Identification and classification of fine-grained sedimentary fabrics in coal measures: A case study from the northwestern Dongsheng Coalfield

Abstract

Based on detailed analyses of fine-grained sedimentary characteristics and rock assemblages in coal measures, as well as correlations between macroscopic and microscopic sedimentary components, this study systematically investigates the types of fine-grained sedimentary fabrics in coal-bearing strata. A combination of macroscopic description, petrographic characterization, experimental analyses, and maceral identification was employed to classify the fabrics. According to mineralogical composition—particularly the abundance of key minerals—together with textural attributes and sedimentary structures, the fine-grained sedimentary fabrics of the coal measures are classified into carbonaceous clayey fabric, siliceous-clayey mixed fabric, organic-clayey composite fabric, and clayey silty fabric. On this basis, the lithofacies associations of the coal measures, the dominant sedimentary facies, and their spatial and temporal variations are further analyzed. The results demonstrate that systematic characterization of fine-grained sedimentary fabrics in coal measures is of considerable significance for reconstructing the paleogeography of coal-forming basins and for evaluating the reservoir potential of fine-grained sedimentary rocks.

Keywords

Coal measures sedimentary fabric lithofacies sedimentary facies Dongsheng coalfield

Introduction

Fine-grained fabrics in coal-forming systems constitute an important component of fine-grained sedimentary deposits and exhibit distinctive characteristics. Fine-grained fabric in coal measures refers to the integrated properties of fine-grained sediments containing coal-type organic matter within stratigraphic sequences. These include mudstone, siltstone, gangue layers within coal seams, and dispersed coal-type organic matter and are characterized by their mineralogical composition, microstructural features, distribution and lamination of organic matter, as well as pore characteristics. Investigation of these fabrics is essential for understanding sedimentary processes, evaluating reservoir properties, and predicting the development potential of coal-measure resources such as coalbed methane, shale gas, and rare-metal mineralization.

Extensive studies have been conducted on fine-grained sedimentation theory, particularly in the context of unconventional oil and gas exploration and development. However, fine-grained deposits in coal measures are characterized by greater compositional and structural complexity than those in typical marine or lacustrine systems. Given the uniqueness of fine-grained fabrics in coal measures, this study focuses on the relationships among fabric characteristics, the identification of Milankovitch cyclicity, fabric evolution, and climatic responses during coal-measure deposition.

Recent studies on fine-grained sedimentary characteristics and reservoir properties have demonstrated that fine-grained sedimentation is controlled by multiple dynamic processes, including aeolian activity, gravity flows, bottom currents, and hyperpycnal flows, among which gravity-flow deposits in continental lacustrine basins are particularly well developed (Stow et al., 2001; Zhai, 2025). Meanwhile, research on material sources has evolved from a singular terrigenous perspective to a dual coupling framework involving both extrabasinal detrital inputs and intrabasinal authigenic components. Minerals such as quartz and dolomite have been shown to originate from multiple processes, including biological activity and diagenesis (Aplin and Macquaker, 2011). Consequently, the study of fine-grained sedimentation has been elevated to the level of Earth system science (Zou and Qiu, 2021).

Looking forward, fine-grained sedimentology is expected to advance toward intelligent core analysis, forward modeling of sedimentary processes, and data-driven evaluation of reservoir heterogeneity, thereby providing more precise theoretical support for shale oil and gas exploration and development (Zhu et al., 2025, 2026a, 2026b). In recent years, significant progress has been achieved in shale lithofacies research, primarily focusing on lithofacies identification and classification (Wang and Carr, 2013; Wang et al., 2014). The research paradigm is undergoing a fundamental transition from traditional qualitative descriptions to quantitative, data-driven, and multi-scale integrated approaches. In terms of lithofacies classification, ternary diagrams based on mineral composition, combined with total organic carbon (TOC) content, are widely adopted (Wu et al., 2018; Ou et al., 2018; Tang et al., 2021).

In addition, reservoir pore structure and the geochemical conditions of source–reservoir systems constitute key research topics (Li H et al., 2025). In studies of pore structure characterization and heterogeneity in coal-measure reservoirs, integrated multi-technique approaches—including low-field nuclear magnetic resonance (LF-NMR), micro-computed tomography (μ-CT), high-pressure mercury intrusion, and low-temperature nitrogen adsorption—have been employed to reveal the pore characteristics of coal-measure reservoirs (Li et al., 2026; Huang et al., 2022; Yang et al., 2023).

In sediment petrology, fabric is defined as the spatial arrangement of rock constituents and the boundaries between them. It describes the internal geometric configuration of physical and structural elements within rocks, reflecting the arrangement and interaction of sedimentary components during deposition. When temporal and spatial variations of these components are considered, sedimentary fabric exhibits both static and dynamic characteristics. The fabric characteristics and evolutionary processes of fine-grained sediments—particularly in coal measures—are therefore fundamental to understanding the mechanisms controlling the accumulation of coal-related unconventional gas resources.

The concept of fabric originates from petrofabric studies, which describe the constituent elements of rocks, commonly referred to as fabric elements. These elements can be regarded as discontinuities at different scales and are generally classified into geometric fabric and physical fabric, with the latter being influenced by the former. Fabric thus represents the three-dimensional spatial arrangement of surfaces and linear features, which directly affects properties such as porosity, permeability, and gas absorption capacity. Fabric characteristics include geometry, symmetry, continuity, permeability, and statistical homogeneity and can be further subdivided into overall fabric and sub-fabrics.

Recent research has advanced toward micro- and ultra-microfabric analysis. For example, studies of microbial rocks have investigated microfabric features formed through organic-mineral interactions (Tang Dongjie, 2013). Research on the relationship between microstructure and mechanical properties of continental coal-measure mudstones has also emerged (Sun Chengcai, 2015). These studies employ multivariate statistical methods, such as principal component analysis, to quantitatively characterize microstructural features, thereby providing more comprehensive evaluation frameworks than single-parameter approaches.

