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Article

Eocene Sedimentary–Diagenetic Environment Analysis of the Pingtai Area of the Qaidam Basin

by
Guoqiang Sun
1,2,*,
Shuncun Zhang
1,2,
Yetong Wang
1,3,
Yaoliang Li
1,2,
Hui Guo
1,2 and
Shangshang Bo
1,3
1
Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences, Lanzhou 730000, China
2
National Engineering Research Center of Offshore Oil and Gas Exploration, Beijing 100028, China
3
University of Chinese Academy of Sciences, Beijing 100049, China
*
Author to whom correspondence should be addressed.
Appl. Sci. 2022, 12(14), 6850; https://doi.org/10.3390/app12146850
Submission received: 10 May 2022 / Revised: 4 July 2022 / Accepted: 5 July 2022 / Published: 6 July 2022
(This article belongs to the Special Issue Safe and Efficient Exploitation and Utilization of Deep Earth Resources)
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Abstract

:
Based on the petrological characteristics and elemental geochemical analysis of core samples from the Pingtai area in the northern structural belt of the Qaidam Basin, this study shows that the clay mineral assemblage of Lulehe Formation sandstone is dominated by high contents of smectite, chlorite and illite, and does not contain illite–smectite mixed layers or kaolinite. The chlorite and illite in the Xiaganchaigou Formation decreased gradually and the smectite disappeared. In addition, illite–smectite mixed layers began to appear and kaolinite was not present. These results indicate that the diagenetic environment of the Pingtai area in the early Eocene was dominated by alkaline media poor in K+ and rich in Mg2+, Na+ and Ca2+. In the late Eocene, K+ content in the diagenetic medium increased significantly, and smectite began to transform into illite. From the early Eocene to the late Eocene, the overall climate and environmental characteristics showed a transition from cold and dry to a cold climate that alternated between dry and wet. The content changes of common oxides, such as CaO, MgO, K2O, Na2O, SiO2, Al2O3, Fe2O3 and TiO2, showed obvious correlation. Based on the content and ratio change tendencies of Sr, Ba, Cu, Zn, U, Th, Ce and other trace elements, combined with the variation characteristics of rare earth element contents, it is suggested that the Lulehe Formation was dominated by a dry and cold freshwater sedimentary environment, and that paleoclimatic conditions were relatively cold and arid during this sedimentary period. However, the climate in the sedimentary period of the Xiaganchaigou Formation was dominated by a cold environment alternating between dry and wet, which also reflected the finding that the global climate was mainly cold and dry in the early Eocene and gradually became warm and humid in the late Eocene.

1. Introduction

Clay minerals are widely distributed in all kinds of sediments and are products of the comprehensive influence of sedimentation and diagenesis under certain climatic, water media and provenance conditions. They are significant indicators of the sedimentary environment, climatic conditions and diagenesis. Therefore, through the study of clay minerals and their assemblages, it is possible to understand characteristics related to provenance, paleoclimate, sedimentary environment and diagenetic environment, and to help reconstruct the processes of sedimentation and diagenesis [1,2,3,4]. Extracting information on environmental evolution from the fluctuations in element content in sediments is a commonly used method to study environmental changes. Different environmental conditions have different effects on the decomposition, migration and enrichment of various elements with different properties. Thus, the variations in element content in sediments can reflect the changes of environmental conditions during sediment deposition to a certain extent [5,6,7,8]. The common oxides of major elements, such as CaO, MgO, K2O, Na2O, SiO2, Al2O3, Fe2O3 and TiO2 [9,10], and trace elements, such as Sr, Ba, Ti, Fe, P, Mn, U, V and Ni [11,12], are sensitive to the paleoenvironment and have clear indicative significance [13,14,15]. Consequently, it is feasible to analyze the sedimentary–diagenetic environment and paleoclimatic conditions that existed during the formation of sedimentary rocks by studying the changes related to clay minerals in those rocks, as well as the content and ratio changes of major and trace elements.
In the Cenozoic era, the collision between the Indian plate and the Eurasian plate has caused the Tibetan Plateau to rise greatly [2]. The complex intracontinental deformation process has had a profound impact on climate change, sedimentary processes, tectonic evolution and the source–sink system in the inland region of Asia, resulting in a huge effect on environmental resources. In particular, it plays an important role in controlling the hydrocarbon accumulation, migration and transformation of the basins in the Tibet Plateau. The Qaidam Basin (Figure 1), which is enclosed by the East Kunlun, Altun and Qilian mountains, is the largest inland basin in the Tibetan Plateau and one of the most important petroleum basins in China. In recent years, hundreds of millions of tons of oil and gas resources have been discovered successively in the Kunbei fault belt and Yingxiong Ridge in the southwestern margin of the basin and the Dongping area and Niudong slope in the northwest [15], while oil and gas exploration in the structural belt in the northern margin of the Qaidam Basin has made slow progress and no significant discoveries have been made. Previous studies on Eocene clastic rocks in the Pingtai area believed that Eocene sediments mainly came from the western section of Saishiteng Mountain, which belongs to the product of near-source rapid accumulation [16]. and was developed in the sedimentary environment of braided-river facies [17], with a high-quality clastic reservoir which is mainly controlled by the sedimentary–diagenetic environment [18]. As the Pingtai area is close to the south Qilian mountains, the natural conditions are bad, the geological conditions are complex and there are few wells in this area. Thus, there are few studies on the sedimentary–diagenetic environment in this area which is one of the main controlling factors of the sandstone reservoir. In order to promote oil and gas exploration and discover the history of environmental changes, this paper takes drilling cores from the Pingtai area as the research object and uses petrological and geochemical methods to study the Eocene sedimentary–diagenetic environment and paleoclimatic changes, which lays a foundation for studying the environmental and resource effects caused by the collision between the Indian plate and Eurasian plate.

