Microfacies analysis of marine shale: A case study of the shales of the Wufeng–Longmaxi formation in the western Chongqing, Sichuan Basin, China

Yana Chen; Jia Liu; Nan Wang; Yiqing Zhu; Wei Lin; Quansheng Cai; Yuchuan Chen; Mingtao Li

doi:10.1515/geo-2022-0583

Article Open Access

Microfacies analysis of marine shale: A case study of the shales of the Wufeng–Longmaxi formation in the western Chongqing, Sichuan Basin, China

Yana Chen , Jia Liu , Nan Wang , Yiqing Zhu , Wei Lin , Quansheng Cai , Yuchuan Chen and Mingtao Li

Published/Copyright: February 15, 2024

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From the journal Open Geosciences Volume 16 Issue 1

Abstract

It is a great challenge for sedimentologists to perform the facies analysis of shales due to the lack of macro-structures and high heterogeneity in shale, and thus, conventional research methods are poorly applied in the field of shale sedimentology. To establish a typical depositional model for marine shales, a new method adopted from marine carbonate rocks was employed to perform the microfacies analysis of marine shales of the Wufeng and Longmaxi Formation in Sichuan Basin. This method emphasizes the fact that many skeletal constituents in marine shales have specific bathymetric distributions, allowing for a better understanding of the depositional process of shales. With this approach, a total of nine microfacies were identified and two depositional models for marine shales were proposed. The depositional models show that sea levels were high during the Middle to Late Wufeng period, which was followed by a rapid regression that led to a significant sea-level drop by over 50 m at the topmost of the Wufeng Formation, and consequently, widespread fine-grained sandy shales with large amounts of benthic bioclasts were deposited in the study region. The eustatic sea-level changes recovered by using the method of microfacies analysis are in line with the global trend, indicating that the method has promising applications in the field of marine shale sedimentology.

Keywords: Sichuan basin; Wufeng Formation–Longmaxi formation; shale; microfacies; depositional model

1 Introduction

In recent years, with the exploration and development of shale gas, shale sedimentology has gradually become a hot spot in the field of sedimentology [1,2]. Compared with traditional clastic rocks, shale has the following characteristics: (1) the sediment is fine-grained, with over 50% of grains smaller than 62.5 μm and small particle size differentiation [3]; (2) the composition is highly heterogeneous in mineral composition, lithofacies, textural variation, and physical properties [4,5,6,7]; (3) the dynamic of shale deposition is complex since fine-grained sediments mostly deposited in deep-water environments in which bottom currents induced by gravity, storm wave, or hyperpycnal flow can occasionally exert influence on the process of deposition [8,9]; and (4) there is a lack of typical depositional models that can be widely applied to the shales both in marine and lacustrine settings [10,11,12].

To resolve the challenges of shale sedimentology, some advanced technologies and methods have been put forward and listed as the following: (1) sequence stratigraphy proxied by the TOC content and Th/U ratio has been employed to reconstruct the depositional history and to explore the factors that influence the organic enrichment [13,14,15] which had gained good achievements; (2) detailed petrological observations of millimeter- or micron-sized structures caused by the flows of bottom currents [1,16] provide new evidence for depositional processes and depositional dynamics in shales. These studies provide important clues to understand the depositional processes of shales, but it is difficult to characterize the depositional bathymetry of shales, which is a challenge in fine-grained sedimentology.

The Longmaxi Formation shale is an important object for the study of shale because it is not only rich in shale gas but also has accumulated a large amount of drilling, coring, and logging data. Shale gas reserves in the Sichuan Basin are very promising in the Wufeng Formation-Longmaxi Formation [17,18], which has shown potential for shale gas exploration and development. A national standard demonstration area for shale gas development has been established for the Longmaxi Formation shale in Fuling area, the eastern part of Sichuan Basin. Depositional conditions lay the foundation for the formation of high-quality shales, and previous works have been conducted to explore the deposition condition of the Longmaxi Formation shales in the Sichuan Basin, including (1) recovering the sedimentary palaeogeography of the Longmaxi Formation based on petrological observations, grain size analysis, and the test of TOC [18,19,20]; (2) the identification and classification of shales based on cores and logs [21]; and (3) the study of sequence stratigraphy of the Longmaxi Formation in the Sichuan Basin based on geochemical data [22]. These researches proposed different classification schemes and depositional models for marine shales of the Longmaxi Formation which provide the framework for shale gas development in the Sichuan Basin. However, due to the lack of a standard classification for shale, it still remains debated on the depositional process of marine shales and a generally accepted depositional model is an urgent need. Here, we propose a new method that heavily relies on the biotic skeletal constituents to perform the microfacies analysis of the marine shales of the Wufeng to Longmaxi Formations, with a goal to establish a better constrained depositional model and to uncover the hydrodynamic mechanism underlying deposition of the fine-grained sediments.

