Mineralogical and Geochemical Characterization of Argillaceous Rocks in the Upper Wuerhe Formation in the Mahu 1 Well Block of the Junggar Basin, NW China

Abstract

The Mahu Sag, where the Mahu 1 well block is located, is one of the most important hydrocarbon-rich depressions in the Junggar Basin, NW China. The Permian Upper Wuerhe Formation (UWF) constitutes the primary layer of the unconventional tight oil reservoir in the Mahu Oilfield. To explore the provenance and sedimentary environment during the deposition of the UWF in the study area, we determined the clay mineralogy and whole-rock geochemical composition of argillaceous rocks. The results show that the primary minerals in argillaceous rock are feldspar, clay minerals, quartz, and a minor amount of hematite. The clay minerals identified included illite, smectite, kaolinite, chlorite, and illite/smectite mixed layers. The tectonic setting of the provenance area for the UWF is a continental island arc, associated with a cutting magmatic arc. The main provenance area is related to the Baogutu tectonic belt (the Zhayier Mountain and the Hala’alate Mountain). The bedrock primarily consists of acidic igneous rocks, with minor occurrences of intermediate–basic igneous and sedimentary rock. The chemical index of alteration (CIA) shows that the parent rocks of the argillaceous rocks have experienced moderate–strong chemical weathering. Combining the Sr/Cu and ΣLREE/ΣHREE ratios, δEu values, and clay mineral characteristics, we determined that the paleoclimate during the deposition of the UWF was generally warm and humid, with occasional short-term dry and cold periods. The UWF gradually changes, according to the relative humidity and enhanced chemical weathering from the bottom to the top. An analysis of trace elements, paleosalinity, and paleowater depth indicate that the studied argillaceous rocks were deposited in a shallow-water oxidation environment of continental fresh water with weak hydrodynamic conditions.

1. Introduction

A sedimentary environment and the provenance of sediment characteristics are key factors affecting sedimentary systems, controlling the generation, migration, distribution, and efficient development of oil and gas reservoirs and resources [1,2]. Therefore, their clay mineral and geochemical characteristics are of great significance to characterize the sedimentary environments, paleoclimate, weathering degree, provenance properties, and tectonic settings of provenance areas throughout geological history [3,4,5,6,7,8,9,10,11,12,13,14].

The Permian UWF in the Mahu 1 well block is the main reservoir of the unconventional tight oil reservoirs in the Mahu Oilfield. Recently, it has emerged as a key layer for oil and gas reserve augmentation and production replacement for the Xinjiang Oilfield Company, with significant exploration and development potential [15,16]. Previous studies have investigated various aspects of the UWF sand-conglomerate reservoir in the study area, including petrological and physical properties, diagenesis, sedimentary facies, hydrocarbon source rocks, geochemical characteristics, source-to-sink systems, reservoir sensitivity, tectonic evolution, petroleum accumulation systems, and exploration strategies [17,18,19,20,21,22,23]. However, the research on clay mineral associations and the elemental geochemistry of argillaceous rocks in the UWF reservoir remains relatively limited.

At present, there are disagreements concerning the sedimentary environment and provenance attributes of the UWF in the northwestern margin of the Junggar Basin. Huang Y.F. et al. [24] believe that the UWF in the northern Mahu Sag experienced strong low-grade chemical weathering, with a cold and dry paleoclimate. The parent rocks are felsic volcanic rocks, and a few are intermediate volcanic rocks. The tectonic setting may be an oceanic island arc. Yu Y.J. et al. [25] believe that the tectonic setting of the UWF’s provenance area in the Hongche, Zhongguai, Kewu and Wuxia areas around the study area is a continental island arc and related to an uncut magmatic arc. The western Junggar orogenic belt constitutes the provenance system of sediments in the northwestern margin. The parent rock types are mainly a combination of pyroclastic rocks, sedimentary rocks, intermediate-acid volcanic rocks, a small amount of metamorphic rock and intrusive rock. Ding C. et al. [2] believe that the UWF in the Zhongguai area has a warm and humid paleoclimate and that the sedimentary water body is a saline/brackish water environment under oxidizing conditions. Its source may be from the upper crust. Li J.S. et al. [26] believe that the UWF in the Jinlong 30 well block of the Zhongguai area has a warm and humid climate and that the sedimentary water body belongs to the brackish water environment under oxidizing conditions. The tectonic setting of the provenance area is a continental island arc, related to the cutting magmatic arc. The source comes from the upper crust, and the parent rocks are mainly felsic, including some basic rocks and felsic migmatites. Zhang Z.J. et al. [27] determined that the Permian provenance in the northwestern margin of the Junggar Basin is mainly from the Upper Carboniferous rocks in the central part of the West Junggar area. These disagreements hinder our comprehensive understanding of the regularity of distribution of oil and gas reservoirs in the region and the identification of favorable exploration targets. Therefore, this study presents new results on the clay mineral features and geochemical characteristics of argillaceous rocks in the Permian UWF reservoir, located in the Mahu 1 well block of the Junggar Basin, NW China. These data are used to define the provenance characteristics and sedimentary environment, offering insights that can aid in the subsequent exploration and development of oil and gas resources.

2. Geological Setting

The Junggar Basin is located in the southern part of the Paleo–Asian Ocean tectonic domain (Figure 1a) [28,29,30,31,32]. This basin is significant for petroleum reserves, having formed during the Paleozoic era and been subsequently modified by numerous tectonic events since the Mesozoic [33,34,35]. It is bordered by the Bogda and Yilinheibiergen Mountains in the south; the Kelameili, Qinggelidi, and Altai Mountains in the northeast; and the Zhayier and Hala’alate Mountains in the northwest. Based on the basement morphology and late tectonic evolution of the basin, it can be divided into six first–order tectonic units: the Western Uplift, Luliang Uplift, Eastern Uplift, Wulungu Sag, Central Depression, and South Thrust Belt. Additionally, there are 44 second–order tectonic units (Figure 1b) [36,37]. The Mahu Sag, a secondary tectonic unit trending NE–SW, is located on the northwestern margin of the Junggar Basin. It borders the Zhayier and Hala’alate Mountains in the west; the Zhongguai Uplift in the southwest; the Dabasong and Xiayan Uplifts; the Yingxi Sag in the southeast; and the Shiyingtan Uplift in the north. Notably, the Mahu Sag is the most significant hydrocarbon-rich sag in the basin [38].

The Mahu 1 well block is located on the southern slope of the Mahu Sag (Figure 1c) [39], with Carboniferous, Permian, Triassic, Jurassic, Cretaceous, and Quaternary strata from bottom to top. The Permian strata belong to a foreland basin-type continental clastic rock formation [40] and comprise the Lower Permian Jiamuhe Formation (P₁j), Fengcheng Formation (P₁f), Middle Permian Xiazijie Formation (P₂x), Lower Wuerhe Formation (P₂w), and Upper Permian UWF (P₃w) (Figure 1d) [41]. The UWF is an angular unconformity with the underlying Lower Wuerhe Formation and is in conformable contact with the overlying Triassic Baikouquan Formation [42]. The UWF is characterized by fan delta deposits, primarily comprising sandy conglomerate with minor amounts of fine sandstone and argillaceous rocks, with a groundmass featuring tuff, andesite, and rhyolite rock fragments [41]. Based on its sedimentary characteristics, this formation is divided into three lithologic members, P₃w¹, P₃w², and P₃w³, from bottom to top. P₃w¹ primarily consists of gray and grayish-brown thick-layered sandy conglomerate with a small amount of thin–layered argillaceous rock, while P₃w² is characterized by interbedded gray thick-layered sandy conglomerate and argillaceous rocks. P₃w³ is predominantly composed of argillaceous rocks and sandy mudstone [43].

Figure 1. (a) Structural location of the Junggar Basin [30,44]; (b) division of tectonic units in the Junggar Basin (modified from Xinjiang Oilfield Company); (c) geographical location and tectonic division map of the Mahu 1 well block showing the studied well’s location information [41]; (d) stratigraphic column of the study area [41,42].