2. Geologic Setting

The Qaidam Basin is the largest inland basin in the northeastern part of the Qinghai–Tibet Plateau [19] (Figure 1) with a total area of 120,000 square km [20]. The Pingtai area is located in the northern margin of the Qaidam Basin, adjacent to Saishiteng Mountain. Based on the lithostratigraphic framework, microfossils, magnetic stratigraphy and isotopic chronology in the Qaidam Basin, the Eocene mainly comprises the Lulehe Formation (early Eocene, ~55.8–45 Ma) and Xiaganchaigou Formation (middle Eocene to late Eocene, ~45–35.5 Ma) from bottom to top [21,22,23,24]. As one of the most favorable oil and gas exploration areas in the northern margin of the Qaidam Basin, the Pingtai area has provided great breakthroughs in recent years [25], especially after wells Ping 1 and Ping 3 obtained industrial oil flow indicating that this area has broad prospects for oil and gas exploration [26].

3. Materials and Methods

The Paleogene strata in the Pingtai area are relatively shallow, with an average depth of about 1000 m. The samples in this study are mainly from the drill cores of Well Ping 2. Mudstone is the main lithology of the samples, followed by siltstone, sandstone and conglomerate. Well Ping 2 is 1300 m deep, and the well head is 3135 m above sea level. A 9.00 m segment of core was obtained from between 644.77 m and 653.77 m (Figure 2a), with a harvest rate of 100%; 3.33 m of core was obtained from between 928.23 m and 934.81 m (Figure 2b), with a harvest rate of 50.6%; the length of core harvested between 995.50 m and 1003.50 m was 8.00 m (Figure 2c), with a harvest rate of 100%. The lithology and logging characteristics of the core section of Well Ping 2 are shown in Figure 2.
X-ray diffraction (XRD) was performed following techniques outlined by the Standard of the Petroleum and Natural Gas Industry of the People’s Republic of China (SY/T 5163-2018, Analysis Method for Clay Minerals and Ordinary Non-Clay Minerals in Sedimentary Rocks by X-Ray Diffraction). The experimental method is shown in reference [27]. Illite/smectite mixed layer minerals generally have broad diffraction peaks between 1.0~15.4 Å. After ethylene glycol treatment, the diffraction peak between 1.0~15.4 Å moved to 1.7 Å, and after heat treatment the peak moved to 1.0 Å. The XRD peak of the smectite natural orientation sheet was 1.50 Å. After ethylene glycol treatment, a diffraction peak at 1.70 Å appeared. After heating, the peak at 1.70 Å moved to 1.0 Å, and the other peaks disappeared. Illite was present in all samples. The characteristic diffraction peaks of illite, d (001) = 1.0 Å and d (002) = 0.5 Å, appeared in the XRD pattern of the directional plate. After ethylene glycol saturation and heat treatments, the position of the diffraction peak remained basically unchanged. Chlorite was also a common clay mineral. The characteristic diffraction peaks of d (001) = 1.42 Å, d (002) = 0.71 Å, d (003) = 0.47 Å and d (004) = 0.353 Å did not change after ethylene glycol treatment. After heating to 490 °C, the peak position of d (001) moved to about 13.8 Å and the intensity of the other peaks weakened and disappeared (Figure 3). The relative content of clay minerals was calculated from the ethylene glycol curve (the area of each diffraction peak was obtained with Macdiff software). In the specific calculation process, the relative content of the illite/smectite mixed layer and smectite was expressed by the area of the 1.7 Å diffraction peak, the relative content of illite was expressed by the area of the 1.0 Å diffraction peak, and the relative content of chlorite was expressed by the area of the 0.7 Å diffraction peak. See reference [28] for the specific calculation method and Table 1 for calculation results.
Before elemental analysis, the samples were observed under a microscope for mineral composition and structure to ensure that the samples used for elemental geochemical analysis were not altered, secondarily mineralized or secondarily weathered. All samples were ground with a pollution-free crusher and passed through 200-mesh sieves then baked in an oven for about 3 h to remove moisture for accurate weighing. Determination of the major elements was carried out by a 3080E3X fluorescence spectrometer produced by the Japanese Science Company. We used HF + HNO3 to seal and dissolve samples during trace element analysis using laser coupled plasma mass spectrometry (ICP-MS). Clay mineral and element analyses were both completed at the Key Laboratory of Oil and Gas Resources Research of the CAS.

