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Article

Paleoenvironment Comparison of the Longmaxi and Qiongzhusi Formations, Weiyuan Shale Gas Field, Sichuan Basin

by
Qin Zhang
1,2,
Feng Liang
1,2,*,
Jingbo Zeng
3,
Zhen Qiu
1,2,*,
Shangwen Zhou
1,2,
Wen Liu
1,2 and
Weiliang Kong
1,2
1
PetroChina Research Institute of Petroleum Exploration and Development, Beijing 100083, China
2
National Energy Shale Gas R&D (Experiment) Centre, Langfang 065007, China
3
China National Logging Corporation, Xi’an 710000, China
*
Authors to whom correspondence should be addressed.
Processes 2023, 11(7), 2153; https://doi.org/10.3390/pr11072153
Submission received: 28 April 2023 / Revised: 25 June 2023 / Accepted: 26 June 2023 / Published: 19 July 2023
(This article belongs to the Special Issue Advances in Gas Adsorption and Porosity for Enhanced Recovery of Shale Gas)
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Versions Notes

Abstract

:
The Lower Cambrian Qiongzhusi formation and the Lower Silurian Longmaxi Formation are the two most important shale strata. Although differences between these two shales have become the focus of current research, a comparative study of the depositional environments has not been performed. Using cores of both Longmaxi and Qiongzhusi formations of well W201, the in situ comparison of the sedimentary environment was realized, and the interference of other factors was eliminated, which made the results more reliable. In this study, 72 samples from both formations were collected from well W201, Weiyuan shale gas field, Sichuan Basin. A systematic study, including total organic carbon (TOC) content, mineral composition, and major/trace elemental analyses, was conducted to elucidate the paleoenvironments of the Qiongzhusi and Longmaxi formations. The results show both formations were deposited in non-sulfidic environments. The depositional conditions of the Longmaxi formation varied from reducing to oxidizing from bottom to top. The detrital flow happened during the deposition of the Qiongzhusi formation, which resulted in three stages of the redox conditions, from anoxic to oxic and then to anoxic from bottom to top of the Qiongzhusi formation. The anoxic conditions of the Qiongzhusi formation were considerably stronger than those of the Longmaxi formation. Both formations were deposited in warm and humid climates. Ratios of Eu/Eu*, Y/Y*, LaN/YbN, light rare earth element (LREE) and heavy rare earth element (HREE) revealed that the Longmaxi formation was primarily controlled by seawater, whereas the Qiongzhusi formation was jointly influenced by seawater and hydrothermal fluid. The organic matter enrichment for the Longmaxi and Qiongzhusi formations was controlled by paleoproductivity and redox conditions. Due to the slightly lower paleoproductivity and influence of detrital input, the degree of organic matter enrichment in the Qiongzhusi formation was lower than that in the Longmaxi formation.

