J. Cent. South Univ. (2020) 27: 1814-1827

DOI: https://doi.org/10.1007/s11771-020-4410-2

Discovery and identification of clay-sized tuffaceous rocks in Chang 7 Member of Upper Triassic Yanchang Formation in Ordos Basin, China

ZHENG Qing-hua(郑庆华)¹, LIU Xing-jun(刘行军)², LIU Yi-qun(柳益群)³,

ZHOU Ding-wu(周鼎武)⁴, YOU Ji-yuan(尤继元)¹, YANG Kai-yan(杨开艳)⁵

1. School of Chemistry & Chemical Engineering, Yulin University, Yulin 719000, China;

2. Changqing Division, China Petroleum Well Logging Limited Company, Xi’an 710201, China;

3. Department of Geology, Northwest University, Xi’an 710069, China;

4. College of Earth Science and Engineering, Shandong University of Science and Technology,Qingdao 266510, China;

5. Fengcheng Field Operation District, PetroChina Xinjiang Oilfield Company, Karamay 834000, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020

Abstract: This study for the first time demonstrates that some of the so-called clay-sized mudstones observed by the naked eye, such as clay-sized black mudstones and clay-sized oil shales, which are rich in black organic matter (including oil and asphaltene), in the Chang 7₃ Submember of the Upper Triassic Yanchang Formation in the Ordos Basin of China are actually clay-sized tuffaceous rocks (including tuff, sedimentary tuff and tuffaceous sedimentary rock) with high hydrocarbon generation capacities. Thus, these rocks can be defined as clay-sized tuffaceous source rocks. Identification of this lithology has important theoretical and practical significance for the exploration and development of shale oil in the Chang 7 Member. Through the macroscopic observation of drill cores and outcrop profiles, microscopic observation of electron probe thin sections and whole-rock inorganic geochemical analysis (including major, trace and rare earth elements), this work demonstrates that the organic matter-rich clay-sized tuffaceous rocks, especially clay-sized tuffs, have the following characteristics. First, the clay-sized tuffaceous rocks with little black organic matter are mainly greyish white, yellowish brown and purplish grey, and mixed colors occur in areas with strong bentonite lithification. Second, the clay-sized tuffaceous rocks have experienced strong devitrification and recrystallization, forming abundant flaky aluminosilicate minerals with directional arrangement. In thin sections under a polarizing microscope, the interference colors generally show regular alternation between the lowest interference color of first-order yellow and the highest interference color of second-order blue-green. Third, the rock samples plot in the igneous rock field in the TiO₂-SiO₂ cross-plot and exhibit similar trace element and rare earth element patterns on spider diagrams, indicating that the samples are derived from the same source. The results prove that clay-sized tuffaceous rocks may be widespread in the Chang 7₃ Submember of the Upper Triassic Yanchang Formation in the Ordos Basin, China.

Key words: Ordos Basin; Chang 7 Member; clay-sized mudstone; clay-sized tuffaceous rock; lithologic identification

Cite this article as: ZHENG Qing-hua, LIU Xing-jun, LIU Yi-qun, ZHOU Ding-wu, YOU Ji-yuan, YANG Kai-yan. Discovery and identification of clay-sized tuffaceous rocks in Chang 7 Member of Upper Triassic Yanchang Formation in Ordos Basin, China [J]. Journal of Central South University, 2020, 27(6): 1814-1827. DOI: https://doi.org/10.1007/s11771-020-4410-2.

