The Sedimentary Geochemical Characteristics and Geological Significance of the Wufeng-Longmaxi Formation Accumulation of Organic Matter Black Shale on the Southeastern Sichuan Basin, China

Wu, Bin

doi:https://doi.org/10.1155/2022/1900158

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Research Article | Open Access

Volume 2022 | Article ID 1900158 | https://doi.org/10.1155/2022/1900158

The Sedimentary Geochemical Characteristics and Geological Significance of the Wufeng-Longmaxi Formation Accumulation of Organic Matter Black Shale on the Southeastern Sichuan Basin, China

Bin Wu¹

Academic Editor: Yu Wang

Received22 Apr 2022

Accepted14 Jun 2022

Published07 Jul 2022

Abstract

At present, the understanding of the sedimentary paleoenvironment and organic matter enrichment law of black shale in the Wufeng-Longmaxi Formation is still insufficient. This time, we investigated the total organic carbon (TOC) content, X-ray diffraction mineralogical composition, and the major and trace element abundances of the newly recovered cores of three shale gas exploration wells in Changning, Qianjiang, and Xiushan areas which were, respectively, selected to discuss and compare the varying paleoenvironmental conditions and the factors that control organic matter accumulation, such as terrigenous input, redox conditions, primary productivity, and the degree of water retention in the basin. The results show that black siliceous shale lithofacies, gray-black shale lithofacies, gray-black silty shale lithofacies, gray argillaceous siltstone lithofacies, and gray shell marl lithofacies are mainly deposited in the Wufeng-Longmaxi Formation in the southeastern Sichuan Basin. Organic-rich black shale is mainly concentrated in the lower part of the Wufeng Formation and Longmaxi Formation. The continuous thickness of organic carbon content greater than 2% in the Changning area is 32 m, the continuous thickness in the Qianjiang area is 22 m, and the continuous thickness in the Xiushan area is 11 m. The hydrocarbon generation potential gradually weakens from west to east. The identification of the geochemical behavior characteristics of major and trace elements shows that enrichment of the organic matter black shale section in the Changning-Qianjiang-Xiushan areas is mainly formation in the anoxic-moderate water mass restriction environment with high primary productivity and less terrigenous detritus input under the low hydrodynamic conditions. However, the correlations of terrigenous detrital proxies Ti/Al, redox proxies V/Cr, Ni/Co, and paleoproductivity proxies Babio with the TOC contents show that the Changning area is a favorable area for the formation of organic-rich black shale and the Qianjiang area is affected by the Wuling underwater uplift; the thickness of the black shale deposits becomes thinner; the Xiushan area is adjacent to the Xuefeng uplift, and the deposition conditions of the accumulation of organic matter black shale deteriorate, which is unfavorable for shale gas exploration.

1. Introduction

Since the organic genesis theory of oil and gas was put forward, scholars at home and abroad have attached great importance to the study of black shale. A large number of studies have been carried out on its sedimentary environment, material composition, structure, and occurrence state, and many understandings have been obtained [1–9]. With the flourishing development of shale gas exploration and development in China, organic-rich black shale has attracted a lot of attention from Chinese scholars, promoting the development of Chinese shale gas in Fuling Jiaoshiba, Weiyuan-Changning, Zhaotong, etc. Large-scale development has been achieved in the region where tens of billions of shale gas fields have been built [10, 11]. Exploration practice has confirmed that organic-rich black shale lithofacies are the material basis for the occurrence of shale gas, and the selection of high-quality lithofacies is the key to the success of hydraulic fracturing, and trace elements in black shale lithofacies have recorded a large amount of the sedimentary environment information which is the key indicator of organic matter enrichment. The Wufeng-Longmaxi Formation in the southeastern Sichuan Basin is a typical organic-rich black shale, which is currently the key stratum for shale gas exploration and development in this area. A large number of studies have been carried out on the characteristics, paleontology, and geochemistry of shale gas, and a relatively complete theory of shale gas exploration and development has been formed [12–15]. However, some shale gas exploration wells implemented in the peripheral areas of the Sichuan Basin have not made breakthroughs, hindering the process of shale gas exploration and development in these areas. Some key geological problems, such as black shale lithofacies combination, organic matter enrichment law, and the conditions for shale gas accumulation, have not been fully solved. In this study, we investigated the total organic carbon content (TOC), X-ray diffraction mineralogical composition, and the major and trace element abundances of the newly recovered core of three shale gas exploration wells from Changning, Qianjiang, and Xiushan areas to elucidate the varying paleoenvironmental conditions and the factors that control organic matter accumulation, such as terrigenous input, redox conditions, primary productivity, and the degree of water retention in the basin, which can provide reference for the exploration of shale gas in the basin margin area.