1. Introduction

Shale gas, a new unconventional energy source, has effectively relieved the contradiction between global oil and gas supply and demand, and facilitated energy independence in the United States [1,2]. Owing to advanced horizontal drilling and fracturing technologies, shale gas has become one of the most significant energy resources [3,4]. Behind the USA, China has become the second-largest shale gas producer. The large-scale development of shale gas, on the other hand, has promoted shale gas research, with relevant papers accounting for 33% of global shale gas publications [5]. Shale gas resources in China are 134.42 × 1012 m3, of which the resources in the Lower Paleozoic strata account for 43.58%. These resources are primarily distributed in the Longmaxi and Qiongzhusi formations [6,7], which are widespread in the Yangtze Platform. The TOC content, organic matter maturity, thickness, and brittle mineral content all meet shale gas requirements. Shale gas in the Longmaxi formation has been commercially developed in the Changning, Weiyuan, Zhaotong, and Fuling shale gas fields. In 2020, shale gas production in the Longmaxi formation reached 200 × 108 m3 [8]. In contrast, the Qiongzhusi formation has not developed as successfully as the Longmaxi formation. Industrial gas flows have only been achieved in wells W201 (1.08 × 104 m3/d) and W201-H3 (2.83 × 104 m3/d) of the Weiyuan block by PetroChina [9] and in Wells Jinshi-1 (2.88 × 104 m3/d) [10] and Jinye-1HF (8 × 104 m3/d) [11] in the southwest Weiyuan block by Sinopec. Because of the large number of dry wells and the low single-well production, the development of shale gas in the Qiongzhusi formation has plateaued. The reasons for the failure of the Qiongzhusi formation are complex and remain highly controversial. Comparative analysis of the mineral composition [12], pore characteristics [13,14], thermal maturity [15], methane adsorption capacity [16], and preservation conditions [9] of the Qiongzhusi and Longmaxi formations have been extensively studied. However, their depositional environments have not been compared.
Depositional conditions control the TOC content, mineral composition, and lamination development, and in turn determine hydrocarbon potential, storage capacity, methane adsorption, and the fracturability of shale [17,18,19]. As breakthroughs have been achieved in the Longmaxi formation, water restrictions [20,21], redox conditions, paleoproductivity [22,23,24], terrestrial input [25,26], lamina development [27], lithofacies [28], and authigenic quartz origins [29,30] have been systematically investigated. The Ediacaran to Early Cambrian transition is a critical interval in Earth’s history characterized by global environmental and biological changes. The interval is typically studied as an integrated period, whose biological fossils, sedimentary and geochemical characteristics [31], deposits associated with hydrothermal flux [32], and carbon and sulfur isotope excursions [33,34] have been researched. Previous studies on paleoenvironments of the Qiongzhusi formation were primarily based on outcrop sections in northern Guizhou and Hunan Provinces [34,35,36]. Due to weathering influence, the outcrops are inferior to the cores. Weiyuan, as the demonstration area for shale gas development, currently focuses on the Longmaxi formation. However, it is under great pressure to increase shale gas reserves and production by only relying on the Longmaxi formation. It is necessary to explore new targets and if the Qiongzhusi formation makes a breakthrough, scale production can be achieved rapidly using the existing facilities. Well W201, the first evaluation well of shale gas, retrieved cores from the Longmaxi and Qiongzhusi formations and provided excellent conditions for a detailed in situ comparison of the corresponding paleoenvironments. The comparison studies can provide theoretical support for shale gas exploration and the development of the Qiongzhusi formation.
In this study, shale samples from the Longmaxi and Qiongzhusi formations were systematically collected from Well W201. The mineral composition and high-resolution geochemical profiles of the shale samples were measured to better elucidate the geochemical conditions during their deposition. The focus was to characterize the sedimentary environments and to determine the differences between the deposition periods of the Longmaxi and Qiongzhusi formations.

2. Geological Background

The Weiyuan block is in the southwest of the Sichuan Basin and geographically straddles Weiyuan, Zizhong, and Rong counties, with an area of 6500 km2 (Figure 1a). Structurally, it belongs to the low-amplitude fold belt of the Guzhong Slope in southwestern Sichuan Province and develops the Weiyuan anticline. The Upper Yangtze region, centered on the Sichuan Basin, entered the evolution stage of the craton basin in the Nanhua period and was in a tensional tectonic environment from the Sinian to the Middle Ordovician period. In the Early Cambrian, the Pangea disintegrated, and the rapid expansion of the sea floor led to rapid sea level rise, forming the Qiongzhusi formation. From the Late Ordovician to Early Silurian, two global-scale transgressions occurred [37], resulting in the deposition of the Wufeng and Longmaxi formations. The Qiongzhusi formation is widespread across the Yangtze Block and in unconformable contact with the Ediacaran Dengying Formation. The lithology of the Qiongzhusi formation is primarily basal organic-rich black shale and black shale interbedded with gray siltstone (Figure 1c). Small shelly fossils are commonly observed in the lowermost part of the black shale [38]. The Lower Silurian Longmaxi Formation is in conformable contact with the Upper Ordovician Wufeng Formation and the burial depth of the Wufeng Formation in the study area is 1500~4000 m, deepening to the southeast direction (Figure 1a). The Longmaxi formation is dominated by black carbonaceous and black silty shales (Figure 1b). The formation is darker in color and finer in grain size with increasing depth. The lower Longmaxi formation is rich in graptolite fossils and can be laterally correlated with other wells in the Yangtze area. Well W201 is the first shale gas evaluation well deployed by the Southwest Oil & Gasfield Company in China, which is located on the paleo-carbonate platform [38]. The well was drilled to the Sinian Dengying formation with a depth of 2840 m. The purpose of drilling is to obtain reservoir parameters and evaluate the gas-bearing properties of both the Longmaxi and Qiongzhusi formations.

3. Materials and Methods

3.1. Experimental Methods

A total of 72 shale samples, including 25 samples from the Longmaxi formation and 47 samples from the Qiongzhusi formation of Well W201 were collected. The samples were tested for TOC content, mineral composition, and major/trace elements.
The TOC content was tested using a Leco CS-230 carbon and sulfur analyzer (LECO Corporation, St. Joseph, MI, USA) following the Chinese National Standard (GB/T 19145-2003). The shale samples were first crushed using a 200 mesh, then a 12.5% hydrochloric acid (HCl) solution was added to the powder to remove the inorganic carbon. Finally, the samples were completely dried after washing with deionized water.
The mineral composition was determined by X-ray diffraction analysis using a Bruker Corp. AXS D8 Discover X-ray diffractometer (Bruker, Billerica, MA, USA) according to the national standard of China (SY/T5163-1995).
The major and trace elements were characterized by X-ray fluorescence spectrometry at the Beijing Research Institute of Uranium Geology, and the trace elements were measured using a VG PQ2 Turbo inductively coupled plasma source mass spectrometer (ICP-MS, Perkin Elmer, Waltham, MA, USA).