Rock fabric analysis, including investigations of biogenic deposits and trace fabrics, plays a critical role in reconstructing the sedimentary environments, classifying depositional sequences, and elucidating genetic mechanisms (Gong Yiming et al., 1997; Zhang Guocheng et al., 2004; Gong Yunyun, 2016; Xiao Enhao et al., 2020). For instance, detailed studies of benthic ooids have identified distinct microfabric related to depositional energy conditions and microbial activity (Mei et al., 2011). Additionally, analyses of lithofabric characteristics and sediment magnetic fabrics demonstrate close relationships between particle size, magnetic susceptibility, and environmental changes, providing valuable proxies for paleoenvironmental reconstruction (Zhu Dagang et al., 2000, 2003; Yang Xiaoqiang and Huamei, 2000).

Conventional fine-grained sedimentological and petrological studies provide important references for investigating fine-grained fabrics in coal measures. For example, core-based observations and experimental analyses have been used to examine the sedimentary characteristics, genetic mechanisms, controlling factors, and reservoir properties of lacustrine fine-grained deposits (Peng Jun et al., 2022a). Similarly, integrated analyses of lithofabric, biological remnants, and geochemical indicators have been applied to reconstruct carbonate depositional environments and sedimentary processes. These studies highlight the close relationship between sedimentary origin, fabric characteristics, and reservoir quality (Huang Daojun et al., 2021).

In China, sedimentologists and petroleum geologists have conducted systematic investigations into fine-grained sedimentary rocks associated with oil and gas resources (Xie Zongkui, 2009; Sun Longde et al., 2010, 2015; Zhu Rukai et al., 2011; Wu Yinye et al., 2013; Jiang Zaixing et al., 2013). Further studies have addressed sedimentary sequences and sequence stratigraphy of fine-grained coal-measure deposits to better understand unconventional hydrocarbon accumulation in coal-measure mudstones (Qin Yong et al., 2016, 2018, 2020).

The evolution of fine-grained sedimentary fabrics in coal measures constitutes a fundamental basis for research on coal-measure gas reservoirs. Coal measures may form in a wide range of depositional environments, including marine, continental, and transitional settings such lagoons, tidal flats, rivers, lakes, and alluvial fans. They are characterized by abundant sediment supply, diverse lithologies, thin individual beds, frequent interbedding, strong cyclicity, and high organic matter content, all of which favor the development of coal-measure gas reservoirs (Qin Yong, 2018). Consequently, coal measures can occur in nearly all sedimentary systems.

Fine-grained sedimentary fabrics constitute the fundamental basis for defining and identifying fine-grained lithofacies. The relationship between the two can be understood from two dimensions. On the one hand, fine-grained sedimentary fabrics serve as key secondary criteria for lithofacies classification. The primary criterion, namely the mineralogical composition of fine-grained materials, determines the major categories of fine-grained rocks, whereas the secondary criteria reflect the spatial assemblages of material composition and lamination types. On the other hand, fine-grained fabrics and lithofacies in coal measures represent a distinctive category characterized by the presence of coal-type organic matter and its laminar associations, exhibiting pronounced and unique features.

Nevertheless, the sedimentary fabric of coal measures remains highly complex, and several key scientific issues require further investigation. These include the formation mechanisms of fine-grained coal-measure deposits, the sedimentary processes controlling fabric development and their influence on gas accumulation, the transport and redistribution of organic debris during deposition, and the ways in which these processes collectively shape coal-measure fabric architecture. Addressing these challenges is essential for advancing the understanding of coal-measure sedimentology and improving resource evaluation and prediction.

Overview of the study area and analytical methods

Overview of the study area

This study takes the coal-bearing area in the northwestern part of Dongsheng Coalfield as a case example to investigate the sedimentary fabric characteristics of the coal measures of the Yan'an Formation.

The Dongsheng uplift is located between the Tianhuan Depression, the Yishan Slope, and the Yimeng Uplift and lies in the northwestern part of the Dongsheng City and Hangjin Banner (Figure 1). The study area is situated in a transitional zone from the interior sag of the basin to the uplifted basin margin. Structurally, it marks the transition from the gently sloping region in the southern part of the basin to the uplifted zone along the northern basin margin (Wang Shuangming, 1996; Yu Hanwen, 2023).

Figure 1.

Geographic location and transportation network of the study area.

Overall structural deformation in the Dongsheng Coalfield is weak, and the main structural framework is characterized by a gently dipping monocline with dip angles of approximately 1–5°. Structural features are mainly manifested as (1) sporadically developed NE-trending normal faults in the northern part of the study area, with fault throws generally around 10 m; (2) a small number of short-axis anticlines and synclines; and (3) localized gentle undulations. In general, the coalfield lacks large-scale faults and folds and has not been affected by magmatic intrusions, making it a typical example of a simple structural setting (Liang Shengjian, 2023).

The Yan'an Formation (J_1–2y) is the principal coal-bearing unit in the study area. Its thickness ranges from 88.68 to 239.50 m, with an average thickness of approximately 180.28 m. The formation is widely developed throughout the area but is not exposed at the surface. A variety of bedding structures occur, and the lithology mainly includes conglomerate, sandstones of varying grain sizes, siltstone, argillaceous rocks, and coal seams. The Yan’an Formation is subdivided into five lithologic sections, corresponding to coal groups 2 through 6.

The study area lies in the northwestern part of the Dongsheng Coalfield. Influenced by paleosedimentary basement fluctuations, sedimentation is locally incomplete, resulting in sporadic development of the 5th and 6th coal groups. Regionally, the formation exhibits a south-north thinning trend and displays typical margin basin characteristics, commonly described as “deficient lower strata and truncated upper strata.” The Yan'an Formation is in parallel unconformable contact with the underlying Yanchang Formation.

Based on drilling data, the stratigraphic succession in the study area, from oldest to newest, includes the Triassic Yanchang Formation (T₃y); the Jurassic Yan'an Formation (J_1–2y) (Figure 2), Zhiluo Formation (J₂z), and Anding Formation (J₂a); the Cretaceous Zhidan Group (K₁z), Yijun Formation (K₁y), Luohe Formation (K₁l), and Huanhe Formation (K₁h); and the Quaternary System (Q).

Figure 2.

Comprehensive stratigraphic column of the Yan'an Formation based on drilling data from borehole ZK04.