Figure 1. (a) Structural location of the Junggar Basin [30,44]; (b) division of tectonic units in the Junggar Basin (modified from Xinjiang Oilfield Company); (c) geographical location and tectonic division map of the Mahu 1 well block showing the studied well’s location information [41]; (d) stratigraphic column of the study area [41,42].

3. Sampling and Analytical Methods

A total of 13 argillaceous rock samples of the UWF were collected from 7 wells, including MH31, MH11, MH027, K206, MH013, MH032 and K044 (Figure 1c). The samples were fresh, unweathered, gray, and brown argillaceous rocks; silty mudstone; and sandy mudstone, and their locations and depths are shown in Figure 2. Petrographic observation, ESEM analysis, and major and trace element analysis of the samples were completed at the Key Laboratory of Mineralization and Dynamics, Chang’an University, Xi’an, China. Bulk XRD analysis was completed at the State Key Laboratory of Continental Dynamics, Northwest University, Xi’an, China. The clay mineral test was completed at the Key Laboratory for the Study of Focused Magmatism and Giant Ore Deposits, MNR, Xi’an Center of Geological Survey, CGS, China.

The samples were ground into standard rock slices with a thickness of about 0.003 cm. The petrographic analysis was determined using the LEICA ICC50 W polarizing microscope (Wetzlar, Germany).

The samples were treated with washing oil below the fluorescence level 4. They were ground to below 200 mesh with an agate mortar. Mineralogical analysis was performed using a Bruker D8 Advance X-ray diffractometer (Massachusetts, U.S.A.) Test conditions: Cu target; voltage, 40 kV; current, 40 mA; scanning range, 5°–70°; step size, 0.02°; slit width, 0.6°. Mineral identification with DIFFRAC.EVA was used to compare the diffraction pattern with the standard cards of the International Center for Diffraction Database (ICDD-2022). TOPAS (Raunheim, Germany) was used for semi-quantitative analysis to fit the diffraction patterns of the samples.

Clay minerals were analyzed using the Rigaku D/max 2500 X-ray diffractometer (Tokyo, Japan). Test conditions: Cu target; voltage, 40 kV; current, 100 mA; step size, 0.02°; scanning speed, 4°/min. The experimental steps were as follows: First, the sample was crushed to 200 mesh and soaked in ultrapure water for 24 h. EDTA was used to remove carbonates; hydrogen peroxide was used to remove organic matter and clean the sample to a suspended state; and the sample was suspended for 4 h. We sampled 8 cm below the liquid level and centrifuged to obtain an enriched clay sample. The enriched clay samples were diluted with an appropriate amount of ultrapure water and applied to glass slides. After natural drying, a N slide was prepared for the first test. The N slide was placed in ethylene glycol vapor (constant temperature, 60 °C; 8 h) for the saturation treatment to make an EG slide for the second test. The EG slide was placed in a muffle furnace (450 °C, 2.5 h) to make a T slide for the third test. The identification and interpretation of the clay minerals were based on a comparative analysis of XRD overlay maps obtained from the N, EG, and T slides. The data were processed with Jade 6.5. After determining the types of clay minerals in the sample, the diffraction spectrum was subjected to peak separation and fitting processing. The areas of illite and I/S mixed layer peaks overlapping at 1.000 nm were calculated using the “symmetric Gauss–Lorentz” function. Secondly, the peak areas of the smectite and C/S mixed layer were calculated. When the interlayer ratio was lower than 15, the 1.0 nm peak height of the EG was recorded. The 1.0000 nm peak was fitted using the “symmetric Gauss–Lorentz” function. The peak shape parameters were adjusted so that the right side of the fitted peak coincided with the N slide’s right half height and width (1.0 nm). The fitted peak was identified as the illite peak. The formulas [45] for calculating the percentage of mineral assemblages for smectite (S), the I/S mixed layer (I/S), illite (It), kaolinite (K), and chlorite (C) are as follows: (1) K + C = [I_{0.7 nm}(N)/1.5)]/[I_{0.7 nm}(N)/1.5 + I_{1.0 nm}(H)] × 100%; (2) K = [h_{0.358 nm}(EG)/h_{0.358 nm}(EG) + h_{0.353 nm}(EG)] × (K + C); (3) C = (K + C) − K; (4) S = (I_{1.7 nm}(EG)/4)/I_{1.0 nm}(H) × [100 − (K + C)]; (4) It = {I_{1.0 nm}(EG) × [h_{0.7 nm}(N)/h_{0.7 nm}(EG)]}/I_{1.0 nm}(H) × [100 − (K + C)]; (5) I/S = 100 − (S + It + K + C). I_{0.7 nm}(N) is the intensity of the 0.7000 nm, I_{1.0 nm}(H) is the intensity at 1.000 nm in the T; h_{0.358 nm}(EG) is the height at 0.358 nm in EG; h_{0.353 nm}(EG) is a height of 0.353 nm in EG; I_{1.7 nm}(EG) is the intensity at 1.700 nm of smectite in EG; I_{1.0 nm}(EG) is a intensity at 1.000 nm in EG; h_{0.7 nm}(N) is a height at 0.700 nm in N; h_{0.7 nm}(EG) is a height of 0.700 nm in EG.

Microstructure and elemental analyses of the samples were conducted using an FEI Quanta 650 ESEM and INCA energy spectrometer. The samples were processed into appropriate sizes and sprayed with gold before testing.

The major element test was performed using the Shimadzu XRF-1800. A 0.5 g powder sample was fully mixed with 5 g of composite cosolvent and loaded into a platinum crucible. After adding two drops of release agent, the sample was heated to 1200 °C for 20 min. After cooling and solidification, it was tested with the machine. The test method strictly complied with the GB/T 14506.28-2010 standard [46]. The elemental analysis error was less than 2.5%, and the total amount of oxide was 99.75%–100.25%. The loss on ignition (LOI) value was obtained by weighing after baking at 1000 °C for 90 min in a muffle furnace. Trace elements were measured using a Thermo-X7 inductively coupled plasma mass spectrometer. The implementation standard was GB/ T14506.30-2010 [47], and the error was less than 3%. Instrument parameters: power, 1200 W; atomizing gas, 0.64 L/min; auxiliary gas, 0.80 L/min; plasma gas, 13 L/min [48].

The CIA is an effective index for distinguishing the weathering degree of provenance areas and the evolution of paleoclimates [49]. The formula is CIA = [Al₂O₃/(Al₂O₃ + CaO* + Na₂O + K₂O)] × 100%, in which the oxide content is the molar percentage, and CaO* represents CaO in the silicate of the rock (a detailed calculation method is provided in [50]). A CIA of 50–65 reflects weak weathering conditions under cold and dry conditions; a CIA of 65–85 indicates a moderate chemical weathering intensity under warm and humid climate conditions, and a CIA of 85–100 represents strong chemical weathering intensity under hot and humid conditions [51,52]. Notably, the effects of potassium metasomatism and sedimentary recycling should be excluded when calculating the CIA using argillaceous rock samples [53]. In this study, CIA_corr is used to calculate the CIA of potassium metasomatism that has not occurred [54], and the index of compositional variability (ICV) is used to exclude the influence of recycling [55].

4. Results

4.1. Mesoscopic and Petrographic Observations

Mesoscopic observations indicate an extremely fine mineral grain size, with shell-like fractures and minute horizontal bedding structures visible on hand specimens (Figure 3a–d). Microscopic observation shows that the studied argillaceous rock samples mostly comprised clay minerals with a small number of opaque minerals, feldspars, quartzes, and rock fragments. The clay mineral particles (75–95 wt%) were yellow–brown under plane-polarized light and generally experienced sericitization. The opaque minerals (<5 wt%) were irregular particles and 0.01–0.05 mm in diameter. The feldspars (<5 wt%) were lath-shaped, irregularly granular, and sub-angular with a particle size of 0.02–0.20 mm, and they generally experienced sericitization and argillization. The quartzes (<5 wt%) generally exhibited anhedral granular structures with poor roundness, sub-angular shapes, no color, and transparency, and they were 0.02–0.30 mm in diameter. The debris (<5 wt%) was irregularly shaped with moderate roundness and consisted chiefly of intermediate-acidic extrusive rocks, tuff, chlorite schist, and mudstones with particle sizes of 0.05–0.40 mm (Figure 3e–l).

