Organic-Inorganic Geochemical Characteristics of the Upper Permian Pusige Formation in a High-Saline Lake Basin, Tarim Basin: Implications for Provenance, Paleoenvironments, and Organic Matter Enrichment

Wang, Jingbin; He, Zhiliang; Zhu, Dongya; Gao, Zhiqian; Huang, Xiaowei; Liu, Quanyou

doi:https://doi.org/10.1155/2021/6651747

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Volume 2021 | Article ID 6651747 | https://doi.org/10.1155/2021/6651747

Organic-Inorganic Geochemical Characteristics of the Upper Permian Pusige Formation in a High-Saline Lake Basin, Tarim Basin: Implications for Provenance, Paleoenvironments, and Organic Matter Enrichment

Jingbin Wang,^1,2,3Zhiliang He,^1,2,4Dongya Zhu,^2,3Zhiqian Gao,¹Xiaowei Huang,^1,2,3and Quanyou Liu^2,3

Academic Editor: Kun Zhang

Received06 Jan 2021

Revised27 Feb 2021

Accepted03 Mar 2021

Published07 Apr 2021

Abstract

The third member (M3) of the Upper Permian Pusige Formation is a prominent organic-rich lacustrine mudstone sequence within the Yecheng-Hetian Sag, Tarim Basin, hosting major petroleum resources. However, its depositional history and organic matter (OM) enrichment mechanism have received little attention. Therefore, various organic and inorganic geochemical analyses were performed on thirty-four core samples from the Well DW1, to elucidate their depositional paleoenvironments, provenance, and tectonic setting, as well as the controlling factors of OM enrichment. Results showed that the M3 mudstones are classified as poor- to fair-quality hydrocarbon source rocks with mature type II-III kerogen, considering their low organic geochemical parameters. Paleosalinity indexes (e.g., Beq, Sr/Ba, and B/Ga) indicated the typical high-saline lacustrine water body, in which redox state was the oxic-dysoxic as suggested by multiple indicators. Many paleoclimate and weathering proxies suggest a dominant semiarid condition and low weathering degree in the Yecheng-Hetian Sag, which led to that weathered felsic rocks from the West Kunlun Orogen to the southwest of basin were quickly transported into the lake basin. Detrital materials carrying nutrient elements finally promoted the development of relatively high paleoproductivity indicated by fairly high P/Ti and Ba/Al ratios. The negative relationship between P/Ti and total organic carbon (TOC) indicates that paleoproductivity was not the main controlling factor. The correlations among TOC and P/Ti and other multiple proxies suggest that the OM enrichment can be interpreted as both the “preservation model” and “dilution model.” Although the water body was relatively oxygen-riched, high sedimentation rate could largely shorten the exposure time of OM with oxygen, thus decreased the decomposition of OM. In particular, the high-saline, stratified lake water may also restrain the degradation of OM. Furthermore, detrital dilution exerted a potential effect on TOC abundances. On the basis of the above results, a developing model was established to decipher the formation mechanism of OM in these M3 mudstones.

1. Introduction

In recent decades, much attention has been drawn to the formation mechanism of organic matter (OM) in both marine and lacustrine fine-grained sediments. Three fundamental models have been put forward, including (i) higher organic productivity, (ii) enhanced OM preservation (associated with oxygen-deficiency condition in bottom water and sedimentation rate), and (iii) OM dilution (minimal detrital material input) [1–11]. However, the main controlling factors on the formation of organic-rich sediments remain to be controversial. This is because that in most cases, the formation of OM is the result of the complicated, nonlinear interactions among these controlling factors, and also each basin has its own specific factors [1, 4, 12, 13].

Organic-rich mudstones in lacustrine basins can be regarded as favorable hydrocarbon source rocks or direct unconventional petroleum resources (e.g., shale oil and shale gas) [14, 15]; however, their formation processes are much more complicated than those in marine basins, since terrigenous influx and paleoclimate can exert a substantial influence on the evolution of restricted lacustrine basins [16, 17]. Fortunately, continuous geochemical data of lacustrine sediments are used to reconstruct the depositional paleoenvironments at that time, which contributes to a better understanding of OM enrichment in lacustrine basins [9, 12, 18–20].

The Yecheng-Hetian Sag is currently one of the most potential petroliferous areas in the Tarim Basin [21–26]. The Permian organic-rich marls (Qipan Formation) and lacustrine mudstones (Pusige Formation) are two sets of important source rocks because of their widespread distributions in the study area and relatively high organic geochemical properties [23–25]. According to the results of the oil-source correlation results [21, 23, 24], the lacustrine mudstones in the third member (M3) of the Upper Permian Pusige Formation studied herein are potentially the dominant hydrocarbon source rocks for the Kekeya oil/gas field and Well KD-1. The Pusige Formation sediments were just developed during one basin-mountain transition process that basin type was transferred from the Carboniferous-late Early Permian retroarc extensional basin to late Late Permian typical foreland basin. Previous studies mainly focused on their stratigraphic sequences, paleontology, sedimentary facies, and hydrocarbon source rock evolution [27–31]. However, few studies have been made regarding the depositional paleoenvironments (e.g., paleoclimate, weathering, sedimentation rate, and water-mass properties), provenance attribute, and tectonic setting during the deposition of Pusige Formation, which limits the understanding of controlling factors for OM accumulation within the mudstones in the Yecheng-Hetian Sag.