2. Geological Setting

The Sichuan Basin, located in southwestern China, is a subcratonic basin within the Yangtze Plate (Figure 1(b)). In the Late Ordovician, the entire South China Block, as an independent unit, was located in the equatorial waters of the northern margin of the Gondwana continent and was composed of the Yangtze Plate and the Cathaysia Plate on the southeastern margin (Metcalfe, 1994) (Figure 1(a)). The Longmaxi period of the Early Silurian was the tectonic transition stage of the Middle and Upper Yangtze, and the margin of the continental block was in the process of compression and fold orogeny. On the Yangtze Craton, the great tectono-paleogeographical changes manifested as the peak stage of the formation of the paleouplift (Figure 1(b)). In addition to the expansion of the western Sichuan-Dianzhong ancient land and the Hannan ancient land on the edge, the scope of Chuanzhong uplift continued to expand, and the Qianzhong uplift, Wuling uplift, Xuefeng uplift, and Miaoling uplift on the southern margin of the Yangtze were basically connected, forming “Dian-Qian-Gui Uplift Belt” [16, 17]. The Lower Silurian Longmaxi Formation in the Sichuan Basin is relatively stable in lithology, which has the black shale and gray-black sandy shale in the lower part and gray-green shale and yellow-green sandy shale in the middle and upper parts and sometimes has siltstone or argillaceous gray rock, carbonaceous, and pyrite (Figure 1(c)). The middle and lower parts are rich in graptolites, especially in the lower part, including Glyptograptus persculptus, Pristiograptus cyphus, Demirastrites triangulatus, Monograptus sedgwickii, and Spirograptus turriculata. The thickness of the Wufeng-Longmaxi Formation in the Sichuan Basin and its surrounding areas is generally 150-450 m, and the thickness is the thickest in the northern Guizhou-Southern Sichuan, western Hubei, and northern Sichuan regions [18, 19]. Part or all of the top of the paleohigh in the Sichuan Basin and the western part of the basin are missing. The organic carbon content (TOC) of the Wufeng-Longmaxi Formation in the southern Sichuan-Wanzhou area is generally 1.88%-4.36%, the highest is 9.84%, and the Ro is generally 1.83%-3.26%, while the organic carbon content (TOC) in western Hunan and Hubei is generally 0.53%-3%, the average is 1.74%, and the Ro is generally 2.0%-3.0% [20, 21]. The parent material type is mainly sapropel kerogen, and a small amount is humic type. The thickness of shale with organic carbon greater than 2% in the study area can reach 80-100 m, and the thickest ones in southern Sichuan and western Hubei are a set of high-efficiency source rocks.

3. Samples and Testing

The samples in this study were collected from the Wufeng-Longmaxi formation in well N1 in the Changning area, well ZY1 in the Qianjiang area, and well XY3 in the Xiushan area. All samples were fresh rock samples from downhole cores without weathering modification. A total of 45 samples were collected for analysis, 15 fresh samples of which were collected from well N1 for analysis and testing of major and trace elements, including 2 black shale members of the Wufeng Formation and 13 black shale members of the Longmaxi Formation; 15 samples of which were collected from well ZY1, including 3 black shale sections of the Wufeng Formation and 12 black shale sections of Longmaxi Formation; and 15 samples of which were collected from well XY3, including 5 black shale sections of the Wufeng Formation and 10 black shale sections of the Longmaxi Formation. All samples were sent to the Chongqing Mineral Resources Supervision and Testing Center of the Ministry of Natural Resources for testing in strict accordance with the shale gas sample analysis specifications.

4. Lithofacies and Element Geochemical Characteristics

4.1. Lithofacies and Mineralogy Characteristics

In this time, the black shale of the Wufeng-Longmaxi Formation in Changning, Qianjiang, and Xiushan is mainly collected for analysis. According to the color, composition, structure, and mineralogy of the rock, five major lithofacies are identified, namely, black silica shale lithofacies, gray-black shale lithofacies, gray-black silty shale lithofacies, gray argillaceous siltstone lithofacies, and gray shell marl lithofacies. (1)Black siliceous shale lithofacies

Black siliceous shale lithofacies are distributed in the Wufeng Formation and the lower part of the Longmaxi Formation. This lithofacies is characterized by black and gray-black shale (Figures 2(a) and 2(c)), in which abundant quartz minerals, feldspar minerals, and pyrite can be observed (Figure 2(b)). Graptolites are occasionally observed in this lithofacies (Figure 2(c)), and the TOC ranges from 2.4 wt% to 5.3 wt%. It is a favorable interval for shale gas hydraulic fracturing. According to XRD analysis, this lithofacies has high siliceous (quartz+feldspar) content in the range between 58.3 wt% and 91.4 wt% (), which is the main mineral component (Figure 3). Abundant quartz particles and fossil detritus can be observed in thin sections, such as radiolarians and sponge spicules (Figure 2(b)). (2)Gray-black shale lithofacies

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

(i)

(j)

(k)

(l)

Figure 2

Photos of cores and thin sections: (a) well N1, Longmaxi Formation, 2518.42 m, black siliceous shale lithofacies, horizontal bedding; (b) well N1, Longmaxi Formation, 2518.42 m, black siliceous shale lithofacies, the siliceous sponge spicules, partly replaced by calcite, dispersed in quartz, calcite, and dolomite, and enriched in organic matter; (c) well N1, Longmaxi Formation, 2517.13 m, black siliceous shale lithofacies, a large number of graptolites; (d) well N1, Longmaxi Formation, 2509.34 m, gray-black shale lithofacies, horizontal bedding and bedded pyrite nodules; (e) well N1, Longmaxi Formation, 2509.34 m, gray-black shale lithofacies, the dark color is clay and organic matter; (f) well N1, Longmaxi Formation, 2509.34 m, gray-black shale lithofacies, black is berry pyrite, contemporaneous-quasi-contemporaneous origin; (g, h) well ZY1, Longmaxi Formation, 2611.13 m, gray-black silty shale lithofacies, horizontal bedding and microfractures developed, black shale and gray silty interbedded, clay mineral content is greater than silty grains; (i, j) XY3 well, Longmaxi Formation, 1779.55 m, gray-black argillaceous siltstone lithofacies, wavy bedding and eyeball bedding are developed, black shale and gray silty interbeds of unequal thickness, clay mineral content is less than silty grains; (k, l) well N1, Guanyinqiao Member of the Wufeng Formation, 2520.34 m, grayscale limestone, containing a lot of biological debris, mainly trilobites (Dalmanitina) and brachiopod (Hirnantia).