2 Geological settings

The Sichuan Basin belongs to the Upper Yangtze Platform [23], and since the Middle and Late Ordovician (Figure 1a), it has undergone the Caledonian, Indochinese, Yanshanian, and Himalaya orogeny [24], which led to the complex and diverse tectonic history of the Sichuan Basin. Sichuan Basin can be subdivided into the following five structural units including the Eastern Steep Structural Zone, the Western Depression, the Northern Gentle Structural Uplift, the Central Uplift, as well as Southern Gentle Structural Zone [25] (Figure 1b). The study area is located within the transitional zone between the Eastern Steep Structural Zone and the Southern Gentle Structural Zone and is strongly affected by the large-scale orogeny of the Himalaya movement, which led to a series of NNE-oriented folds and reverse faults in the study area. Compared to the ejective fold structure developed in the eastern and southern parts of the Sichuan Basin, the strata in the study region were less folded and characterized by the development of wide and gentle anticlines.

Figure 1

Palaeogeographic and tectonic maps of the Sichuan Basin and distribution of the studied wells in the study area. (a) The palaeogeographic map of the Yangtze Platform and Sichuan Basin during the Late Ordovician (modified from [16]). (b) The tectonic map of the Sichuan Basin and the location of the studied area (yellow rectangle). (c) The distribution of studied wells in the western part of Chongqing.

The grey-black shales of the Late Ordovician Wufeng Formation-Early Silurian Longmaxi Formation are widespread in the western part of Chongqing. Previous works have systematically studied these shales and established a high-resolution graptolite biostratigraphic zone. The Long 1₁ Submember can be subdivided into four beds based on the graptolite biostratigraphic zones in combination with a comparative logging stratigraphy [26,27]. The lower part of the Wufeng Formation is mainly composed of shale, siltstone, or greywacke shale, corresponding to the graptolite zone WF1–WF3; the Guanyinqiao Member at the top of the Wufeng Formation is characterized by the presence of fine sandy mudstone enriched in bioclasts especially of benthic Hernantian fossils, which corresponds to the graptolite zone WF4. The overlying Long 1₁ Submember of the Longmaxi Formation is dominated by siliceous shale, occasionally intercalated with thin layers of siltstone shale, with beds Long 1₁ ¹ to Long 1₁ ⁴ corresponding to graptolite zone LM1, LM2–LM3, LM4, and LM5, respectively (Table 1).

Table 1

Stratigraphic sequence of the Wufeng Formation–Longmaxi Formation in the Sichuan Basin (modified from Shi et al. [27])

Formation	Member	Submember	Bed	Graptolite zone	Stage	Series
Longmaxi	Long 1 (L-1)	Long 1₂ (L-1₂)		LM6–8	Rhuddanian	Lower Silurian
		Long 1₁ (L-1₁)	Long 1₁ ⁴ (L-1₁ ⁴)	LM5
			Long 1₁ ³ (L-1₁ ³)	LM4
			Long 1₁ ² (L-1₁ ²)	LM2–3
			Long 1₁ ¹ (L-1₁ ¹)	LM1	Hirnantian	Upper Ordovician
Wufeng	Guanyinqiao			WF4	Hirnantian
Wufeng	Graptolite shale			WF2–3	Katian

Graptolite zone: WF2 – dicellograptus complexus, WF3 – paraorthograptus pacificus, WF4 – persculptograptus extraordinarius, LM1 – normalograptus persculptus, LM2 – akidograptus ascensus, LM3 – parakidograptus acuminatus, LM4 – cystograptus vesiculosus, LM5 – coronograptus cyphus, LM6 – demirastrites triangulatus, LM7 – lituigraptus convolutus, and LM8 – stimulograptus sedgwickii.

3 Methodology

At present, a unified standard classification for shale has not yet been reached, and different scholars have proposed different schemes in terms of shale properties such as mineral composition, sedimentary structure, and elemental content [28,29]. Given that fine-grained marine sediments are rich in fossils that are sensitive to changes in the marine environments and thus have specific bathymetric distributions. Therefore, a method adopted from carbonate rocks [30] that emphasizes the importance of biotic skeletal constituents in marine shales was employed to perform the microfacies analysis. The classification of microfacies relies on both lithology and bioclasts with emphasis on the latter especially those that are indicative of bathymetry such as radiolaria, corals, sponges, and trilobites. A total of 376 thin sections (63 from Zu201, 52 from Zu202, 58 from Zu203, 56 from Zu205, 55 from Zu206, 59 from Zu207, 33 from Zu208, the well locations; see Figure 1c) were observed to establish the standard classification of marine shales in western part of Chongqing. In addition, a general consensus on the topography of the shelf in the Sichuan Basin during the early Paleozoic age is lacking, here a model for a Cretaceous muddy shelf is cited as a matched analog for the Sichuan Basin as both share common features such as widespread distribution of dark grey shale with thin intercalations of silty or calcareous laminae [12].