Therefore, in this study, many reliable organic parameters and inorganic elemental indicators were well screened to clarify the depositional paleoenvironments, provenance characters, and mechanism of OM enrichment, as well as the developing model of these Permian Pusige Formation lacustrine mudstones. This study not only provides a more comprehensive cognition of the depositional history for Pusige Formation sediments and guide the future petroleum explorations in Southwestern Tarim Basin but also acts as a reference for OM enrichment in other high-saline lake basins.

2. Geological Setting

Tarim Basin is a typical superimposed polycyclic intracratonic basin developed on the Precambrian basements [22, 32–34]. The Southwestern Tarim Basin is sandwiched between the Tianshan Mountain to the north and Kunlun Mountain to the southwest. The investigations of tectonic cycles, basin prototype, stratigraphic sequence, and geomorphologic recovery suggest that the sedimentary and tectonic evolution of the southwestern Tarim Basin is closely correlated to the uplift and extrusion of the West Kunlun Orogen [22, 32–34]. Six tectonic-stages can be identified, namely, the Stage I: Sinian-Ordovician passive continental margin stage; Stage II: Silurian-Middle Devonian peripheral foreland basin; Stage III: Late Devonian-Early Permianpassive continental margin stage; Stage IV: Late Permian-Triassic back-arc foreland basin; Stage V: Jurassic-Paleogene intracontinental depression; and Stage VI: Neogene-Quaternary compound foreland basin. The investigated Permian continental sediments were just deposited at the transition of Stage III and IV. In addition, following the opening and closure of the Tethys oceans, many Paleozoic intrusive plutons were widely formed within the Kegang, Kudi, Mazha, and Kangxiwar regions (Figure 1), which are mainly composed of granite and granodiorite associated with continental island arc- and continental margin-related tectonic settings [35–40].

The Yecheng-Hetian Sag is a subtectonic unit located in the southwestern Tarim Basin. The evolution of the Permian sedimentary basin is thought to be directly controlled by the closure of the Kangxiwar Paleo-Tethys [27, 41]. Kangxiwar Paleo-Tethys is particularly one of the key regions to investigate the geological evolution of West Kunlun Orogen and Tarim plate. At the beginning of the Early Carboniferous, open marine platform facies, commenced with a large-scale transgression that represents one of the greatest sea-level rises in geological history, covered extensively in Yecheng-Hetian Sag and developed marine carbonate rocks with thousands of meters in thickness until the late Early Permian [41]. At the beginning of the late Early Permian, the Kangxiwar Paleo-Tethys moved intensively northward towards the Tarim Plate with a relatively high angle, leading to an enhanced uplift and the intense regression of sea-level in the Hetian region relative to the Yecheng region [27–29, 41]. As a result, depositional systems in the Yecheng and Hetian regions show remarkable differences [27]. A marine environment was still dominated in the Yecheng region, whereas continental clastic depositions began to be widely developed in the Hetian region (Figure 1(c)) [42]. Two stratigraphic successions were then established and include the Keziliqiman, Qipan, and Daliyueer formations in Yecheng region, and the Keziliqiman, Pusige, and Duwa formations in the Hetian region from Early to Late Permian (Figure 2) [27]. Notably, the Pusige Formation spans the Lower, Middle, and Upper series of the Permian [31] and can be informally divided into three continuous members: Member 1 (M1), Member 2 (M2), and Member 3 (M3) [23, 25, 29, 30].

The investigated Well DW1 was an important exploratory well, which was located in the west of Duwa town (Figure 1(c)) [42]. This well explored the Pusige Formation and Duwa Formation strata, with a total thickness of 1 050 m (Figure 2). The lithology included the conglomerate, sandstone, siltstone, and mudstone interbedded with some gypsum and limestone layers, which were likely deposited in fan-delta to shore-shallow-deep lacustrine facies [23, 28, 29]. In the upper part of M3, black-gray, maroon, and gray-green mudstones are the dominant lithofacies, with a total thickness of approximately 170 m in this profile. This mudstone section can be correlated with other drilling wells (e.g., Yang 1 and Fusha 2) and outcrops (e.g., Duwa and Wuluwusitang), which were deposited in the semideep lacustrine facies based on the sedimentary structures (e.g., horizontal bedding), fossils (e.g., fish skeleton) and lithofacies [23, 28]. The M3 black mudstones are widely distributed within the Yecheng-Hetian Sag with a vertical thickness up to 565.0 m [23].

3. Samples and Methods

A total of thirty-four core samples were collected from the mudstone section (82.0~223.0 m) in Well DW1, with an average spacing of 3.4 m (Figure 2), and then subjected to various geochemical analyses.

TOC and total sulfur (S) were determined using a Leco CS-400 apparatus. Prior to analysis, powdered samples (approximately 100 mesh) were dissolved by dilute HCl (~8%) to remove carbonate minerals (inorganic carbon) for 12 hours. Rock-Eval pyrolysis was conducted on the powdered samples using a Rock-Eval VI instrument. The free hydrocarbon was measured at 300°C, and generated hydrocarbon was identified at a temperature of 600°C. Potential generation index (PG, ) and hydrocarbon index (HI, ) were calculated to reveal the hydrocarbon generation potential. These experiments were performed at the Yangtze University, China.