3.2. Data Presentation

Trace elements in sediments generally have detrital and authigenic origins. Only the authigenic fraction can reflect the variations of the paleoenvironment and chemical composition of seawater throughout geological history [35]. Trace elemental concentrations are normalized to the content of the detrital indicators to eliminate the interference of terrestrial input to authigenic trace elements and to reduce the dilution effects of the carbonate to trace-element abundance in the sediments [39]. The concentrations are then compared with the reference values (enrichment factors [EFs]). Specifically, the EF of element X is determined using the formula XEF = (X/detrital indicator)sample/(X/detrital indicator)PAAS.
The Post-Archean Australian Shale (PAAS) data were obtained from Taylor and McLennan [40] and McLennan [41]. Ti and Al are commonly used as indicators of detrital input; however, some researchers found that soluble Ti and Al precipitate to the sea floor in the form of hydroxides and organometallic compounds in the continental shelf, and become trapped within sedimentary rocks [42]. Therefore, identifying whether Al and Ti primarily originate from terrigenous siliciclastic sources is necessary to precisely assess element enrichment. A customary method to determine the origin is to plot Al and Ti content against the content of Sc, Zr, or Th, which are also overwhelmingly of detrital origin [16,20,39]. A positive correlation of Al and Ti with Sc, Zr, and Th in the samples indicates a terrigenous origin. Otherwise, Al and Ti are a mixture of detrital and authigenic sources.

4. Results

4.1. TOC and Mineral Composition

The TOC contents of the Longmaxi and Qiongzhusi formations ranged from 0.05 to 4.28% (average of 2.02%) and 0.48 to 3.71% (average of 1.4%), respectively (Supplementary Table S1 and Figure 1b).
The mineral compositions of the Longmaxi and Qiongzhusi formations are listed in Supplementary Table S1. The shale samples of the Longmaxi formation are primarily composed of quartz, clay minerals, and carbonate, with small amounts of pyrite, plagioclase feldspar, potassium feldspar, and pyrite. The quartz content ranges from 12.6 to 44.1% (average of 26.2%). The clay mineral content ranges from 21.3 to 70.4% (average of 48.6%), and the carbonate minerals vary from 0 to 49.4% (average of 18.5%). Other minerals (including pyrite, plagioclase, and potassium feldspar) each account for an average of <5%. The shale samples of the Qiongzhusi formation are primarily composed of quartz and feldspar (including plagioclase and potassium feldspar), with contents of 19~42% and 24~53%, respectively. The clay minerals, carbonate, and pyrite are minor constituents, ranging from 6 to 23%, 2 to 16%, and 1 to 7%, respectively.

4.2. Major Elements

The major/trace elements of the Longmaxi and Qiongzhusi formations of well W201 are presented in Supplementary Tables S2 and S3. The samples of the Longmaxi formation are composed primarily of SiO2, Al2O3, and TFe2O3, which account for a total of 57.78~85.49%. SiO2, Al2O3, and CaO have contents of 29.28~76% (average of 51.33%), 3.43~18.42% (average of 12.88%), and 0.272~33.01% (average of 8.9%), respectively. The average contents of TFe2O3, K2O, and MgO account for 5.2%, 3.4%, and 3.6%, respectively. Other major elements (including Na2O, MnO, TiO2, and P2O5) have mean concentrations below 1%. SiO2, Al2O3, and Fe2O3 are also the major components of the Qiongzhusi formation, accounting for a combined 75.61~83.98%. SiO2 is the dominant component, with contents ranging from 58.48 to 65.85% (average of 62.4%). Al2O3 is the second most abundant, with contents ranging from 11.54 to 19.2% (average of 11.2%). The samples have average TFe2O3 contents of 3.88%, 7.65%, and 5.26%. The mean values of CaO, Na2O, and MgO are 3.3%, 2.91%, and 2.49%, respectively. Other major elements (including TiO2, P2O5, and MnO) account for less than 1% each.
The chemical index of alteration (CIA) quantitatively reflects the degree of chemical weathering of the source rock [43] and was adopted to study paleoclimatic changes during sediment deposition [44]. The expression of CIA is as follows: CIA = [(Al2O3)/(Al2O3 + CaO* + Na2O + K2O)] × 100, where CaO* is the CaO composition of silicate minerals [43]. All oxide concentrations are given in molecular proportions.
The TOC contents of the Longmaxi and Qiongzhusi formations ranged from 0.05 to 4.28% (average of 2.02%) and 0.48~3.71% (average of 1.4%), respectively (Supplementary Table S1 and Figure 1b).

