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Article

Characteristics and Paleoenvironment of High-Quality Shale in the Triassic Yanchang Formation, Southern Margin of the Ordos Basin

by
Mengsi Sun
1,*,
Congjun Feng
2 and
Yipu Li
1
1
College of Energy & Environment Engineering, Yan’an University, Yan’an 716000, China
2
Department of Geology, State Key Laboratory of Continental Dynamics, Northwest University, Xi’an 710069, China
*
Author to whom correspondence should be addressed.
Minerals 2023, 13(8), 1075; https://doi.org/10.3390/min13081075
Submission received: 6 June 2023 / Revised: 28 July 2023 / Accepted: 6 August 2023 / Published: 13 August 2023
Browse Figures
Versions Notes

Abstract

:
A set of high-quality lacustrine shales at the bottom of the Chang 7 member of the Yanchang Formation in the Ordos Basin is one of the main source rocks of tight oil and gas and shale oil in the Yanchang Formation. Based on outcrop, core, drilling and seismic data, by the quantitative characterization of outcrops, fine characterization of logging facies and seismic facies, and geochemical tests, the lithofacies types, geophysical response characteristics and organic geochemical characteristics of this high-quality shale are clarified, and the formation paleoenvironment, including redox conditions, paleoclimate, paleosalinity and paleowater depth, is analyzed. The high-quality shale at the bottom of the Chang 7 member is divided into three lithofacies types: black shale, dark massive mudstone and silty mudstone. The organic matter in black shale is mainly interbedded or stratified, the organic matter in dark massive mudstone is dispersed and the organic matter content in silty mudstone is lower. The shale shows high gamma (more than 260 API), a high acoustic time difference (more than 280 μs/m), a high resistivity (more than 330 Ω m) well-logging phase and strong-amplitude parallel–subparallel seismic phase characteristics. Based on the logging and seismic facies characteristics, the plane distribution range of this set of shales is defined. The sedimentary thickness gradually increases from the edge (5–10 m) to the center of the basin, among which the Jiyuan–Huachi–Yijun black shale has the largest thickness (more than 30 m). This set of high-quality shales was mainly formed under a warm and humid paleoclimate, in water depths of 60–120 m, and in an anaerobic reducing and continental freshwater paleoenvironment. The fine identification, distribution range and formation conditions of black shale lithofacies are of practical significance for predicting the distribution of favorable lithofacies of shale oil and gas and the deployment of horizontal wells.

1. Introduction

According to incomplete statistics, the total amount of shale oil resources in the world is more than 50% of traditional oil resources [1,2], so research on shale and the oil and gas in shale has become a current research focus [3], and many emerging technologies are also being applied [4,5,6]. Among them, related disciplines, such as geochemistry [7,8,9,10], geophysics [11] and organic petrology [12], have successively developed related technologies in studies of shale depositional environments and have made great progress. China is one of the countries with the most abundant oil shale reservoirs in the world, and the Ordos Basin is one of the basins with the richest oil shale resources. Shale, which is characterized by a large thickness, a wide distribution and rich organic matter, is the main Mesozoic source rock in the Ordos Basin [13]. Predecessors have performed much research on this set of high-quality shales. Zhang, W. [14] analyzed the trace elements of the source rocks of the Chang 7 and found a low Sr/Ba value and a high V/Ni value, indicating that it formed in a continental freshwater and reducing environment. The B content was low, indicating that the salinity of the water in the lake basin was not high. Zhang Cail [15] analyzed the geochemical indicators and biological remains as identification markers and believed that the Chang 7 was part of the central continental area of the western part of the basin, and the results showed that the salinity of the Chang 7 member was slightly higher than that of the Chang 8 and Chang 9 members.
Zheng Yiding [16] tested trace elements in the Chang 7 shale in Well Y1 in Zhangjiatan and believed that the sedimentary environment during the Chang 7 member deposition was warm and humid, with fresh–brackish water and oxygen deficiency. In previous studies, the sedimentary environment of high-quality shale in Yanchang Formation was mostly discussed with the method of element geochemistry [17], and few scholars conducted studies with the combination of geophysical data [18,19]. Therefore, this paper analyzes the lithology, lithofacies, geophysical response characteristics and organic geochemical characteristics of this set of high-quality shales through field sedimentary investigations, core observations and thin section observations, combined with logging, seismic, organic geochemical and other data. On this basis, the trace element ratio is comprehensively applied to analyze the redox conditions, paleoclimate, paleosalinity and paleowater depth of high-quality shale, helping the interpretation of the formation environment of high-quality shale more comprehensively and may provide guidance for oil and gas exploration in the Ordos Basin.