4.2. XRD Analysis

Bulk XRD analysis (Figure 4,

Table 1. Whole-rock XRD analysis results for argillaceous rocks.

Sample ID	Lithologic Member	Lithology	Depth/m	wt/%
				Feldspar	Quartz	Clay Mineral	Hematite
MH11-1-14x	P3w3	Silty mudstone	3236.2	38.2	23.8	38.0	0.03
MH013-3-16x		Silty mudstone	3535.0	37.4	31.1	31.5	0.01
MH013-3-21x		Sandy silty mudstone	3540.2	41.5	28.8	29.7	/
K044-2-1x	P3w2	Silty mudstone	3219.4	42.0	15.5	42.5	/
K044-4-33x		Silty mudstone	3239.1	49.1	20.1	30.6	0.1
K044-4-35x		Silty mudstone	3240.1	50.4	19.9	29.6	0.1
K206-9-1x		Silty mudstone	3607.2	36.4	21.6	41.6	0.3
K206-9-6x		Silty mudstone	3607.3	36.4	28.2	35.3	0.1
K206-9-8x		Silty mudstone	3608.5	31.9	29.4	38.6	0.1
K206-15-23x	P3w1	Silty mudstone	3698.1	66.1	15.2	18.5	0.2
MH032-5-21x		Silty mudstone	3506.8	31.1	31.5	37.4	/
MH032-10-2x		Silty mudstone	3529.4	35.6	23.2	41.1	/
MH027-4-4x		Silty mudstone	3352.2	1.5	39.5	59.0	/

) reveals that the argillaceous rocks studied primarily consist of feldspar, clay minerals, and quartz, with corresponding average contents of 38.3 wt% (31.1–66.1 wt%: only one sample: 1.5 wt%), 36.4 wt% (18.5–59.0 wt%), and 25.2 wt% (15.5–39.5 wt%), respectively. The feldspar is mainly albite, with a small amount of orthoclase. The identified clay minerals are mainly illite, chlorite, and kaolinite. Smectite only exists in sample K044-2-1x.

The XRD superposition spectra of N (natural orientation), EG (ethylene glycol) and T (high-temperature heating) reveal that the diffraction peak intensities of non-clay minerals, including quartz (Q), albite (Ab), and calcite (Cc), are extremely low. By contrast, the diffraction intensities of clay minerals, such as smectite (S), kaolinite (K), chlorite (C), illite (I), and illite/smectite mixed layers (I/S), are relatively high (Figure 5).

The mineral composition of the samples generally remains stable across the three lithologic members of the UWF. The diffraction peaks of quartz at (100) and (101) are located at 0.427 nm and 0.335 nm, respectively, overlapping with the illite peak at (003). The diffraction peak of albite (040) is located at 0.320 nm. Calcite (104) is located at 0.303 nm. The clay minerals present include illite, smectite, kaolinite, chlorite, and an I/S mixed layer.

The diffraction peak of illite (001) in the N slide is at 1.000 nm. In the EG slide, illite has three diffraction peaks of (001), (002), and (003) at 1.000 nm, 0.500 nm, and 0.333 nm, respectively, while the diffraction peak of illite (001) in the T slide remains unchanged at 1.000 nm (Figure 5a). The diffraction peaks of kaolinite (001) and chlorite (002) in the N slide are at 0.720 nm and 0.710 nm, respectively, overlapping with each other in the diffraction pattern. In the EG slide, the diffraction peaks of kaolinites (001) and (002) are located at 0.720 nm and 0.358 nm. Kaolinite (001) overlaps with chlorite (002). The crystal lattice of kaolinite in the T slide has been completely destroyed, and its diffraction peak does not exist (Figure 5b). The diffraction peak of calcium smectite (001) in the N slide can be observed at 1.500 nm. The diffraction peaks of smectites (001), (002), and (003) in the EG slide are located at 1.700 nm, 0.852 nm, and 0.562 nm. The (001) peak moves to 1.000 nm in the T slide (Figure 5c). In the N slide, the diffraction peak of chlorite (001) is at 1.420 nm. In the EG slide, four diffraction peaks can be observed: (001) at 1.420 nm; (002) at 0.710 nm (overlapping with kaolinite (001)); and (003) and (004) at 0.480 nm and 0.353 nm, respectively. In the T slide, the diffraction peak of chlorite (001) moves to 1.380 nm, and the intensity of the (002) diffraction peak is greatly weakened (Figure 5e). The diffraction peak of the I/S mixed layer (001) can be observed in a range of 1.000–1.540 nm in the N slide. In the EG slide, the peak moves to the low angle side, but less than 1.700 nm. Conversely, in the T slide, it shifts to 1.000 nm at a high angle (Figure 5b). This shows that it is an I/S-ordered interstratification mineral. Consequently, the clay mineral assemblage of the UWF primarily comprises I/S mixed layers, illite, kaolinite, chlorite (

Table 2. Analysis results for clay mineral content of argillaceous rocks.

Sample ID	Lithologic Member	wt/%					Interstratified Ratio (%S)	Kübler Index (KI)	Clay Mineral Index
		S	I/S	I	K	C	I/S		I/C	(K+I/S)/(I+C)
MH11-1-14x	P3w3	/	6	18	34	42	25	1.1	0.4	1.3
MH013-3-16x		/	57	20	15	8	25	1.3	2.5	2.6
MH013-3-21x		/	59	16	15	10	25	0.4	1.6	2.9
K044-2-1x	P3w2	68	/	7	10	15	70	0.3	0.5	/
K044-4-33x		/	57	33	4	6	45	1.3	5.5	1.6
K044-4-35x		59	/	24	5	12	60	1.0	2.0	/
K206-9-1x		/	69	24	/	7	45	1.4	3.4	/
K206-9-6x		/	72	22	/	6	40	1.3	3.7	/
K206-9-8x		/	49	29	10	12	15	1.0	2.4	1.4
K206-15-23x	P3w1	/	71	25	2	2	20	1.6	12.5	2.7
MH032-5-21x		/	51	24	13	12	45	1.2	2.0	1.8
MH032-10-2x		/	66	15	10	9	55	0.4	1.7	3.2
MH027-4-4x		/	47	17	17	19	50	0.4	0.9	1.8

).

4.3. Clay Mineral Content and Vertical Variation

The XRD results for the clay minerals are shown in Table 2. The clay minerals in P₃w¹ exhibit high and variable I/S mixed layer content (ranging from 47 wt% to 71 wt%, with an average of 58.8 wt%) and moderate illite content (ranging from 15 wt% to 25 wt%, with an average of 20.3 wt%). This lithologic member also includes low kaolinite (2–17 wt%, 10.5 wt% on average) and chlorite contents (9–19 wt%, except one at 2 wt%, 10.5 wt% on average). Compared with P₃w¹, the clay minerals in P₃w² display a high I/S mixed layer (49–72 wt%, 61.8 wt% on average) and illite contents (22–33 wt%, 27 wt% on average), and lower chlorite (6–15 wt%, 7.8 wt% on average) and kaolinite contents (4–10 wt%, 7 wt% on average). Compared with the previous two lithologic sections, the clay minerals in P₃w³ display the lowest I/S mixed layer (57–59 wt%, except one at 6 wt%, 40.7 wt% on average) and illite contents (16–20 wt%, 18 wt% on average), and the highest kaolinite (15–34 wt%, 21.3 wt% on average) and chlorite contents (8–42 wt%, 20 wt% on average). Smectite is only present in samples K044-2-1x and K044-4-35x, with contents of 59 wt% and 68 wt%, respectively.

The data indicate that the vertical variations in the compositions of the studied clay minerals exhibit a relatively regular pattern. In both P₃w¹ and P₃w², the clay minerals are predominantly an I/S mixed layer and illite, with lower kaolinite and chlorite contents. P₃w² has even lower kaolinite and chlorite contents. In P₃w³, the clay minerals are predominantly I/S mixed layer and kaolinite, followed by chlorite and illite, with the highest contents observed among the three lithologic members. In the UWF, the overall I/S mixed layer content increases from the bottom to the top. The illite content increases and decreases from the bottom to the top. The kaolinite and chlorite contents exhibit an overall downward trend. The I/S mixed layer, illite, kaolinite, and chlorite contents in the UWF exhibit mutual growth and decline.