4 Results

4.1 Microfacies types and characteristics of the shales of the Wufeng Formation-Longmaxi formation

Nine microfacies were identified in this study based on their petrographic features and paleontological fossils, which are shown in Figure 2 and Table 2. The nine microfacies can be grouped into sedimentary environments corresponding to the outer shelf, middle shelf, and inner shelf. Microfacies 1–3 are dominated by siliceous shales, in which radiolarian fossils can be seen, corresponding to the outer shelf environment; Microfacies 4–6 are dominated by siliceous shales and siltstone shales, containing coral and sponge fossils that show fractured structure indicators of redeposition by bottom currents, corresponding to the middle shelf environment; Microfacies 7–9 are characterized by calcareous shales, fine sandy shales, and bioclastic limestone, in which fine-grained quartz and typical shallow-water fossils such as benthic trilobites and brachiopods are common, corresponding to the inner shelf environment.

Figure 2

Photomicrographs of microfacies from 1 to 9. (a) Microfacies 1: radiolaria-bearing siliceous shale, Zu208, 4371.58 m, Wufeng Formation, plane-polarized light; (b) Microfacies 2: siliceous shale, Zu208, 4366.2 m, Bed Long 1₁ ¹, plane-polarized light; (c) Microfacies 3: tuffaceous shale, Zu203, Bed Long 1₁ ⁴, plane-polarized light; (d) Microfacies 4: silty shale, Zu208, 4352.85 m, Bed Long 1₁ ³, plane-polarized light; (e) Microfacies 5: sponge-bearing siliceous shale, Zu207, 4392.0 m, Wufeng Formation, cross-polarized light; (f) Microfacies 6: coral-bearing siliceous shale, Zu201, 4368.02 m, plane-polarized light; (g) Microfacies 7: calcareous shale, Zu202, 3884.25 m, Long 1₁ ², plane-polarized light; (h) Microfacies 8: fine-sandy shale, Zu202, 3891.86 m, Guanyinqiao Member, cross-polarized light; and (i) Microfacies 9: bioclastic limestone, Zu205, 4298.98 m, Wufeng Formation, plane-polarized light.

Table 2

Microfacies, characteristics, and corresponding depositional environments in the study area

Microfacies	Description	Depositional environment
MF1: radiolaria-bearing siliceous shale	Dark grey laminated siliceous shale rich in radiolaria and organic matter	Outer shelf
MF2: siliceous shale	Dark grey laminated shale with abundant silt-sized quartz
MF3: tuffaceous shale	Dark grey shale containing banded or lenticular pale yellow volcanic tuff
MF4: silty shale	Shale containing occasional layers of silt-sized quartz, showing lenticular or flaser bedding	Middle shelf
MF5: sponge-bearing siliceous shale	Dark grey shale with fragmented siliceous sponges and occasional sponge spicules
MF6: coral-bearing siliceous shale	Relatively well-reserved corals floated in the dark grey shale
MF7: calcareous shale	Grey shale with abundant bioclasts including echinoderms	Inner shelf
MF8: fine-sandy shale	Dark grey shale containing abundant medium- to well-sorted fine-grained quartz, with occasional fragments of echinoderms and brachiopods
MF9: bioclastic limestone	Limestone containing abundant bioclasts including benthic trilobites, ostracods and brachiopods

4.2 Microfacies sequence at Well Zu201

The microfacies types of the Wufeng Formation at Well Zu201 are various and change rapidly. The lower part of the Wufeng Formation mainly develops siliceous shale and radiolarian-bearing siliceous shale, reflecting a deep-water depositional setting (Figure 3). The top of the Wufeng Formation abruptly transformed into silty shale and coral-bearing siliceous shale, with well-reserved corals floating in gray-black shale matrix (Figure 2f). Long 1₁ Submember shows monotonous microfacies types, dominated by siliceous shale that is interbedded with thin layers of silty shale. The corals in MF6 are recognized as Tetracoralla, which has been reported by He (1978) [31]. Given that the growth of most corals requires hard substrate like the carbonate platform, the floating corals in sandy shales suggest that they were transported into the black shale after the collapse of reefs due to exposure to erosion caused by the sudden drop in sea level. Taken together, the above evidence suggests that Well Zu201 was located in an inner shelf region during the late period of the Wufeng Formation and was strongly influenced by the Hernantian maximum regression.

Figure 3

Stratigraphical sequence and vertical distribution of microfacies at Well Zu201. MF – microfacies, For – formation, Sub – subformation, GY – Guanyinqiao, GR – Gamma Ray, KTH – potassium and thorium, TOC – total organic carbon, and Wufen – Wufeng. The sampling position for each photomicrograph is marked at the bottom right in red.

4.3 Microfacies sequence at Well Zu202

The Wufeng Formation at Well Zu202 mainly consists of siliceous shale, with occasional thin layers of radiolaria-bearing siliceous shale. The topmost part of the Wufeng Formation is characterized by the deposition of fine sandy shale (MF8) (Figure 4), which preserves highly fragmented echinoderms and brachiopods, along with poorly sorted quartz grains ranging from 300 to 500 μm in size (Figures 2h and 4). Upwards, the Longmaxi Formation is dominated by siliceous shales, with occasional thin-bedded silty shales. The presence of quartz grains at the top of the Wufeng Formation suggests that Well Zu202 is more proximal to a land source, and the enhanced input of debris from land sources following the regression led to the deposition of fine sandy shales.

