Palaeoenvironmental Evolution Based on Elemental Geochemistry of the Wufeng-Longmaxi Shales in Western Hubei, Middle Yangtze, China

Abstract

The organic-rich shales found in the Wufeng–Longmaxi Formation are typically deposited in oxygen-deficient reducing environments. One of the primary sources of debate revolves around the question of whether the anoxic bottom water found in these shales is either euxinic or ferruginous, and this matter remains unresolved. Previous studies have mostly focused on the Wufeng–Longmaxi Formation as a whole in order to understand the key factors that control organic matter accumulation (OMA). However, research on OMA for each member, including the Wufeng Formation (WF), the lower Longmaxi Formation (LLM), and the upper Longmaxi Formation (ULM), has been insufficient. This paper aims to investigate the palaeoenvironmental conditions and OMA mechanisms of the Wufeng–Longmaxi shales in western Hubei by integrating data on total organic carbon (TOC) content, mineral compositions, major and trace elements, and iron speciation. The results indicate that the Wufeng–Longmaxi shales were deposited under highly restricted hydrographic conditions, except for relatively open and upwelling conditions in the upper WF. Silica in the upper WF was primarily biogenic origin and not hydrothermal. Ferruginous conditions were the primary redox conditions for organic-rich shales except for minor formations in the lower LLM that were deposited under euxinic conditions. Due to the tectonic uplift caused by the Kwangsian Orogeny in the upper LLM, the palaeoenvironment was characterized by a warmer and wetter climate, high terrigenous influx, oxic conditions, and low productivity as the result of the insufficient nutrients caused by the weak upwelling, leading to the turnover of graptolite biozones from LM5 to LM6. The factors influencing OMA changed vertically. TOC contents have a highly positive correlation with Al content, indicating that terrigenous influx was the main factor affecting OMA in the WF, which significantly differed from patterns found in other regions. This suggests that the sedimentation rate of organic matter was higher than the terrigenous dilution rate during the WF stage. The combination of redox conditions and productivity were the main factors affecting OMA in the LLM, while terrigenous influx was the key factor controlling OMA in the ULM, resulting in the dilution of organic matter. Regions in the eastern Yiling block, which are close to the Qinling Ocean, show better prospects for shale gas exploration. This research will further facilitate the development of shale gas in this area.

1. Introduction

During the late Ordovician to early Silurian period, black shales in the Wufeng–Longmaxi Formation were widespread on the upper and middle Yangtze Platforms [1,2,3,4,5], and are now considered among the most promising hydrocarbon source rocks in South China [6,7]. In recent years, large-scale commercial developments in shale gas have achieved success in the Jiaoshiba block of the southeastern Sichuan Basin [8,9,10], and significant shale gas sources have been discovered in the Yiling block of western Hubei [2,3,11]. Consequently, considerable attention has been given to studying the palaeoenvironmental characteristics and organic matter accumulation (OMA) mechanisms of these blocks [5,12,13,14,15], which are vital for hydrocarbon generation and play a crucial role in determining the physical properties of shale reservoirs [1,2,3].

Previous studies have demonstrated that the Wufeng Formation (WF) and lower Longmaxi Formation (LLM) are organic-rich shales [3,13,16] deposited in the deep shelf [15,17] during a large-scale transgression [16,18]. These formations experienced warm and humid climates [5,14], dysoxic to anoxic conditions [13], and high productivity [3,15,16]. Moving upward to the upper Longmaxi Formation (ULM), the palaeoenvironmental became characterized by a warmer and wetter climate, high terrigenous influx [15,16], and the oxic conditions of bottom water due to the fall in sea level caused by the intense tectonic uplift of the Chuanzhong and Xuefeng uplifts [16,19], leading to the formation of organic-lean shales. Remarkably, the fall in sea level during the ULM stage was caused by the Kwangsian Orogeny, which was a tectonic event dominated by horizontal movement with local vertical uplift movement, occurring from the southeast of Guangxi province to the northwest of Hunan province during the late Ordovician to the late Silurian [16,20,21], leading to the change of the environment from anoxic to oxic conditions, and turnover of the graptolite biozone LM5 in the Rhuddanian to LM6 in the Aeronian (Figure 1d) [21]. However, further research is necessary to fully understand the transition from LM5 to LM6.

Recently, iron speciation has been utilized to distinguish ferruginous and euxinic bottom water conditions in the Wufeng–Longmaxi Formation of the Sichuan Basin [22,23,24,25]. For instance, formations in the upper WF and lower LLM in well PY1 in the Sichuan Basin and in well Shenci-1 near the Qinling Ocean are apparently deposited under euxinic conditions [23]. However, the anoxic bottom water conditions in the Yichang area of western Hubei may be different from those in the regions near the depocenter of the Sichuan Basin and Qinling Sea in northern Yangtze, due to the different sedimentary facies. Therefore, further studies are required to determine the anoxic-ferruginous or anoxic-euxinic conditions of organic-rich shales in the WF and LLM in western Hubei. Additionally, the upwelling intensity of the bottom water may differ in different regions of the middle and upper Yangtze, owing to the varying sedimentary facies [12].

Generally, the OMA mechanism can be explained by the preservation model and productivity model [2,26], which indicate that redox conditions [5,27] and productivity [3,28] are the two main factors that influence OMA in the Wufeng–Longmaxi Formation. Previous studies have mostly focused on the Wufeng–Longmaxi Formation as a whole in order to understand the key factors that control organic matter accumulation (OMA). However, research on OMA for each member, including the WF, LLM, and ULM, has been insufficient.

Considering the inadequate research on the palaeoenvironmental evolution and OMA mechanism, we collected 24 samples from well YD1 of the Yiling block for geochemical testing to analyze the associated palaeoenvironmental conditions and OMA mechanism (Figure 1d). Moreover, we conducted further studies on the differences in geological conditions between the Yiling block and Jiaoshiba block [6,13]. The geological significance of this study lies in providing a geochemical basis for shale gas exploration in the area.

2. Geological Setting

The Yangtze Platform, located in central China, is divided into the upper, middle, and lower regions based on sedimentary facies (Figure 1a). The middle-upper Yangtze Platform lies in the central and western parts of the region and is bordered by various uplifts and blocks, such as the Kangdian uplift to the southwest, Chuanzhong uplift to the northwest, Qinling Ocean and North China Block to the north, Qianzhong uplift to the south, and Xuefeng uplift and Cathaysia Block to the southeast (Figure 1b) [4,16]. During the late Ordovician to early Silurian, the Cathaysia Block subducted and extruded the middle-upper Yangtze Platform due to the Caledonian movement, forming a tectonic pattern with alternating uplifts and depressions and a semi-closed marine environment [4,15]. The Wufeng–Longmaxi shales were deposited in these depressions, with two main depocenters in the southern and eastern Sichuan Basin, respectively [29]. Additionally, the Hunan–Hubei submarine high was formed between the cities of Chongqing and Yichang (Figure 1b and c) during the WF stage of the late Ordovician due to the collision between the Cathaysia Block and the Yangtze Platform [15,16,30].

The Fuling Jiaoshiba block, located in the southeastern Sichuan Basin and depicted in the black rectangle in Figure 1c, is the oldest commercial shale gas development block in China [6]. Recently, significant shale gas sources have also been discovered in the Yiling block in the study area [2], which is situated southeast of the Huangling basement and indicated by the red rectangle in Figure 1c. Well YD1, which targets the Wufeng–Longmaxi shales, is located in the Yiling block (Figure 1c). Generally, the Wufeng–Longmaxi Formation can be vertically divided into the Katian, Hirnantian, Rhuddanian, and Aeronian stages according to graptolite biozones [21]. According to lithological characteristics, it can be further divided into the LX, WF, GYQ, LLM, and ULM sections [2]. Among these, the LX section is composed of dark grey carbonaceous mudstone, the WF section is primarily composed of black carbonaceous siliceous rock interbedded with minor dolomite siltstone, the lower and upper LLM sections are mainly composed of black carbonaceous siliceous rock and black carbonaceous argillaceous shale, respectively, and the ULM section is mainly composed of dark grey carbonaceous shale (Figure 1d).

