Element Geochemical Characteristics and Provenance Conditions of the 1st Member of Jurassic Zhongjiangou Formation in Wudun Sag, Dunhuang Basin

Abstract

In order to clarify the provenance of the 1st member of the Zhongjiangou formation in Wudun sag, Dunhuang basin, the structural attributes, weathering and sedimentary characteristics of the provenance area were analyzed by means of element geochemistry, so as to determine the differences of sediment sources in different well areas. The results show that the higher the Al₂O₃ and K₂O contents, the higher the enrichment of large ion lithophile elements and high field strength elements, while the iron and magnesium elements are relatively deficient, and there are characteristics of medium degree differentiation of light and heavy rare earth elements in Well XC1 and Well D2. The lower the Al₂O₃ content and the higher the SiO₂ content, a loss of large ion lithophile elements and high field strength elements are observed, while the ferrophilic magnesium elements show serious loss, as shown in the characteristics of the high degree of differentiation of light and heavy rare earth elements in Well D1. In the UCC-normalized element spidergrams, the trend of Well XC1 and Well D2 is similar, which is different from that of well D1, indicating that the sediments of Well XC1 and Well D2 come from the same provenance area, while the sediment of Well D1 comes from a different provenance area. The provenance area of Well XC1 and Well D2 shows strong tectonic activity and strong weathering, while the provenance area of well D1 exhibits relatively weak tectonic activity and weathering. Combined with previous research results, Wudun sag is mainly characterized by a faulted lacustrine basin controlled by the southern boundary fault in the Jurassic layer. Therefore, the sediments of Well XC1 and Well D2 mainly come from the southern Sanweishan uplift provenance area, with strong tectonic activity; the sediments of Well D1 mainly come from the northern Beishan provenance area, with relatively weak tectonic activity.

1. Introduction

Wudun sag is located in the middle of the Andun depression in the Dunhuang basin in Western China, with an area of about 2000 km². It is confirmed by oil and gas exploration that the 1st member of Middle Jurassic Zhongjiangou Formation (J₂z₁) is its main oil−bearing series. Good oil and gas display and oil flow are seen in Well XC1 and Well D1, indicating better oil and gas resource potential. Early studies suggested that oil and gas mainly came from the Lower Jurassic Dashankou Formation (J₁d) and was transported upward to the J₂z₁ Formation for accumulation. However, recent studies showed that a collection of high-quality source rocks were also developed in the J₂z₁ Formation [1,2], forming a good reservoir seal assemblage with the reservoirs developed in this section. The study of provenance conditions will play a key role in determining how to find favorable source rocks and reservoir development areas.

However, the degree of exploration of Wudun sag is low. At present, there are only three exploration wells with data and seventeen 2D poor−quality seismic lines, and there are few Jurassic outcrops around Wudun sag. In view of the provenance conditions of the J₂z₁ Formation, our predecessors used drilling and logging data, combined with the development degree of coarse clastic sediments and heavy mineral analysis, and considered that the northern Beishan provenance area and the Southern Sanweishan uplift provenance area have material source contributions to the sediments. This area mainly develops distal braided river delta deposits, dominated by relatively fine−grained sediments in the northern provenance system, and proximal fan delta deposits, dominated by relatively coarse−grained sediments, in the southern provenance system [3,4]. These studies generally suggest that the Zhongjiangou Formation in Wudun sag is developed in a sedimentary environment, with multiple provenance supplies, and the provenance may come from Beishan and Sanweishan. However, these results are limited by drilling and seismic data and cannot accurately reflect the provenance attribution of different well areas. It is necessary to use other methods to further determine the provenance attribution in different well areas, so as to implement the possible provenance conditions.

Clastic sedimentary rocks are an important part of the earth’s material cycle. Their chemical composition characteristics contain information closely related to the evolution of the sedimentary environment. These chemical compositions are mainly controlled by the material composition, weathering and climatic conditions, sedimentary transportation process, diagenesis and other factors of the provenance area [5]. Therefore, in recent years, with the progress of modern analytical side testing technology and the improvement of analysis accuracy, the majority of scholars have begun to analyze provenance conditions by using the element geochemical method as an important tool. The analysis contents mainly include area tectonic background, weathering intensity, material source, sediment maturity and sediment transportation distance from the provenance area [6,7,8,9,10].

In this study, to achieve element geochemical analysis and understanding, the method of element geochemical analysis is introduced to establish the corresponding relationship between clastic sedimentary rocks and source rocks in different well areas of Wudun sag, in order to clarify provenance conditions in the study area and lay a foundation for the further study of source rock and reservoirs in the study area.

2. Geological Settings

Wudun sag is located in the middle of Andun depression in the Dunhuang basin, with the Yumenguan slope in the north, the Sanweishan uplift in the south, Nanhu basin in the West and Tianshuiquan High in the East. It is characterized by a southern fault and a northern uplift in profile. Structurally, it can be divided into three structural units: a northern slope zone, a central depression zone and southern fault zone [3,11] (Figure 1).

The main structural pattern of Wudun sag was initially formed in the Indosinian period, which experienced a fault−depression stage under the background of a near−SN strike slip extension structure in the Yanshan period and a depression stage under the background of overall subsidence in the Himalayan period, forming the current structural pattern [11]. In the Early and Middle Jurassic period, the north−south depression belt of Sanweishan was formed. Affected by the southern boundary fault activity, the fault depression of Wudun sag was obvious. In the late Jurassic, the eastern part of the Dunhuang basin was simultaneously subjected to the left strike slip of the Altun fault and the continuous compression of North Qilian orogenic belt. Many small fault depressions gradually developed into the whole basin depression, and the nature of the Wudun depression gradually changed from fault−depression type to depression type. At the end of the late Cretaceous period, the middle Tethys ocean in the southern Dunhuang basin were closed, and the Lhasa block collided with the Eurasian plate, resulting in the uplift of the Altun orogenic belt, resulting in the Cretaceous sedimentary discontinuity of the Dunhuang basin, the overall uplift of the Jurassic formations and severe denudation; In the Neogene, the Tethys residual sea continued to expand, the Dunhuang basin declined as a whole, and the Wudun sag continued to receive sedimentation [11].

Figure 1. Tectonic location and geological section of Wudun sag.

Figure 1. Tectonic location and geological section of Wudun sag.

According to the analysis of the structural evolution process, Wudun sag is characterized by an obvious faulted lacustrine basin in the early Jurassic period, and the southern boundary fault controls the development and evolution of the sag.

The strata in Wudun sag are underdeveloped. The Sinian (PT), Jurassic (J), Neogene (N) and Quaternary (Q) layers are developed from bottom to top. The Jurassic is the main exploration target, and the Dashankou Formation (J₁d), Zhongjiangou Formation (J₂z), Xinhe Formation (J₂x) and Boluo Formation (J₃b) are also developed. The J₂z₁ formation contains the most active oil and gas display, mainly exhibiting gray sandy conglomerates and sandstone with medium sorting, along with locally developing mudstone. The mudstone color changes from gray to gray black from bottom to top. Well XC1 and Well D1 obtained low oil production flow, while Well D2 encountered good oil and gas display. Previous studies propose that the J₂z₁ Formation develops four main sedimentary facies types: braided river, braided river delta, fan delta and lacustrine [3,4].

3. Sample and Experimental Methods

The research samples used herein are come mainly from three coring wells: XC1, D2 and D1 in Wudun sag. A total of 34 elemental analysis samples are collected, including 11 samples in Well XC1, 13 samples in Well D2 and 10 samples in Well XC1. The major element analysis experiment is carried out in the Analysis and Test Center of the Qingdao Institute of Oceanography. The detection instrument is the S8 tiger, a German Brooke wavelength dispersion X−ray fluorescence spectrometer, which analyzes eight major elements, including Na₂O. The trace element analysis experiment was carried out in the Oil and Gas Exploration and Development Experimental Center of Changjiang University. The detection instrument is a Shimadzu inductively coupled plasma mass spectrometer (ICP−MS−2030), Japan. A total of 33 trace elements, including as Sc, were analyzed.

