Carbonaceous rock is a special soft rock containing TOC organic carbon 6%∼40%. In order to reveal the influence of engineering characteristics of carbonaceous rock on the engineering construction, firstly the stratigraphic distribution of carbonaceous Rocks in Guangxi is investigated, and the genetic mechanism and tectonic environment of carbonaceous rocks are discussed. Secondly, the influence of pore microstructure on the disintegration characteristics of carbonaceous rocks is analyzed. The role of geochemical characteristics of carbonaceous rocks (mineral composition, TOC total organic matter content, and type) on engineering properties is revealed. Finally, combined with the distribution, structure, microstructure, and microscopic characteristics of the previous studies, the disintegration mechanism of carbonaceous rocks in water swelling and heat dehydration is discussed. The results are as follows. (1) Carbonaceous rocks in Guangxi are mainly distributed in Devonian, Carboniferous, and Cambrian systems. It is mainly formed in anoxic and reductive deep water basins, slopes, and relatively confined coastal lagoons and swamps. The carbonaceous rocks in the Devonian Luofu formation are most typical. (2) The pores of carbonaceous rocks are divided into mineral pores, organic matter pores, and microfracture, which are mainly mineral pores. The more developed pores in mineral pores are intragranular dissolved pores. Secondly, mineral intergranular pores and a small amount of intergranular dissolution pores and less inner pores. Organic matter porosity increases with the increase of shale organic carbon content and maturity, but the shale porosity and adsorption capacity decrease when shale maturity reaches more than 2.4%. (3) Clay minerals in carbonaceous shale mainly consist of illite and illite/montmorillonite layer, which have water swelling and heat dehydration. The total average value of TOC (total organic matter content) is more than 1%, which belongs to carbonaceous rocks of medium high grade hydrocarbon source rocks. Organic matter is mainly dominated by type I and II1-II2 type with large hydrocarbon generating potential, which is prone to oxidation-reduction reaction and cause rock disintegration.
Carbonaceous rock is a rock containing a large number of dispersed carbonized organic matter. It can be divided into carbonaceous mudstone and carbonaceous shale, as shown in Figure
Main types of carbonaceous rocks. (a) Carbonaceous mudstone and (b) carbonaceous shale.
The petrophysical and geochemical characteristics of carbonaceous shale in Guangxi are different from those of different strata in different ages, but they all have many common engineering characteristics. They are heterogeneous, soft and hard interbedded multilayered structure. The structure of folds, joints, fissures, and faults in the strata are more developed. Carbonaceous rocks have higher initial strength. The lower CBR value and acidity are affected by the natural forces, the sun and the rain. It is very easy to be weathered and disintegrated. Through comprehensive field investigation and rock sample analysis and test results, the main reasons and mechanisms leading to the disintegration of carbonaceous rocks are as follows: the influence of tectonic activities; the water swelling and the dehydration of clay minerals; and oxidation and decomposition of organic matter and reductive minerals.
Carbonaceous rock research at home and abroad is mostly the literature on oil shale and coal. The purpose of finding energy and mineral resources is to analyze the origin distribution, indoor microstructure, organic matter and mineral composition, and carbon isotopic elements and to evaluate the distribution and reserves of resources. The main work is to investigate energy and mineral resources. According to the Guangxi Region geological survey and geological survey and reconnaissance report [
Discriminant map of the tectonic background of the Luofu formation [
In the southwest section of Hechi, the value of type III1 Ro is 3.47%∼3.73%, which is an all-over mature stage. The source rocks of the Luofu formation are mainly from lower aquatic organisms, with the input of terrigenous higher plants, and the sedimentary environment is reduced brackish water marine sedimentary environment.
