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Article

Deciphering Nano-Resolution Petrological Characteristics of the Siliceous Shale at the Bottom of the Longmaxi Formation in the Zigong Area, Sichuan Basin, China: Deep-Water Microbialites

by
Xiaofeng Zhou
1,2,*,
Wei Guo
3,
Xizhe Li
3,
Pingping Liang
3,
Junmin Yu
2 and
Chenglin Zhang
4
1
State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum (Beijing), Beijing 102249, China
2
College of Petroleum Engineering, China University of Petroleum (Beijing), Beijing 102249, China
3
PetroChina Research Institute of Petroleum Exploration and Development, Beijing 100083, China
4
Shale Gas Institute of PetroChina Southwest Oil & Gas Field Company, Chengdu 610051, China
*
Author to whom correspondence should be addressed.
Minerals 2024, 14(10), 1020; https://doi.org/10.3390/min14101020
Submission received: 5 August 2024 / Revised: 5 October 2024 / Accepted: 7 October 2024 / Published: 10 October 2024
(This article belongs to the Section Mineral Deposits)
Browse Figures
Versions Notes

Abstract

:
Three nano-resolution petrological microtextures were discovered in the siliceous shale at the bottom of the Longmaxi Formation in the Zigong area, Sichuan Basin. Based on observations of the occurrences of the minerals, organic matter, and organic matter pores in the different microtextures and analysis of their relationships by means of nano-resolution petrological image datasets obtained using the Modular Automated Processing System (MAPS 3.18), the formation mechanism of the siliceous shale was studied. The results show that the strong modification of clay-rich sediments by a deep-water traction current was the basis for the formation of the siliceous shale. The clay-rich sediments were converted into flocculent sediments rich in oxygen and nutrients via agitation and transport by the deep-water traction current, providing space and a material basis for microbes to flourish. Under the continuous activity of the deep-water traction current, the clay-rich sediments were transformed into microbial mats, in which in situ terrigenous detrital quartz and feldspar, endogenous detrital calcite, authigenic dolomite, and dolomite ringed by ferrodolomite were scattered. During the burial stage, the microbial mats were lithified into the siliceous shale composed of three petrological microtextures. Microtexture I was mainly transformed by microbes. Microtexture II was formed via lithification of the residual clay-rich sediments. Microtexture III was composed of migratory organic matter filling hydrocarbon-generating pressurized fractures. Due to the universality of deep-water traction flow and the diversity of microbes in deep-water sediments, we firmly believe that more and more deep-water microbialites will be discovered worldwide through systematic characterization of nano-resolution petrology with the booming development of the shale gas industry.

Graphical Abstract

1. Introduction

China is the second largest oil- and gas-producing country after North America. The Longmaxi Formation in the Sichuan Basin is the main shale gas-producing layer, and the siliceous shale at its bottom is the horizontal well target for shale gas [1,2,3,4,5,6,7]. The lithofacies, including the siliceous shale, are the product of certain sedimentary environments, so researchers have strived to determine the distribution pattern of the siliceous shale by analyzing the relationship between the complex lithofacies and sedimentary environments [8,9,10,11,12]. Whole rock and trace element data, supplemented by total organic carbon (TOC) data and thin section observations, are common information and methods for analyzing shale lithofacies and sedimentary environments. Previous studies have revealed that the organic matter in the siliceous shale was mainly derived from the plankton at the sea surface; that is, it was the product of an anoxic deep-water environment [1,2,8,12,13,14]. However, based on thin section observations, some researchers have identified some benthos in black shale, which indicated that there was a certain amount of oxygen in the deep seabed [15,16]. Strong hydrodynamic sedimentary structures in black shale and non-black shale with a gradual transition indicated the existence of periodic or intermittent hydrodynamic conditions in a fairly deep-water environment [17,18,19,20,21,22,23,24,25,26,27], which provided oxygen-rich conditions for the development of benthos. By observing nano-resolution petrological images of the siliceous shale, several researchers have reported that some nano-scale siliceous particles may be the product of the metabolic activities of anaerobic bacteria or other microbes near the water–sediment interface [20,28,29]. These findings indicated the complexity and diversity of deep-water sedimentary environments, in which periodic or intermittent oxygenation events occurred under a background of seawater stratification and deep-water anoxia.
Nano-resolution petrological evidence, including the occurrence of minerals, organic matter, organic matter pores, and their relationships, is the primary information for deciphering the characteristics of a shale reservoir. Great progress has been made in the characterization of nano-scale organic matter pores [30,31,32,33,34,35,36], but studies of deep-water shale lithofacies and sedimentary environments through the systematic analysis of nano-resolution petrological characteristics are still scarce. Based on the systematic analysis of nano-resolution petrological characteristics of shale samples from the Wufeng–Longmaxi Formation in the Sichuan Basin, China, we discovered that the studied siliceous shales at the bottom of the Longmaxi Formation in the Zigong area were the lithification products of deep-water microbial mats. The goals of this study were to determine (1) the characterization of the siliceous shale; (2) the origins of the minerals and organic matter; and (3) the lithification process of the deep-water microbial mats. The results of this study improve our understanding of the environment and processes that produced the siliceous shale at the bottom of the Longmaxi Formation and can be extended to other similar shales, further promoting the exploration and development of shale gas.

2. Geologic Background

The Sichuan Basin in China is a multicycle sedimentary basin that is rich in oil and gas resources [37,38,39,40,41]. From the Linxiang Period to the Wufeng Period in the Late Ordovician, the Sichuan Basin changed from a shallow-water shelf to a deep-water shelf, and consequently, the sedimentary deposits changed from marl to laminated black shale [8,42]. Near the end of the Ordovician, under the influence of a catastrophic event that caused sudden climate cooling and ice sheet accumulation, the Sichuan Basin turned into a shallow-water shelf, and less than 1 m of shell limestone was deposited, which was the marker rock of the Ordovician–Silurian transition [43,44]. At the beginning of the Silurian, the melting of the ice sheet and the sharp rise in sea level led to the formation of a deep-water shelf sedimentary system in the Sichuan Basin and its peripheral area and about 20–80 m of black shale with a lithological sequence of siliceous shale, mixed shale, calcareous shale, and claystone from bottom to top were deposited in the Longmaxi Formation (Figure 1). The black shale, which is rich in graptolites and radiolarians and has an organic carbon mass fraction of greater than 3%, provides the material conditions for shale gas [1,2,12].
According to the evolution characteristics of the graptolites, the Wufeng–Longmaxi formations were divided into 13 graptolite zones [47,48]. Biostratigraphic correlation of outcrops and cores has revealed that a considerable number of underwater highlands occurred in the Sichuan Basin and its surrounding areas [49,50,51,52]. These underwater highlands controlled the distribution of the sedimentary facies and lithofacies and have influenced shale gas exploration and development [1,2,8,53]. In the Zigong area, there was a NE-SW underwater highland. Its northeast end was connected to the Central Sichuan paleouplift, and its southwest end was submerged in the deep-water shelf, and the siliceous shale developed in the bottom of the Longmaxi Formation between the underwater highland and the Central Sichuan paleouplift [54,55,56]. The results on the regional tectonic evolution indicated that at the beginning of the Silurian, the Guangxi movement expanded to include the Sichuan Basin and its surrounding areas, accelerating the change from a shallow-water shelf to a deep-water shelf and forming numerous underwater highlands [49,50,51,52].
The Longmaxi Formation in the Sichuan Basin has undergone two uplift events and one subsidence event (Figure 2). In the Early Cretaceous, its burial depth was greater than 6000 m, the sedimentary organic matter was transformed into pyrobitumen and gas, and shale gas formed [39,57]. Since the Late Cretaceous, the thickness eroded during the uplift process was generally 2000–4000 m. At present, the burial depth of the Longmaxi Formation is generally 2000–5500 m [39]. The annual gas production (m3) of shallow shale gas from the reservoir above 3500 m has reached more than 200 × 108 m3, and the deep shale gas below 3500 m is the key to increasing production in China [6]. Exploration and development have proven that the siliceous shale at the bottom of the Longmaxi Formation is a shale gas hot spot with high amounts of siliceous and brittle minerals, a high TOC content, and a high gas content [1,2,58,59,60,61,62].