4. Clay Minerals

The data of 18 whole-rock components and 18 clay mineral components were obtained through testing of core samples from Well Ping 2 (Table 1). Based on test results combined with observations of rock slices under a microscope, scanning electron microscope and electron probe microanalyzer, this study systematically revealed the clay mineral development characteristics of clastic rocks of the Paleogene Lulehe Formation and Xiaganchaigou Formation in the Pingtai area of the Qaidam Basin.
According to the test results of the samples (Table 1), the content of smectite was highest in all samples of the Lulehe Formation, followed by illite and chlorite, while no kaolinite was found in any of these samples. The lowest quartz content was 50.3%, the highest was 70.5% and the average content was 61.49%. The minimum content of clay minerals was 4.9%, the maximum clay content was 13.0% and the average clay content was 8.45%. The highest content of smectite among the clay minerals was 77%, the lowest was 32% and the average was 53.75%. The second most abundant clay was illite, with a maximum content of 53%, a minimum content of 11% and an average content of 24.63%. The highest content of chlorite was 45%, the lowest was 9% and the average was 21.63%. There was no kaolinite and few illite–smectite mixed layers. The lowest content of quartz in the Xiaganchaigou Formation was 15.4%, the highest was 56.5% and the average was 35.95%. The lowest content of clay minerals was 14.9%, the highest was 41.2% and the average was 25.29%. Smectite was not found in the clay minerals, whereas the content of illite–smectite mixed layers was the highest, between 56% and 76%, and the average content reached 68%. The second most abundant clay mineral was chlorite, which comprised between 14% and 20% of the clays with an average content of 16.67%. The content of illite varied greatly, between 4% and 29%, with an average content of 16.67%.
Kaolinite is generally considered to form by strong leaching of feldspar, mica and pyroxene in a humid climate and acid medium [4,29]. A warm and humid climate is conducive to the formation and preservation of kaolinite [30]. Illite is formed in warm or cold climate conditions with little rain and is formed by aluminosilicate minerals, such as feldspar and mica in the case of weathering and removal of K+ [31,32,33]. Therefore, a dry climate and weak leaching are favorable to the formation and preservation of illite [34]. If the climate becomes hot and humid with thorough chemical weathering, illite will gradually decompose into kaolinite [31]. Chlorite is formed in alkaline environments and is unstable under oxidizing conditions; thus, chlorite can generally only survive in areas where chemical weathering is inhibited (such as glaciers or arid surfaces) [35,36]. Increases in chlorite and illite content are commonly considered to represent climatic conditions that gradually become more arid [37,38,39]. Smectite forms easily in a climate with alternating wet and dry conditions [40,41] and readily forms in salt-rich conditions, especially in alkaline media poor in K+ but rich in Na+ and Ca2+. The presence of smectite reflects cold climate characteristics [42], the existence of illite–smectite mixed layers reflects the gradual transformation of smectite into illite in an alkaline environment rich in K+, and chlorite is formed in an alkaline environment rich in Mg2+ [34,43]. Smectite coexisting with hematite indicates a semi-arid climate [44], whereas illite–smectite mixed layers represent a gradual change of climate to a humid environment [45].
Since the Paleogene, the sedimentary sources in the Pingtai area have mainly been from the western section of Saishiteng Mountain [16]; thus, the deposits are considered near-source, and the source area and sedimentary area have the same sedimentary–diagenetic environment. As a result, the characteristics of the clay mineral assemblages in the samples reflect the sedimentary–diagenetic environment in the study area. The clay minerals in the lower Eocene (Lulehe Formation depositional period) clastic rocks in the Pingtai area are mainly composed of high contents of a combination of smectite, chlorite and illite without illite–smectite mixed layers or kaolinite. The middle to upper Eocene (sedimentary period of the Xiaganchaigou Formation) is dominated by high contents of illite–smectite mixed layers, chlorite and illite without smectite or kaolinite. These assemblage characteristics indicate that the sedimentary and diagenetic environment of the Pingtai area in the early Eocene was dominated by alkaline media poor in K+ and rich in Mg2+, Na+ and Ca2+. Chlorite and illite contents gradually decrease from bottom to top, indicating that the cold and dry climate gradually changed to a cold climate that alternated between dry and wet conditions. In the middle-late Eocene, smectite disappeared, illite–smectite mixed layers appeared and the contents of illite and chlorite decreased slightly, indicating that the diagenetic medium was still alkaline. However, K+ increased notably and smectite began to transform into illite, indicating that the climate was relatively humid.

5. Elemental Geochemistry

5.1. Major Element Analysis

Different natural environmental conditions have diverse effects on the decomposition, migration, enrichment and other behavioral characteristics of various elements with different properties and analyzing the fluctuations in content of these elements is a commonly used method to extract environmental change information in the study of environmental evolution [9]. The common oxides of major elements (such as CaO, MgO, K2O, Na2O, SiO2, Al2O3, Fe2O3 and TiO2) are more sensitive to the paleoenvironment than other oxides [10], and the contents of oxides fluctuate little, which is generally related to the continuous and stable macroscopic supergene sedimentary environment [46]. A hot and humid environment is conducive to desiliconization and aluminization; thus, it is more beneficial for the enrichment of Al2O3, Fe2O3, TiO2, K2O and MgO. In contrast, SiO2, Na2O and CaO are more easily preserved under dry and cold conditions [9].
The results for common oxides in the Pingtai area are shown in Table 2 and show obvious correlation between the oxides (Figure 4). For a closed or semi-closed inland lake, high CaCO3 content in the formation at the early stage of chemical deposition indicates an arid climate, whereas low CaCO3 content represents a relatively humid climate [47,48,49]. Compared to the Lulehe Formation, the CaO content of the Xiaganchaigou Formation showed greater fluctuation (Figure 5), with a maximum content of 19.94% and a minimum content of 4.46% (Table 2), which shows that the deposition period of the Xiaganchaigou Formation was dominated by an alternately dry and wet climate environment, whereas the Lulehe Formation deposition period was relatively stable with little change. The content of Na+ in nature is generally very low; however, the content of Na+ in arid enclosed basins increases significantly, whereas K+ content is easily decreased by the adsorption of plants or clay minerals [9]. In contrast to the Xiaganchaigou Formation, the Na+ content in sediments of the Lulehe Formation is higher whereas the K+ content is lower, indicating that the climate was more arid and the lake basin more closed during the deposition period of the Lulehe Formation. Through comparison with North American shale [50] (Figure 4), both the Lulehe Formation and the Xiaganchaiguo Formation showed obvious characteristics of high CaO and Na2O content, revealing that the overall depositional period was dominated by arid environments. Compared to the Lulehe Formation, the hygrophilic oxides, such as K2O, MgO, TiO2 and Fe2O3, in the Xiaganchaigou Formation are more enriched, whereas xerophilic oxides, such as SiO2 and Na2O, in the Lulehe Formation are more enriched. These findings show that the climate during the deposition of the Lulehe Formation was arid, whereas the climate of the Xiaganchaigou Formation gradually became relatively humid. The changes in the content of elements such as Ti reflect the degree to which terrigenous substances were added; higher values represent more abundant terrigenous substances, indicating a warm and humid climate background [27,51]. The change of Ti content also shows that the paleoclimate of the Xiaganchaigou Formation in the Pingtai area gradually became humid during the deposition period.
Loss on ignition not only indicates the deposition process of organic matter and carbonate, but also reflects the input of terrestrial organic matter and the production and preservation capacity of organic matter in the lake [52,53,54,55,56,57]. Climate and environment are the main factors affecting the productivity and preservation of lake organic matter. Under cold and dry conditions, the land-based input to the lake decreases and the productivity of organic matter decreases; this is not conducive to the preservation of organic matter and will reduce loss on ignition. On the contrary, loss on ignition will increase under warm and humid climate conditions [58]. The average loss on ignition of the Lulehe Formation is 6.66%, which is significantly lower than the 12.07% observed in the Xiaganchaigou Formation, indicating that the climate during the deposition of the Lulehe Formation was relatively cold and dry, while climate during the deposition of the lower Ganchaigou Formation was relatively warm and humid.

