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Article

Deep-Water Traction Current Sedimentation in the Lower Silurian Longmaxi Formation Siliceous Shales, Weiyuan Area, Sichuan Basin, China, Using Nano-Resolution Petrological Evidence

by
Xiaofeng Zhou
1,
Jun Zhao
2,
Baonian Yan
2,
Zeyu Zhu
3,
Nan Yang
4,
Pingping Liang
5,* and
Wei Guo
5
1
College of Petroleum Engineering, China University of Petroleum (Beijing), Beijing 102249, China
2
Research Institute of Exploration and Development, PetroChina Yumen Oilfield Company, Jiuquan 735019, China
3
PetroChina Yumen Oilfield Company Supervision Center, Jiuquan 735019, China
4
PetroChina Yumen Oilfield Company Lao Junmiao Oil Production Plant, Jiuquan 735019, China
5
PetroChina Research Institute of Petroleum Exploration and Development, Beijing 100083, China
*
Author to whom correspondence should be addressed.
Minerals 2025, 15(7), 723; https://doi.org/10.3390/min15070723
Submission received: 13 May 2025 / Revised: 7 July 2025 / Accepted: 7 July 2025 / Published: 10 July 2025
(This article belongs to the Special Issue Deep-Time Source-to-Sink in Continental Basins)
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Abstract

Despite the shale revolution triggering global shale oil and gas exploration, our understanding of the sedimentary environments of deep-water organic-matter-rich shale remains unclear. The sedimentary environment and facies of some siliceous shales at the bottom of the Longmaxi Formation in the Weiyuan area of the Sichuan Basin, China, were therefore analyzed. Nano-resolution petrological characterization and genesis analysis of the siliceous shales studied were conducted using nano-resolution petrologic image datasets. We identified these siliceous shales as microbial mats formed by deep-water traction current sedimentation. The microbial mats’ formation and burial diagenesis processes were divided into seven stages. The silt-grade bioclastic carpet deposits initially, colonizing mud-grade siliceous microbes and forming the siliceous microbial mat. Subsequently, carbohydrate-rich microbes thrive in sediment voids, forming the carbohydrate-rich microbial mat. Additionally, SOM undergoes four stages of burial diagenesis process, progressing from kerogens to pre-oil bitumen generation and ultimately transforming into porous pyrobitumen and nonporous pyrobitumen. This study will improve the understanding of deep-water traction current sedimentation and has implications for guiding shale gas exploration and development.

Graphical Abstract

1. Introduction

Deep-water sedimentation mainly includes hydrostatic sedimentation, gravity flow sedimentation, and traction current sedimentation. Hydrostatic sediments are often impacted by the deep-water gravity flow and the deep-water traction current, and the deep-water gravity flow sediments and the deep-water traction current sediments are usually transformed into each other [1,2,3,4]. The deep-water traction current includes the internal wave, the internal tide, and the contour current. The internal wave is the underwater wave formed at the interface of two water layers with different densities or in a water body with a density gradient [5]. The internal wave with the same tidal cycle as the sea surface is known as the internal tide [6]. The contour current is the bottom current flowing horizontally along the seafloor isobaths [1]. The deep-water traction current generally has flow velocities of less than 50 cm/s and carries silt to fine sand particles, which forms large-scale, well-sorted clastic sediments with bedding structures [1,7,8,9].
Since the discovery of the deep-water traction current in the 1960s, research on its sedimentation has remained in its initial stage of discovering new examples and summarizing identification signs with more modern sediments and less ancient strata, and the research objects are mainly coarse-grained clastic sediments and rocks [1,3,4]. In recent years, the US shale boom has triggered global shale oil and gas exploration and development, which has led to many studies on shale sedimentation. Based on geochemical data, shale has been traditionally considered to develop in the hydrostatic environment [10,11,12,13,14]. By observing morphological and mineralogical characters through thin sections, deep-water shale was discovered to be composed of the lamina from hydrostatic sedimentation, gravity flow sedimentation, traction current sedimentation, and microbial mat [15,16,17]. The opinions of deep-water shale sedimentation study remain inconsistent by using different evidence without the nano-resolution petrological characterization.
Using nano-resolution petrological evidence, the authors discovered deep-water traction current sedimentation in some siliceous shales from the Longmaxi Formation, Sichuan Basin, China [18]. Recently, the authors identified a new kind of deep-water traction current sedimentation in some siliceous shales of the Longmaxi Formation, Weiyuan area, Sichuan Basin, China. In this study, we aim to determine (1) the occurrences of minerals and organic matters (OMs), (2) the origins of minerals and OMs, and (3) the sedimentation and diagenesis processes of the siliceous shales studied.