4.4. Microstructure of Clay Minerals

SEM images of typical clay minerals are presented in Figure 6. A morphological analysis of these minerals mainly shows fine detrital grains. Most crystals exhibit unclear boundaries and strong fragmentation. The detrital grains are tightly packed with minimal pores, and SEM images (Figure 6a) reveal obvious bedding structures. Dissolution is also evident on the surfaces of some irregularly shaped minerals, indicating their detrital origin. Compared with authigenic clay minerals, the clay minerals in the study area exhibit poor crystallization. Kaolinite is an irregular plate shape (Figure 6b), and chlorite is mostly leaf-like or inlaid between debris particles (Figure 6c). The I/S mixed layers are primarily honeycomb-like or curved fold-like, distributed between the pores of detrital particles, with illite developing in a silk-like shape within these pores (Figure 6d).

4.5. Major Elements

Major elements results for the argillaceous rocks are presented in

Table 3. Major element (wt%) analysis results for argillaceous rocks.

Sample ID	Member	SiO2	TiO2	Al2O3	TFe2O3	MnO	MgO	CaO	Na2O	K2O	P2O5	Al2O3/TiO2	K2O/Al2O3	ICV	CIAcorr
MH11-4-34b	P3w1	68.2	0.8	16.0	5.9	0.1	1.9	1.8	1.5	3.8	0.12	19.8	0.2	1.0	71.1
MH31-2-36b		66.8	0.9	19.9	4.9	0.1	2.1	1.5	1.0	2.8	0.03	21.9	0.1	0.7	80.8
MH032-10-2b		63.6	1.2	21.6	5.8	0.1	1.4	1.6	0.8	3.9	0.17	18.2	0.2	0.7	80.3
MH027-6-9b		63.7	1.0	21.5	7.1	0.1	1.6	1.1	0.2	3.7	0.02	21.7	0.2	0.7	84.2
K206-15-23b		59.6	1.0	19.2	11.9	0.1	1.3	1.0	0.7	5.1	0.03	20.3	0.3	1.1	74.8
average		64.4	1.0	19.7	7.1	0.1	1.7	1.4	0.9	3.8	0.07	20.4	0.2	0.8	78.2
K206-9-1b	P3w2	59.3	1.1	17.1	12.6	0.1	2.8	2.1	0.9	3.9	0.21	15.2	0.2	1.4	75.9
K206-12-33b		62.0	1.0	17.8	8.7	0.1	3.1	2.4	1.3	3.3	0.21	17.8	0.2	1.1	75.6
K044-2-1b		60.1	1.2	19.6	8.6	0.1	3.1	2.6	1.4	2.9	0.23	15.8	0.2	1.0	78.0
K044-4-33b		63.2	0.8	17.5	8.8	0.1	2.4	1.6	1.3	4.1	0.14	20.8	0.2	1.1	73.0
K044-4-35b		61.7	0.9	17.7	10.0	0.1	2.5	1.5	1.9	3.7	0.13	19.4	0.2	1.2	72.9
average		61.3	1.0	17.9	9.7	0.1	2.8	2.0	1.4	3.6	0.18	17.8	0.2	1.2	75.1
MH11-1-14b	P3w3	63.3	0.9	19.6	9.0	0.2	1.8	0.8	1.6	2.7	0.02	22.6	0.1	0.9	79.4
MH013-3-21b		63.9	0.9	19.1	8.3	0.1	1.4	1.0	1.8	3.4	0.09	21.5	0.2	0.9	76.4
average		63.6	0.9	19.4	8.7	0.1	1.6	0.9	1.7	3.1	0.06	22.0	0.2	0.9	77.9
PAAS		62.8	1.0	18.9	6.5	0.1	2.2	1.3	1.2	3.7	0.16	/	/	/	/
UCC		66.6	0.6	15.4	5.0	0.1	2.5	3.6	3.3	2.8	0.15	/	/	/	/

Note: The major elements are the results recalculated to 100% after removing the LOI value; CIA_corr = [Al₂O₃(Al₂O₃ + CaO* + Na₂O + K₂O_corr)] × 100%, K₂O_corr = [mAl₂O₃ + m(CaO* + Na₂O)]/(1 − m), m = K₂O/(Al₂O₃ + CaO* + Na₂O + K₂O), K₂O_corr is the K₂O content in argillaceous rock without potassium metasomatism, m is the K₂O in the parent rock, and the calculation formula is based on [54]; ICV = (Fe₂O₃ + K₂O + Na₂O + CaO + MgO + MnO + TiO₂)/Al₂O₃, the calculation formula is based on [55]; PAAS data are from [56]; UCC data are from [57].

. The SiO₂ content of the P₃w¹, P₃w², and P₃w³ samples range from 59.6 wt% to 68.2 wt% (average of 64.4 wt%), 59.3 wt% to 63.2 wt% (average of 61.3 wt%), and 63.3 wt% to 64.0 wt% (average of 63.6 wt%), respectively. These values are comparable to the average SiO₂ content of post-Archean Australian Shale (PAAS, 62.8 wt%) [56] and slightly lower than that of the upper continental crust (UCC, 66.6 wt%) [57]. In addition, the average Al₂O₃, Fe₂O₃, and TiO₂ contents in the studied samples are remarkably close to those of PAAS, while the average MgO, CaO, Na₂O, K₂O, and MnO contents are very close to those of UCC (Table 3).

4.6. Trace and Rare Earth Elements

The trace element composition of the argillaceous rocks is presented in

Table 4. Trace element (ppm) analysis results for argillaceous rocks.

Sample ID	Member	Li	Ga		Zr		Sc		V		Cr	Co	Ni		Cu		Zn		Rb		Sr		Ta
MH11-4-34b	P3w1	18.9	21.5		194		18.7		115		39.6	16.2	32.5		71.0		100		101		121		0.6
MH31-2-36b		36.0	23.3		207		18.1		87.9		48.5	10.8	21.2		38.6		109		52.1		163		0.6
MH032-10-2b		33.7	24.5		212		16.2		144		58.8	14.9	31.4		68.6		82.1		73.8		112		0.9
MH027-6-9b		40.1	24.9		201		18.9		139		32.1	12.1	24.5		36.5		89.1		50.6		82.4		0.5
K206-15-23b		13.5	26.8		201		22.4		141		35.0	9.9	22.1		38.5		102		105		78.6		0.5
average		28.4	24.2		203		18.9		125		42.8	12.8	26.3		50.7		96.5		76.4		111		0.6
K206-9-1b	P3w2	12.0	19.2		144		22.7		182		107.5	23.6	75.1		50.5		82.3		67.8		190		0.5
K206-12-33b		25.1	21.9		152		20.7		141		88.8	21.9	57.2		59.7		88.2		54.2		176		0.5
K044-2-1b		32.3	24.5		188		20.2		249		94.2	17.1	52.6		29.6		88.0		52.8		262		0.8
K044-4-33b		16.4	22.0		167		18.4		110		42.9	18.6	39.1		40.5		93.4		95.8		146		0.5
K044-4-35b		17.3	21.3		167		21.0		84.1		53.6	21.7	44.1		58.7		100		88.4		173		0.5
average		20.6	21.8		164		20.6		153		77.4	20.6	53.6		47.8		90.4		71.8		189		0.5
MH11-1-14b	P3w3	29.1	31.9		166		18.6		64.9		36.2	12.6	35.5		15.5		114		72.3		105		0.7
MH013-3-21b		18.6	23.6		224		17.7		97.4		56.7	12.2	38.3		49.7		78.6		70.0		157		0.6
average		23.8	27.8		195		18.1		81.2		46.5	12.4	36.9		32.6		96.4		71.1		131		0.7
PAAS		75.0	20.0		210		16.0		150		110	23.0	55.0		50.0		85.0		160		200		/
UCC		21.0	17.5		193		14.0		97.0		92.0	17.3	47.0		28.0		67.0		84.0		320		0.9
Sample ID	Member	Ba		Th		U		Nb		Hf		Sr/Cu		Sr/Ba		Cu/Zn	Rb/K	Rb/Zr		Ni/Co		U/Th
MH11-4-34b	P3w1	255		7.7		2.0		8.2		5.0		1.7		0.5		0.7	0.0032	0.5		2.0		0.3
MH31-2-36b		356		6.4		2.1		9.1		4.7		4.2		0.5		0.4	0.0022	0.3		2.0		0.3
MH032-10-2b		396		6.5		1.7		13.5		5.2		1.6		0.3		0.8	0.0023	0.4		2.1		0.3
MH027-6-9b		305		5.0		1.6		7.3		5.2		2.3		0.3		0.4	0.0016	0.3		2.0		0.3
K206-15-23b		347		5.8		1.8		8.6		4.5		2.0		0.2		0.4	0.0025	0.5		2.2		0.3
average		332		6.3		1.8		9.4		4.93		2.4		0.3		0.5	0.0024	0.4		2.1		0.3
K206-9-1b	P3w2	686		3.8		1.0		7.6		3.6		3.8		0.3		0.6	0.0021	0.5		3.2		0.3
K206-12-33b		506		4.7		1.3		8.9		3.4		2.9		0.4		0.7	0.0020	0.4		2.6		0.3
K044-2-1b		445		5.4		2.3		12.		4.6		8.9		0.6		0.3	0.0022	0.3		3.1		0.4
K044-4-33b		394		5.8		1.5		7.7		4.4		3.6		0.4		0.4	0.0028	0.6		2.1		0.3
K044-4-35b		429		5.3		1.4		7.8		3.9		3.0		0.4		0.6	0.0029	0.3		2.0		0.3
average		492		5.0		1.5		8.8		4.0		4.4		0.4		0.5	0.0024	0.4		2.6		0.3
MH11-1-14b	P3w3	445		10.1		1.6		12.		4.0		6.8		0.2		0.1	0.0032	0.4		2.8		0.2
MH013-3-21b		471		6.6		2.2		10.5		5.2		3.2		0.3		0.6	0.0025	0.3		3.1		0.3
average		458		8.4		1.9		11.3		4.6		5.0		0.3		0.4	0.0028	0.4		3.0		0.2
PAAS		650		14.6		3.1		19.0		5.0		/		/		/	/	/		/		/
UCC		624		10.5		2.7		12.0		5.3		/		/		/	/	/		/		/

