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Article

Petrology, Mineralogy, and Geochemical Characterization of Paleogene Oil Shales of the Youganwo Formation in the Maoming Basin, Southern China: Implication for Source Rock Evaluation, Provenance, Paleoweathering and Maturity

by
Fei Hu
1,2,3,
Qingtao Meng
1,2,3,*,
Zhaojun Liu
1,2,3,
Chuan Xu
1 and
Xun Zhang
1
1
College of Earth Sciences, Jilin University, Changchun 130061, China
2
Key-Lab for Oil Shale and Paragenetic Minerals of Jilin Province, Changchun 130061, China
3
Key Laboratory for Evolution of Past Life and Environment in Northeast Asia, Jilin University, Ministry of Education, Changchun 130061, China
*
Author to whom correspondence should be addressed.
Energies 2023, 16(1), 514; https://doi.org/10.3390/en16010514
Submission received: 22 November 2022 / Revised: 23 December 2022 / Accepted: 29 December 2022 / Published: 3 January 2023
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Abstract

:
Oil shale is a crucial unconventional energy source to supplement conventional oil and gas. The oil shale in the Maoming Basin of China has excellent resource potential. In this study, through systematic geochemical testing, the industrial quality and geochemical characteristics of oil shale are revealed, and the hydrocarbon generation potential of oil shale, the parent rock type, and the tectonic setting of the source area are discussed. It is comprehensively assessed that Maoming oil shale has a medium-oil yield (avg. 6.71%) with high ash content (avg. 76.1%), a high calorific value (avg. 7.16 M J/kg), and ultra-low sulfur (avg. 0.54%). The mineralogical compositions primarily consist of clay minerals and quartz, and barely pyrite. Maoming oil shale is in an immature evolution stage, with high TOC and I-II1 kerogen type, and could be considered an excellent hydrocarbon source rock. The chemical index of alteration (CIA), the index of chemical variability (ICV), and the Th/U ratio indicate that the Maoming oil shale parent rock area is strongly weathered. Multitudinous geochemical diagrams also show that the oil shale was mainly derived from Late Cretaceous felsic volcanic rock and the granite zone, and the tectonic setting was a continental island arc environment related to the active continental margin. This is consistent with the tectonic history of southern China in the Late Cretaceous.

1. Introduction

Driven by the international background of increased demands on conventional oil and gas resources, countries worldwide have gradually increased the exploration and development of unconventional oil and gas resources [1,2]. As an essential unconventional oil and gas supplementary energy source, oil shale has attracted global attention with its vast resources and diversified comprehensive utilization methods [3,4,5,6,7]. Oil shale can not only be directly exploited as a solid mineral product but also can continue to be buried as a source rock to produce liquid hydrocarbons. Over the past decades, oil shales have been widely investigated in resource evaluation and petrology, coupled with their paleoenvironment and paleoclimate [8,9,10,11,12,13,14].
Oil shale is defined as a solid combustible organic sedimentary rock with a high ash content [1,2]. Shale oil can be obtained by low-temperature retorting [15,16,17]. The oil yield is more than 3.5%, and the organic matter content is high. It is mainly sapropel and mixed type, and its calorific value is generally not less than 4.18 MJ/kg [1,2]. During the development and utilization of oil shale, its industrial indicators may change with the change in economic and technical conditions [1,2]. Maoming Basin, a very important oil shale basin in southeast China, is rich in oil shale resources (Figure 1a,b). There are three oil shale mining areas in the basin, namely the Jintang, Yanjiao, and Gaozhou oil shale mining areas (Figure 1b), and its high-quality oil shale developed in Paleogene is characterized by good quality, large thickness, and wide distribution. The resource of Maoming oil shale is 16.5 billion tons, equivalent to 1 billion tons of oil shale oil. Although the research on Maoming oil shale began at the beginning of the last century and its exploitation began in 1970, so far, only 200 million tons of Maoming oil shale have been exploited, and there are still a large number of oil shale resources buried underground, with high national production value and strategic value [2].
Several studies have been conducted on the oil shale in Maoming Basin, but most of them focus on the characteristics of oil shale [18,19,20,21], the evaluation of resource potential [2,22], comprehensive development and utilization [23,24,25], biomarkers [26,27], the paleoclimate and paleoenvironment [21,28,29], hydrocarbon generation evolution and pyrolysis kinetics of oil shale [30,31,32], and the lack of research on the parent rock type and tectonic setting of oil shale. However, the research methods for the characteristics of fine sediment source areas are mature. Although the traditional Dickenson diagram cannot be used for the source area characteristics of fine sediment [33], inorganic elements and their related series of discrimination diagrams have good effects and are widely used to study the weathering intensity, rock type, and tectonic setting of the source area [34,35,36]. At the same time, mineral content, particularly the variation characteristics of clay minerals, has also played an active role in the study of the characteristics of fine sediment source areas [37,38].
This paper considers the oil shale of the Paleogene Youganwo Formation as the research object, based on the field outcrop data, through systematic geochemical testing. Moreover, the geochemical characteristics of oil shale in the study area are revealed, and the parent rock type and source area tectonic setting of oil shale are discussed. This allows studying the composition, quality characteristics, and formation mechanism of oil shale. In addition, the evaluation of the industrial quality of Maoming oil shale ended in 2012 [19]. In the context of improving current experimental technology, it is also necessary to redefine the quality of Maoming oil shale and provide new parameters for the future evaluation of oil shale in the Maoming Basin.