2. Geologic Setting

Sichuan Basin, in southwestern China, is a multi-cycle sedimentary basin whose resources are abundant [19,20]. At the initial stage of Silurian, a bay opened northward; this consisted of a shallow-water shelf and a deep-water shelf (Figure 1a) [18,21]. Approximately 80 m of black shale (claystone) deposited in the deep-water shelf and formed strata rich in graptolites and radiolarians at the bottom of the Longmaxi Formation (Figure 1b) [18,22], which provided the material conditions for shale gas [23].
Biostratigraphic correlations showed that some underwater highlands occurred in the Sichuan Basin (Figure 1a) [21]. These underwater highlands controlled the lithofacies of shale and influenced the enrichment of shale gas [24,25]. In the Weiyuan area, a narrow NE–SW deep-water shelf is sandwiched between the paleo-uplift and the highland (Figure 1a), where siliceous shale developed [26,27]. Research results on tectonic evolution have showed that, at the initial stage of the Silurian, the Caledonian Orogeny spread to the Sichuan Basin; this formed some underwater highlands [21].
The Longmaxi Formation went through two uplifts and one subsidence (Figure 2). In the Cretaceous, its maximum burial depth was approximately 6500 m [19], and the sedimentary organic matter (SOM) was turned into porous pyrobitumen and hydrocarbon gas, forming shale gas [28]. Since the Cenozoic, its burial depth has been raised to 2000–5500 m, and the structural pattern of alternating faults, folds, uplifts, and depressions led to the differential enrichment of shale gas [19,29,30,31]. The shale gas above 3500 m has produced more than 250 × 108 m3 annually, and that below 3500 m is important to increasing production in China [32,33].

3. Research Methods

Sampling wells W2 and W3 were located in the narrow deep-water shelf sandwiched between the paleo-uplift and the highland (Figure 1a). Based on the collected data, the profiles of TOC data and the bulk rock analysis data were compiled for the Wufeng–Longmaxi Formations of wells W2 and W3 (Figure 3). At depths of 2572.87 m and 2573.82 m of well W2 and 2852.76 m and 2853.10 m of well W3, the contents of quartz are all approximately 90%. One core sample was collected from each depth (Figure 3, black solid circles).
Four core samples were cut into 1 × 1 cm argon-ion-polished slices for the nano-resolution petrological image datasets by Modular Automated Processing System (MAPS). The MAPS technology is an effective means of obtaining high-resolution, large-view petrological image datasets; this provides first-hand information for the nano-resolution petrological characterization of shale [18,28,37,38]. This technology divides the argon-ion-polished surface into a series of regular grids, obtains secondary electron scanning images with a resolution of 4 nm on each grid, concatenates all the images, and obtains a two-dimensional large-view image dataset. An offline version of the image editor (ATLASTM Browser-Based Viewer (Carl Zeiss, Oberkochen, Germany)) is used to obtain petrological images. The preparation of argon-ion-polished slices and the acquisition of nano-resolution MAPS petrological image datasets were completed at the National Energy Shale Gas Research and Development (Experiment) Center, Langfang Branch, Research Institute of Petroleum Exploration and Development, China. The argon ion polishing instrument model is a Fischione Model 1060 (Fischione Instruments, Inc, Export, PA, USA), the scanning electron microscope model is a Fei Helios 650 (Thermo Fisher Scientific, Waltham, MA, USA), and the mineral scanner model is an Apreo 2S (Thermo Fisher Scientific, Waltham, MA, USA). Argon-ion-polished slices were cut into the slices oriented perpendicular to the bedding plane. These slices were placed on an objective table according to the natural state of the samples. The acquired 4 nm resolution petrological image datasets were used to obtain continuous nano-resolution images (Figure 4, Figure 5 and Figure 6) in order to observe and describe the occurrences of minerals and OMs and their compaction information in the siliceous shales.

4. Results

4.1. Occurrence of Minerals

The bulk rock analysis data showed that the quartz contents in the four siliceous shales studied were approximately 90% and pyrite, feldspar, and clay were not detected (Figure 3). However, sporadic clays (micas) (Figure 4f,h,j–l, Figure 5g,h,j and Figure 6g,h) and pyrites (Figure 5b,d–f,j and Figure 6h) were observed in every nano-resolution petrological image dataset.