2. Geological Background

The Ordos Basin is the second largest oil- and gas-bearing basin in China, covering an area of 25 × 104 km2. The basin is characterized by a wide and gradual slope in the east and a large slope with rapid subsidence in the west, and the internal structure is relatively simple, mainly consisting of six first-level structural units: the Yimeng uplift in the north, the thrust fault zone and Tianhuan depression in the west, the Weibei uplift in the south, the Jinxi tortuous belt in the east and the Yishan slope in the middle. A series of tectonic belts is present in the basin periphery, including the Qinling–Qilian orogenic belt in the south, Yinshan fold structural belt in the north, Helan Mountains in the west and Luliang Mountains in the east [20]. The Ordos Basin is rich in oil, gas, coal, uranium and other resources, among which the Triassic Yanchang Formation is an important oil and gas exploration target. The Yanchang Formation can be divided into 5 member and 10 reservoir groups from bottom to top [21], and the sedimentary period of the Chang 7 was the maximum flooding period of the lake basin. A set of high-quality shales rich in organic matter was deposited in the center of the lake basin, and this set has a large thickness and a wide distribution range and is the main Mesozoic source rock [22,23]. The organic matter abundance of the source rocks is higher than that of other oil layers in the Yanchang Formation, and the total organic carbon (TOC) contents range from 0.3% to 36.83%, with an average content of 6.93% [24,25,26]; the organic matter vitrinite reflectance Ro values range from 0.53% to 1.09%, with an average of 0.82% [27,28], and the source rocks are characterized by high resistivity values, high gamma values and low potential values. This set of strata is stable in the whole area and is also a marker for stratigraphic correlation in this area (Figure 1).

3. Samples and Analytical Methods

This study used 2D seismic line and well log data. The seismic data are a zero-phase migration stack profile with a sampling interval of 2 ms, and the main frequency is 35 Hz; 16 drilling data points are used in this study, of which 13 wells have gamma curves, density curves and neutron curves. The well curve was used to identify shale, and the sampling interval is 0.125 m, which can clearly display the lithology of the formation and identify high-quality shale. The core photographs of the other three wells were used to analyze the lithofacies characteristics.
In this paper, three typical outcrops were selected for actual measurement. In this paper, three typical outcrops were selected for actual measurement. First, we measure the thickness of each set of strata. Then, the outcrop section was observed and described, systematically photographed and, finally, returned to the laboratory to stitch together the cross-sectional view of the outcrop, to analyze the depositional environment of the Yanchang Formation.
The samples used in this study were taken from three outcrops. In these three outcrops, the Chang 7 formation has a stable distribution and good continuity and is suitable for studying the sedimentary environment through its main and trace element contents, which basically represent the entire study area. Four indexes are used to analyze the redox conditions in the sedimentary period of the Yanchang Formation shale: the U/Th ratio, V/(V + Ni) ratio, V/Cr ratio and Ce/La ratio. The basic principle is as follows: U and Th have different activities and different lithologies. U is more active than Th and is prone to oxidation, while Th is less active. U is often adsorbed in mudstone and Th is often adsorbed in fine-grained sediments; V, Ni and Cr are mainly adsorbed and precipitated by colloidal particles or clay. V is easily adsorbed under reducing conditions, and Ni, Cr and Co are easily enriched in a reducing environment. Thin sections of shale samples were prepared at Northwestern University’s State Key Laboratory of Continental Dynamics and studied in detail using light microscopy. Major and trace element testing of shale samples was performed at the State Key Laboratory of Continental Dynamics, Northwestern University. Inductively coupled plasma optical emission spectroscopy (ICP–OES; Al2O3, CaO, Fe2O3, K2O, MgO, MnO, Na2O, P2O5, TiO2, Barium, and Strontium) and inductively coupled plasma–mass spectrometry (ICP–MS; Vanadium, Zinc, Zirconium, Scandium, Chromium, Cobalt, Niobium and Cuprum) were used.

4. Results and Discussion

4.1. Quality Shale Characteristic

There are two sets of shales in the Yanchang Formation in the Ordos Basin: the Zhangjiatan shale of the Chang 7 member and the Lijiapan shale of the Chang 9 member. The Zhangjiatan shale is widely distributed and relatively thick, and it is also the main object of this paper. Based on detailed observations of the Chang 7 Shale in the Yaoqu, Jinsuoguan and Xunyi outlying sections, the lithofacies characteristics are summarized in this paper, and the distribution range is described by combining logging, drilling and seismic data.