4. Results

4.1. Characteristics of Major Element

Table 1 shows the major element analysis test results and relevant calculation parameters of 34 core samples from 3 wells in Wudun sag.

4.1.1. Well XC1

The SiO₂ content is 57.88~67.91%, with an average of 64.04%; the Al₂O₃ content is 14.36~20.81%, with an average of 16.96%; the Fe₂O₃ content is 3.51~5.71%, with an average of 4.43%; the K₂O content is 2.77~3.66%, with an average of 3.14%; the Na₂O content is 1.06~1.79%, with an average of 1.43%; the CaO content is 0.21~0.47%, with an average of 0.31%; and the MgO content is 0.84~2.16%, with an average of 1.43%.

4.1.2. Well D2

The SiO₂ content is 55.80~67.13%, with an average of 61.85%; the Al₂O₃ content is 15.15~23.93%, with an average of 19.41%; the Fe₂O₃ content is 2.34~5.16%, with an average of 3.69%; the K₂O content is 3.01~4.29%, with an average of 3.51%; the Na₂O content is 0.77~1.46%, with an average of 1.01%; the content of CaO is 0.29~0.79%, with an average of 0.43%; and the content of MgO is 1.07~2.62%, with an average of 1.91%.

4.1.3. Well D1

The SiO₂ content is 71.78~80.23%, with an average of 75.70%; the Al₂O₃ content is 9.06~12.35%, with an average of 10.96%; the Fe₂O₃ content is 1.00~2.47%, with an average of 2.01%; the K₂O content is 2.28~3.18%, with an average of 2.77%; the Na₂O content is 0.86~1.84%, with an average of 1.35%; the content of CaO is 0.26~0.91%, with an average of 0.41%; the content of MgO is 0.21~0.78%, with an average of 0.53%.

The samples from the three wells have high SiO₂ content, indicating that the main material comes from quartz debris or SiO₂ rich minerals, and the mineral composition maturity is high. The composition maturity of Well D1 is higher than that of Well XC1 and Well D2. Al₂O₃ mainly comes from clay minerals and feldspar. The high content indicates that the study area has a sufficient supply of terrigenous clastic materials. The K₂O/Na₂O ratios of Well XC1, Well D2 and Well D1 are 2.25, 3.71 and 2.22, respectively, indicating that the content of potassium feldspar or potassium−rich minerals in the sedimentary rocks is high. The samples from the three wells show a small amount of MnO and P₂O₅ content, which is inferred to be due to the performance of heavy minerals. Compared with the Upper Continental Crust (UCC), the three wells have lower Na₂O, MgO, CaO and Fe₂O_3, as a whole. In addition, Well XC1 and Well D2 also show higher Al₂O₃ and K₂O contents, while Well D1 shows a lower Al₂O₃ content and a higher SiO₂ content.

Bhatia et al. (1986) proposed that there is a certain correlation between geochemical elements of sandstone, and analyzed the correlation by using the Harker diagram [12]. This study found that the Harker variation diagrams of three wells showed that SiO₂ has an obvious negative correlation with TiO₂, Fe₂O₃, Al₂O₃, MgO and K₂O (Figure 2), indicating that silicate mineral particles, such as quartz, affect the chemical properties of the whole rock. On the ratio plot of (Al₂O₃/SiO₂) and (Fe₂O₃ + MgO), there is an obvious positive correlation (Figure 3), indicating that the increase in ferromanganese content has a certain relationship with the supply of terrigenous clastic materials. The ratio plot of (K₂O/Na₂O) and (Fe₂O₃ + MgO) (Figure 3) shows a weak positive correlation, indicating that the content of potassium feldspar and plagioclase in terrigenous clastic materials has little to do with the increase in ferromagnetic content.

Generally, the change trend of major elements is used to reflect the overall characteristics of mineral composition in rock samples, which can further speculate the differences between sediment sources [13]. The average value of Upper Continental Crust (UCC), the average value of North American Shale Composite (NASC) and the average value of Post Archean Average Shale in Australia (PAAS) are generally used for the standardization of elements. Since the average value of the NASC and PAAS could be affected by the difference in provenance supply and provenance age, the samples in this study mainly use the UCC average value [13], with better universal applicability for standardized treatment and analysis. In the UCC−normalized major element spidergrams, the overall similarity of the trend of sample major element/UCC ratio between Well D2 and Well XC1 is good, and there is a notable difference in the composition of the major element of Well D1, indicating that the provenance of Well D1 is inconsistent with Well D2 and Well XC1 (Figure 4).

Figure 2. Harker variation diagrams for Wudun sag samples.

Figure 2. Harker variation diagrams for Wudun sag samples.

Table 1. Characteristics of major elements of samples from the Zhongjiangou Formation of Wudun sag.