3. Research Methods

Three samples of siliceous shale were collected from the bottom of the Longmaxi Formation in wells Z201 and Z203 (Figure 1). The samples were cut into 1 cm × 1 cm argon ion polished slices in order to analyze the nano-resolution petrological characteristics of the siliceous shale using a high-resolution scanning electron microscope and mineral scanner. The preparation of argon ion polished slices, nano-resolution petrological observations, and mineral quantitative analysis 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 was a Fischione Model 1060 (Fischione Instruments, Inc, Export, PA, USA), the scanning electron microscope model was a Fei Helios 650 (Thermo Fisher Scientific, Waltham, MA, USA), and the mineral scanner model was an Apreo 2S (Thermo Fisher Scientific, Waltham, MA, USA). The argon ion polished slices were cut perpendicular to the bedding plane. The horizontal state of the bedding plane or banded organic matter was taken as the basic condition when observing in order to roughly determine the compaction information about the siliceous shale.
MAPS (Modular Automated Processing System) was adopted to obtain high-resolution and large-view microscopic images for nano-resolution petrological characterization of the shale [33,45,72,73,74,75,76,77]. The argon ion polished slice was divided into a series of regular grids, and each grid was scanned to obtain an image. The images of all the grids were spliced into a two-dimensional large-view scanning image. First, a low-resolution image dataset for the entire slice was obtained to identify the representative areas. Second, a high-resolution image dataset for these areas was obtained. Finally, using an offline map editor (ATLASTM BROWSER-BASED VIEWER), different resolution images were obtained for deciphering the petrological characteristics of the studied siliceous shale. For example, a series of images with a pixel range of 10.3 μm to 500 nm was obtained from the image dataset with a width and height of 19.8 mm × 8.7 mm and a resolution of 500 nm, and a series of images with a pixel range of 45.5 nm to 4 nm was obtained from the image dataset with a width and height of 87.0 μm × 38.0 μm and a resolution of 4 nm (Figure 3).
Because the three siliceous shale samples had similar nano-resolution petrological characteristics, composition scanning was conducted on a single representative selected area to obtain the planar distribution characteristics of the minerals and organic matter (Figure 4). Although the mineral scanning image could be used to quantitatively analyze the minerals and organic matter, its resolution was only 1 μm; thus, it was not as clear as the MAPS images with higher resolutions, from which the nano-resolution petrological characteristics were observed.

4. Results

The studied siliceous shales are composed of three microtextures. Microtexture I mainly consists of a micro-quartz and nano-quartz skeleton, and the voids contain porous organic matter. Microtexture II is comprised of clay-rich patches. Microtexture III is composed of non-porous dendritic organic matter. Microtexture I constitutes the main part of the siliceous shale, and both Microtexture II and Microtexture III are sporadically encased in Microtexture I (Figure 4, Figure 5, Figure 6, Figure 7, Figure 8, Figure 9 and Figure 10).