3. Methodology

3.1. Samples and Methods

Twenty-four fresh shale samples were collected from well YD1 in the Yiling block, comprising one sample from the LX (Y1), eight samples from the WF (Y2–Y9), six samples from the LLM (YA10–YA15), and nine samples from the ULM (YA16–YA24). After recovery, the samples were immediately placed in paper bags and analyzed by the State Key Laboratory of Biogeology and Environmental Geology in China for their TOC content, mineral compositions, major, trace, and rare earth elements, as well as iron speciation.

Among these, the TOC content was tested with a Multi EA 4000 carbon-sulfur tester, and 200 mesh samples of 0.1~0.3 g were treated with dilute hydrochloric acid to remove inorganic carbon and dried at 60~80 ℃ before the experiment, using the test methods and procedures following Chinese National Standard GB/T19145-2003. Mineral compositions were tested by a Rigaku SmartLab SE X-ray diffractometer, and approximately 5 g of evenly mixed sample powder was taken out for X-ray diffraction analysis, using test methods and procedures following China Petroleum Industry Standard SY/T 5163-2018. Major elements were analysed by a ZSXPrimus Ⅱ X-ray fluorescence spectrometer, using test methods and procedures following Chinese National Standard GB/T 14506.28-2010. Trace and rare earth elements were determined with a multireceiver cup inductively coupled plasma source mass spectrometer, using test methods and procedures following Chinese National Standard GB/T14506.30-2010. Total iron content (FeT) was determined by the sulfosalicylic acid spectrophotometric method. Highly reactive iron (FeHR) includes iron in pyrite (Fepy), carbonate (Fecarb), magnetite (Femag), and ferric oxides (Feox). Fepy content was calculated from the concentration of pyrite sulfur (FeS2) extracted using CrCl2 reduction and precipitated as Ag2S in silver nitrate traps [26]. Fecarb, Feox, and Femag fractions were extracted by the sequential procedure described in Poulton and Canfield (2005) [31] and measured by atomic absorption spectroscopy. All of the tests were conducted under an ambient laboratory temperature approximately 24 °C; relative humidity was maintained at 65%; and analytical errors were less than 5%.

3.2. Data Presentation

The enrichment factor (EF) can be applied to evaluate the degree of elements enrichment in sediments using the following equation [32]:

EF = (X/Al)_sample/(X/Al)_PAAS

(1)

where (X/Al)_sample is the ratio of element X to Al measured in the sample, and (X/Al)_PAAS is the ratio of element X to Al in post Archean Australian shale (PAAS) [33]. Generally, EF > 3, 1 < EF < 3, and EF < 1 were interpreted to reflect apparent enrichment, moderate enrichment, and depletion, respectively, relative to the PAAS standard [34].

In palaeomarine environments, the element concentration in sediments includes both non-biogenic and biogenic components. The concentration of elements derived from biogenic sources equals the total concentration in every sample minus its non-biogenic influx, and can therefore be calculated as follows [33,35]:

X_bio = X_Sample − Al_Sample × (X/Al)_PAAS

(2)

where X_bio is the biogenic part of element X, and X_sample, and Al_sample are the contents of elements X and Al measured in the sample, respectively. (X/Al)_PAAS is the ratio of X to Al in the PAAS.

The chemical index of alteration (CIA), which was originally proposed by Nesbitt and Young (1982) to determine the chemical weathering intensity of the provenance of clastic rock, was calculated following the formula below [36]:

CIA = molar (Al₂O₃/[(Al₂O₃ + CaO* + Na₂O + K₂O)]×100)

(3)

All major element concentrations are calculated in mole fractions. CaO* refers to CaO in silicate. The method of indirect calculation of the value of CaO* as follows [5]: CaO_Remain = molar (CaO-P₂O₅ × 10/3), if molar (CaO_Remain) < molar (Na₂O), then molar (CaO*) = molar (CaO_Remain); if molar (CaO_Remain) > molar (Na₂O), then molar (CaO*) = molar (Na₂O). Generally, CIA values change greatly in different palaeoclimates, and CIA values range from 50 to 70, 70 to 80, and 80 to 100, indicating low, moderate, and high degrees of chemical weathering, respectively [14]. In general, weak weathering is associated with cold and arid climates, whereas intense weathering is associated with hot and wet climates. New K was introduced by K metasomatism during diagenesis, resulting in low CIA values. Therefore, it was necessary to correct it. Generally, the CIA values before K metasomatism (CIA_corr) value can be calculated by Al₂O₃–CaO* + Na₂O–K₂O ternary diagrams [35].

Additionally, La–Th–Sc and Th–Sc–Zr/10 ternary discrimination diagrams are used to infer the tectonic settings [37,38]. The CIA_corr value, Sr/Cu, and Rb/Sr ratios are used to reconstruct palaeoclimate conditions [39,40]. The Al/(Al + Fe + Mn) and (Fe+Mn)/Ti ratios are used to evaluate hydrothermal deposition [15,41]. The Si_bio/(Si_bio + Fe_bio + Al + Ca_bio) ratio is used to distinguish biogenic sources, where a ratio of > 0.8 indicates an absolute biogenic source [17,35]. The Al content is used to study the level of terrigenous influx [15,16]. The V/Cr, Ni/Co, U/Th, and Fe/Al ratios are used to reflect palaeoredox conditions [14,42,43,44,45]. A Fe_py/Fe_HR vs. Fe_HR/Fe_T discrimination diagram is used to distinguish euxinic and ferruginous conditions [46,47]. The Mo vs. TOC discrimination diagram and proxy Co × Mn are used to evaluate palaeohydrography [48,49]. The proxies Si_bio, (Ni + Cu + Zn)_bio, P/Ti, and Cu/Al are used to evaluate palaeoproductivity [3,13,15,40]. The La_N/Yb_N ratio is used to reflect the deposition rate [35], where subscripts N represents the chondrite normalization [50].

4. Results

4.1. Lithology and Graptolite Biozones

Core and thin section images are shown in Figure 2. The LX is composed of dark grey carbonaceous mudstone (Figure 2a). The WF is primarily composed of black carbonaceous siliceous rock (Figure 2b) with larger quartz grains and many bioclasts (Figure 2c) interbedded with minor dolomite siltstone (Figure 2d) containing much calcite and dolomite (Figure 2e). Rectograptus of the WF2-3 graptolite biozones was clearly observed in the bedding plane (Figure 2f). The LLM is mainly composed of black carbonaceous argillaceous shale with clear laminated bedding (Figure 2g), the mineral compositions are slightly oriented, and the particle size of quartz is obviously smaller than that in the WF (Figure 2h). Campograptus of graptolite biozone LM6 was clearly observed in the bedding plane (Figure 2i). The ULM is mainly composed of dark grey carbonaceous shale (Figure 2j), and the mineral compositions are apparently oriented with many hydromica minerals (Figure 2k). Demirastrite graptolites of the LM6 biozone are clearly observed in the bedding plane (Figure 2l).

4.2. TOC Content

Generally, organic-rich and organic-lean shales are characterized by TOC contents of greater than 2%, and lower than 2%, respectively [51]. The TOC values are presented in

Table 1. TOC and major elements contents of the Wufeng–Longmaxi Formation in well YD1.