Major elements were detected by a wavelength dispersive X-ray fluorescence spectrometer at Beijing Research Institute of Uranium Geology (China). Sample was firstly heated in a muffle furnace at 105°C for 4 hours, and then, anhydrous lithium tetraborate (~5.2 g), lithium fluoride (~0.4 g), and ammonium nitrate (~0.3 g) were added to dissolve at 1150°C for another 15 minutes, in order to measure major oxides and loss on ignition. To determine trace elements, powered samples (~25 mg) were dissolved in a mixture of hydrofluoric acid (1 ml) and nitric acid (0.5 ml) in a tightly sealed Teflon bomb at 185°C for 24 hours. The dissolved samples were diluted to 25 ml in a clean bottle for trace element analyses using a Finnigan MAT high-resolution inductively coupled plasma mass spectrometer. The relatively analytical precision is better than 5.0%. The detailed procedures and experimental parameters for major and trace element analyses are followed by the Chinese National Standard GB/T 14506-28-2010 and GB/T 14506-30-2010, respectively.

4. Results

4.1. Bulk Geochemical Characteristics

The results of TOC, S, and Rock-Eval pyrolysis are given in Table 1. All the samples have relatively low TOC values of 0.2-1.11% with an average of 0.45% (), displaying less variability upward from the bottom of this profile (Figure 3). Pyrolysis parameters and can represent the hydrocarbon generation potential of the organic matter [43]. The PG () and HI values are distinctly low, varying from 0.02 to 0.30 mg HC/g rock (average = 0.11 mg HC/g rock), and 5.0 to 49.0 mg HC/g TOC (average = 17.8 mg HC/g TOC), respectively. For the kerogen types of organic matter, these samples are commonly dominated by type II kerogen and following type III kerogen, as evidenced by the bulk geochemical and biomarker parameters studied by Du et al. [23] and Wang et al. [25]. On the cross-plot of versus TOC (Figure 4) [44], the studied samples in Well DW1 are classified as the poor to fair-quality source rocks with mature characters (Figure 4). However, equivalent black mudstones of the M3 in other drilling wells (e.g., Yang1 and Fusha2) show relatively higher TOC, PG, and HI contents than those in Well DW1, which can be generally regarded as poor to good-quality source rocks (Figure 4) [23].

Moreover, the S contents of these lacustrine mudstones range from 0.07% to 0.88% with a median of 0.20%. The TOC/S ratios, which reflect the vertical changes in paleosalinity during the deposition [45, 46], are lower than 10.0, varying between 1.01 and 9.97.

4.2. Major Elements

Major element concentrations of the studied lacustrine mudstones are listed in Table 2. SiO₂, Al₂O₃, and CaO are the predominant oxides, with their contents ranging from 36.93% to 55.45%, 10.31% to 16.32%, and 4.44% to 16.35%, respectively. The second abundant oxides are the TFe₂O₃ (3.66-6.419%), MgO (2.51-5.37%), K₂O (1.92-3.32%), and Na₂O (1.07-2.77%), while contents of other oxides (e.g., TiO₂, P₂O₅, and MnO) are generally lower than 1.0%. When normalized to the Post-Archaean Australia Shale (PAAS) [47], these samples show uniform distribution patterns (Figure 5(a)). The contents of CaO, MgO, and Na₂O are strongly enriched, whereas others, such as the SiO₂, Al₂O₃, MnO, K₂O, TiO₂, TFe₂O₃, and P₂O₅, are substantially depleted relative to those of the PAAS. In the well, these major element compositions in each sample are relatively uniform and exhibit slight vertical fluctuations.

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4.3. Trace Elements

Some specific trace element concentrations of the studied samples are shown in Table 3. In the descending order of average concentration, trace elements include Sr (~519 ppm), Ba (~346 ppm), B (~147 ppm), Zr (~128 ppm), Rb (~112 ppm), V (~90.9 ppm), Zn (~78.4 ppm), Cr (~69.2 ppm), Ni (~41.3 ppm), Cu (~28.4 ppm), Co (~14.9 ppm), Sc (~12 ppm), Th (~9.91 ppm), Hf (~3.91 ppm), U (~3.51 ppm), and Mo (~0.76 ppm). Vertically, these trace elements do not also display distinctive changing trends along with the depths in the profile. Enrichment factor (EF) has been used to characterize the relative enrichment degree of trace elements [19]. Aluminium (Al) is often used as a normalized element due to its relatively high abundance and stability during weathering, transportation, and diagenesis. was calculated using the following equation:

where the background values refer to those of PAAS in the present study [47]. indicates a depletion, whereas or greater corresponds to a distinct enrichment relative to the PAAS. The EF results are shown in Figure 5(b), showing enrichment or depletion of trace elements with varying degrees. Remarkably, Sr is strongly enriched, with EF values varying from 2.72 to 7.5. In contrast, other trace elements, such as B (EF = 1.8-2.93), Zn (EF = 1.01-2.08), U (EF = 1.14-3.15), Hf (EF = 1.06-1.64), Ni (EF = 0.78-1.53), Sc (EF = 1.07-1.38), Th (EF = 0.97-1.32), Rb (EF = 0.91-1.34), Co (EF = 0.7-1.83), Cr (EF = 0.82-1.35), V (EF = 0.71-1.13), Zr (EF = 0.79-1.27), Cu (EF = 0.39-1.57), and Ba (EF = 0.46-1.2), show roughly narrow EF values being close to 1.0, suggesting slight enrichment or depletion. However, Mo element exhibits a wide range of EF values (0.27-5.0), which indicates various degrees of depletion to enrichment.