Note: PAAS data are from [56]; UCC data are from [57].

. All samples exhibit significant depletion in large ion lithophile elements (LILEs), such as Rb, Sr, and Ba, relative to PAAS and UCC, with average contents of 76.4 × 10⁻⁶, 111 × 10⁻⁶, and 332 × 10⁻⁶, respectively. In addition, the average concentrations of compatible elements (Cr, Co, Ni, and Cu) are lower than those in PAAS and UCC, whereas Zn is higher. The V concentration in P₃w¹ and P₃w³ is lower than in PAAS but higher than in UCC. The high-field-strength elements (HFSEs) Zr and Hf are slightly elevated compared with PAAS but slightly depressed compared with UCC. The concentrations of Ta and Nb are comparable to those in UCC. These samples exhibit consistent trace element patterns, with enrichment in K, La, Ce, Nd, Sm, and Tb and depletion in Rb, Nb, Sr, and Ti. These patterns are similar to those of PAAS and UCC (Figure 7a), indicating potential genetic affinities.

The analytical results for the rare earth elements (REEs) are presented in

Table 5. Rare earth element (ppm) analysis results for argillaceous rocks.

Sample ID	Member	La	Ce	Pr	Nd	Sm	Eu	Gd	Tb		Dy	Ho	Er	Tm
MH11-4-34b	P3w1	25.7	63.9	8.1	31.2	7.1	1.5	6.8	1.1		6.5	1.3	3.6	0.6
MH31-2-36b		18.2	42.5	4.8	17.3	3.6	0.8	3.5	0.6		3.4	0.8	2.5	0.4
MH032-10-2b		25.3	51.2	6.4	24.0	5.1	1.4	5.2	0.8		4.9	1.0	2.6	0.4
MH027-6-9b		17.8	40.5	5.0	21.3	5.1	1.3	5.2	0.9		5.6	1.2	3.4	0.5
K206-15-23b		29.4	69.8	8.3	32.2	5.8	1.4	5.2	0.8		4.2	1.0	2.9	0.5
K206-9-1b	P3w2	21.4	40.4	4.8	19.3	4.3	1.3	4.7	0.7		4.4	0.9	2.4	0.4
K206-12-33b		19.5	41.6	5.3	19.8	4.2	1.4	4.4	0.7		3.6	0.8	2.3	0.4
K044-2-1b		24.6	46.4	5.4	21.0	4.3	1.2	4.5	0.7		4.2	0.9	2.4	0.4
K044-4-33b		22.2	49.4	5.6	22.4	5.0	1.3	5.1	0.8		4.9	1.0	2.8	0.4
K044-4-35b		22.2	45.3	5.4	19.9	4.3	1.2	4.5	0.7		3.9	0.9	2.6	0.4
MH11-1-14b	P3w3	11.9	25.4	3.4	12.5	2.4	0.6	2.4	0.4		2.6	0.7	2.1	0.4
MH013-3-21b		27.0	63.2	8.0	32.0	6.3	1.4	6.1	0.9		4.9	1.1	3.2	0.5
average		22.1	48.3	5.9	22.7	4.8	1.2	4.8	0.8		4.4	1.0	2.7	0.4
PAAS		38.0	80.0	8.9	32.0	5.6	1.1	4.7	0.8		4.4	1.0	2.9	0.4
UCC		31.0	63.0	7.1	27.0	4.7	1.0	4.0	0.7		3.9	0.8	2.3	0.3
NASC		32.0	73.0	7.9	33.0	5.7	1.2	5.2	0.8		5.8	1.0	3.4	0.5
Sample ID	Member	Yb	Lu	Y	ΣLREE	ΣHREE	ΣREE	ΣLREE/ ΣHREE		(La/Yb)N		δCe	δEu	(Dy/Sm)N
MH11-4-34b	P3w1	3.7	0.6	37.9	137	24.0	161	5.7		5.0		1.1	0.7	0.6
MH31-2-36b		2.8	0.4	25.0	87.1	14.5	102	6.0		4.8		1.1	0.7	0.6
MH032-10-2b		2.7	0.4	27.3	113	17.9	131	6.4		6.8		1.0	0.8	0.6
MH027-6-9b		3.7	0.6	30.6	91.0	20.9	112	4.4		3.5		1.0	0.8	0.7
K206-15-23b		3.1	0.5	26.0	147	18.0	165	8.2		6.8		1.1	0.7	0.4
K206-9-1b	P3w2	2.6	0.4	24.6	91.5	16.5	108	5.5		5.9		0.9	0.9	0.6
K206-12-33b		2.3	0.4	24.4	91.9	14.8	107	6.2		6.0		1.0	0.9	0.5
K044-2-1b		2.6	0.4	25.4	103	16.1	119	6.4		6.7		0.9	0.8	0.6
K044-4-33b		3.0	0.5	29.3	106	18.4	124	5.7		5.4		1.1	0.8	0.6
K044-4-35b		2.7	0.4	26.9	98.4	16.1	114	6.1		6.0		1.0	0.8	0.5
MH11-1-14b	P3w3	2.5	0.4	18.9	56.2	11.4	67.7	4.9		3.4		1.0	0.7	0.7
MH013-3-21b		3.4	0.5	31.6	138	20.5	158	6.7		5.8		1.0	0.7	0.5
average		2.9	0.4	27.3	105	17.4	122	6.0		5.5		1.0	0.8	0.6
PAAS		2.8	0.4	27.0	166	17.4	183	9.5		9.7		0.7	1.1	0.5
UCC		2.0	0.3	21.0	134	14.3	148	9.3		11.1		0.7	1.0	0.5
NASC		3.1	0.5	27.0	153	20.4	173	7.5		7.4		0.7	1.1	0.6

Note: PAAS data are from [56]; UCC data are from [57]; NASC data are from [59]; δCe = 2(Ce/Ce_C1)/(La/La_C1 + Pr/Pr_C1); δEu = 2(Eu/Eu_C1)/(Sm/Sm_C1 + Gd/Gd_C1); (La/Yb)_N = (La/La_C1)/(Yb/Yb_C1); (Dy/Sm)_N = (Dy/Dy_C1)/(Sm/Sm_C1); C1 chondrite values are from [58].