Sample Name	Well Name	Depth /m	Na2O /%	MgO /%	Al2O3 /%	SiO2 /%	K2O /%	CaO /%	Fe2O3 /%	TiO2 /%	MnO /%	P2O3 /%	(Fe2O3 + MgO) /%	SiO2/Al2O3	K2O/Na2O	(CaO + Na2O) /%	CIA	ICV
D01	XC1	2379.75	1.75	1.43	18.45	63.07	3.4	0.29	3.51	0.65	0.03	0.04	4.94	3.42	1.94	2.04	77.23	0.60
D02	XC1	2358.65	1.79	2.14	20.81	56.88	3.66	0.36	5.7	0.72	0.05	0.07	7.84	2.73	2.04	2.15	78.17	0.69
D03	XC1	2352.08	1.74	2.16	18.92	59.14	3.62	0.37	5.71	0.71	0.06	0.05	7.87	3.13	2.08	2.11	76.75	0.76
D04	XC1	2351.38	1.71	1.47	15.13	65.23	2.94	0.47	4.44	0.71	0.06	0.10	5.91	4.31	1.72	2.18	74.72	0.78
D05	XC1	2304.89	1.32	0.84	19.84	62.23	3.42	0.25	3.51	0.71	0.02	0.04	4.35	3.14	2.59	1.57	79.90	0.51
D06	XC1	2294.50	1.2	0.91	15.58	67.25	2.8	0.25	4.41	0.56	0.02	0.03	5.32	4.32	2.33	1.45	78.57	0.65
D07	XC1	2293.24	1.06	1.03	14.78	67.57	2.98	0.23	3.92	0.68	0.02	0.04	4.95	4.57	2.81	1.29	77.59	0.67
D08	XC1	2289.95	1.27	1.29	14.36	67.91	2.77	0.21	4.19	0.66	0.03	0.04	5.48	4.73	2.18	1.48	77.16	0.73
D09	XC1	2288.25	1.31	1.43	15.33	65.94	2.97	0.23	4.22	0.70	0.03	0.04	5.65	4.30	2.27	1.54	77.27	0.71
D10	XC1	2286.11	1.09	1.38	18.15	63.45	3.11	0.41	4.33	0.69	0.04	0.04	5.71	3.50	2.85	1.50	79.75	0.61
D11	XC1	2283.45	1.51	1.62	15.23	65.79	2.84	0.38	4.77	0.71	0.03	0.05	6.39	4.32	1.88	1.89	76.30	0.78
D12	D2	2282.50	1.46	2.35	16.58	63.03	3.41	0.73	4.25	0.74	0.06	0.03	6.60	3.80	2.34	2.19	74.75	0.78
D13	D2	2282.15	0.84	1.07	18.48	65.8	3.04	0.32	2.55	0.64	0.02	0.03	3.62	3.56	3.62	1.16	81.48	0.46
D14	D2	2281.80	0.79	1.78	23.21	57.1	3.57	0.34	4.64	0.74	0.02	0.04	6.42	2.46	4.52	1.13	83.16	0.51
D15	D2	2281.45	0.78	2.19	23.93	55.8	3.7	0.34	4.82	0.73	0.03	0.04	7.01	2.33	4.74	1.12	83.23	0.53
D16	D2	2280.60	1.01	2.62	21.88	57.32	4.02	0.30	4.70	0.74	0.04	0.05	7.32	2.62	3.98	1.31	80.41	0.61
D17	D2	2280.00	0.77	2.02	21.72	58.21	4.29	0.39	4.42	0.73	0.03	0.03	6.44	2.68	5.57	1.16	79.94	0.58
D18	D2	2279.50	1.23	1.66	15.15	67.13	3.04	0.37	3.15	0.65	0.03	0.03	4.81	4.43	2.47	1.60	76.55	0.67
D19	D2	2279.00	0.84	2.25	22.08	57.22	4.19	0.46	5.16	0.74	0.03	0.04	7.41	2.59	4.99	1.30	80.09	0.62
D20	D2	2278.40	0.85	1.66	18.92	64.26	3.47	0.59	2.34	0.56	0.03	0.04	4.00	3.40	4.08	1.44	79.40	0.50
D21	D2	2277.75	0.93	1.81	16.79	65.63	3.01	0.79	2.77	0.57	0.03	0.04	4.58	3.91	3.24	1.72	78.02	0.59
D22	D2	2277.30	0.88	1.52	21.67	60.55	3.69	0.29	3.07	0.71	0.02	0.03	4.59	2.79	4.19	1.17	81.68	0.47
D23	D2	2277.15	1.33	2.01	15.99	65.95	3.18	0.33	3.07	0.54	0.03	0.04	5.08	4.12	2.39	1.66	76.76	0.66
D24	D2	2275.70	1.41	1.95	15.98	65.99	3.05	0.35	2.98	0.53	0.04	0.04	4.93	4.13	2.16	1.76	76.86	0.65
D25	D1	1932.40	0.99	0.78	11.44	76.32	2.88	0.28	2.05	0.41	0.02	0.05	2.83	6.67	2.91	1.27	73.38	0.65
D26	D1	1932.00	1.39	0.47	10.69	78.57	2.66	0.28	2.47	0.38	0.02	0.02	2.94	7.35	1.91	1.67	71.17	0.72
D27	D1	1927.00	1.61	0.73	12.26	71.78	3.18	0.91	2.34	0.37	0.04	0.03	3.07	5.85	1.98	2.52	68.26	0.75
D28	D1	1923.70	1.68	0.21	10.28	80.23	2.56	0.31	1.00	0.40	0.01	0.03	1.21	7.80	1.52	1.99	69.32	0.60
D29	D1	1922.20	1.84	0.31	11.49	76.99	2.87	0.54	1.22	0.39	0.02	0.05	1.53	6.7	1.56	2.38	68.64	0.63
D30	D1	1921.60	0.89	0.54	12.35	73.15	3.07	0.26	2.25	0.39	0.02	0.03	2.79	5.92	3.45	1.15	74.53	0.6
D31	D1	1919.22	0.86	0.61	12.03	73.05	3.02	0.49	2.19	0.38	0.01	0.03	2.80	6.07	3.51	1.35	73.35	0.63
D32	D1	1918.95	1.38	0.58	10.16	75.23	2.66	0.36	2.09	0.33	0.02	0.03	2.67	7.40	1.93	1.74	69.78	0.73
D33	D1	1918.35	1.23	0.49	9.82	75.32	2.47	0.41	2.17	0.34	0.02	0.03	2.66	7.67	2.01	1.64	70.50	0.73
D34	D1	1917.20	1.59	0.55	9.06	76.33	2.28	0.29	2.36	0.40	0.01	0.04	2.91	8.42	1.43	1.88	68.53	0.83

Notes: CIA = 100 × Al₂O₃/(Al₂O₃ + CaO + Na₂O + K₂O); ICV = (Fe₂O₃ + K₂O + Na₂O + CaO + MgO + MnO + TiO₂)/Al₂O_3.

Table 1. Characteristics of major elements of samples from the Zhongjiangou Formation of Wudun sag.

Sample Name	Well Name	Depth /m	Na2O /%	MgO /%	Al2O3 /%	SiO2 /%	K2O /%	CaO /%	Fe2O3 /%	TiO2 /%	MnO /%	P2O3 /%	(Fe2O3 + MgO) /%	SiO2/Al2O3	K2O/Na2O	(CaO + Na2O) /%	CIA	ICV
D01	XC1	2379.75	1.75	1.43	18.45	63.07	3.4	0.29	3.51	0.65	0.03	0.04	4.94	3.42	1.94	2.04	77.23	0.60
D02	XC1	2358.65	1.79	2.14	20.81	56.88	3.66	0.36	5.7	0.72	0.05	0.07	7.84	2.73	2.04	2.15	78.17	0.69
D03	XC1	2352.08	1.74	2.16	18.92	59.14	3.62	0.37	5.71	0.71	0.06	0.05	7.87	3.13	2.08	2.11	76.75	0.76
D04	XC1	2351.38	1.71	1.47	15.13	65.23	2.94	0.47	4.44	0.71	0.06	0.10	5.91	4.31	1.72	2.18	74.72	0.78
D05	XC1	2304.89	1.32	0.84	19.84	62.23	3.42	0.25	3.51	0.71	0.02	0.04	4.35	3.14	2.59	1.57	79.90	0.51
D06	XC1	2294.50	1.2	0.91	15.58	67.25	2.8	0.25	4.41	0.56	0.02	0.03	5.32	4.32	2.33	1.45	78.57	0.65
D07	XC1	2293.24	1.06	1.03	14.78	67.57	2.98	0.23	3.92	0.68	0.02	0.04	4.95	4.57	2.81	1.29	77.59	0.67
D08	XC1	2289.95	1.27	1.29	14.36	67.91	2.77	0.21	4.19	0.66	0.03	0.04	5.48	4.73	2.18	1.48	77.16	0.73
D09	XC1	2288.25	1.31	1.43	15.33	65.94	2.97	0.23	4.22	0.70	0.03	0.04	5.65	4.30	2.27	1.54	77.27	0.71
D10	XC1	2286.11	1.09	1.38	18.15	63.45	3.11	0.41	4.33	0.69	0.04	0.04	5.71	3.50	2.85	1.50	79.75	0.61
D11	XC1	2283.45	1.51	1.62	15.23	65.79	2.84	0.38	4.77	0.71	0.03	0.05	6.39	4.32	1.88	1.89	76.30	0.78
D12	D2	2282.50	1.46	2.35	16.58	63.03	3.41	0.73	4.25	0.74	0.06	0.03	6.60	3.80	2.34	2.19	74.75	0.78
D13	D2	2282.15	0.84	1.07	18.48	65.8	3.04	0.32	2.55	0.64	0.02	0.03	3.62	3.56	3.62	1.16	81.48	0.46
D14	D2	2281.80	0.79	1.78	23.21	57.1	3.57	0.34	4.64	0.74	0.02	0.04	6.42	2.46	4.52	1.13	83.16	0.51
D15	D2	2281.45	0.78	2.19	23.93	55.8	3.7	0.34	4.82	0.73	0.03	0.04	7.01	2.33	4.74	1.12	83.23	0.53
D16	D2	2280.60	1.01	2.62	21.88	57.32	4.02	0.30	4.70	0.74	0.04	0.05	7.32	2.62	3.98	1.31	80.41	0.61
D17	D2	2280.00	0.77	2.02	21.72	58.21	4.29	0.39	4.42	0.73	0.03	0.03	6.44	2.68	5.57	1.16	79.94	0.58
D18	D2	2279.50	1.23	1.66	15.15	67.13	3.04	0.37	3.15	0.65	0.03	0.03	4.81	4.43	2.47	1.60	76.55	0.67
D19	D2	2279.00	0.84	2.25	22.08	57.22	4.19	0.46	5.16	0.74	0.03	0.04	7.41	2.59	4.99	1.30	80.09	0.62
D20	D2	2278.40	0.85	1.66	18.92	64.26	3.47	0.59	2.34	0.56	0.03	0.04	4.00	3.40	4.08	1.44	79.40	0.50
D21	D2	2277.75	0.93	1.81	16.79	65.63	3.01	0.79	2.77	0.57	0.03	0.04	4.58	3.91	3.24	1.72	78.02	0.59
D22	D2	2277.30	0.88	1.52	21.67	60.55	3.69	0.29	3.07	0.71	0.02	0.03	4.59	2.79	4.19	1.17	81.68	0.47
D23	D2	2277.15	1.33	2.01	15.99	65.95	3.18	0.33	3.07	0.54	0.03	0.04	5.08	4.12	2.39	1.66	76.76	0.66
D24	D2	2275.70	1.41	1.95	15.98	65.99	3.05	0.35	2.98	0.53	0.04	0.04	4.93	4.13	2.16	1.76	76.86	0.65
D25	D1	1932.40	0.99	0.78	11.44	76.32	2.88	0.28	2.05	0.41	0.02	0.05	2.83	6.67	2.91	1.27	73.38	0.65
D26	D1	1932.00	1.39	0.47	10.69	78.57	2.66	0.28	2.47	0.38	0.02	0.02	2.94	7.35	1.91	1.67	71.17	0.72
D27	D1	1927.00	1.61	0.73	12.26	71.78	3.18	0.91	2.34	0.37	0.04	0.03	3.07	5.85	1.98	2.52	68.26	0.75
D28	D1	1923.70	1.68	0.21	10.28	80.23	2.56	0.31	1.00	0.40	0.01	0.03	1.21	7.80	1.52	1.99	69.32	0.60
D29	D1	1922.20	1.84	0.31	11.49	76.99	2.87	0.54	1.22	0.39	0.02	0.05	1.53	6.7	1.56	2.38	68.64	0.63
D30	D1	1921.60	0.89	0.54	12.35	73.15	3.07	0.26	2.25	0.39	0.02	0.03	2.79	5.92	3.45	1.15	74.53	0.6
D31	D1	1919.22	0.86	0.61	12.03	73.05	3.02	0.49	2.19	0.38	0.01	0.03	2.80	6.07	3.51	1.35	73.35	0.63
D32	D1	1918.95	1.38	0.58	10.16	75.23	2.66	0.36	2.09	0.33	0.02	0.03	2.67	7.40	1.93	1.74	69.78	0.73
D33	D1	1918.35	1.23	0.49	9.82	75.32	2.47	0.41	2.17	0.34	0.02	0.03	2.66	7.67	2.01	1.64	70.50	0.73
D34	D1	1917.20	1.59	0.55	9.06	76.33	2.28	0.29	2.36	0.40	0.01	0.04	2.91	8.42	1.43	1.88	68.53	0.83