To sum up, from the aspect of engineering construction, first of all, through data collection and field geological survey, the main distribution horizon, exposed area, and lithological combination characteristics of carbonaceous rocks in Guangxi area were identified. Large scale comprehensive profile measurement and sampling test were carried out in key areas. Secondly, the influence of microscopic characteristics of main minerals and chemical composition, total organic carbon content, and chemical reaction on the characteristics of carbonaceous rock disintegration engineering is studied. The main mechanism and development law of carbonaceous rock disintegration are analyzed. The research results provide theoretical basis and engineering geological research data for engineering personnel and have far-reaching economic and social value in improving the safety and quality of engineering construction [
Guangxi is located in the western part of Southern China, China, as shown in Figure
Distribution map of Guangxi and carbonaceous rocks in China. (a) Map of Guangxi, China, (b) distribution map of carbonaceous rocks in Guangxi, and (c) the stratigraphy column of Middle Devonian Luofu formation.
Guangxi is located in the southwestern section of the Yangtze belt and Wuyi-Yun Kai arc basin and has undergone four cycles of tectonic movement, including the “the three fortress movement,” “the Caledonian Movement,” “the Hercynian Indosinian movement,” “the Yanshan movement,” “the Himalaya movement,” and “the neotectonic movement.” A complex geological structure system was formed. Carbonaceous rocks were mainly formed in anoxic and reductive deep water basins, slopes, and relatively confined coastal lagoons and swamps. Sediments were mainly composed of fine-grained argillaceous and silty sand, and the deposition rate was slow in the process of diagenesis. It belongs to tectonic weak layer. These structural weak layers are more prone to produce various kinds of folds (such as Figure
Guangxi carbonaceous siliceous rock fold structure.
Carbonaceous rocks in Guangxi are widely distributed. However, due to their different genetic environments, carbonaceous rocks in different strata are different in composition and lithological characteristics.
The typical carbonaceous rock formation is the Devonian Luofu (D2l) in Western Guangxi. It is mainly distributed in Guangxi Nandan five Hechi, Shanglin Binyang, Wuming Taiping Gan Wei, Baise Yang Xu, and Tianlin eight crossing. In the area of Luofu Xiang in Nandan County, the lower carbonaceous rock formation is mainly composed of black horizontal mudstone and marble stone. The upper part consists of thin mudstone and marlite and a few calcareous siltstones. Occasionally, phosphorite nodules and siderite belts occur. From north to the Huang Jiang and Luo Tun areas, the lithology is mainly mudstone, but the rocks contain more silty or calcareous, and some fine-grained quartz sandstone appears at the bottom, and the limestone intercalation increases obviously. There are corals and laminar reefs near the Dachang mountain area near the Dachang. From southeast to Hechi, the area of north Xiang and five polders is thin-thick marl, calcareous mudstone, or calcareous mudstone containing marl and mudstone. The lower part of the eight-crossing area in Tianlin county is a thin mudstone in gray black, and the upper part is dark gray black mudstone, siliceous mudstone, fine-grained dolomite, and silt and mudstone. In Shanglin County and Wuming District, the group has an integrated contact relationship with the lower Tangding formation and the overlying GRANJIANG River group. The lithological difference is relatively large on the lateral side. The Lei Huang-Xiang Xian area is a gray thin siliceous rock, and the siliceous rock contains a small amount of siliceous mudstone. The lower part of the lower part of the Ma Tou Bai Shi area is interbedded in the deep gray-gray black thin layer with bioclastic powder limestone and gray-gray thin mudstone.
In the relationship between different strata, the Luofu formation is a series of black mudstone, carbonaceous mudstone, calcareous mudstone, marl and a small amount of sandstone, and siltstone and a small amount of marl. The upper part of the strata is black mudstone or the gray thin layer siliceous mudstone intercalated with the siliceous rocks. As a result, the rift formation is adjacent to the Tangding formation. The carbonaceous rocks in the group are more pure, the thickness is large, the sedimentary thickness is 102∼913 m, and the thickness of the strata is the largest in the Luofu area of the Nandan depression. The developed part of the part of the Bauma sequence is deeper than that of the deep slope facies. The lithology is gray and black carbonaceous mudstone, calcareous mudstone and biotic marl lens, locally cladding tuff, phosphorous tuffaceous gravel bearing mudstone, siliceous shale, siltstone, and quartz fine sandstone. Fossils are developed as coral and Permian reefs in the leading mountain area of Dachang, Bioclones Nowakiaotomari, Viriatellinaguangxiensis, V. minuta, V. multicostata, and fossils such as stronidas, and three species.