Sample No.	TOC (%)	SiO2 (%)	TiO2 (%)	Al2O3 (%)	Fe2O3 (%)	MnO (%)	MgO (%)	CaO (%)	Na2O (%)	K2O (%)	P2O5 (%)	FeT (%)	Fepy (%)	FeHR (%)	Al (%)	Sibio (%)	Sibio/(Sibio + Febio + Al + Cabio)	Al/(Al + Fe+Mn)	(Fe + Mn)/Ti	CIAcorr	P/Ti	Fepy/FeHR	FeHR/FeT	FeT/Al
Y24	0.76	61.5	0.69	16.79	6.55	0.06	2.68	0.48	1.21	3.9	0.11	4.59	0.79	1.44	8.89	2.66	0.24	0.66	11.19	76.8	0.12	0.55	0.31	0.52
Y23	0.80	61.6	0.7	17.1	6.38	0.05	2.71	0.43	1.18	4.05	0.11	4.47	0.68	1.2	9.05	2.23	0.21	0.67	10.73	77.4	0.11	0.57	0.27	0.49
Y22	1.74	60.2	0.67	16.41	6.34	0.05	2.59	0.51	1.04	3.92	0.11	4.44	0.97	1.57	8.69	2.64	0.24	0.66	11.14	78.7	0.12	0.62	0.35	0.51
Y21	1.37	60.4	0.67	16.62	6.65	0.05	2.66	0.55	1.04	3.95	0.11	4.65	1.02	1.67	8.80	2.42	0.22	0.65	11.68	79.0	0.12	0.61	0.36	0.53
Y20	1.11	59.3	0.7	16.88	6.44	0.05	2.71	0.62	1.09	4.04	0.1	4.51	0.89	1.62	8.94	1.48	0.15	0.66	10.83	78.2	0.10	0.55	0.36	0.50
Y19	1.73	61	0.69	16.12	6.23	0.05	2.6	0.61	1.04	3.84	0.1	4.36	0.94	1.61	8.53	3.48	0.30	0.66	10.63	78.3	0.11	0.58	0.37	0.51
Y18	1.58	61.3	0.65	15.74	6.23	0.04	2.5	0.42	0.98	3.76	0.11	4.36	1.06	1.6	8.33	4.20	0.34	0.65	11.26	79.0	0.12	0.66	0.37	0.52
Y17	2.74	59.3	0.62	14.24	6.33	0.03	2.28	0.57	1.1	3.32	0.13	4.43	0.85	2.45	7.54	5.57	0.41	0.63	11.97	75.9	0.15	0.35	0.55	0.59
Y16	1.71	62.8	0.6	14.8	6.2	0.04	2.38	0.36	1.01	3.51	0.1	4.34	1.06	2.14	7.84	6.34	0.45	0.64	12.14	77.7	0.12	0.50	0.49	0.55
Y15	2.61	61.1	0.6	13.8	6.61	0.04	2.36	0.94	0.98	3.3	0.1	4.63	1.67	2.71	7.31	7.11	0.46	0.61	12.94	77.0	0.12	0.62	0.59	0.63
Y14	3.05	58.4	0.62	13.83	6.57	0.03	2.08	0.75	1.09	3.27	0.12	4.6	1.23	3.27	7.32	5.80	0.42	0.61	12.43	75.4	0.14	0.38	0.71	0.63
Y13	6.18	70.4	0.48	8.9	3.65	0.02	1.24	0.96	0.98	1.98	0.11	2.56	1.41	2.09	4.71	19.04	0.79	0.65	8.93	69.7	0.17	0.67	0.82	0.54
Y12	6.29	48.9	0.6	11.01	7.55	0.06	2.41	2.5	1.27	2.33	0.2	5.29	3.19	4.41	5.83	5.76	0.38	0.52	14.81	68.8	0.24	0.72	0.83	0.91
Y11	3.45	82	0.26	4.99	1.87	0.02	0.6	1.28	0.61	0.99	0.07	1.31	0.83	1.22	2.64	30.55	0.90	0.67	8.49	67.8	0.20	0.68	0.93	0.50
Y10	3.23	82	0.26	4.55	1.98	0.03	0.71	1.75	0.66	0.88	0.07	1.38	0.78	1.28	2.41	31.19	0.90	0.63	9.03	64.2	0.20	0.61	0.92	0.58
Y9	2.99	78	0.24	6.64	2.7	0.02	0.62	1.45	0.61	1.47	0.06	1.89	1.31	1.93	3.52	26.09	0.86	0.65	13.23	73.0	0.18	0.68	1.02	0.54
Y8	2.58	34.7	0.37	8.34	4.34	0.22	3.62	18.9	0.58	1.85	0.03	3.04	1.9	3.18	4.42	3.28	0.15	0.58	14.45	77.4	0.06	0.60	1.05	0.69
Y7	2.05	82.1	0.2	3.73	1.85	0.04	0.62	2.87	0.46	0.75	0.06	1.29	0.82	1.25	1.97	32.55	0.89	0.60	11.05	67.6	0.22	0.66	0.97	0.66
Y6	3.86	83.9	0.22	4.57	1.66	0.01	0.63	0.7	0.42	1.04	0.05	1.16	0.66	0.98	2.42	32.07	0.92	0.67	8.86	72.9	0.17	0.67	0.84	0.48
Y5	4.49	74.3	0.44	8.07	2.91	0.02	1.28	0.94	0.65	1.99	0.08	2.03	0.85	1.34	4.27	22.17	0.83	0.68	7.77	75.2	0.13	0.63	0.66	0.48
Y4	1.9	89.9	0.13	2.95	0.94	0.01	0.41	0.81	0.26	0.66	0.04	0.66	0.27	0.46	1.56	37.39	0.95	0.70	8.54	73.7	0.22	0.59	0.70	0.42
Y3	2.88	77.2	0.33	7.12	2.81	0.03	1.03	1.41	0.47	1.78	0.07	1.96	0.78	1.25	3.77	24.99	0.85	0.65	10.05	77.9	0.15	0.62	0.64	0.52
Y2	4.94	63.6	0.68	13.98	4.29	0.03	2.17	0.65	0.92	3.71	0.11	3	0.55	1.01	7.40	7.99	0.55	0.71	7.42	78.1	0.12	0.54	0.34	0.41
Y1	0.7	60.8	0.84	17.07	5.52	0.03	2.81	0.95	1.01	4.6	0.09	3.86	0.56	1.12	9.04	1.89	0.19	0.70	7.71	79.5	0.08	0.50	0.29	0.43
PAAS	/	62.8	1	18.9	7.22	0.11	2.2	1.3	1.2	3.7	0.16	/	/	/	/	/	/	/	/	/	/	/	/	/

and Figure 3a. The TOC content is less than 1% in the LX with a value of only 0.7%, and it ranges from 1.90% to 4.94% (avg. 3.21%), 2.61% to 6.29% (avg. 4.14%), and 0.76% to 2.74% (avg. 1.50%) in the WF, LLM, and ULM, respectively. The results show that the WF and LLM include organic-rich shales, and the LX and ULM include organic-lean shales. Additionally, the results show a significant decrease in TOC content from 6.18% of sample Y13 to 3.05% of sample Y14 in the LLM.

4.3. Mineral Compositions and Lithofacies

Mineral compositions are mainly composed of quartz and clay in all of the samples, for which quartz contents reach 59.3%, 33.0%–89.9% (avg. 68.3%), 48.1%–81.5% (avg. 61.6%), and 40.8%–61.9% (avg. 48.0%) in the LX, WF, LLM, and ULM, respectively, showing a decreasing trend from the bottom to the top. In contrast, clay contents reach 26.0%, 7.8%–36.6% (avg. 19.6%), 10.4%–41.9% (avg. 25.3%), 24.9%–48.2% (avg. 42.2%), showing an increasing trend from bottom to top. Contents of feldspar, carbonate, and pyrite are low in all of the samples, and the feldspar, carbonate, and pyrite contents are higher in the ULM, WF, and LLM, respectively (Figure 3b).

Shale can be divided into four lithofacies types (siliceous, calcareous, clay, and mixed shales) [52]. These can further be subdivided into sixteen lithofacies types according to a three-end element map of clay, carbonate, and felsic rocks content [53]. The results show that most samples are plotted in the siliceous shale field except for sample Y8 of the WF, which is plotted in the mixed shale field, for which organic-rich shales in the WF and LLM are primarily plotted as subclass lithofacies type S, and organic-lean shales in the LX and ULM are primarily plotted as subclass lithofacies type S-3 (Figure 3c).