Notes: CIA = 100 × Al₂O₃/(Al₂O₃ + CaO + Na₂O + K₂O); ICV = (Fe₂O₃ + K₂O + Na₂O + CaO + MgO + MnO + TiO₂)/Al₂O_3.

Figure 3. Plot of Al₂O₃/SiO₂ and K₂O/Na₂O to Fe₂O₃ + MgO content.

Figure 3. Plot of Al₂O₃/SiO₂ and K₂O/Na₂O to Fe₂O₃ + MgO content.

Figure 4. UCC−normalized major element spidergrams of the samples.

Figure 4. UCC−normalized major element spidergrams of the samples.

4.2. Characteristics of Trace Elements

Table 2. Characteristics of trace elements of samples from the J₂z₁ Formation of Wudun sag.

Sample	D01	D02	D03	D04	D05	D06	D07	D08	D09	D10	D11	D12
Well name	XC1	XC1	XC1	XC1	XC1	XC1	XC1	XC1	XC1	XC1	XC1	D2
Depth/m	2379.8	2358.7	2352.1	2351.4	2304.9	2294.5	2293.2	2290	2288.3	2286.1	2283.5	2282.5
Ti/10−6	3892.6	4321.7	4271.6	4279.4	4256.7	3342.2	4052.1	3931.2	4219.4	4112.7	4286.4	4434
Mn/10−6	219.6	421.9	444	483.9	119	164.5	187.3	204.3	245.3	301.8	198.5	433
Rb/10−6	92.3	102.9	100.1	82.6	58.6	75.6	89	92.2	101.9	95.2	88.9	124.1
V/10−6	60.9	112.7	101.7	93.8	106.8	60.4	72.1	87.6	98.3	77.8	90.3	79
Cr/10−6	42.5	73.6	62.4	63.9	73.4	38.6	47.8	50.5	53.9	59.4	61.9	58.3
Co/10−6	10	12.2	12.5	14	10.4	10	11.8	12.3	13.2	11.3	12.1	16.1
Ni/10−6	13.6	19	19.7	20.6	11.4	16.8	22.1	19.6	21.1	17.9	20.5	24
Ga/10−6	18.4	28.8	24	16.9	19.2	17.3	18.5	19.3	22.7	20.9	23.1	20.7
Sr/10−6	118	116	129	121	73	73	102	107	110	109	116	148
Ba/10−6	783	748	756	613	478	458	649	718	625	688	744	608
Pb/10−6	12.3	17.8	16	14.8	17	12.9	13.1	16.9	14.5	16.2	15.8	22.8
Th/10−6	6.7	13.1	11.9	11.7	10.6	7.1	8.5	9.3	11.2	7.9	12.6	12.1
U/10−6	2.6	3	3	2.8	2.1	2.4	2.8	2.5	3.1	3.4	2.7	3.1
Zr/10−6	177	369	279	453	373	186	321	308	298	311	356	267
Hf/10−6	4.9	10.6	7.6	14.6	9.5	5	7.2	8.5	11.3	8.9	12.1	7.5
Nb/10−6	16.3	33.8	24.6	30.2	25.5	16.7	19.7	22.8	21.9	28.1	26.5	19.5
Sc/10−6	10.4	15.7	16.6	12.3	13.7	9.6	13.6	11.9	12.3	13.3	15.8	16.9
Cu/10−6	13.1	18	12.1	13.6	13.4	10.8	14.5	12.9	13.3	13.8	11.1	20.9
Zn/10−6	56	79.7	75.3	56.6	51.6	45.7	69.8	70.9	62.9	58.9	50.2	76.7
As/10−6	3.3	0	0	0.3	4.6	1.1	2.1	1.3	1.1	0	1.3	4.2
Br/10−6	0.3	0.6	0.2	0.5	0.5	0.5	0.4	0.2	0.5	0.3	0.3	0.8
Mo/10−6	1.5	1.5	1.4	1.6	1.9	1.8	1.5	1.7	1.3	1.3	1.6	0.9
Sn/10−6	4.3	2.7	3	1.4	3.3	1.6	2.2	1.9	1.5	1.1	3.1	5.4
Sb/10−6	3.6	0.9	2.6	3.3	0.2	0.8	3.2	2.6	2.1	0.9	2.2	5.1
W/10−6	20.6	21.6	21.1	27.3	18.7	15.9	22.1	25.1	27.1	22.9	19.6	11.7
Bi/10−6	0.4	0.2	0	1.3	0.8	0	0.6	0.7	0.5	0	0.3	1.2
Y/10−6	15.9	28.8	22.9	30.2	18.4	16.5	20.6	23.3	19.6	25.1	29.3	23.5
La/10−6	29.9	48.5	56.8	37	37.4	38	47.4	39.7	49.7	37.3	48.1	37.5
Ce/10−6	64.1	96.8	96.7	65.2	45.3	66.5	78.4	81.3	79.8	91.3	89.2	80
Pr/10−6	9.9	11.7	12.1	8.6	8.4	8.2	8.9	9.1	9.1	10.3	10.1	10.2
Nd/10−6	28.9	41.1	45.4	31.1	29.6	27.5	33.9	37.4	39.4	40.2	36.2	29.9
Sm/10−6	7.4	7.3	7.7	5.9	6.6	5.1	6.1	6.3	7.2	8.1	7.5	8.1
Eu/10−6	1.3	1.7	1.8	1.2	2.2	1.1	1.8	1.9	2.1	1.6	1.9	1.6
Gd/10−6	5.1	6.4	6.7	5.1	5.2	4.2	5.7	6.2	5.9	5.4	6.2	5.3
Tb/10−6	0.8	1	1.2	0.8	0.8	0.7	0.9	1.1	1.2	1	1.2	0.9
Dy/10−6	4	6.3	6.5	4.7	3.6	3.8	4.1	5.5	4.9	6.2	5.8	4.9
Ho/10−6	0.7	1.3	1.4	0.9	0.8	0.8	0.9	0.8	1.3	1.2	0.9	0.9
Er/10−6	1.9	3.6	3.8	2.5	1.9	2.1	1.9	1.8	2.2	2.6	2.9	2.3
Tm/10−6	0.3	0.6	0.7	0.4	0.4	0.4	0.5	0.7	0.6	0.4	0.5	0.5
Yb/10−6	1.9	3.8	4.1	2.6	1.9	2.5	2.3	2.6	3.1	1.7	3.7	2.3
Lu/10−6	0.3	0.5	0.6	0.3	0.4	0.4	0.4	0.4	0.5	0.4	0.5	0.5
Th/U	2.6	4.4	4	4.2	5	3	3	3.7	3.6	2.3	4.7	3.9
La/Sc	2.9	3.1	3.4	3	2.7	4	3.5	3.3	4	2.8	3	2.2
Rb/Sr	0.8	0.9	0.8	0.7	0.8	1	0.9	0.9	0.9	0.9	0.8	0.8
Ti/Zr	22	11.7	15.3	7.7	11.4	18	12.6	12.8	14.2	13.2	12	16.6
La/Th	4.5	3.7	4.8	3.2	3.5	5.4	5.6	4.3	4.4	4.7	3.8	3.1
La/Y	1.9	1.7	2.5	1.2	2	2.3	2.3	1.7	2.5	1.5	1.6	1.6
Sc/Cr	0.2	0.2	0.3	0.2	0.2	0.2	0.3	0.2	0.2	0.2	0.3	0.3