5.2. Trace Element Analysis

The distributions of different elements in the rock strata depend on the physical and chemical properties of the elements themselves and are also greatly affected by the paleoclimate and paleoenvironment. Therefore, the distribution and ratio changes of trace elements also indicate the evolution process of the paleoclimate and environment to some extent [11,12,14,59,60,61]. Among them, the content of P and the ratio of Sr/Ba are the most sensitive geochemical indexes and environmental parameters of changes of the sedimentary environment [15].
There is a linear relationship between B content and salinity in the water body. Specifically, the higher the salinity of the water body, the greater the B content and the more B ions are adsorbed by the sediment [62]. However, for a continental lake basin, when the sedimentary area is far from the center of the salt lake, the B content is low or normal (less than or equal to 135 × 10−6 on average), which represents the arid to semi-arid salt lake sedimentary environment. This is mainly because of low salinity, which leads to low B content. Therefore, B content can be used to analyze the sedimentary environment [15]. The sediments in the Pingtai area deposited since the Paleogene are mainly near-source deposits far from the sedimentary center [16], and the P content in all samples is lower than 135 × 10−6. As it was far from the sedimentary center, the Pingtai area was dominated by an arid and semi-arid sedimentary environment in the Paleogene. In general, low Sr content indicates a humid climate background, and high Sr content indicates a dry climate background [15]. Compared to average shale [63], the content of Sr in the Paleogene was obviously higher (Figure 6), which also reflects the overall arid sedimentary environment during the Paleogene period. In lake sediments without seawater intrusion, Sr/Ba > 1 is considered to indicate that the lake water has begun to alkalinize (the concentration of soluble ESiO in lacustrine deposits in arid areas is generally higher, and lake water in extremely dry environments can be alkalized), and Sr/Ba < 1 is considered to indicate freshwater sedimentation [9,60]. The Sr/Ba values (Table 3) of all samples of the Lulehe Formation were less than 1, whereas the values of the Xiaganchaigou Formation were from 0.38 to 1.41, showing great variation, which indicates that the climate fluctuated greatly during the sedimentary period of the Xiaganchaigou Formation and that climatic alternation between dry and wet conditions occurred.

5.3. Rare Earth Elements

Rare earth elements (REEs), as a group of special elements, occupy a very significant position in geochemical research. Due to the similar chemical properties of REEs, they always coexist in nature; however, there are slight differences in their atomic structures, leading to differences in the chemical properties of each element. Therefore, REEs undergo fractionation during different geological processes, resulting in distribution patterns with different characteristics [64]. The REE content of the Xiaganchaigou Formation in the Pingtai area is generally high (Table 4), with a distribution range of 157.33–212.36 mg/kg and an average of 184.69 mg/kg. Among them, the average value for light REEs is 135.16 mg/kg, accounting for 73.16% of total REEs; the average value of heavy REEs is 49.53 mg/kg, accounting for 26.84% of the total. The distribution range of rare earth content in the Lulehe Formation is 70.57–243.94 mg/kg with an average value of 130.49 mg/kg. The average value of light REEs is 94.72 mg/kg, accounting for 72.06% of total REEs, and the average value of heavy REEs is 35.11 mg/kg, accounting for 27.94% of the total.
The Lulehe Formation and the Xiaganchaigou Formation are very similar in terms of their REE distribution patterns normalized to chondrites [65] and North American shales [63] (Figure 7a), indicating that the sediments had the same material source and formation process [64,66]. The vertical distribution of total REEs is very close to the variation trend of REEs such as La, Ce and Er (Figure 7b). The change of ∑REE is closely related to the change of climate environment; ∑REE is higher in a warm and humid climate and lower in a cold and dry climate [10,67,68,69]. The ∑REE value of the Lulehe Formation in the vertical direction is significantly lower than that of the Xiaganchaigou Formation, indicating that the paleoclimate conditions during the deposition of the Lulehe Formation were relatively cold and arid, whereas the climate during the deposition of the Xiaganchaigou Formation was warmer and moister, which is consistent with the climatic characteristics reflected by clay minerals.