4.1. Nano-Resolution Petrological Characteristics of Microtexture I

Figure 4 presents the mineral scanning result. It can be seen that the area percentages of the minerals and organic matter are as follows: quartz (74.9%), organic matter (16.0%), carbonate (5.0%), clay (2.2%), feldspar (1.5%), and pyrite (0.4%). Both the quartz and scattered minerals such as carbonate, clay, feldspar, and pyrite constitute the rock’s framework, and the voids contain organic matter, which generally belongs to Microtexture I. The carbonate minerals include calcite (2.5%), dolomite (1.1%), and ferrodolomite (1.4%). The calcite particles are irregular. The dolomite has two occurrences: rhombic dolomite and rhombic dolomite ringed by ferrodolomite. The clay minerals include illite (1.9%) and chlorite (0.3%), which generally belong to Microtexture II.
The particle size of the quartz has two distribution ranges: 300 nm–7 μm and 10–40 μm in Microtexture I. The former forms the main part of the rock’s framework, while the latter is sporadically distributed (Figure 5a). Due to the occurrence difference of the quartz with particle sizes of 300 nm to 7 μm, the observations and descriptions of the nano-quartz and the micro-quartz needed to be carried out separately. The nano-quartz occurs in the form of aggregates and individual crystals, and the aggregates have three occurrences: a part of the rock’s framework (Figure 5a,b), rims on the surfaces of both the quartz particle and the feldspar particle with sizes of 10 μm to 40 μm (Figure 5a,d–f,v,w), and a material filling the carbonate dissolution pores (Figure 5g,h). The individual crystals fill the carbonate dissolution pores (Figure 5j,l,m,o). The micro-quartz occurs in the form of aggregates and individual crystals, and the aggregates have one occurrence: a part of the rock’s framework (Figure 5a,c). The individual crystals fill the carbonate dissolution pores (Figure 5j,l). The sporadic clays occur in the nano-quartz aggregates (Figure 5b,d,f,w), while no clay occurs in the micro-quartz aggregates (Figure 5a,c). The quartz and feldspar with particle sizes of 10 μm to 40 μm occur in the form of individual crystals, and their surfaces are covered by rims that are typically composed of nano-quartz aggregates containing sporadic clays (Figure 5a,d–f,v,w). Pores occur in the center of the quartz with sizes of 300 nm to 7 μm, while no pores occur at the edges (Figure 5a–d,f,h,l,o,w,x). There are no pores in the quartz and the feldspar with sizes of 10–40 μm (Figure 5a,d–f,v,w).
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Figure 5. Images of Microtexture I. (a) Microtexture I is generally composed of nano-quartz aggregates, micro-quartz aggregates, and amorphous organic matter. The quartz aggregates form the main part of the rock’s framework, and the organic matter fills the voids in the rock’s framework. In these quartz aggregates, the size distribution of the nano-quartz particles ranges from 300 nm to 1 μm, and the size distribution of the micro-quartz particles is 1–7 μm. One micro-quartz particle with a size of 11.7 μm can be seen in Microtexture I. (b) In the nano-quartz aggregate containing sporadic clays (red arrows), a few pores occur in the central part of the nano-quartz particle, but there are no pores in its edge. (c) In the micro-quartz aggregate, pores occur in the central part of the micro-quartz particle, but there are no pores in its edge. (d) No pores appear in the quartz particle with a size of 11.7 μm. Flaky clays (red arrow) and nano-quartz aggregates usually adhere to their surface. (e,f) A rim on the surface of one non-porous micro-quartz particle with a size above 10 μm, which is composed of a porous nano-quartz aggregate containing sporadic clays (red arrows). (g–i) A porous nano-quartz aggregate and non-porous organic matter fill the calcite dissolution pores. (j–l) Clay (red arrows), a nano-quartz particle (yellow arrow), and a micro-quartz particle (purple arrows) exist in the calcite dissolution pores. The voids in the calcite particle were formed by the destruction of fluid inclusions during the process of argon ion polishing. (m–p) Non-porous organic matter, clay, and nano-quartz fill in the dolomite dissolution pores. (q,r) Honeycomb-like pores occur in the organic matter filling the calcite dissolution pores. The voids in the calcite particle were formed by the destruction of fluid inclusions during the process of argon ion polishing. (s,t) Honeycomb-like pores appear in the organic matter filling in the dolomite dissolution pore. (u) In the regular dolomite ringed by ferrodolomite, the range of which is delineated by the solid purple line, the latter intermittently encases the former due to dissolution. (v,w) A rim on the surface of one non-porous micro-quartz particle with a size of >10 μm is composed of a porous nano-quartz aggregate containing sporadic clays (red arrows). The grayscale and morphology of the feldspar and quartz in the MAPS images are too similar to distinguish, so the feldspar is distinguished according to the mineral scanning results (Figure 4). (x) Honeycomb-like pores occur in the organic matter filling the quartz aggregate.
Figure 5. Images of Microtexture I. (a) Microtexture I is generally composed of nano-quartz aggregates, micro-quartz aggregates, and amorphous organic matter. The quartz aggregates form the main part of the rock’s framework, and the organic matter fills the voids in the rock’s framework. In these quartz aggregates, the size distribution of the nano-quartz particles ranges from 300 nm to 1 μm, and the size distribution of the micro-quartz particles is 1–7 μm. One micro-quartz particle with a size of 11.7 μm can be seen in Microtexture I. (b) In the nano-quartz aggregate containing sporadic clays (red arrows), a few pores occur in the central part of the nano-quartz particle, but there are no pores in its edge. (c) In the micro-quartz aggregate, pores occur in the central part of the micro-quartz particle, but there are no pores in its edge. (d) No pores appear in the quartz particle with a size of 11.7 μm. Flaky clays (red arrow) and nano-quartz aggregates usually adhere to their surface. (e,f) A rim on the surface of one non-porous micro-quartz particle with a size above 10 μm, which is composed of a porous nano-quartz aggregate containing sporadic clays (red arrows). (g–i) A porous nano-quartz aggregate and non-porous organic matter fill the calcite dissolution pores. (j–l) Clay (red arrows), a nano-quartz particle (yellow arrow), and a micro-quartz particle (purple arrows) exist in the calcite dissolution pores. The voids in the calcite particle were formed by the destruction of fluid inclusions during the process of argon ion polishing. (m–p) Non-porous organic matter, clay, and nano-quartz fill in the dolomite dissolution pores. (q,r) Honeycomb-like pores occur in the organic matter filling the calcite dissolution pores. The voids in the calcite particle were formed by the destruction of fluid inclusions during the process of argon ion polishing. (s,t) Honeycomb-like pores appear in the organic matter filling in the dolomite dissolution pore. (u) In the regular dolomite ringed by ferrodolomite, the range of which is delineated by the solid purple line, the latter intermittently encases the former due to dissolution. (v,w) A rim on the surface of one non-porous micro-quartz particle with a size of >10 μm is composed of a porous nano-quartz aggregate containing sporadic clays (red arrows). The grayscale and morphology of the feldspar and quartz in the MAPS images are too similar to distinguish, so the feldspar is distinguished according to the mineral scanning results (Figure 4). (x) Honeycomb-like pores occur in the organic matter filling the quartz aggregate.
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The carbonate minerals consist of three types: irregular calcite (Figure 5g,j,q), rhombic dolomite (Figure 5m,s), and rhombic dolomite ringed by ferrodolomite (Figure 5u). Their particle sizes are generally between 5 μm and 40 μm. The dissolution of these carbonate minerals is obvious. Clay (Figure 5j,k,m,p), nano-quartz aggregate (Figure 5g,h), individual nano-quartz crystals (Figure 5j,l,m,o), individual micro-quartz crystals (Figure 5j,l), and organic matter (Figure 5g,i,m,n,q–t) fill the dissolution pores. The organic matter in the dissolution pores in Figure 5g,m lacks pores (Figure 5i,n), while the organic matter in Figure 5q,s contains honeycomb-like pores (Figure 5r,t). The voids inside the carbonate minerals were formed by the destruction of fluid inclusions during the process of argon ion polishing (Figure 5j,l,q).
The voids in the framework of the minerals are filled with two types of organic matter: amorphous organic matter and spherical and short rod-like organic matter aggregates. The former accounts for the majority. The shape of the amorphous organic matter varies with the shape of the filling space, and honeycomb-like pores are developed inside it (Figure 3c and Figure 5x). In the aggregate, no pores occur in the spherical and short rod-like organic matter particles with sizes of less than 500 nm, but porous organic matter exists among these particles (Figure 6).
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Figure 6. Images of organic matter aggregates in Microtexture I. (a) Spherical and short rod-like organic matter aggregates among the rock’s framework minerals. (b) Spherical and short rod-like organic matter particles with sizes of less than 500 nm. There are no pores in the spherical and short rod-like organic matter particles, but there are honeycomb-like pores in the organic matter among the particles.
Figure 6. Images of organic matter aggregates in Microtexture I. (a) Spherical and short rod-like organic matter aggregates among the rock’s framework minerals. (b) Spherical and short rod-like organic matter particles with sizes of less than 500 nm. There are no pores in the spherical and short rod-like organic matter particles, but there are honeycomb-like pores in the organic matter among the particles.
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4.2. Nano-Resolution Petrological Characteristics of Microtexture II