4.1.1. Outcrop Profile Performance

The outcrop profile has many advantages, such as intuitiveness, large scale and high precision. There are many outcrop profiles in the Yanchang Formation. The results of previous studies on these outcrops have laid a good foundation for this study. The Yaoqu, Jinsuoguan and Xunyi sections (Figure 1b) are on the southern edge of the basin outcrop along the highway and have large or small dip angles and clear lithological and lithofacies characteristics. Therefore, these three outcrop profiles were selected for study. Observations and descriptions were carried out for the three selected observation points, and systematic photographing and splicing of outcrop and landscape sections were carried out for further indoor analysis and research. To analyze the real sedimentary environment of the Yanchang Formation, a high-precision total station instrument was used to measure the outcrop strata, and a compass was used to measure the orientation and dip angle of the rock strata.

Nijiahe Outcrop

The Nijiahe outcrop is located in Yaoqu Town, Tongchuan City. The Chang 7 member has a dip angle of 10° and dips NNE. The measured cumulative thickness is approximately 2.2 m (Figure 2). A total of 23 small layers are present from bottom to top, and they are mainly black shale facies, dark massive mudstone facies and yellow–brown tuffite facies. The three lithologies are interbedded, and the thickness of each small tuffite layer is uneven; the thinnest is 15 mm and the thickest is up to 420 mm. The interbedded deposition of multilayer tuffite and shale reflects frequent volcanic activity and different intensities of volcanic activity at that time. Tuffite deposits are widely distributed in the Yanchang Formation and are commonly light gray, yellow–brown, purplish red, etc. [29] (Zhang et al. 2009).

Tangnihe Outcrop

The Tangnihe outcrop is located in Jinsuoguan, Yijun County, Tongchuan City. The Chang 7 member presents an inverted formation with a dip angle of 58° and NNE dip. A rolling anticline is present in this outcrop. Based on the contact relationship between the formation and the fault, it is inferred that the fault is a synsedimentary fault. Three lithofacies types, i.e., black shale facies, dark massive mudstone facies and silty mudstone facies, are identified in the outcrop profile (Figure 3). The black shale takes the form of black flakes, and foliation is developed. The bedding of dark massive mudstone is not well-developed, and the mudstone is mainly dark gray mudstone or carbonaceous mudstone. Silty mudstone is mainly gray–brown. In the outcrop profile, black shale, dark mudstone and silty mudstone are layered and interbedded, which reflects the sedimentary background of high-frequency lacustrine level variations under a deep lacustrine sedimentary background.

Shanshuihe Outcrop

The Shanshuihe outcrop is located at the entrance of Shicaogou and is distributed in the northwest direction. Two layers of black shale with thicknesses of 25 cm and 5 cm, one layer of silty mudstone with a thickness of 90 cm and one layer of dark mudstone with a thickness of 25 cm can be identified in this outcrop. A set of thick massive sandy detrital fine sandstones developed between silty mudstone and dark mudstone; it is an oil-bearing, fine sandstone with great thickness variation in the transverse direction (Figure 4). A sliding deformation layer with a thickness of more than 10 cm is visible in the shale, and this layer is presumed to be a small synsedimentary fault caused by tectonic activity. Due to the blocking of the front mound and the blocking of the ditch, this feature cannot be explored at a close distance, and its cause needs to be further confirmed.

4.1.2. The Core and the Microscopic Chip Performance

Three lithofacies types can be identified in the Chang 7 member: black shale facies, dark massive mudstone facies and silty mudstone facies. Black shale is grayish black–black, hard, has developed bedding (Figure 5a,b) and includes animal fossils, such as bivalves and squamata. The dark massive mudstone is dark gray, with undeveloped bedding, a high carbon content and a large number of plant fossil fragments (Figure 5c,d). The silty mudstone is mainly gray–grayish brown and contains a small number of plant fossil fragments. The high-quality shale of the Yanchang Formation was formed during basin development. During this period, the lake water was the stillest, and the organic matter was the most abundant. Therefore, this set of strata was also the most widely and stably distributed in the basin. Tuffite deposits are also found in the core (Figure 5e), indicating that there may have been multistage volcanic eruptions in the adjacent areas of the basin at the same time. Through microscopic observations of core slices of the Chang 7 member, we found that the organic matter is mainly interbedded or laminated in black shale and contains spherical algae fossils, terrigenous mud and a small amount of silt (Figure 6a), indicating that algae may be the source of the organic material. In the dark massive mudstone, the contents of terrigenous mud and silt are high, and the organic matter is dispersed (Figure 6b). In silty mudstone, the contents of terrigenous mud and silt are higher, and the contents of organic matter are lower. Crystal fragments and crystal pyroclasts are visible in some core slices (Figure 6c,d). These volcanic materials indicate that volcanic ash may be involved in the formation and development of high-quality shale, and volcanic ash indicates that there may be synchronous volcanic eruptions in the areas adjacent to the basin.