6. Discussion

Both sedimentary processes and diagenesis may cause the migration and loss of elements. Therefore, when the geochemical properties of elements are used to reconstruct the sedimentary environment and paleoclimate conditions, the sources of elements should be analyzed. Only trace elements that are mainly autogenous and maintain their initial contents can be used to accurately evaluate paleoenvironmental conditions [70]. The chemical components in sedimentary rocks generally come from three main sources: terrigenous clastic materials, biological processes and authigenic sources [71]. To better determine the enrichment pattern of redox-sensitive elements, it is first necessary to determine whether there are terrigenous debris-derived components in the sediments. Among them, Th, as an important index to measure terrigenous clastic composition, is less affected by the grain size of sediments [72], and the diagenesis of sedimentary rocks has little influence on Zr and Y/Ho. Therefore, the ratios of Th to Zr and Y/Ho can effectively indicate the contribution of terrigenous clasts to diagenesis [73]. The relationships between Th and Zr and between Th and Y/Ho in Figure 8 show that there were no obvious correlations between these elements, indicating that the sources of the elements in the clastic rock were not solely terrigenous clastic materials, but that some elements were authigenic. In nature, Y and Ho have very similar geochemical properties; therefore, they migrate or precipitate together in many geological processes. Bau and Dulski (1996) [74] found that the Y/Ho ratio in clastic sediments is about 28, and the closer to this value, the smaller the contribution of terrigenous clastics in sedimentary rocks which are mainly authigenic [75]. The ratio of Y/Ho in Paleogene samples in the Pingtai area is between 22.5 and 27.7, with an average value of 26, which is close to the Y/Ho ratio in the clastic sediments, indicating that the chemical components of sandstone in the study area are mainly authigenic and that the geochemical characteristics can be used to reconstruct the sedimentary environment and paleoclimatic conditions [75].
Previous researchers have considered both Cu and Zn to be transition metal elements. According to the Nernst equation, the pH of the medium will affect the oxidation–reduction process, and Cu and Zn are separated in the environmental medium as the oxygen fugacity decreases. Therefore, the oxidation–reduction environment during deposition can be divided according to the Cu/Zn ratio. A ratio of less than 0.21 indicates a reducing environment, 0.21–0.38 represents a weakly reducing environment, 0.38–0.5 denotes a reduction–oxidation transitional environment, 0.5–0.63 indicates a weakly oxidizing environment and greater than 0.63 specifies an oxidizing environment [76]. The Cu/Zn ratio of the Lulehe Formation in the Pingtai area ranges from 0.28 to 0.59 with an average of 0.43, and the Cu/Zn ratio of the Xiaganchaigou Formation ranges from 0.28 to 0.47 with an average of 0.35. These results indicate that the Pingtai area was dominated by a weakly reducing to reducing–weakly oxidizing transitional environment during the Paleogene sedimentary period, and that the Xiaganchaigou Formation was more hypoxic and reducing. The geochemical properties of Th and U are very different in oxidizing environments but similar in reducing environments, which can be used to reconstruct the sedimentary environment. Based on the formula &U = U/[0.5 × (Th/3 + U)], a value that is greater than 1 represents a reducing environment, and a value less than 1 denotes an oxidizing environment [77]. A Th/U ratio between 0 and 2 indicates a reducing environment, and this ratio can reach 8 under strongly oxidizing conditions [78]. In the Pingtai area, the &U value of the Lulehe Formation is between 1.13 and 4.15 with an average of 2.24, and the Th/U ratio is between 3.01 and 4.75 with an average of 3.91. In the Xiaganchaigou Formation, &U is between 2.68 and 3.71 with an average of 3.12, and Th/U ratio is between 2.34 and 4.23 with an average of 3.36. The lower &U ratio and higher Th/U ratio of the Xiaganchaigou Formation also represent stronger reducing conditions. Among the REEs, Ce is the only element with redox characteristics; it is depleted in oxidizing environments and enriched in oxygen-deficient reducing environments. Wright et al. (1987) [79] proposed that positive and negative values of Ceanom = lg[3Cen/(2Lan + Ndn)] can be used to determine whether Ce is enriched. A positive value of Ceanom indicates enrichment, whereas a negative value indicates depletion. In this formula, “n” signifies shale-normalized concentrations (using the convention established by Gromet et al., 1984 [49]). The value of Ce/La can also reflect the redox environment. A value greater than 2.0 indicates that Ce is enriched and the environment is reducing; a value less than 1.5 indicates a negative Ce anomaly and an oxidizing environment [80]. The Ce/La values of all Paleogene samples in the Pingtai area are greater than 1.5, and the Ce/La values of the Lulehe Formation are between 1.73 and 2.07 with an average of 1.86; the Ce/La values of the Xiaganchaigou Formation are between 1.83 and 2.07 with an average of 1.97. These results indicate that, on the whole, the samples are enriched in Ce representing a weakly oxidizing to reducing sedimentary environment, and that the Xiaganchaigou Formation was more reducing overall. The element V is easily adsorbed and enriched in an oxidizing environment, and most V in sediments is authigenic and migrates little during the diagenetic process. It retains the original state of the sedimentary period and is an ideal indicator of discrimination. In contrast, Ni is relatively depleted in oxidizing environments and relatively enriched in reducing environments. The value of V/(V + Ni) can be used to indicate the redox environment; a ratio greater than 0.7 indicates an anoxic environment [81]. The V/(V + Ni) values of the samples vary little, ranging from 0.69 to 0.80 with an average value of 0.75, which indicates that the Paleogene sedimentary period was dominated by an anoxic environment.
The most important climate warming event since the Cenozoic occurred during the middle Paleocene to early Eocene. During 55–45 Ma (the deposition period of the Lulehe Formation), the climate gradually became colder, global sea level slowly declined and the Antarctic continent formed a permanent ice sheet, forming a long-term gradual cooling process. It was not until the climate warmed again in the late Oligocene that the ice sheet began to melt [82,83,84]. Studies on sedimentation of the North American continent [85], vegetation change [86] and animal evolution [87] have shown that from the early Eocene (the Lulehe Formation deposition period) to the late Eocene (the Xiaganchaigou Formation deposition period) the climate underwent significant changes; the climate changed from cold and dry to alternately dry and wet. Studies on the European continent [88], Asia and Oceania [89] have also suggested that the climate in the middle to late Eocene was relatively humid. The global climate entered a dry and cold period again in the early Oligocene. Both clay mineral composition and geochemical components in the Pingtai area indicate that, from the early Eocene to the middle and late Eocene, the climate gradually changed from cold and dry to cold and alternately dry and wet, and the depositional environment was dominated by anoxic reducing conditions. Oxygen content decreased slightly from the early to late Eocene, representing a humid climate environment and the tendency for sedimentary water bodies to become deeper. Thus, the changes in the depositional environment of the clastic rocks of the Lulehe Formation and Xiaganchaigou Formation in the Pingtai area were responses to global climate evolution.