4.4. Major and Trace Elements

The results regarding major elements are listed in Table 1. Major oxides mainly include SiO₂ and Al₂O₃ in all of the formations, where organic-rich shales have higher contents of SiO₂ in the WF (34.74%–89.92%, avg. 72.97%) and LLM (48.92%–82.04%, avg. 67.13%), and organic-lean shales have higher contents of Al₂O₃ in the LX (17.07%) and ULM (14.24%–17.10%, avg. 16.08%). Additionally, organic-rich shales are relatively rich in CaO, while organic-lean shales are relatively rich in MgO, Na₂O, and K₂O. The EF value shows that Si, Ca, Na, P, and Fe contents in the organic-rich shales are higher than those of the organic-lean shales with moderate to apparent enrichment relative to PAAS (Figure 4a).

The results of the trace elements are listed in

Table 2. Trace elements contents of the Wufeng–Longmaxi Formation in well YD1.

Sample No.	Y24	Y23	Y22	Y21	Y20	Y19	Y18	Y17	Y16	Y15	Y14	Y13	Y12	Y11	Y10	Y9	Y8	Y7	Y6	Y5	Y4	Y3	Y2	Y1	PAAS
Sc (ppm)	16.33	16.85	16.52	16.87	17.54	15.45	15.16	14.15	13.90	13.75	13.40	7.65	10.20	3.88	3.52	4.82	1.57	2.69	3.96	8.00	2.44	6.31	16.02	16.84	16
V (ppm)	139.18	134.34	158.23	175.24	166.84	171.58	166.72	131.83	125.06	134.55	144.00	218.36	228.99	128.61	99.23	123.32	77.90	83.90	279.34	571.00	234.40	96.02	329.21	134.65	150
Cr (ppm)	83.48	86.69	84.90	87.13	87.04	81.64	77.71	74.68	71.64	70.79	71.10	47.48	59.30	28.37	23.18	26.20	4.11	19.80	34.60	64.70	27.00	56.40	115.48	85.18	110
Co (ppm)	18.08	18.26	18.68	20.02	19.15	18.37	18.31	16.00	17.66	18.66	24.40	16.34	29.40	26.89	30.98	16.02	18.20	70.00	22.51	21.60	66.77	24.77	12.00	21.14	23
Ni (ppm)	64.23	64.53	87.47	92.86	83.46	87.25	83.68	64.84	75.25	110.77	144.00	132.13	340.22	93.91	97.50	120.29	195.00	154.00	105.60	150.00	105.29	98.04	83.62	60.83	55
Cu (ppm)	50.48	39.34	46.07	51.68	48.93	48.63	50.48	49.17	47.11	64.75	75.30	58.07	78.58	31.12	26.74	32.53	14.80	19.30	51.26	82.40	32.03	74.03	159.46	113.71	50
Zn (ppm)	124.79	111.17	123.66	122.29	124.99	120.96	107.02	107.72	102.87	116.80	119.00	140.06	355.78	85.89	69.91	110.44	308.00	102.00	135.52	258.00	135.07	172.61	244.17	245.27	85
Rb (ppm)	184.08	194.66	193.04	193.38	193.17	180.57	175.65	164.80	167.13	162.99	153.00	88.32	117.18	42.99	37.66	53.26	54.80	29.40	46.27	87.80	29.80	71.74	172.53	197.96	160
Sr (ppm)	86.66	89.16	107.48	88.65	90.98	86.69	96.85	91.79	74.15	84.31	85.90	65.02	260.65	60.25	53.33	58.84	257.00	61.90	38.54	59.70	33.19	53.26	81.58	97.36	200
Zr (ppm)	146.84	144.92	150.92	146.13	144.68	139.68	137.26	137.50	125.47	130.30	144.00	134.52	213.58	91.31	152.60	146.99	186.00	243.00	56.26	113.00	77.54	98.31	216.38	236.25	210
Mo (ppm)	2.16	3.25	8.17	5.45	6.20	10.52	6.27	31.29	12.75	16.23	34.10	45.00	53.05	13.54	12.68	11.75	33.00	33.50	43.26	85.20	7.51	6.58	25.20	2.84	1
Ba (ppm)	1606.3	2037.3	6096.1	1374.8	1202.8	1662.1	5645.0	4164.0	1127.6	879.7	958.0	873.2	26770.3	896.7	1025.6	1284.2	2732.0	1106.0	893.3	951.0	743.9	995.6	818.6	981.3	650
Hf (ppm)	3.88	3.98	4.03	3.97	3.88	3.63	3.60	3.72	3.34	3.58	3.89	3.61	5.53	2.21	2.89	3.18	7.98	5.15	1.32	2.95	0.87	2.69	4.31	6.37	5
U (ppm)	4.82	4.91	6.78	4.98	6.13	5.90	5.83	8.43	6.38	10.13	14.50	14.97	51.17	8.54	14.53	25.56	87.70	21.50	8.06	13.50	4.31	12.88	7.89	4.22	3.1
Th (ppm)	17.54	17.61	18.57	18.54	18.63	17.69	16.78	17.29	16.12	17.21	17.50	12.65	17.71	6.69	7.05	10.96	11.50	6.15	5.81	11.10	3.24	9.94	18.80	23.42	14.6
La (ppm)	45.95	47.08	48.20	48.57	47.14	45.75	45.31	44.07	42.62	43.59	44.10	32.83	46.47	18.88	23.05	29.09	62.10	25.50	19.57	34.10	11.10	26.54	52.84	60.33	38
Yb (ppm)	2.79	2.93	2.91	2.95	2.80	2.78	2.62	2.73	2.51	2.69	2.84	2.27	3.90	1.46	1.67	2.27	1.36	2.10	1.21	2.19	1.03	1.57	4.43	3.89	2.8
Sr/Cu	1.72	2.27	2.33	1.72	1.86	1.78	1.92	1.87	1.57	1.30	1.14	1.12	3.32	1.94	1.99	1.81	17.36	3.21	0.75	0.72	1.04	0.72	0.51	0.86	/
Rb/Sr	2.12	2.18	1.80	2.18	2.12	2.08	1.81	1.80	2.25	1.93	1.78	1.36	0.45	0.71	0.71	0.91	0.21	0.47	1.20	1.47	0.90	1.35	2.11	2.03	/
Sr/Ba	0.05	0.04	0.02	0.06	0.08	0.05	0.02	0.02	0.07	0.10	0.09	0.07	0.01	0.07	0.05	0.05	0.09	0.06	0.04	0.06	0.04	0.05	0.10	0.10	/
V/Cr	1.67	1.55	1.86	2.01	1.92	2.10	2.15	1.77	1.75	1.90	2.03	4.60	3.86	4.53	4.28	4.71	18.95	4.24	8.07	8.83	8.68	1.70	2.85	1.58	/
U/Th	0.28	0.28	0.37	0.27	0.33	0.33	0.35	0.49	0.40	0.59	0.83	1.18	2.89	1.28	2.06	2.33	7.63	3.50	1.39	1.22	1.33	1.30	0.42	0.18	/
Ni/Co	3.55	3.53	4.68	4.64	4.36	4.75	4.57	4.05	4.26	5.94	5.90	8.08	11.57	3.49	3.15	7.51	10.71	2.20	4.69	6.94	1.58	3.96	6.97	2.88	/
Cu/Al	5.68	4.35	5.30	5.87	5.47	5.70	6.06	6.52	6.01	8.86	10.28	12.32	13.48	11.78	11.10	9.25	3.35	9.77	21.19	19.29	20.51	19.64	21.55	12.58	/
Mo/TOC (×10−4)	2.85	4.04	4.70	3.98	5.59	6.08	3.97	11.42	7.46	6.22	11.18	7.28	8.43	3.92	3.93	3.93	12.79	16.34	11.21	18.98	3.95	2.28	5.10	4.06	/
Co × Mn (×10−8)	0.84	0.71	0.72	0.78	0.74	0.71	0.57	0.37	0.55	0.58	0.57	0.25	1.37	0.42	0.72	0.25	3.10	2.17	0.17	0.33	0.52	0.58	0.28	0.49	/
(Ni + Cu + Zn)bio (ppm)	70.72	49.03	92.23	99.75	87.68	94.79	82.95	78.58	76.46	153.60	199.27	240.79	663.90	160.75	148.41	196.50	441.22	237.80	246.44	409.27	242.73	273.10	346.71	248.21	/
LaN/YbN	10.96	10.71	11.06	10.96	11.22	10.98	11.54	10.77	11.32	10.78	10.35	9.62	7.94	8.61	9.18	8.55	30.44	8.10	10.76	10.38	7.20	11.27	7.95	10.34	9.05

. V, Co, Ni, Mo, and U are rich in organic-rich shales, while Sc, Cr, Rb, Hf, and Th are rich in organic-lean shales. The EF values show that V, Co, Ni, Cu, Zn, Mo, Ba, and U contents in organic-rich shales are higher than those of organic-lean shales with moderate to apparent enrichment relative to PAAS (Figure 4b).