2. Geological Setting

The Maoming Basin is located in the southwestern Guangdong Province of China (Figure 1a) and is a Cenozoic half-graben basin sandwiched between the Pacific and Indian Oceans, and it is bordered by the Gaopengling Fault to the northeast with an area of approximately 360 km2 [39,40]. The collision of the Western Pacific plate and the Asian continent in the Late Jurassic and Early Cretaceous resulted in the Yanshan cycle, which caused the region to suffer from strong tension or extensional shearing, forming the NNW-trending Nanshan Basin and depositing the Late Cretaceous strata dominated by red clastic rock and volcanic rock. Following this regional tectonic movements, under the influence of the increasing tension during the Himalayan cycle, with the reactivation of the Gaopengling fault, the Cenozoic Maoming Basin was superimposed based on the Nanshan Basin, which mainly deposited a set of delta clastic rock and Paleogene strata containing oil shale [18,41].
The basin is controlled by the fault system, and the NW-trending faults control the sedimentation and filling. The Gaopengling fault in the north mainly controls the formation and evolution of the basin, while the Jintang fault in the south controls the formation and thickness of oil shale (Figure 1c).
Maoming Basin is a Cenozoic basin superimposed on Nanshan Basin. The basement of Nanshan Basin is the Sinian Yunkai Group (Zyn), which is a set of metamorphic rocks mixed with migmatites. Sedimentary caprocks include Lower Cretaceous Luoding (K1l), Upper Cretaceous Luoyeling (K2l), Aiwu (K2a), Daling (K2d), Xitangling (K2x), and Tongguling Formations (K2t), primarily a set of intermediate acid volcanic rocks with coarse clastic rocks [18,41]. The Maoming Basin contains an up to 3000 m thick succession of clastic deposits [19]. The Cenozoic strata are composed of the early Eocene Shangtong (E2s), middle Eocene Youganwo (E2y), late Eocene Huangniuling (E2h), Oligocene Shangcun (E3sh), Miocene Laohuling(N1l), and Pliocene Gaopengling Formations (N2g) (Figure 2a). However, oil shale is mainly concentrated in the Youganwo Formation.

3. Materials and Methods

The sampling profiles are located in the west of Jintang and Yangjiao mining areas, and the profile heights are 7 m and 11 m, respectively. Eighteen samples were taken at an interval of 1 m (Figure 2b,c). All samples were investigated by industrial analysis (oil yield, calorific value, ash content, volatile content, and total sulfur), source rock evaluation (organic petrography, total organic carbon, and Rock-Eval pyrolysis), XRD, and inorganic geochemistry (major, trace, and rare earth element) analysis. The detailed data reported below are courtesy of the industrial analysis and source rock evaluation conducted at the Key Laboratory of Oil Shale and Coexistent Energy Minerals of Jilin Province, China. XRD and element analyses were conducted at the Beijing Research Institute of Uranium Geology and Research Institute of Petroleum Exploration and Development, China.

3.1. Oil Yield, Calorific Value, Ash Content, and Volatile Content

The oil yield was measured with a Chinese Fushun retort using the Fischer assay, and the shale oil was obtained from low-temperature carbonization at approximately 520 °C, following the ASTM Standard D3904-80 (1984). Calorific value, ash content, and volatile content were separately carried out using an SDC5015 Calorimeter and an Xl-2000 Muffle Furnace (all Sande Technology Limited Liability Company, Hunan Province, China) following the standard of GB212-91 (Industrial analysis method of coal).

3.2. C, S and Rock-Eval Pyrolysis

C and S were measured using a Leco CS-230 instrument following the criteria of GB/T 19145-2003 and using the method according to Hu et al. (2021) [36]. Rock-Eval pyrolysis analysis was carried out on a Rock-Eval 6 instrument. The main parameters include S1 (free hydrocarbons), S2 (pyrolyzable kerogen), and Tmax (temperature of maximum pyrolysis yield; unit: °C). The method was in accordance with Xu et al. (2022) [29].

3.3. Organic Petrography

Samples were polished and observed for organic petrographic investigation. Using a Caisi MPV microscope and a 50× oil immersion objective lens to count the maceral, no less than 500 observation points were collected for each sample.

3.4. XRD and Element Analysis

XRD analyses were tested with a Philips PW1830 diffractometer system, following the criteria of SY/T 5163-2010, with a minimum relative deviation of 10%. The specific methods were in accordance with Hu et al. (2021) [36]. Major element analysis was performed using a Philips PW 2404 X-ray fluorescence spectrometer following the standard of GB/T14506.28-93. Trace elements were tested vis spectrometry (ICP-MS; ELEMENT XR), and the specific methods were in accordance with Xu et al. (2022) [29].

4. Results

4.1. Petrology

The fresh surface of Maoming oil shale is brown and brown-black. After long-term exposure to sunlight and weathering, its surface is bleached yellow or yellowish-white, with uneven fractures (Figure 3a). The hardness is minimal and can be carved with nails, but it is ductile. The foliation is relatively developed. Along the layer direction, it is easy to crack into paper flakes (Figure 3b). The crack surface is flat, showing an earthy luster, in which animal fossils can be seen (Figure 3c). The oil shale fragments can be directly ignited and smell of oil (Figure 3d). The gravity of oil shale is relatively light, and generally, the darker in color and lighter in specific gravity it is, the better the quality it is.
Maceral analysis shows that oil shale in the study area is dominated by lamalginite, which is mainly flocculent and mat-like (Figure 3e). Secondly is telalginite with large and isolated individual forms (Figure 3e) and sporinite with an elongated thread shape with obvious sporocoel structures (Figure 3f). In addition, a small amount of vitrinite and pyrite can also be observed. Vitrinite is banding (Figure 3g), and pyrite is often observed in a framboidal form (Figure 3h).

4.2. Mineralogy

The X-ray diffraction results show that the oil shale in Maoming Basin mainly contains quartz and clay minerals, and the quartz content is 17.0–39.9%, with an average of 26.2%. The clay mineral content is 53.6–83.0%, with an average of 73.0%. Individual samples contain a small amount of pyrite (JT01-6). Further quantitative analysis of clay minerals was carried out, primarily including an illite/montmorillonite mixed layer (I/S) and kaolinite (K), accounting for 49.8% and 37.4% of the total clay minerals, respectively, followed by a small amount of illite (I) and chlorite (C), accounting for 5.3% and 9.6% of the total clay minerals, respectively (Table 1).

4.3. Bulk of Industrial Analysis Data

4.3.1. Oil Yield and Calorific Value

Oil yield refers to the mass fraction of shale oil (tar) in oil shale, which is one of the important indicators for defining the concept of oil shale and evaluating oil shale resources [42,43]. The oil yield test results show that the oil yield distribution of Maoming oil shale is 3.56–13.07% (Figure 4), with an average of 6.71%, mainly concentrated between 5% and 10% (Table 1). Calorific value is an important evaluation parameter to evaluate the value of oil shale industrial fuel. Generally, the larger this parameter is, the higher the value of industrial fuel is [2,7]. The calorific value of Maoming oil shale is 4.19–17.80 MJ/kg, with an average of 7.16 MJ/kg. The calorific value is primarily distributed between 5 MJ/kg and 9 MJ/kg. The calorific value is positively correlated with the oil content; therefore, some shale samples display relatively high calorific values, similar to low-grade coals (Figure 5a).