Figure 7 shows one type of the petrological characteristics of Microtexture II. The clay encases the dolomite and the calcite. The dolomite occurs in the form of euhedral crystals and lacks the dissolution phenomenon. The calcite is irregular, and it does not contain dissolution pores.
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Figure 7. Image of one type of Microtexture II. The regular dolomite without dissolution (red arrows) and the irregular calcite without internal dissolution (yellow arrow) are encased by clay. The voids in the carbonate particles were formed by the destruction of fluid inclusions during the process of argon ion polishing.
Figure 7. Image of one type of Microtexture II. The regular dolomite without dissolution (red arrows) and the irregular calcite without internal dissolution (yellow arrow) are encased by clay. The voids in the carbonate particles were formed by the destruction of fluid inclusions during the process of argon ion polishing.
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Microtexture II, a nearly horizontal patch, is composed of clay and nano-quartz aggregates encasing calcite, dolomite, micro-quartz, and a small amount of organic matter (Figure 8). A few pores occur in the nano-scale quartz (Figure 8b). The aggregates tend to be attached to the surface of the non-porous micro-quartz (Figure 8e). The clay fills the carbonate dissolution pores (Figure 8c,g), but rhombic dolomite with a lack of dissolution phenomenon is still present (Figure 8d). A small amount of non-porous organic matter occurs with the calcite, clay, and nano-quartz aggregates (Figure 8f).
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Figure 8. Images in one type of Microtexture II. (a) Panorama of Microtexture II, the range of which is delineated by the yellow solid lines. (b) The clay and porous nano-quartz aggregate. (c) Clays (red arrows) fill the dissolution pores of the irregular calcite. (d) Rhombic dolomite encased by the clay and porous nano-quartz aggregate. (e) A non-porous micro-quartz particle encased by the clay and porous nano-quartz aggregate. (f) Calcite, clay (red arrow), porous nano-quartz aggregate (yellow arrow), and non-porous organic matter occur together, and the clay exhibits flow phenomena. (g) Clays (red arrow) fill the dissolution pores in the rhombic dolomite.
Figure 8. Images in one type of Microtexture II. (a) Panorama of Microtexture II, the range of which is delineated by the yellow solid lines. (b) The clay and porous nano-quartz aggregate. (c) Clays (red arrows) fill the dissolution pores of the irregular calcite. (d) Rhombic dolomite encased by the clay and porous nano-quartz aggregate. (e) A non-porous micro-quartz particle encased by the clay and porous nano-quartz aggregate. (f) Calcite, clay (red arrow), porous nano-quartz aggregate (yellow arrow), and non-porous organic matter occur together, and the clay exhibits flow phenomena. (g) Clays (red arrow) fill the dissolution pores in the rhombic dolomite.
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In Figure 9a,b, the discontinuous patch is mainly composed of clay and nano-quartz aggregates. The nano-quartz contains a few pores (Figure 9b). In the void in the clay and nano-quartz aggregates, there is a nano-quartz aggregate, among which amorphous organic matter containing honeycomb-like pores occurs (Figure 9d). The non-porous micro-quartz is usually coated by clay and nano-quartz aggregates (Figure 9c). At its edge, the nano-quartz aggregate coexists with non-porous organic matter and clay (Figure 9e).
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Figure 9. Images in one type of Microtexture II. (a) Panorama of a discontinuous clay and nano-quartz aggregate. (b) The clay and porous nano-quartz aggregate. (c) A non-porous micro-quartz particle with a size of 10.4 μm is encased by the clay and porous nano-quartz aggregate. (d) The porous amorphous organic matter is distributed in the nano-quartz aggregate. (e) Nano-quartz particles and clays coexist with non-porous organic matter.
Figure 9. Images in one type of Microtexture II. (a) Panorama of a discontinuous clay and nano-quartz aggregate. (b) The clay and porous nano-quartz aggregate. (c) A non-porous micro-quartz particle with a size of 10.4 μm is encased by the clay and porous nano-quartz aggregate. (d) The porous amorphous organic matter is distributed in the nano-quartz aggregate. (e) Nano-quartz particles and clays coexist with non-porous organic matter.
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4.3. Nano-Resolution Petrological Characteristics of Microtexture III

Figure 10b presents a sketch of the image of Microtexture III in Figure 10a. The width of the organic matter reaches more than 10 μm, but it is less than 1 μm in the narrow part. Flaky clays exhibiting flow phenomena are scattered in the interior of the dendritic organic matter (Figure 10c,d). The organic matter passes through the spherical and short rod-like organic matter aggregate (Figure 10e), where porous organic matter and the spherical and short rod-like organic matter disappear completely (Figure 10f). When the dendrites intersect the polished surface vertically or at a large angle, they create the illusion that it is the non-porous organic matter filling in Microtexture I (Figure 10g). The open cracks in Figure 10a,c are artificial and were produced during the sample production process.
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Figure 10. Images of one patch of dendritic organic matter. (a,b) Panorama of one patch of dendritic organic matter and a corresponding sketch. The boxes in (c) to (g) in the two images correspond to (c–g). (c) and (d) The nearly vertical extension of flaky clays (red arrows) is consistent with that of the non-porous dendritic organic matter, indicating that they have experienced flow before. (e) Honeycomb-like pores occur in the organic matter among the non-porous spherical and short rod-like organic matter particles. (f) The dendritic organic matter passes through the spherical and short rod-like organic matter aggregates, causing the honeycomb-like pores and the spherical and short rod-like organic matter to be submerged. (g) The part of the dendritic organic matter encased by quartz aggregates is usually considered to be isolated blocky organic matter.
Figure 10. Images of one patch of dendritic organic matter. (a,b) Panorama of one patch of dendritic organic matter and a corresponding sketch. The boxes in (c) to (g) in the two images correspond to (c–g). (c) and (d) The nearly vertical extension of flaky clays (red arrows) is consistent with that of the non-porous dendritic organic matter, indicating that they have experienced flow before. (e) Honeycomb-like pores occur in the organic matter among the non-porous spherical and short rod-like organic matter particles. (f) The dendritic organic matter passes through the spherical and short rod-like organic matter aggregates, causing the honeycomb-like pores and the spherical and short rod-like organic matter to be submerged. (g) The part of the dendritic organic matter encased by quartz aggregates is usually considered to be isolated blocky organic matter.
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Figure 11b presents a sketch of the image of Microtexture III in Figure 11a. The clay inside the organic matter exhibits obvious flow phenomena (Figure 11c,d). There is an abrupt contact between the non-porous dendritic organic matter and the porous organic matter in Microtexture I (Figure 11e,f), indicating that the former is the exogenous invader.