4.5. Iron Speciation

The iron speciation results are listed in Table 1. Fe_py and Fe_HR contents are characterized as follows: the LLM (avg. 1.52% and 2.50%, respectively) > the ULM (avg. 0.92% and 1.70%, respectively) > the WF (avg. 0.89% and 1.43%, respectively) > the LX (0.56% and 1.12%, respectively). Average contents of Fe_T are characterized as follows: the ULM (avg. 4.46%) > the LX (3.86%) > the LMX (avg. 3.29%) > the WF (avg. 1.88%).

5. Discussion

5.1. Tectonic Setting

Trace element discrimination diagrams have also been used to distinguish tectonic settings [37,38]. In the diagrams of La-Th-Sc and Th-Sc-Zr/10 (Figure 5a,b) proposed by Bhatia and Crook (1986) [37], most samples mainly plot within or near the field of the active continental margin, and minor samples fall within the field of the continental island arc, which is consistent with that of the Huadi-1 well in the southeastern Sichuan Basin [38]. These results show that from the late Ordovician to the early Silurian, the subduction of the Proto-Tethys oceanic crust towards the South China Plate led to the convergence and compression between the Cathysia Block and Yangtze Platform [16], forming a tectonic setting of mainly active continental margins.

5.2. Palaeoenvironment

5.2.1. Palaeoclimate

The palaeoclimate controls levels of the humidity and weathering, thus affecting terrigenous influx and the sediment supply in sedimentary rocks [41]. The chemical index of alteration (CIA) has been widely used to reconstruct the palaeoclimate, and the formula of CIA is shown in Section 3.2 [5,16,35]. The Sr/Cu ratio is an effective indicator for evaluating palaeoclimate change, with ratios of <5, 5–10, and >10 indicating humid, semihumid-semiarid, and arid palaeoclimates, respectively [40,54]. Additionally, because Rb is relatively stable under weathering, whereas Sr is prone to leaching during weathering, the Rb/Sr ratio is often used to study palaeoclimate in provenance, with high and low ratios indicating a warm and humid climate with a high degree of weathering and an arid climate with a low degree of weathering, respectively [39,55].

The Al₂O₃–CaO* + Na₂O–K₂O ternary diagrams (Figure 6) show that the predicted weathering trend line (T’) is parallel to the Al₂O₃–CaO* + Na₂O line and points towards the endmember of Al₂O₃. Starting from the K₂O end member, crossing the sample data point and intersecting with T’, the CIA_corr values can be obtained [35].

The results show that the CIA_corr value is highest in the LX with the value of 79.5, and it ranges from 67.6 to 78.1 (avg. 74.5), 64.2 to 77.0 (avg. 70.5), 75.9 to 79.0 (avg. 77.9), and 57.4 to 75.7 (avg. 72.1), respectively, which indicate mainly a moderate degree of chemical weathering and imply a warm and humid climate in the Wufeng–Longmaxi Formation. Comparatively speaking, the CIA_corr values of the LX and ULM are higher than that of the WF and LLM, which indicates a much warmer and wetter climate in the organic-lean shales (Table 1; Figure 6 and Figure 7).

As shown in Figure 7, the Sr/Cu ratios of most samples are lower than 5 except for sample Y8 of dolomite siltstone from the WF, which has a Sr/Cu ratio of 17.4, denoting a primarily warm and humid climate in the Wufeng–Longmaxi Formation and a much drier climate in the formation enriched with carbonate minerals.

The profiles of Rb/Sr ratios and CIA_corr values are nearly consistent, which shows that from the bottom to the top, the climate was moderately warm and humid in the LX and the lower WF. Then, there was slight warmth and humidity in the upper WF. Further upwards, cold and arid conditions occurred in the GYQ due to Gondwanan continental glaciation at the end of the late Ordovician [16,56]. Subsequently, with the melting of Gondwanan continental glaciers in the early Silurian [16,18], the climate gradually changed to slight warmth and humidity in the lower LLM. Then, the Kwangsian Orogeny intensified in the Yangtze Platform with strong tectonic uplift, leading to the sea level dropping rapidly [20,21] and the palaeoenvironment changing significantly. Remarkably, there is a significant decrease in TOC content from 6.18% in sample Y13 below the red dotted line to 3.05% in sample Y14 above the red dotted line, a significant increase in Al content from 8.9% to 13.83%, and a significant increase in Rb/Sr ratios and CIA_corr value, indicating that the climate rapidly changed from slightly warm and humid in the lower LLM to moderately warm and humid in the upper LLM. Accordingly, the significant variation in the palaeoclimate during the LLM stage was one of the key factors that led to the turnover of graptolite biozones from LM5 to LM6. After this, the climate was much hotter and wetter in the ULM with high Rb/Sr ratios, leading to a high degree of chemical weathering [13,16].

The variation of the palaeoclimate in well YD1 is consistent with that of well YY2 in the Yiling block [2] and well JY8 in the Jiaoshiba block [16], indicating that the palaeoclimate of the middle Yangtze Platform is similar to that of the upper Yangtze Platform. Additionally, we found that the climate of organic-rich shales in the WF and LLM is neither cold and arid like that of the GYQ nor hot and wet like that of organic-lean shales in the LX and ULM. The climate during the WF and LLM stages can be described as moderately warm and humid.

5.2.2. Origin of Silica and Terrigenous Influx

Previous studies have identified three primary sources of siliceous sediments in marine environments: hydrothermal fluids, siliceous organisms, and silica-rich terrigenous clastics [57]. In sedimentary rocks, Fe and Mn are generally enriched with hydrothermal deposits, and in typical sediments influenced by hydrothermal events, the (Fe + Mn)/Ti and Al/(Al + Fe + Mn) ratios are generally valued at >15 and < 0.4, respectively [41]. The analysis shows that all of the samples plot in the nonhydrothermal field, implying that silica mainly had a nonhydrothermal origin (Figure 8a).

Al is generally considered an indicator element for terrigenous clastic material [5,15]; therefore, the crossplots of SiO₂ vs. Al₂O₃ are usually used to distinguish terrestrial and biogenic silica with a positive correlation, no apparent correlation, and a negative correlation reflecting mainly terrestrial silica, mixed origin of terrestrial and biogenic silica, and biogenic silica, respectively [35]. The results show that silica mainly had a biogenic origin in the WF and LLM, and a mixed terrestrial and biogenic origin in the ULM (Figure 8b). The results show that graptolites are the main sources of biogenic silica in the Wufeng–Longmaxi Formation [13,48] (Figure 2), and there is also a small amount of sponge spicules and radiolarians [13,48,58].