According to the on-site investigation, the main lithology of carbonaceous rocks in the Luofu formation is deep gray, black thin middle carbon mudstone, sludge mudstone, calcareous mudstone, carbon-dyed bioclad mudstone, siltstone with very thin layer of silty mudstone (such as Figure
Field investigation of carbonaceous rocks in the Luofu formation, Western Guangxi province. (a) Combination of silty mudstone and carbonaceous mudstone. (b) Pyrite-enriched layer in carbonaceous mudstone. (c) Iron and manganese nodules in carbonaceous mudstone. (d) Precipitated from semiweathered carbonaceous rocks. (e) Horizontal bedding structure in semiweathered cleavage carbonaceous shale. (f) Carbonaceous mudstone.
Pore system is an important space for gas storage of carbonaceous shale, and also an important channel for fluid migration. Therefore, the types, development characteristics, and relationships of pores have great influence on the stability and swelling capacity of carbonaceous shale in the later stage. The types of pore developed in the sample include 3 types of reservoir space, including mineral pore, organic matter pore, and microfracture (Table
Main pore types of carbonaceous rocks.
Pore type | Features | Form |
---|---|---|
Organic pore | The pore size of organic matter is generally micron level, and some can reach nanometer level | Round, oval, long strip and irregular shape, etc. |
Intergranular pore | The aperture is nanometer and micron level | Triangle, strip, and irregular shape |
Intergranular corrosion pore | Round, oval, long, triangular, and irregular | |
Intragranular pore | Elliptical, irregular, etc. | |
Intragranular dissolution pore | Round, oval, irregular, etc. | |
Microfracture | The width is generally nanometer, and the length is generally micron level | Strip shape |
Mineral pores mainly include intergranular pores, intergranular dissolution pores, intragranular pores, and intragranular dissolved pores. Organic pores belong to pores formed by organic matter in the late thermal evolution process, while microcracks are mainly developed in the laminar development stage and easy to form bedding joints between rigid minerals and plastic minerals [
According to the different locations of mineral crystals and minerals, mineral pores can be divided into four types: mineral intergranular pores, intergranular dissolution pores, intragranular pores, and intragranular dissolved pores.
Intergranular pore is one of the main pore types of carbonaceous shale. It is mainly developed in quartz crystal, feldspar, pyrite, and calcite, which are brittle, well compacted, and intercalated with clay minerals such as illite, kaolinite, montmorillonite, and chlorite. They are triangular, long, and irregular. The diameter of these pores is mostly 1–200 nm. It can be further divided into two kinds: intergranular pore and clay intergranular pore. Among them, the intergranular framework hole is shown in Figure
Intergranular pore of carbonaceous rock: (a) intergranular pores and (b) between the clay minerals and the edge pores of the rigid grains.
The clay interlayer pores are mainly developed between the clay soil. When the clay is deposited, flocs are thickly floc, and the intermespores of clay are located between the supporting clay sheets. Because the clay is a plastic mineral, when the clay particles and the quartz and other brittle mineral particles are contacted, due to the different compaction resistance of the two materials, the deformation of the clay and brittle particles will produce pores. Clay sheets may also be formed by pores due to compaction, or parallel to clay sheets, or bent into crescent forms (strongly caused by compaction during the diagenetic stage). Intergranular pores occur mainly in the depositional stage. Under special circumstances, the dehydration of clay minerals in the diagenetic stage can also form clay intermespores. There are some intergranular pores formed between clay particles such as illite, kaolinite, and montmorillonite, which have great influence on the stability of carbonaceous rocks. The more intergranular pores are, the easier the carbonaceous rocks are disintegrated.
The intergranular dissolution pores, as shown in Figure
Intergranular corrosion pores of carbonaceous rocks.
Figure
Intragranular pores are mainly developed in pyrite, quartz, calcite, and clay. The shape of strawberry is as shown in Figure
Carbonaceous rock strawberry pyrite grain hole.
As shown in Figure
The dissolution pores of carbonaceous shale are shown in Figure
Intragranular dissolution pores of carbonaceous rocks.