The depositional environment of the Yan’an Formation is dominated by a lacustrine-deltaic system, characterized by multistage sedimentary cycles reflecting transitions from fluvial to lacustrine facies. During the period of deposition, the climate was generally warm and humid, and the basin remained tectonically stable. Sedimentation was primarily controlled by the development of fluvial channels, peat-forming swamps, lakes, and shallow lacustrine shore–shoal environments, leading to the formation of important coal-bearing strata. Overall, the sedimentary system is typified by the widespread interbedding of meandering river channel sand bodies and coal-accumulating swamp deposits.

Analytical techniques and methods

Macroscopic characterization of fine-grained fabric types

Macroscopic description and measurement methods were employed to characterize sedimentary laminae in the coal measures, including their assemblage patterns, thickness variations, and contact relationships. These observations provide independent evidence for interpreting the fine-grained depositional environment and the sedimentary evolution of the coal measures.

Microfabric and maceral analysis and their relationship to fine-grained fabrics

A range of analytical techniques was applied to investigate the microfabric characteristics of coal measures. Rapid mineral identification and semi-quantitative mineralogical analysis were conducted using X-ray diffraction (XRD), with particular emphasis on clay mineral assemblages and interlayer structures. Scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM-EDS) was used to observe mineral morphologies and the occurrence of organic matter at high spatial resolution.

For diagenetic and depositional environment analysis, cathodoluminescence (CL) was employed to identify cementation sequences of quartz, calcite and other minerals. Elemental geochemical proxies, such as Sr/BA, V/(V + Ni), were used to infer paleo-water depth and redox conditions during coal-measure deposition. Thin-section petrography served as the fundamental and most critical analytical method, ensuring the reliability and rationality of data interpretation.

XRD: XRD was employed for mineralogical analysis of the samples. The samples were ground using an agate mortar to a particle size of 200–300 mesh until no granular sensation was detected by manual grinding. Mineral composition was then determined using an X-ray powder diffractometer. During sample preparation, the powder was carefully packed into the sample holder to ensure a smooth surface level with the reference plane. The XRD patterns were acquired over a 2θ scanning range of 3°–70°, with continuous scanning at a constant angular velocity of the detector.

Maceral Quantification: For maceral quantification, coal samples were first crushed to less than 1 mm and sieved to obtain uniform particle sizes. The samples were then prepared as polished particulate sections. Under reflected light and oil-immersion conditions, macerals were identified using an optical microscope. For quantitative analysis, at least 500 points were counted per polished sample to ensure statistical reliability. Vitrinite random reflectance (R_o) was measured using a Leica DM4P microscope equipped with a Craic UCC-300 spectrophotometer under a 50× oil-immersion objective.

(3) TOC determination: Coal samples commonly contain carbonate minerals (inorganic carbon), which must be removed prior to the determination of pure organic carbon. The samples were first crushed, ground, and dried to constant weight. An appropriate amount of sample was then treated with excess dilute hydrochloric acid to fully dissolve the carbonate minerals, during which CO₂ was released. Care was taken to ensure the complete removal of inorganic carbon. After acid treatment, the samples were rinsed with distilled water to eliminate residual acid and subsequently dried again.

The treated samples were analyzed using an Elementar Vario MACRO cube elemental analyzer. Under high-temperature, oxygen-rich conditions, the samples were combusted, converting all organic carbon into CO₂. The generated gas was then quantified, and the measured total carbon content corresponds to the TOC of the sample.

It should be noted that fine-grained sedimentary rocks in coal measures are fragile, and artificial fractures may be introduced during sample preparation, potentially leading to analytical uncertainties. Additionally, the vertical superposition of coal-measure strata and pronounced lateral heterogeneity make single-point analyses insufficient. Accordingly, this study adopts an integrated, multi-scale analytical approach to minimize these adverse effects and improve the robustness of the results.

Results

Fine-grained sedimentary and rock assemblage characteristics of coal measures

The primary objective of fine-grained fabric analysis in coal measures is to elucidate the formation mechanisms, diagenetic evolution, and reservoir physical properties of fine-grained sediments. This includes (1) the composition and spatial distribution of fine-grained minerals in coal measures, such as clay minerals, quartz, and carbonate minerals; (2) the characteristics of organic matter within the fine-grained sedimentary fabric, including abundance, type (predominantly humic), maturity, and microscopic occurrence; (3) the features of fine-grained pore systems, including pore morphology, connectivity, and pore size distribution; and (4) microscopic analysis of diagenetic processes—such as compaction, cementation, and dissolution—that influence fabric transformation.

Petrofabric characteristics

The petrological type provides the fundamental basis for fine-grained fabric development in coal measures. At the macroscopic scale, the main fine-grained lithologies in the study area include siltstone, sandy mudstone, argillaceous rock, carbonaceous mudstone, and coal. As a distinct organic sedimentary rock, coal itself is not included in the fine-grained sedimentary fabric classification in this study.

In the study area, siltstone is predominantly gray to dark gray in color and consists mainly of quartz and feldspar, with argillaceous cement. Minor constituents include mica flakes and other dark minerals, along with abundant carbonaceous debris and plant fossils. The rock typically exhibits a massive structure with flat fractures, moderate hardness, well-developed horizontal bedding, and gently undulating laminae.

Petrographic observations indicate that fine-grained rocks in the coal measures are dominated by the following lithotypes:

Feldspathic siltstone with biotite and lithic fragments (Figure 3A1, plane polarized light; Figure 3A2, cross-polarized light): this rock displays a clastic texture with oriented detrital grains. The mineral assemblage includes quartz, plagioclase, microcline, mudstone fragments, acidic extrusive rocks, and biotite. Biotite flakes are preferentially aligned parallel to bedding. Intergranular spaces are filled with clay matrix and ferruginous cement, and the argillaceous matrix occurs in thin layers and is locally iron-stained.

Biotite-bearing feldspathic lithic siltstone (Figure 3B1, plane polarized light; Figure 3B2, cross-polarized light): this lithology also exhibits a clastic structure with well-oriented grains. Its composition includes quartz, plagioclase, shale fragments, acidic extrusive rocks, and biotite. Biotite shows strong preferred orientation along bedding. The intergranular spaces are filled with clay matrix and iron cement, and the thin argillaceous matrix is aligned parallel to lamination.