Additionally, the Si_bio/(Si_bio + Fe_bio + Al + Ca_bio) ratios of most samples of organic-rich shales in the WF and the lower LLM are higher than 0.8, indicating an absolute biogenic silica [17,59]. In contrast, the low Si_bio/(Si_bio + Fe_bio + Al + Ca_bio) ratios of samples from the LX, upper LLM, and ULM suggest that silica partly originated from terrigenous clasts. Remarkably, the Si_bio/(Si_bio + Fe_bio + Al + Ca_bio) ratios of the LM6 graptolite biozone of the Aeronian above the red dotted line are significantly lower than those of the LM5 graptolite biozone of the Rhuddanian below the red dotted line (Table 1; Figure 7). The reason is that, as a result of the Kwangsian Orogeny [60], the sea level dropped rapidly due to the large-scale uplift in the Yangtze Platform, which led to a decrease in the quantity of graptolites and contents of biogenic silica.

The influx of terrigenous clastic material is a key factor that influences black shale composition and OMA [15,35,42]. The results show that Al contents are higher in organic-lean shales in the LX (9.04%) and ULM (avg. 8.51%) and lower in organic-rich shales in the WF (avg. 3.67%) and LLM (avg. 5.04%) (Table 1; Figure 7). The variation of Al content in the Yiling block is consistent with that in the JY8 well and PG section of the southeastern Sichuan Basin (Figure 1c) [16,34]. Specifically, Al content of the LM6 graptolite biozone in the Aeronian is significantly higher than those of the LM5 graptolite biozone in the Rhuddanian (Table 1; Figure 7), which indicates that as a result of the Kwangsian Orogeny [58], the Chuangzhong and Qianzhong uplifts were significantly uplifted, coupled with a warm and wet climate (Figure 7) [16], resulting in a significant increase in terrigenous influx.

As shown in Figure 9, Al contents show a highly positive correlation with the palaeoclimate proxy Rb/Sr ratio, implying that terrigenous influx is significantly influenced by the palaeoclimate. Generally, warm and humid palaeoclimates lead to a relatively high degree of weathering [14,35,36], which facilitates the weathering and transport of terrigenous clasts from surrounding palaeohighs into the Yangtze Sea. Additionally, Xuefeng uplift was exposed to erosion and became one of the source areas during the early Silurian [16], which is a direct influencing factor leading to a much stronger terrigenous influx in the ULM (Table 1 and Figure 7).

5.2.3. Palaeoredox Conditions

Some trace elements such as Ni, Co, V, Cr, Th, U, and Mo in sedimentary rocks are redox sensitive. Thus, they are usually used to reflect redox conditions due to their differentiation under different bottom water conditions [32]. The ratios of some trace elements such as V/Cr, Ni/Co, and U/Th are widely used tools in the reconstruction of benthic oceanic redox conditions since these trace metals can coprecipitate with organic matter or metal sulphides under anoxic conditions [61,62].

In general, V/Cr < 2, Ni/Co < 5, and U/Th < 0.75 indicate oxic conditions; 2 < V/Cr < 4.25, 5 < Ni/Co < 7, and 0.75 < U/Th < 1.25 indicate dysoxic conditions; and V/Cr > 4.25, Ni/Co > 7, and U/Th > 1.25 indicate anoxic conditions [14,15,44,45,61,62]. Additionally, iron speciation proxies Fe_HR/Fe_T and Fe_py/Fe_HR are usually used to evaluate the bottom water redox conditions of ancient oceans [46,63]. In general, Fe_HR/Fe_T values < 0.38 and > 0.38 indicate oxic and anoxic conditions, respectively. For anoxic conditions, Fe_py/Fe_HR < 0.7−0.8 and > 0.7−0.8 indicate ferruginous and euxinic conditions, respectively [47]. Furthermore, the Fe_T/Al ratio is an effective indicator with which to evaluate bottom water redox conditions [43]. Generally, Fe_T/Al ratios are valued at > 0.5 in deep-water basins under anoxic conditions. Remarkably, the Fe_T/Al ratios in the sediments under euxinic conditions are 2~3 times those of the sediments under oxic conditions.

The results show that the curve graphs of V/Cr and U/Th are relatively consistent, and they are apparently different from the Ni/Co curve graph (Figure 10). Therefore, the V/Cr and U/Th ratios are used to evaluate redox conditions. From the bottom to the top, the LX and the bottom of the WF are mainly deposited under oxic conditions that rapidly changed to anoxic conditions in the middle-upper WF. At the end of the late Ordicivian, the GYQ was largely deposited under oxic conditions in the middle Yangtze Platform as a result of the significant drop in global sea level [56]. Subsequently, with the melting of the Gondwanan continental glaciers and the influxing of the freshwater in the early Silurian, the sea level rose rapidly, and the seawater formed a stable stratified water body due to density differences, which resulted in clear vertical difference between the surface and bottom layers with the low content of dissolved oxygen in the bottom water [18,64,65], leading to mainly anoxic conditions of bottom water in the lower LLM. During the upper LLM and ULM stages, as the Kwangsian Orogeny with compressive stress continuously transferred from the South China Plate to the Yangtze Platform in the northwest, Xuefeng uplift was elevated rapidly and even exposed to erosion [16,60], leading to the fall of sea level in the long term. Thus, from sample Y14 in the upper LLM, the bottom water started to change from anoxic to mainly oxic, which is not beneficial to the preservation of organic matter such as the species of graptolite, radiolarian, and sponge spicule, leading to a significant drop in TOC content above the red dotted line.

As shown in Figure 11, the average Fe_py/Fe_HR and Fe_T/Al ratios of the LX, WF, LLM, and ULM are 0.50 and 0.43, 0.62 and 0.52, 0.61 and 0.63, and 0.55 and 0.53, respectively. Therefore, the bottom water in the LLM shows the highest level of anoxia, especially in sample Y12, and the bottom water is in a euxinic condition with peak values of Fe_py/Fe_HR ratios, Fe_T/Al ratios, and TOC content. Subsequently, the Fe_py/Fe_HR ratios significantly decreased from graptolite biozones LM5 to LM6, which implies that bottom water conditions transformed from ferruginous conditions in the Rhuddanian to oxic conditions in the Aeronian (Figure 11a).

The Fe_py/Fe_HR vs. Fe_HR/Fe_T discrimination diagram shows that the LX and ULM were mainly deposited under oxic conditions and that the WF and the LLM were primarily deposited under ferruginous conditions except for sample Y12 with a Fe_py/Fe_HR ratio of 0.72, implying euxinic conditions during this period (Figure 11b). There are three main reasons for the euxinic conditions found in sample Y12 from the LLM. with the first reason regards the melting of Gondwanan continent glaciers and the influencing of the salinity stratification, which led to an anoxic reducing environment in bottom water [18,65]. In addition, in the early Silurian, tectonic compression and uplift continuously transferred from Xuefeng uplift in the southeast to the Yangtze Platform in the northwest [16], leading to strong tectonic subsidence and the formation of a deep-water basin during the Longmaxi stage. Furthermore, the seawater in the study area was confined by the Huangling basement (a submarine high), Hunan–Hubei submarine high, and Xuefeng uplift (Figure 1c), leading to a strongly restricted basin, which was beneficial for forming euxinic conditions.

The results show that the redox conditions of organic-rich shales in well YD1 of the Yiling block are similar to those in well Qianqian-1 [24] and different from those in well PY1 in the southeastern Sichuan Basin [23] and well Shenci-1 in northern Hubei [22]. The bottom water of well Qianqian-1 is mainly under oxic to dysoxic conditions because it is close to the Qianzhong uplift, and the bottom water is primarily under ferruginous to euxinic conditions in well PY1 in the southeastern Sichuan Basin and well Shenci-1 in northern Hubei close to the Qingling Ocean because of the oxygen-deficient environment in the deep shelf resulting in anoxic conditions. In contrast, the bottom water of well YD1 mainly shows oxic to dysoxic conditions except for minor formations deposited under euxinic conditions (Figure 11a,b), which infer a much shallower palaeoocean in the Yiling block relative to the regions of the southeastern Sichuan Basin and northwestern Qinling Ocean.