5. Discussion

5.1. Origin of the Types of Organic Matter

The sedimentary organic matter underwent a condensation reaction shortly after burial to form kerogen. The kerogen was classified into four types [63,64,65]. The type I and II kerogens are mainly composed of microbes and amorphous organic matter with a high hydrocarbon generation ability, the type III kerogen is mainly composed of vitrinite with gas generation ability, and the type IV kerogen is mainly composed of inertinite with little hydrocarbon generation ability. The amorphous organic matter may be pre-oil bitumen. As the burial depth and temperature increased, both the type I and II kerogens were successively degraded into pre-oil bitumen, solid bitumen associated with the oil, pyrobitumen and gas, and graphite [68,78,79,80,81]. The type IV kerogen retained its original morphology [63,64,65,66,67]. The pre-oil bitumen, oil, solid bitumen, and pyrobitumen in the location of the kerogen from which they formed were referred to as in situ organic matter, while these types of organic matter in other locations were referred to as migratory organic matter [32,61]. The pyrobitumen derived from the type I and II kerogens can be divided into two types: porous pyrobitumen converted from solid bitumen and non-porous pyrobitumen converted from oil [72].
For the shale in the Longmaxi Formation in the Sichuan Basin, the vitrinite reflectance (Ro) of the organic matter is greater than 2.5%, which indicates that the organic matter has evolved into pyrobitumen [57,61], and the main types of kerogens are type I and II kerogens and a small amount of type III and IV kerogens [63,64,66,67,82,83,84]. There are three types of pyrobitumen in the studied siliceous shales: porous amorphous pyrobitumen, porous pyrobitumen among the spherical and short rod-like organic matter aggregate, and non-porous dendritic pyrobitumen. Based on the relationship among the organic matter pores, the pyrobitumen, and the hydrocarbon generation ability of the kerogen degradation [63,64,65,66,67,68,72,82,85], the origins of the three types of pyrobitumen were determined. The porous amorphous pyrobitumen was derived from the solid bitumen converted from type I and II kerogens. The spherical and short rod-like organic matter aggregate consists of a porous part and a non-porous part. The porous part was derived from the solid bitumen converted from the type I and II kerogens, which may have come from the epidermis, while the non-porous part was converted from the type IV kerogen that came from the main part of the spherical and short rod-like microbes. The non-porous dendritic pyrobitumen was a derivative of the oil filling the dendritic fractures formed under the action of abnormally high oil pressure.
In the non-porous dendritic pyrobitumen, the extension direction of the flaky clay is parallel to that of the pyrobitumen, especially when the vertically distributed flaky clay matches the vertically extended pyrobitumen. These phenomena indicate that the oil flowed with the clay in the mature stage of the organic matter, and the flaky clay only had a minimum migration resistance in the direction parallel to the flow of the oil. In the high to over-mature stages of the organic matter, the oil was converted into the pyrobitumen, in which the clay was encased.
When the pre-oil bitumen formed, the shale was characterized by a shallow burial depth, low degree of consolidation, and easy peeling off, making it easy for the high-viscosity migratory pre-oil bitumen to carry the flaky clay and form a flow structure. With the pre-oil bitumen converted into solid bitumen and oil, some flaky clays remained in the solid bitumen, while others were carried away by the oil. When the solid bitumen and oil were turned into pyrobitumen, some flaky clays occurred in the porous amorphous pyrobitumen derived from the solid bitumen. However, the fact that the porous amorphous pyrobitumen with a lack of clays was derived from the in situ solid bitumen in the studied siliceous shales indicated that there were no clays in the sedimentary organic matter.
At the beginning of the Silurian, plants had not yet appeared on land, and the sedimentary organic matter was from marine microbes, including microbial residues, fecal pellets, flocculent organic matter, and soluble organic matter [86,87,88,89,90]. The microbial residues and fecal pellets could settle independently, while the flocculent and soluble organic matter often combined with the clays and settled as organic matter–clay aggregates in still, stratified, and anoxic water bodies [86,87,91,92,93,94,95]. The total organic content of the black shale at the bottom of the Longmaxi Formation in the Sichuan Basin is generally 3%–7%, and the sedimentary organic matter is mainly composed of biological residues from plankton algae, acritarchs, bacteria, graptolites, and chitinozoans [66,69,70,71,96], organic matter–clay aggregates [72], organic matter–silicon aggregates [45,77], and spherical and short rod-like microbial aggregates [82], of which, the organic matter–clay aggregates were the main source of the type I and II kerogens [72,86,91,92,93,94,95]. However, in the studied siliceous shales, the porous amorphous pyrobitumen does not contain clay formed from the sedimentary organic matter, indicating that this sedimentary organic matter is a benthic microbial aggregate. The spherical and short rod-like microbes may have been a type of chemoautotrophic sulfate-reducing bacteria, which could secrete or adsorb siliceous particles to form micro-quartz and nano-quartz aggregates [20,28,29,82,97], and the amorphous pyrobitumen may have been benthic microbes that lost their own morphologies during the thermal evolution of the sedimentary organic matter. That is, all the sedimentary organic matter in the studied siliceous shales was derived from benthons. The benthons were transformed into three types of kerogens: type I, II, and IV kerogens. The type I and II kerogens became porous amorphous pyrobitumen, and the type IV kerogen retained its microbial morphology.

5.2. Origin of Carbonate Minerals

The carbonate minerals occur in the form of irregular calcite, rhombic dolomite, and rhombic dolomite ringed by ferrodolomite. The calcite in the black shales has three possible sources: terrigenous clastics, precipitation from the surface seawater [45,98,99], and authigenic minerals formed during burial [32,100,101]. Because calcite has a poorer stability than quartz, calcite was rapidly broken down and dissolved during transportation [102], and thus, the content of terrigenous quartz is much higher than that of terrigenous calcite in black shale. In the studied siliceous shale, the calcite and terrigenous quartz (non-porous micro-quartz, see Section 5.3 for details) have similar particle sizes, and the calcite content is higher than that of the terrigenous quartz, indicating that the calcite is not a terrigenous clastic material. The sedimentary organic matter among the minerals of the rock framework is generally smaller than the calcite, and the sediments, including clay, porous micro-quartz, and nano-quartz, fill the calcite dissolution pores (see Section 5.3 for details), which implies that the calcite is not an authigenic mineral formed during burial. Therefore, precipitation from the surface seawater was the only source of calcite.
Calcite precipitated from the surface seawater is a by-product of the photosynthesis conducted by microbes that secret calcium carbonate [98,103]. The 10 m surface layer of seawater near the equator is known as the carbonate factory, in which the intensity of the biological metabolism of the calcium carbonate-secreting microbes is so high that calcite is deposited as flocculation-like suspension because of the provision of sufficient sunlight and photosynthesis [103]. The most significant change in calcite settling from the seawater is dissolution at a key depth, such as the carbonate compensation depth (CCD), carbonate lysocline depth (CLD), and carbonate saturation depth (CSD) [104,105,106,107]. The CCD is similar to the snow line on land, with calcite-rich shallow sediment and calcite-poor deep sediment. In practical operations, the boundary is often the depth where the calcite content of the sediment is 10%. The CCD gradually deepened during the Phanerozoic, from a few hundred meters in the Early Paleozoic to below 4000 m at present [16].
In the Silurian, the Sichuan Basin was located near the equator [108]. Calcite was continuously deposited in the surface water and subsequently settled. The average calcite content of the studied siliceous shale is less than 10% (Figure 4), which indicates that the water depth was generally below the CCD, and only a small amount of calcite settled to the seabed.
The rhombic dolomite and the rhombic dolomite ringed by ferrodolomite form a harbor shape when strongly dissolved, while the crystal edge is straight and smooth when undissolved, exhibiting the characteristics of strong chemical weathering and weak physical weathering, indicating that they are products of the in situ dissolution of authigenic minerals. Previous research [99] has shown that rhombic dolomite precipitates near the water–sediment interface, and subsequently, ferrodolomite is deposited on the surface of the dolomite, which may be related to exopolymeric substances produced by sulfate-reducing bacteria [109].
As shown in Figure 7, the dolomite has a complete rhomboid morphology, from which it can be inferred that it is an undissolved diagenetic mineral. Accordingly, there are no dissolution pores within the calcite. These phenomena imply that this patch composed of clay encasing carbonate has not undergone physical or chemical weathering. In Figure 8, the dolomite particles exhibit a complete rhombic morphology and incompleteness due to dissolution, and the dissolution pores in the carbonate are filled with clay, indicating that this patch composed of clay and nano-quartz aggregate encasing minerals such as dolomite, calcite, and non-porous micro-quartz has undergone in situ chemical weathering. In Figure 9, there is a nano-quartz aggregate in this patch, which is mainly composed of clay and nano-quartz aggregate, and porous organic matter occurs in the voids among the nano-quartz aggregate, indicating that this patch has undergone a certain degree of modification, resulting in the disappearance of clay and increases in the amounts of nano-quartz and organic matter in local areas. The porous organic matter is one in situ derivative of the sedimentary organic matter, implying that the modification occurred during or before the period of benthic activity. Based on the occurrence of carbonate and sporadic clays inside the nano-quartz aggregate in Microtexture I, we conclude that Microtexture I is the product of modification of Microtexture II during the period of benthic activity, and this modification involved a decrease in the amounts of clays and an increase in the amounts of nano-quartz particles.