4.3.2. Ash Content, Volatile Content, and Total Sulfur

Ash content is not only an important indicator to distinguish oil shale from coal but also a key parameter to evaluate the quality of oil shale. Volatile content is the sum of volatile or lost components in the combustion process of oil shale, which can also be used to reflect the content of organic matter in oil shale. Generally, the lower the ash content and the higher the volatile content, the higher the organic content and the better the quality of oil shale. The ash content of oil shale of the Youganwo Formation in Maoming Basin ranges from 59.0% to 84.2% (Figure 4), with an average of 75.2%, and the volatile content ranges from 8.04% to 27.85%, with an average of 16.4% (Table 1). There is a negative correlation between ash content and oil content, with a correlation coefficient of 0.76 (Figure 5b), while there is a positive correlation between volatile matter and oil content, with a correlation coefficient of 0.74 (Figure 5c).
Total sulfur content is the sum of all kinds of sulfur in rock samples. The sulfur in oil shale is an essential source of environmental pollution during its development and utilization [2]. The higher the total sulfur content, the greater the pollution. The test results show that except for sample JT01-6 (2.73%) (Figure 4), the sulfur content of Youganwo oil shale in Maoming Basin is low, ranging from 0.08% to 1.75% (Table 1), with an average of 0.41%, belonging to ultra-low-sulfur oil shale.

4.4. Evaluation Parameters of Hydrocarbon Source Rock

Hydrocarbon source rock evaluation mainly includes organic matter abundance, organic matter type, and organic matter maturity.
The assessment of the abundance of organic matter in oil shale in this study area mainly relies on the organic carbon content (TOC). The TOC content of Youganwo oil shale is between 8.2% and 21.6%, with an average value of 12.5%. There is a good positive correlation between TOC and oil content, with a correlation coefficient of 0.86 (Figure 5d).
The Rock-Eval pyrolysis data could be helpful to estimate the organic matter (kerogen) composition of organic-rich sediments, such as coal and shale; however, these data should be used with caution since the organic matter and mineral composition of organic-rich rocks could affect Rock-Eval pyrolysis measurements [44,45,46]. The content of S1 in oil shale is between 0.61 and 2.60 mg/g, with an average of 1.14 mg/g, and the content of S2 is between 49.01 mg/g and 172.28 mg/g, with an average of 82.07 mg/g.

4.5. Inorganic Elements

4.5.1. Major Elements

Several factors control major element concentrations in sedimentary rocks, the most critical being mineral composition [47]. For example, the K2O/Na2O ratio of sedimentary rocks is controlled by the relative proportions of K-feldspar and plagioclase. Furthermore, the abundance of quartz content could control distributions of the SiO2 contents. Moreover, the provenance can also affect the major element concentrations [48,49,50]. For example, Ti, Si, and P are generally related to coarse sediments in high-energy environments, while Al is mainly related to fine-grained claystone in low-energy environments. Therefore, higher Ti and Si contents usually reflect the supply of high-energy clastic materials, while higher Al is often related to the supply of low-energy fine sediment. The major elements in the oil shale measured primarily included SiO2, Al2O3, Fe2O3, MgO, CaO, Na2O, K2O, MnO, TiO2, P2O5, etc. SiO2 is the main component of Youganwo oil shales, ranging from 36.22 to 49.54% (average = 45.63%), and there is a relatively high Al2O3 content, ranging from 14.07 to 21.48% (average = 18.45%). It is worth mentioning that Maoming oil shale has a high loss on ignition (LOI), ranging from 21.01 to 34.86% (average = 26.50%), due to high clay mineral contents. Minor components of the oil shales include Fe (5.70%), K2O (1.70%), MgO (0.73%), and TiO2 (0.55%), along with lesser Na2O, MnO, CaO, and P2O5 (Table 2).

4.5.2. Trace Elements

Barium, Cd, Co, Cr, Cs, Cu, Ga, Hf, Li, Mn, Mo, and U, were identified in the oil shales. The paleoclimate, paleoenvironment, and provenance are closely related to the enrichment of trace elements. Therefore, the change in trace elements can be used to reconstruct the environment. For example, Sr, Cu, Th, U, Rb, and Sr can be used to indicate the paleoclimate [51,52,53,54], Sr, Ba, V, Cr, Ni, and Cu can be used to reconstruct the water environment [55,56,57], and Th, Sc, Zr, and Co are relevant to the provenance area [58,59]. Table 3 lists the selected trace element concentrations of the studied oil shale samples. The contents of Li, V, Zn, Rb, Sr, and Ba are relatively high, with an average content of 66.99 μg/g, 79.93 μg/g, 100.17 μg/g, 139.70 μg/g, 63.91 μg/g, and 468.50 μg/g, while the contents of Th, Sc, Zr, and Co are low, with an average of 36.77 μg/g,13.54 μg/g,45.51 μg/g, and 18.99 μg/g (Table 3).
When normalized to the upper continental crust (UCC) [60] and plotted on a spider diagram, the trace elements showed consistent distribution patterns in all samples, indicating that the sources of the oil shale of the detrital mineral matrix barely changed. Meanwhile, the sample/UCC is used to represent the enrichment coefficient, and a sample/UCC of more than 1 represents enrichment while a sample/UCC of less than 1 represents a deficit. Some elements close to the crust, such as Rb, Ba, Nb, Ta, Sc, Co, Y, Th, and U, showed various enrichment, and Cr, Sr, Zr, and Hf showed various depletion (Figure 6).

4.5.3. Rare Earth Elements

The light rare earth elements (LREEs) include La to Eu, and the heavy rare earth elements (HREEs) include Gd to Lu. Generally, the REE, HREE, and LREE content averages of Youganwo oil shales are greater than those of the upper continental crust (average = 146 μg/g) [61]. The LREE and ΣREE average values are 293.09 and 323.88 μg/g, respectively. The sample is mainly composed of light rare earth elements, which account for 90.5%, and the HREE content average is 30.79 μg/g, which makes up 9.95% of REE (Table 4). When normalized to the chondrite, the chondrite-normalized patterns of the LREEs are distinctly sloping compared with those of the HREEs. This is consistent with the LREE enrichment and HREE depletion described above. Meanwhile, Eu and Tm show obvious negative anomaly characteristics (Figure 7).