Calcareous-cemented lithic arkose fine sandstone (Figure 3C1, plane-polarized light; Figure 3C2, cross-polarized light): the rock has a clastic structure with oriented grains and is composed of quartz, plagioclase, shale fragments, siltstone fragments, acidic extrusive rocks, and biotite. Biotite shows good orientation and partial chloritization. Intergranular pores are filled with calcareous and ferruginous cements. During diagenesis, some plagioclase grains were partially replaced by calcite, forming relict crystals or pseudomorphic textures.

Calcite-bearing feldspathic fine siltstone (Figure 3D1, plane-polarized light; Figure 3D2, cross-polarized light): this rock displays a clastic texture with oriented grains and consists mainly of quartz, plagioclase, and biotite. Intergranular spaces are filled with clay matrix, carbonaceous material, calcareous cement, and iron cement. Diagenetic alteration includes partial replacement of plagioclase by calcite and localized sericitization of clay minerals.

Figure 3.

Clastic lithologic petrographic characteristics of fine-grained rocks. Note: A1–A2. Biotite-bearing lithic feldspathic siltstone: the rock is composed mainly of quartz, plagioclase, microcline, mudstone, acidic extrusive rocks, and biotite. Biotite is well oriented and distributed parallel to bedding. Intergranular spaces are filled with an argillaceous matrix and ferruginous cement. B1–B2. Biotite-bearing lithic feldspathic siltstone: the constituent minerals include quartz, plagioclase, mudstone (shale), siltstone, acidic extrusive rocks, and biotite. C1–C2. Calcite-cemented lithic feldspathic fine sandstone: composed of quartz, plagioclase, mudstone (shale), siltstone, acidic extrusive rocks, and biotite. Biotite shows good preferred orientation and partial chloritization. D1–D2. Calcite-bearing feldspathic fine siltstone: the rock exhibits a clastic structure with oriented detrital grains. The principal components are quartz, plagioclase, and biotite. Intergranular spaces are filled with argillaceous matrix, carbonaceous material, and calcareous and ferruginous cements.

Clay minerals are highly sensitive to paleoclimatic conditions due to their fine grain size, variable composition and structure, and their depositional differentiation and assemblage characteristics, which make them effective indicators of paleosedimentary environments (Zhang Tianfu et al., 2016). The presence of kaolinite is generally interpreted as evidence of weakly acidic conditions and intense chemical weathering under warm and humid climates. (Zhang Tianfu et al., 2016).

In the Yan'an Formation, clay minerals are dominated by kaolinite, which exhibits generally good crystallinity (Figure 4A), indicating a warm and humid depositional climate. Minor amounts of chlorite (Figure 4B), illite (Figure 4C), and montmorillonite (Figure 4D) are also present, indicating that the clay minerals underwent varying degrees of diagenesis modification under specific post-depositional conditions.

Figure 4.

Major clay mineral types in the Yan'an Formation of the study area.

Kaolinite (Kln): an aluminosilicate mineral composed mainly of Si and Al and typically formed by feldspar alteration. SEM observations show well-crystallized hexagonal plate-like kaolinite, commonly occurring within intergranular pores or on grain surfaces. Locally, kaolinite plates overlap to form worm-like aggregates. Kaolinite often coexists with minerals such as ilmenite and chlorite, indicating formation under warm and humid climate conditions (Figure 4A).

Chlorite (Chl): composed mainly of Si, Al, Fe, and Mg, chlorite in the Yan’an Formation is rich in iron and magnesium and is commonly derived from the alteration of biotite. Under SEM, chlorite typically appears as flaky aggregates (Figure 4B).

Illite (Ill): composed primarily of Si, Al, and K, illite generally forms in potassium-rich, alkaline environments and may result from the transformation of kaolinite. SEM observations show illite coexisting with chalcopyrite (Figure 4C).

Montmorillonite (Sme): typically formed from feldspar alteration under alkaline conditions, montmorillonite is relatively stable in such environments. It consists mainly of Si, Al, Ca, and Na and appears as cotton-like aggregates under SEM, indicating formation during late diagenetic alteration (Figure 4D).

Major sedimentary structures of coal measures

Sedimentary structures in the study area are dominated by flow-related structures, with deformation structures occurring secondarily. The coal measures commonly exhibit parallel bedding (Figure 5A) and horizontal bedding (Figure 5B), which are particularly well developed in the roofs and floors of coal seams. Other sedimentary structures include tabular, wedge-shaped, and trough cross-bedding (Figure 5C); climbing ripple cross-lamination (Figure 5D); wavy ripple bedding (Figure 5E); graded bedding (Figure 5F); massive bedding (Figure 5G); scoured-and-filled structures (Figure 5H); and localized deformation structures (Figure 5I).

Figure 5.

Sedimentary structures of the Yan'an Formation in the study area.

Macroscopic fine-grained fabric types and characteristics of coal measures

Basis for classification of fine-grained sedimentary fabric in coal measures

“Sedimentary fabric” refers to the spatial arrangement, orientation, and organization of components within sediments or sedimentary rocks. This concept encompasses three principal aspects. First, it includes the characteristics of material structure, such as the orientation and arrangement of mineral clasts and the relationships among interstitial materials. Second, it involves tectonic or depositional fabric, which describes macroscopic physical structures formed during sedimentation, including bedding styles and stratification. Third, it comprises macro- and micro-lamination types and their spatial relationships with framework components (e.g., rigid mineral particles and organic matter), which represent key elements of fine-grained sedimentary fabric.

At both macroscopic and microscopic scales, sedimentary fabric can be regarded as a system of discontinuous planes and lineations within sedimentary structures. Accordingly, fine-grained sedimentary fabric reflects the continuous spatial arrangement of planes and line elements in three-dimensional space.

The classification of sedimentary fabric types aims to systematically distinguish different genetic types based on geometry, scale, internal structure, and relationships with adjacent strata. In coal measures, the classification of fine-grained sedimentary fabric types is based on the following criteria:

Mineralogical composition, including the relative proportions of clay minerals, rigid (siliceous or feldspathic) particles, or organic matter, and other mineral components, as well as their morphologies and symbiotic relationships.

Sedimentary structural characteristics, referring primarily to macroscopic features of fine-grained fabric, such as bedding style (e.g., horizontal or wavy lamination) and the degree of biological disturbance.