4.1.1. Quartz

Based on size, quartz was divided into mud-grade quartz particles and silt-grade quartz particles (Figure 4a,c, Figure 5g and Figure 6a). These two kinds of quartz constitute the main part of the siliceous shales studied. The sizes of the mud-grade particles are concentrated in 1 nm to 4 nm, and the sizes of the silt-grade particles are concentrated in the range of 6 to 30 μm. The mud-grade quartz particles occur in the forms of euhedral crystals and anhedral crystals, with many pores in their cores and no pore at their edges (Figure 4b and Figure 6a). Silt-grade quartz particles are circular, elliptical, and elongated (Figure 4c). According to the degree of intragranular pores, the silt-grade quartz particles were further subdivided into nonporous quartz particles (Figure 4c,e,f) and porous quartz particles with many pores in their cores and no pores at their edges (Figure 4c,d,g). In these siliceous shales, the porous mud-grade quartz particles and the porous silt-grade quartz particles dominate, whereas the nonporous silt-grade quartz particles sporadically scatter (Figure 4a,c).
The porous silt-grade quartz particles are mainly fused through the nonporous edge to form the aggregate (Figure 4a,c), and the porous mud-grade quartz particles are also fused through the nonporous edge to form the aggregate (Figure 4b and Figure 5a,b). The porous silt-grade quartz aggregate is the outline of the siliceous shale′s framework, and other minerals and OMs are within this aggregate (Figure 4a,c). The porous mud-grade quartz aggregate exists in the forms of five occurrences, that is, among the porous silt-grade quartz aggregates (Figure 4a,c), on the surfaces of the nonporous silt-grade quartz particles (Figure 4c,e,f) and the fecal pellets (Figure 4h and Figure 6g,h), and in the calcite and dolomite dissolution pores (Figure 5a,b).
Each silt-grade quartz particle has a mineral rim, but there is a difference between the rims on the porous silt-grade quartz particle and that on the nonporous silt-grade quartz particle. Some clays exist between the nonporous silt-grade quartz particle and its mud-grade quartz rim (Figure 4i,j). Sometimes, a clay rim separates the nonporous silt-grade particle from the mud-grade quartz rim (Figure 4k,l). The porous silt-grade quartz particle is completely fused with the mud-grade quartz rim, which usually manifests as the nonporous edge with an uneven surface (Figure 4m–p). In these siliceous shales, the nonporous edges of the quartz particles serve as hubs, connecting all the quartz particles to form a strong framework; this indicates that these shales have a strong resistance to compaction.

4.1.2. Other Minerals

Other minerals include calcite, dolomite, pyrite, and clay. The calcite is in the forms of irregular morphology and intense dissolution, and porous mud-grade quartz particles fill in its dissolution pores (Figure 5a). The dolomite is in the forms of rhombic morphology with a size generally greater than 10 μm and intense dissolution, and pyrite, mud-grade quartz aggregate, and nonporous OM fill in its dissolution pores (Figure 5b,c).
Pyrite appears in two forms, namely, pyrite crystal (Figure 5b) and pyrite framboid (Figure 5d–f). These pyrite framboids were divided into overgrowth pyrite framboid and deformed pyrite framboid. The pyrite crystal fills in the carbonate dissolution pore (Figure 5b). The overgrowth pyrite framboid scatters among the quartz aggregate (Figure 5d). The deformed pyrite framboid is composed of dozens of nano-grade pyrite crystals with sizes between 300 and 500 nm, and the porous OMs fill in its intercrystalline pores (Figure 5e,f). Typically, the deformed pyrite framboid with irregular morphology is the mixture of pyrite crystals and mud-grade quartz particles (Figure 5e,f).
Multiple clay occurrences have been observed. Flaky clays (micas) occur among the quartz aggregate (Figure 5g). Clay rim or sporadic clays adhere on the nonporous silt-grade quartz particle (Figure 4i–l). Flocculent clays appear in the fecal pellets (Figure 4h and Figure 6h). Sporadic clays scatter in the porous OM (Figure 5h) and the nonporous OM (Figure 5j).

4.2. Occurrence of OM

According to the degree of pores, the OMs were divided into porous OMs and nonporous OMs. The porous OM only fills among the mineral particles (Figure 6a). the nonporous OM occurs among the mineral particles (Figure 6c,d), in the fecal pellet (Figure 4h), in carbonate dissolution pores (Figure 5b,c), in the secondary pores in the fecal pellets (Figure 6g,h), and in the secondary pore in a mud-grade quartz aggregate (Figure 6i).
When the porous OM fills among the porous quartz particles, the nonporous edges of the quartz particles are thin and rarely develop into complete crystal prisms (Figure 6a). When the nonporous OM appears among the porous quartz particles, the nonporous edges of the quartz particles are thick and show the prominent crystal prisms (Figure 6c). When both the porous OM and the nonporous OM coexist in the same pore, the development of quartz edges in contact with the porous OM is weak and that in contact with the nonporous OM is relatively prominent (Figure 6e,f).
The nonporous OM fills in the secondary pores in the fecal pellets (Figure 6g,h) and the mud-grade quartz aggregate (Figure 6i). These secondary pores whose opposing walls coincide manifest as the split features of the mud-grade quartz aggregate and the fecal pellets. The secondary pores and voids among the mineral particles usually interconnect and are filled with the nonporous OMs (Figure 6h,i); this indicates that the nonporous OMs in these two occurrences are homologous.