5. Discussion

5.1. Comprehensive Oil Shale Quality

The oil yield quality classification of oil shale was in accordance with the following standards [2]: Low-quality, 3.5%~5% oil yield; medium-quality, 5%~10% oil yield; and high-quality, ≥10% oil yield. The proportions of low-, medium-, and high-quality oil shale in Maoming Basin are 22%, 67%, and 11%, respectively, with an average of 6.71%, indicating that Maoming oil shale is classified as medium-quality oil shale.
Although an international evaluation standard for evaluating the power generation capacity of oil shale has not been established, production practice shows that the Palmer oil shale of Israel and the Luozigou oil shale of China, with calorific values as low as 3 MJ/kg, still perform well during combustion in circulating fluidized bed boilers (CFBBs) and are used to generate electricity [7]. The average calorific value of Maoming oil shale is 7.16 MJ/kg, making it a high-calorific oil shale. From the year 1990 to 2005, most of the Maoming oil shale was developed for power generation [2].
Generally, oil shale with an ash content between 66 and 83% belongs to the high-ash oil shale category [62,63]. Although high ash content causes trouble in the oil shale industry, it is a crucial parameter for evaluating the quality of building materials. The average ash content of Maoming oil shale is 75.2%, making it high-ash oil shale.
Oil shale can be classified into five grades according to the total sulfur content: Ultra-low-sulfur oil shale (<1.0%), low-sulfur oil shale (1.0–1.5%), medium-sulfur oil shale (1.5–2.5%), sulfur-rich oil shale (2.5–4.0%), and high-sulfur oil shale (>4.0%) [2]. The average total sulfur of Maoming oil shale is 0.41%, classifying it as ultra-low-sulfur oil shale.
It is generally believed that the oil shale in Maoming Basin is of medium quality with high ash content, high calorific value, and ultra-low sulfur.

5.2. Hydrocarbon Generation Potential

TOC could be an important parameter for evaluating the hydrocarbon generation potential of source rocks. A TOC lower than 0.5% is a poor source rock, a TOC between 0.5% and 1.0% is a medium source rock, a TOC between 1% and 2% is a good source rock, and a TOC greater than 2% is an excellent source rock [64]. The average TOC of Maoming oil shale is 12.5%, which makes it an excellent source rock.
Hydrocarbon potential (S1 + S2) refers to the sum of free hydrocarbons (S1) and pyrolytic hydrocarbons (S2) in rocks. The hydrocarbon potential is another useful parameter for evaluating the quality of source rocks. S1 + S2 between 0.5 and 2.0 mg/g is a poor source rock, S1 + S2 between 2.0 and 5.0 mg/g is a medium source rock, S1 + S2 between 5.0 and 10.0 mg/g is a good source rock, and S1 + S2 greater than 10.0 mg/g is an excellent source rock [65]. The results of rock pyrolysis show that the hydrocarbon potential of Maoming oil shale is between 49.62 and 174.88 mg/g, with an average of 83.21 mg/g. Whether from the perspectives of TOC or S1 + S2, the oil shale of the Youganwo Formation in the Maoming Basin can be considered a good to excellent source rock, and the two have a good positive correlation (Figure 8).
With the help of the HI-Tmax chart (HI = S2/TOC * 100) (Figure 9a) and S2-TOC chart (Figure 9b), it is found that the organic matter of Youganwo oil shale is mainly Type I and Type II1 kerogen.
In general, the sediment Tmax of type I kerogen is less than 437 °C, or that of type II kerogen is less than 435 °C, indicating that it is in the immature stage [66]. Tmax of Youganwo oil shale is between 427 °C and 434 °C (Table 1), which is in an immature stage, which is also supported by the fluorescent color of sporinites (Figure 3e,f).
Energies 16 00514 g009 550
Figure 9. Organic matter type discrimination of oil shale in Youganwo Formation of Maoming Basin. (a): HI-Tmax diagram (after peters,1986 [67]); (b): S2-TOC diagram (after Langford and Blanc-Valleron, 1990 [68]).
Figure 9. Organic matter type discrimination of oil shale in Youganwo Formation of Maoming Basin. (a): HI-Tmax diagram (after peters,1986 [67]); (b): S2-TOC diagram (after Langford and Blanc-Valleron, 1990 [68]).
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5.3. Weathering of Source Area

During weathering, stable cations (e.g., Al3+ and Ti4+) accumulate in weathering products, whereas unstable cations (e.g., Na+, Ca2+, and K+) break down and are lost from the rock [66,69]. The extent of enrichment or loss indicates the intensity of chemical weathering [70,71]. To quantify the degree of weathering, Nesbitt and Young (1982) proposed the CIA: CIA = [Al2O3/(Al2O3 +CaO* + Na2O + K2O)] × 100, where CaO* refers only to the CaO in siliceous minerals. Generally, a CIA between 50 and 65 suggests a cold-dry climate with low-grade weathering, between 65 and 85 suggests a warm-humid climate with medium-grade weathering, and between 85 and 100 suggests a hot-humid climate with high-grade weathering [72,73]. A-CN-K diagram is also used to reflect the weathering intensity. On the ternary plot of A-CN-K, the samples with CIA values of ≥80 trend approximately parallel to the A-K boundary but generally tend toward the A apex. This trend corresponds to the average composition of the upper crust of the Interior North China Craton (INCC). If it deviates from the ideal weathering trend line, it indicates that it is affected by K-metasomatism [74,75,76]. The calculation formula of CIA and the resulting Al2O3−(Na2O + CaO*)−K2O ternary diagram (Figure 10a) show that the CIA values in the present samples range from 85.95 to 92.83 (average = 89.91) and are not affected by potash metasomatism, suggesting a hot-humid climate with strong weathering (Table 2). Predecessors have also confirmed by vegetation research that the climate of the Youganwo Formation is humid subtropical and is characterized by hot summers and warm winters [77].
The Th/U ratio is another parameter that can be used to reflect weathering intensity. Generally, when Th/U > 4, it can be attributed to strong weathering [58,61]. In the present study, the Th/U ratio ranges from 3.48 to 6.93 (average 5.13: Figure 10b), indicating parent-rock weathering of a strong intensity.
The index of compositional variability (ICV) can also be used to indicate the weathering: ICV = (Fe2O3 + K2O + Na2O + CaO + MgO + MnO + TiO2)/Al2O3. When The ICV is applied to mud rocks to quantify the compositional maturity and weathering intensity, low ICV values indicate strong weathering, while high ICV values reflect the opposite [78,79,80]. The ICV values of the present samples range from 0.28 to 0.89 (average = 0.51), indicating strong weathering (Table 2).
The comprehensive analysis shows that the parent rock area of Maoming oil shale has intense weathering, which results in high clay mineral content in Maoming oil shale, thus promoting the development of high-quality oil shale.