. These argillaceous rocks have moderate REE concentrations of 67.7–165 × 10⁻⁶ (122 × 10⁻⁶ on average), significantly lower than those in North American shale composite (NASC, ΣREE = 173 × 10⁻⁶) [59]. The rocks exhibit enriched LREEs patterns relative to HREEs, with (La/Yb)_N ratios ranging from 3.4 to 6.8 in the chondrite-normalized REE diagram. These patterns are analogous to those observed in PAAS and UCC (Figure 7b).

5. Discussion

5.1. The Influence of Diagenesis

Fine-grained sediments, such as argillaceous rocks, are valuable geological indicators of the sedimentary paleoenvironment [60,61]. Diagenesis impacts the enrichment and migration of elements, leading to decreased accuracy in geochemical discriminant indicators. Therefore, it is crucial to eliminate the effects of diagenesis before utilizing element geochemical data [62]. The correlation between REEs can indicate the impact of diagenesis on major and trace elements. Significant diagenesis in our samples resulted in a positive correlation between δCe and δEu, a positive correlation between δCe and ΣREE, and a negative correlation between δCe and (Dy/Sm)_N [63]. Our diagram illustrates a negative correlation between δCe and δEu and a weak correlation between δCe and ΣREE, as well as δCe and (Dy/Sm)_N (Figure 8), suggesting a minimal impact from diagenesis. In addition, as a variable valence element, Eu is prone to the chemical reaction of Eu³⁺+e→Eu²⁺ in low-temperature alkaline environments, making it migrate and deplete, resulting in negative Eu anomalies. However, in a high-temperature system, Eu³⁺ is easily oxidized and loses electrons to become insoluble Eu⁴⁺, with positive Eu anomalies [64]. The slightly negative Eu anomalies (δEu = 0.7–0.9, averaging 0.8) observed in the studied argillaceous rocks suggest a low-temperature alkaline sedimentary environment for the UWF, with minimal involvement from hydrothermal solutions in their diagenetic processes. Consequently, the geochemical data of the samples can be deemed representative of the original sedimentary environment’s geochemical characteristics.

5.2. Diagenetic Transformation of Clay Minerals

The type and content of clay minerals in argillaceous rocks correlate well with diagenesis, which can indicate the degree of diagenesis evolution [65]. Generally, with increased burial depth, diagenesis is enhanced, and smectite is transformed into illite through the I/S mixed layer [66]. The process of oil generation is often consistent with this diagenetic change and clearly corresponds to the thermal evolution of organic matter [67,68]. The organic matter is in the immature stage in the early diagenetic stage, where there is a large amount of smectite. The clay minerals are composed of smectite, I/S disordered interlayer, illite, kaolinite, and chlorite. The transformation stage of smectite into illite can be found in the smectite–I/S disordered interlayer. The content of the smectite layer in interlayer minerals is 100% to 50% (R = 0), and there is a 1.700 nm diffraction peak in the EG slide [69]. The organic matter is in a mature stage in the middle diagenesis stage of a large amount of I/S ordered interlayer. The clay minerals comprise I/S ordered interlayer, illite, kaolinite, and chlorite. The transformation of smectite into illite can be seen in the I/S ordered interlayer. The smectite content in the interlayer minerals is 50% to 10%, and there is no diffraction peak at 1.700 nm in the EG slide [70]. The organic matter is in the over–mature stage in the late diagenesis stage, where illite is abundant, and the clay mineral composition comprises illite and chlorite. The diagenetic stage of smectite into illite can be seen in the illite section, and smectite layer content in the interlayer minerals is less than 10% [71,72].

The average depth of argillaceous rock samples in the UWF is more than 3100 m (Figure 2). The clay minerals mainly comprise I/S ordered interlayer, illite, kaolinite, and chlorite (Table 2). The transformation of smectite into illite can be seen in the I/S ordered interlayer, and the smectite layer content in the interlayer minerals is between 15% and 50% (average 35.5%, Table 2). Almost none of the samples have a 1.700 nm diffraction peak in the EG slide (Figure 5). The results show that the argillaceous rocks of the UWF are in the middle diagenesis stage, and the organic matter is in the mature stage, consistent with the practical results of oil exploration in the UWF of the Mahu Sag [73].

5.3. Paleoclimate

Clay minerals are usually formed by the weathering of source rocks in supergene environments, and their formation and transformation are closely related to climate, which can produce different types of these minerals [74,75,76]. The compositions and types of clay minerals vary significantly under different climatic conditions [77,78]. In dry and cold climates, source rocks undergo weak weathering and leaching, making it difficult to remove alkali metal elements, thus forming illite as the primary weathering product [79]. In warm and humid climates, chemical weathering is prevalent, significantly increasing element activity [78,80]. With weathering, alkali metal elements like K⁺ are partially lost, forming smectite as an intermediate weathering product and, subsequently, kaolinite upon the complete loss of K⁺ [81]. During dry–wet alternations, the transformation of smectite into illite involves the weathering and leaching of K⁺ in the lattice, resulting in the formation of a transitional I/S mixed layer product, indicative of a gradual shift toward a humid climate. Chlorite forms in alkaline, cold, and dry environments where physical weathering is dominant [76,78]. Based on the clay mineral types in the sample, this study uses the I/C ratio, the (K + I/S)/(I + C) ratio, and the variation characteristics of clay mineral contents to reflect the paleoclimate evolution. Diagenesis and changes in provenance significantly impact the composition and content of clay minerals, thereby affecting the precision of paleoclimate discrimination [82]. The Kübler index (KI) [83] is an effective parameter for assessing whether samples have undergone diagenesis [84]. Most samples in the study area have a KI value greater than 0.42° (except for four samples), suggesting minimal impact from diagenesis (Table 2).

In addition, the micro-morphology of clay minerals indicates whether samples are affected by diagenesis [85]. Clay minerals can be subdivided into terrestrial and authigenic types based on their genesis [86]. Terrestrial clay minerals are formed through the weathering of parent rocks in provenance areas under supergene conditions, exhibiting irregular morphologies and mixing with other minerals due to erosion and subsequent diagenesis [75]. Authigenic clay minerals exhibit clean crystal surfaces and well-defined morphologies, facilitating SEM–based identifications of their origins [87,88]. The clay minerals in the study area mainly have a detrital genesis, indicating that they are unaffected by diagenesis and can be used to infer the characteristics of paleoclimate change in the provenance area [78]. Based on the characteristics of clay mineral assemblage and content variation (Table 2), the following conclusions can be drawn: (1) Across the three lithologic sections of the UWF, the I/S mixed layer exhibits the highest average content. It is an intermediate product in the transformation of smectite into illite. Its formation signifies a shift in the climate toward a humid environment during the Late Permian in the study area [78]. Kaolinite reflects hot and humid climatic conditions. The kaolinite content in P₃w³ is significantly higher than in the other two lithologic sections. This suggests that the late sedimentary period of the UWF experienced warmer and wetter conditions. Smectite reflects a cold-climate environment [89], indicating a short-term cold climate during the deposition of P₃w². At the same time, the illite and chlorite contents in the samples account for one/third± of the total, indicating that there may have been a short-term dry and cold climate during this period. Consequently, the paleoclimate of the northwestern margin of the Junggar Basin during the Late Permian was predominantly warm and humid, with occasional short-term dry and cold climates. (2) From the bottom to the top of the UWF, the average I/S and kaolinite contents increased, while the average I/C and (K + I/S)/(I + C) exhibited an upward trend. This suggests that the climate in the study area evolved from P₃w¹ to P₃w³, transitioning to a relatively humid state accompanied by increasing chemical weathering.