Total REE (∑REE) contents range between 116.3 ppm and 201.3 ppm (Table 4), with an average of 134.8 ppm. However, most of the ∑REE concentrations are not above 150 ppm, lower than that of PAAS (183.0 ppm) [47]. ∑REE contents are positively correlated with Al₂O₃ (Figure 6(a)), suggesting typical detrital sources that were not affected by chemical alteration and sedimentary sorting. The ∑LREE/∑HREE (L/H) ratio varies from 7.4 to 12.5, reflecting a moderate enrichment in LREE relative to HREE. When normalized to the chondrite (Figure 7) [48], samples are characterized by fractionated LREE patterns [ on average], flat HREE patterns [], obvious Eu negative anomalies (Eu/Eu = 0.61-0.7), and weak Ce negative anomalies (Ce/Ce = 0.92-0.97).

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5. Discussion

5.1. Sedimentary Recycling and Diagenesis

Sedimentary recycling and diagenesis can affect the redistribution of major and trace elements in sediments [49, 50]. Index of Compositional Variability [, all in weight percentages (%)] was defined as an index to evaluate the recycling process and compositional maturity of rocks [50]. An ICV value > 1.0 indicates that sediments were likely sourced from the first-cycle materials in active tectonic settings with a low degree of sediment recycling, while an ICV value < 1.0 is suggestive of either recycled weathered materials in cratonic environments or first-cycle products but enhanced by intense weathering. All the studied samples except for DW1-41 have high ICV values from 1.21 to 2.17, which implies that they are compositionally immature, and received first-recycling detrital materials in an active tectonic setting.

Moreover, cross-plot of Th/Sc against Zr/Sc ratios is widely used to constrain the degree of recycling and sorting [49]. The Th/Sc and Zr/Sc ratios of the samples vary between 0.74 and 1.12 averaging 0.82 and between 8.99 and 15.49 averaging 10.64 (Table 2). These data were plotted away from the trending line of mineral sorting and recycling (Figure 8(a)). Similarly, the Al₂O₃-(CaO+Na₂O)-K₂O (A-CN-K; all in molar percentages) ternary diagram can also be applied to deduce the postdepositional diagenetic and metasomatic processes and parental rock composition [51–53]. On the A-CN-K diagram (Figure 8(b)), all the samples plot above the plagioclase-potash feldspar joint, and fall between the granodioritic and granitic rocks. The predicted weathering trend of these studied samples is subparallel to the A-CN boundary and does not display any implication toward the K₂O apex, revealing insignificant potash-metasomatism during burial diagenesis.

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Furthermore, although REEs in sediments are generally not easily mobile during weathering and transportation, diagenetic exchanges after the depositions would cause the apparent enrichment of Ce, depletion of Eu, and decrease in (Dy/Sm)_N ratio [54]. Negative correlations between Ce/Ce and Eu/Eu or (Dy/Sm)_N may be the result of diagenetic alterations. However, no detectable negative covariations between the Ce/Ce and Eu/Eu and (Dy/Sm)_N ratio are observed in these lacustrine mudstones (Table 4), suggesting minor secondary chemical alterations.

In summary, the geochemical data of lacustrine mudstones, to a large extent, represent their original chemical compositions of the source rocks and could be used to identify their provenance, paleoclimate, and water conditions as the following texts.

5.2. Provenance and Tectonic Setting

Geochemical data of fine-grained sediments have been widely employed to investigate the source-rock lithotypes and their tectonic settings, due to their robust chemical behaviors during weathering, transportation, and diagenesis [55–62]. The ratios of Al₂O₃/TiO₂ (Al/Ti) and TiO₂/Zr (Ti/Zr) are often invariable and represent the chemical compositions of their parental rocks [61]. Al/Ti ratios > 21 and Ti/Zr ratios < 55 are indicative of felsic rocks, whereas mafic rocks have Al/Ti ratios lower than 8 and Ti/Zr ratios more than 200. For the studied samples, Al/Ti and Ti/Zr values range from 21 to 25 and 35 to 49, respectively, indicating a relatively stable provenance supply that consists principally of felsic rocks (Figures 9(a) and 9(b)). The above interpretation is inferred in the A-CN-K and Zr/Sc versus Th/Sc diagrams, in which provenance was mainly composed of the granodioritic and granitic rocks (Figure 8).