Microstructural features, including mineral particle size, micro-lamination characteristics, spatial correlations among laminae, and the relationships between organic-matter laminae and inorganic laminae, as well as the presence of micro-cyclic sedimentary structures.

Therefore, the fine-grained fabric of coal measures is composed of three fundamental elements: (1) mineral composition and interrelationships; (2) sedimentary lamination and its spatial organization; and (3) sedimentary dynamics reflected by compositional and structural characteristics, particularly at the microstructural scale.

Types of fine-grained sedimentary fabric in coal measures

Macro- and micro-laminated fabric. Fine-grained laminated fabric represents a comprehensive expression of rock fabric and sedimentary structure. The uneven distribution of mineral components and fine particles within this fabric reflects variations in sedimentary dynamics and is closely linked to petrological, sedimentological, and paleoclimatic conditions. Figure 6 illustrates representative lamination types developed in key fine-grained strata of coal measures (i.e., roofs and floors of coal seams) in the study area.

Figure 6.

Sedimentary structural characteristics of coal-measure cores.

Micro-laminated cycles commonly exhibit regular variations in color, grain-size grading, and fine sedimentary structures. Among these, organic-rich laminae are of particular significance, as they record subtle changes in the depositional environment during micro-sedimentary cycles. Dark laminae enriched in organic matter are especially prominent and are mainly composed of carbonaceous, argillaceous, and fine-grained detrital materials.

Particular emphasis is placed on the distinctive significance of organic matter laminae within micro-laminated structures, which constitute a key innovative aspect in the classification of fine-grained sedimentary fabrics in coal measures. In addition to the general characteristics of fine-grained fabrics, these organic-rich laminae highlight the unique role of organic matter within micro-sedimentary cycles, reflecting subtle variations in depositional environments recorded by coal-measure lamination.

Of particular importance are the dark laminae enriched in coal-type organic matter. These laminae are characterized by distinctive fabric assemblages formed through the spatial association of carbonaceous material, argillaceous components, and fine-grained detrital particles, representing a critical feature of fine-grained sedimentary fabrics in coal-bearing strata (Figure 6).

Microstructural fabric types and characteristics. Based on mineral composition, structural attributes, and sedimentary features, the fine-grained sedimentary fabric of coal measures can be classified into the following principal types (Figure 7):

Carbonaceous clayey fabric (Figure 7A): This fabric is dominated by clay minerals (e.g., kaolinite, illite, and montmorillonite), typically accounting for more than 50% of the mineral composition. It commonly develops in swamp or low-energy, hydrodynamically stable environments. The fabric is characterized by massive to weakly foliated structures and uneven bedding, with localized enrichment of carbonaceous laminae that provide internal support. Plant debris and occasional pyrite nodules are commonly present.

Siliceous clayey mixed fabric (Figure 7B): In this fabric, quartz (20–50%) is closely associated with clay minerals, reflecting deposition in delta-front or shallow lacustrine environments with relatively weak hydrodynamic conditions. The fabric is characterized by irregular micro-bedding formed by the interlayering of siliceous and clayey materials. Siliceous minerals serve as the skeletal framework of the fabric, and bioturbation structures may locally occur.

Organic-matter-clayey composite fabric (Figure 7C): This fabric contains abundant organic matter (TOC >5%) and exhibits a sandy-argillaceous structure composed of argillaceous and felsic clasts with particle sizes smaller than 0.05 mm. The argillaceous components show uneven sericitization, mudstone clasts are commonly iron-stained, and felsic clasts are relatively evenly distributed. Local iron enrichment occurs in granular form. A prominent laminar structure is developed, with organic matter and inorganic minerals interbedded in strip-like layers. This fabric is typically formed in semi-deep lacustrine environments.

Clayey silty fabric (Figure 7D): This fabric is dominated by silt-sized particles (>50%) and contains relatively abundant clay minerals, including kaolinite, illite, and montmorillonite. Detrital components include biotite-bearing feldspar, quartz, plagioclase, acidic extrusive rocks, and mudstone fragments. Plagioclase grains exhibit moderate clay alteration, biotite flakes are preferentially oriented parallel to bedding and locally chloritized, and thin argillaceous laminae occur intermittently.

Figure 7.

Types and characteristics of microstructures in coal-measure fine-grained rocks. Note: (A) Carbonaceous peat sedimentary fabric: the rock as a whole exhibits an argillaceous structure and is composed primarily of clay minerals with a small amount of carbonaceous debris. The clay matrix shows uneven sericitization, and the carbonaceous debris is irregularly distributed along bedding planes. Local enrichment of carbonaceous material forms carbonaceous laminae, which provide structural support for the lamination. (B) Siliceous clayey mixed fabric: overall, the rock displays a clastic structure. The clastic particles consist mainly of quartz, plagioclase, microcline, argillite (shale), siltstone, acidic extrusive rocks, and biotite. Biotite is locally enriched, well-oriented, and partially chloritized. The intergranular spaces are filled with an argillaceous matrix and ferruginous cement, resulting in a dense rock with little to no pore space. (C) Organic matter-clayey composite fabric: the rock exhibits a sandy argillaceous structure composed of clay minerals and felsic clasts with particle sizes smaller than 0.05 mm. The clay matrix shows uneven sericitization; mudstone clasts are infected by iron staining, while felsic clasts are relatively uniformly distributed. Iron is locally enriched and occurs in granular form. (D) Clayey silty fabric: this fabric is represented by biotite-bearing clastic feldspathic siltstone composed of quartz, plagioclase, acidic extrusive rocks, mudstone, and biotite. Plagioclase is moderately altered to clay minerals, and biotite is aligned parallel to bedding, with some grains showing chloritization. Mudstone layers occur intermittently as thin beds.

Main sedimentary facies and evolution of coal measures

Lithofacies association of coal measures

Lithofacies association represents the products of continuous sedimentary processes operating with specific depositional environments. Different lithofacies associations and their vertical successions can be used to infer sedimentary microfacies and environmental evolution (Sun Shi, 2019). Analyses of mineral and rock composition, together with sedimentary structures, form the foundation of lithofacies interpretation and provide the basis for reconstructing depositional environments and paleogeography.