Organic matter pores (as shown in Figure
Organic pores in carbonaceous rocks.
The study shows that organic matter pores can be produced when the maturity of organic matter in carbonaceous shale is greater than 0.6%. This kind of pore is greatly influenced by the organic carbon content and maturity of shale and increases with the increase of organic carbon content and maturity. However, when the maturity of shale is increased to a certain extent, the microscopic pores and specific surface of shale can be reduced by more than 2.4%. The porosity and adsorption capacity of shale decreased.
According to the size of fractures, they are divided into microcracks and macrofractures. The macrofractures are mainly structural fractures, diagenetic contraction joints, and interlayer foliage joints, which extend relatively far away (Figure
Microfracture between clay minerals of carbonaceous shale in the Luofu formation.
It should be noted that, although the above microfractures are not very far away, because of the large quantity and enrichment distribution, the pore permeability conditions of carbonaceous shale often change greatly, especially in the development of underground fluids and other active fluids. They will have a great influence on the weathering and collapse of carbonaceous mud sheets.
In general, the samples of the Devonian Carboniferous carbonaceous shale are mainly composed of mineral pores, organic pores, and microfractures, which are dominated by mineral pores, belonging to micron-sized pores and generally developed pores. The most developed pore types in mineral pores are intragranular dissolved pores, followed by mineral intergranular pores and a small amount of intergranular dissolution pores. The intergranular pores in mineral grains are mostly residual intergranular pores. Their morphology is mostly irregular and slit shaped due to compaction. There are more cloth disposed at the boundary of lamellar, flaky, plate columnar, long columnar rigid minerals, and soft plastic minerals. The connectivity is poor. The microfracture caused by the deformation of the sedimentary structure and brittle minerals is more developed in the local layer, and the seam width is between several tens of nanometers and several microns.
The porosity and permeability of carbonaceous shale samples in the test section of the Devonian Carboniferous section and the shale gas geological survey well were collected and summarized. The characteristics of each segment are as follows.
It can be seen from Table
Physical properties of carbonaceous shale in different areas of Northern Guangxi.
Number | Horizon | Porosity | Permeability (mD) | ||
---|---|---|---|---|---|
Range value (%) | Average value (%) | Range value | Average value | ||
1-a | Upper part | 0.06∼2.61 | 1.0 | 0.03∼5.21 | 0.21 |
1-b | Lower part | 0.06∼2.61 | 0.77 | 0.03∼5.21 | 0.66 |
2 | Luofu group | 0.81–6.90 | 2.69 | 0.0012–0.056 | 0.0095 |
Because different types of minerals are different in basic physical properties such as heat dehydration, water swelling, and porosity development, mineral composition analysis is the basis for studying the classification of carbonaceous rocks and analyzing the disintegration of carbonaceous rock slope. The minerals in carbonaceous shale include clay minerals, quartz, calcite, dolomite, Xie Changshi, potash feldspar, siderite, pyrite, and a small amount of sulfate rock.
Minerals have a significant effect on the engineering properties of carbonaceous rocks. Quartz, feldspar, calcite, dolomite, siderite, and pyrite are brittle minerals. The higher their content is, the more brittle the rock is and the more susceptible it is to external forces, resulting in a series of structural fissures. The quartz is mainly derived from terrigenous clasts and biotic siliceous shells. During the diagenesis process, quartz minerals can also be formed. They usually appear in the form of the self-form crystal, which enhances the strength of carbonaceous rocks. Carbonate minerals are mainly calcite, usually in the form of eutectic crystals or filled with cementation. In addition, dolomite, siderite, and manganese carbonate are also found. The feldspar of carbonaceous rocks is mainly derived from terrigenous clasts. Feldspar minerals are usually replaced by clay minerals due to the alteration of diagenesis and alteration. Besides main minerals, pyrite, siderite, and other authigenic minerals are also common in carbonaceous rocks, and their contents are usually low (<5%). However, some layers of slope basin facies deposits are more enriched in the deeper water shelf, and sometimes they can form ribbons with pyrite as the main mineral. Later oxidation may cause greater damage to the carbonaceous rock slope. Clay minerals refer to water-containing layered structure. Mudstone is mainly composed of silicate minerals, the main components are kaolinite, montmorillonite, and illite. It is a hydrophilic mineral, which has a significant effect on the disintegration characteristics of carbonaceous rocks. The higher the hydrophilic mineral content, the stronger the disintegration.