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Other binary diagrams, such as La/Sc versus Co/Th and Hf versus La/Th, are also used to evaluate the source-rock compositions [59, 62, 63]. In the present study, these immobile elements show strong positive relationships with terrigenous Al₂O₃ and TiO₂ (Tables 2 and 3), preserving useful geochemical information of the parental rocks. Most of the studied samples are plotted within the felsic fields but possibly mixed with minor intermediate rocks and old sediments (Figures 9(c) and 9(d)). Typically, sediments derived from the felsic provenance are characterized by relatively low ∑REE contents, strong fractionated REE, and distinct negative Eu anomalies, whereas contradict REE features are always indicative of the mafic rocks [59, 64]. The chondrite-normalized patterns further support a predominant felsic provenance with distinctive negative Eu anomalies (Eu/Eu = 0.61~0.7; Table 4).

To further constrain the tectonic setting of source rocks, several discrimination diagrams were used in this study [56, 57, 65]. For example, on the cross-plot of K₂O/Na₂O versus SiO₂/Al₂O₃, these samples are scattered into the fields of active continental margin (ACM) and continental island arc (CIA) (Figure 10(a)). Further, the ternary discrimination diagrams of La-Th-Sc, Th-Sc-Zr/10, and Th-Co-Zr/10 were applied to decipher the tectonic settings [57]. Almost all the samples fall within the field of continental island arc (CIA) but adjacent to the ACM area (Figures 10(b)–10(d)). Moreover, according to the criteria of Bhatia [56], the contents of La, Ce, REE, (La/Yb)_N, and Eu/Eu (Table 4) of the studied samples are much more similar with the continental island arc (dissected magmatic arc) than the ACM tectonic setting.

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Paleocurrent data indicate that the provenance during the deposition of the Pusige Formation was the West Kunlun Orogen located on the southwest of the study area [34]. The above results have indicated that the study area received detritus from a predominant felsic provenance consisting of granodiorite and granite with continental island arc-related tectonic signatures. By coincidence, previous studies have reported that large-scale Paleozoic granodiorite and granite are widely distributed within the West Kunlun Orogen (Figure 1(b)), which were formed under continental island arc (mostly) and active continental margin-related tectonic settings [35–39]. Chondrite-normalized REE patterns of some Paleozoic felsic plutons from the Kudi, Mazha, and Kangxiwar regions to the southwest of the study area can be compared with those of lacustrine mudstones (Figure 7) [35–39]. Thus, it is credible that the dominant provenance for the study area during the Permian was the West Kunlun Orogen to the southwest.

5.3. Paleoweathering and Paleoclimate Implications

Chemical Index of Alteration (CIA) is commonly employed to quantitatively evaluate the paleoweathering conditions [49, 51–53] and calculated using the following equation:

where all elements are presented as molar ratios, of which CaO only represents silicate minerals (excluding calcite, dolomite, and apatite), and is calibrated using the method of McLennan et al. [49]. As discussed in Section 5.1, none of the studied samples were not influenced by sedimentary sorting, recycling, and diagenesis, and thus, CIA values are credible to examine the chemical weathering intensity in the study area. CIA values of the studied samples range between 53.0 and 66.0 with an average of 57.9 (Table 2), which are lower than that of PAAS (CIA = 70.4) [47]. These low CIA data indicate a low paleoweathering condition. This CIA value can be corrected in the A-CN-K diagram (Figure 8(b)), in which these samples are plotted between 50.0 and 65.0.

In general, CIA is also used to reconstruct the paleoclimatic conditions [49, 51, 66]. A CIA value > 80~100 is indicative of a hot climate, > 70~80 indicates a warm and humid climatic setting, and > 50~70 corresponds to a relatively cool and arid paleoclimate. Therefore, the low CIA values (53.0~65.0) in the present study indicate a low chemical weathering with cool and arid paleoclimatic condition.

Previous studies have documented that Fe, Mn, V, Cr, Co, and Ni are relatively enriched under the humid conditions, whereas Ca, K, Sr, Na, Mg, and Ba, being associated with saline mineral precipitates, are comparatively concentrated under arid climatic conditions [67–70]. Taken into consideration of their geochemical behaviors, the C-value was then proposed as an indicator for inferring the paleoclimate [7, 67, 71]. The C-value was defined as follows:

The C-value of 0~0.2, 0.2~0.4, 0.4~0.6, 0.6~0.8, and 0.8~1.0 corresponds to arid, semiarid, semiarid to semihumid, semihumid, and humid climatic conditions, respectively [72]. C-values for all the studied samples vary from 0.2 to 0.59 averaging 0.35 (Table 2). As most C-values are restricted between 0.2 and 0.4, this suggests a relatively stable semiarid paleoclimate during the depositions. Other samples with relatively moderate C-values (>0.4-0.6) are indicative of semiarid to semihumid climatic conditions (Figure 11(a)).

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Besides, ratios of Sr/Cu and Rb/Sr can also be employed for characterizing the paleoclimatic changes [67–69]. The Sr/Cu ratio will increase, while the Rb/Sr ratio decreases under drier and/or hotter climatic conditions. Sr/Cu and Rb/Sr ratios for all the studied samples range from 5.22 to 34.17 and 0.11 to 1.05, respectively. The statistically high Sr/Cu (>5.0) and low Rb/Sr (<0.4) ratios reflect a relatively arid climatic conditions (Figures 11(a) and 11(b)). From the bottom to top in the M3 section, the paleoclimate exhibits a relatively stable semiarid condition (Figure 3).