Based on lithology, sedimentary structures, logging responses, and microfabric characteristics, the Yan'an Formation in the study area is subdivided into four lithofacies assemblages: FA1, FA2, FA3, and FA4. The characteristics of each association are described below and illustrated in Figure 8 .

Figure 8–1.

Lithofacies association types of the Yan’an Formation in the study area. Note: (A) FA1, ZK04, 665.75–678.34 m, Yan’an Formation, member 2; (B) FA2, ZK22, 647.8–665 m, Yan’an Formation, Member 2; (C) FA3, ZK22, 589–603.7 m, Yan’an Formation, Member 4; (D) FA4, ZK01, 632.35–644.6 m, Yan’an Formation, Member 3.

Figure 8–2.

FA1 (Gc, Scb, Sh, Sir, Sih, C): this lithofacies association is dominated by thick-bedded, coarse-grained sandstone with large individual bed thicknesses. Sedimentary structures include parallel bedding, massive bedding, and large-scale tabular, wedge-shaped, and trough cross-bedding. The vertical succession commonly exhibits an asymmetric binary structure: coal seams may occur at the top, overlain by thin fine-grained sediments such as mudstone, siltstone, or fine sandstone. Logging curves typically display a box-shaped pattern. FA1 represents braided-river depositional systems, mainly developed in parts of the Yan-1, Yan-2, and late Yan-5 members (Figure 8A).

FA2 (Gc, Sm, Sh, Sir, Sih, Mg, C, Mc): this association consists of sandstones with variable grain sizes and displays a typical fluvial binary structure. Grain size generally fines upward, with sandstone and pebbly sandstone in the lower part and sandy mudstone and siltstone in the upper part. Coal seams commonly occur near the maximum lake flooding surface, indicating deposition in meandering-river or delta-plain environments. Logging curves exhibit a bell- or pine-tower-shaped pattern. FA2 is widely distributed throughout the Yan'an Formation and represents one of its most common lithofacies associations (Figure 8B).

FA3 (Sir, Sih, Mg, C, Mc): FA3 is mainly composed of interbedded siltstone, fine sandstone, sandy mudstone, and coal seams. Coal seams are relatively well developed and commonly occur as multiple layers, forming sandy mudstone-coal seam assemblages. This association is interpreted as having formed in floodplain, alluvial plain, or delta-plain environments. Logging curves are characterized by low-amplitude zigzag patterns, except for coal seams, which display high-amplitude finger-shaped responses. FA3 is mainly developed in the third and fourth members of the Yanchang Formation (Figure 8C).

FA4 (Mg, Sir, Scb, C, Sm): In FA4, sandy mudstone dominates the lower part of the succession, while sandstone is more upward. Sandstone layers exhibit parallel bedding, cross-bedding, and rhythmic bedding, forming an inverse rhythmic sequence that coarsens upward. Logging curves display a funnel-shaped pattern, indicating gradual weakening hydrodynamic conditions from bottom to top. This lithofacies association typically represents delta-front mouth-bar deposits (Figure 8D).

It can be clearly recognized that fine-grained lithofacies fall within the category of lithofacies classification, whereas sedimentary fabric pertains to the domain of material composition and its intrinsic relationships, reflecting fundamentally different emphases. Sedimentary fabric is defined on the basis of the primary characteristics of fine-grained materials and the structural relationships among their components, and thus represents one of the most fundamental and essential aspects of sedimentological analysis.

In contrast, the analysis and classification of fine-grained lithofacies are primarily aimed at reconstructing sedimentary paleogeography, operating at a broader spatial scale than sedimentary fabrics and integrating multiple fabric characteristics into a more comprehensive geological framework.

Characteristics and evolution of the main sedimentary facies

Based on integrated analyses of petrology, sedimentary structures, and fine-grained fabric classification of the coal measures, vertical and horizontal sedimentary facies profiles were constructed to illustrate the spatial variation of coal seams and coal-measure sedimentary fabrics within the study area. Although these profiles are similar in form to sedimentary facies sections used in paleogeographic reconstructions, they place particular emphasis on the spatial distribution and evolutionary characteristics of fine-grained sedimentary fabrics within coal seams and associated strata.

In this study, cross-section 3 and longitudinal-section 2 were selected to represent the sequence stratigraphic framework and sedimentary facies evolution of the Yan’an Formation.

Cross-section 3 (Figure 9) is located near the central part of the basin, where stratal thickness increases significantly. During the LSC1 stage, sedimentation was significantly reduced or absent in the central and eastern areas. Braided-river deposits were mainly developed in the central and western regions, while most of the central and eastern areas experienced non-disposition or erosion. During LSC2, meandering-river systems were present in the western region but were absent in the central and eastern parts during the MSC3 stage. Meandering rivers became widespread during MSC4. In the LSC3 stage, meandering rivers dominated the western area, floodplains developed mainly in the central area, and floodplain-lacustrine environments prevailed in the eastern area. During LSC4, braided rivers dominated the western region, whereas meandering rivers developed in the central and eastern regions. In LSC5, meandering-river deposits were mainly preserved in the western area, while the central and eastern strata were largely denuded.

Figure 9.

Sequence stratigraphic division and sedimentary facies profile of the Yan'an Formation along the E–W direction (cross-section 3).

Longitudinal-section 2 (Figure 10) reveals five long-term base-level cycles, with four cycles developed in both the southern and northern parts of the study area. During LSC1, braided-river systems were mainly developed in the south-central region, while paleohighs dominated the southern and northern margins, resulting in limited sedimentation. In the LSC2, meandering-river systems expanded across most areas, except for the northern highlands, where sedimentation was minimal. By LSC3 and LSC4, basin infilling and topographic leveling were largely completed, leading to the development of extensive delta-plain environments under relatively flat terrain conditions. During LSC5, meandering-river deposits again became the dominant sedimentary facies.

Figure 10.

Sequence stratigraphic division and sedimentary facies profile of the Yan'an Formation along the N–S direction (longitudinal section 2).