4.3. Trace Elements

Al in the Longmaxi formation is positively correlated with Zr, Sc, and Th, demonstrating a terrigenous source. In contrast, no significant correlations were observed in the Qiongzhusi formation among Al and Ti and Sc, Zr, and Th; therefore, Al and Ti originated from a mixture of authigenic and detrital sources (Figure 2). Accordingly, for EF calculation, the trace-element concentrations of the Longmaxi and Qiongzhusi formations were normalized to the Al concentration and Sc content, respectively. Figure 3 shows the EFs. The trace-element abundances of the Longmaxi formation (except for the W201-101, W201-103, and W201-105 upper Longmaxi samples) share similarities with the samples of the Qiongzhusi formation, exhibiting the highest enrichment of Mo; moderate enrichment of U, V, Zn, Ni, Th, Cu, Ba, and Zr; and the lowest enrichment of Cr and Co. The Qiongzhusi formation samples have considerably higher MoEF and UEF values (42.5% and 7.1%, respectively) than those of the Longmaxi formation samples (15.2% and 4.8%, respectively).

4.4. Rare Earth Elements (REEs)

The Longmaxi samples have average total rare earth element (∑REE) concentrations of 47.25~645.8 ppm (average of 207.84 ppm) and LREE and HREE ratios of 0.50~1.29 (average of 0.90). The PAAS-normalized Longmaxi shale samples exhibit a flat REE pattern (LaN/YbN: 0.62~2.60, average of 1.23), weak Ce depletion (Ce/Ce* = 0.61~0.90, average of 0.85), weakly negative Eu anomalies (Eu/Eu* = 0.82~1.0, average of 0.95)—except for sample W201-124 (Eu/Eu* = 1.30)—and positive Y anomalies (Y/Y* = 0.97~1.7, average of 1.10) (Figure 4a). The Qiongzhusi samples have ∑REEs ranging from 98.52 to 207.48 ppm (average of 158.11 ppm) and LREE and HREE ratios of 0.56~0.89 (average of 0.68). The REE distribution patterns of the Qiongzhusi formation samples show a progressive enrichment toward heavier REE (LaN/YbN = 0.4~1.04, average of 0.71), weak Ce depletion (Ce/Ce* = 0.85~1.03, average of 0.91), moderately positive Eu anomalies (Eu/Eu* = 1.09~1.34, average of 1.18) and negative Y anomalies (Y/Y* = 0.61~1.05, average of 0.91) (Figure 4b). The Qiongzhusi samples have considerably lower ∑REEs and LREE and HREE ratios than the Longmaxi samples.
To prevent analytical interference in the calculation of Eu anomalies caused by Ba concentration in ICP-MS analysis, the method by Dulski [45] was adopted to correct:
Eu/Eu* = (3 × EuN)/(2 × SmN + TbN)
Ce/Ce* and Y/Y* were calculated using the following formulas:
Ce/Ce* = CeN(1/2 × LaN + 1/2×PrN)
Y/Y* = YN/(DyN × HoN)1/2
N refers to normalized concentrations against the PAAS.