The mineral composition of carbonatite in the Luofu formation is analyzed, as shown in Figure
Mineral composition map of carbonaceous shale in Luofu formation, Luofu section, Nandan county.
Clay mineral composition characteristics of carbonaceous shale in the Luofu formation.
Sample number | Lithology | Kaolinite (%) | Chlorite (%) | Illite (%) | Montmorillonite (%) | Illite/montmorillonite layer | Green/montmorillonite layer | Mixed layer ratio | |
---|---|---|---|---|---|---|---|---|---|
— | — | K | C | I | S | I/S | C/S | I/S | C/S |
D2092-1 | Carbonaceous mudstone | — | 11 | 42 | — | 46 | 1 | 5 | 13 |
PM08-6-1 | — | 5 | 41 | — | 54 | — | 5 | — | |
PM08-9-1 | — | 3 | 49 | — | 48 | — | 5 | — | |
PM08-26-1 | — | 3 | 45 | — | 52 | — | 5 | — | |
D0269-1 | — | — | 32 | — | 68 | — | 5 | — |
The main mineral components in carbonaceous shale samples are clay minerals and quartz. Clay minerals are a kind of silicate or aluminosilicate, which can be divided into two types: amorphous and crystalline. The latter can be divided into two different structural types: lamellar and chain stratiform. One is the repeated stacking of the same type of clay mineral structural units, which are called simple layered clay minerals, such as kaolinite, halloysite, montmorillonite, illite, and chlorite. The other is made up of two or more than two types of clay mineral structural units called mixed layer clay minerals. It can be further divided into two types: ordered mixed layer and disorder mixed layer. Besides layered and chain layered clay minerals, there are also amorphous and semicrystalline clay minerals, such as hydrargillite, and hairy stripe. They are less distributed in clay rock.
It should be specially pointed out that there is a certain pore between the structural units of clay minerals, which often fills with more water molecules or other hydration cations. This layered structure determines their water sensitivity, and the adsorbed water and interlayer water content of different minerals are different.
Clay minerals have strong water absorption ability. They are the manifestation of hydration of clay minerals. Previous studies have shown that hydration of clay minerals can be divided into two stages. The first stage is hydration of clay adsorbed by exchanged cation. The second stage is hydration between clay minerals at the crystal level. In clay minerals, the hydration capacity of montmorillonite is the strongest and hydration volumetric expansion can exceed 50%. The hydration of some sodium montmorillonite can increase 8–10 times the volume of the dry soil. The reason is that the molecular structure of the montmorillonite crystal structure is linked by molecular attraction, and the bond force is weak. The water molecules can easily enter the structural layer along the silicon oxygen layer so that the adjacent lamellar bodies are separated and the spacing between layers increases, thus causing the volume to expand. The cation absorbed by montmorillonite (Na+ and Ca2+) will form thicker hydration film on the surface of montmorillonite, which will increase the cell distance and expand the volume. Besides montmorillonite, the water absorption and expansion of illite and halloysite are next, and kaolinite is the weakest. Besides, clay minerals contain a large amount of intragranular dissolution pores and intergranular pores. It also provides a certain physical space for mineral water swelling. These clay minerals often affect the formation of plastic deformation, such as landslides, rock disintegration, and other geological disasters.
From the Devonian carbonaceous rocks sampled from the investigation, it is concluded that the clay minerals are mainly illite and illite/montmorillonite mixed layer and a small amount of kaolin and chlorite are found in the clay layer. The content of montmorillonite and other minerals is relatively small. Therefore, the illite and illite/illite mixed layer are the decisive factors for the water swelling of these carbonaceous rocks.