Th/Sc	0.6	0.8	0.7	1	0.8	0.7	0.6	0.8	0.9	0.6	0.8	0.7
Zr/Sc	17	23.5	16.8	45	27.2	19.4	23.6	25.9	24.2	23.4	22.5	15.8
Co/Th	1.5	0.9	1.1	1.2	1	1.4	1.4	1.3	1.2	1.4	1	1.3
Th/Co	0.7	1.1	1	0.8	1	0.7	0.7	0.8	0.8	0.7	1	0.8
∑REE/10−6	156.5	230.4	245.5	166.3	144.4	161.2	193.2	194.8	207	207.7	214.7	184.9
∑LREE/10−6	141.5	207	220.5	149	129.5	146.4	176.5	175.7	187.3	188.8	193	167.3
∑HREE/10−6	15	23.4	25	17.3	14.9	14.8	16.7	19.1	19.7	18.9	21.7	17.6
δEu	0.64	0.76	0.77	0.67	0.96	0.7	0.93	0.93	0.99	0.74	0.85	0.75
Sample	D13	D14	D15	D18	D17	D20	D19	D21	D22	D16	D23
Well name	D2	D2	D2	D2	D2	D2	D2	D2	D2	D2	D2
Depth/m	2282.2	2281.8	2281.5	2280.6	2280	2279.5	2279	2278.4	2277.8	2277.3	2277.2
Ti/10−6	3838	4459.1	4377.7	4443	4395.1	3889.3	4448.9	3378.3	3409.2	4232.9	3258.2
Mn/10−6	142.2	185.7	234.8	338.7	217.6	264.6	249.8	226.3	196.9	168.9	214.6
Rb/10−6	101.1	162.3	173.7	158.1	164.8	117.3	170.8	131.8	122.9	113.7	105.9
V/10−6	63	112	118.3	93.8	88.9	103.4	117	98.2	85.3	77	91.3
Cr/10−6	41.2	72.7	78.3	67.8	65.4	53.6	77	59.8	61.3	47.5	68.2
Co/10−6	11.7	15.1	15.5	16	11.2	12.7	10.6	13.5	11.9	9.1	10.9
Ni/10−6	11.9	27.9	29.6	29.2	16.8	13.7	21	15.6	19.9	9.5	21.4
Ga/10−6	20.3	29	31.4	28.6	24.7	22.8	27.2	27.6	24.5	21.7	23.9
Sr/10−6	94	75	83	128	110	102	104	109	99	95	105
Ba/10−6	802	681	757	858	797	799	759	714	879	687	852
Pb/10−6	14	15.7	17.8	21.3	16.8	15.8	28	17.1	19.2	12.3	16.3
Th/10−6	8.9	18.6	18.1	13.6	13.1	10.3	16.9	9.8	14.7	11.4	12.9
U/10−6	2.3	5.4	4.8	4.1	4.1	2.1	4.5	3.5	2.8	3.3	3.2
Zr/10−6	275	289	263	450	449	256	402	229	297	448	268
Hf/10−6	7.7	8	7	12.3	11.6	9.1	10.8	10.1	8.9	11.8	11.2
Nb/10−6	22.7	36.9	32.7	40.1	28.1	33.8	55.3	48.9	29.5	30.7	39.1
Sc/10−6	8.4	14.3	17.4	15.5	16.5	15.2	13.6	11.8	12.6	13.6	12.9
Cu/10−6	13.7	27.9	32	14.9	20.1	28.6	77.7	30.1	29.5	14.5	22.9
Zn/10−6	39.7	70.3	95.9	98.3	76.9	91.3	92.9	86.5	77.9	53.6	97.1
As/10−6	0.3	3.4	0	0	0	1.3	0	1.9	0	0	2.5
Br/10−6	0.4	0.2	0.3	0.5	0.5	0.5	0.5	0.3	0.4	0.3	0.6
Mo/10−6	3	3.7	1.3	0.4	0.5	2.2	0.8	1.9	2.1	0.8	1.7
Sn/10−6	3.2	12.5	13.5	1.7	3.6	9.8	4.3	11.5	10.6	4.4	9.9
Sb/10−6	5.2	12.6	17.2	0	0.8	15.8	3	7.1	9.7	3	5.2
W/10−6	13.9	6.2	3.7	16.3	21.9	19.8	11.8	18.4	20.1	25.8	17.9
Bi/10−6	0	0	0.7	0.4	0	0.8	0	0.6	0	0	0.6
Y/10−6	20.7	33.7	32.6	35.5	31.5	29.5	55.6	30.8	35.1	20	31.8
La/10−6	37	57.9	53.5	67.2	55.2	54.1	56.7	46.9	50.6	41.7	49.3
Ce/10−6	55	112.7	111.6	143.1	109.1	112.9	88.1	93.4	99.9	74.7	111.4
Pr/10−6	11.3	11.6	10.9	16.9	12.8	10.7	13.3	12.4	11.7	10.7	10.3
Nd/10−6	32	49.7	44.2	61.3	39.3	33.8	44.9	37.7	40.1	28.9	32.9
Sm/10−6	7.9	7.7	8.3	10.5	7.8	7.2	8.2	7.5	8.4	6.3	7.5
Eu/10−6	1.3	1.7	1.4	1.9	1.5	1.5	1.4	1.8	1.9	1.4	1.7
Gd/10−6	5.5	6.2	7.9	8.8	6.6	7.2	6.5	7.4	6.6	5.2	6.2
Tb/10−6	1.1	1.3	1.4	1.5	1.2	0.9	1.1	0.9	1.2	0.8	1.1
Dy/10−6	6.1	5.5	5.3	8.6	6.9	6.2	6	7.1	6.1	4.5	5.3
Ho/10−6	1.1	1.4	1.2	1.7	1.3	0.8	1.2	1.5	1.1	0.8	1.4
Er/10−6	3.5	2.8	2.9	4.4	3.5	3.1	3	3.5	2.7	2.2	2.5
Tm/10−6	0.3	0.6	0.5	0.7	0.6	0.6	0.5	0.4	0.7	0.3	0.6
Yb/10−6	3.1	2.7	2.5	4.7	3.8	3.3	3	2.8	2.9	2.2	3.2
Lu/10−6	0.3	0.5	0.4	0.6	0.5	0.6	0.4	0.4	0.5	0.3	0.4
Th/U	3.9	3.4	3.8	3.3	3.2	4.9	3.8	2.8	5.3	3.5	4
La/Sc	4.4	4	3.1	4.3	3.3	3.6	4.2	4	4	3.1	3.8
Rb/Sr	1.1	2.2	2.1	1.2	1.5	1.2	1.6	1.2	1.2	1.2	1
Ti/Zr	14	15.4	16.6	9.9	9.8	15.2	11.1	14.8	11.5	9.4	12.2
La/Th	4.2	3.1	3	4.9	4.2	5.3	3.4	4.8	3.4	3.7	3.8
La/Y	1.8	1.7	1.6	1.9	1.8	1.8	1	1.5	1.4	2.1	1.6
Sc/Cr	0.2	0.2	0.2	0.2	0.3	0.3	0.2	0.2	0.2	0.3	0.2
Th/Sc	1.1	1.3	1	0.9	0.8	0.7	1.2	0.8	1.2	0.8	1
Zr/Sc	32.7	20.2	15.1	29	27.2	16.8	29.6	19.4	23.6	32.9	20.8
Co/Th	1.3	0.8	0.9	1.2	0.9	1.2	0.6	1.4	0.8	0.8	0.8
Th/Co	0.8	1.2	1.2	0.9	1.2	0.8	1.6	0.7	1.2	1.3	1.2
∑REE/10−6	165.5	262.3	252	331.8	250	242.9	234.2	223.7	234.4	180	233.8