7. Conclusions

The clay minerals of the Lulehe Formation are mainly composed of high smectite, chlorite and illite content without illite–smectite mixed layers or kaolinite; the Xiaganchaigou Formation is dominated by high illite–smectite mixed layer, chlorite and illite content without smectite or kaolinite. Chlorite and illite contents gradually decreased, smectite disappeared and illite–smectite mixed layers appeared in the Xiaganchaigou Formation. These findings show that the sedimentary basin was in a cold and dry climate environment during the early Eocene, and that the climate gradually changed to dry and wet alternately in the late Eocene.
The diagenetic medium was mainly alkaline, poor in K+, rich in Mg2+, Na+ and Ca2+ and leaching and chemical weathering were weak. Both the Lulehe Formation and the Xiaganchaigou Formation show obvious characteristics of high CaO and Na2O contents. Xerophilic oxides, such as SiO2 and Na2O, are more enriched in the Lulehe Formation, whereas hygrophilic oxides, such as K2O, MgO, TiO2 and Fe2O3, are more enriched in the Xiaganchaigou Formation. In addition, CaO content fluctuates more. This indicates that the overall climate was dominated by aridity during the deposition period and gradually became relatively humid in the later period. The V/(V + Ni) values of the Lulehe Formation and the Xiaganchaigou Formation have little variation, ranging from 0.69 to 0.80 with an average value of 0.75. In the Lulehe Formation, the average Cu/Zn ratio is 0.43, the average &U is 2.24, the average Th/U ratio is 3.91 and the average Ce/La ratio is 1.86. In contrast, in the Xiaganchaigou Formation the average Cu/Zn ratio, &U, Th/U ratio and Ce/La ratio values are 0.35, 3.12, 3.36 and 1.97, respectively. These results indicate that the Eocene sedimentary period was dominated by an anoxic reducing environment and the climate was cold and dry. Compared to the Lulehe Formation, the Xiaganchaigou Formation had a wetter climate, deeper water body and stronger reduction conditions in the sedimentary period. In terrigenous sedimentary environments, stronger reducibility and deeper water bodies are not conducive to the development of high-quality reservoir rocks. This shows that the reservoir of the Lulehe formation is better than that of the lower Ganchaigou Formation, which provides a basis for reservoir prediction and future oil and gas exploration in the Pingtai area.
In the early Eocene, global tectonic activities intensified and the Tibetan Plateau and Kunlun, Atun, and Qilian Mountains continued to rise which blocked water vapor from the Indian Ocean and the Pacific Ocean, making the Qaidam Basin a closed inland basin with a cold and dry climate. During the middle and late Eocene, the global climate became wetter, and the climate of the Qinghai-Tibet Plateau changed accordingly. Weak leaching and weathering also reflect the characteristics of cold, arid and near-source deposition in the enclosed inland basin.

Author Contributions

Conceptualization, G.S., S.Z. and Y.W.; Methodology, G.S., S.Z. and Y.W.; Formal Analysis, G.S., Y.W. and Y.L.; Investigation, G.S., H.G. and S.B.; Writing—Original Draft Preparation, G.S., S.Z., Y.L. and Y.W.; Writing—Review and Editing, G.S., S.Z., Y.W., Y.L. and Y.W. All authors have read and agreed to the published version of the manuscript.

Funding

This work was financially supported by the National Natural Science Foundation of China (Grant No. 41872145), and Key Laboratory Project of Gansu Province (Grant No. 1309RTSA041).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Informed consent was obtained from all subjects involved in the study.

Data Availability Statement

All analysis data used to support the findings of this study are available from the corresponding author upon request.