Overall, the redox conditions of the Wufeng–Longmaxi Formation in the Yiling block show oxic conditions mainly in the LX, lower WF, upper LLM, and ULM, and anoxic conditions in the middle-upper WF and lower LLM, which is consistent with the Jiaoshiba block [13]. Previous studies have shown that anoxic conditions are directly caused by the rapid transgressive events during the middle-upper WF and lower LLM stages [12,16]. Oxic conditions are mainly attributed to the shallow water environment present during the LX and lower WF stages [12], while oxic conditions are directly caused by the fall of sea level as a result of the continuous elevation of the Chuanzhong and Xuefeng uplifts during the upper LLM and ULM stages [16].

5.2.4. Palaeohydrography

Hydrographic conditions are essential to controlling nutrients obtained from the deep ocean to the shallow shelf, and thus can influence palaeoproductivity and OMA [35]. Usually, Mo is a common redox-sensitive element and is abundant only under sulfurated conditions [66], and the Mo−TOC discrimination diagram is usually used to determine hydrographic conditions. Basins can be classified as presenting three levels of restriction: strongly restricted (Mo/TOC < 15 × 10⁻⁴), moderately restricted (15 × 10⁻⁴ < Mo/TOC < 35 × 10⁻⁴), and weakly restricted (Mo/TOC > 35 × 10⁻⁴) [67]. The results show that the average Mo/TOC ratios in the LX, WF, LLM, and ULM are 4.06 × 10⁻⁴, 9.32 × 10⁻⁴, 6.83 × 10⁻⁴, and 5.56 × 10⁻⁴, respectively (Table 2), and the Mo/TOC ratios of most samples are lower than 15 × 10⁻⁴ except for two samples in the WF with Mo/TOC ratios of greater than 15 × 10⁻⁴, indicating mainly a strongly restricted basin in the Wufeng-Longmaxi Formation, and minor formations in the middle-upper WF were deposited under a moderately restricted basin (Figure 11c).

Given that the Co × Mn values were observed to show significant differences between sediments from the upwelling and restricted settings, Sweere et al. (2016) [49] proposed that Co × Mn values can be used to evaluate upwelling intensity, with values of <0.4 and >0.4 indicating upwelling and restricted settings, respectively. The results show that most samples plot within the restricted field except for minor samples of organic-rich shales mainly found in the WF that fall within the open and upwelling field, implying that the WF showed much stronger upwelling intensity, followed by the lower LLM, and that the LX, upper LLM and ULM showed weak upwelling intensity (Figure 12a,b), leading to the insufficient nutrients transportation from the deep ocean to the shallow shelf [2,3], which was not beneficial for the survival of graptolites at the stages of the upper LLM and ULM. Thus, the graptolite abundance and quantity rapidly decreased due to the oligotrophic seawater caused by the weak upwelling, which was probably another important factor that led to the turnover of graptolite biozones from LM5 to LM6.

Overall, both the Mo/TOC and Co × Mn proxies show that the Wufeng–Longmaxi Formation of well YD1 mainly formed in a highly restricted setting except for minor samples that formed in a moderately restricted setting in the WF. In fact, the hydrographic conditions of the Wufeng–Longmaxi Formation are closely related to the sedimentary location [12]. The sedimentary basin in the Yiling block is a highly restricted setting, as it is surrounded by the Xuefeng uplift, the Hunan–Hubei submarine high, and Huangling basement, a submarine high formed in the Sinian (Figure 1c) [68]. The southeastern Sichuan Basin, where wells PY1 and JY143 are located, is a moderately restricted setting because it is a depocentre that has a relatively good connection with the Qinling Ocean [12,68]. The sedimentary basin, where well EHD1 is located, is weakly restricted because it is much closer to the Qinling Ocean, showing the highest level of upwelling intensity [12].

5.2.5. Palaeoproductivity

TOC is the surface productivity of the ocean reflected in sediments, and thus, it is regarded as a most direct primary productivity indicator [69]. Cu, Ni, and Zn are micronutrients for marine microbes that are complex with organic matter under reducing conditions [15,32]. Thus, these elements are typical indicators of marine primary productivity. A large number of radiolarians and graptolites in the shale confirmed the biological contribution of silica in the Wufeng–Longmaxi Formation. Thus, Si_bio can be used as a productivity indicator [16,70]. P is involved in most of the metabolic activities of organisms and is usually regarded as one of the most extensively applicable and reliable productivity indicators [71]. To eliminate the terrestrial part of P content, proxy P/Ti is used to estimate primary productivity [72].

The results show that the profiles of TOC, Si_bio, (Ni + Cu + Zn)_bio and P/Ti generally have similar variation trends with only a small difference in local formation (Figure 10). In the LX and lower WF, palaeoproductivity was low to moderate, apparently increased to high palaeoproductivity in the middle-upper WF, and suddenly decreased in the upper WF. Palaeoproductivity was generally low in the GYQ, which was caused by mass extinction as a result of Hirnantian glaciation at the end of the late Ordovician [14,16]. In the lower LLM in the early Silurian, palaeoproductivity was high and peaked in sample Y12 of graptolite biozone LM5, and then gradually decreased in graptolite biozone LM6 in the upper LLM shown above the red dotted line, maintaining a low level of palaeoproductivity throughout the ULM stage. The decline in palaeoproductivity in the upper LLM and ULM is closely related to the Kwangsian Orogeny, which resulted in the uplift of the Longmaxi Formation and the fall of sea level [20,21,61], and the sedimentary facies gradually changed from deep shelf to shallow shelf, which resulted in a large number of graptolites that cannot be deposited and preserved due to the influence of tides and waves in the shallower water, leading to a decrease in graptolite abundance and quantity [21,73,74,75], as well as a low palaeoproductivity level with low TOC and Si_bio content and Si_bio/(Si_bio + Fe_bio + Al + Ca_bio) ratios (Figure 7 and Figure 10). Overall, theses analyses show that palaeoproductivity was high in organic-rich shales with peak values occurring in the lower LLM and significantly low values occurring in organic-lean shales in the Yiling block, and the trend of palaeoproductivity variation is consistent with that found in other regions of the middle-upper Yangtze Platform [12,13,15].

Previous studies have shown that palaeoproductivity is strongly related to the upwelling of nutrients [3] because upwelling can usually bring nutrients from the deep ocean to the shallow shelf, which results in plankton flourishing in surface seawater. The scatter plots show that the Co × Mn values have a perceptible negative correlation with palaeoproductivity proxies TOC and Si_bio in well YD1 (Figure 12a,b), which indicates that palaeoproductivity is apparently influenced by upwelling. Remarkably, palaeoproductivity is primarily controlled by upwelling in the WF and the high TOC and Si_bio contents of the four samples with Co× Mn values of less than 0.4. Additionally, in the region close to the Qinling Ocean (well EHD1) with strong upwelling, palaeoproductivity is higher than that far from the Qinling Ocean (well JY143) (Figure 1c) [12].

5.3. OMA Mechanism

5.3.1. Lithofacies Types Influencing OMA

The sedimentary environment controlled the lithofacies types of shale. Previous studies have suggested that siliceous shale in the Wufeng–Longmaxi Formation has relatively high TOC content, which is beneficial for preserving organic matter [51,53]. The results show that siliceous shale has the highest TOC content (Figure 3d), which is consistent with previous research results [52]. TOC levels are higher in subclass lithofacies S and S-2, and relatively lower in subclass lithofacies M-1 and S-3. Lithofacies S and S-2 are mainly distributed in the WF and LLM with anoxic conditions, high palaeoproductivity, and low terrigenous influx, which are conducive to OMA (Figure 3 and Figure 10). In contrast, lithofacies M-1 is distributed in the WF with low palaeoproductivity and moderate terrigenous influx, and the lithofacies S-3 is mainly distributed in the ULM with oxic conditions, low palaeoproductivity, and high terrigenous influx (Figure 3 and Figure 10). Thus, they are generally not beneficial to OMA.