∑LREE/10−6	144.5	241.3	229.9	300.8	225.7	220.2	212.6	199.7	212.6	163.6	213.1
∑HREE/10−6	21	21	22.1	31	24.3	22.7	21.7	24	21.8	16.4	20.7
δEu	0.6	0.75	0.53	0.6	0.64	0.64	0.58	0.74	0.78	0.72	0.76
Sample	D24	D25	D26	D27	D28	D29	D30	D31	D32	D33	D34	UCC
Well name	D2	D1	D1	D1	D1	D1	D1	D1	D1	D1	D1
Depth/m	2275.7	1932.4	1932	1927	1923.7	1922.2	1921.6	1919.2	1919	1918.4	1917.2
Ti/10−6	3201.7	2448.9	2289.6	2190.7	2382.9	2313.1	2356.9	2288.4	1992.8	2059.3	2378.4	3765
Mn/10−6	272.1	127.3	152.7	306.4	96.3	141.4	119.4	98.1	153.2	132.9	102.7	-
Rb/10−6	115.6	64.5	55.3	60.5	43.2	46.9	49.5	51.3	55.8	53.7	59.5	82
V/10−6	83.9	30.5	22.6	35	22.8	20.6	24.1	26.8	29.4	27.3	22.1	97
Cr/10−6	70.5	17.3	13.3	21.9	11.2	15.4	11.8	16.2	15.5	13.1	15.3	92
Co/10−6	14.2	9.9	8	8.3	5.1	6.7	6.1	7.8	7.2	5.9	8.8	17
Ni/10−6	20.8	4.4	5.3	9.8	3.3	4.7	6.3	3.9	8.2	7.1	5.8	47
Ga/10−6	27.8	11.6	10.8	12.6	8.6	8.8	9.3	9.9	8.9	11.1	10.4
Sr/10−6	107	82	96	125	90	100	92	98	103	89	91	320
Ba/10−6	904	519	471	665	523	553	593	652	637	661	489	628
Pb/10−6	17.9	10	14.7	10.4	9.7	11.2	11	10.7	10.5	9.9	10.4	17
Th/10−6	15.1	2.6	2.6	5	3.2	3.6	3.1	4.2	2.8	3.5	3.7	10.5
U/10−6	2.6	1.7	1.6	2.6	1.2	1.4	1.3	1.7	1.5	1.3	1.8	2.7
Zr/10−6	241	185	202	220	238	229	261	274	241	187	258	193
Hf/10−6	10.3	4.7	4.9	4.3	6.2	6.2	4.1	3.9	4.4	4.5	4.2	5.3
Nb/10−6	36.8	14.8	10	13.6	14.6	13.5	13.8	14.1	14.5	13.3	13.7	12
Sc/10−6	14.2	4.4	5.5	7.8	3.5	4.7	3.1	5.1	4.5	3.8	3.6	14
Cu/10−6	27.7	4.4	6.6	9.2	5	6.1	7.8	8.1	9.1	8.9	9.1	28
Zn/10−6	90.5	22.8	21.8	28.1	11.7	15.8	24.8	23.4	25.1	27.6	28.1	67
As/10−6	1.9	0	0	3.4	5.7	3.4	4.1	3.9	4.6	5.3	3.8	-
Br/10−6	0.4	0.5	0.6	0.4	0.7	0.9	0.8	0.6	0.7	0.8	0.7	-
Mo/10−6	1.5	2	1.9	1.7	2.7	2.3	2.2	2.7	2.4	2.6	2.5	-
Sn/10−6	10.7	2	1.5	3.3	1.6	1.2	1.5	2.2	2.8	3.2	2.9	-
Sb/10−6	13.4	0	3.6	4	0.6	0	3.8	3.1	3.5	3.3	3.1	-
W/10−6	19.8	39.5	23.3	23.3	22.6	35	28.7	30.9	32.1	22.9	30.1	-
Bi/10−6	0.5	0.5	1.4	0.4	0.7	0.5	1.3	1.4	1.3	1.1	0.9	-
Y/10−6	22.9	8.7	10.1	15.6	12.7	12.7	9.6	13.5	10.8	11.2	9.9	21
La/10−6	52.8	19.8	25.2	42.5	19.3	30	22.3	25.1	27.8	18.9	24.1	31
Ce/10−6	109.5	27.2	38.5	42.1	24.7	26.5	29.8	31.4	33.7	39.1	32.5	63
Pr/10−6	11.2	3.8	5	9.6	3.6	6.4	5.9	4.8	5.5	7.1	5.8	7.1
Nd/10−6	42.1	16.9	19.1	34	14.7	24.3	28.9	26.5	32.8	41.1	23.8	27
Sm/10−6	9.1	2.1	3.6	6.8	2.1	4.1	3.8	5	4.7	3.9	5.5	4.7
Eu/10−6	1.6	1.2	0.8	2.2	1.2	0.9	0.9	1.3	1.1	0.8	1.1	1
Gd/10−6	7.7	1.6	3	5.2	1.6	2.8	1.9	2.4	3.1	2.9	1.7	4
Tb/10−6	0.9	0.2	0.5	0.7	0.3	0.5	0.4	0.5	0.6	0.4	0.7	0.7
Dy/10−6	5.5	0.8	2.8	3.6	1.1	2.2	1.6	1.9	2.5	2.9	2.6	3.9
Ho/10−6	1.6	0.2	0.6	0.7	0.2	0.4	0.3	0.5	0.4	0.6	0.4	0.8
Er/10−6	2.9	0.4	1.5	1.7	0.4	0.7	0.9	1.1	0.7	1.3	0.8	2.3
Tm/10−6	0.6	0.1	0.2	0.3	0.1	0.2	0.2	0.1	0.2	0.3	0.1	0.3
Yb/10−6	2.5	0.4	1.5	1.6	0.4	1.1	0.8	0.5	1	1.2	0.9	2
Lu/10−6	0.5	0.1	0.2	0.2	0.1	0.2	0.2	0.1	0.2	0.1	0.1	0.3
Th/U	5.8	1.5	1.6	1.9	2.7	2.6	2.4	2.5	1.9	2.7	2.1	-
La/Sc	3.7	4.5	4.6	5.4	5.5	6.4	7.2	4.9	6.2	5	6.7	-
Rb/Sr	1.1	0.8	0.6	0.5	0.5	0.5	0.5	0.5	0.5	0.6	0.7	-
Ti/Zr	13.3	13.2	11.3	10	10	10.1	9	8.4	8.3	11	9.2	-
La/Th	3.5	7.6	9.7	8.5	6	8.3	7.2	6	9.9	5.4	6.5	-
La/Y	2.3	2.3	2.5	2.7	1.5	2.4	2.3	1.9	2.6	1.7	2.4	-
Sc/Cr	0.2	0.3	0.4	0.4	0.3	0.3	0.3	0.3	0.3	0.3	0.2	-
Th/Sc	1.1	0.6	0.5	0.6	0.9	0.8	1	0.8	0.6	0.9	1	-
Zr/Sc	17	42	36.7	28.2	68	48.7	84.2	53.7	53.6	49.2	71.7	-
Co/Th	0.9	3.8	3.1	1.7	1.6	1.9	2	1.9	2.6	1.7	2.4	-
Th/Co	1.1	0.3	0.3	0.6	0.6	0.5	0.5	0.5	0.4	0.6	0.4	-
∑REE/10−6	248.5	74.6	102.5	151.2	69.7	100.3	97.9	101.2	114.3	120.6	100.1	-
∑LREE/10−6	226.3	70.9	92.2	137.2	65.6	92.2	91.6	94.1	105.6	110.9	92.8	-
∑HREE/10−6	22.2	3.7	10.2	14	4.1	8.1	6.3	7.1	8.7	9.7	7.3	-
δEu	0.58	1.94	1.13	1.15	2	1.05	1.02	1.15	1.05	1.08	1.1	-