Table 3 shows that the ICV of most samples is greater than or close to one, indicating that they may be products of the first deposition. Conversely, a few samples exhibit lower ICV, hinting at potential recycling or intense weathering processes [90]. In the Zr/Sc–La/Sc diagram [91,92], all samples align closely with or intersect with the composition trend line (Figure 9), suggesting that they have not undergone sedimentary recycling. The results show that the average CIA_corr values of P₃w¹, P₃w², and P₃w³ are 78.2, 75.1, and 77.9, respectively (Table 3). This suggests that these rocks underwent intense chemical weathering during deposition in a warm and humid Late Permian climate. Furthermore, the high SiO₂, Al₂O₃, and Fe₂O₃ contents, coupled with low alkaline earth metal concentrations (Mg and Ca), suggest that the studied argillaceous rocks were deposited in a stable, warm, and humid paleoclimate [93].

To investigate the sedimentary paleoclimate in the study area, we conducted further analysis using the Sr/Cu, δEu, and ΣLREE/ΣHREE ratios. A Sr/Cu ratio between 1.3 and 5 indicates a warm and humid climate, while Sr/Cu > 5 indicates an arid climate [94]. Negative δEu anomalies can indicate warm and humid climate conditions [95]. In warm and humid environments, HREEs are more soluble and mobile in solution than LREEs, enriching LREEs. Therefore, higher ΣLREE/ΣHREE ratios indicate warmer and more humid environments [96]. The samples analyzed exhibited low Sr/Cu ratios of 1.7–6.8 (3.7 on average), weak negative anomalies with δEu values of 0.7–0.9 (0.8 on average), and relatively high ΣLREE/ΣHREE ratios of 4.4–8.2 (Table 3), indicating a warm and humid paleoclimate.

5.4. Paleowater Depth

Authigenic minerals record natural conditions during the formation of the sedimentary environment. They indirectly reflect paleowater depths. Iron–bearing authigenic minerals exhibit regular distribution patterns based on redox conditions related to water depth: hematite at 0–1 m, limonite at 1–3 m, siderite at 3–15 m, and pyrite dominating beyond 15 m [97]. The XRD bulk analysis reveals that the primary iron–bearing authigenic mineral in the study area is hematite (averaging 0.1%, Table 1), suggesting a shallow–water sedimentary environment for UWF deposition.

Zirconium (Zr), a relatively immobile element, typically occurs in detrital rocks and readily precipitates in shallow-water environments [98]. Rubidium (Rb), on the other hand, is chemically reactive and readily migrates into clay minerals, ultimately precipitating in deeper water [99]. Consequently, the Rb/Zr ratio is commonly utilized as an indicator for paleowater depth determination [100,101]. The Rb/Zr ratio exhibits a positive correlation with water depth and a negative correlation with hydrodynamic intensity [102,103]. The Rb/Zr ratios of the samples studied ranged between 0.3 and 0.6, with an average of 0.3 (Table 4), suggesting deposition of the UWF in a shallow–water environment characterized by low hydrodynamic activity.

Previous studies have suggested a method to estimate paleowater depth using the deposition rate derived from the Co and La content in lacustrine mudstone [104]. The calculation formulas are as follows: V_s = V₀ × N_Co/(S_Co-t × T_Co), h = 3.05 × 10⁵/V_s^1.5. V₀ represents the deposition rate of normal water, with a typical range of 200–300 m/Ma for lacustrine mudstone. N_Co denotes the Co abundance in normal water (20 × 10⁻⁶), while S_Co represents the average Co abundance in the sample (average 15.26 × 10⁻⁶). The symbol t represents the ratio of average La content in the sample to that in terrigenous clastic rocks (22.08 × 10⁻⁶/38.99 × 10⁻⁶). T_Co represents the average abundance of Co in terrigenous clastic rocks (4.68 × 10⁻⁶). The average deposition rate, V_s, of the argillaceous rock samples of the UWF is 475.8 m/Ma. Using the deposition rate results, we calculated an average paleowater depth (h) of 29.4 m during the deposition of argillaceous rocks in the study area’s UWF. Yang H. et al. [105] defined the paleowater depth of shallow lakes as 15–35 m based on paleontology phase zones of lake facies. Analysis of authigenic iron mineral types and trace elements suggests that the UWF in the study area was deposited in a shallow-water environment.

5.5. Paleosalinity

The concentrations of Li, Ni, Ga, and Sr in sedimentary rocks are highly sensitive to the salinity of sedimentary water and are, therefore, used to distinguish the paleosalinity of sedimentary environments [106]. The concentrations of Li, Ga, and Sr in the studied samples are 12.0 × 10⁻⁶–36.0 × 10⁻⁶, 21.2 × 10⁻⁶–75.1 × 10⁻⁶, and 78.6 × 10⁻⁶–262 × 10⁻⁶, respectively, falling within the range of a continental (freshwater) environment. The Ni concentration, ranging from 21.2 × 10⁻⁶ to 75.1 × 10⁻⁶, is slightly lower than in marine (salty water) environments [107], suggesting that the UWF may have been deposited in a continental brackish-to-freshwater environment. In addition, the enrichment of Sr/Ba ratios in water bodies positively correlates with salinity. When Sr/Ba > 1, it indicates a saltwater environment; a Sr/Ba ratio between 0.5 and 1 represents a brackish water environment; and Sr/Ba < 0.5 corresponds to a freshwater environment [108]. The Sr/Ba ratios of the P₃w¹, P₃w², and P₃w³ samples are 0.2–0.47 (average, 0.34), 0.3–0.4 (average, 0.4, with one outlier at 0.6), and 0.2–0.3 (average, 0.3), respectively (Table 4), indicating a freshwater environment. The Rb/K ratio is another effective parameter for determining sedimentary water salinity and distinguishing between marine and continental environments. The Rb/K ratio increases with increasing salinity [109]. In normal marine sediments, the Rb/K ratio exceeds 0.006; in saltwater sediments, it ranges from 0.004 to 0.006; and in freshwater sediments, it is approximately 0.0028 [110]. The Rb/K ratios of the UWF samples range from 0.0016 to 0.0033, averaging 0.0025, consistent with a continental freshwater environment. The majority of samples plotted on the Sr–Ba diagram (Figure 10a) [111] and V–Ba diagram (Figure 10b) [5] reside within the continental freshwater region. The northwestern margin of the Junggar Basin underwent an ocean–continent transition from the Late Carboniferous to the Early Permian, entering the intracontinental evolution stage and transitioning from marine to continental deposits. This reinforces the notion that the UWF in the study area was a continental sedimentary environment during the Late Permian [112]. Consequently, the sedimentary period of the UWF is defined as a continental freshwater environment.

5.6. Redox Conditions

The trace elements sensitive to redox conditions mainly include U, Th, Ni, Co, Cu, and Zn. Their enrichment levels and ratios are reliable indicators of a sedimentary environment’s redox properties [113]. A U/Th ratio less than 0.75 indicates an oxidizing environment; between 0.75 and 1.25 suggests an oxygen-poor environment; and greater than 1.25 corresponds to an anoxic environment [114,115]. The U/Th ratios of the samples studied range from 0.2 to 0.4, with an average of 0.3 (Table 4), indicating that the UWF was deposited in an oxidizing environment during the Late Permian. In addition, the Ni/Co and Cu/Zn ratios are also important parameters for determining the redox properties of water bodies. When Ni/Co is less than five, it indicates an oxidizing environment; when the ratio is between five and seven, it reflects an oxygen-poor environment; and when the ratio is greater than seven, it corresponds to an anoxic environment [116]. Similarly, Cu/Zn > 0.35 indicates an oxidizing environment; Cu/Zn = 0.25–0.35 reflects a weakly reducing environment; and Cu/Zn < 0.21 represents a reducing environment [117]. The Ni/Co ratios of the samples range from 2.0 to 3.2 (2.4 on average), coupled with high Cu/Zn ratios (0.3–0.8; only one sample, 0.1; 0.5 on average) (Table 4), further suggesting that the UWF was deposited in a typical oxidizing environment.

Using discriminant analysis of trace element ratios, the redox environment was further characterized through a Ni/Co–U/Th diagram [118]. The results show that all the data fall into the oxidizing environment area (Figure 11), verifying the reliability of the trace element ratio discrimination results. Therefore, this suggests that the UWF was deposited in an oxidizing environment.