Acknowledgments

We would like to thank J.-L.Q. for their technical assistance with chemical analyses.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Structural location of the Pingtai area in the northern margin of the Qaidam Basin.
Figure 1. Structural location of the Pingtai area in the northern margin of the Qaidam Basin.
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Figure 2. Core and logging characteristics of the coring section of Well Ping 2. (a) 9.00 m of core was obtained from between 644.77 m and 653.77 m; (b) 3.33 m of core was obtained from between 928.23 m and 934.81 m; (c) 8.00 m of core was obtained from between 995.50 m and 1003.50 m.
Figure 2. Core and logging characteristics of the coring section of Well Ping 2. (a) 9.00 m of core was obtained from between 644.77 m and 653.77 m; (b) 3.33 m of core was obtained from between 928.23 m and 934.81 m; (c) 8.00 m of core was obtained from between 995.50 m and 1003.50 m.
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Figure 3. XRD patterns of the representative samples of clay fractions from Well Ping 2 in the Qaidam Basin. S is smectite, I/S is a mixed layer of illite/smectite, I is illite and C is chlorite. N: oriented section; EG: ethylene glycol saturated section; T: heating section.
Figure 3. XRD patterns of the representative samples of clay fractions from Well Ping 2 in the Qaidam Basin. S is smectite, I/S is a mixed layer of illite/smectite, I is illite and C is chlorite. N: oriented section; EG: ethylene glycol saturated section; T: heating section.
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Figure 4. Variation curves of main oxide contents of the Eocene in the Pingtai area.
Figure 4. Variation curves of main oxide contents of the Eocene in the Pingtai area.
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Figure 5. Comparison of main oxide contents in the Pingtai area with average shale. EF (enrichment factor) = Csediment/Cstandard rock.
Figure 5. Comparison of main oxide contents in the Pingtai area with average shale. EF (enrichment factor) = Csediment/Cstandard rock.
Applsci 12 06850 g005
Applsci 12 06850 g006 550
Figure 6. Comparison of trace elements between samples and average shale. EF (enrichment factor) = Csediment/Cstandard rock.
Figure 6. Comparison of trace elements between samples and average shale. EF (enrichment factor) = Csediment/Cstandard rock.
Applsci 12 06850 g006
Applsci 12 06850 g007 550
Figure 7. Chondrite-normalized REE patterns (a) and NASC-normalized REE patterns (b) of sandstones from the Eocene in the Pingtai area.
Figure 7. Chondrite-normalized REE patterns (a) and NASC-normalized REE patterns (b) of sandstones from the Eocene in the Pingtai area.
Applsci 12 06850 g007
Applsci 12 06850 g008 550
Figure 8. Diagrams of Th–Zr and Th–Y/Ho relationships among samples of the Lulehe Formation and Xiaganchaigou Formation.
Figure 8. Diagrams of Th–Zr and Th–Y/Ho relationships among samples of the Lulehe Formation and Xiaganchaigou Formation.
Applsci 12 06850 g008
Table
Table 1. X-ray diffraction analysis results of all Eocene rock and clay minerals in the Pingtai area.
Table 1. X-ray diffraction analysis results of all Eocene rock and clay minerals in the Pingtai area.
FmBuried
Depth
(m)		Quartz
(w%)		Potassium
Feldspar
(w%)		Sodium
Feldspar
(w%)		Calcite
(w%)		Dolomite
(w%)		Hematite
(w%)		Total
Amount
of Clay
(w%)		Relative Content of Clay
Minerals (w%)		Mixed Layer
Ratio (w%)
SI/SIKCI/S
XiaganchaigouFormation644.7715.40.53.416.623.52.236.0/6122/1723
645.7731.91.07.110.84.01.241.2/6914/1763
647.7647.52.311.2183.0/16.4/764/2053
648.2626.91.06.522.710.41.828.8/7113/1653
648.7618.20.43.728.49.21.436.6/7214/1456
649.7640.71.411.723.92.5/17.1/6914/1754
650.2656.51.911.49.51.8/18.0/6913/1857
651.2642.80.98.527.23.8/14.9//////
652.2642.81.49.120.93.0/20.7/6915/1656
653.2636.81.39.423.23.5/23.2/5629/1559
Lulehe Formation928.2370.51.59.69.5//7.977/11/12/
929.6860.86.615.64.81.1/8.963/14/23/
995.864.52.38.318.8//4.974/13/13/
997.367.93.112.38.1//7.968/12/20/
998.2850.33.112.322.9//9.634/21/45/
1000.8858.83.815.811.21.0/7.632/28/40/
1002.2861.51.66.413.1//7.846/45/9/
1002.7857.62.013.38.1//13.036/53/11/
Note: Standard: SY/T5163-2018. The reason why S + I/S + I +K + C = 101 or 99 is rounding rather than data bias. According to the relevant standards, I/S with a mixed layer ratio greater than 70% is classified as smectite. Description: S is smectite, I/S is a mixed layer of illite/smectite, I is illite, K is kaolinite and C is chlorite. The mixed-layer ratio (e.g., 20%) indicates that the smectite content in the mixed layer of illite/smectite is 20%.
Table
Table 2. Major oxide contents of the Eocene in the Pingtai area.
Table 2. Major oxide contents of the Eocene in the Pingtai area.
FmBuried
Depth
(m)		LithologyNa2O
(%)		MgO
(%)		Al2O3
(%)		SiO2
(%)		K2O
(%)		CaO
(%)		Fe2O3
(%)		P2O5
(%)		TiO2
(%)		MnO2
(%)		LOI
Xiaganchaigou Formation644.77Brown-red mudstone0.928.6712.8233.872.7918.975.990.040.680.1014.72
645.77Brown-red mudstone1.205.7117.3048.673.486.438.330.110.740.107.86
647.76Brown-red mudstone1.633.8013.2951.252.259.495.480.090.720.1311.44
648.26Brown-red mudstone1.106.3212.2337.862.4417.355.680.200.680.1115.34
