
标准物质编号 样品类型 研制单位 δ51V推荐值
(‰)AA 纯钒溶液 Alfa Aesar公司 0.00 USTC-V 纯钒溶液 中国有色金属及电子材料分析测试中心 −0.07±0.08 BDH 纯钒溶液 BDH公司 −1.24±0.08 NIST-3165 纯钒溶液 美国国家标准与技术研究院(NIST) 0.7±0.08 GSP-2 花岗闪长岩 美国国家地质调查局(USGS) −0.62 BIR-1 玄武岩 美国国家地质调查局(USGS) −0.92 GBW07454 土壤 中国地质科学院地球物理地球化学勘查研究所(IGGE) −0.74 JA-1 安山岩 日本地质调查局(GSJ) / JB-3 玄武岩 日本地质调查局(GSJ) / JB-1b 玄武岩 日本地质调查局(GSJ) / GBW07105 玄武岩 中国地质科学院地球物理地球化学勘查研究所(IGGE) / JGb-1 辉长岩 日本地质调查局(GSJ) / GBW07123 辉绿岩 中国地质科学院地球物理地球化学勘查研究所(IGGE) / 注：USTC-V是由两瓶GSB(国家实物标准样品)单元素纯钒溶液混合均匀制备而成，表格中δ51V推荐值来源于Prytulak等(2011)［12］、Wu等(2016)［15］和Zeng等(2024)［44］。 Note: USTC-V is made by mixing two bottles of GSB (National Physical Standard Sample) single element pure vanadium solution evenly，and the recommended values for δ51V in the table are sourced from Prytual et al (2011)［12］，Wu et al (2016)［15］ and Zeng et al (2024)［44］. 
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表 2 MC-ICP-MS仪器测量钒同位素的主要工作条件
Table 2 Operating conditions for vanadium isotopic determination of MC-ICP-MS instrument
工作参数 实验条件 射频功率 1160～1200W 冷却气流速 ～16L/min 辅助器气体流速 ～0.8L/min 样品气流速 0.85L/min 灵敏度 51V为200V/(μg/g) 锥 X截取锥和镍Jet样品锥 膜去溶系统 Aridus 3 进样速率 50μL/min 分辨率 中分辨率(＞5500) 法拉第杯 L4 L2 L1 C H2 H3 48Ti 49Ti 50V 51V 52Cr 53Cr 电阻 1011Ω 1011Ω 1011Ω 1010Ω 1011Ω 1011Ω
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表 3 地质标准物质的钒同位素组成测量结果
Table 3 Vanadium isotopic composition of geological reference materials from different laboratories and our date
标准物质编号 样品类型 V含量
(μg/g)δ51V
(‰)2SD n 文献来源 BIR-1 玄武岩 310 −0.91 0.03 3 本文研究 −0.94 0.15 52 Prytulak等(2011)［12］ −0.92 0.09 52 Wu等(2016)［15］ −1.05 0.22 7 Sossi等(2018)［20］ −0.96 0.03 3 Wu等(2018)［27］ −0.89 0.23 3 Hopkins等(2019)［22］ −1.01 0.08 3 Qi等(2022)［34］ GSP-2 花岗闪长岩 52 −0.60 0.02 3 本文研究 −0.63 0.1 6 Prytulak等(2011)［12］ −0.62 0.07 26 Wu等(2016)［15］ GBW07454 黄土 77 −0.78 0.06 12 本文研究 −0.74 0.08 12 Zeng等(2024)［44］
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表 4 7个地质标准物质的钒同位素组成测量结果
Table 4 Determined values of vanadium isotopic composition of seven geological reference materials
标准物质编号 样品类型 V含量
（μg/g）δ51V测定值
（‰）2SD
（‰，n=3）δ51V总平均值
（‰）总测试数据的2SD
（‰）GBW07454 黄土
(loess)77 −0.81a 0.06 −0.78
（−0.74*）0.06 （n=12）
（0.08*）−0.74a 0.04 −0.78a 0.01 −0.80a 0.04 GBW07123 辉绿岩
(diabase)268 −0.75a 0.07 −0.72 0.06 （n=15） −0.71a 0.02 −0.70b 0.01 −0.69a 0.07 −0.74b 0.02 JGb-1 辉长岩
(gabrro)635 −1.11a 0.03 −1.05 0.08 （n=9） −1.03a 0.04 −1.03a 0.03 GBW07105 玄武岩
(basalt)167 −0.83a 0.05 −0.80 0.06 （n=12） −0.82a 0.04 −0.78a 0.05 −0.77a 0.04 JB-1b 玄武岩
(basalt)214 −0.79a 0.10 −0.79 0.08 （n=9） −0.74a 0.04 −0.83a 0.08 JB-3 玄武岩
(basalt)372 −0.83a 0.06 −0.81 0.09 （n=9） −0.84a 0.04 −0.76a 0.07 JA-1 安山岩
(andesite)105 −0.35a 0.03 −0.34 0.06 （n=12） −0.37a 0.02 −0.33a 0.03 −0.30b 0.05 注：a代表相同样品粉末单独进行化学纯化得到的数据；b代表相同的溶液重复测试得到的数据；n代表单次测量次数；*代表Zeng等(2024)[44]报道的δ51V数据值。 Note: a. Data obtained from chemical purification of the same sample powder separately; b. Data obtained from repeated testing of the same solution;n. Single measurement frequency; *. The data values of δ51V reported by Zeng et al. (2024)[44].
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