Figure 4

Stratigraphical sequence and vertical distribution of microfacies at Well Zu202.

4.4 Microfacies sequence at Well Zu203

Siliceous shale develops in the lower and middle parts of the Wufeng Formation and abruptly changes to a fine sandy shale at the topmost beds at Well Zu203 (Figure 5). The quartz grains in the fine sandy shale are poorly sorted and well rounded (Figure 6a), suggesting that the location of Well Zu203 is proximal from the land source and quartz grains were deposited after a long-distance transport. Upward, the abrupt lithological change to siliceous shale at the base of the Longmaxi, which mainly comprises siliceous shale. Microfacies analysis shows that a thin layer of fine sandy shale was also deposited at the topmost of the Wufeng Formation at Well Zu203, suggesting that it was deposited in the inner shelf, proximal to a terrestrial source.

Figure 5

Stratigraphical sequence and vertical distribution of microfacies at Well Zu203.

Figure 6

Photomicrographs of quartz grains in Wells Zu203 and Zu206; (a) fine sandy shale, Zu203, 4105.97 m, Wufeng Formation, cross-polarized light, and (b) fine sandy shale, Zu206, 4268.92 m, Wufeng Formation, plane-polarized light.

4.5 Microfacies sequence at Well Zu205

Bioclastic limestone and calcareous shale are developed in the middle part of the Wufeng Formation at Well Zu205, where the bioclasts are mainly trilobite and brachiopod fossils. The top of the Wufeng Formation is characterized by a sudden transition from siliceous shale into a calcareous shale, which lacks bioclasts. Upward, the Long 1₁ Submember is dominated by siliceous shales (Figure 7). The abrupt change from siliceous shale to calcareous shale at the top of the Wufeng Formation indicates that the changes in lithology were most likely caused by the Hernantian regression. However, it is noteworthy that the lithology changes at the top of the Wufeng Formation at Well Zu205 are not obvious, compared to other wells, which is probably due to the relatively deep-water depth.

Figure 7

Stratigraphical sequence and vertical distribution of microfacies at Well Zu205.

4.6 Microfacies sequence at Well Zu206

The bioclastic limestone develops in the lower and middle parts of the Wufeng Formation at Well Zu206, and the bioclasts are mainly benthic trilobites and brachiopods. Upward, the microfacies gradually change to calcareous shale, siltstone, and siliceous shale, and at the top, the siliceous shale abruptly changes to fine sandy shale. Quartz grains are abundant in fine sandy shale with an average grain size of approximately 500 μm and are moderately rounded and sorted (Figure 6b). Upward, the Long 1₁ Submember consists mainly of siliceous shale (Figure 8). The fine sandy shales at the top of the Wufeng Formation at Zu206 are similar to those of wells Zu202 and Zu203, suggesting that Well Zu206 was also deposited in inner shelf during the late period of the Wufeng Formation and received clastic input from terrestrial sources.

Figure 8

Stratigraphical sequence and vertical distribution of microfacies at Well Zu206.

4.7 Microfacies sequence at Well Zu207

Bioclastic limestone developed in the lower and middle parts of the Wufeng Formation, and gradually transformed upward into silty shale (Figure 9), a trend similar to that of Wells Zu205 and Zu206. The top of the Wufeng Formation in Well Zu207 consists of siliceous shales containing sponges, which are siliceous and fractured (Figure 2e), suggesting possible offshore transportation by storm wave currents. It is notable that the sponges are associated with a few radiolarians with the absence of in-situ sponge spicules. The occurrence of sponges in the Guanyinqiao Member in this area has also been reported by previous research, but the sponges were not systematically described and identified due to poor preservation [32]. However, previous authors have proposed that sponge fossils coexist with shallow-water bioclasts, indicating a possible shallow-water depositional environment. Based on the sedimentary records described above, it is speculated that Well Zu207 was deposited in a middle-shelf environment during the late Wufeng Formation and received shallow-water bioclasts from the inner shelf.

Figure 9

Stratigraphical sequence and vertical distribution of microfacies at Well Zu207.

4.8 Microfacies sequence at Well Zu208

The lower part of the Wufeng Formation at Well Zu208 develops thin layers of radiolaria-bearing siliceous shale and siltstone, while the upper part of the Wufeng Formation is silty shale. Upward, the Long 1₁ Submember is mainly composed of siliciclastic shale (Figure 10). Overall, the microfacies of Well Zu208 are relatively homogeneous compared to other wells, indicating that Well Zu208 was deposited at a deeper-water depth and was less affected by the regression during the late period of the Wufeng Formation.

Figure 10

Stratigraphical sequence and vertical distribution of microfacies at Well Zu208.