5.4. Parent Rock Types

Provenance rock types and tectonic settings can be determined by the samples that have not experienced sediment recycling. Firstly, the REE Chondrite-normalized patterns suggest that the REEs of Youganwo oil shale may be derived from a similar terrigenous source (Figure 8). The Zr/Sc and Th/Sc ratios record the composition, differentiation, and heavy mineral changes of the parent rock [80]. The Zr/Sc–Th/Sc diagram shows that the studied samples have not experienced sediment recycling (Figure 11a).
The contents of trace elements and rare earth elements are closely related to the type of parent rock [38,80,81,82]. Generally, mafic rocks are rich in Sc, Ni, Cr, and Co, while La, Th, Hf, Zr, and REEs are more abundant in acidic rocks [59]. Felsic rocks usually contain higher LREE/HREE ratios and negative Eu anomalies, whereas mafic rocks contain low LREE/HREE ratios and no Eu anomalies [61,83,84]. Therefore, La/Th versus Hf, Co/Th versus La/Sc, and La/Yb versus REE diagrams were employed to discriminate the sediment provenance in different tectonic environments [85,86,87].
In the La/Th-Hf diagram, the studied samples show that the sediments in the oil shale were mainly derived from mixed sources of felsic, basic rocks. Furthermore, dropping the samples in the Co/Th-La/Sc plot and La/Yb–REE plot, the samples show the oil shales were mainly derived from felsic volcanic rock and the granite zone (Figure 11b–d).

5.5. Tectonic Setting of Parent Rock Area

The unique chemical elements in siliceous rocks from the sedimentary basins can be used to identify the tectonic environment of the source area [88,89,90]. According to the nature of the crust, Bhatia [88] divided tectonic settings into four categories: (a) Oceanic island arc, (b) continental island arc, (c) active continental margin, and (d) passive continental margin.
The (Fe2O3T + MgO) − TiO2, (Fe2O3T + MgO) − (Al2O3/SiO2), and log (K2O +Na2O) − SiO2 diagrams can be used to distinguish the tectonic settings with a good effect [87,91]. The oil shale samples from the Maoming Basin have been plotted primarily on the continental island arc area (Figure 12a–c). In addition, La–Th–Sc, Th–Sc–Zr/10, and Th–Co–Zr/10 diagrams can also be used to reflect the tectonic setting [91,92,93]. From the plot, the same result was acquired. It appears that most samples of the oil shales met in the active continental margin area, while a few fell in the passive continental margin area (Figure 12d–f). According to the comprehensive study, the parent rock of oil shales of the Youganwo Formation in Maoming Basin was formed in a continental island arc environment related to the active continental margin.
Maoming Basin, as a part of the Meso-Cenozoic volcanic belt around the Pacific Ocean, had intense magmatic activity in the late Cretaceous but became calm since the Cenozoic. The Maoming Basin belongs to the Wuchuan-Sihui volcanic eruption belt in western Guangdong. In the Late Cretaceous, a large area of a lava flow or banded distribution of intermediate-acid extrusive rocks was formed, mainly located southwest of the Maoming Basin. This set of intermediate-acid volcanic rocks has the tectonic setting of a continental island arc, and its formation is related to the subduction of the proto-South China Sea plate to the South China block in the Late Cretaceous [41,94,95]. At present, the intermediate-acid volcanic rocks southwest of the basin are most likely the parent rocks of the oil shale deposits of the Paleogene Youganwo Formation in the Maoming Basin, which is consistent with the felsic volcanic rocks in the continental island arc tectonic setting. However, the transition between the Late Cretaceous volcanic and the Paleogene clastic rocks in the basin may also represent the transition from the active continental margin of the proto-South China Sea to the modern passive continental margin of the South China Sea [94].

6. Conclusions

1. The comprehensive test results show that the oil shale in Maoming Basin is of medium quality with high ash content, high calorific value, and ultra-low sulfur. The oil shale is in an immature stage, and the organic matter type is I-II1, with excellent hydrocarbon generation potential.
2. The chemical index of alteration (CIA), the index of chemical variability (ICV), and the Th/U ratio indicate that the parent rock area of Maoming oil shale is strongly weathered.
3. Multitudinous geochemical diagrams show that the oil shales were mainly derived from the Late Cretaceous felsic volcanic rock and granite zone, and the tectonic setting was a continental island arc environment related to the active continental margin. This is consistent with the tectonic activity background of southern China in the Late Cretaceous.

Author Contributions

Software, X.Z.; Investigation, C.X.; Data curation, Z.L.; Writing—original draft, F.H.; Writing–review & editing, Q.M. All authors have read and agreed to the published version of the manuscript.

Funding

This study was financially supported by the program of the National Natural Science Foundation of China (No. 41872103) and the program for the JLU Science and Technology Innovative Research Team (No. 2021-TD-05).

Data Availability Statement

The data is unavailable due to privacy.

Acknowledgments

The authors thank the Opening Foundation of Key Laboratory for Oil Shale and Paragenetic Energy Minerals, Jilin Province, for the support. We also want to thank the reviewers for their valuable comments.

Conflicts of Interest

The authors declare no conflict of interest.