4.1.3. Geophysical Response Characteristics

Seismic Response Characteristics

Gu Lijing [30] summarized the seismic reflection characteristics of mudstone and coal. Among them, lacustrine sedimentary shale has low-frequency, high-continuity and strong-amplitude seismic reflection characteristics. In this paper, the Chang 7 Formation is also identified through fine seismic data. The black shale of the Chang 7 Formation obviously shows low-frequency, high-continuity, strong-amplitude and parallel–subparallel seismic facies characteristics, and it is a typical marker layer for dividing the strata of the Yanchang Formation (Figure 7).

Logging Response Characteristics

Previous researchers have used logging data to identify the shale at the bottom of the Chang 7 member. Generally, the logging curves of this set of shales show high-anomaly natural gamma curves, low-anomaly density curves, high-anomaly acoustic moveout curves and high-anomaly resistivity curves, especially black shale. Therefore, it is easy to distinguish shale sections from non-shale sections through logging curves [31,32].
According to the black shale logging curve characteristics and seismic response characteristics, we identify the black shale of the Chang 7 member (Figure 8 and Figure 9). The black shale is mainly distributed at the bottom of the Chang 7 member (Figure 8 and Figure 9), with good connectivity, and the thickness ranges from a few meters to tens of meters, gradually thickening from the edge to the center of the basin. Among them, the black shale of the Jiyuan–Huachi–Yijun line is the thickest, i.e., more than 30 m (Figure 10).

4.1.4. Geochemical Characteristics

The test data and analytical results of the main elements in the black shale samples of the Yanchang Formation are shown in Table 1. The data show that the CaO values in the Chang 7 member range from 0.1% to 0.76% (average = 0.56%); the MgO values range from 0.01% to 0.08% (average = 0.036%); the SiO2 values range from 34.62% to 50.78%; and the average value is 41.2% (Table 1).
Table 2 shows the results of the trace element analysis of black shale samples in the Yanchang Formation. The ratio method is a commonly used method to indicate the contents and differentiation characteristics of rare earth elements. One method is to compare the measured values of samples with chondrites, and the other method is to compare the measured values of samples with standard shale samples. Because the rare earth elements of chondrites mainly represent the rare earth element content of the original composition of the Earth, they are mostly used for studying igneous rocks, and the rare earth elements in shale mainly reflect the rare earth element abundance in the upper crust. Therefore, the ratio method that compares the sample value to North American shale is used to analyze the rare earth element differentiation characteristics of samples. Figure 11 shows that the total rare earth element (∑REE) contents of shale in this area have relatively little change, ranging from 69.34 × 10−6 to 154.71 × 10−6, with an average of 109.7 × 10−6, which is lower than that of North American shale (193.18 × 10−6 μg/g) [33], indicating that the ∑REE content was relatively deficient, which may be due to the lack of terrigenous detritus of higher plants in the shale, and the (Lanthanum/Ytterbium)S ratio is close to 1. The normalized curve shows a flat trend, and the differentiation of light and heavy rare earth elements is not obvious, indicating that the diagenetic environment of shale was reducing [34,35].

4.2. Analysis of Sedimentary Paleoenvironment of High-Quality Shale

4.2.1. Redox Condition

In this paper, four indexes are used to analyze the redox conditions in the sedimentary period of the Yanchang Formation shale: the U/Th ratio, V/(V + Ni) ratio, V/Cr ratio and Ce/La ratio. The basic principle is as follows: U and Th have different activities and different lithologies. U is more active than Th and is prone to oxidation, while Th is less active. U is often adsorbed in mudstone and Th is often adsorbed in fine-grained sediments; V, Ni and Cr are mainly adsorbed and precipitated by colloidal particles or clay. V is easily adsorbed under reducing conditions, and Ni, Cr and Co are easily enriched in a reducing environment.
The U/Th ratios of the Chang 7 reservoir were between 0.87 and 6.14, with an average value of 4.1. Only one sample had a ratio of 0.87, and the U/Th ratios of the other samples were greater than 1.25. The V/(V + Ni) ratios were between 0.77 and 0.96, and the average value was 0.90, which is greater than 0.84. The Ce/La ratios were between 1.53 and 2.09, and the average was 1.77, which is greater than 1.5. According to the traditional discriminant indexes of redox conditions [36] (Table 3) (Figure 12), the Chang 7 member formed in an anaerobic reducing environment. Qiu Xinwei [37] and Xin Bushe [38] studied the distribution characteristics of tuffite in the Yanchang Formation and believed that the formation of high-quality source rocks in Chang 7 was related to volcanic eruptions; these eruptions easily led to an anoxic environment, which was conducive to the development of source rocks.