5. Discussion

5.1. Redox Proxies

Cr and Co in sedimentary rocks are likely affected by terrestrial input, whereas U and V are barely affected [39,46]. Therefore, U and V enrichment and the Th/U and V/Sc ratios are used as redox proxies in the following discussions.
The U/Al ratios of the Longmaxi formation samples decrease in the upwards direction. Compared with the U/Al ratio (×10−4) in the PAAS (0.310), most Longmaxi formation samples exhibit significant U enrichment (0.25~9.47, average of 1.5) (Figure 5). The Qiongzhusi formation samples have U/Sc ratios of 0.45~3.88, which are considerably higher than that in the PAAS (0.19). The U/Sc ratios show a three-section pattern that enriched in the lower (0.54~3.88, average of 1.89) and upper (0.63~3.54, average of 1.79) sections and depleted in the middle section (0.44~2.50, average of 0.7) (Figure 6). This may have resulted from the abundant detrital input in the middle section. U enrichment exhibits a strong positive correlation with TOC under non-sulfidic anoxic conditions, whereas a weak correlation is observed under sulfidic conditions [46]. In the present study, the U/Al ratios of the Longmaxi formation—except for the W201-124 sample—were highly correlated with the TOC content (correlation coefficient of 0.61). The U/Sc ratios of the Qiongzhusi formation samples are also positively correlated with TOC content (correlation coefficient of 0.88), demonstrating that the redox condition was generally anoxic but not sulfidic; these results were contrary to the conclusion reached by [32].
The Longmaxi formation samples have V/Al ratios (×10−4) of 11.4~82.1 (average of 31.2), which are considerably higher than that in the PAAS (15.0) (Figure 5). The V/Al ratios of the samples exhibit a similar decreasing trend from bottom to top as the U/Al ratios, indicating that the paleoenvironment of the Longmaxi formation gradually varied from strongly anoxic to oxic from bottom to top. The Qiongzhusi formation samples have V/Sc ratios of 7.07~150.1 (average of 28.3) that are higher than those of the PAAS (9.4). Similar to the distribution pattern of U/Sc ratios, the V/Sc ratios are highest in the lower section (7.07~125.02, average of 48.3), followed by the upper section (8.4~150.1, average of 24.7) and the middle section (7.1~13.3, average of 10.1), except for sample 9 (57.8) (Figure 6). These results imply that the seawater was strongly anoxic, weakly anoxic, and oxic during the deposition periods of the lower, upper, and middle intervals of the sections. The V/Al and V/Sc ratios are highly correlated with the TOC content in the Longmaxi and Qiongzhusi formations (Figure 7), demonstrating that redox conditions were generally anoxic but not sulfidic [46]. These results are consistent with the conclusions obtained by the U/Al and U/Sc ratios.
Kimura et al. [47] proposed that Th/U = 2 was an appropriate cut-off point between oxic and anoxic bottom waters. Th/U ratios of 0~2 represent anoxic conditions, whereas values higher than 2 indicate oxic seawater. The Th/U ratios of the Longmaxi formation exhibit significant variations (0.3–7.34, average of 2.73). The value is lower than 1.5 at the bottom and increases in the upward direction (Figure 5); thus, the seawater of the Longmaxi formation changed from anoxic to oxic from bottom to top. The Th/U ratios of the Qiongzhusi samples are considerably lower (average value < 1.2) than those of the Longmaxi samples. The samples from the lower, upper, and middle sections have Th/U ratios of 0.22~1.27 (average of 0.66), 0.28~21.84 (average of 0.83), and 0.33~2.58 (average of 2.12), respectively (Figure 6); therefore, the lower section of the Qiongzhusi formation was deposited under a more reducing environment than the upper section and the middle section deposited under oxic conditions. Because both V and Sc are insoluble, V/Sc ratios can also be used as a redox-sensitive proxy [47]. The samples from the lower part of the Longmaxi formation show significant V enrichment over Sc (9.12~34.09, average of 21.8) and Th/U ratios < 1.5. The samples from the lower and upper sections of the Qiongzhusi formation are characterized by high V/Sc ratios and low Th/U ratios. The Th/U ratios and V/Sc ratios all reveal that the redox conditions of the Longmaxi formation changed gradually from bottom to top, whereas the redox conditions of the Qiongzhusi formation experienced three stages, from strongly anoxic to oxic conditions to anoxic. Except for the middle section, the V/Sc and Th/U ratios all revealed that the anoxic conditions of the lower and upper sections of the Qiongzhusi formation were considerably stronger than those of the Longmaxi formation.

5.2. Paleoclimate

CIA values higher than 80, from 65 to 80, and from 50 to 65 indicate hot and humid, warm and humid, and cold and arid climates, respectively [48]. The shale samples of the Longmaxi formation generally have high corrected CIAs from 65.4 to 76.1 (average of 71.6) that are equivalent to those of average shale (70~75) [49] and indicate warm and humid climates. Vertically, the CIAs are the lowest in the lower Longmaxi formation and increase from bottom to top (Figure 5), suggesting that the paleoclimate was cold in the Early Silurian and warmed as the Gondwana glaciers melted. The CIAs of 44.4~63.9 (average of 51.8) of the Qiongzhusi formation samples are considerably lower than those of the Longmaxi formation (Figure 6), reflecting a cold and arid paleoclimate. Previous studies show that a warm climate and increased atmospheric oxygen were major factors leading to the Early Cambrian biological explosion [32,50]. Recent glendonite findings in the Lower Cambrian suggest the possibility of at least momentary cooling events [51]. The cause of the low CIAs in the Qiongzhusi formation is unknown. The Qiongzhusi formation samples have high potassium feldspar and plagioclase contents (24~53%, average of 39.5%) that are equivalent to quartz content (19~42%, average of 33.2%) but extremely low clay contents (6~23%, average of 13.5%), which reflect the short-distance transportation of the parent rocks. Therefore, feldspar particles directly accumulated rather than being changed to clay; consequently, Al-abundant clay did not form, resulting in extremely low CIAs. Thus, detritus flow occurred during the deposition of the Qiongzhusi formation, leading to poor sorting and high feldspar content during that period.
The Sr/Cu ratio is also commonly used to reflect paleoclimates. Sr/Cu ratios of 1.3~5.0 denote warm and humid paleoclimates, whereas ratios higher than 5 indicate a dry and hot paleoclimate [16]. The samples of the Longmaxi formation have Sr/Cu ratios between 1.9 and 11.1 (average of 4.6) (Figure 5); and 61.2% of the samples have Sr/Cu ratios between 1.3 and 5.0, suggesting a warm and humid paleoclimate. The samples of the Qiongzhusi formation have Sr/Cu ratios between 1.1 and 9.1 (average of 4.3) (Figure 6), and 72% of the samples have Sr/Cu ratios between 1.3 and 5.0; thus, the paleoclimate was warm and humid during the deposition of the Qiongzhusi formation. This further proves that detrital flow occurred during the Early Cambrian, leading to an inaccurate CIA paleoclimate indication.