Conclusions and perspectives

Analysis of the sedimentary fabric of the Yan'an Formation demonstrates that the fine-grained lithological composition of coal measures constitutes the material foundation of sedimentary fabric development. Fabric classification primarily depends on the spatial relationships between macro- and micro-scale components, particularly the interaction between framework-forming materials and sedimentary structures. Therefore, research on fine-grained sedimentary fabrics and their classification in coal measures has the following major scientific and practical significance:

The classification of sedimentary fabric types in coal measures is of fundamental significance in that it establishes a critical linkage between the “morphology” and “genesis” of material composition and structural characteristics. Such classification is not merely a taxonomic exercise of fabric types, but also serves as a key approach for reconstructing the sedimentary evolution history, restoring paleoenvironmental conditions, and guiding the exploration and development of coal-measure resources.

Fine-grained lithofacies and sedimentary fabrics represent distinct conceptual terms with clear differences in definition and scope. Fine-grained lithofacies fall within the domain of lithofacies analysis, whereas sedimentary fabrics pertain to the composition of materials and their intrinsic structural relationships, reflecting fundamentally different emphases.

Sedimentary fabrics are established on the basis of the primary characteristics of fine-grained materials and their compositional and structural relationships, constituting one of the most fundamental and essential components of sedimentological research. In contrast, the analysis and classification of fine-grained lithofacies are primarily aimed at reconstructing sedimentary paleogeography, operating at a broader spatial scale compared to sedimentary fabrics.

According to mineralogical composition—particularly the abundance of key minerals—together with textural characteristics and sedimentary structural features, the fine-grained sedimentary fabrics of coal measures can be classified into carbonaceous–clayey fabric, siliceous–clayey mixed fabric, organic matter–clayey composite fabric, and clayey–silty fabric. Each type of fabric represents the product of a specific depositional environment.

On the basis of the classification of fine-grained sedimentary fabric types in coal measures, further analyses were conducted on lithofacies associations, dominant sedimentary facies, and their variation characteristics. The results indicate that the study of fine-grained sedimentary fabrics in coal measures is of significant importance for reconstructing the paleogeography of coal-forming basins and for evaluating the potential of fine-grained reservoirs.

The value and practical significance of identifying and classifying fine-grained sedimentary fabrics in coal measures are mainly manifested in two aspects:

Application to the prediction of unconventional gas reservoirs: Different sedimentary fabrics exert significant control over the physical properties of reservoirs. Accurate identification of fine-grained fabric types in coal measures facilitates the establishment of refined geological reservoir models. In particular, the classification of fine-grained sedimentary fabrics provides a fundamental basis for understanding gas storage and migration conditions in coal-measure systems.

Guidance for source–reservoir–seal assemblage evaluation: The classification results of coal-measure sedimentary fabrics, combined with analyses of lithofacies and sedimentary facies, enable the determination of the spatial and temporal distribution of favorable source rocks, reservoirs, and cap rocks. This integrated approach provides an effective framework for guiding hydrocarbon exploration and deployment strategies.

In summary, detailed analysis of coal-measures sedimentary fabrics is not only a core component of sedimentological research but also a critical bridge between geological theory and practical applications. Studies of fine-grained fabrics in coal measures are of great significance for paleogeographic reconstruction of coal-forming basins and for the evaluation and development of fine-grained reservoirs.

Footnotes

Author contribution

This article is jointly written by the authors. YL was the organizer of the article and the author of the first draft, and divided and classified the laminae. HZ and XLH provided specimens and data for experimental analysis, and carried out fine-grain deposition analysis. YW and YL analyzed and studied the fine-grained facies.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This research was supported by funds provided by the National Science Foundation of China (42372201), Open Research Fund of Geophysical Prospecting Survey Team of Shandong Coalfield Geology Bureau 2024.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

ORCID iD

Ying Li

References

Aplin

Macquaker

JHS

(2011) Mudstone diversity: Origin and implications for source, seal, and reservoir properties in petroleum systems. AAPG Bulletin 95(12): 2031–2059.

Gong

(2016) Sedimentary fabrics for the Cambrian thrombolite bioherm: An example from the Zhangxia Formation in Western Shandong Province. Geoscience 30(02): 436–444.

Gong

Benpei

Shiyu

, et al. (1997) Marine Devonian ichnofabrics of China and their relations to sedimentary sequences. Earth Science-Journal of China University of Geosciences 02(9–14): 113–114.

Huang

Wei

Liubin

, et al. (2021) Reconstruction of carbonate sedimentary palaeo-environment by using rock fabric, biological remains and geochemistry: A case study of the Ma 5-5 submember at Guanjiaya, Xingxian, Shanxi, China. Journal of Chengdu University of Technology 48(5): 539–548.

Huang

Sun

Chang

, et al. (2022) Experimental investigation of pore characteristics and permeability in coal-measure sandstones in Jixi Basin, China. Energies 15(16): 5898.

Jiang

Chao

Jing

, et al. (2013) Several issues in sedimentological studies on hydrocarbon-bearing fine grained sedimentary rocks. Acta Petrolei Sinica 34(6): 1031–1039.

Tang

, et al. (2025) Source reservoir configuration and geological geochemical control of coal and shale. Scientific Reports 15(1): 5753.

Cui

Zhang

, et al. (2026) Experimental study on dynamic occurrence and migration of gas and water in deep coal measure reservoirs in Yan’an area of Eastern Ordos Basin. Fuel 405: 136654.

Liang

Xinglong

Jiamin

, et al. (2023) Sequence and coal-forming paleogeographic characteristics of Yan’an Formation in the Northwestern Margin of Dongsheng Coalfield. Coal Geology of China 35(12): 10–16.

10.

Mei

Rongtao

Yuan

(2011) Sedimentary fabrics for the stromatolitic bioherm of the Cambrian Gushan Formation at the Xiaweidian section in the western suburb of Beijing. Acta Petrologica Sinica. Acta Petrologica Sinica 27(08): 2473–2486.

11.

Rui

, et al. (2018) Lithofacies distribution and gas-controlling characteristics of the wufeng–longmaxi black shales in the southeastern region of the Sichuan Basin, China. Journal of Petroleum Science and Engineering 165: 269–283.

12.

Peng

Yiming

Huimin

, et al. (2022) Sedimentary characteristics and genetic mechanism of fine-grained hybrid sedimentary rocks in continental lacustrine basin: A case study of the upper submember of Member 4 of Shahejie Formation in Chenguanzhuang area, southern slope of Dongying sag. Acta Petrolei Sinica 43(10): 1409–1426.