4.3. Occurrence of Fecal Pellet

Flocculent blocks that are composed of nano-grade silicon, clays, and nonporous OMs sporadically scatter in the siliceous shales with lack of clay. It is preliminarily inferred that they are fecal pellets. The fecal pellets are encased in mud-grade quartz rims to form silt-grade fossil particles (Figure 4h and Figure 6g,h). The secondary pores formed through the splitting of the fecal pellets are full of nonporous OMs (Figure 6g,h).

5. Discussions

Using MAPS technology, we showed that the siliceous shales studied were the lithification of the microbial mats formed through deep-water traction current sedimentation (Figure 7). The formation process of the microbial mats was divided into three stages, elucidating the growth sequences between the siliceous microbial mats and the carbohydrate-rich microbial mats. Subsequently, the petrogenetic process of the microbial mats was delineated into four stages, highlighting the transformation of biogenic silica and SOM into the various forms of quartz and pyrobitumen.

5.1. Origin of Minerals and OMs

Minerals mainly include quartz, carbonate, and pyrite. OMs consist of porous OMs and nonporous OMs.
Quartz occurs in the forms of porous mud-grade quartz, porous silt-grade quartz, and nonporous silt-grade quartz. The porous mud-grade and silt-grade quartz originate from biogenic silica [18,39,40], and the nonporous silt-grade quartz is the terrigenous clast [18]. Carbonate minerals include irregular calcite and rhombic dolomite. The irregular calcite is from the microbes that secrete calcium carbonate [41], and the phenomenon that the porous mud-grade quartz particles fill in its dissolution pores indicates that siliceous microbes lived in the pores. The rhomboid dolomite is an authigenic mineral near the water–sediment interface [41], and the phenomenon that the porous mud-grade quartz aggregates fill in the rhomboid dolomite dissolution pores implies that the dolomite underwent strong chemical weathering and weak physical weathering and siliceous microbes lived in its dissolution pores. The pyrite framboid usually developed near the interface of aerobic and anoxic water bodies [42,43], and the phenomenon that some porous mud-grade quartz fills in the irregular pyrite framboids indicates that the framboids were transformed into different shapes during microbial activity.
SOM went through four stages of evolution [18,28]. In the shallow burial diagenesis stage, SOM turned into the Type I and II kerogens [34,35,36]. In the early diagenesis stage of the moderate burial, the kerogens transformed into pre-oil bitumen. In the late diagenesis stage of the moderate burial, pre-oil bitumen converted to solid bitumen and oil. In the deep burial diagenesis stage, solid bitumen evolved into porous pyrobitumen and the oil turned into nonporous pyrobitumen. In the siliceous shale studied, the porous pyrobitumen (the porous OM) from the solid bitumen filled among the quartz aggregate, and the nonporous pyrobitumen (the nonporous OM) from the oil filled among the quartz aggregate and the secondary pores.
The phenomenon that porous mud-grade quartz rim lacks between the nonporous silt-grade quartz particle and the calcite particle indicates that the activity time of the siliceous microbes was later than the deposit time of these silt-grade quartz and calcite particles.

5.2. Sedimentary Environment of Siliceous Shales Studied: Deep-Water Traction Current Sedimentation

At the initial stage of Silurian, with the wave-like push of the Caledonian Orogeny from south to north in the Sichuan Basin, a northeast-trending underwater highland in the Weiyuan area appeared, connecting to the paleo-uplift toward the north and submerging toward the south [21,26,27,44]. There was a narrow, deep-water bay (Figure 1a), affording the favorable topographic condition for the formation of a deep-water traction current [1,3,4].
The deep-water traction current on seafloor sediments has a certain flow velocity [7,9], which has two functions. On the one hand, the vertically settling clays and OMs are carried away while the vertically settling silt-grade biogenic silica and calcite particles are deposited. On the other hand, unconsolidated sediments are reworked, and clays and OMs are carried away while the silt-grade terrigenous quartz with the residual clays on its surface, irregular calcite particles, and rhomboid dolomite particles are in situ redeposited. Under the influence of the two functions, both the vertically settling silt-grade particles and the in situ silt-grade particles are mixed together and the biogenic silica content is much higher than other mineral content, forming the silt-grade bioclastic carpet in which the deep-water traction current flows. The calcite content is less than 10% in each sample point (Figure 3), which indicates that the bioclastic carpet lies under the carbonate compensation depth (CCD) and calcite and dolomite particles strongly dissolve [41].
The deep-water traction current is rich in oxygen and nutrients [15,16,45], so mud-grade siliceous microbes become active in the bioclastic carpet. Microbes aggregate and inhabit on the surfaces of grains and in the carbonate dissolution pores to resist erosion through the deep-water traction current, forming a microbial mat. In the siliceous microbial mat, an increasing number of microbes grow; this decreases the flow velocity of deep-water traction current and is unsuitable for mud-grade siliceous microbes because of lack of oxygen and nutrients. This provides favorable conditions for carbohydrate-rich microbes, and the carbohydrate-rich microbial mat replaces the siliceous microbial mat. The predecessor of the siliceous shales studied, siliceous soft mud, is formed at this point.
The occurrences of the particles reworked by the deep-water traction current vary significantly. Almost all the clays were carried away, resulting in sporadic residue in the siliceous shales studied, which was not detected in the whole rock data (Figure 3). Based on clays or the clay rim on the surface of the silt-grade terrigenous quartz, this quartz was inferred from the reworked sediments. The absence of clays on the silt-grade biogenic silica indicates that the silica is from the siliceous microbes in seawater and that the fecal pellet is the metabolic material of microbes. The calcite and pyrite may be either from the endogenous clasts deposited in seawater or from the reworked sediments.