(a) N1

(b) ZY1

(c) XY3

Gray-black shale lithofacies can be seen in the entire Wufeng-Longmaxi Formation, and the color gradually becomes lighter from dark black at the bottom to the upper parts. (Figures 2(d)–2(f)). This lithofacies has abundant graptolite and pyrite, and the TOC ranges from 1.7 wt% to 5.6 wt%. According to XRD analysis, this lithofacies has high quartz and clay content in the range between 35.4 wt% and 73.2 wt% (), which are the main mineral components (Figure 3). Abundant black clay minerals can be observed in thin sections (Figure 2(e)), and fossil debris such as radiolarians and sponge spicules are occasionally produced. (3)Gray-black silty shale lithofacies

Gray-black silty shale lithofacies are distributed in the Wufeng Formation and the middle and lower parts of Longmaxi Formation, in which quartz minerals, feldspar minerals, and pyrite are relatively abundant, and the TOC ranges from 0.4 wt% to 3.7 wt% (Figures 2(g) and 2(h)). According to XRD analysis, this lithofacies has high quartz and clay content in the range between 48.6 wt% and 74.2 wt% (), which are the main mineral components (Figure 3). (4)Gray argillaceous siltstone lithofacies

The gray argillaceous siltstone lithofacies are mainly found in the Qianjiang and Xiushan areas, where black mudstone and argillaceous siltstone are interbedded, which has quartz minerals and feldspar minerals, the content of graptolite is low, and the total organic carbon contents (TOC) range from 0.8 wt% to 2.3 wt% (Figures 2(i) and 2(j)). According to XRD analysis, this lithofacies has high detrital quartz mineral content in the range between 68.6 wt% and 79.3 wt% (). Abundant quartz grains can be observed in thin sections (Figure 2(j)), and fossil debris is occasionally seen. (5)Gray shell marl lithofacies

The gray shell marl lithofacies are distributed in the Guanyinqiao Member of the Wufeng Formation, where black mudstone and argillaceous limestone are interbedded, and a large number of Dalmanitina and Hirnantia organisms developed. Abundant brachiopod fossil debris can be observed in thin sections (Figures 2(k) and 2(l)).

4.2. Total Organic Carbon Contents (TOC)

The TOC contents are widely used internationally as the lower limit standard value for oil and gas exploration (Zhang et al., 2002; Zhong et al., 2004). The Changning area is located between the Chuanzhong uplift and the Qianzhong uplift and was affected by multistage tectonic movements, especially the Caledonian movement. According to the TOC content test (Table 1 and Figure 3), the TOC contents of well N1 vary between 0.82 wt% and 5.55 wt% (), and the continuous thickness greater than 2 wt % reaches 32 m. It has great hydrocarbon generation potential and is a favorable interval for shale gas exploration in the Sichuan Basin. The Qianjiang area is adjacent to the Wuling underwater uplift, in which the TOC contents of well ZY1 vary between 0.3 wt% and 5.91 wt% (), and the continuous thickness greater than 2 wt % reaches 22 m. Compared with well N1, the thickness of high-quality shale is relatively thin. The Xiushan area is relatively close to the Xuefeng uplift, in which the TOC contents of well XY3 vary between 0.13 wt% and 6.15 wt% (), and the continuous thickness greater than 2 wt % is only 11 m, and it is mainly distributed in the Wufeng Formation. The hydrocarbon generation conditions are poor, which is not conducive to shale gas exploration.

4.3. Major Element Characteristics

SiO₂ (detrital quartz and/or biosilica), Al₂O₃ (clay fraction), and CaO (carbonate content) can be regarded as the main constituents of marine shales and mudrocks (Ross and Bustin, 2009). So in the analysis of major elements, 7 main oxides including SiO₂, Al₂O₃, CaO, Fe₂O₃, MgO, K₂O, and TiO₂ were selected for analysis (Table 1). The measurement shows that the contents of SiO₂ and Al₂O₃ in the three regions are the dominant oxide. Among them, SiO₂ is relatively high in Changning and Qianjiang, ranging from 17.82 wt% to 65.43 wt% (), and relatively low in the Wufeng Formation, while the changes in Xiushan are relatively consistent, ranging from 36.8 wt% to 65.91 wt% (). Followed by Al₂O₃, well N1 varies between 6.21 wt% and 17.44 wt% (); well ZY1 varies between 7.05 wt% and 19.9 wt% (); and well XY3 varies between 8.83 wt% and 14.38 wt% (). The CaO in the three areas generally showed a trend of being lower and higher, and the average value of the N1 well and ZY1 well was relatively high. In addition, the negative covariance of the terrigenous detrital index TiO₂ with the TOC contents was observed, and the overall trend is that the lower part is lower and the upper part is higher.

4.4. Trace Element Characteristics

(1)In this study, the trace elements related to the accumulation of organic matter were mainly selected for analysis. Oceanographic studies have shown that the enrichment of barium in deep-sea sediments is related to biological deposition [22, 23] and is positively correlated with the TOC contents and paleoproductivity (Mazumdar, 1999; Chen, 2002). Due to the fact that the sulfate solubility of barium is very low, it is easy for barium to combine with SO42- to form BaSO₄ precipitation after entering the sea (lake) basin, so the migration ability of barium is very poor. The contents of barium are related to the change of carrier minerals potassium feldspar and biotite, and the contents are higher in fine-grained sedimentary rocks such as siltstone and mudstone. There are three main sources of barium in the ocean (Paytan A., 1996; Dickens G.R., 2003; Carter S.C., 2020; Jin C.Y., 2021). Barium required for biological activities is called biological barium, which is affected by terrigenous detrital barium ions [24–27]. It is usually necessary to correct in the study of ancient productivity, and the correction equation iswhere Basa and Alsa are the measured elemental concentrations in the sample, and (Ba/Al) PAAS is the average value of aluminosilicate in terrigenous debris [28, 29], which is calculated by PAAS here (WEDEPOHL, 1971; He, 2020; Huang and Wang et al., 2021).