648.76Brown-red mudstone1.034.8512.6437.272.6219.945.490.050.730.1114.71
649.76Brown-red mudstone1.243.2912.3244.032.3215.734.990.110.740.1314.42
650.26Brown-red mudstone1.664.2312.1859.852.764.466.490.130.720.107.29
651.26Brown-red mudstone1.203.7413.9945.532.7212.645.570.210.720.1113.09
652.26Brown-red mudstone1.424.2915.0848.702.809.696.340.110.750.1110.25
653.26Brown-red mudstone1.433.6914.0948.852.5811.055.540.100.750.1211.58
Average1.284.8613.5945.592.6812.585.990.120.720.1112.07
Lulehe Formation928.23Grayish-brown siltstone1.681.459.4272.301.683.143.000.090.510.135.84
929.68Grayish-brown siltstone3.080.9511.5471.122.384.312.620.050.210.133.41
995.8Grayish-white fine sandstone1.871.017.9566.471.507.592.30.060.470.149.89
997.3Grayish-white medium sandstone2.551.3610.1668.681.735.422.870.070.490.136.17
998.28Grayish-white medium sandstone2.710.9310.6465.261.907.673.950.070.260.226.27
1000.88Grayish-white conglomerate2.670.8610.6166.352.297.452.70.060.230.145.95
1002.28Brown-red mudstone0.985.5910.2862.434.314.643.680.190.740.127.77
1002.78Brown-red mudstone1.194.5615.1859.953.464.553.150.140.700.117.95
Average2.092.0910.7266.52.415.603.030.090.450.146.66
Table
Table 3. Trace element contents of the Paleogene in the Pingtai area.
Table 3. Trace element contents of the Paleogene in the Pingtai area.
FmBuried
Depth
(m)		Element Content(mg/kg)Ratio
VCrCoNiCuZnSrBaThUSr/CuSr/Ba
Xiaganchaigou Formation644.77111.85114.5417.0743.2222.6977.83603.14447.4110.283.9926.581.35
645.77132.07107.1222.4655.9035.00116.58275.48506.2112.523.077.870.54
647.7695.4987.1914.9326.9621.4369.22241.51420.499.332.6211.270.57
648.2685.2387.8315.0326.2336.0476.32669.95475.258.283.5418.591.41
648.7691.2190.1715.3129.3535.2081.55457.44423.398.132.9713.001.08
649.7683.7182.0614.6525.0022.2869.52252.86401.228.392.5711.350.63
650.26118.48106.1817.8534.3026.1493.64186.30486.2512.512.967.130.38
651.2679.7788.9214.4025.6026.0581.72255.99468.119.162.729.830.55
652.2694.3984.4716.0829.6334.2984.52225.85472.2510.432.766.590.48
653.2665.4158.8810.1325.9122.5163.04203.10469.379.302.519.020.43
Lulehe Formation928.2344.6866.147.7219.1114.3637.95139.25465.937.481.829.700.30
929.6836.2464.784.8911.6612.4123.26243.711333.974.220.9619.640.18
995.839.1754.715.9010.4319.5333.07132.27531.376.112.036.770.25
997.353.7268.236.2015.9114.1731.73185.69482.287.701.7313.100.39
998.2851.81105.456.0714.9013.3327.86233.871442.844.391.4117.540.16
1000.939.6359.095.9910.1410.7025.81227.13622.134.160.8821.230.37
1002.3140.47103.2822.4961.0033.57118.48156.86535.2313.703.734.670.29
1002.8103.7288.5717.3847.3229.9499.18148.11499.4012.253.274.950.30
Table
Table 4. REE contents of the Eocene in the Pingtai area.
Table 4. REE contents of the Eocene in the Pingtai area.
FmBuried
Depth
(m)		Rare Earth Element Content (mg/kg)∑REE
ScYLaCePrNdSmEuGdTbDyHoErTmYbLu
Xiaganchaigou Formation644.7711.6219.7426.8551.915.9322.354.420.953.920.623.590.712.070.312.020.32157.33
645.7714.1320.3030.0362.287.2025.665.071.044.370.704.060.812.390.372.400.37181.17
647.7614.6020.8829.2556.397.0425.504.851.004.330.704.080.812.340.372.330.35174.82
648.2611.0924.7934.5767.798.7333.096.531.385.960.904.760.902.410.352.200.33205.79
648.7610.4918.6930.1855.206.7925.084.750.974.190.673.760.742.100.322.050.31166.28
649.7612.0920.0329.6457.267.1926.175.021.044.460.714.020.792.240.352.200.33173.53
650.2620.1023.2134.6269.288.3532.236.091.225.200.814.510.882.540.392.540.39212.36
651.2611.4023.6134.9368.938.5531.766.091.265.550.864.730.912.510.372.340.35204.15
652.2612.3221.7231.8165.597.7729.375.691.154.970.794.420.862.450.382.420.37192.07
653.267.3618.5732.3063.437.4428.315.271.064.620.744.200.832.350.352.220.34179.39
Lulehe Formation928.237.2516.8424.1345.025.3920.273.950.823.460.553.160.631.840.281.830.29135.70
929.684.259.4612.6822.002.7310.262.050.611.850.301.730.351.010.151.000.1570.57
995.86.6915.2119.6036.504.6917.523.300.672.910.472.730.551.620.261.700.27114.67
997.37.3515.9621.5337.744.7217.853.530.813.180.512.950.591.730.261.700.26120.67
998.286.2910.1813.9925.333.0211.562.250.632.140.352.050.411.180.181.190.1880.94
1000.885.789.2112.9722.962.9310.862.070.571.860.301.680.330.960.150.970.1573.73
1002.2816.9826.4642.2885.229.4635.246.971.466.060.945.261.032.920.442.790.43243.94
1002.7814.9422.3534.7772.137.8528.165.561.154.860.774.440.882.560.392.490.38203.70
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Sun, G.; Zhang, S.; Wang, Y.; Li, Y.; Guo, H.; Bo, S. Eocene Sedimentary–Diagenetic Environment Analysis of the Pingtai Area of the Qaidam Basin. Appl. Sci. 2022, 12, 6850. https://doi.org/10.3390/app12146850

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Sun G, Zhang S, Wang Y, Li Y, Guo H, Bo S. Eocene Sedimentary–Diagenetic Environment Analysis of the Pingtai Area of the Qaidam Basin. Applied Sciences. 2022; 12(14):6850. https://doi.org/10.3390/app12146850

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Sun, Guoqiang, Shuncun Zhang, Yetong Wang, Yaoliang Li, Hui Guo, and Shangshang Bo. 2022. "Eocene Sedimentary–Diagenetic Environment Analysis of the Pingtai Area of the Qaidam Basin" Applied Sciences 12, no. 14: 6850. https://doi.org/10.3390/app12146850

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