4.2.2. Paleoclimate and Paleosalinity

Climate conditions have a certain influence on the sedimentary environment. In a humid climate, Cu, Cr, Ni and Mn elements are often enriched, while in arid conditions, water evaporation and alkalinity increase, and Ca, Mg, K, Na, Sr, and Ba are enriched [38,39]. We mainly used the trace element Sr/Cu ratio to assess the paleoclimate characteristics. The Sr/Cu ratios were between 1 and 10, which may indicate that the paleoclimate was warm and humid, and the Sr/Cu ratios were greater than 10, which may indicate that the paleoclimate was arid. In this paper, the Sr/Cu ratios of black shale samples from the Yanchang Formation ranged from 0.47 to 3.34 (average = 1.4), indicating that the paleoclimate of the Yanchang Formation shale depositional period was warm and humid.
There are many methods to determine and distinguish ancient salinity, and the commonly used method is the geochemical semiquantitative division of major and trace elements. In this paper, the Sr/Ba ratio was used to distinguish ancient salinity. The Sr/Ba ratio is one of the commonly used methods to restore ancient salinity [40,41]. When the salinity in the water body is low, Sr and Ba do not change. When the salinity of the water body gradually increases, Ba and Sr precipitate in the form of sulfate. Due to the strong migration ability of Ba, BaSO4 precipitates preferentially, the migration ability of Sr is relatively weak and SrSO4 precipitates behind BaSO4; therefore, the Sr/Ba ratio in sediments can be used to assess the ancient salinity during sediment formation. In the continental freshwater environment, because Sr and Ba do not precipitate, the Sr/Ba ratio in sediments was relatively low. With the slow entry of fresh water into the sea, a mixed sea–land environment formed, the salinity of the water body increased, and the Sr/Ba ratio increased. When the continental freshwater environment became a seawater environment, the salinity of the water body increased sharply, and the Sr/Ba ratio also reached a relative maximum. Therefore, predecessors have summarized the Sr/Ba ratio in different environments [42]. Generally, the Sr/Ba ratio of terrestrial freshwater environments is less than 0.6, that of brackish water environments is between 0.6 and 1, and that of marine saline water environments is greater than 1. However, different basins can have different comparison standards. Chen Hongde [43] analyzed the trace element content in the Ordos Basin and considered a Sr/Ba ratio greater than 0.8 to indicate a seawater sedimentary environment, a Sr/Ba ratio greater than 0.5 and less than 0.8 to indicate a mixed water sedimentary environment and a Sr/Ba ratio less than 0.5 to indicate a freshwater sedimentary environment. According to the mass fraction table of major and trace elements in the Yanchang Formation shale (Table 3) (Figure 13), the Sr/Ba ratio of the samples ranges from 0.15 to 0.75, and the average value is 0.31. One of the samples has a ratio of 0.75. The samples were deposited in a terrestrial freshwater environment, and only some were deposited in a brackish water environment.

4.2.3. Paleowater Depth

Many methods are used to calculate ancient water depth, including micro-paleontological fossil discrimination [44], sedimentary structure discrimination [45,46], stratum thickness [47], authigenic mineral indication [48] and redox condition discrimination. These discrimination methods have achieved good results in specific research areas. The paleowater depth in the Yanchang period has always been a research focus. This paper mainly uses the Co content in trace elements to calculate the paleowater depth during the sedimentary period of black shale formation in the Yanchang Formation. The main principle is that the Co content in rock is different under different deposition rates, so the Co content can be used to calculate the deposition rate of rocks, especially the Co content in mudstone, which can better reflect the ancient water depth of mudstone at the time of deposition (Formulas (1)–(3)). It is generally considered that the deposition rate of mudstone in shore–shallow lake sedimentary facies belts is 0.2 mm/a, that in delta front facies belts is 0.3 mm/a and that in river sedimentary facies belts is 0.4 mm/a. The calculation results show that the paleowater depth of the Yanchang Formation is between 50 m and 120 m, and the average water depth is 80 m.
The specific calculation formula is as follows [49]:
Vs = V0 × NCo/(SCo − t × TCo)
t = SLa/NLa
h = 3.05 × 105/(Vs1.5)
where h is the paleowater depth (m); NCo is the Co abundance in normal lake sediments (20 μg/g); NLa is the average abundance of La in terrigenous clastic rocks (38.99 μg/g); SCo is the Co abundance in the sample (μg/g); SLa is the La abundance in the sample (μg/g); t is the contribution of source cobalt (Co) to the sample; TCo is the Co abundance in terrigenous clastic rocks (4.68 μg/g); vs. is the velocity of sediment (mm/a); and V0 is the normal velocity of the lake (0.2~0.4 mm/a).
The sedimentary environment has three main effects on the development of high-quality shale: (1) the level of paleoproductivity; (2) preservation conditions; and (3) sediment input. We sampled the black shale of the Yanchang Formation and tested its main and trace element contents. The Sr/Cu ratio in the sample was less than 10, and the Sr/Ba ratio was less than 1, indicating that the shale may have developed in a warm and humid terrestrial freshwater environment. The V/(V + Ni) ratio was greater than 0.6, the V/Cr ratio was greater than 2 and the Ce/La ratio was greater than 1.5, indicating that shale may have developed under redox conditions. We believe that this environment was conducive to the growth of organisms, improved the deposition rate of organic matter in the lake basin and laid a material foundation for the deposition of thick organic matter in this period. At the same time, during this period, earthquakes and volcanic activity around the lake basin were more frequent, and the volcanic sediments around the basin were easier to transport to the water body of the lake basin, which increased the nutrients in the water body, had an obvious “fertilization” effect on the lake basin and may have improved the original productivity of the lake basin [50]. Previous studies have suggested that there was hidden tectonic activity in the basin during this period [51], which promoted the upwelling of some hydrothermal fluids. Hydrothermal fluids may have carried minerals into the lake basin, improved the nutrient content of the water body in the lake basin, and promoted the growth of organisms [52,53,54]. During the depositional period of black shale, the lake basin expanded continuously, and the water body deepened, resulting in obvious stratification of the lake water. The deep-water anaerobic reducing environment of the lake basin provided favorable preservation conditions for the deposited thick organic matter. The late Chang 7~Chang 4 + 5 sedimentary period was the progressive filling stage of the lake basin, and the increasing rate of accommodation space was much less than the sediment supply rate. A series of highly active river delta sedimentary systems deposited sediment into the lake basin, covering the early deposited mudstone that was rich in organic matter and providing good coverage and preservation conditions.