5 Discussion

5.1 The priority of microfacies analysis in the interpretation of marine shales

The schemes of classification for marine shales are critical for the interpretation of shale deposition, and many studies have proposed different schemes based on the property of shales such as TOC, mineral composition, and sedimentary structures [18,19,20]. Based on these classification schemes, the depositional environments of shales can be broadly classified as clastic shelf and carbonate shelf [33,34], but precise interpretation of the water depth when the shales were deposited is hardly feasible. The method adopted from the marine carbonate in this study provides a practicable approach to trace the depositional process of the shales as the skeletal constituents in marine shales have specific bathymetric distributions. But given the fact that some shallow-water benthic bioclasts might be re-worked and transported into deep water regions by gravity flows on high-relief shelf, this would affect the reliability of the method [6,7]. Thus, it is suggested that the method proposed here is more suitable for low-relief shelf where gravity flows tend to occur less frequently.

5.2 Sea level variations from the Wufeng Formation to Longmaxi Formation in the western Chongqing

During the late period of the Wufeng Formation (Hernantian), a large glaciation cap formed along the northern margin of Gondwana, resulting in a global sea level fall [35,36]. This sea level fall is documented globally and can be well correlated among different regions, such as North America, Russia, and China [37,38]. Global sea level curves for this period have been generated based on the study of sedimentary records and have been applied to the comparison of sea levels on the global scale with good consistency. The intensity of this regression is still controversial, as evidence from depositional records suggests a sea level drop by over 50 m [39]. Also, the rapid drop in sea level had been considered to be one of the causes of the biological mass extinction at the end of the Ordovician [40].

In this study, we recovered the sea level variations from the Wufeng Formation to the Long 1₁ Submember based on the results of the microfacies analysis (Figure 11). It is concluded that: the sea level was relatively high in the early period of the Wufeng Formation, and consequently radiolarian-bearing siliceous shales were extensively deposited; in the late period of the Wufeng Formation, the sea level dropped abruptly, and the sea level changed rapidly in the northern (Well Zu202) and southern (Wells Zu203 and 206) part of the study area where fine sandy deposits were deposited; Subsequently, the sea level rose rapidly at the Long 1₁ Bed, resulting in extensive deposition of extensive siliceous shale. The sea level change trend from the Wufeng Formation to the Long 1₁ Submember in the study area is consistent with the global pattern, indicating that the method of microfacies analysis with emphasis on the skeletal constituents has reliable and potential implications for the sedimentology of fine-grained sediments.

Figure 11

Correlation of eustatic sea-level changes among wells in the study area.

5.3 Depositional models of the Wufeng formation

The depositional model of the Wufeng Formation in the western part of Chongqing (Figure 12) is established based on the results of microfacies analysis, paleo sea-level changes, as well as paleoclimate data (paleotemperature data from [41]). The detailed description is as follows: the early period of the Wufeng Formation was featured by high sea levels with warm climates, and widespread deposition of radiolarian-bearing siliceous shale or siliceous shale; in the late period of the Wufeng Formation, the sea levels dropped sharply due to the onset of global glaciations, consequently, widespread deposition of fine sandy shale containing poor-rounded quartz in association with shallow-water bioclasts developed due to the enhanced input of debris from terrestrial sources. Meanwhile, it is noteworthy that benthic debris preserved in fine sandy shales or shales, such as corals, sponges, brachiopods, echinoderms, and other fossils, indicates the offshore transport of bioclasts by storm wave currents. This depositional model indicates that strong hydrodynamic currents, i.e., storm wave currents and gravity flow, can impose an impact on the transportation of fine-grained sediments, which is supported by previous reports [42]. It is thus concluded that the dynamics mechanism of fine-grained sedimentology is exceptionally complex and needs further work with respect to advanced techniques to better constrain the depositional process.

Figure 12

Depositional model for the shales of the Wufeng Formation in this study (the paleothermometry data is from [41]). FWWB – Fair Weather Wave Base, SWB – Storm Wave Base, and NE – Northeast.

6 Conclusions

A method that adopted from the marine carbonate rocks is employed to perform the microfacies analysis of marine shales of the Wufeng to Longmaxi Formations in Sichuan Basin. The achievements can be concluded as follows:

A total of nine types of microfacies were identified and classified into three major depositional environments including inner shelf, middle shelf, and outer shelf, based on their mineralogical composition and biotic skeletal constituents.
The sea level changes recovered by the microfacies analysis show that sea levels were high during the middle period of the Wufeng Formation, and dropped abruptly in the latest period of the Wufeng Formation, and then were immediately followed by a rapid transgression in the earliest period of the Longmaxi Formation. The paleo sea level change trend in this study is consistent with the global pattern, indicating that the method has promising applications in the shale sedimentology.
A depositional model for shales of the Wufeng Formation in the western part of Chongqing was established, which was closely related to the cooling event. The shales of the Wufeng Formation witnessed a sudden transition from the radiolarian-bearing siliceous shales to fine sandy siliceous shales at the top of the Wufeng Formation, indicating that the glaciation led to a significant impact on the depositional model of marine shales.