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Energies 16 00514 g001 550
Figure 1. Regional location and mining area distribution map of Maoming Basin: (a) Location of Maoming Basin; (b) tectonic outline and oil shale mining areas distribution of Maoming Basin; (c) cross-sectional profiles of the study area showing the oil shale distribution controlled by the Jintang fault, locations of cross-sections are shown in (b). Abbreviations: OSMA, oil shale mining areas.
Figure 1. Regional location and mining area distribution map of Maoming Basin: (a) Location of Maoming Basin; (b) tectonic outline and oil shale mining areas distribution of Maoming Basin; (c) cross-sectional profiles of the study area showing the oil shale distribution controlled by the Jintang fault, locations of cross-sections are shown in (b). Abbreviations: OSMA, oil shale mining areas.
Energies 16 00514 g001
Energies 16 00514 g002 550
Figure 2. Comprehensive stratigraphic histogram and sampling section of Maoming Basin. (a): comprehensive stratigraphic histogram of Maoming Basin; (b): sampling profile of Jintang mining area; (c): sampling profile of Yangjiao mining area.
Figure 2. Comprehensive stratigraphic histogram and sampling section of Maoming Basin. (a): comprehensive stratigraphic histogram of Maoming Basin; (b): sampling profile of Jintang mining area; (c): sampling profile of Yangjiao mining area.
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Energies 16 00514 g003 550
Figure 3. Oil shale characteristics of Youganwo Formation in Maoming Basin: (a) Brown-black oil shale; (b) the weathered color is yellowish white in the form of paper sheets; (c) animal fossils; (d) oil shale can be ignited; (e) JT01-7, Lamalginite and telalginite characteristics; (f) JT01-2, Sporinite characteristics; (g) YJ01-2, Vitrinite characteristics; (h) JT01-6, Pyrite characteristics.
Figure 3. Oil shale characteristics of Youganwo Formation in Maoming Basin: (a) Brown-black oil shale; (b) the weathered color is yellowish white in the form of paper sheets; (c) animal fossils; (d) oil shale can be ignited; (e) JT01-7, Lamalginite and telalginite characteristics; (f) JT01-2, Sporinite characteristics; (g) YJ01-2, Vitrinite characteristics; (h) JT01-6, Pyrite characteristics.
Energies 16 00514 g003
Energies 16 00514 g004 550
Figure 4. Mineral types and geochemical parameters of oil shale in Maoming Basin. (a): mineral types and geochemical parameters of oil shale in Yangjiao mining area; (b): mineral types and geochemical parameters of oil shale in Jintang mining area.
Figure 4. Mineral types and geochemical parameters of oil shale in Maoming Basin. (a): mineral types and geochemical parameters of oil shale in Yangjiao mining area; (b): mineral types and geochemical parameters of oil shale in Jintang mining area.
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Energies 16 00514 g005 550
Figure 5. Series correlation diagram: (a) Oil yield–calorific value; (b) oil yield–ash content; (c) oil yield–volatile content; (d) oil yield–TOC.
Figure 5. Series correlation diagram: (a) Oil yield–calorific value; (b) oil yield–ash content; (c) oil yield–volatile content; (d) oil yield–TOC.
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Energies 16 00514 g006 550
Figure 6. Trace elements UCC-normalized patterns for the samples.
Figure 6. Trace elements UCC-normalized patterns for the samples.
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Energies 16 00514 g007 550
Figure 7. Rare earth element Chondrite-normalized patterns for the samples.
Figure 7. Rare earth element Chondrite-normalized patterns for the samples.
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Energies 16 00514 g008 550
Figure 8. TOC-S1 + S2 intersection map of oil shale in Youganwo Formation of Maoming Basin (After Peters and Cassa, 1994).
Figure 8. TOC-S1 + S2 intersection map of oil shale in Youganwo Formation of Maoming Basin (After Peters and Cassa, 1994).
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Energies 16 00514 g010 550
Figure 10. Relative abundances of selected major and trace elements of Oil shales, showing the weathering status of its source rocks: (a) Al2O3 − (Na2O + CaO*) − K2O ternary diagram (after Condie et al., 1992) and (b) Th/U versus Th plot (after McLennan et al., 1993).
Figure 10. Relative abundances of selected major and trace elements of Oil shales, showing the weathering status of its source rocks: (a) Al2O3 − (Na2O + CaO*) − K2O ternary diagram (after Condie et al., 1992) and (b) Th/U versus Th plot (after McLennan et al., 1993).
Energies 16 00514 g010
Energies 16 00514 g011 550
Figure 11. Kinds of discrimination diagrams reflect the parent rock type. The templates are from (a) Mclennan et al., 1993; (b) Floyd and Leveridge, 1987; (c) Wronkiewicz and Condie, 1987; and (d) Allegre and Minster, 1978.