5.3. Burial Diagenesis Process of Microbial Mat

The microbial mats in the burial diagenesis process were converted into the siliceous shales studied through a series of physical and chemical changes. Of these, the most important changes include the conversion of SOM into pyrobitumen in various forms and the conversion of biogenic silica into porous quartz. In this study, by the thermal evolution of SOM, the burial diagenesis process of these microbial mats was divided into four stages; that is, kerogen, pre-oil bitumen, solid bitumen and oil, and pyrobitumen and hydrocarbon gas formation stages (Figure 7).
In the kerogen formation stage, SOM underwent a condensation reaction. The biogenic silica underwent a dissolution–precipitation reaction with a large amount of Si discharged [18,46]. Here, the fluid environment is more favorable for quartz overgrowth. During the pre-oil bitumen formation stage, the kerogen is converted into pre-oil bitumen, and quartz overgrowth continues on the walls of the remaining intergranular pore space. In the solid bitumen and oil formation stage, pre-oil bitumen is converted to solid bitumen and oil, which caused the volume expansion to produce an abnormally high pressure. The abnormally high fluid pressure led to the formation of secondary pores and fractures, which became paths for the primary migration of oil. The dissolution–precipitation reaction of biogenic silica was halted. In the pyrobitumen and hydrocarbon gas formation stage, the solid bitumen is converted to porous pyrobitumen (the porous OM), whereas the oil is converted to nonporous pyrobitumen (the nonporous OM).

6. Overview

Integrating the sedimentation and diagenesis processes, a coupled pattern of deep-water traction current sedimentation–diagenesis processes was established for the siliceous shales studied (Figure 7). This pattern can explain all the observed nano-resolution petrological phenomena. As shown in Figure 7, the dissolution of carbonate minerals mainly occurs in the deep-water traction current sedimentation. The quartz overgrowth develops from the sedimentation process to the solid bitumen and oil formation stage. Therefore, the study of the formation mechanism of shale reservoirs requires an integrating analysis of sedimentation and diagenesis, and nano-resolution petrological characterization is one way to realize the analysis.
There have been three views on the formation of the mud-grade quartz aggregate in siliceous shales. One view suggests that it is the authigenic mineral aggregate [47,48,49,50,51,52,53,54,55,56,57,58], another view suggests that it is the floating microbial aggregate or mats in deep-water sulfate-reducing environments [59,60,61], and the third view suggests that it is the recrystallization of biogenic silica [62,63,64,65]. This paper found that the mud-grade quartz particle has a bilayer structure with core and edge. The core is the porous quartz converted from the biogenic silica and the edge is the secondary quartz overgrowth by the dissolution–precipitation reaction of biogenic silica, which proves that the mud-grade quartz aggregate is the siliceous microbial aggregate rather than the authigenic mineral aggregate. Based on the following evidence, the mud-grade quartz aggregate is almost impossible from the floating microbial aggregate or mat: (1) the clay rim occurs on the terrigenous quartz rather than that of the biogenic silica; (2) the pyrite framboid is transformed into irregular nano-grade pyrite and quartz aggregate; (3) the lack of clay and OM-clay aggregate [18]; (4) the pyrite and the mud-grade biogenic silica fill in the dissolution pores of the rhomboid dolomite particles; and (5) the content of pyrite and clay is too low (Figure 3). However, the petrogenesis of microbial mats formed through deep-water traction current sedimentation systematically explains these features.
Although the Weiyuan and Zigong areas are not far apart (Figure 1a), the deep-water traction flow sedimentation is significantly different [18]. In the Zigong area, the siliceous shales lithified from the deep-water traction current sediments consist of three microtextures. Microtexture I is mainly a micro-quartz and nano-quartz skeleton among which the porous organic matter fills. Microtexture II is a clay-rich patch. Microtexture III is a nonporous dendritic organic matter. By deciphering the microtextures, in Microtexture I, the porous micro-quartz and the porous nano-quartz are from the siliceous microbial mats and the porous OM is from the carbohydrate-rich microbial mat; Microtexture II is a residual clay-rich sediment; and Microtexture III is a pyrobitumen derived from oil. In the Weiyuan area, the porous silt-grade quartz aggregate is the main body of the bioclastic carpet, the porous mud-grade quartz aggregate is from the siliceous microbial mat, and the porous OM is from the carbohydrate-rich microbial mat. Due to the weak heterogeneity of the rock structure, hydrocarbon-generating pressurized dendritic fractures do not develop, and the oil is migrated out through the pore network that consists of the secondary pores and the primary voids among the mineral particles.
The findings of this paper will enhance our understanding of siliceous shale’s sedimentation process in the Longmaxi Formation. It is possible to extend the approach outlined in this paper to other regions within the Sichuan Basin and worldwide. The siliceous shales formed through deep-water traction current sedimentation are high-quality reservoirs because of their high content of quartz and porous OM. With shale oil and gas boom, an increasing number of deep-water traction current sediments will be identified by nano-resolution petrological characterization in the Sichuan Basin and worldwide.