5.3. Paleoproductivity

Studies of modern seas have revealed that high surface productivity accelerates the consumption of dissolved oxygen in water, resulting in anoxic conditions favorable for the preservation of organic matter [52]. Surface productivity is primarily contributed by plankton, which rely on P, Ba, Cu, and Ni for growth. Under strong reducing conditions, Ba and P are likely to precipitate from sediment, whereas Cu and Ni are hardly affected [39,46]. Under anoxic conditions, Cu and Ni are reduced by sulfate-reducing bacteria and subsequently precipitate in an independent sulfide phase in sediment [39,46]. Therefore, the original information on productivity can be analyzed using Cu and Ni. In this study, (Ni + Cu)/Al and (Ni + Cu)/Sc were used as paleoproductivity proxies for the Longmaxi and Qiongzhusi formations [26], respectively, to eliminate the influence of detrital input. Longmaxi formation samples have Ni + Cu/Al ratios of 5.15~58.6 (average of 18.5), which are mostly higher than that of the PAAS (10.5). Paleoproductivity tends to decrease gradually from bottom to top and peaks in the lower section of the Longmaxi formation, with values ranging from 14.6 to 58.6 (average of 22.3) (Figure 5). The Qiongzhusi formation samples have Ni + Cu/Sc ratios of 5.1~36.3 (average of 11.3), which are mostly lower than that of the PAAS (17.5), and reflect lower paleoproductivity than that of the Longmaxi formation. Moreover, from bottom to top, the Ni + Cu/Sc ratios of the Qiongzhusi formation are divided into three different sections. The samples from the lower, upper, and middle sections, except for sample W201-9 (36.3), have Ni + Cu/Sc ratios of 5.1~20.3 (average of 14.2), 6.2~23.1 (average of 11.5), and 5.3~8.9 (average of 6.5), respectively (Figure 6).

5.4. Hydrothermal Depositional Condition

Sediments influenced by hydrothermal input have low ∑REEs, low LREE/HREE ratio, significant Ce depletion, and positive Eu and Y anomalies. The features of the normal seawater sediments are contrary to those of the sediments influenced by hydrothermal input [31,53].
Positive Eu anomalies are typically the result of a strong reducing environment [31,54]. However, hydrothermal input can also result in positive Eu anomalies, which we calculated using the formula proposed by Dulski [45] to eliminate Ba interference. The samples of the Longmaxi formation—except W201-124 (Eu/Eu* = 1.30, Ce/Ce* = 0.61; Y/Y* = 1.7; LaN/YbN = 0.62; LREE/HREE = 0.50)—show typical seawater features of weak negative Eu and Ce anomalies (Eu/Eu* = 0.82~1.0, average of 0.95; Ce/Ce* = 0.78~0.91, average of 0.86), positive Y anomalies (Y/Y* = 0.97~1.27, average of 1.10), a high LaN/YbN ratio (LaN/YbN = 1.1~2.6, average of 1.3), and a relatively high LREE/HREE ratio (LREE/HREE = 0.83~1.29, average of 0.93).
The Qiongzhusi formation samples—except for those with extremely high Ba content (samples 2 and 5)—showed no significant positive linear correlation between Ba and Eu/Eu* (Figure 8a), indicating the relatively weak effect of Ba interference. Previous research indicates that detrital feldspar could contribute to positive Eu anomalies [55]. However, this possibility was ruled out, because the samples show no correlation between Sc and Eu/Eu* (Figure 8b). The Qiongzhusi formation samples show moderately positive Eu anomalies (Eu/Eu* = 1.09~1.34, average of 1.18), weak Ce depletion (Ce/Ce* = 0.85~1.03, average of 0.91), negative Y anomalies (Y/Y* = 0.61~1.05, average of 0.91), a lower LaN/YbN ratio (LaN/YbN = 0.4~1.04, average of 0.71), a considerably lower LREE/HREE ratio (LREE/HREE = 0.56~0.89, average of 0.68), and considerably higher ∑ REE content; these features are typical of mixed hydrothermal fluid and seawater. The influence of hydrothermal activities on black shale deposition in the Qiongzhusi formation is also discussed by Chen et al. [56].