The thermal dehydration of clay minerals is the phenomenon that the clay minerals lose the interlayer water and structural water when heated to a certain extent. According to the data research, the temperature of different clay minerals is quite different from that of thermal dehydration. Kaolinite starts to lose structural water between heating 400°C∼525°C and has no significant dehydration phenomenon below 400°C and upper 550°C; below 400°C and even below 100°C, some interlayer water has been dehydrated. The main dehydration stage at 400°C∼500°C is mainly the interlayer water and structural water. Some of the structural water has been deactivated until 800°C. Because of a large amount of interlayer water, montmorillonite has started to remove some interlayer water at 0°C∼100°C, and then lost a lot of interlayer water at about 100°C∼350°C. At 300°C∼600°C, the structure water is mainly lost. When the illite is heated, it will first lose the adsorbed water and interlayer water below 100 degrees. After that, it is mainly the stage of structural water loss at 200°C∼500°C.
According to the results of previous rock mineral tests, the clay minerals in the Guangxi Devonian Carboniferous carbonaceous shale samples were mainly illite, which belonged to rock types which were more susceptible to thermal dehydration. In addition, the fresh surface of carbonaceous shale was mainly black, and it was easier to absorb and store heat under the sun’s illumination, thus increasing the temperature of the rock. The destruction of rock formations and the long summer time and sufficient illumination in Guangxi will cause the collapse of these carbonaceous shale slopes in a relatively short period of time due to continuous heat and dehydration.
It can be concluded that carbonaceous shale can produce a series of mechanical deformation, forming a series of cracked fissures, cracks, and so on, due to the influence of the swelling and shrinkage of the clay minerals and the heat and dehydration of the sun when exposed to the surface.
Organic matter in rocks is a residual organism that is deposited and retained in rocks in relatively reduced, blocked, or deeper water environments. Organic matter is the material basis for hydrocarbon generation of shale gas. Its abundance determines the hydrocarbon generation and expulsion ability of carbonaceous shale. The abundance of organic matter is mainly evaluated by parameters such as total organic carbon (TOC), hydrocarbon generation potential (s1 + s2), and chloroform green “a.” The total organic carbon (TOC) is an important reference standard for the abundance of organic matter. In particular, it has strong adaptability to marine organic shale. According to the technical specification for calculation and evaluation of shale gas resources/reserves, the TOC content of carbonaceous rocks is divided into 5 grades: high (>4%), high (2%∼4%), medium (1%∼2%), low (0.5%∼1%), and ultralow (<0.5%).
The carbonaceous shale with high organic matter content is easier to disintegrate or produce other deformation due to oxidation and decomposition of organic matter and reducing minerals. Therefore, it is necessary to test and analyze the content and type of TOC in carbonaceous rock slope. TOC test results of carbonaceous shale in the Middle Devonian Luofu formation are shown in Table
TOC statistics of carbonaceous shale in the Middle Devonian Luofu formation.
Group class | Sampling position | Minimum (%) | Max (%) | Average (%) | Number of samples |
---|---|---|---|---|---|
Luofu group | Che He town, Nandan | 0.11 | 0.81 | 0.37 | 6 |
Nandan Dachang | 0.54 | 4.74 | 3.14 | 14 |
According to Table
The type of organic matter largely determines the stability of carbonaceous rocks. Different organic matter types have different source of material, sedimentary environment, rock composition, and structure, so the potential for further decomposition is also quite different. The damage caused by oxidative decomposition to rock stability is also very different under the condition of surface exposure. By using kerogen macerals test to determine the parent material types of organic matter, combined with previous research results, the types of kerogen in the investigation area are discriminated. The classification criteria are shown in Table
List of kerogen types for classification of kerogen (sy/t5735-1995 three, four divisions).
Type | Type I (rotten clay type) | II1 (rotten rot type) | II2 (rotten mud rot type) | III (rot type) | |
---|---|---|---|---|---|
Kerogen inspection | Chitin group (%) | 70∼90 | 70∼50 | 50∼10 | <10 |
Vitrinite group (%) | <10 | 10∼20 | 20∼70 | 70∼90 | |
TI | 80∼100 | 80∼40 | 40∼0 | <0 |
According to the results of Table
Macerals and organic matter types of carbonaceous kerogen in the Nandan Dachang section of the Luofu formation.