7. Conclusions

The application of MAPS technology to acquire a nano-resolution petrological image dataset was used to observe and describe the occurrences of minerals and OMs and then to carry out nano-resolution petrological characterization, and achieved the following results.
(1)
The siliceous shales studied from the Longmaxi Formation in the Weiyuan area, Sichuan Basin, were proposed to be microbialites formed by deep-water traction current sedimentation. This understanding can systematically explain all the nano-resolution petrological phenomena observed.
(2)
The formation process of the microbial mat was divided into three formation stages, namely, the silt-grade bioclast carpet, the mud-grade siliceous microbial mat, and the carbohydrate-rich microbial mat.
(3)
The petrogenetic process of microbial mats was divided into four stages. Through the kerogen formation stage, the pre-oil bitumen formation stage, the solid bitumen and oil formation stage, and the pyrobitumen and hydrocarbon gas formation stage, SOM evolves into porous pyrobitumen and nonporous pyrobitumen.
(4)
Siliceous shales formed through deep-water traction current sedimentation are high-quality reservoirs because of their high content of quartz and porous OM. With shale oil and gas boom, an increasing number of deep-water traction current sediments will be identified by nano-resolution petrological characterization in the Sichuan Basin and worldwide.

Author Contributions

Conceptualization, Investigation, Methodology, Writing—original draft, Writing—review and editing, X.Z.; Writing—original draft, Fund acquisition, Investigation, Methodology, Project administration and Resources, P.L.; Fund acquisition, Investigation, Project administration and Resources, W.G.; Data curation, Writing—original draft, J.Z. and B.Y.; Investigation, Data curation, Editing all Figures, Z.Z. and N.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the 14th Five-Year Plan of the Ministry of Science and Technology of PetroChina, grant: 2021DJ1901, and by the National Science and Technology Major Project of the Ministry of Science and Technology of China Project, grant: 2016ZX05037006.

Data Availability Statement

Data are contained within the article.

Acknowledgments

The authors give special thanks to the State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum (Beijing) and the National Energy Shale Gas Research and Development (Experiment) Center, Langfang Branch, Research Institute of Petroleum Exploration and Development, China, for their assistance in the preparation of argon-ion-polished slices, nano-resolution petrological observations, and mineral quantitative analysis. In addition, the authors thank the editors and reviewers for their help in revising and improving the article.