5. Conclusions

(1).
The high-quality shale at the bottom of the Chang 7 member of the Yanchang Formation includes black shale, dark massive mudrock and silty mudstone. The organic matter in black shale is mainly interbedded or stratified, the organic matter in dark massive mudstone is dispersed, and the content of organic matter in silty mudstone is low. The shale shows high gamma, high acoustic time difference, high resistance well-logging phase and strong-amplitude parallel–subparallel seismic phase characteristics.
(2).
The sedimentary thickness of this set of high-quality shales gradually increases from the edge of the basin (5–10 m) to the center of the basin, among which the black shale of the Jiyuan–Huachi–Yijun line has the largest thickness (more than 30 m). This set of high-quality shales was mainly formed under a warm and humid paleoclimate, in water depths of 60–120 m, and in an anaerobic reducing and continental freshwater paleoenvironment.

Author Contributions

Conceptualization, M.S. and C.F.; methodology, M.S. and C.F.; software, M.S. and Y.L.; validation, M.S. and C.F.; formal analysis, M.S.; investigation, M.S.; writing—original draft preparation, M.S.; writing—review and editing, M.S., C.F. and Y.L.; visualization, M.S.; supervision, M.S. All authors have read and agreed to the published version of the manuscript.

Funding

This work was financially supported by Natural Science Basic Research Program of Shaanxi (2020JQ-798, 2017JM4013).

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.

Nomenclatures

hPaleowater depth (m);
NCoAbundance of Co in normal lake sediments (20 μg/g);
NLaAverage abundance of La in terrigenous clastic rocks (38.99 μg/g);
SCoAbundance of Co in sample (μg/g);
SLaAbundance of La in sample (μg/g);
tContribution value of source cobalt (Co) to the sample;
TCoAbundance of Co in terrigenous clastic rocks (4.68 μg/g);
VsVelocity of sediment (mm/a);
V0Normal velocity of the lake (0.2~0.4 mm/a).