5.5. Organic Matter Enrichment Mechanism

The controlling factors for black shale include climate fluctuations, sedimentation rate, nutrient flux, terrestrial input, hydrodynamic setting, or a combination of these factors [57]. Samples from the Longmaxi and Qiongzhusi formations, which formed in warm and humid climates, show positive correlations between TOC content and paleoproductivity proxies (Ni + Cu/Al, Ni + Cu/Sc), with correlation coefficients of 0.66 and 0.61, respectively (Figure 9a,b). The samples of the Longmaxi and Qiongzhusi formations also show a close correlation between TOC content and redox-sensitive proxies (Th/U and V/Sc), with correlation coefficients of −0.83 and 0.60 and −0.53 and 0.53, respectively (Figure 9c,d). Thus, anoxic conditions and nutrient input play important roles in organic matter enrichment in the Longmaxi and Qiongzhusi formations. The redox environment and paleoproductivity are not independent but affect each other [57]. The lower part of the Longmaxi formation is characterized by maximum TOC content, which may reflect the effect of climatic-oceanic changes. At the beginning of the Longmaxi deposition, the climate warmed, promoting algal blooms. However, the increased nutrient flux enhanced oxygen consumption, promoted the development of anoxic conditions, and enriched organic matter. In addition to redox conditions and paleoproductivity, the hydrothermal input of the Qiongzhusi formation promoted element enrichment and intensified the reducing environment and algal blooms. However, the detritus flow during the middle period of the Qiongzhusi formation deposition introduced oxygen into the bottom water, which led to lean organic matter in the shale.

6. Conclusions

The analysis of petrography, TOC content, mineral composition, and major/trace elements provides constraints on the paleoenvironments of the Longmaxi and Qiongzhusi formations, and the conclusions are as follows:
(1)
Both the Longmaxi and Qiongzhusi formations were deposited in non-sulfidic seawater, and the paleoclimate was warm and humid.
(2)
The redox conditions of the Longmaxi formation varied from reducing to oxidizing from bottom to top, while the redox conditions of the Qiongzhusi formation experienced three stages, from anoxic to oxic to suboxic. The anoxic conditions in the Qiongzhusi formation are considerably stronger than those in the Longmaxi formation.
(3)
The Longmaxi formation was primarily controlled by seawater, whereas the Qiongzhusi formation was jointly influenced by seawater and hydrothermal fluid. Paleoproductivity and the covariation of redox conditions jointly controlled organic matter enrichment for the Longmaxi and Qiongzhusi formations.
(4)
Due to the low detrital input and high TOC content, the bottom sections of the Longmaxi and the Qiongzhusi formations are favorable sections for shale gas exploration.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/pr11072153/s1, Table S1: X ray diffraction (XRD) mineral analytical result; Table S2: Major oxides (as %) and minor elements (reported as ppm) of the Longmaxi Formation samples from well W201; Table S3: Major oxides (as %) and minor elements (reported as ppm) of the Qiongzhusi Formation samples from well W201.

Author Contributions

Conceptualization, formal analysis, and writing—original draft: Q.Z. and F.L.; supervision and project administration: F.L.; funding acquisition and validation: Z.Q.; writing—reviewing and investigation: J.Z., S.Z., W.L. and W.K. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Scientific Research and Technological Development Programs of RIPED (No. 2021yjcq02) and CNPC (No. 2021DJ2001).