1 Introduction

In this paper, we defined tuffaceous rocks (including tuff, sedimentary tuff and tuffaceous sedimentary rock) with hydrocarbon generation capacities that reach the standard of an effective source as tuffaceous source rocks [1]. The discovery and proposal of tuffaceous source rocks, a common research field in China, has a long history and includes tuffaceous oil-generating rocks (including tuffaceous shale and tuffaceous siliceous shale, etc.) [2], silt-sized to sand-sized sedimentary tuff source rocks [3, 4], tuffaceous source rocks (such as tuffaceous mudstone) [5], diamictic and dolomitic sedimentary tuff source rocks [6, 7] and tuffaceous source rocks (such as silt-sized to sand-sized sedimentary tuff and tuffaceous mudstone) [8]. Generally, the above tuffaceous source rocks, especially silt-sized to sand-sized tuffaceous rocks with relatively coarse grain sizes, are easily identified because they are not rich in black organic matter (such as black oil or asphaltene). Currently, an increasing number of researchers have recognized that the silt-sized to sand-sized tuffaceous rocks above can also act as effective source rocks. However, whether some of the so-called clay-sized mudstones observed by the naked eye, in which more than 50% of the grains are <0.0039 mm, such as clay-sized black mudstones and clay-sized oil shales, are clay-sized tuffaceous rocks rich in black organic matter (such as black oil and asphaltene) has not been explored deeply.

2 Geological and sedimentary background

The Ordos Basin is the largest petroleum production basin in China, and it is divided into six second-order tectonic units by its basement and fault characteristics, including the Yimeng uplift, Xiyuan obduction zone, Tianhuan depression, Yishan slope, Jinxi flexural fold zone and Weibei uplift (Figure 1(a)). The Upper Triassic Yanchang Formation is the most important oil-producing formation, and its oil is mainly located in the south-central Ordos Basin (research area) (Figure 1(a)). This formation can be divided in descending order into ten members (Chang 1- Chang 10) by lithology, sedimentary cycle, palaeontology and so on. In the sedimentary period of the Yanchang Formation, the Ordos Basin is a large fresh water depression basin, and the Chang 7 Member represents the peak of the Ordos Basin development, which is characterized by semi-deep and deep lake sedimentary environments and abundant clay-sized oil shale (Figure 1(b)) [9-21]. Unconventional oil and gas resources of the Chang 7 Member of the Upper Triassic Yanchang Formation in the Ordos Basin have developed on a large scale, and they include the oil accumulation in tight sandstone and clay-sized mudstone in the Chang 7 Member source rocks, which formed without long-distance migration. FU et al [20] refered to these sources as shale oil. At present, this conception is generally accepted by PetroChina Changqing Oilfield Company, China.

The Chang 7 member can be divided in descending order into the Chang 7₁ Submember, Chang 7₂ Submember and Chang 7₃ Submember [13]. The so-called clay-sized mudstones observed by the naked eye, such as clay-sized black mudstones and clay-sized oil shales, in the Chang 7₃ Submember at the bottom of the Chang 7 Member of the Upper Triassic Yanchang Formation in the Ordos Basin have a large thickness, wide distribution and high hydrocarbon generation capacity. Thus, this submember is the most important oil-generation layer in the Yanchang Formation. This unit is rich in black organic matter, such as black oil and asphaltene. Thus, it has long been regarded as a so-called clay-sized mudstone [9-21]. However, some studies in recent years have shown that some of the so-called clay-sized mudstones observed by the naked eye, such as clay-sized black mudstones and clay-sized oil shales, in the Chang 7₃ Submember might be clay-sized tuffaceous rocks with high hydrocarbon generation capacities and can thus be considered clay-sized tuffaceous source rocks (Figure 1(c)) [1]. Hence, re-examining the lithology of the source rocks in the Chang 7₃ Submember and determining whether some of the source rocks are indeed clay-sized tuffaceous rocks have theoretical and practical significance for confirming the source rock type and genetic mechanism. Through macroscopic observation of drill cores and outcrop profiles, microscopic observation of electron probe thin sections (approximately 0.04 mm thick) and whole-rock inorganic geochemical analysis (including major elements, trace elements and rare earth elements) of clay-sized black mudstone, clay-sized oil shale and clay-sized tuffaceous rock rich in black organic matter (such as black oil and asphaltene) in the Chang 7₃ Submember, we discerned the identifying features (such as petrology, mineralogy, diagenesis and geochemistry) of the clay-sized tuffaceous rocks rich in black organic matter and developed an identification method and lithology type classification.