Acknowledgments

We applied the EC approach for the sequence of authors. We are grateful for the constructive comments by the Editor and the two anonymous reviewers. This work is funded by the Scientific Research and Technological Development Project of PetroChina: Main controlling factors of high production and beneficial development technology of deep shale gas in western Chongqing (2022KT1205).

Author contributions: Yana Chen: conceptualization, data curation, formal analysis, investigation, methodology, resources, bisualization, writing – original draft. Jia Liu: resources, investigation, validation, writing – review & editing. Nan Wang: conceptualization, supervision, validation, writing – review & editing. Yiqing Zhu: investigation, methodology, resources, writing – review & editing. Wei Lin: methodology, validation, visualization, writing – review & editing. Quansheng Cai: writing – review & editing. Yuchuan Chen: writing – review & editing. Mingtao Li: conceptualization, investigation, methodology, resources, validation, visualization, writing – review & editing.
Conflict of interest: All authors state no conflict of interest.

References

[1] Ghadeer SG, Macquaker JHS. The role of event beds in the preservation of organic carbon in fine-grained sediments: Analyses of the sedimentological processes operating during deposition of the whitby mudstone formation (Toarcian, lower jurassic) preserved in northeast England. Mar Pet Geol. 2012;35(1):309–20. 10.1016/j.marpetgeo.2012.01.001.Search in Google Scholar

[2] Peng J, Larson TE. A novel integrated approach for chemofacies characterization of organic-rich mudrocks. Am Assoc Pet Geol Bull. 2022;106(2):437–60. 10.1306/05112120210.Search in Google Scholar

[3] Milliken K. A compositional classification for grain assemblages in fine-grained sediments and sedimentary rocks. J Sediment Res. 2014;84(12):1185–99. 10.2110/jsr.2014.92.Search in Google Scholar

[4] Lazar OR, Bohacs KM, Macquaker JHS, Schieber J, Demko TM. Capturing key attributes of fine-grained sedimentary rocks in outcrops, cores, and thin sections: Nomenclature and description guidelines. J Sediment Res. 2015;85(3):230–46. 10.2110/jsr.2015.11.Search in Google Scholar

[5] Peng J, Milliken KL, Fu Q, Janson X, Hamlin HS. Grain assemblages and diagenesis in organic-rich mudrocks, upper Pennsylvanian cline shale (Wolfcamp D), Midland Basin, Texas. Am Assoc Pet Geol Bull. 2020;104(7):1593–624. 10.1306/03022018240.Search in Google Scholar

[6] Peng J. Sedimentology of the upper Pennsylvanian organic‐rich cline shale, midland basin: From gravity flows to pelagic suspension fallout. Sedimentology. 2021;68(2):805–33. 10.1111/sed.12811.Search in Google Scholar

[7] Peng J, Hu Z, Feng D, Wang Q. Sedimentology and sequence stratigraphy of lacustrine deep-water fine-grained sedimentary rocks: The lower jurassic dongyuemiao formation in the sichuan Basin, Western China. Mar Pet Geol. 2022;146:105933. 10.1016/j.marpetgeo.2022.105933.Search in Google Scholar

[8] Yang R, Fan A, Van Loon (Tom) AJ, Han Z, Zavala C. The influence of hyperpycnal flows on the salinity of deep-marine environments, and implications for the interpretation of marine facies. Mar Pet Geol. 2018;98:1–11. 10.1016/j.marpetgeo.2018.08.005.Search in Google Scholar

[9] Oluwajana OA, Opatola AO, Adamolekun OJ, Ndukwe OS, Olawuyi GT, Ofiwe CU, et al. Sedimentation, depositional environments, and hydrocarbon potential of the maastrichtian-paleocene araromi formation, Eastern Dahomey (Benin) Basin, Southwestern Nigeria. J Pet Explor Prod Technol. 2021;11(11):3917–34. 10.1007/s13202-021-01262-7.Search in Google Scholar

[10] Sun L, Fang C, Li F, Zhu R, Zhang Y, Yuan X, et al. Innovations and challenges of sedimentology in oil and gas exploration and development. Shiyou Kantan Yu Kaifa/Pet Explor Dev. 2015;42(2):129–36. 10.11698/PED.2015.02.01.Search in Google Scholar

[11] Birgenheier LP, Horton B, McCauley AD, Johnson CL, Kennedy A. A depositional model for offshore deposits of the lower blue gate member, Mancos Shale, Uinta Basin, Utah, USA. Sedimentology. 2017;64(5):1402–38. 10.1111/sed.12359.Search in Google Scholar

[12] Li Z, Schieber J. Detailed facies analysis of the upper cretaceous tununk shale member, henry mountains region, Utah: Implications for mudstone depositional models in epicontinental seas. Sediment Geol. 2018;364:141–59. 10.1016/j.sedgeo.2017.12.015.Search in Google Scholar