5.3.2. Main Controlling Factor of OMA

In general, OMA occurred for three reasons: organic matter input (palaeoproductivity), organic matter preservation (palaeoredox conditions and deposition rates), and organic matter dilution (terrigenous detrital influx) [35,76], and the correlations between TOC content and the terrigenous influx proxy Al, deposition rate proxy La_N/Yb_N, palaeoredox proxy Ni/Co, and palaeoproductivity proxy (Ni + Cu + Zn)_bio are used to study the main controlling factors of OMA in well YD1. The results show that TOC contents have a weak positive correlation with Al content (Figure 13a), have no obvious correlation with the La_N/Yb_N ratio (Figure 13b), and have an apparent correlation with the Ni/Co ratios and (Ni + Cu + Zn)_bio concentrations (Figure 13c,d). These analysis results are consistent with previous studies showing redox conditions and productivity to be the key factors of OMA in the Wufeng–Longmaxi Formation [3,5].

However, we further found that the factors that influence OMA changed vertically (Figure 13e–h). In the WF, TOC contents have a highly positive correlation with Al content, indicating that terrigenous influx is the main controlling factor of OMA, which is significantly different from other regions, inferring that the sedimentation rate of organic matter is higher than the terrigenous dilution rate in well YD1. In the LLM, TOC contents have no correlation with Al content, a weak negative correlation with the La_N/Yb_N ratios, and positive correlations with the Ni/Co ratios and (Ni + Cu + Zn)_bio concentrations, indicating that combined redox conditions and productivity influence OMA. In the ULM, TOC contents have strongly negative correlations with Al content and no significant correlations with the La_N/Yb_N and the Ni/Co ratios, as well as the (Ni + Cu + Zn)_bio concentrations, indicating that terrigenous influx is the main controlling factor of OMA and implying that terrigenous influx led to the dilution of organic matter, which was not conducive to OMA.

5.4. Evolution Model

A palaeoenvironmental evolutionary model of the Wufeng–Longmaxi Formation is established for the study area, including the LX, WF, GYQ, LLM, and ULM stages (Figure 14). The routing of the model starts from the Chuanzhong uplift (point A), passes through the Jiaoshiba block, the Hunan–Hubei submarine high, the Yiling block, and ends in the Qinling Ocean (point A’) (Figure 1c).

During the LX stage, the middle-upper Yangtze Platform was deposited under a tension setting [16] with a broad, flat, and shallow sea environment [30]. The palaeoenvironment was characterized by a moderately warm and humid climate, moderate terrigenous influx, low productivity, and oxic bottom water conditions, which were not beneficial to OMA (Figure 7, Figure 10 and Figure 14a).

During the WF stage, the sea level started to rise, and the middle-upper Yangtze platform was deposited under a compressive tectonic setting caused by the subduction of the Cathaysia Block towards the Yangtze Platform [16], which led to the formation of the Hunan-Hubei submarine high and sedimentary depressions. The palaeoenvironment was characterized by a slightly warm and humid climate, low terrigenous influx, anoxic bottom water conditions, and high productivity with strong upwelling, which was conducive to OMA and the formation of black siliceous rock (Figure 7, Figure 10 and Figure 14b).

During the GYQ stage, the Hirnantian glaciation caused a significant drop in global sea levels [16,56], and the Hunan–Hubei submarine high was gradually exposed at Earth’s surface [30]. At this time, the climate was cold and arid [14,77], and weak upwelling led to low productivity in both the middle and upper Yangtze Platforms. The depositional basins were mainly deposited under oxic conditions, except for minor formations in the depocentre of the Sichuan Basin that were still deposited under anoxic conditions [14,78]. The cold glacial climate and poor nutrition in the water body are the important reasons for the global biological extinction event in late Ordovician, with a large number of graptolites disappearing [79]. However, Hirnantia Fauna (Brachiopoda), which benefited from a cold climate and oxygen-rich bottom water, survived (Figure 7, Figure 10 and Figure 14c) [80].

During the LLM, the sea level rose rapidly and seawater formed a stable stratified water body due to the melting of the Gondwanan Glacier [16,64,65]. The palaeoenvironment was characterized by a slightly warm and humid climate, low terrigenous influx, anoxic bottom water and even euxinic conditions (Figure 11), and high palaeoproductivity (Figure 7, Figure 10 and Figure 14d). In addition, the accommodating space of the deep basin significantly increased as a result of continuous tectonic compression [16], leading to the deposition of the thickest organic-rich siliceous and argillaceous shales in the Wufeng–Longmaxi Formation. Remarkably, during the upper LLM stage, the climate became much warmer and wetter, and the sea level gradually dropped due to the strong tectonic uplift as a result of the Kwangsian Orogeny [21,81], which led to dysoxic to oxic bottom water conditions. Meanwhile, the graptolite biozones changed significantly from LM5 in the Rhuddanianto LM6 in the Aeronian (Figure 7 and Figure 10).

During the ULM stage, the palaeoenvironment was characterized by a highly warm and humid climate, similar to that of the upper LLM stage, as well as oxic and low productivity conditions. Remarkably, the Xuefeng uplift continued to rise and was gradually exposed at Earth’s surface, finally forming an uplift due to continuous orogenic movement [16], which became a provenance and supplied terrigenous clasts to the middle-upper Yangtze Platform, leading to the highest terrigenous influx in the Wufeng–Longmaxi Formation, which diluted organic matter and was not conductive to OMA (Figure 7, Figure 10 and Figure 14e).

Overall, organic-rich shales in the WF and LLM of the Yiling block are the high-quality shale gas reservoirs, which are characterized by shale thicknesses of 10–20 m, TOC contents of 3%–4%, and total gas contents of 1–3 m³/t [11]. Although gas contents in the Yiling block are slightly lower than those in the Jiaoshiba block [6,13], prospects for shale gas exploration are still excellent due to high TOC contents, anoxic conditions, and high productivity [2,11]. Remarkably, regions in the eastern Yiling block close to the Qinling Ocean show better prospects for shale gas exploration because of the thicker organic-rich shales caused by deeper water, euxinic bottom water conditions, and higher productivity as a result of the strong upwelling.

6. Conclusions

The Wufeng–Longmaxi shales were primarily deposited in a highly restricted basin, except for the relatively open and upwelling setting of the upper WF. These organic-rich shales were formed in a warm and humid climate with low terrigenous influx, anoxic conditions, and high productivity. The silica in the shales had a primarily nonhydrothermal origin, with an entirely biogenic source only in the upper WF.

In the southeastern Sichuan Basin, the bottom water of organic-rich shales is mainly characterized by anoxic–euxinic conditions, while in the study area, it is primarily dominated by anoxic–ferruginous conditions, suggesting a shallower water depth.

Compared to previous studies, our research provides further evidence that the Kwangsian Orogeny had a rapid and significant impact on the palaeoenvironment of the Wufeng–Longmaxi Formation in western Hubei. This environmental shift is observed in the black shale of the LLM stage, resulting in a warmer and wetter climate, a notable decrease in productivity due to inadequate nutrients caused by weak upwelling, an evident increase in terrigenous input, and a shift in bottom water from anoxic to oxic conditions. These changes led to a decrease in graptolite abundance and the transition of graptolite biozones from LM5 to LM6.

Previous studies have established that redox conditions and productivity were the primary controlling factors for OMA, while our research indicates that the factors influencing OMA change vertically, with terrigenous influx mainly influencing OMA in the WF and ULM, and both redox conditions and productivity affecting OMA in the LLM. Notably, the WF stands out from other regions in that TOC contents have a highly positive correlation with Al content, which indicate that the terrigenous influx is the primary factor influencing OMA, suggesting a higher sedimentation rate of organic matter during this stage.

Despite having slightly lower gas contents than the Jiaoshiba block, the organic-rich shales in the WF and LLM of the Yiling block are still considered high-quality shale gas reservoirs with the organic-rich shale thicknesses exceeding 10 meters. Notably, the eastern regions of the Yiling block that are closer to the Qinling Ocean exhibit even better prospects for shale gas exploration.