5.3. Origin of Quartz

The quartz in the black shale can be divided into clastic particles and cement [20,28,29]. The clastic quartz includes terrigenous and endogenous particles. The terrigenous quartz is derived from the weathering products of sedimentary rocks, magmatic rocks, and metamorphic rocks. The endogenous quartz is mostly composed of the remnants of siliceous microbes such as radiolarians, sponges, algae, and bacteria [20,28,29]. The quartz cement is derived from biogenic silica dissolution, silica secreted by bacteria, and clay mineral transformation [20,29,45,110,111]. The quartz in the siliceous shale at the bottom of the Longmaxi Formation in the Sichuan Basin is mainly biogenic silicon, such as radiolarian and sponge spicule fossils [82,112]. Biogenic silicon, mineralogically known as opal A, consists of colloidal SiO2, silica, and a small amount of water [70]. During diagenesis, biogenic silicon is gradually transformed into porous quartz [113,114,115,116], such as the conversion of radiolarian silicon into porous quartz [45,72]. Therefore, in the studied siliceous shale, the porous central part of the nano-quartz and the micro-quartz are the products of biogenic silicon. The non-porous marginal part is the cement material that originated from the dissolution of biogenic silica. The non-porous micro-quartz is a terrigenous clastic material.
Clay inhibits microbial activity and increases the concentration of silica precipitate, even when the clay mineral content is extremely low [117,118,119]. The presence of more or less clay minerals in the nano-quartz aggregates and the lack of clay within the micro-quartz aggregates in Microtexture I indicate that the sedimentary environment of the nano-quartz aggregates was worse than that of the micro-quartz aggregates. That is, the former environment was not conducive to the growth of siliceous microbes and the precipitation of siliceous cement, while the latter was conducive to microbial growth and silica precipitation in cleaner water.
In Microtexture I, the nano-quartz aggregates containing sporadic clays were as the rims encasing the non-porous micro-quartz particles. However, In Microtexture II, the clays and nano-quartz aggregates occur on the surface of the non-porous micro-quartz particles. The difference in the occurrence of the non-porous micro-quartz in Microtexture I and Microtexture II indicates that the former may be a product of the redeposition of the latter, which led to a decrease in the amounts of clays and an increase in the amounts of nano-quartz particles.

5.4. Formation Process of Siliceous Shale

Based on analysis of the occurrence and origin of the minerals and types of organic matter, as well as the sedimentary background of the underwater highland, it was concluded that the studied siliceous shales were formed via the lithification of microbial mats that formed through the transformation of clay-rich sediments near the water–sediment interface by a deep-water traction current.
At the beginning of the Silurian, the Zigong area was a shallow shelf adjacent to the Central Sichuan paleouplift, and the sediments contained abundant terrigenous clay and endogenous calcite, as well as small amounts of terrigenous quartz and feldspar. Near the water–sediment interface, pyrite framboids, rhombic dolomite, and rhombic dolomite ringed by ferrodolomite precipitated [99]. Subsequently, as the Guangxi movement expanded to include the Zigong area, the shallow shelf evolved into a deep shelf and an underwater highland [49,50,51,54,55,56], which made the sea floor uneven and triggered a deep-water traction current [19,120,121,122,123]. The traction current had two important effects on the water–sediment interface. First, it provided oxygen and nutrients for microbes. Second, it agitated and transformed the clay-rich sediments, providing a place for the microbes.
The loose and porous clay-rich sediments provided a straight channel for fluids, which was conducive to deep-water traction current activity. Most of the clays were carried away by the traction current because of their weak resistance to the fluid flow, and the remaining flocculent material became Microtexture II. The microbes aggregated together to resist fluid erosion. Siliceous microbes gathered together to form the precursor of the porous micro-quartz and nano-quartz aggregates. Carbonhydrate-rich microbes gathered together to form the precursor of the porous amorphous pyrobitumen and the spherulitic and rod-like organic matter aggregates. All of the microbial aggregates accumulated together to form a microbial mat. The energy of the traction current could only agitate and transport the clays while the coarse and dense particles, such as quartz, feldspar, carbonate, and pyrite framboids, remained in place. Nano-scale siliceous microbes attached to the terrigenous clastic surface, producing a rim of micro-scale quartz and feldspar. Below the CCD, carbonate minerals dissolved, and clay minerals and microbes filled the resulting dissolution pores.
As the burial depth and temperature increased, the sedimentary organic matter was converted into type I, II, and IV kerogens. Both the type I and II kerogens were transformed into pre-oil bitumen without a microbial morphology [63,64,69], and then it was transformed into solid bitumen and oil. Finally, it was transformed into porous pyrobitumen [72]. The type IV kerogen has a poor hydrocarbon generation potential and retains its spherical and rod-shaped microbial morphology.
During the transformation of the pre-oil bitumen into oil, the volume expansion of the fluid produced an abnormally high fluid pressure. Under its action, dendritic fractures were formed, which became channels for the primary migration of oil, namely, the precursor of Microtexture III. Organic acids are associated with the process of the transformation of the pre-oil bitumen into oil [124,125]. These acids dissolved the carbonate minerals to form intragranular pores, which became part of the primary migration pathway of the oil. The oil in these pores was converted into non-porous pyrobitumen [72]. The resistance of Microtexture II to compaction was so weak that the honeycomb-like pores in the organic matter were crushed and disappeared. In Microtexture I, siliceous and carbonate minerals constitute the rock framework, which has a strong resistance to compaction and protects the honeycomb-like pores.