[13] Slatt RM, Rodriguez ND. Comparative sequence stratigraphy and organic geochemistry of gas shales: Commonality or coincidence? J Nat Gas Sci Eng. 2012;8:68–84. 10.1016/j.jngse.2012.01.008.Search in Google Scholar

[14] Blocho RM, Smith RW, Noll MR. Analyses of depositional environments of the marcellus formation in new york using biomarker and trace metal proxies. J Pet Explor Prod Technol. 2021;11(8):3163–75. 10.1007/s13202-021-01237-8.Search in Google Scholar

[15] Cai Q, Hu M, Kane OI, Li M, Zhang B, Hu Z, et al. Cyclic variations in paleoenvironment and organic matter accumulation of the Upper Ordovician–Lower Silurian Black Shale in the Middle Yangtze Region, South China: Implications for tectonic setting, paleoclimate, and sea-level change. Mar Pet Geol. 2022;136:105477. 10.1016/j.marpetgeo.2021.105477.Search in Google Scholar

[16] Wang H, Shi Z, Sun S, Zhao Q, Zhou T, Cheng F, et al. Microfacies types and distribution of epicontinental shale: A case study of the Wufeng–Longmaxi Shale in Southern sichuan basin, China. Pet Explor Dev. 2023;50(1):57–71. 10.1016/S1876-3804(22)60369-0.Search in Google Scholar

[17] Xiao B, Liu S, Li Z, Ran B, Ye Y, Yang D, et al. Geochemical characteristics of marine shale in the Wufeng formation–Longmaxi formation in the Northern sichuan basin, South China and its implications for depositional controls on organic matter. J Pet Sci Eng. 2021;203:108618. 10.1016/j.petrol.2021.108618.Search in Google Scholar

[18] Xiao B, Xiong L, Zhao Z, Fu X. Sedimentary tectonic pattern of Wufeng and Longmaxi Formations in the northern margin of Sichuan Basin, South China. Int Geol Rev. 2022;64(15):2166–85. 10.1080/00206814.2021.1972477.Search in Google Scholar

[19] Wang S, Dong D, Wang Y, Li X, Huang J, Guan Q. Sedimentary geochemical proxies for paleoenvironment interpretation of organic-rich shale: A case study of the lower Silurian Longmaxi Formation, Southern Sichuan Basin, China. J Nat Gas Sci Eng. 2016;28:691–9. 10.1016/j.jngse.2015.11.045.Search in Google Scholar

[20] Liang C, Jiang Z, Cao Y, Zhang J, Guo L. Sedimentary characteristics and paleoenvironment of shale in the Wufeng-Longmaxi formation, North Guizhou Province, and its shale gas potential. J Earth Sci. 2017;28:1020–31. 10.1007/s12583-016-0932-x.Search in Google Scholar

[21] Wei Y, Jianbo W, Shuai L, Kun W, Yinan Z. Logging identification of the longmaxi mud shale reservoir in the Jiaoshiba area, sichuan basin. Nat Gas Ind B. 2014;1(2):230–6. 10.1016/j.ngib.2014.11.016.Search in Google Scholar

[22] Zhu Y, Chen G, Liu Y, Shi X, Wu W, Luo C, et al. Sequence stratigraphy and lithofacies paleogeographic evolution of katian stage - aeronian stage in southern sichuan basin, SW China. Shiyou Kantan Yu Kaifa/Pet Explor Dev. 2021;48(5):974–85. 10.1016/S1876-3804(21)60096-4.Search in Google Scholar

[23] Xiao B, Xiong L, Zhao Z, Fu X, Zhao Z, Hou H, et al. Late Ordovician-early silurian extension of the northern margin of the upper Yangtze platform (South China) and its impact on organic matter accumulation. J Pet Sci Eng. 2023;220:111238. 10.1016/j.petrol.2022.111238.Search in Google Scholar

[24] Feng Q, Qiu N, Borjigin T, Wu H, Zhang J. Tectonic evolution revealed by thermo-kinematic and its effect on shale gas preservation. Energy. 2022;240:122781. 10.1016/j.energy.2021.122781.Search in Google Scholar

[25] Feng Q, Qiu N, Fu X, Li W, Liu X, Ji R. Maturity evolution of permian source rocks in the sichuan basin, Southwestern China: The role of the Emeishan Mantle Plume. J Asian Earth Sci. 2022;229:105180. 10.1016/j.jseaes.2022.105180.Search in Google Scholar

[26] Fan JX, Melchin MJ, Chen X, Wang Y, Zhang YD, Chen Q, et al. Biostratigraphy and geography of the Ordovician-Silurian Lungmachi Black Shales in South China. Sci China Earth Sci. 2011;54(12):1854–63. 10.1007/s11430-011-4301-3.Search in Google Scholar

[27] Shi Z, Wang H, Sun S, Guo C. Graptolite zone calibrated stratigraphy and topography of the late Ordovician-early silurian Wufeng-Lungmachi Shale in Upper Yangtze Area, South China. Arab J Geosci. 2021;14(3):1–17. 10.1007/s12517-021-06517-5.Search in Google Scholar