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Figure 1. (a) Regional location and (b) simplified structure of the Ordos Basin and data used in this study, the red star is the sampling location of the samples and the gray shadow shows the range of black shale, (c) general stratigraphy of Ordos basin showing major tectonic and depositional events.
Figure 1. (a) Regional location and (b) simplified structure of the Ordos Basin and data used in this study, the red star is the sampling location of the samples and the gray shadow shows the range of black shale, (c) general stratigraphy of Ordos basin showing major tectonic and depositional events.
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Figure 2. Outcrop and measured stratigraphic of Yanchang Formations of Nijiahe in Tongchuan City; Black shale, tuffite and dark mudstone are developed in this outcrop, which are interbedded.
Figure 2. Outcrop and measured stratigraphic of Yanchang Formations of Nijiahe in Tongchuan City; Black shale, tuffite and dark mudstone are developed in this outcrop, which are interbedded.
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Figure 3. Outcrop and measured stratigraphic of Yanchang Formations of Tangnihe in Tongchuan City; black shale, dark mudstone and silty mudstone are developed in this outcrop, which are interbedded.
Figure 3. Outcrop and measured stratigraphic of Yanchang Formations of Tangnihe in Tongchuan City; black shale, dark mudstone and silty mudstone are developed in this outcrop, which are interbedded.
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Figure 4. Outcrop and measured stratigraphic of Yanchang Formations of Shanshuihe in Xunyi County; black shale, dark mudstone and silty mudstone are developed in this outcrop.
Figure 4. Outcrop and measured stratigraphic of Yanchang Formations of Shanshuihe in Xunyi County; black shale, dark mudstone and silty mudstone are developed in this outcrop.
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Figure 5. Shale core characteristics of Yanchang Formations. (a) Black shale of well zhuang144; (b) Black shale of well zhuang80; (c) Dark massive mudstone of well Zhuang80, containing bivalve fossils; (d) Dark massive mudstone of well Zhuang144, containing plant fragment fossil; (e) Tuffite of well Zhuang144; (f) Silty mudstone of well Zhuang57 containing plant fragment fossil.
Figure 5. Shale core characteristics of Yanchang Formations. (a) Black shale of well zhuang144; (b) Black shale of well zhuang80; (c) Dark massive mudstone of well Zhuang80, containing bivalve fossils; (d) Dark massive mudstone of well Zhuang144, containing plant fragment fossil; (e) Tuffite of well Zhuang144; (f) Silty mudstone of well Zhuang57 containing plant fragment fossil.
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Figure 6. Microscopic characteristics of shale in Yanchang Formations. (a) Laminar organic matter in black shale facies; (b) Dispersed organic matter in dark massive mudstone facies; (c) Crystalline volcanic material in silty mudstone facies; (d) Glassy volcanic material in silty mudstone facies.
Figure 6. Microscopic characteristics of shale in Yanchang Formations. (a) Laminar organic matter in black shale facies; (b) Dispersed organic matter in dark massive mudstone facies; (c) Crystalline volcanic material in silty mudstone facies; (d) Glassy volcanic material in silty mudstone facies.
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Figure 7. Seismic facies characteristics of black shale in Yanchang formations; the yellow dotted line in the figure indicates the black shale of the Chang 7, which obviously shows the characteristics of low-frequency, high-continuous, strong-amplitude, parallel-subparallel seismic facies.
Figure 7. Seismic facies characteristics of black shale in Yanchang formations; the yellow dotted line in the figure indicates the black shale of the Chang 7, which obviously shows the characteristics of low-frequency, high-continuous, strong-amplitude, parallel-subparallel seismic facies.
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Figure 8. Profile of East–West connecting wells in Ordos Basin (see Figure 10a for plane position); the logging curve characteristics of black shale are very obvious, mainly characterized by high natural gamma and low density, which can clearly identify the black shale section.
Figure 8. Profile of East–West connecting wells in Ordos Basin (see Figure 10a for plane position); the logging curve characteristics of black shale are very obvious, mainly characterized by high natural gamma and low density, which can clearly identify the black shale section.
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Figure 9. Profile of South–North connecting wells in Ordos Basin (see Figure 10b for plane position).
Figure 9. Profile of South–North connecting wells in Ordos Basin (see Figure 10b for plane position).
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Figure 10. Plan distribution of black shale thickness in Yanchang Formation, the black shale is well-connected, and the stratum gradually thickens from the edge of the basin to the center of the basin, and the thickness of the thickest stratum is more than 30 m (a and b are two connected well profiles).