Figure 1 (a) Tectonic zoning of the Ordos Basin and location of research area (modified from Ref. [21]);(b) Distribution of clay-sized oil shale of the Chang 7 Member, wells and outcrop profiles in research area (modified from Ref. [14]); (c) Lithology of the lower Chang 7₃ Submember in Motiangou, Jinsuoguan Town, Tongchuan City, Shaanxi Province (modified from Ref. [1])

3 Identification of clay-sized tuffaceous rock

The key to identify clay-size tuffaceous rocks is the identification of clay-size tuff. If we can identify clay-sized tuff, then we can also identify tuff materials (such as clay-sized to sand-sized crystal fragments and vitric fragments) and then determine the types of clay-sized tuffaceous rocks by combining the content and grain size data of the tuffaceous materials in the rock. Some studies have shown that the clay-sized tuff in the Chang 7₃ Submember differs greatly from clay-sized mudstone in terms of physical properties, rock mineral composition, diagenesis, fabric, structure, geochemical features and other characteristics. Thus, these factors can be taken as the major identifying features in the determination of lithology [1, 22].

3.1 Physical properties

Mainly distributed along bedding planes, clay-sized tuffs are microns to metres thick and variable in color. When not fully coated in black organic matter (such as black oil), the tuffs are mainly greyish white, yellowish brown and purplish grey and therefore easy to identify. The parts coated in black organic matter are greyish black and black and are easily mistaken for clay-sized mudstone (Figures 2(a)-(c)). After weathering, the clay-sized tuffs generally exhibit secondary colors (such as white, yellowish brown and tan) (Figures 3(a)-(d)). In contrast, clay-sized mudstone hardly changes in color and remains largely grey or black. Clay-sized tuff is usually harder than clay-sized mudstone and generally shows conchoidal fractures after being struck with a hammer; in contrast, clay-sized mudstone generally breaks along bedding or lamellae after being struck with a hammer. Additionally, clay-sized tuff is generally sparser than clay-sized mudstone.

Figure 2 Characteristics of tuffaceous rocks in the Chang 7₃ Submember in the Motiangou profile in Yaoqu Town, Tongchuan City, Shaanxi Province:

3.2 Petrological and mineralogical characteristics

Because of the strong K-bentonite lithification, clay minerals in the clay-sized tuff of the Chang 7₃ Submember in the studied basin are dominated by illite-smectite mixed-layer minerals (I/S) and illite (accounting for more than 60%). The silt-sized to sand-sized crystal fragments are predominantly alkaline feldspar (potash feldspar and albite) and quartz, with minor biotite. Vitric fragments have been almost entirely altered, and intact lithic fragments are rare. Under plane-polarized light, the feldspar is generally fresh, clean and bright.

The clay-sized mudstone contains silt-sized to sand-sized detritus from sedimentary rocks, metamorphic rocks and magmatic rocks, a high mica content, and features illite, chlorite and kaolinite as dominant clay minerals. The surfaces of the silt-sized to sand-sized feldspar usually appear to be “dirty” under the plane-polarized light as the result of illitization and kaolinization.

3.3 Diagenetic characteristics

Similar to their compositional differences, the clay-sized tuff and clay-sized mudstone in the Chang 7₃ Submember have different diagenetic characteristics, which are also an important identifying characteristic in terms of lithology.

3.3.1 Bentonite lithification

In general, in an alkaline environment, clay- sized tuff undergoes bentonite lithification, which mainly generates smectite first [23] and mixed-layer illite-smectite (I/S) and illite second [24, 25]. In acidic environments, clay-sized tuff experiences kaolinization, which mainly generates kaolinite [26]. In contrast, clay-sized mudstone does not experience bentonite lithification.

Exhibiting strong K-bentonite lithification, the clay-sized tuff in the Chang 7₃ Submember in the study basin has high illite-smectite mixed-layer (I/S) and illite contents, a low density, a loose texture, and a tendency to swell when in contact with water [12]. Units with K-bentonite lithification feature the following major identifying characteristics: they generally exhibit secondary colors after weathering (such as white, yellowish brown and tan) in outcrops and drill cores (Figures 3(a)-(d)); feldspar crystals along surfaces and borders are often blurry because of K-bentonite lithification (Figures 3(e) and (f)) and may even be altered into several residual parts (Figure 3(g)); and under scanning electron microscopy, clay minerals are predominantly mixed-layer illite-smectite (I/S) and illite in flake and curly flake shapes (Figures 3(h) and (i)).