5.4. Paleoredox, Paleosalinity, and Primary Productivity

5.4.1. Paleoredox Condition

Redox state is generally divided into four gradients: oxic (>2 ml O₂ L^-1), dysoxic (2-0 ml O₂ L^-1 ml_O2/L_H2O), anoxic (0 ml O₂ L^-1), and euxinic (0 ml O₂ L^-1, H₂S > 0) conditions [13, 20, 73]. Recently, many redox proxies have been established to interpret redox conditions in water columns [12, 18–20].

In general, under reducing water columns, U (VI) is more easily reduced to U (IV), which will accelerate the removal of U from the water column to sediments (especially when organic materials exist) [20]. In contrast, insoluble Th (IV) is unaffected by the redox changes. In the present study, U and Th elements display negative () and positive () correlations with Al₂O₃ (Figures 6(b) and 6(c)), and to a certain extent, using U/Th ratio to study the redox states are also suitable. Some authors suggest that a U/Th ratio <0.27, 0.27-0.5, and >0.5 corresponds to oxidizing, dysoxic, and anoxic environments, respectively [74–76]. These samples exhibit moderate U/Th values (0.24~0.65; Table 3), but most values are distributed in the range of 0.27~0.49 (), implying dominant oxic-dysoxic bottom water conditions (Figure 3).

Another quantitative index for the redox state is the Cu/Zn ratio [77–79]. With a decrease in oxygen fugacity, vertical zonation from Cu enrichment towards Zn enrichment can develop in water columns during depositions [79]. A Cu/Zn ratio < 0.21, 0.21-0.38, 0.38-0.5, 0.5-0.63, and > 0.63 represents anoxic, weak anoxic, weak anoxic to weak oxidizing (viz., anoxic to oxidizing transition), weak oxidizing, and oxidizing conditions, respectively [79]. Cu/Zn values for all the studied samples range from 0.17 to 0.75 (Table 3) and reflect an undulatory anoxic-oxic water condition. However, most of the Cu/Zn values vary between 0.21 and 0.50 (), suggesting weak anoxic-weak oxidizing (dysoxic) water columns. Other samples () show Cu/Zn values lower than 0.21 or higher than 0.5, which are indicative of some episodes of anoxic or oxidizing water environments, respectively.

Algeo and Tribovillard [19] investigated the covariations in U and Mo enrichment factors in some modern low-oxygen marine basins and established three Mo-U covariation patterns associated with different specific redox conditions and enrichment processes like “unrestricted marine,” “particulate shuttle,” and “strongly restricted basin” [19, 80]. These Mo_EF-U_EF covariation patterns have successfully been applied to reconstruct the redox environments in both ancient oceans and lakes [80, 81]. The U_EF values for all the studied samples are greater than 1.0, varying between 1.21 and 3.15 with an average of 1.88, revealing a relatively stable enrichment of U in these mudstones. In contrast, Mo_EF displays a wide range of 0.27-5.0 averaging 1.19, suggesting variable degrees of depletion or enrichment. Relatively low MoEF leads to the (Mo/U)_sample ratio to be 0.1~0.3 times of seawater for most studied samples. When plotted into a cross-plot of U_EF versus Mo_EF, U_EF exhibits a positive correlation with Mo_EF (Figure 12), suggesting relatively less fluctuation of redox conditions during the depositions. Additionally, Mo and U accumulation may not be influenced by an active particulate shuttle, but are largely controlled by oxic to dysoxic conditions (Figure 12). In dysoxic waters close to the Fe (III)-Fe (II) transition, soluble U (VI) can be largely reduced to insoluble U (IV) and, then, preferentially incorporated into the sediments over Mo element (especially in the presence of OM) [29, 81]. This is consistent with that the level of U enrichment is greater than that of Mo enrichment in most samples. However, the Mo accumulation in these mudstones may not be due to the coprecipitation or absorption onto the Mn-Fe oxyhydroxides but may have been captured by sulfurized OM or Fe-S phase with different enrichment degrees under dysoxic conditions and subsequently converted into particle-reactive thiomolybdate ions (MoO_x^2-S_4-x, x = 0-3) [20]. This inference is further indicated by the obvious positive relationships between Mo_EF and TOC and S (Figure 3).

5.4.2. Paleosalinity

Salinity is a fundamental feature of water-masses that is useful for paleoenvironment reconstruction in ancient oceans and lakes. Paleosalinity in this study was estimated using Sr/Ba, B, TOC/S, and biomarker parameters. The Sr/Ba ratio typically increases along with enhanced water salinity [46, 82–86]. Sr/Ba values of all the samples range from 0.56 to 2.36, and only five samples have relatively low Sr/Ba values that are lower than 1.0 (Table 3), suggesting typical marine or high-saline environments but interspersed with several episodes of brackish water. According to the reassessment of Wei and Algeo [46], the TOC/S ratio is much more accurate than the Sr/Ba ratio in predicting paleosalinity, and ratios of >10, 2-10, and <2 are broadly suggestive of freshwater, brackish, or marine, and marine facies, respectively. About 27% of the studied samples have TOC/S values lower than 2.0, while other samples show moderate TOC/S values of 2.39~9.97. These data possibly suggest a dominant brackish water column but with some episodes of a marine (or high saline) environment. The above inference is slightly different from the high-saline-dominated paleosalinity derived from Sr/Ba ratios; however, it corresponds to the results from the B proxy as discussed below.