Figure 10. Plan distribution of black shale thickness in Yanchang Formation, the black shale is well-connected, and the stratum gradually thickens from the edge of the basin to the center of the basin, and the thickness of the thickest stratum is more than 30 m (a and b are two connected well profiles).
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Figure 11. North American shale composite-normalized (NASC) rare earth element (REE) distribution patterns of Yanchang formation, Ordos Basin; the value of the Y-axis is the logarithmic function value, which is the ratio of the trace element content in the sample to the trace element in the North American shale species.
Figure 11. North American shale composite-normalized (NASC) rare earth element (REE) distribution patterns of Yanchang formation, Ordos Basin; the value of the Y-axis is the logarithmic function value, which is the ratio of the trace element content in the sample to the trace element in the North American shale species.
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Figure 12. Variation trend of trace element ratio of black shale of Yanchang Formation in Ordos Basin, (a) the ratio of V/V+Ni; (b) the ratio of V/Cr; (c) the ratio of Ce/La; (d) the ratio of U/Th.
Figure 12. Variation trend of trace element ratio of black shale of Yanchang Formation in Ordos Basin, (a) the ratio of V/V+Ni; (b) the ratio of V/Cr; (c) the ratio of Ce/La; (d) the ratio of U/Th.
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Figure 13. Trace element ratio of black shale of Yanchang Formation in Ordos Basin, (a) the ratio of Sr/Cu; (b) the ratio of Sr/Ba.
Figure 13. Trace element ratio of black shale of Yanchang Formation in Ordos Basin, (a) the ratio of Sr/Cu; (b) the ratio of Sr/Ba.
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Table 1. Mass fraction of major elements in black shale of Yanchang formation, Ordos Basin.
Table 1. Mass fraction of major elements in black shale of Yanchang formation, Ordos Basin.
SamplesSiO2TiO2Al2O3Fe2O3MnOMgOCaONa2OK2OP2O5
BWZ-2440.150.486.5711.50<0.010.340.100.512.170.31
BWZ-2737.080.5211.225.070.020.580.460.752.370.26
BWZ-3438.460.3010.178.020.020.560.660.581.720.26
YQ-4034.620.5512.399.020.020.580.681.042.960.22
FDH-1946.120.5214.355.100.010.850.721.122.630.42
FDH-2250.780.5616.675.670.081.720.761.122.910.25
average41.200.4911.907.400.030.770.560.852.460.29
North American shale15.40 4.023.110.653.242.440.121.30
Table 2. Mass fraction of trace elements in black shale of Yanchang Formation in Ordos Basin.
Table 2. Mass fraction of trace elements in black shale of Yanchang Formation in Ordos Basin.
SamplesBWZ-24BWZ-27BWZ-34YQ-40FDH-19FDH-22
Hf2.692.932.893.072.663.00
Ta0.530.600.650.620.790.87
Pb23.023.226.930.140.239.2
Th9.235.917.346.2212.813.9
U39.736.339.833.332.312.1
Li17.022.325.031.530.045.4
Be1.062.081.912.222.462.74
Sc10.46.775.539.4911.315.0
V166224223214212127
Cr36.851.436.269.766.673.1
Co14.96.8320.612.85.2528.8
Ni6.9718.727.218.011.837.5
Cu10313312212411071.3
Zn8.3812.448.823.723.1101
Ga14.317.213.717.521.722.5
Ge1.461.631.181.471.621.52
Rb11676.376.1104148159
Sr10862.790.1105366143
Y8.637.8113.69.0314.821.2
Zr10010296.610392.7103
Nb7.358.527.848.2810.111.0
Cs6.109.507.568.7411.916.5
Ba601428432339491504
La31.6916.4120.0817.9835.7034.47
Ce49.9827.4630.7637.5366.8365.59
Pr5.203.024.333.477.417.31
Nd18.0011.9316.5712.6525.9626.70
Sm2.892.733.192.374.545.07
Eu0.520.550.610.460.841.00
Gd2.162.202.832.033.524.23
Tb0.280.320.420.290.490.63
Dy1.621.822.451.742.853.76
Ho0.320.360.490.350.560.76
Er1.021.071.441.061.642.24
Tm0.160.170.220.170.250.35
Yb1.121.151.431.151.632.26
Lu0.170.180.210.170.250.34
∑REE115.1369.3685.0181.44152.47154.71
LREE108.2962.1075.5374.47141.27140.14
HREE6.847.269.496.9711.2014.56
LREE/HREE15.828.557.9610.6812.629.62
Table 3. Discriminant indexes of redox condition.
Table 3. Discriminant indexes of redox condition.
Redox ConditionPaleo-Oxygenation FaciesU/ThV/(V + Ni)V/CrNi/CoCe/La
ReductionAnaerobism>1.250.84–0.89>4.25>7>2
Weak reductionExtremely oxygen-poor0.6–0.841.8–2
Weak oxidationOxygen-poor0.75–1.250.46–0.62–4.255–71.5–1.8
OxidationOxygen-rich<0.75<0.46<2<5<1.5
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Sun, M.; Feng, C.; Li, Y. Characteristics and Paleoenvironment of High-Quality Shale in the Triassic Yanchang Formation, Southern Margin of the Ordos Basin. Minerals 2023, 13, 1075. https://doi.org/10.3390/min13081075

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Sun M, Feng C, Li Y. Characteristics and Paleoenvironment of High-Quality Shale in the Triassic Yanchang Formation, Southern Margin of the Ordos Basin. Minerals. 2023; 13(8):1075. https://doi.org/10.3390/min13081075

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Sun, Mengsi, Congjun Feng, and Yipu Li. 2023. "Characteristics and Paleoenvironment of High-Quality Shale in the Triassic Yanchang Formation, Southern Margin of the Ordos Basin" Minerals 13, no. 8: 1075. https://doi.org/10.3390/min13081075

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