Note: - means not detected.

shows the analysis results of the trace elements and relevant calculation parameters. It can be seen that the content of the trace elements in the J₂z₁ Formation in Wudun sag changes greatly.

4.2.1. Well XC1

The average contents of large ion lithophile elements (LILE) such as Rb, Sr, Pb and Ba are 89.0 × 10⁻⁶, 106.7 × 10⁻⁶, 15.2 × 10⁻⁶ and 660.0 × 10⁻⁶, respectively, slightly higher than the average value of UCC, except for Sr. The average contents of incompatible high field−strength elements (HFSE) such as Nb, Hf, Zr, Th, U and Y are 24.2 × 10⁻⁶, 9.1 × 10⁻⁶, 311.9 × 10⁻⁶, 10.1 × 10⁻⁶, 2.8 × 10⁻⁶ and 22.8 × 10⁻⁶, respectively, slightly higher than the average value of UCC. The average contents of ferrophilic magnesium mineral elements such as Cr, Co, Sc, Ni and V are 57.1 × 10⁻⁶, 11.8 × 10⁻⁶, 13.2 × 10⁻⁶, 18.4 × 10⁻⁶ and 87.5 × 10⁻⁶, respectively, lower than the average value of UCC.

4.2.2. Well D2

The average contents of LILE such as Rb, Sr, Pb and Ba are 135.5 × 10⁻⁶, 104.5 × 10⁻⁶, 18.1 × 10⁻⁶ and 776.7 × 10⁻⁶, respectively, higher than the average value of UCC, except for Sr. The average contents of HFSE such as Nb, Hf, Zr, Th, U and Y are 34.9 × 10⁻⁶, 9.7 × 10⁻⁶, 318.0 × 10⁻⁶, 13.5 × 10⁻⁶, 3.5 × 10⁻⁶ and 31.0 × 10⁻⁶, respectively, which are higher than the average value of UCC. The average contents of ferrophilic magnesium mineral elements such as Cr, Co, Sc, Ni and V are 63.2 × 10⁻⁶, 13.0 × 10⁻⁶, 14.0 × 10⁻⁶, 20.1 × 10⁻⁶ and 93.2 × 10⁻⁶, respectively, slightly lower than the average value of UCC.

4.2.3. Well D1

The average contents of LILE such as Rb, Sr, Pb and Ba are 54.0 × 10⁻⁶, 96.9 × 10⁻⁶, 10.9 × 10⁻⁶ and 576.3 × 10⁻⁶, respectively, lower than the average value of UCC. The average contents of HFSE such as Nb, Hf, Zr, Th, U and Y are 13.6 × 10⁻⁶, 4.7 × 10⁻⁶, 229.5 × 10⁻⁶, 3.4 × 10⁻⁶, 1.6 × 10⁻⁶ and 11.5 × 10⁻⁶, respectively, lower than the average value of UCC, except Nb. The average contents of ferrophilic magnesium mineral elements such as Cr, Co, Sc, Ni and V are 15.1 × 10⁻⁶, 7.4 × 10⁻⁶, 4.6 × 10⁻⁶, 5.9 × 10⁻⁶ and 26.1 × 10⁻⁶, respectively, much lower than the average value of UCC.

Therefore, the trace element contents of Well XC1 and Well D2 show the characteristics of the enrichment of large ion lithophile elements and high field strength elements and the relative loss of iron and magnesium elements, while the trace element content of Well D1 shows the characteristics of loss of large ion lithophile elements and high field strength elements, as well as the serious loss of iron and magnesium elements.

In the UCC−normalized trace element spidergrams, the trend of the trace element/UCC ratio from Well D2 and Well XC1 is relatively consistent, and there is a certain difference with Well D1, which is consistent with the analysis results of the major elements, indicating that the provenance of Well D1 is different from that of Well D2 and Well XC1 (Figure 5).

4.3. Characteristics of Rare Earth Elements

The analysis and test results of rare earth elements and relevant calculation parameters are shown in Table 2. It can be seen that the total amount of rare earth elements (ΣREE) of the three wells in the J₂z₁ Formation in Wudun sag varies greatly: 144.4 × 10⁻⁶~245.5 × 10⁻⁶, with an average of 192.9 × 10⁻⁶ for Well XC1, 165.5 × 10⁻⁶~331.8 × 10⁻⁶, with an average of 234.2 × 10⁻⁶ for Well D2, and 69.7 × 10⁻⁶~151.2 × 10⁻⁶, with an average of 103.2 × 10⁻⁶ for Well D1. The ratio of LREE/HREE is 6.9~19.4, of which Well XC1 is 8.6~10.6, with an average of 9.3, Well D2 is 6.9~11.5, with an average of 9.6, and Well D1 is 9.0~19.4, with an average of 13.0. Well XC1 and Well D2 show moderate differentiation characteristics of light and heavy rare earth elements, while Well D1 shows high differentiation characteristics of light and heavy rare earth elements.

In the UCC−normalized rare earth element spidergrams, Well D2 and Well XC1 show relatively consistent distribution characteristics, showing obvious “V” right dip characteristics of light rare earth element enrichment and heavy rare earth element flatness, with Eu as a negative anomaly. Well D1 shows the characteristics of light rare earth element enrichment, heavy rare earth element loss and slight positive Eu anomaly. It reflects that Well D1 is different from Well D2 and Well XC1 (Figure 6), which is consistent with the analysis results of major and trace elements.

5. Discussions

The geochemical composition of terrigenous clastic sediments is often controlled by many factors, such as tectonic environment, the nature of the source rock, weathering, transportation and sedimentation, diagenesis and metamorphism in the provenance area [14]. Therefore, the geochemical characteristics of terrigenous clastic sediments can be used to reflect the tectonic environment background, weathering degree and composition of the source rack, sediment transport distance and maturity, etc. The differences between provenance areas can be distinguished below:

5.1. Analysis of Tectonic Background in Provenance Area

Because the tectonic environment can not only control the material source of terrigenous clastic sediments, but also affect the differentiation degree of elements in the process of weathering, denudation, transportation and deposition, different tectonic environments correspond to different element geochemical characteristics. Bhatia (1986) established a series of discriminant plates for distinguishing the tectonic provenance by using the content of Fe₂O₃ + MgO, TiO₂, Al₂O₃, the ratio of K₂O/Na₂O, SiO₂/Al₂O₃ and other parameters. The sedimentary tectonic provenance of sandstone could be divided into four types: passive continental margin (PM), active continental margin (ACM), continental island arc (CIA) and oceanic island arc (OIA) [12], and the intensity of tectonic activity gradually increased with each type. Wudun sag is a typical continental faulted lacustrine basin in the Jurassic period [11], which is different from the four kinds of sandstone sedimentary tectonic provenance established by Bhatia. However, this study restores the structural attributes of the provenance area and tracks the material source of sediments by using the strength of the intensity of the tectonic activity of different tectonic backgrounds and the tectonic evolution background.