Conflicts of Interest

Authors Jun Zhao, Baonian Yan, Zeyu Zhu, Nan Yang were employed by the company PetroChina Yumen Oilfield Company. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. (a) Early Silurian sedimentary environment in the Sichuan Basin and its adjacent area [18]. (b) Stratigraphic histogram of the Longmaxi Formation [18].
Figure 1. (a) Early Silurian sedimentary environment in the Sichuan Basin and its adjacent area [18]. (b) Stratigraphic histogram of the Longmaxi Formation [18].
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Figure 2. Burial history and thermal evolution of organic matter of the Longmaxi Formation in the Weiyuan area, Sichuan Basin [18]. SOM was converted into kerogen with a burial depth of less than 1500 m and a geothermal temperature of less than 60 °C (First gray line) [34,35,36]. The kerogen was converted into pre-oil bitumen with burial depths of 1500–2500 m and geothermal temperatures of 60–90 °C (Pink lines). The pre-oil bitumen was converted into solid bitumen and oil with burial depths of 2500–3500 m and geothermal temperatures of 90–120 °C (Red line). The solid bitumen and the oil were converted into pyrobitumen and hydrocarbon gas with burial depths of 3500–6500 m and geothermal temperatures of 120–210 °C (Yellow line).
Figure 2. Burial history and thermal evolution of organic matter of the Longmaxi Formation in the Weiyuan area, Sichuan Basin [18]. SOM was converted into kerogen with a burial depth of less than 1500 m and a geothermal temperature of less than 60 °C (First gray line) [34,35,36]. The kerogen was converted into pre-oil bitumen with burial depths of 1500–2500 m and geothermal temperatures of 60–90 °C (Pink lines). The pre-oil bitumen was converted into solid bitumen and oil with burial depths of 2500–3500 m and geothermal temperatures of 90–120 °C (Red line). The solid bitumen and the oil were converted into pyrobitumen and hydrocarbon gas with burial depths of 3500–6500 m and geothermal temperatures of 120–210 °C (Yellow line).
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Figure 3. TOC and the bulk rock analysis data profiles in Wufeng–Longmaxi Formations of sampling wells W2 (a) and W3 (b). Shales with quartz content greater than 50% are siliceous shales. The dots present the bulk rock analysis data of the Wufeng–Longmaxi Formations, among which the black dots present the bulk rock analysis data of the core samples studied.
Figure 3. TOC and the bulk rock analysis data profiles in Wufeng–Longmaxi Formations of sampling wells W2 (a) and W3 (b). Shales with quartz content greater than 50% are siliceous shales. The dots present the bulk rock analysis data of the Wufeng–Longmaxi Formations, among which the black dots present the bulk rock analysis data of the core samples studied.
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Figure 4. Nano-resolution images of quartz in the siliceous shales studied. (a) The framework of the siliceous shale is mainly composed of porous mud-grade quartz particles and porous silt-grade quartz particles. The porous mud-grade quartz particles and the porous silt-grade quartz particles both appear in the form of aggregate, and the porous mud-grade quartz aggregates are nested within the porous silt-grade quartz aggregates. The OMs (dark materials) are among these aggregates. (b) The OMs are among the porous mud-grade quartz aggregates whose particle diameters range from 1 μm to 4 μm. (c–g) The silt-grade quartz particles with the largest size of 30 μm appear in the forms of porous quartz particle and nonporous quartz particle. A mud-grade quartz rim exists on the surface of each silt-grade quartz particle, but this rim is absent at the contact between the calcite particle and the silt-grade quartz particle (blue arrows). Sporadic clays (red arrows) are present between the nonporous silt-grade quartz particle and the mud-grade quartz rim. (h) The porous mud-grade quartz particles rim the fecal pellet. (i,j) Sporadic clays (red arrows) are between the nonporous silt-grade quartz particle and the porous mud-grade quartz rim. (k,l) The clay rim is between the nonporous silt-grade quartz particle and its porous mud-grade quartz rim. (m–p) The porous mud-grade quartz rim with no clay develops on the porous silt-grade quartz particle.
Figure 4. Nano-resolution images of quartz in the siliceous shales studied. (a) The framework of the siliceous shale is mainly composed of porous mud-grade quartz particles and porous silt-grade quartz particles. The porous mud-grade quartz particles and the porous silt-grade quartz particles both appear in the form of aggregate, and the porous mud-grade quartz aggregates are nested within the porous silt-grade quartz aggregates. The OMs (dark materials) are among these aggregates. (b) The OMs are among the porous mud-grade quartz aggregates whose particle diameters range from 1 μm to 4 μm. (c–g) The silt-grade quartz particles with the largest size of 30 μm appear in the forms of porous quartz particle and nonporous quartz particle. A mud-grade quartz rim exists on the surface of each silt-grade quartz particle, but this rim is absent at the contact between the calcite particle and the silt-grade quartz particle (blue arrows). Sporadic clays (red arrows) are present between the nonporous silt-grade quartz particle and the mud-grade quartz rim. (h) The porous mud-grade quartz particles rim the fecal pellet. (i,j) Sporadic clays (red arrows) are between the nonporous silt-grade quartz particle and the porous mud-grade quartz rim. (k,l) The clay rim is between the nonporous silt-grade quartz particle and its porous mud-grade quartz rim. (m–p) The porous mud-grade quartz rim with no clay develops on the porous silt-grade quartz particle.
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Figure 5. Nano-resolution images of calcite, dolomite, pyrite, and clay in the siliceous shales studied. (a) Porous mud-grade quartz particles (red arrows) filled in dissolution pores of the calcite particle. (b,c) A pyrite crystal, nonporous OM, and porous mud-grade quartz aggregate (blue arrows) occur in the dissolution pores of the rhombic dolomite. (d) An overgrowth pyrite framboid exists among the porous mud-quartz aggregate. (e,f) Sporadic porous mud-grade quartz particles (red arrows) are in the irregular pyrite framboids. (g) Sporadic flaky clays (micas) occur among the quartz aggregate. (h,i) A flaky clay is in the porous OM. (j) Sporadic flaky clays are in the nonporous OM.
Figure 5. Nano-resolution images of calcite, dolomite, pyrite, and clay in the siliceous shales studied. (a) Porous mud-grade quartz particles (red arrows) filled in dissolution pores of the calcite particle. (b,c) A pyrite crystal, nonporous OM, and porous mud-grade quartz aggregate (blue arrows) occur in the dissolution pores of the rhombic dolomite. (d) An overgrowth pyrite framboid exists among the porous mud-quartz aggregate. (e,f) Sporadic porous mud-grade quartz particles (red arrows) are in the irregular pyrite framboids. (g) Sporadic flaky clays (micas) occur among the quartz aggregate. (h,i) A flaky clay is in the porous OM. (j) Sporadic flaky clays are in the nonporous OM.
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Figure 6. Nano-resolution images of OMs in the siliceous shales studied. (a,b) Porous OMs occur among the porous quartz aggregates. (c,d) Nonporous OMs are among the porous quartz aggregate. (e,f) Porous (red arrows) and nonporous (blue arrows) OMs coexist in a void. The thickness of the quartz overgrowth in contact with the nonporous OM is bigger than that of the quartz overgrowth in contact with the porous OM. (g) A nonporous OM fills in the secondary pore in the fecal pellet. (h) A nonporous OM, coming into direct contact with that among the porous quartz aggregate, fills in in the secondary pore in the fecal pellet. (i) A nonporous OM, coming into direct contact with that among the mud-grade quartz aggregate, fills in the secondary pore in the mud-grade quartz aggregate.
Figure 6. Nano-resolution images of OMs in the siliceous shales studied. (a,b) Porous OMs occur among the porous quartz aggregates. (c,d) Nonporous OMs are among the porous quartz aggregate. (e,f) Porous (red arrows) and nonporous (blue arrows) OMs coexist in a void. The thickness of the quartz overgrowth in contact with the nonporous OM is bigger than that of the quartz overgrowth in contact with the porous OM. (g) A nonporous OM fills in the secondary pore in the fecal pellet. (h) A nonporous OM, coming into direct contact with that among the porous quartz aggregate, fills in in the secondary pore in the fecal pellet. (i) A nonporous OM, coming into direct contact with that among the mud-grade quartz aggregate, fills in the secondary pore in the mud-grade quartz aggregate.
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Figure 7. Pattern of the sedimentation–diagenetic processes of some siliceous shales studied from the Longmaxi Formation in the Weiyuan area, Sichuan Basin.
Figure 7. Pattern of the sedimentation–diagenetic processes of some siliceous shales studied from the Longmaxi Formation in the Weiyuan area, Sichuan Basin.
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Zhou, X.; Zhao, J.; Yan, B.; Zhu, Z.; Yang, N.; Liang, P.; Guo, W. Deep-Water Traction Current Sedimentation in the Lower Silurian Longmaxi Formation Siliceous Shales, Weiyuan Area, Sichuan Basin, China, Using Nano-Resolution Petrological Evidence. Minerals 2025, 15, 723. https://doi.org/10.3390/min15070723