3.3.2 Devitrification and recrystallization

According to current rock section identification methods in “Oil and gas industry standard of People’s Republic of China (SY/T 5368-2000)”, it is difficult to distinguish clay-sized mudstone from clay-sized tuff [27, 28].

Figure 3 Diagenetic characteristics of the tuffaceous rocks in the Chang 7₃ Submember of the Triassic Yanchang Formation in the Ordos Basin:

ZHANG et al [27, 29] proposed a practicable identification method for identifying clay-sized mudstone and clay-sized tuff in the Permian Pingdiquan Formation of the Huoshaoshan Oilfield in the Eastern Junggar Basin, Xinjiang. The basic principle is that single micro-grained minerals in clay-sized mudstone and clay-sized tuff both take on oriented pinpoint shapes. Although the shape and interference color of single micro-grained minerals cannot be distinguished under the transmitted light system of a polarized light microscope, their lithologies can be distinguished by the overall optical features of fine mineral aggregates [30, 31]. The clay minerals (mainly illite with an average content of approximately 56.0%) in clay-sized mudstone are in a directional arrangement with a positive elongation feature. Observed under the cross-polarized light of a polarized light microscope with the gypsum test plate inserted and the microscope stage rotated, the thin section of the clay-sized mudstone shows a regular change in interference color from the lowest interference color of first-order yellow to the highest interference color of purple. The clay-sized minerals of clay-sized tuff are compositionally uniform (mainly andesite), and the andesite and clay minerals exhibit directional arrangement with a negative elongation feature. When thin sections are under cross-polarized light, the gypsum test plate of the polarized light microscope is inserted, and the microscope stage is rotated, the interference color generally shows regular alteration between the lowest interference color of first-order yellow and the highest interference color of second-order blue [27, 29].

Many studies have shown that the main clay mineral in clay-sized mudstone in the Chang 7₃ Submember Ordos Basin is illite (with an average content of approximately 61.7%). Observed under the cross-polarized light of a polarizing light microscope, clay-sized mudstone also shows a regular change in interference colors from the lowest interference color of first-order yellow to the highest interference color of purple (Figures 4(a) and (b)). However, because of differences in composition and diagenesis, clay-sized tuff shows a regular change in interference colors from the lowest interference color of first-order yellow to the highest interference color of second-order blue-green under the cross-polarized light of a polarized light microscope (Figures 4(c)-(h)). This change mainly occurs because the clay-sized tuff with little or no black organic matter in the Chang 7₃ Submember in the study basin has experienced strong devitrification and recrystallization, which generated a large amount of long, flake-shaped aluminosilicate minerals with positive elongation features and a directional arrangement along the long axis (Figures 4(c)-(f)). Obviously, the difference between the clay-sized tuff and clay-sized mudstone is that the highest interference color of the clay-sized tuff is second-order blue- green, which can be taken as a major identifying feature of clay-sized tuff. Even when the clay-sized tuff contains considerable black organic matter or has experienced stronger bentonite lithification, this feature is still effective (although it is less apparent) (Figures 4(i)-(o)).