The B concentration and relevant indicators can provide a quantitative assessment of paleosalinity [82–84]. The proxy of equivalent B (B_eq) is usually calculated using the method of Walker and Price [85], and values of , , and are indicative of freshwater, brackish water, and marine or high-saline facies. Furthermore, Adams et al. [82] proposed another indicator of S-value to estimate the paleosalinity of ancient water-masses, as

where the S-value represents the paleosalinity (‰). In general, the calculated S-value < 10‰ indicates a freshwater environment, >10-25‰ is suggestive of a brackish condition, and >25-35‰ reflects a high-saline water (or marine) water. The studied samples have relatively high B_eq and S-values, ranging from to and from 15.6‰ to 30.1‰, respectively (Table 3). These data reflect a relatively stable, stratified brackish water column with some episodes of high-saline condition during the lacustrine mudstone deposition (Figure 3). Additionally, B/Ga ratio can also be a common index to evaluate the paleosalinity, and B/Ga ratio for <3.0, 3.0~6.0, and >6.0 is indicative of freshwater, brackish, and marine facies, respectively [46]. These samples have higher B/Ga ratios, ranging from 7.85 to 12.09 (average 9.6), pointing out the typical marine facies. Considering the fact that this period was the regressive lacustrine deposition, we thus interpret that the higher B/Ga ratios represent the high-saline water body, consistent with the above inferences.

Moreover, some biomarker parameters, such as pristine/phytane (Pr/Ph) and gammacerane/C₃₀ hopane (G/C₃₀), are also effectively applied to infer the paleosalinity [87]. Relatively high G/C₃₀ (G/C₃₀ = 0.18-0.3) and low Pr/Ph ratios (Pr/Ph = 0.06-0.62), along with their negative correlations reported in our previous study [25], also suggest relatively high-saline water conditions.

Overall, the relatively high-saline paleosalinity might result from the strong evaporation induced by drier and/or hotter climate in a relatively closed lake during the M3 deposition period [88], as indicated by the positive or negative correlations between C-value, Sr/Cu, Sr/Ba, B_eq, and S-value (Figure 3).

5.4.3. Primary Productivity

The primary productivity is a crucial indicator to reveal the degree of OM enrichment. Several geochemical proxies, such as TOC, organic phosphorus (P_org), and biogenic barium (Ba_bio), are often useful for evaluating the paleoproductivity during the formation of organic-rich sediments (as discussed by Schoepfer et al. [9]). Although the TOC content provides the most direct implication for paleoproductivity, the application of this index is limited in the present study due to relatively high thermal maturation (which can largely accelerate the loss of organic carbon). Phosphorus is regarded as a fundamental nutrient element for all organisms and can originate from detrital and organic phosphorus sources [9]. However, in organic-rich sediments, detrital P can be neglected because of its low proportion (<20%) in total P, based on statistical analysis [9, 75]. Therefore, the P_org content is approximately equivalent to the measured P. Similarly, biogenic barium (Ba_bio) is also associated with the sinking flux of OM and is another important proxy of primary productivity [9, 20]. The Ba and P contents exhibit relatively negative correlations with TiO₂ (, respectively) (Figures 6(d) and 6(e)), to a large extent, reflecting the organic origins. The Ti- and Al-normalized indicators (e.g., P/Ti and Ba/Al) can be used to eliminate the impact of terrigenous fractions [75], and thereafter, the primary productivity can be effectively evaluated using these indicators. The P/Ti and Ba/Al ratios of the samples show relatively narrow ranges of 0.14-0.36 (0.26 on average) and 15.69-41.32 (29.24 on average) (Table 2). In terms of the P/Ti ratio, this ratio for the studied samples is higher than that of PAAS (P/Ti = 0.13) [47], and Permian Lucaogou Formation fine-grained rocks (P/Ti = 0.16 on average) with high TOC contents (~4.22% on average) in the Santanghu Basin [88], implying a relatively high primary productivity with stable vertical variations during the M3 deposition period.

5.5. Enrichment Mechanism and Developing Model of Organic Matter

The accumulation, preservation, and enrichment of OM are usually affected by several fundamental factors, such as paleoclimate, primary productivity, detrital influx, redox states, sedimentation rate, and sea/lake-level [1–7]. However, each sedimentary environment may have its specific elements; and therefore, the enrichment mechanism of OM cannot be explained by a single controlling factor. In this study, the covariations between various controlling factors and TOC and P/Ti will be used to evaluate the controlling factors on the enrichment process of OM.

The northward subduction of Kangxiwar Paleo-Tethys downwards the Tarim Plate at the beginning of the late Early Permian ended the marine sedimentation and developed such thick (> 2 000 m) continental clastic depositions in the Hetian region. Along with the gradual release of tectonic stress, the Permian sedimentary basin entered a relatively stable period of tectonic setting, provenance supply, and depositional environments during the M3 deposition period (Figure 13). This is supported that most geochemical parameters exhibit weak fluctuations upward from the bottom to top in the Well DW1 (Figure 3). Under such stable sedimentary backgrounds, the semiarid and low weathering dominated conditions resulted in low detrital influx input with carrying a certain amount of soluble nutrients, promoting the relatively high primary OM generation, as supported by the negative relationships between P/Ti and C-value, and Al₂O₃ (, respectively; Figures 14(a) and 14(b)). The CaO contents are negatively or positively correlated with Ti and P/Ti (, respectively; Figures 6(f) and 14(c)). This illustrates that CaO possibly derived from the authigenic carbonate precipitation due to the low annual precipitation and strong evaporation, which were closely associated with the formation process of OM in M3 lacustrine mudstones.