Region | Horizon | Lithology | Macerals (%) | Type index | Type | |||
---|---|---|---|---|---|---|---|---|
Sapropelite | Chitin group | Vitrinite group | Inert group | |||||
Nandan | D2l | Gray-black carbonaceous mudstone | 60.0 | 0.0 | 40.0 | 0.0 | 30.0 | II2 |
Nandan | D2l | Gray-black carbonaceous mudstone | 69.3 | 0.0 | 30.3 | 0.3 | 46.3 | II1 |
Nandan | D2l | Gray-black carbonaceous mudstone | 75.3 | 0.0 | 24.0 | 0.7 | 56.6 | II1 |
Nandan | D2l | Gray-black carbonaceous mudstone | 59.3 | 0.0 | 40.7 | 0.0 | 28.8 | II2 |
Nandan | D2l | Gray-black carbonaceous mudstone | 81.7 | 0.0 | 18.3 | 0.0 | 68 | II1 |
Nandan | D2l | Gray-black carbonaceous mudstone | 83.0 | 0.0 | 17.0 | 0.0 | 70.3 | II1 |
Nandan | D2l | Gray-black carbonaceous mudstone | 83.3 | 0.0 | 16.3 | 0.3 | 70.8 | II1 |
Nandan | D2l | Gray-black carbonaceous mudstone | 60.0 | 0.0 | 39.0 | 1.0 | 29.8 | II2 |
Nandan | D2l | Gray black carbonaceous mudstone | 71.0 | 0.0 | 28.3 | 0.7 | 49.1 | II1 |
Nandan | D2l | Gray-black carbonaceous mudstone | 80.3 | 0.0 | 19.3 | 0.3 | 65.5 | II1 |
Nandan | D2l | Gray-black carbonaceous mudstone | 87.7 | 0.0 | 12.0 | 0.3 | 78.4 | II1 |
Nandan | D2l | Gray-black carbonaceous mudstone | 75.3 | 0.0 | 24.0 | 0.7 | 56.6 | II1 |
Nandan | D2l | Gray-black carbonaceous mudstone | 85.3 | 0.0 | 14.0 | 0.3 | 74.5 | II1 |
Therefore, the total average value of TOC (total organic matter content) in the carbonaceous rock samples sampled is over 1%. It belongs to the carbonaceous rocks of medium high source rocks and has higher organic matter content. In terms of organic matter type, the organic matter in carbonaceous shale mainly consists of type I and II1-II2, which has higher hydrocarbon generating potential and has great potential for oxidation and decomposition under natural exposure. The organic matter in siliceous mudstone, carbonaceous calcareous mudstone, and carbonaceous mudstone is dominated by type III, which is less hydrocarbon generating potential. It is not easy to cause further oxidative decomposition. Therefore, carbonaceous shale is more prone to rock disintegration and collapse than other rocks. In addition, for the fresh rock samples in core, the TOC content of the semiweathered rock samples in the surface section is relatively low. It should be related to the weathering and decomposition loss of organic matter in carbonaceous shale.
Carbonaceous shale is mainly formed in the lagoon, swamp, bay, and deep water slope, basin of block reduction, quiet and low energy, rich in massive reductive metal sulfide such as reduced organic matter and pyrite. Under the condition of natural exposure, they will undergo oxidative decomposition reaction under the combined action of oxygen and water, thus causing certain damage to the structure of the rock.