The variation law of Babio in the study area is shown in Figure 3, and its variation law is similar to that of Ni and TOC (Table 2), indicating the enrichment characteristics of organic matter in the Wufeng Formation and the lower part of the Longmaxi Formation. (2)The redox conditions of the depositional environment play an important role in controlling the migration, coexistence, and precipitation of transition metal elements such as V, Mo, Cr, U, and Th and are also one of the main controlling factors for the source rock deposition. Therefore, the content and ratio of the trace elements in sediments can be used to reconstruct the sedimentary paleoenvironment [30–32] (Algeo and Lyons, 2006; Ross and Bustin, 2009). Since fine-grained sediments are easy to adsorb trace elements, the enrichment factor (EF) is often used to characterize the elemental concentrations of specific redox-sensitive trace elements [18, 28, 33], and the calculation equation is

where and Al are the contents of a certain element and Al in the sample (ppm); PAAS is used as the standard to calculate (WEDEPOHL, 1971). In general, indicates that the sample element is enriched for PAAS shale; is strongly enriched, and is depleted [34, 35] (Tribollivard et al., 2006, 2012).

According to the analysis of test results (Table 2 and Figure 3), the black shale of the Wufeng-Longmaxi Formation in Changning has the highest concentration of redox-sensitive elements such as V, Ni, Mo, U, and Co, followed by the Qianjiang; Xiushan has the weakest enrichment. Changning was moderately enriched in VEF (1.57-13.99, ), strongly enriched in MoEF (1.31-45.29, ), strongly enriched in UEF (4.11-43.45, ), and weakly enriched in NiEF (1.35-6.03, ). Qianjiang was moderately enriched in VEF (0.48-10.51, ), strongly enriched in MoEF (1.44-29.21, ), moderately enriched in UEF (0.89-15.49, ), and weakly enriched in NiEF (0.38-4.79, ). Xiushan was moderately enriched in VEF (0.52-5.83, ), strongly enriched in MoEF (1.56-77.42, ), moderately enriched in UEF (1.01-9.40, ), and weakly enriched in NiEF (0.66-2.29, ). In summary, the described redox-sensitive element profile is similar to the TOC contents of the Wufeng-Longmaxi Formation, which are higher in the Wufeng Formation and the lower part of the Longmaxi Formation and gradually decrease upward.

5. Discussion

5.1. Paleooceanic Redox Conditions

The analysis of paleooceanic redox conditions is of great significance to the study of the accumulation of organic matter, which is usually judged by the degree of enrichment of oxygen. Rhoads and Morse (1971) established terms for three levels of oxygenation in the water column—oxic () and dysoxic () and anoxic (); the latter two are collectively referred to as the anoxic phase [36, 37] (Algeo T J, Maynard JB, 2004; Potter, 2005). In the oxic seawater environment, Mo, U, V, etc. are often high-valence ions to form stable compounds that dissolve in seawater and are prone to migration; in the dysoxic seawater environment, these elements are reduced from high-valence ions to lower-valence ions which are prone to precipitation; in the anoxic seawater environment, these elements are reduced to a lower valence state, and they are authigenic deposits in the form of oxides and hydroxides or are directly captured by authigenic pyrite and enriched in sediments. Therefore, the redox-sensitive trace elements such as U, Th, V, Ni, Cr, and Co were selected for reconstructing paleoocean environments, and we employed , Ni/Co, V/Cr, and U/Th to reconstruct the redox conditions associated with deposition of the Wufeng-Longmaxi Formation [38] (Table 3).

The measurement elemental concentrations of the redox-sensitive trace elements are shown in Table 2 and Figure 3; in well N1 in Changning, (1) the V/Cr ratios range from 2.06 to 8.44 (); (2) the Ni/Co ratios range from 3.18 to 9.41 (); (3) the ratios vary between 0.62 and 0.95 (); (4) the U/Th ratios vary between 0.25 and 2.85 (); and (5) the autogenous uranium (AU) varies between 9.4 ppm and 64.8 ppm (). In well ZY1 in Qianjiang, (1) the V/Cr ratios range from 1.28 to 8.21 (); (2) the Ni/Co ratios range from 2.08 to 7.91 (); (3) the ratios vary between 0.61 and 0.86 (); (4) the U/Th ratios vary between 1.21 and 10.02 (); and (5) the autogenous uranium (AU) varies between 0.14 ppm and 27.47 ppm (). In well XY3 in Xiushan, (1) the V/Cr ratios range from 0.98 to 6.93 (); (2) the Ni/Co ratios range from 3.27 to 9.69 (); (3) the ratios vary between 0.59 and 0.85 (); (4) the U/Th ratios vary between 0.4 and 2.87 (); and ⑤ the autogenous uranium (AU) varies between 0.14 ppm and 22.33 ppm ().