Observed by the naked eye, the upper section of the core at 1649.50 m of Well ZH22 is a typical yellowish-brown clay-sized tuff containing silt-sized crystal fragments with minor black organic matter and strong bentonite lithification (Figures 5(a) and (b)). Observed under cross- polarized light, its highest interference color is relatively apparent second-order blue-green (Figure 5(c)). Although some silt-sized to sand- sized crystal fragments are not obvious under reflected light due to bentonite lithification (Figure 5(d)), they are still fresh, clean and bright when observed under the plane-polarized light of a polarized light microscope (Figure 5(b)). The lower section of this sample would be considered a so-called clay-sized black mudstone containing silt based on naked eye observations (Figure 5(a)) because of the abundant black organic matter and strong bentonite lithification (Figure 5(e)). However, when observed under cross-polarized light, the highest interference color is still relatively apparent second-order blue-green (Figure 5(f)). The whole-rock major element data further support this point. The mass percentages of SiO₂, TiO₂, Al₂O₃, Fe₂O₃, MnO, MgO, CaO, Na₂O, K₂O, P₂O₅, loss on ignition (LOI) and total in the upper section of the core at 1649.50 m in Well ZH22 are 67.33%, 0.11%, 19.08%, 0.99%, 0.01%, 0.94%, 1.11%, 1.00%, 2.88%, 0.02%, 6.03% and 99.50%, respectively, while those in the lower section are 69.87%, 0.08%, 15.59%, 1.53%, 0.02%, 1.02%, 1.17%, 0.78%, 2.43%, 0.02%, 7.10% and 99.61%, respectively. These samples appear to have very similar geochemical features, and the differences might be caused by different organic matter contents in the form of black oil or asphaltene. Both samples are crystal clay-sized tuff.

In addition, the so-called clay-sized oil shale (Figure 5(g)) and silt-sized crystal tuff with normally graded bedding (between 0.5 and 4.0 mm thick) observed by the naked eye in the outcrop profile in the Chang 7₃ Submember in Bawangzhuang Village (Figures 5(h), (i), and (j)) both contain horizontal bituminous dykes, and the so-called clay-sized oil shale has oil drops seeping out (Figure 5(h)), which indicates that both units have a certain storage capacity. In the area at the margins of the thin sections where most of the organic matter has washed away, the so-called clay-sized oil shale observed by the naked eye generally shows a regular change between the lowest interference color of first-order yellow and the highest interference color of second-order blue-green (Figures 5(k), (l), (m) and (n)), the typical characteristic of clay-sized tuff. The identification of the following clay-sized tuff rich in black organic matter is also based on observations at the margins of the electron probe thin sections.

Figure 4 Devitrification and recrystallization characteristics of the tuffaceous rocks in the Chang 7₃ Submember of the Triassic Yanchang Formation in the Ordos Basin:

Figure 5 Devitrification and recrystallization characteristics of the tuffaceous rocks in the Chang 7₃ Submember of the Triassic Yanchang Formation in the Ordos Basin:

3.4 Fabric and structural features

The results of the above studies on certain types of diagenesis of tuff can be used to determine the original textural and structural characteristics. The clay-sized tuff in the Chang 7₃ Submember was formed by the rapid deposition of volcanic ash carried by wind in calm water. According to the thickness, this unit can be mainly classified as a medium- to thin-bedded, laminated clay-sized tuff. This medium-to thin-bedded clay-sized tuff rich in black organic matter (Figures 6(a)-(c)) is usually associated with medium- to thin-bedded, relatively light-colored (such as light grey) silt-sized to sand-sized tuff (Figures 6(a), (d) and (e)). The two might have formed during the same period of volcanism. This type of medium- to thin-bedded clay-sized tuff generally exhibits a massive structure and is usually mistaken as clay-sized black mudstone source rock. In fact, it generally has no hydrocarbon generation material; thus, it cannot be categorized as a clay-sized tuff source rock. The black laminated clay-sized tuff rich in black organic matter might have formed by slow deposition of volcanic ash erupting in multiple phases and usually contains interbedded laminated clay-sized tuff and laminated black organic matter (Figures 6(f)-(i)). Hence, this unit generally has false horizontal bedding and laminae, and is often mistaken as clay-sized black mudstone, clay-sized oil shale and other source rocks (Figure 6(f)). Abundant in kerogen, this rock type can be categorized as a clay-sized tuffaceous source rock and commonly has thick lenticular clay-sized tuff laminae that can be easily identified. Generally, this rock type shows a regular change from the lowest interference color of first-order yellow to the highest interference color of second-order blue-green, the typical feature of clay-sized tuff (Figures 6(f)-(i)).