6. Overview

Microbes, according to their simplest definition, include organisms at all microscopic levels, generally including bacteria, fungi, microalgae, and protozoa [126]. Because of their enormous biomass and metabolic diversity, microbes are geobiological agents in the Earth’s biosphere [127]. In particular, microbial mats formed by microbial communities were the earliest ecosystems on Earth [128] and have existed on Earth for more than 3 billion years [129,130]. Microbial mats and microbialites have been described and studied since as early as the era of Paracelsus, and after years of neglect, scientists have finally rediscovered the importance of microbial mats and microbialites [131].
Until recently, it has been thought that stromatolites are the most representative microbialites distributed in shallow-water environments, and the corresponding petrological evidence mainly comes from field outcrops, drill cores, and microscope observations [132,133,134,135,136]. In this study, by analyzing the nano-resolution petrological characteristics, for the first time, we determined that the studied siliceous shales are deep-water microbialites. The growth phenomena of the deep-water microbial mats are as follows. (1) The traction current transforms clay-rich sediments to form a fluid migration channel in which microbes flourish. (2) In the early stage of the traction current activity, nano-scale siliceous microbes cling to the clays and aggregate to resist being transported, and as a result, the clay-rich sediments are transformed into clay and nano-scale siliceous microbe aggregates. (3) In the peak stage of the traction current activity, due to a lack of inhibition by the clay, the microbes fully grow, and their individual sizes increase, forming micro-scale siliceous microbial aggregates. (4) In the declining stage of the traction current, the carbohydrate-rich microbes thrive and occupy the voids in the framework, which is mainly composed of nano-quartz and micro-quartz aggregates. (5) The non-porous micro-quartz and micro-feldspar coated with nano-quartz aggregates are terrigenous clastic remnants of the clay-rich sediments. (6) The calcite is composed of endogenous clastic remnants of the clay-rich sediments. (7) The rhombic dolomite and dolomite ringed by ferrodolomite characterized by strong dissolution in Microtexture I are authigenic mineral residues from the clay-rich sediments. (8) Under the action of the traction current, the carbonate minerals are continuously dissolved, and the resulting dissolution pores are filled with clays and microbes.
The diagenetic phenomena of the microbial mats are as follows. (1) The microbial opal is transformed into porous quartz and is the source of the quartz overgrowth, which may occur during the deep-water traction current activity [45,72,110]. (2) With increasing burial depth and temperature, the sedimentary organic matter is transformed into type I, II, and IV kerogens. Both the type I and II kerogens are transformed into porous amorphous pyrobitumen and non-porous dendritic pyrobitumen, while the type IV kerogen retains its microbial morphology. (3) The solid bitumen is transformed into porous pyrobitumen, and the oil is transformed into non-porous pyrobitumen. (4) The oil in the dendritic cracks formed via oil-generating pressurization is converted into non-porous pyrobitumen and becomes Microtexture III. (5) The oil in the carbonate dissolution pores formed by organic acids is converted into non-porous pyrobitumen. (6) The pyrobitumen derived from the pre-oil bitumen in the edge of Microtexture II lacks pores because of the pressure imposed by the overlying formation.
Due to the universality of deep-water traction current [19,120,121,122,123] and the diversity of microbes [137,138,139] in deep-water sediments, we firmly believe that there are a large number of deep-water microbial mats, especially in areas with underwater highlands in the Sichuan Basin [49,50,51] and similar submarine topographies [140,141,142,143,144]. Siliceous and carbonate minerals, as the hard rock framework of the microbialites, have a strong resistance to compaction and protect pores. The content of organic matter with honeycomb-like pores is as high as 16%, providing rich reservoir space for shale gas. In view of this, the microbialites are high-quality reservoirs for shale oil and gas. With increased shale oil and gas exploration and development, as well as the continuous promotion and application of nano-resolution petrological characterization techniques, more and more deep-water microbialites will be discovered worldwide.

7. Conclusions

MAPS and quantitative techniques for scanning minerals and organic matter were used to characterize the nano-resolution petrological characteristics of the siliceous shale at the bottom of the Longmaxi Formation in the Zigong area, Sichuan Basin, in order to study the formation mechanism of the siliceous shale and the evolution of the sedimentary organic matter. The main conclusions of this study are as follows.
(1)
The quartz content is as high as 75%, the organic matter content is greater than 15%, and the total content of the carbonate minerals, clay minerals, feldspar, and pyrite is less than 10%. The quartz mainly exists in the form of micro-quartz and nano-quartz aggregates. The organic matter can be divided into porous amorphous organic matter, porous organic matter among non-porous spherical and rod-like organic matter aggregates, and non-porous dendritic organic matter.
(2)
The transformation of clay-rich sediments by a deep-water traction current provided the basis for the formation of the siliceous shale. The traction current agitated and transported the clays, forming fluid transport channels. The oxygen and nutrients supplied provided the materials for microbes to flourish. Gradually, the clay-rich sediments were transformed into microbial mats.
(3)
The microbialites consist of three microtextures. Microtexture II and Microtexture III are sporadically encased in Microtexture I. The main body of Microtexture I is transformed by microbes. Microtexture II is composed of residual clay-rich sediments. Microtexture III is composed of the pyrobitumen derived from the oil in hydrocarbon-generating pressurized dendritic fractures.
(4)
During the transformation of the microbial mats into microbialites, the biogenic silica was transformed into porous micro-quartz and nano-quartz, and the sedimentary organic matter was transformed into porous amorphous pyrobitumen and non-porous dendritic pyrobitumen, but the spherical and rod-like organic matter retained their biological morphologies.
(5)
The results of this study improve our understanding of the environment and processes that produced the siliceous shale at the bottom of the Longmaxi Formation and can be extended to other areas, such as other underwater highlands in the Sichuan Basin and similar reservoirs worldwide.

Author Contributions

Conceptualization, X.Z.; Data curation, X.Z. and J.Y.; Funding acquisition, W.G., P.L. and X.L.; Investigation, W.G. and X.L.; Methodology, X.Z. and P.L.; Project administration, W.G. and P.L.; Resources, W.G. and P.L.; Writing—original draft preparation, X.Z. and J.Y.; Writing—review and editing, X.Z., J.Y. and C.Z. 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

All used data are included in this publication as a digital supplement.