5.7. Tectonic Setting of Provenance Area

The parent rocks with different tectonic settings and provenance areas show different geochemical characteristics after weathering and denudation. Therefore, the tectonic setting of the parent rocks can be defined by the geochemical characteristics of clastic rocks [5,119]. Major elements are susceptible to weathering and subsequent diagenesis, whereas trace elements like La, Sc, Zr, and Ti retain chemical stability throughout the diagenetic process [119]. Therefore, a major–element SiO₂–K₂O/Na₂O diagram [5], supplemented by a trace-element La/Sc–Ti/Zr diagram [119], was used to discriminate the tectonic setting of the provenance area, which can effectively ensure the accuracy of the discriminant result.

Based on an investigation of the correlation between sandstone composition and plate tectonics, Bhatia et al. [119] classified sandstone formation environments into four types: oceanic island arcs (OIAs), continental island arcs (CIAs), active continental margins (ACMs), and passive continental margins (PCMs), utilizing REE and their combination characteristics. In our SiO₂ߝK₂O/Na₂O diagram, the samples fall into the ACM and PCM areas (Figure 12a). In our La/Sc-Ti/Zr diagram, most samples similarly fall into the CIA and nearby areas, and only three samples fall into the OIA area (Figure 12b). In addition, the REE contents and ratios in the studied samples are closer to those in the continental island arc tectonic setting, which are characterized by high ΣREE concentrations and La/Yb ratios, as well as weak negative Eu anomalies (

Table 6. Comparison of geochemical characteristics among different tectonic settings in the provenance area (after [119]).

Tectonic Setting	OIA	CIA	ACM	PCM	Average of UWF
Provenance Area	Uncutting Magmatic Arc	Cutting Magmatic Arc	Upward Basement	Craton Internal Tectonic Highlands
La (×10−6)	8 ± 1.7	27 ± 4.5	37	39	22.1
Ce (×10−6)	19 ± 3.7	59 ± 8.2	78	85	48.3
ΣREE (×10−6)	58 ± 10	146 ± 20	186	210	122
ΣLREE/ΣHREE	3.8 ± 0.9	7.7 ± 1.7	9.1	8.5	6.02
(La/Yb)N	2.8 ± 0.9	7.5 ± 2.5	8.5	8.5	5.49
δEu	1.04 ± 0.11	0.79 ± 0.13	0.6	0.56	0.77

). This indicates that the tectonic setting of the provenance areas of the studied argillaceous rocks is a continental island arc, related to a cutting magmatic arc. Previous studies have analyzed major and trace elements in argillaceous rocks of the UWF in the adjacent Zhongguai area. The results show that the tectonic setting of the provenance areas of the Zhongguai area is a continental island arc [24]. In addition, Yu Y.J. et al. [25] used the Gazzi–Dickinson detrital skeleton composition statistical method to statistically analyze the sandy conglomerate detrital components of the UWF in the Hongche, Zhongguai, Kewu, and Wuxia areas around the study area. The tectonic setting of the provenance area of the Upper Permian UWF has been identified as a continental island arc environment. Therefore, this study shows that the tectonic setting of the UWF argillaceous rocks’ source area is a continental island arc, consistent with a series of Carboniferous to Early Permian magmatic island arc orogenic belts in the northwestern margin of the Junggar Basin [120].

5.8. Provenance Characteristics

The Al₂O₃/TiO₂ ratio was utilized to classify the parent rock types in the source area, with mafic igneous rocks (<14) and felsic igneous rocks (18–26) defined based on the specified ratio range [121]. The samples exhibited Al₂O₃/TiO₂ ratios ranging from 15.2 to 22.6, averaging 19.6 (Table 3), suggesting a primary felsic igneous rock composition. In addition, two P₃w² samples had low Al₂O₃/TiO₂ ratios of 15.2 and 15.8, indicating that there may be a small amount of intermediate igneous rock in their parent rocks. The K₂O/Al₂O₃ ratio can determine the alkaline feldspar content in the parent rocks of provenance areas [55]. A ratio greater than 0.5 indicates a large amount of alkali feldspar in the parent rock. The samples had K₂O/Al₂O₃ ratios ranging from 0.1 to 0.3 (0.2 on average) (Table 3), indicating low-alkali feldspar content in the parent rock, aligning with XRD bulk analysis results. Therefore, we speculate that the parent rock may be acidic granodiorite or dacite, with a small amount of intermediate andesite or diorite. In the La/Th–Hf diagram (Figure 13a) [122], the majority of samples fall within the felsic provenance area of the upper crust, with some in the mixed provenance area with felsic rock and basic rock. In addition, the samples studied exhibit an REE distribution pattern similar to UCC and NASC (Figure 7), suggesting that the UWF originates from upper–crust rocks. The source rocks are primarily felsic, with some basic rock contributions, aligning with the La/Yb–ΣREE discrimination diagram in Figure 13b [123]. The abundance of major elements in the UWF are analogous to those of PAAS and UCC, indicating that the material may be derived from the upper crust; the components are mainly silicates, aluminosilicates, and a small amount of carbonate (Table 3).

Large inland debris lake basins are usually characterized by near-source and multi-directional provenance [124]. The study area is adjacent to the Baogutu tectonic belt (including the Zhayier Mountain and the Hala’alate Mountain) of southern West Junggar. There are two types of large–scale Late Paleozoic intermediate-acid granitic intrusions exposed on both sides of the Darbut Fault. One type is represented by huge granite batholiths, such as Miaoergou, Akebasitao, and Hongshan, while the other consists of quartz diorite porphyry, diorite porphyry, and granodiorite porphyry occurring as small stocks or rock dikes. These are primarily distributed in the Hatu–Baogutu area [125,126,127] and are provenance areas for felsic sediments in the UWF. The Carboniferous rocks in the Baogutu tectonic belt consist of marine clastic–volcanic formations, with the largest exposed area [128]. The Upper Carboniferous Hala’alate and Aladeyikesai Formations mainly comprise intermediate–basic volcanic rocks [129,130], providing source components for the study area. The Upper Carboniferous Chengjisihanshan Formation comprises fine and coarse clastic rocks intercalated with basic volcanic rocks and thick limestone layers [131]. The Lower Carboniferous Baogutu Formation primarily comprises fine clastic rocks with a minor component of basic volcanic rocks [132]. The Xibeikulasi Formation, on the other hand, comprises coarse clastic rocks, serving as the primary sediment source for the study area. The Permian strata, predominantly exposed on the Hala’alate Mountain, encompass the Lower Permian Jiamuhe and Baiyanghe Formations. Their lithologies are characterized by coarse clastic rocks and intermediate–basic volcanic rocks [133]. The Devonian strata are the Upper Devonian Hongshanliang Formation [134,135]. These strata significantly contribute to sediment provenance. In summary, we believe that the main provenance area of the UWF in the Mahu 1 well block is related to the Baogutu tectonic belt (Zhayier Mountain and Hala’alate Mountain). The parent rocks are predominantly acidic igneous rocks, with a minor presence of intermediate–basic igneous rocks and sedimentary rock.

6. Conclusions

Our XRD results reveal that the argillaceous rocks of the UWF mainly comprise feldspar, clay minerals, and quartz, and contain a small amount of hematite. The clay minerals are illite, smectite, kaolinite, chlorite, and I/S mixed layer. The clay mineral assemblages of the UWF are I/S mixed layers, illite, kaolinite, chlorite.

The CIA demonstrated that the argillaceous parent rock experienced moderate–strong chemical weathering. Combining the Sr/Cu, δEu, ΣLREE/ΣHREE values, and clay mineral characteristics, we determined that there was a warm and humid paleoclimate environment, accompanied by short-term dry and cold climates in the northwestern margin of the Junggar Basin during the Late Permian. From the bottom to the top, it gradually entered a relatively humid state with enhanced chemical weathering. The trace element, paleosalinity, and paleowater depth results jointly indicate that the argillaceous rocks of the UWF were deposited in a shallow-water oxidation environment of continental fresh water with weak hydrodynamic conditions.

The tectonic setting of the UWF’s provenance area in the Mahu 1 well block is a continental island arc, associated with a cutting magmatic arc. The main provenance area is related to the Baogutu tectonic belt (the Zhayier Mountain and the Hala’alate Mountain). The parent rocks are mainly acidic igneous rocks, with a small amount of intermediate–basic igneous and sedimentary rock.