Figure 6 Textural and structural characteristics of the tuffaceous rocks in the Chang 7₃ Submember of the Triassic Yanchang Formation in the Ordos Basin:

3.5 Geochemical characteristics

Based on the collection of geochemical data of rocks in the Chang 7₃ Submember reported by previous researchers [1, 9, 11, 12, 32], we used geochemical methods to re-examine the lithology of some of the tuff and the so-called clay-sized mudstones observed by the naked eye, such as clay-sized black mudstones and clay-sized oil shales.

3.5.1 TiO₂-SiO₂ diagram method

The TiO₂-SiO₂ diagram method presented by TARNEY [33] can effectively distinguish sedimentary rocks from igneous rocks. In the TiO₂-SiO₂ diagram, most of the tuff and the so-called clay-sized mudstones in the Chang 7 Member of the Triassic Yanchang Formation in the Ordos Basin plot in the igneous rock region (Figure 7).

Figure 7 TiO₂-SiO₂ cross-plot for rock samples from Chang 7 Member of the Triassic Yanchang Formation in Ordos Basin (element data from Refs. [1, 9, 11, 12, 32])

Therefore, some of the so-called clay-sized mudstones observed by the naked eye, such as clay-sized black mudstones and clay-sized oil shales in the black rock series of the Chang 7 Member, might contain tuffaceous materials; many of them may be clay-sized tuffaceous rocks.

3.5.2 Spider graphs of trace elements and rare earth elements

The tuff and the so-called clay-sized mudstones observed by the naked eye, such as clay-sized black mudstones and clay-sized oil shales in the Chang 7 Member of the Triassic Yanchang Formation in the Ordos Basin, have similar primitive mantle- and chondrite-normalized features for trace elements and rare earth elements (Figures 8 and 9), which mean that some of the above so-called clay-sized mudstones observed by the naked eye, such as clay-sized black mudstones and clay-sized oil shales, might contain tuffaceous materials, which derived from the same source; many of them may even be clay-sized tuffaceous rocks.

Figure 8 Chondrite-normalized trace element patterns for rock samples from the Chang 7 Member of Triassic Yanchang Formation in Ordos Basin (element data from Refs. [1, 9, 11, 12, 32])

Figure 9 Chondrite-normalized rare earth element (REE) patterns for rock samples in Chang 7 Member of Triassic Yanchang Formation in Ordos Basin (element data from Refs. [1, 9, 11, 12, 32])

Recent studies have shown that the tuffaceous materials of tuffaceous rocks may have come from the eruptions of a volcano in the Qinling orogenic belt or the Qilian orogenic belt [9, 34-37] (Figure 1(a)).

4 Types and distribution features of clay-sized tuffaceous rocks

The volcanic particles in the Chang 7 Member in the Ordos Basin generally have a grain size of less than 2.0000 mm; thus, these units are classified as tuffaceous. According to the standard for the classification and naming scheme of volcaniclastic rocks in China (SY/T 5830-93) [38] and the ratios between tuffaceous materials and terrigenous clastic rocks, these units can be classified as tuff, sedimentary tuff, and tuffaceous sedimentary rocks (Table 1), which are together referred to as tuffaceous rocks [1]. In addition, to unify the grain classification of tuffaceous rocks and normal terrigenous clastic rocks in the Chang 7 Member, the rocks are classified into the following classes depending on whether more than 50% of the grains are <0.0039 mm, 0.0039-0.0625 mm or 0.0625- 2.0000 mm: clay-sized tuff, silt-sized tuff and sand-sized tuff for tuffs; clay-sized sedimentary tuff, silt-sized sedimentary tuff and sand-sized sedimentary tuff for sedimentary tuffs; and tuffaceous clay-sized mudstone, tuffaceous siltstone and tuffaceous sand for tuffaceous sedimentary rocks, respectively (Table 1).

We collected many samples of the so-called clay-sized mudstones observed by the naked eye, such as clay-sized black mudstones and clay-sized oil shales, in the Chang 7₃ Submember in Motiangou profile in the Ordos Basin and identified their lithology based on the study results above. The results show that most of the so-called clay-sized mudstones observed by the naked eye, such as clay-sized black mudstones and clay-sized oil shales, in the Chang 7₃ Submember are clay-sized tuffaceous rocks, mainly clay-sized tuff (Figure 1(c)) [1].