Figure 11. Kinds of discrimination diagrams reflect the parent rock type. The templates are from (a) Mclennan et al., 1993; (b) Floyd and Leveridge, 1987; (c) Wronkiewicz and Condie, 1987; and (d) Allegre and Minster, 1978.
Energies 16 00514 g011
Energies 16 00514 g012 550
Figure 12. Kinds of discrimination diagrams reflect the tectonic setting. The templates from (a) Allegre and Minster, 1978; (b) Flord and Leveridge, 1987; (c) Roser and Korsch, 1988; (d) Bhatia and Crook, 1986; (e) Mclennan, 2001; and (f) Bhatia, 1985.
Figure 12. Kinds of discrimination diagrams reflect the tectonic setting. The templates from (a) Allegre and Minster, 1978; (b) Flord and Leveridge, 1987; (c) Roser and Korsch, 1988; (d) Bhatia and Crook, 1986; (e) Mclennan, 2001; and (f) Bhatia, 1985.
Energies 16 00514 g012
Table
Table 1. Industrial analysis and geochemical test parameters in oil Shale of Youganwo Formation in Maoming Basin.
Table 1. Industrial analysis and geochemical test parameters in oil Shale of Youganwo Formation in Maoming Basin.
SamplesQuartz
(%)		Pyrite
(%)		Clay
(%)		Clay MineralOil Yield
(%)		Calorific
(MJ/kg)		Industrial AnalysisTOC
(%)		Pyrolysis Parameter
Smectite Mixed
Layer (%)		Illite
(%)		Kaolinite
(%)		Chlorite
(%)		Ash
(%)		Volatile
(%)		Total Sulfur
(%)		S1
(mg/g)		S2
(mg/g)		S1 + S2
(mg/g)		Tmax
(°C)
YJ01-118.8/81.247548/6.445.31 76.215.410.16 10.9 0.96 74.41 75.37 431
YJ01-220.8/79.259437/6.635.59 78.612.660.26 10.9 0.80 68.76 69.56 429
YJ01-324.4/75.655528128.407.42 76.817.380.09 15.4 1.32 106.37 107.69 429
YJ01-420.3/79.749634114.574.27 7913.790.17 8.9 0.61 49.01 49.62 429
YJ01-520.4/79.643542106.235.12 80.114.250.11 10.1 0.65 67.21 67.86 431
YJ01-622.7/77.34764344.364.98 81.48.040.08 8.2 0.65 53.76 54.41 430
YJ01-722.4/77.640543125.905.35 82.612.20.10 10.5 0.83 65.84 66.67 433
YJ01-822.5/77.550644/6.275.69 78.215.490.20 11.5 0.90 73.62 74.52 432
YJ01-924.1/75.955430116.185.43 75.416.050.12 11.0 1.10 70.46 71.56 432
YJ01-1021.3/78.749432155.195.57 82.312.680.19 10.7 1.14 59.27 60.41 430
YJ01-1117.0/8346537125.654.89 76.615.680.18 9.9 1.30 66.14 67.44 434
JT01-134.1/65.96152776.886.57 73.118.420.44 12.4 1.20 82.09 83.29 434
JT01-221.8/78.25453383.564.19 84.28.511.75 10.7 0.90 57.91 58.81 428
JT01-336.5/63.555639/6.828.84 72.620.670.94 16.8 1.07 140.78 141.85 427
JT01-439.9/60.14564094.526.72 76.415.990.61 10.1 2.05 111.87 113.92 432
JT01-537.4/62.63934997.288.43 70.416.580.59 13.1 1.49 85.62 87.11 434
JT01-632.214.253.654732712.8916.66 5927.852.73 21.4 2.60 172.28 174.88 434
JT01-735.1/64.949835813.0717.79 6622.921.00 21.6 1.01 71.84 72.85 430
Average26.2 0.8 73.0 49.8 5.3 37.4 9.6 6.71 7.16 76.1 15.81 0.54 12.5 1.14 82.07 83.21 431
Table
Table 2. Major elements (in wt.%) in oil shale of Youganwo Formation in Maoming Basin.
Table 2. Major elements (in wt.%) in oil shale of Youganwo Formation in Maoming Basin.
SamplesSiO2Al2O3Fe2O3MgOCaONa2OK2OMnOTi2OP2O5LOICIAICV
YJ01-146.93 19.94 4.66 0.88 0.25 0.06 1.67 0.02 0.59 0.06 24.79 90.97 0.41
YJ01-247.37 18.58 5.12 0.84 0.47 0.18 1.63 0.03 0.55 0.16 24.96 89.07 0.47
YJ01-343.84 17.23 6.03 0.88 0.47 0.13 1.56 0.04 0.55 0.09 28.73 88.86 0.56
YJ01-447.61 21.01 5.50 0.80 0.17 0.10 1.60 0.02 0.64 0.07 22.31 91.83 0.42
YJ01-547.47 20.21 5.33 0.79 0.26 0.09 1.52 0.02 0.61 0.10 23.53 91.53 0.43
YJ01-649.03 21.48 4.89 0.75 0.23 0.08 1.58 0.02 0.65 0.08 21.01 91.91 0.38
YJ01-747.05 20.78 5.17 0.76 0.14 0.07 1.62 0.01 0.62 0.07 23.49 91.91 0.40
YJ01-845.38 19.75 6.18 0.77 0.20 0.07 1.66 0.02 0.57 0.11 25.00 91.10 0.48
YJ01-946.17 19.81 5.51 0.82 0.14 0.07 1.74 0.02 0.61 0.06 24.83 91.04 0.45
YJ01-1044.80 20.70 6.17 0.69 0.12 0.07 1.67 0.01 0.62 0.09 24.69 91.76 0.45
YJ01-1149.54 21.24 3.14 0.50 0.07 0.07 1.50 0.01 0.67 0.06 23.13 92.83 0.28
JT01-144.48 15.81 10.79 0.71 0.12 0.08 1.84 0.02 0.44 0.12 25.26 88.57 0.89
JT01-248.37 16.52 4.99 0.80 0.21 0.10 2.14 0.01 0.49 0.04 25.74 87.08 0.53
JT01-339.33 14.07 8.00 0.67 0.43 0.11 1.76 0.08 0.42 0.13 34.86 85.95 0.82
JT01-445.12 16.35 3.30 0.57 0.12 0.03 1.48 0.02 0.54 0.03 32.10 90.93 0.37
JT01-546.18 18.80 3.73 0.61 0.14 0.14 1.74 0.02 0.48 0.19 27.49 90.30 0.36
JT01-636.22 14.27 7.64 0.62 0.37 0.10 1.71 0.03 0.41 0.10 37.94 86.75 0.76
JT01-746.48 15.46 6.48 0.75 0.28 0.13 2.10 0.04 0.48 0.08 27.13 86.03 0.66
Average45.63 18.45 5.70 0.73 0.23 0.09 1.70 0.02 0.55 0.09 26.50 89.910.51
LOI: Loss on ignition; CIA: Chemical index of alteration, CIA = [Al2O3/(Al2O3 + CaO* + Na2O + K2O)] × 100; ICV: Index of compositional variability; and ICV = (Fe2O3 + K2O + Na2O + CaO + MgO + MnO + TiO2)/Al2O3.
Table
Table 3. Trace elements (μg/g) in oil shale of Youganwo Formation in Maoming Basin.
Table 3. Trace elements (μg/g) in oil shale of Youganwo Formation in Maoming Basin.
SamplesLiBeScVCrCoNiCuZnRbSrThZrUSbCsBa