The ratios of V/Cr, Ni/Co, , U/Th, etc. indicate anoxic-dysoxic water conditions that have good organic matter enrichment environment in the Changning, Qianjiang, and Xiushan areas. The stratigraphic distribution of the redox-sensitive trace elements and , Ni/Co, V/Cr, and U/Th of the vertical section is shown; however, the redox conditions of the three regions are quite different. The thickness of the Wufeng Formation in the Changning area is relatively thin, only about 4 m, the V/Cr ratios range from 3.45 to 6.64 (), which indicates anoxic water conditions. The V/Cr ratios range from 4.66 to 8.44 (average =6.14) in the lower Longmaxi Formation (well N1 ranges from 2521 m to 2493 m), which is in a strong reducing environment, while the V/Cr ratios range from 2.06 to 3.32 () in the upper Longmaxi Formation (well N1 ranges from 2493 m to 2479 m), which is in a weak reducing environment, and the stratigraphic profiles of U/Th and Ni/Co ratios exhibit similar varying patterns as the V/Cr ratios for the Longmaxi Formation, exhibiting a decreasing trend upward (Figure 3(a) and Table 2), so the Wufeng-Longmaxi Formation in Changning is in an anoxic environment, which provides a good paleoenvironment for the occurrence and transformation of organic matter. The thickness of the Wufeng Formation in Qianjiang is about 9 m, and the V/Cr ratios range from 5.01 to 8.21(), which indicates anoxic water conditions. The V/Cr ratios range from 5.19 to 6.32 () in the lower Longmaxi Formation (well ZY1 ranges from 2619 m to 2603 m), which is in a strong reducing environment, while the V/Cr ratios range from 1.28 to 4.44 () in the upper Longmaxi Formation (well ZY1 ranges from 2603 m to 2562 m), which is in a weak reducing and oxidizing environment with relatively strong hydrodynamic conditions (Figure 3(b) and Table 2); compared with the Changning area, the occurrence and transformation conditions of its organic matter are worse. The thickness of the Wufeng Formation in Xiushan is about 8 m, and the V/Cr ratios range from 4.26 to 6.93 (), which indicates anoxic water conditions. The V/Cr ratios range from 1.91 to 3.03 () in the lower Longmaxi Formation (well ZY3 ranges from 1802 m to 1782 m), which is in a weak reducing environment, while the V/Cr ratios range from 0.99 to 1.20 () in the upper Longmaxi Formation (well XY3 ranges from 1782 m to 1760 m), which is in an oxidizing environment with strong hydrodynamic conditions; compared with the Changning and Qianjiang areas, its organic matter occurrence and transformation conditions are the worst (Figure 3(c) and Table 2).

In summary, the paleoocean sedimentary environment of the Wufeng-Longmaxi black shale in Changning, Qianjiang, and Xiushan areas has the characteristics of a gradual transition from an anaerobic-oxygen-depleted environment to an oxic environment (Figure 3), but the change pattern is distinct. In contrast, the Changning area is located in the euxinic environment. And the stratigraphic profiles of the redox-sensitive trace element ratios suggest that anoxic conditions prevailed through the Wufeng Formation and the lower Longmaxi Formation (Figure 3) and then slowly transform into a weak reducing-oxic environment; the Qianjiang area is in a relatively strong hydrodynamic condition, in which the stratigraphic profile exhibits a slow transition from a strong reducing environment at the bottom to an oxidizing environment, while the Xiushan area shows an oscillating transition from an anoxic environment at the bottom to an oxic environment. The reasons for the above-mentioned different phenomena may be related to the depositional structures of the three regions.

5.2. Paleooceanic Productivity

The paleoproductivity has been widely used as the evaluation of source rocks [39–41], where the biogenic barium (Babio) and the TOC contents indicate the strength of paleoocean productivity (Tribovillard et al., 2006; Ross and Bustin, 2009). The distribution of barium in modern ocean waters is usually controlled by biochemical reactions, combining with SO42- to form BaSO₄ precipitates. Previous studies suggest that the accumulation rate of barium is often positively correlated with the TOC contents and biological productivity, and its enrichment degree can indicate the productivity of the upper water column. Generally, the higher the enrichment degree, the higher the productivity of the upper water column. Therefore, biogenic barium calculated based on barium concentration can better reflect paleoproductivity under the paleooceanic redox conditions (Ross et al., 2009; He, 2020; Wang et al., 2021).

The Babio content has been widely used as paleooceanic primary productivity proxies in shale sediments (Tribovillard et al., 2006; Liu et al., 2019). The average Babio contents in the studied well N1 and well ZY1 samples are 414.74 ppm and 432.29 ppm, respectively, suggesting moderate primary productivity (Figures 3(a) and Figure 3(b)), while for the well XY3, the average Babio contents were only a few tens of ppm, suggesting extremely low primary productivity (Figure 3(c)). The stratigraphic profiles of the Babio contents exhibit similar varying patterns as the TOC contents for the Wufeng-Longmaxi Formation, exhibiting a decreasing trend upward (Figure 3), indicating a declining paleoproductivity upward from the Wufeng-lower Longmaxi Formation to the upper Longmaxi Formation. Previous studies suggest that the content of biogenic barium in the modern equatorial Pacific Ocean waters is about 1000-5000 ppm [42] (Murray, 1996). The average Babio contents in the studied Wufeng Formation and the Longmaxi formation samples are 631.21 ppm and 403.2 ppm, respectively; however, the samples from the bottom of the Longmaxi Formation display the greatest Babio contents and suggest high paleoproductivity during the deposition of the lower Longmaxi Formation. The reason may be that the biological extinction event during the Hirnanth Ice Age temporarily weakened the paleooceanic productivity (Rong J.Y., 1979, 1999, 2019; Chen X., et al., 2004, 2015; Wang et al., 2021) and then entered the Longmaxi sedimentary period of the Early Silurian that the rising sea level was due to climate warming and related glacial melting [43–47]. During this period, the transgression almost spread over the entire Upper Yangtze Basin and the primary productivity began to recover rapidly due to some residual graptolites, and organisms adapted to the new environment flourished again and reached a high value during the depositional period of the lower Longmaxi Formation. In addition, transgression is also manifested in changes in lithology. Compared with the underlying strata, siliceous black shale with richer organic matter, higher siliceous composition, and darker color is deposited in the strata.