5 Conclusions

1) We propose that some of the so-called clay-sized mudstones observed by the naked eye, such as clay-sized black mudstones and clay-sized oil shales, in the Chang 7₃ Submember of the Upper Triassic Yanchang Formation in the Ordos Basin are actually clay-sized tuffaceous rocks rich in black organic matter (black oil, asphaltene and so on) with high hydrocarbon generation capacities; thus, these rocks represent clay-sized tuffaceous source rocks.

2) There are different petrological, mineralogical, diagenetic and geochemical features of clay-sized tuffaceous rocks and clay-sized mudstones, which are rich in black organic matter (black oil, asphaltene and so on) and observed alike by the naked eye in the Chang 7₃ Submember; therefore, it is feasible for the tentatively proposed method to identify the lithology of the clay-sized tuffaceous source rocks.

3) The scale of clay-sized tuffaceous rocks among the so-called clay-sized mudstones observed by the naked eye, such as clay-sized black mudstones and clay-sized oil shales, in the Chang 7₃ Submember of the Upper Triassic Yanchang Formation in the Ordos Basin remains a challenge, and more solid evidence from petrology, elemental geochemistry and micro-mineralogy is needed from future studies to support the potential inferences and conclusions of this work, but this study opens a window into future research on the hydrocarbon generation ability of clay-sized tuffaceous source rocks in the Chang 7₃ Submember.

Table 1 Classification of tuffaceous rocks in Chang 7 Member of Triassic Yanchang Formation, Ordos Basin

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(Edited by ZHENG Yu-tong)

中文导读

鄂尔多斯盆地上三叠统延长组长7段泥级凝灰质岩的发现及识别

摘要：在鄂尔多斯盆地上三叠统延长组长7₃亚段中，发现一些能肉眼观察到的 “泥级泥岩”(如泥级黑色泥岩、泥级油页岩等)，实际为富含黑色石油、沥青质等黑色有机质并具有较强生烃能力的泥级凝灰岩、泥级沉凝灰岩、泥级凝灰质沉积岩等泥级凝灰质岩，可定义为泥级凝灰质烃源岩。泥级凝灰质岩的识别对长7段页岩油的勘探开发具有重要意义。通过钻井取心及野外露头剖面观察，电子探针薄片的显微镜镜下观察，全岩主量元素、微量元素、稀土元素等无机地球化学分析，认为富含黑色石油、沥青质等黑色有机质的泥级凝灰质岩，特别是泥级凝灰岩主要具有以下识别标志：仅少量黑色有机质覆盖的部位主要呈灰白色、土黄褐色和紫灰色，斑脱岩化作用强烈时颜色杂乱；脱玻化-重结晶作用强烈时，生成大量呈片状定向分布的铝硅酸盐矿物，在电子探针薄片的偏光显微镜正交光下插入石膏试板、旋转载物台，干涉色整体呈现最低干涉色一级黄、最高干涉色二级蓝绿的规律变化；在TiO₂-SiO₂图解上落在火成岩范围，微量元素和稀土元素蜘蛛图特征大体相似，反映同源特征。结果表明，鄂尔多斯盆地上三叠统延长组长7₃亚段可能发育很多泥级凝灰质岩。

关键词：鄂尔多斯盆地；长7段；泥级泥岩；泥级凝灰质岩；岩性识别

Foundation item: Project(18GK28) supported by the Doctoral Scientific Research Staring Foundation for Yulin University, China; Project(20106101110020) supported by the University Research Fund of Science and Technology Development Center of Ministry of Education, China; Project(BJ08133-3) supported by the Key Fund Project of Continental Dynamics National Key Laboratory of Northwest University, China

Received date: 2019-04-18; Accepted date: 2020-03-12

Corresponding author: ZHENG Qing-hua, PhD, Lecturer; Tel: +86-15388656689; E-mail: 272594012@qq.com; ORCID: 0000-0002- 1803-2989