YJ01-181.8 4.39 15.6 82.4 46.9 18.2 23 35 94.8 156 68 44.737.59.40.48 23.6 407
YJ01-271.2 4.72 15.6 69.9 45 16.9 21.8 34.1 84.6 154 82.8 39.641.17.890.52 24.9 510
YJ01-370.9 4.51 12.1 71.1 42.6 17.4 25 33 91.7 145 78.5 39.145.97.520.54 23.3 452
YJ01-490 4.78 16 104 53.9 19 24 36.2 115 152 53.9 41.640.28.260.57 27.4 450
YJ01-589.5 5.12 15.8 86 48.9 20.1 25 35.7 104 141 70.9 40.840.38.50.61 25.3 433
YJ01-680.7 5.13 15.3 74.8 44.2 18.6 23.3 30.4 109 125 54.2 41.336.58.10.51 22.1 385
YJ01-786 4.57 17.5 77.1 49.7 19.2 24.6 36.7 104 145 54.2 4841.19.390.25 22.6 417
YJ01-874.7 4.66 14.5 90.5 46.3 18.3 23.1 29.4 98.8 154 49.2 41.834.97.10.57 25.5 422
YJ01-979.9 4.63 16.1 80.6 48.2 19.3 24.2 35.9 111 153 51.6 4440.38.740.27 28.3 415
YJ01-1072.2 6.03 14.6 80.4 45.9 16.8 22 28.5 109 149 42.4 47.734.67.990.38 25.6 902
YJ01-1156.8 3.48 13.4 83.2 43.1 12.5 15.3 27.2 71.6 121 33.3 37.5306.730.51 22.4 236
JT01-156.4 10.7 12.9 85.9 41.5 23.8 32.7 28.4 156 151 71.1 30.4535.860.96 29.6 360
JT01-250 4.52 9.88 71.1 37.8 13.8 36.5 30.1 60.6 166 62.5 13.354.73.821.15 34.9 351
JT01-366.8 5.88 10.1 72.6 28.6 18.2 25.9 23.6 86.8 125 103 29.651.25.60.74 19.1 484
JT01-430.6 4.7 14.6 95.4 42.6 33.7 33.8 38.1 60.8 127 29.8 29.854.36.051.06 24.8 189
JT01-533.7 5.7 9.81 62.8 29.6 18 22.1 29.7 183 92.6 303 37.273.45.371.59 18.6 949
JT01-666.2 4.9 10.3 75.7 34.8 16.8 48.2 44.1 87.6 121 96.6 30.9456.771.40 18.4 499
JT01-748.5 7.46 9.69 75.3 34.5 21.2 28.9 35.7 74.8 137 120 24.565.15.380.89 23.6 572
Average66.99 5.33 13.54 79.93 42.45 18.99 26.63 32.88 100.17 139.7 79.17 36.7746.517.140.72 24.44 468.5
UCC20 3 11 60 35 10 20 25 71 112 350 10.71902.80.2 3.7 550
Table
Table 4. Rare earth elements (μg/g) in oil shale of Youganwo Formation in Maoming Basin.
Table 4. Rare earth elements (μg/g) in oil shale of Youganwo Formation in Maoming Basin.
SamplesLaCePrNdSmEuGdTbDyHoErTmYbLuLREEHREEREE
YJ01-171.4 133 16.3 57.6 10.3 1.97 9.07 1.54 7.65 1.44 3.97 0.69 4.16 0.6 290.5729.12319.69
YJ01-269.2 128 15.4 57.4 10.1 1.98 9.19 1.61 8.42 1.75 4.82 0.85 5.31 0.77 282.0832.72314.8
YJ01-358.5 110 11.9 45.2 7.73 1.38 6.48 1.11 5.44 1.02 2.88 0.49 2.91 0.42 234.7120.75255.46
YJ01-472.7 136 15.9 60.9 10.4 2.02 9.09 1.63 7.75 1.52 4.17 0.72 4.39 0.62 297.9229.89327.81
YJ01-570.7 133 14.9 59 10.1 2.05 8.92 1.59 8.13 1.56 4.39 0.73 4.63 0.63 289.7530.58320.33
YJ01-6132 247 26.7 101 16.9 3.02 14.6 2.42 11.5 2.12 5.66 0.91 5.74 0.76 526.6243.71570.33
YJ01-775.1 141 16.7 64.5 10.8 2.15 9.46 1.66 7.97 1.5 4.2 0.72 4.47 0.63 310.2530.61340.86
YJ01-863.1 114 14.1 50 8.37 1.57 7.54 1.3 6.48 1.25 3.51 0.61 3.81 0.53 251.1425.03276.17
YJ01-962.7 116 13.6 48.9 9.19 1.68 7.71 1.35 6.76 1.28 3.71 0.64 4.01 0.57 252.0726.03278.1
YJ01-10186 255 27.4 108 16.9 3.2 17.7 3.07 15.7 3.42 9.48 1.51 8.1 1.15 596.560.13656.63
YJ01-1192.4 162 18 69 11.1 2.01 9.78 1.68 8.11 1.6 4.34 0.71 4.45 0.61 354.5131.28385.79
JT01-167.1 124 14.4 52.2 9.05 1.8 7.95 1.4 7.5 1.48 4.05 0.72 4.43 0.62 268.5528.15296.7
JT01-281.6 163 19.6 83.7 14.8 3.28 16.9 3.18 16.8 3.83 10.3 1.61 8.75 1.24 365.9862.61428.59
JT01-334.3 56.6 6.45 23.7 3.94 0.77 3.58 0.67 3.58 0.78 2.28 0.42 2.67 0.42 125.7614.4140.16
JT01-458.9 111 12.7 47.5 8.03 1.54 7.29 1.25 6.42 1.3 3.65 0.62 3.85 0.55 239.6724.93264.6
JT01-539.1 73 8.93 34.9 6.98 1.3 5.65 1.05 5.61 1.12 3.29 0.58 3.68 0.53 164.2121.51185.72
JT01-657.2 98.6 11.4 44 7.48 1.34 6.55 1.07 5.73 1.11 3.15 0.53 3.3 0.46 220.0221.9241.92
JT01-750.7 92.8 10.5 43 7.04 1.28 6.42 1.07 5.32 1.06 3.06 0.49 2.96 0.42 205.3220.8226.12
Average74.59 133 15.27 58.36 9.96 1.91 9.1 1.59 8.05 1.62 4.5 0.75 4.53 0.64 293.09 30.79323.88
chondrite0.31 0.81 0.12 0.6 0.2 0.07 0.26 0.05 0.32 0.07 0.21 0.32 0.21 0.03 2.111.473.58
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Hu, F.; Meng, Q.; Liu, Z.; Xu, C.; Zhang, X. Petrology, Mineralogy, and Geochemical Characterization of Paleogene Oil Shales of the Youganwo Formation in the Maoming Basin, Southern China: Implication for Source Rock Evaluation, Provenance, Paleoweathering and Maturity. Energies 2023, 16, 514. https://doi.org/10.3390/en16010514

AMA Style

Hu F, Meng Q, Liu Z, Xu C, Zhang X. Petrology, Mineralogy, and Geochemical Characterization of Paleogene Oil Shales of the Youganwo Formation in the Maoming Basin, Southern China: Implication for Source Rock Evaluation, Provenance, Paleoweathering and Maturity. Energies. 2023; 16(1):514. https://doi.org/10.3390/en16010514

Chicago/Turabian Style

Hu, Fei, Qingtao Meng, Zhaojun Liu, Chuan Xu, and Xun Zhang. 2023. "Petrology, Mineralogy, and Geochemical Characterization of Paleogene Oil Shales of the Youganwo Formation in the Maoming Basin, Southern China: Implication for Source Rock Evaluation, Provenance, Paleoweathering and Maturity" Energies 16, no. 1: 514. https://doi.org/10.3390/en16010514

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