The Ba/Al ratios of sediment are another useful proxy for assessing paleoproductivity that Al is almost unaffected by sediment diagenesis and more able to reflect the level of primary productivity than the TOC [48–51]. Samples from the bottom of the Longmaxi Formation display the relatively high Ba/Al ratios of well N1 and well ZY1 suggesting high levels of productivity. However, the stratigraphic profiles of the Ba/Al ratios are not very consistent with the TOC contents (Figure 3), so we must be cautious when using the Ba/Al ratio when evaluating the paleoproductivity and should refer to other parameters.

5.3. Terrigenous Influx

Studies on the input of terrigenous materials in modern sedimentary basins show that terrigenous detrital mainly comes from rivers, volcanic eruptions, and biological remains. Elements such as Al and Ti are the main components of continental crust, and their contents are relatively low in the oceanic crust dominated by siliceous magnesia in that they are rarely affected during diagenesis [52], so the Al and Ti contents of sediments have been usually used as proxies for the input of terrigenous detritus (Rimmer, 2004; Tribovillard et al., 2006; Liu et al., 2019; He, 2020; Wei et al., 2021). In this time, the analysis of terrigenous debris input in the study area is mainly based on the Ti and Al and the ratio of Ti/Al.

The stratigraphic profiles of the Ti contents exhibit similar varying patterns as the Al contents for the Wufeng-Longmaxi Formation in the Changning area and show an increasing trend upward, while the Ti/Al ratio is basically around 0.02 with little change, indicating that the Changning area is less affected by terrigenous debris and is far from the large input area of coastal terrigenous influx (Table 1 and Figure 3(a)). The Ti and Al and the ratio of Ti/Al contents in the Qianjiang and Xiushan areas fluctuated greatly (Table 1 and Figures 3(b) and 3(c)). During the deposition period of the Wufeng Formation, the Ti and Al and Ti/Al ratios in the two areas did not change much and were relatively stable, representing a quiet environment with continuous deposition. However, after entering the Longmaxi Formation, the content of Ti/Al protracted increases, especially in the Xiushan area, indicating a large amount of terrigenous debris input into the studied areas of Qianjiang and Xiushan. It was argued that it may be closely related to the tectonic activity background during the end of the Late Ordovician in which the tectonic activity in the Xuefeng area of the Upper Yangtze was frequent. Due to the tectonic inversion, the area was gradually uplifted from deep-water shelf deposits to land. The Xiushan area belonged to the deep-water depositional area during the Wufeng depositional period at the end of the Late Ordovician and then was later affected by the Xuefeng uplift; it has evolved from the deep-water sedimentary area to the shallow-water continental shelf and even the coastal environment, and the current drilling and coring show that the lithology of the Longmaxi Formation in this area is mainly gray argillaceous siltstone, typical of shallow-water sedimentary characteristics. The Xiushan area was located near the Xuefeng uplift. The Xiushan area belonged to the deep-water depositional area and was later affected by the Xuefeng uplift. Influenced by the deep-water sedimentary area, it has evolved from the deep-water sedimentary area to the shallow-water continental shelf and even the coastal environment, and the newly recovered cores show that the lithology of the Longmaxi Formation in this area is mainly gray argillaceous siltstone, which is typical of shallow-water sedimentary characteristics. Therefore, studies based on terrigenous debris show that the Xiushan area is located on the edge of the Xuefeng uplift, which is not conducive to shale gas exploration activities.

5.4. Water Mass Restriction

The deposition of organic-rich black shale is affected by the temperature, salinity, water depth, hydrodynamic conditions, water circulation, and the redox conditions. Generally, it can be preserved and enriched in a relatively limited and the euxinic sedimentary paleoenvironment. For studying the water mass restriction of the Wufeng-Longmaxi Formation in the southeastern Sichuan Basin, the Mo and U contents of sediments have been usually used as proxies for the water mass restriction. The relative content of Mo and U in the crust is low, with Clarke values of only 2.6 ppm and 3.7 ppm (WEDEPOHL, 1971; Taylor and McLennan, 1985), and the contents in marine organisms are also very low [53]. Under oxic conditions, Mo and U elements are generally not easy to deposit and stay in seawater for a long time (Algeo and Tribovillard, 2009), but when the depositional environment changes, under anoxic conditions, Mo and U elements will precipitate out in large quantities and be trapped in organic-rich sediments. During the deposition process, there is a difference in the time when Mo and U were captured by sediments, and U was captured earlier than Mo because U precipitates at the redox boundary (Algeo and Lyons, 2006; Tribollivard et al., 2012); the capture of Mo requires the presence of H₂S. Therefore, based on the difference in geochemical behaviors of the Mo and U, the Mo/TOC and the authigenic MoEF-UEF covariation pattern can be used to elucidate paleoredox conditions and the degree of water mass restriction (Algeo and Lyons, 2006; Tribollivard et al., 2012).