In Section
Pyrite and other reducing minerals are common metal sulphides in black carbonaceous shale. They are often locally enriched into ribbons, pellets, etc., which are easily reacted with air, water, and other media. After reaction, hydrogen ions can be generated to make the solution acidified, while the oxidation of sulphide ions and iron ions can react with other minerals in the reaction water solution and surrounding rocks. A series of weathering products which affect the engineering properties of carbonaceous rock slopes have been generated. Previous studies have found that [
The oxidation of pyrite is mainly due to Thiobacillus ferrooxidans, which can oxidize Fe2+, S elements, H2S, and other inorganic components, and belongs to autotrophic aerobic acidophilic bacteria. However, the reaction must take place on the surface of microbiological bacteria and sulfide mineral crystals. The reaction equation is FeS2 + 7/2O2 + H2O ⟶ Fe2+ +
The oxidants for chemical oxidation of pyrite under natural exposure are usually O2 and Fe3+, and their oxidation steps are similar to biological oxidation pathways. The first step is the dissolution oxidation reaction of pyrite with dissolved oxygen and water so that the iron element in the mineral lattice of pyrite is precipitated into ferrous ions, and sulfur is oxidized to form sulfate ion (
Generally speaking, the initial reaction solution of pyrite oxidation is usually high and the rate of oxidation is slower. With the continuous reaction, the hydrogen ion generated by pyrite oxidation reduces the solution value. When the duty drop is less than 2.5, the solubility of iron ion hydrolysates will increase significantly, and the reactivity of iron ions will be significantly enhanced, resulting in the rapid oxidation of pyrite. The oxidation and hydrolysis of pyrite will change the original pH value of rock and then destroy the structure of strata, resulting in collapse, collapse, and so on.
To sum up, the collapse of carbonaceous rock slope is the result of many factors. Therefore, when carbonaceous rock slope treatment and ecological rehabilitation are carried out, we should make full investigation and comprehensive analysis and evaluation before making reasonable and effective treatment methods.
Through comprehensive field investigation, the main distribution horizon, outcropping area, and lithological combination characteristics of carbonaceous rocks in Guangxi area are identified. The influence of the microscopic pore types and geochemical characteristics of carbonaceous rocks on the engineering characteristics is studied in combination with the rock samples analysis and test results. The main cause and mechanism of carbonaceous rock disintegration are obtained. The main conclusions are as follows: Through the investigation of carbonaceous rocks in Guangxi China, it is concluded that carbonaceous rocks are distributed in the Luofu formation of the Devonian system, the Luzhai formation of the Carboniferous system, and the Qingxi formation of the Cambrian. They are mainly formed in the depositional environment of deep water basin, slope, and confined coastal lagoon and swamp, which belong to the tectonic weak layer. It is more prone to produce various types of folds, faults, joints, cleavages, cracks, and other structural deformation and a series of structural weak surfaces, which affect the stability of carbonaceous rock slopes. Scanning electron microscope (SEM) was used to study the mineral pores, organic matter pores, and the microstructure of cracks in carbonaceous rocks. Mineral intergranular pores were mostly residual intergranular pores. There were more cloth disposed at the boundary of lamellar, flaky, plate columnar, long columnar rigid minerals, and soft plastic minerals. The connectivity is poor. The higher the pore content of organic matter is, the more easily the carbon shale is exposed to the surface. The microfracture caused by the deformation of the sedimentary structure and brittle minerals is more developed in the local layer. The porosity of the black shale in the Devonian Luofu formation is 0.81%∼6.90%, with an average of 2.69%, and the permeability range is 0.0012–0.056 mD, with an average of 0.0095 mD. The engineering properties of carbonaceous rocks are related to the oxidation and decomposition of mineral composition, organic matter, and reductive minerals. The clay minerals in carbonaceous rocks are mainly illite and illite/montmorillonite layers, which have the characteristics of water swelling and heat dehydration. They are prone to crevice under the condition of exposure to the surface and break down. The total average value of TOC (total organic matter content) of carbonaceous rock is above 1%. The type I and II1-II2 with large hydrocarbon generating potential is the main type, which has great potential for oxidation and decomposition under the condition of natural exposure, and the oxidative hydrolysis of pyrite will change the original PH value of rock, which will cause rock collapse and collapse.
The data used to support the findings of the study are available from the corresponding author upon request.
The authors declare that there are no conflicts of interest regarding the publication of this paper.
This study was supported by the Science and Technology Project of Guangxi “Research on the Key Technologies of highway construction in Guangxi carbonaceous rock area” (Grant GuikeAC16380119) and “Research and Application of ecological slope protection technology in carbonaceous rock slope” (Grant AD19110124).