With the increase in the intensity of tectonic activity in the basin, the content of the major elements Fe₂O₃ + MgO and TiO₂ increases, while the ratios of SiO₂/Al₂O₃ and K₂O/Na₂O decrease in the terrigenous clastic sediments [12]. Additionally, the content of trace elements such as La, Th, SC, Zr and Ti in clastic sedimentary rocks can also be used as the basis for distinguishing the tectonic environment background, due to their strong stability [15]. With the increase in tectonic activity intensity, the ratio of La/Sc decreased and the ratio of Ti/Zr increased. In the TiO₂−(Fe₂O₃ + MgO) plot (Figure 7a), K₂O/Na₂O−SiO₂ plot (Figure 7b), SiO₂/Al₂O₃−K₂O/Na₂O plot (Figure 7c) and Ti/Zr−La/Sc plot (Figure 7d), the samples of Well D1 mainly fall in the area of the passive continental margin and the active continental margin, while the samples of Well XC1 and Well D2 mainly fall in the area of the continental island arc, indicating the tectonic activity of the provenance area of Well XC1 and Well D2 is stronger than that of Well D1.

In the early and Middle Jurassic period, Wudun sag shown the characteristics of a faulted lacustrine basin affected by the fault activity at the southern boundary [11]. The provenance area of southern Sanwei Mountain is located in the steep slope zone of the faulted lacustrine basin, with strong tectonic activity. The provenance area of northern Beishan is located in the gentle slope zone of the faulted lacustrine basin, with the relatively stable tectonic activity. From the tectonic activity background of different well areas reflected by major and trace elements, the sediments of Well XC1 and D2 are more likely to come from the southern Sanwei Mountain provenance area, while the sediments of Well D1 are probably derived from the northern Beishan provenance area.

5.2. Weathering and Sedimentary Characteristics in Provenance Area

The chemical composition of clastic sedimentary rock is mainly affected by the chemical composition of its provenance area, but the later geological factors such as weathering, transportation, sedimentation and diagenesis will also have a significant impact on its chemical composition [16]. The analysis of the above factors has a certain indicating significance for identifying the source of sediments in different well areas.

The main changes of chemical weathering of rocks in the provenance area are the decomposition of feldspar minerals and the formation of clay minerals, which are manifested in the gradual loss of alkali metal elements (such as Ca, Na, K, etc.) and the relative enrichment of Al₂O₃ in clastic sedimentary rocks. Therefore, Nesbitt and Young (1982) put forward the chemical alteration index (CIA), which is used to quantitatively judge the intensity of chemical weathering in the provenance area, with a certain indication for paleoclimate [17]. Previous CIA studies on sediments propose that the CIA value of clastic sedimentary rocks without chemical weathering is close to that of feldspar minerals (50), and that of clastic sedimentary rocks with complete chemical weathering is close to that of kaolinite (100). A greater CIA value may cause greater chemical weathering intensity in clastic sedimentary rocks. Moreover, a hot and humid climate will also cause a higher CIA value. The content of non−clay minerals decreases and the content of clay minerals increases during the sedimentary cycle of clastic sedimentary rocks. Clay minerals have higher Al₂O₃ content than non−clay minerals. Therefore, Cox et al. (1995) proposed the composition variation index (ICV) to reflect the composition maturity of rocks. Clastic sedimentary rocks with high ICV value indicate an active tectonic environment. Clastic sedimentary rocks with low ICV value are considered to come from mature sedimentary provenance areas containing a large amount of clay minerals, indicating deposition in a passive tectonic environment [18]. It can be seen from the ICV−CIA plot [15] (Figure 8) that the CIA values of all samples range from 68.26 to 83.23, indicating that all the samples are subject to a certain degree of chemical weathering. Among them, the CIA values of samples from Well D1 are significantly different from those of Wells XC1 and Well D2. The ICV value of Well D1 samples is between 0.60 and 0.83, and the CIA is less than 75, showing that the original rock has medium maturity and weak chemical weathering. The ICV values of samples from Well XC1 and Well D2 are between 0.46 and 0.78, and most of the CIA values are greater than 75, indicating that the maturity of the original rock is relatively high and the chemical weathering is strong.

There are also some trace element discrimination methods that can be used to distinguish the weathering and sedimentary characteristics of the provenance area. The influence of different geological factors can be explained by using the characteristics of geochemical behavior of trace elements. The Th element is almost insoluble and stable, while the U element is easy to dissolve and lost in chemical weathering and sedimentary recycling. Therefore, the ratio of Th/U can be used to distinguish chemical weathering and sedimentary recycling. Previous studies on the Th/U ratio show that the ratio of Th/U greater than 3.8 usually indicates that the sediments have experienced weathering or sedimentation for shale [14]. On the other hand, the sediments that have been affected by weathering with the ratio of Th/U greater than 3.0. Both theories support that the ratio of Th/U increases with the increase in kaolinite content during weathering [19]. As shown in Figure 9, the samples of Well XC1 and Well D2 have a large Th/U ratio, which is significantly different from the samples of Well D1, indicating that the source rocks of Well XC1 and Well D2 lost more U elements in the process of weathering or the sedimentary cycle, and experienced stronger chemical weathering intensity or sedimentary recycling. In addition to the Th/U ratio, the ratio of Rb/Sr is also a trace element index commonly used to judge the weathering and sedimentary characteristics of provenance areas [19,20]. During weathering, feldspars are transformed into clay minerals, and Rb⁺ is more likely to remain in the exchange position of clay than Sr²⁺. The ratio of Rb/Sr is often used to distinguish chemical weathering or sedimentary recycling. Previous studies on Rb/Sr ratio show that the ratio of Rb/Sr in the clastic sediments significantly increases to greater than 0.5 with weathering and diagenesis [14,19]. In this study, the Rb/Sr ratio of all samples is greater than 0.5, indicating that the source rocks have been subjected to strong chemical weathering or sedimentary recycling. As shown in Figure 9, the ratio of Rb/Sr of samples from Well D2 (0.8~2.2, with an average of 1.3) and Well XC1 (0.7~1.0, with an average of 0.8) is significantly greater than in those from Well D1 (0.5~0.8, with an average of 0.6), indicating that the source rocks of the samples from Well XC1 and Well D2 experienced stronger chemical weathering or sedimentary recycling.

As a result, the provenance area of Well XC1 and Well D2 is determined to be the southern Sanwei Mountain provenance area, with strong weathering caused by strong tectonic activity, while the provenance area of Well D1 is indicated as the northern Beishan provenance area, with relatively weak tectonic activity and moderate weathering.

6. Conclusions

(1) The content and distribution characteristics of major elements, trace elements and rare earth elements show that the provenance areas of the three wells in the study area are different. The sediments of Well XC1 are similar in provenance area to those from Well D2, whereas the sediments from Well D1 came from different provenance areas.

(2) According to the analysis of the structural background of the provenance area, the structural activity of the provenance area of Well XC1 and Well D2 is stronger than that of Well D1.

(3) The analysis of weathering and sedimentary characteristics in the provenance area shows that the provenance areas of Well XC1 and Well D2 are characterized by strong weathering, under the background of strong tectonic movement. On the other hand, the provenance area of Well D1 is moderately weathered, due to relatively weak tectonic activity.

(4) Combined with the previous research results on the tectonic evolution process of the study area, Wudun sag is a faulted lacustrine basin controlled by the strong activity of the southern boundary faults during the sedimentary period of the J₂z₁ Formation. The tectonic activity in the provenance area of the southern Sanweishan uplift is strong, resulting in strong weathering. The tectonic activity in the provenance area of the northern Beishan is relatively weak, with moderate weathering. Therefore, the sediments in Well XC1 and Well D2 are mainly from the provenance area of the southern Sanweishan uplift, while the sediments in Well D1 are mainly from the provenance area of the northern Beishan area.