5.4.1. MoEF-UEF Covariation Pattern

The UEF-MoEF covariation pattern diagram (Figure 4(a)) shows that the Wufeng Formation in the study area is in an anoxic-anaerobic strong reducing environment, and the U and Mo enrichment coefficients are basically greater than 10, and most of them are above . The Mo/U ratios of well ZY1 in Qianjiang and well N1 in Changning are between 0 and of present-day seawater, while the Mo/U ratio of well N1 is higher than that of well ZY1. The MoEF-UEF covariation pattern is relatively consistent with the Mexico margin, indicating that the Changning-Qianjiang area of the Sichuan Basin was in a moderate water mass restriction environment during the period of the Wufeng Formation. The Mo/U ratios of well XY3 in Xiushan are between and of present-day seawater, indicating that there may be a bottom current connected to the open sea in this area, showing a weak water mass restriction environment [54–57], which is also consistent with the sedimentary paleoenvironment in this area (Mu E.Z., 1954; Feng H.Z. et al., 1993; Rong J.Y., 1999; Guo X.S., 2017; Xi Z.D., 2021).

(a)

(b)

After entering the Longmaxi depositional period, the sedimentary environment in the study areas has undergone great changes when entering the Longmaxi Formation depositional period (Figure 4(b)). During the depositional period of the lower Longmaxi Formation, the Mo/U ratios were between and of present-day seawater, and the Mo and U enrichment coefficients were higher than those of the Wufeng Formation, indicating a weak-moderate water mass restriction environment with poor communication with the open sea. During the upper Longmaxi Formation period, the Mo and U enrichment coefficients were lower than those of the lower Longmaxi Formation, while the Mo/U ratios were still between and 1 of present-day seawater, and the anoxic-hydrostatic conditions gradually changed to suboxic-hydrodynamic conditions, indicating a strong water mass restriction environment due to reduced water depth.

5.4.2. Mo-TOC Cross-Plot

The Mo/TOC ratios are related to the degree of water mass restriction in the sedimentary basin. The water circulates smoothly to the outside in the open seawater, and a large amount of Mo is supplemented by the water circulation, and the enrichment degree of Mo and TOC is positively correlated. While Mo is consumed and cannot be obtained in seawater, the Mo/TOC ratio of sediments also decreases accordingly. Therefore, the Mo-TOC cross-plot of sediments can be used to determine the degree of water mass restriction in sedimentary basins [58–61]. As shown in the Mo-TOC cross-plot (Figure 5), samples from the Wufeng-Longmaxi Formation in the three areas are mostly located between trend lines of the Saanich Inlet and Black Sea, indicating a weak-moderate water mass restriction. The Mo/TOC ratio of the Wufeng-lower Longmaxi Formation is relatively high, and the TOC contents are basically greater than 2 wt%, especially the samples of well XY3 which are located between trend lines of the Saanich Inlet and Cariaco Basin, indicating a weak water mass restriction, which is consistent with the previous MoEF-UEF covariation pattern. For samples from the upper Longmaxi Formation, the Mo and TOC contents are decreased, but it still showed a weak-strong water mass restriction conditions. In summary, both MoEF-UEF covariation patterns and Mo/TOC cross-plot suggest that the Wufeng-Longmaxi Formation in the study area was generally a relatively open water environment with weak to moderate water mass restriction (He, 2020; He J.W. et al., 2021).

5.5. Mechanism of Organic Matter Accumulation

The accumulation of organic matter has been controversial, and the more popular ones are “preservation condition model” and “productivity model” (He, 2020; He J.W., 2021). Through a comprehensive study on the redox conditions, paleoproductivity, input of terrigenous debris, and water mass restriction in the Wufeng-Longmaxi Formation of well N1, well ZY1, and well XY3, it is believed that the enrichment pattern of organic matter is a comprehensive dynamic process, which is not only controlled by the high primary productivity at the beginning but also controlled by the later preservation conditions. Therefore, based on the comprehensive previous research and combined with the tectonic evolution, it is believed that the enrichment of organic matter in the study area is a differential enrichment model [62–69] (Figures 1 and 6).

The Middle Ordovician-Early Silurian was the tectonic transition stage of the Upper Yangtze. The margins of the continental block were in the process of compression and fold orogeny, and the Chuanzhong uplift continued to expand. And then the Qianzhong uplift, Wuling underwater uplift, Xuefeng uplift, and Miaoling uplift on the southern margin of the Yangtze were basically connected (Ma et al., 2004; Wei, 2019). During the Late Ordovician Wufeng period, a large-scale regression occurred in the Upper Yangtze. The Chuanzhong uplift, Xuefeng uplift, and Wuling underwater uplift divided the Sichuan Basin and its surrounding areas into several special undercompensated stagnation basins, which deposited the enrichment of organic matter black shale of the Wufeng Formation. In the Early Silurian Longmaxi period, the rising sea-level due to climate warming and related glacial melting and large-scale transgression occurred in the area, which led to the stratification of the seawater column, and the bottom seawater rapidly changed into the anoxic-moderate water mass restriction environment, and sedimentation of the well-known black siliceous shale of the Longmaxi Formation in the Upper Yangtze area occurred.