Acknowledgments

Our study was supported by the Scientific Research and Technological Development Programs of RIPED (No. 2021yjcq02) and CNPC (No. 2021DJ2001). The editors and anonymous reviewers are gratefully acknowledged. We also want to express our great gratitude to the staff of the National Energy Shale Gas R&D (Experiment) Centre for their hard work and our thanks to Pang Zhenglian for the helpful suggestions and advice.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Bottom structural map of the Wufeng Formation in the Weiyuan block (a) and the lithologic column of the Longmaxi and Qiongzhusi formations of W201 well (b,c) (some Qiongzhusi data adapted from those of the authors in [38]).
Figure 1. Bottom structural map of the Wufeng Formation in the Weiyuan block (a) and the lithologic column of the Longmaxi and Qiongzhusi formations of W201 well (b,c) (some Qiongzhusi data adapted from those of the authors in [38]).
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Figure 2. Al and Ti vs. Zr, Sc, and Th contents of samples from the Longmaxi and Qiongzhusi formations. Blue circles represent samples from the Longmaxi formation, and orange circles represent samples from the Qiongzhusi formation (some Qiongzhusi data adapted from those of the authors in [38]).
Figure 2. Al and Ti vs. Zr, Sc, and Th contents of samples from the Longmaxi and Qiongzhusi formations. Blue circles represent samples from the Longmaxi formation, and orange circles represent samples from the Qiongzhusi formation (some Qiongzhusi data adapted from those of the authors in [38]).
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Figure 3. EFs of trace elements in the Longmaxi (a) and Qiongzhusi (b) formation samples (some Qiongzhusi data adapted from those of the authors in [38]).
Figure 3. EFs of trace elements in the Longmaxi (a) and Qiongzhusi (b) formation samples (some Qiongzhusi data adapted from those of the authors in [38]).
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Figure 4. PAAS-normalized REE patterns of the Longmaxi (a) and Qiongzhusi (b) formations (some Qiongzhusi data adapted from those of the authors in [38]).
Figure 4. PAAS-normalized REE patterns of the Longmaxi (a) and Qiongzhusi (b) formations (some Qiongzhusi data adapted from those of the authors in [38]).
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Figure 5. Environmental proxies of the Longmaxi formation in well W201, Weiyuan shale gas field, Sichuan Basin.
Figure 5. Environmental proxies of the Longmaxi formation in well W201, Weiyuan shale gas field, Sichuan Basin.
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Figure 6. Environmental proxies of the Qiongzhusi formation in Well W201, Weiyuan shale gas field (some Qiongzhusi data adapted from those of the authors in [38]).
Figure 6. Environmental proxies of the Qiongzhusi formation in Well W201, Weiyuan shale gas field (some Qiongzhusi data adapted from those of the authors in [38]).
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Figure 7. TOC vs. U/Al, V/Al and U/Sc, and V/Sc of the Longmaxi and Qiongzhusi formations in Well W201 (some Qiongzhusi data adapted from those of the authors in [38]).
Figure 7. TOC vs. U/Al, V/Al and U/Sc, and V/Sc of the Longmaxi and Qiongzhusi formations in Well W201 (some Qiongzhusi data adapted from those of the authors in [38]).
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Figure 8. Relationships between Ba, Sc, and Eu anomalies in the Longmaxi formation (a) and the Qiongzhusi formation (b) in Well W201.
Figure 8. Relationships between Ba, Sc, and Eu anomalies in the Longmaxi formation (a) and the Qiongzhusi formation (b) in Well W201.
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Figure 9. TOC vs. paleoproductivity (Ni + Cu/Al, Ni + Cu/Sc) (a,b) and redox conditions (Th/U, V/Sc) (c,d) of the samples from the Longmaxiand Qiongzhusi formations of Well W201, Weiyuan shale gas field, Sichuan Basin (some Qiongzhusi data adapted from those of the authors in [38]).
Figure 9. TOC vs. paleoproductivity (Ni + Cu/Al, Ni + Cu/Sc) (a,b) and redox conditions (Th/U, V/Sc) (c,d) of the samples from the Longmaxiand Qiongzhusi formations of Well W201, Weiyuan shale gas field, Sichuan Basin (some Qiongzhusi data adapted from those of the authors in [38]).
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Zhang, Q.; Liang, F.; Zeng, J.; Qiu, Z.; Zhou, S.; Liu, W.; Kong, W. Paleoenvironment Comparison of the Longmaxi and Qiongzhusi Formations, Weiyuan Shale Gas Field, Sichuan Basin. Processes 2023, 11, 2153. https://doi.org/10.3390/pr11072153

AMA Style

Zhang Q, Liang F, Zeng J, Qiu Z, Zhou S, Liu W, Kong W. Paleoenvironment Comparison of the Longmaxi and Qiongzhusi Formations, Weiyuan Shale Gas Field, Sichuan Basin. Processes. 2023; 11(7):2153. https://doi.org/10.3390/pr11072153

Chicago/Turabian Style

Zhang, Qin, Feng Liang, Jingbo Zeng, Zhen Qiu, Shangwen Zhou, Wen Liu, and Weiliang Kong. 2023. "Paleoenvironment Comparison of the Longmaxi and Qiongzhusi Formations, Weiyuan Shale Gas Field, Sichuan Basin" Processes 11, no. 7: 2153. https://doi.org/10.3390/pr11072153

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