AMA Style

Zhou X, Zhao J, Yan B, Zhu Z, Yang N, Liang P, Guo W. Deep-Water Traction Current Sedimentation in the Lower Silurian Longmaxi Formation Siliceous Shales, Weiyuan Area, Sichuan Basin, China, Using Nano-Resolution Petrological Evidence. Minerals. 2025; 15(7):723. https://doi.org/10.3390/min15070723

Chicago/Turabian Style

Zhou, Xiaofeng, Jun Zhao, Baonian Yan, Zeyu Zhu, Nan Yang, Pingping Liang, and Wei Guo. 2025. "Deep-Water Traction Current Sedimentation in the Lower Silurian Longmaxi Formation Siliceous Shales, Weiyuan Area, Sichuan Basin, China, Using Nano-Resolution Petrological Evidence" Minerals 15, no. 7: 723. https://doi.org/10.3390/min15070723

APA Style

Zhou, X., Zhao, J., Yan, B., Zhu, Z., Yang, N., Liang, P., & Guo, W. (2025). Deep-Water Traction Current Sedimentation in the Lower Silurian Longmaxi Formation Siliceous Shales, Weiyuan Area, Sichuan Basin, China, Using Nano-Resolution Petrological Evidence. Minerals, 15(7), 723. https://doi.org/10.3390/min15070723

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