During the late Ordovician Wufeng period, the Upper Yangtze region was affected by the tectonic compression of the Duyun movement in Guangxi, which promoted the expansion of Chuanzhong uplift and the uplift of the Xuefeng area. Under the background that the global sea-level began to decline due to glaciation, the sedimentary environment in the Upper Yangtze region changed from an open platform to a limited basin. The redox proxies V/Cr and Ni/Co show positive correlations with the TOC contents in the anoxic-suboxic intervals of the Wufeng Formation (Figures 6(b) and 6(c)), and the Mo-TOC cross-plot also reflects basin sedimentary environment with moderate water mass restriction. In the oxygen-deficient limited basin, coupled with the frequent eruptions of volcanoes during this period, a large amount of nutrients were provided for the blooms of plankton such as graptolites and the paleoproductivity proxies Babio and TOC contents; obvious correlations also established the high primary production, conducive to the deposition of organic-rich black shale. However, affected by the fall of sea level and the uplift of the paleouplift, the terrigenous flux proxies Ti/Al show negative correlations with the TOC contents in the Wufeng Formation suggesting that the input of terrigenous detritus is not conducive to the preservation of organic matter. In summary, the organic matter enrichment conditions in the Xiushan and Qianjiang areas were better than those in the Changning area, but this period lasted for a short time, and the black shale deposition thickness was thin, which could not reach the effective thickness for shale gas exploration.

During the end of the late Ordovician Guanyinqiao period, the global glaciation developed to its peak, the relative sea level plummeted, the anoxic-moderate water mass restriction environment was destroyed, and then an oxygen-rich sedimentary environment appeared where a large number of brachiopods and shellfish flourished. After entering the Early Silurian, the Upper Yangtze region underwent transgression again caused by the glaciers that began to melt and the relative sea level rising; the primary productivity recovered and the terrigenous detritus input weakened compared with the Wufeng period. During the lower Longmaxi Formation, the Changning and Qianjiang areas are far away from the paleouplift where weak-moderate water mass restriction sedimentary environments are located (Figures 4(b) and 5). The Ti/Al index of terrigenous detritus showed little impact on the area, and core drilling also revealed that plankton such as graptolites flourished and a large number of nodular and infested pyrites developed reflecting the low hydrodynamic conditions and oxygen-depleted stagnant water deposition environment. The redox proxies V/Cr and Ni/Co and the paleoproductivity proxies Babio with the TOC contents are all positively correlated (Figure 7). Therefore, under the combined action of low terrigenous input, high primary productivity, and anoxic retention in the sedimentary environment, massive organic matter is enriched in the lower Longmaxi Formation, which is a favorable area for shale gas exploration. However, due to the proximity of the Wuling underwater uplift in the Qianjiang area, the thickness of the black shale deposits has become thinner, and the enrichment indexes of organic matter, including terrigenous detrital Ti/Al proxies, redox proxies V/Cr, Ni/Co, and paleoproductivity proxies Babio, are lower than those in the Changning area. Under the influence of Xuefeng uplift, the sedimentary water column in the Xiushan area is relatively shallow, which is an suboxic environment, and there is the input of terrigenous detrital (Figure 8). The primary productivity is insufficient and the enrichment of organic matter is poor, so it is not a favorable area for shale gas exploration.

(a)

(b)

(c)

(d)

(a)

(b)

(c)

(d)

During the late Longmaxi period of the Early Silurian, the sea level in the upper Yangtze region decreased. The anoxic sedimentary environment in the study area was destroyed and then entered the suboxic-oxic environment, and the amount of terrigenous input increased; the conditions for depositing enrichment organic matter black shale were destroyed, which was not conducive to the exploration of shale gas.

In summary, we suggest that the deposition of black shale in the Wufeng-Longmaxi Formation and the enrichment of organic matter are controlled by factors such as the regional tectonic movement, paleoproductivity, redox conditions, and the water mass restriction conditions. Only with the cooperation of favorable sedimentary geochemical conditions and good preservation conditions can favorable reservoir areas for enriching shale gas be found.

6. Conclusions

(1)Based on petrographic observations and mineralogical analysis, the Wufeng-Longmaxi Formation in the southeastern Sichuan Basin mainly deposits black siliceous shale facies, gray-black shale facies, gray-black silty shale facies, gray argillaceous siltstone facies, and gray shell marl facies. The accumulation of organic matters black shale is mainly concentrated in the Wufeng Formation and lower Longmaxi Formation. The continuous thickness of the TOC contents greater than 2 wt% in Changning area is 32 m, that in the Qianjiang area is 22 m, and that in the Xiushan area is only 11 m. The hydrocarbon generation potential gradually weakens from west to east. The mineral components are mainly siliceous, carbonate, and clay minerals. Pyrite is mainly formed in the anoxic conditions in the Wufeng-lower Longmaxi Formation(2)The identification of the geochemical behavior characteristics of major and trace elements shows that the enrichment of organic matter black shale section in the Changning-Qianjiang-Xiushan areas is mainly formation in the anoxic-moderate water mass restriction environment with high primary productivity and less terrigenous detritus input under the low hydrodynamic conditions(3)The deposition of black shale in the Wufeng-Longmaxi Formation and the enrichment of organic matter are controlled by factors such as the regional tectonic movement, paleoproductivity, redox conditions, and the water mass restriction conditions. The correlations of terrigenous detrital Ti/Al proxies, redox proxies V/Cr and Ni/Co, and paleoproductivity proxies Babio with the TOC contents show that the Changning area is a favorable area for the formation of organic-rich black shale, and the Qianjiang area is affected by the Wuling underwater uplift; the thickness of the black shale deposits becomes thinner; the Xiushan area is adjacent to the Xuefeng Uplift, and the deposition conditions of the accumulation of organic matter black shale deteriorate, which is unfavorable for shale gas exploration

Data Availability

The data used to support the findings of this study are included within the article.

Conflicts of Interest

The author declares that he has no conflicts of interest.

Acknowledgments

This research was supported by the National Natural Science Foundation of Chongqing (cstc2021jcyj-msxmX1039, cstc2021jcyj-msxmX0624, and cstc2021jcyj-msxmX0771).

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