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

(i)

Unlike typical positive correlations between the primary productivity index (e.g., P/Ti) and TOC contents in most studies [17, 75], TOC contents in the present study display a negative relationship with the P/Ti ratio (; Figure 14(d)). This result may suggest that the preservation conditions instead of primary productivity are much more important for the enrichment of OM and, also, reflect that the primary OM inevitably experienced some degree of postdepositional oxidation. Fe/Mn ratio can be generally used as a common index for the relative paleowater depth [88, 89]. As shown in Figure 3, under relatively deep-waters (low Fe/Mn ratio), the TOC contents, primary productivity, and paleosalinity are relatively higher, and the redox state is relatively more reducing (but not up to dysoxic level) relative to the relatively shallow-waters (high Fe/Mn ratio). Therefore, we assumed that when primary OM reached below the sediment-water interface (SWI), most of them were progressively decomposed through the consumption of oxygen and then released CO₂ into the water columns (Figure 13). As a result, oxic-dysoxic water columns, instead of that prevailed prior to sediment depositions [20], were gradually developed above and below the SWI. Meanwhile, related authigenic carbonates were then precipitated below the SWI as mentioned above. The primary productivity index (e.g., P/Ti ratio) is negatively related with Cu/Zn ratio, and TOC contents (, respectively; Figures 14(d) and 14(e)) can support the above-mentioned conclusion: the relatively high primary productivity corresponds to the relatively more reducing conditions and relatively lower TOC contents. In this circumstance, this process may not only have preserved some amounts of organic carbon below the SWI but also accelerated the removal of U and Mo towards into the sediments with various enrichment degrees (indicated by the strong positive covariations between TOC and Mo_EF and U_EF (, respectively; Figures 14(h) and 14(i)).

Sedimentation rate (SR) and detrital influx may also play potential effects on OM enrichment. Ti, Si, and Zr are mainly derived from aluminosilicate and heavy minerals, and Al-normalized ratios are effective indicators for evaluating the degree of detrital influx [6]. A good negative relationship between the TOC and detrital indicator of Si/Al ratio (; Figure 14(i)) suggests the obvious dilution effect of detrital materials in TOC abundance. The (La/Yb)_n (where “n” represents the North American Shale Composition, NASC [90]) is regarded as a potential proxy to reflect the SR, and (La/Yb)_n ≥ 1.0 represents a rapid SR [75, 90]. The (La/Yb)_n values vary from 0.97 to 2.08 (average 1.2; Table 4), revealing a high SR during the M3 deposition period. The positive correlation between (La/Yb)_n and P/Ti (; Figure 14(f)) reflect that high SR contributes to the preservation of primary OM materials, rather than dilution. However, the high SR exerted a weak effect on TOC abundance because no relationship between them was observed in the present study. That can be interpreted that high SR further shortened the exposure time of OM with oxygen [9, 10] and, thus, largely decreased the decomposition of OM under oxic-dysoxic water conditions (Figure 13). Likewise, the relatively high-saline, stratified water columns may also exert a certain effect on the preservation of primary OM by restraining the degradation of OM [75], as the paleoclimate and paleosalinity are closely related with each other (Figure 3).

In summary, the enrichment mechanism of OM of the M3 lacustrine mudstones can be regarded as the combination of “preservation model” and “detrital dilution model.” Favorable stratified, high-saline, and oxic-dysoxic water conditions, together with high SR, were conducive to the preservation of OM, and the relatively high detrital influx diluted the TOC abundance.

6. Conclusions

(1)The M3 lacustrine mudstones are characterized by the relatively low TOC, , , and HI values and are estimated to be poor- to fair-quality hydrocarbon source rocks with mature type II-III kerogen(2)Geochemical results showed that M3 mudstones had a relatively stable felsic dominated provenance formed in continental island arc and ACM tectonic settings from the West Kunlun Orogen to the southwest of the Yecheng-Hetian Sag. The depositional environment was the stratified, high-saline water body with oxic-dysoxic conditions, and paleoclimate was inferred as the predominant semiarid climate with some episodes of semihumid condition(3)Paleoproductvity was not the main controlling factor, and the OM enrichment in these M3 mudstones should be attributed to the preservation conditions and detrital dilution. High sedimentation rate, together with the high-saline, stratified, oxic-dysoxic lake water largely preserved the OM, while detrital dilution exerted a direct effect on TOC abundances in this study

Data Availability

The data can be found in the manuscript.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

Thanks are due to Dr. Youxing Yang and Prof. Zhihong Kang for their help and providing data. This study was financially supported by the National Key Research and Development Program of China (Grant No. 2019YFA0708504), National Natural Science Foundation of China (U19B6003), and the Frontier Project of the Chinese Academy of Sciences (XDA14010201).

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