13.

Qin

(2018) Research progress of symbiotic accumulation of coal measure gas in China. Nature Gas Industry 38(4): 26–36.

14.

Qin

(2020) Progress in research and thinking on coal measures gas (CMG) paragenetic system based on paragenesis theory. Coal Geology of China 2020(9): 26–32, 58

15.

Qin

Jian

Yulin

(2016) Joint mining compatibility of superposed gas-bearing systems: A general geological problem for extraction of three natural gases and deep CBM in coal series. Journal of China Coal Society 41(1): 14–23.

16.

Stow

DAV

Hue

Bertrand

(2001) Depositional processes of black shale in deep water. Marine and Petroleum Geology 18(4): 491–498.

17.

Sun

(2015) Association Study on Micro Fabric and Mechanical Properties of Terrestrial Coal Series Mudstone. Xuzhou: China University of Mining and Technology.

18.

Sun

Chaoliang

Feng

, et al. (2010) Petroleum exploration and development practices of sedimentary basins in China and research progress of sedimentology. Petroleum Exploration and Development 37(4): 385–396.

19.

Sun

Chaoliang

Feng

, et al. (2015) Innovations and challenges of sedimentology in oil and gas exploration and development. Petroleum Exploration and Development 42(2): 1–8.

20.

Sun

(2019) Study on sedimentary facies characteristics and evolution of the He-8 member in the northeastern margin of Ordos Basin. Chengdu University of Technology.

21.

Tang

(2013) Mesoproterozoic microbialites from North China Platform: microfabrics, organomineralization processes and their palaeoenvironmental distribution. China University of Geosciences (Beijing).

22.

Tang

Zhou

Chen

, et al. (2021) Development characteristics of shale lithofacies in the Longmaxi Formation and their main controlling factors in the Changning area, south Sichuan Basin, SW China. Frontiers in Earth Science 9: 775657.

23.

Wang

Carr

(2013) Organic-rich Marcellus Shale lithofacies modeling and distribution pattern analysis in the Appalachian Basin. AAPG Bulletin 97 (12): 2173–2205.

24.

Wang

Carr

, et al. (2014) Identifying organic-rich Marcellus Shale lithofacies by support vector machine classifier in the Appalachian basin. Computers & Geosciences 64: 52–60.

25.

Wang

(1996) Accumulation Regularity of Coal and Assessment for Coal Resources in Ordos Basin. Beijing: Coal Industry Press, 162–178.

26.

Jiang

, et al. (2018) Pore structure characterization of different lithofacies in marine shale: A case study of the Upper Ordovician Wufeng-Lower Silurian Longmaxi formation in the Sichuan Basin, SW China. Journal of Natural Gas Science and Engineering 57: 203–215.

27.

Rukai

Ping

, et al. (2013) Advance on sedimentology and sequence stratigraphy: A summary from 18th international sedimentology congress. Acta Sedimentologica Sinica 29(1): 199–206.

28.

Xiao

Hao

Yinglun

, et al. (2020) Sedimentary fabrics and environmental characteristics of Leiolite in Cambrian: A case study from the Changshan Formation in Laiyuan city, Hebei province. Acta Sedimentologica Sinica 38(01): 76–90.

29.

Xie

(2009) Research on the Quaternary fine-fraction lithofacies and sedimentation model in Tainan Area, Qaidam Basin. Earth Science Frontiers 16(5): 245–250.

30.

Yang

Xue

, et al. (2023) Pore structure characteristics of shale in coal-bearing strata: In the case of the Shanxi formation of lower Permian in Huainan coalfield in China. Energy Exploration & Exploitation 41(5): 1539–1558.

31.

Yang

Huamei

(2000) Magnetic anisotropy and its environmental significance in limnal faulted basin–taking the Nihewan basin as an example. Marine Geology & Quaternary Geology (03): 43–52.

32.

. Mechanism of Natural Gas Differential Enrichment in the Transition Zone at the Margin of the Basin: A Case Study on the Hangjinqi Area in the Northern Ordos Basin. China University of Geosciences.

33.

Zhai

(2025) Research on the Deep Water Fine grained Sedimentary System of the Chang73 sub-member in the Jiyuan Area of the Ordos Basin. Xi’an Shiyou University.

34.

Zhang

Weixing

Yufeng

(2004) Ichnofabric characteristics of fluvial and lacustrine sediments of the Upper Cretaceous in Xixia Basin, Henan Province. Journal of Palaeogeography.

35.

Zhang

Lixin

Yun

, et al. (2016) Geochemical characteristics of the Jurassic Yan’ an and Zhiluo Formations in the Northern Margin of Ordos Basin and their paleoenvironmental implications. Acta Geologica Sinica 90(12): 3454–3472.

36.

Zhu

Shengqin

Zhenhan

, et al. (2000) Tectonic features of the compressional-extensional systemand dynamic analysis of rock fabric in Yunmengshan Area, Beijing. Acta Geoscientia Sinica (04): 337–344.

37.

Zhu

Xiangang

Xiangyang

, et al. (2003) Tectonic features of metamorphic core complexes in Yiwulushan Area Liaoning Province and a dynamic analysis of rock fabrics. Acta Geoscientia Sinica (03): 225–230.

38.

Zhu

Caineng

Bin

, et al. (2011) Progresses in the global petroleum exploration and its demand in reservoir research. Advances in Earth Science 26(11): 14–25.

39.

Zhu

Sun

Zhang

, et al. (2025) Advances and trends in sedimentological research in oil and gas exploration and development in China. Acta Sedimentologica Sinica 43(1): 1–19.

40.

Zhu

Sun

Zou

, et al. (2026b) Review of the classification and related terminology of fine-grained sedimentary rocks. Petroleum Exploration and Development 53(01): 52–66.

41.

Zhu

Zhang

Wan

(2026a) Non-marine shale sedimentology: Perspectives on lithofacies palaeogeography research. Journal of Palaeogeography (Chinese Edition) 28(01): 68–86.

42.

Zou

Qiu

(2021) New advances in unconventional petroleum sedimentology in China. Acta Sedimentologica Sinica 39(01): 1–9.