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

Author Chenglin Zhang was employed by the company Shale Gas Institute of PetroChina Southwest Oil & Gas Field 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. Sedimentary environment and stratigraphic column of the Longmaxi Formation in the Sichuan Basin and its peripheral area. (a) Map of the sedimentary environment (modified from [45]). A bay that opened northward and was enclosed by the Central Sichuan paleouplift, Central Guizhou paleouplift, and Jiangnan–Xuefeng paleouplift consists of a shallow-water shelf, a deep-water shelf, and a considerable number of underwater highlands, and about 20–80 m of black shale was deposited on the deep-water shelf. (b) Stratigraphic column for Well Z201 (modified from [46]). The lithologic sequence consists of siliceous shale, mixed shale, calcareous shale, and claystone. The thickness of the siliceous shale is 3.4 m at the bottom of the Longmaxi Formation.
Figure 1. Sedimentary environment and stratigraphic column of the Longmaxi Formation in the Sichuan Basin and its peripheral area. (a) Map of the sedimentary environment (modified from [45]). A bay that opened northward and was enclosed by the Central Sichuan paleouplift, Central Guizhou paleouplift, and Jiangnan–Xuefeng paleouplift consists of a shallow-water shelf, a deep-water shelf, and a considerable number of underwater highlands, and about 20–80 m of black shale was deposited on the deep-water shelf. (b) Stratigraphic column for Well Z201 (modified from [46]). The lithologic sequence consists of siliceous shale, mixed shale, calcareous shale, and claystone. The thickness of the siliceous shale is 3.4 m at the bottom of the Longmaxi Formation.
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Figure 2. Burial history and thermal evolution history of organic matter in the Longmaxi Formation of Sichuan Basin modified from [45]. When the Longmaxi Formation had a burial depth of less than 1500 m and a geothermal temperature of less than 60 ℃, sedimentary organic matter was converted into type Ⅰ–Ⅳ kerogens [63,64,65]. When the Longmaxi Formation had burial depths of 1500–2500 m and geothermal temperatures of 60–90 ℃, the type Ⅰ and Ⅱ kerogens were turned into the pre-oil bitumen [66,67,68,69,70,71]. With burial depths of 2500–3500 m and geothermal temperatures of 90–120 ℃, the pre-oil bitumen was transformed into the solid bitumen and the oil. With burial depths of 3500–6500 m and geothermal temperatures of 120–210 ℃, the solid bitumen and the oil were changed into the pyrobitumen and gas [68,72]. Since the Late Cretaceous, the surface thickness eroded during the uplift process was about 3000 m [39].
Figure 2. Burial history and thermal evolution history of organic matter in the Longmaxi Formation of Sichuan Basin modified from [45]. When the Longmaxi Formation had a burial depth of less than 1500 m and a geothermal temperature of less than 60 ℃, sedimentary organic matter was converted into type Ⅰ–Ⅳ kerogens [63,64,65]. When the Longmaxi Formation had burial depths of 1500–2500 m and geothermal temperatures of 60–90 ℃, the type Ⅰ and Ⅱ kerogens were turned into the pre-oil bitumen [66,67,68,69,70,71]. With burial depths of 2500–3500 m and geothermal temperatures of 90–120 ℃, the pre-oil bitumen was transformed into the solid bitumen and the oil. With burial depths of 3500–6500 m and geothermal temperatures of 120–210 ℃, the solid bitumen and the oil were changed into the pyrobitumen and gas [68,72]. Since the Late Cretaceous, the surface thickness eroded during the uplift process was about 3000 m [39].
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Figure 3. MAPS images of the studied siliceous shale. (a) Image resolution of 1.4 μm. The overall outline of the petrological characteristics can be observed. (b) Image resolution of 43.7 nm. Micro-scale particles can be clearly observed. (c) Image resolution of 21.1 nm. Nano-scale particles and pores can be observed.
Figure 3. MAPS images of the studied siliceous shale. (a) Image resolution of 1.4 μm. The overall outline of the petrological characteristics can be observed. (b) Image resolution of 43.7 nm. Micro-scale particles can be clearly observed. (c) Image resolution of 21.1 nm. Nano-scale particles and pores can be observed.
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Figure 4. MAPS image and the corresponding mineral scanning result for the studied siliceous shale. (a) MAPS image with a width of 410.1 μm, a height of 307.6 μm, and a resolution of 4 nm. The image shows the basic petrological characteristics of the studied siliceous shale. (b) Mineral scanning image with a resolution of 1 μm and the result within the range of (a).
Figure 4. MAPS image and the corresponding mineral scanning result for the studied siliceous shale. (a) MAPS image with a width of 410.1 μm, a height of 307.6 μm, and a resolution of 4 nm. The image shows the basic petrological characteristics of the studied siliceous shale. (b) Mineral scanning image with a resolution of 1 μm and the result within the range of (a).
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Figure 11. Images of one patch of dendritic organic matter. (a) and (b) Panorama of one patch of dendritic organic matter and its sketch. The boxes in (c) to (f) in the two images correspond to (c)–(f). (c,d) The extension directions of the flaky clays (red arrows) are consistent with that of the non-porous dendritic organic matter. (e,f) Abrupt contact between the non-porous dendritic organic matter and the porous amorphous organic matter in Microtexture I; sporadic flaky clays (red arrows) occur on the interface.
Figure 11. Images of one patch of dendritic organic matter. (a) and (b) Panorama of one patch of dendritic organic matter and its sketch. The boxes in (c) to (f) in the two images correspond to (c)–(f). (c,d) The extension directions of the flaky clays (red arrows) are consistent with that of the non-porous dendritic organic matter. (e,f) Abrupt contact between the non-porous dendritic organic matter and the porous amorphous organic matter in Microtexture I; sporadic flaky clays (red arrows) occur on the interface.
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Zhou, X.; Guo, W.; Li, X.; Liang, P.; Yu, J.; Zhang, C. Deciphering Nano-Resolution Petrological Characteristics of the Siliceous Shale at the Bottom of the Longmaxi Formation in the Zigong Area, Sichuan Basin, China: Deep-Water Microbialites. Minerals 2024, 14, 1020. https://doi.org/10.3390/min14101020

AMA Style

Zhou X, Guo W, Li X, Liang P, Yu J, Zhang C. Deciphering Nano-Resolution Petrological Characteristics of the Siliceous Shale at the Bottom of the Longmaxi Formation in the Zigong Area, Sichuan Basin, China: Deep-Water Microbialites. Minerals. 2024; 14(10):1020. https://doi.org/10.3390/min14101020

Chicago/Turabian Style

Zhou, Xiaofeng, Wei Guo, Xizhe Li, Pingping Liang, Junmin Yu, and Chenglin Zhang. 2024. "Deciphering Nano-Resolution Petrological Characteristics of the Siliceous Shale at the Bottom of the Longmaxi Formation in the Zigong Area, Sichuan Basin, China: Deep-Water Microbialites" Minerals 14, no. 10: 1020. https://doi.org/10.3390/min14101020

APA Style

Zhou, X., Guo, W., Li, X., Liang, P., Yu, J., & Zhang, C. (2024). Deciphering Nano-Resolution Petrological Characteristics of the Siliceous Shale at the Bottom of the Longmaxi Formation in the Zigong Area, Sichuan Basin, China: Deep-Water Microbialites. Minerals, 14(10), 1020. https://doi.org/10.3390/min14101020

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