[28] Ou C, Li C, Rui Z, Ma Q. Lithofacies distribution and gas-controlling characteristics of the Wufeng–Longmaxi Black Shales in the Southeastern region of the Sichuan Basin, China. J Pet Sci Eng. 2018;165:269–83. 10.1016/j.petrol.2018.02.024.Search in Google Scholar

[29] Wu J, Liang C, Yang R, Xie J. The significance of organic matter–mineral associations in different lithofacies in the wufeng and longmaxi shale-gas reservoirs in the sichuan basin. Mar Pet Geol. 2021;126:104866. 10.1016/j.marpetgeo.2020.104866.Search in Google Scholar

[30] Flügel E. Microfacies of carbonate rocks. Berlin, Heidelberg: Springer; 2010. 10.1007/978-3-642-03796-2.Search in Google Scholar

[31] He X. Late Ordovician Guanyinqiao formation Tetrahymena Coral Faunas, Bijie, Guizhou. Professional Papers of Stratigraphy and Palaeontology. 1978;6:1–45. (in Chinese)Search in Google Scholar

[32] Liang P, Guo W, Wang N, Zhao W, Wang H, Ma X, et al. Lithofacies and sedimentary environments of the uppermost Ordovician Kuanyinchiao formation from the wells in Weiyuan‑Luzhou, Southern Sichuan. Chin J Geol. 2022;57(1):115–26. 10.12017/dzkx.2022.007. (in Chinese with English abstract).Search in Google Scholar

[33] Wang G, Jin Z, Hu Z, Liu G, Zhu T, Du W, et al. Sedimentary Evolution characteristics of fine-grained lithofacies under the high-resolution isochronous shelf system: Insights from the Wufeng-Longmaxi Shales in the Sichuan Basin. Lithosphere. 2021;2021(Special 1):6628867. 10.2113/2021/6628867.Search in Google Scholar

[34] Li J, Li H, Yang C, Wu Y, Gao Z, Jiang S. Geological characteristics and controlling factors of deep shale gas enrichment of the Wufeng-Longmaxi formation in the Southern Sichuan Basin, China. Lithosphere. 2022;2022(Special 12):4737801. 10.2113/2022/4737801.Search in Google Scholar

[35] Trotter JA, Williams IS, Barnes CR, Lécuyer C, Nicoll RS. Did cooling oceans trigger ordovician biodiversification? Evidence from conodont thermometry. Science. 2008;321(5888):550–4. 10.1126/science.115581.Search in Google Scholar

[36] Delabroye A, Vecoli M. The end-ordovician glaciation and the hirnantian stage: A global review and questions about late ordovician event stratigraphy. Earth-Sci Rev. 2010;98(3–4):269–82. 10.1016/j.earscirev.2009.10.010.Search in Google Scholar

[37] Haq BU, Schutter SR. A Chronology of Paleozoic Sea-Level Changes. Science. 2008;322(5898):64–8. 10.1126/science.1161648.Search in Google Scholar PubMed

[38] Munnecke A, Calner M, Harper DAT, Servais T. Ordovician and silurian sea–water chemistry, sea level, and climate: A synopsis. Palaeogeogr Palaeoclimatol Palaeoecol. 2010;296(3–4):389–413. 10.1016/j.palaeo.2010.08.001.Search in Google Scholar

[39] Desrochers A, Farley C, Achab A, Asselin E, Riva JF. A far-field record of the end ordovician glaciation: The ellis bay formation, Anticosti island, Eastern Canada. Palaeogeogr Palaeoclimatol Palaeoecol. 2010;296(3–4):248–63. 10.1016/j.palaeo.2010.02.017.Search in Google Scholar

[40] Harper DAT, Hammarlund EU, Rasmussen CMØ. End ordovician extinctions: A coincidence of causes. Gondwana Res. 2014;25(4):1294–307. 10.1016/j.gr.2012.12.021.Search in Google Scholar

[41] Finnegan S, Bergmann K, Eiler JM, Jones DS, Fike DA, Eisenman I, et al. The magnitude and duration of late ordovician-early silurian glaciation. Science. 2011;331(6019):903–6. 10.1126/science.1200803.Search in Google Scholar PubMed

[42] Gorsline DS. A review of fine-grained sediment origins, characteristics, transport and deposition. Geol Soc Lon Spec Publ. 1984;15(1):17–34. 10.1144/GSL.SP.1984.015.01.0.Search in Google Scholar

Received: 2023-10-06

Revised: 2023-11-16

Accepted: 2023-11-16

Published Online: 2024-02-15

This work is licensed under the Creative Commons Attribution 4.0 International License.

Articles in the same Issue

https://doi.org/10.1515/geo-2022-0583

Keywords for this article

Sichuan basin; Wufeng Formation–Longmaxi formation; shale; microfacies; depositional model

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