Krumbein was the first to propose the concept of fine- grained sediment based on rock grain-size analysis, suggesting the boundary between coarser-grained materials and fine-grained materials to be 0.062 5 mm (or 4ϕ) ^[49]. Fine-grained sedimentary rocks refer to clastic sedimentary rocks have a grain size smaller than 0.062 5 mm (content higher than 50%). They are primarily composed of fine-grained materials such as clay and silt, along with minor amounts of particles including authigenic carbonate minerals, biogenic silica, and phosphate. These rocks constitute over 2/3 of all sedimentary rock types globally distributed. Subsequently, definitions and classifications have been provided by Picard, Weaver, and Lundegard ^[50-52]. Gorsline introduced the concept of “fine-grained sedimentology”, defining it as the fundamental discipline that studies the composition, texture and structure, classification and origin, sedimentary processes, and distribution patterns of fine-grained sedimentary rocks ^[53].

Similar definitions of fine-grained sedimentary rocks have been adopted domestically. According to the textbook Sedimentary Petrology published by YU et al., as well as the textbook Fine-grained Sedimentary Petrology published by HU et al. in recent years, fine-grained sedimentary rocks are defined as sedimentary rocks in which sediments with a grain size below 0.062 5 mm constitute more than 50% of the rock volume. These fine-grained sediments include terrigenous clastics, volcaniclastics, intrabasinal particles, bioclasts, authigenic minerals, lime mud, dolomitic mud, siliceous mud, and organic matter within the clay-sized (less than 0.003 9 mm) to silt-sized (0.003 9-0.062 5 mm) range ^[37,42].

Overall, the concept of fine-grained sedimentary rocks is clear. Rock types include mudstone, mudrock, shale, claystone, silty mudstone (shale), argillaceous siltstone, siltstone, etc. However, a universally accepted and widely applied classification system has not yet been established. Particularly, many of the involved terms lack rigorous definitions, often leading to confusion in research.

The publication of Sedimentary Petrology textbooks in China can be broadly divided into three stages. This section outlines the main viewpoints related to fine-grained sedimentary rocks, showing the gradual transition in domestic textbooks from describing fine-grained sedimentary rocks as claystone to mudstone comprising siltstone and claystone, essentially in a chronological order ^{[13-14,21,27 -33,37 -40,42,54 -56]} (Table 4).

Terms such as laminae, lamina sets, bed sets, bedding, and layers/strata are among the most frequently used in the description of sedimentary rocks. These features are commonly identified based on vertical variations in composition, texture, color, and other related characteristics ^{[13,20,29,37,66 -71]}. In the 1960s, textbooks used in China primarily referred to those edited by ПОЛОВИНКИНА Ю И in the former Soviet Union ^[61]. Subsequently, with the continuous accumulation of observational data and the advancement of research, these concepts have been progressively refined and revised, while remaining generally consistent overall (Table 5).

In international sedimentological literature, Campbell defined a stratum as a rock layer clearly bounded by distinct upper and lower bedding planes. Sedimentary layers or strata were further classified into four types: laminae, lamina sets, beds, and bed sets ^[66]. O’Brien et al. defined a lamina as the thinnest recognizable unit in sediments or sedimentary rocks, primarily identified on the basis of color, composition, and grain size, with a thickness of less than 1 cm (typically 0.05-1.00 mm) ^[67]. Investigations of lamina depositional mechanisms have been closely associated with the development of lacustrine paleoclimate studies, sedimentary geology, and hydrocarbon exploration since the 20th century ^[68] (Fig. 1). Stow regarded bedding (thickness greater than 1 cm) and lamination (thickness less than 1 cm) as descriptive terms for sedimentary stratification or layering. In this framework, a stratum is a genetic term without a strictly defined thickness, whereas lamination represents a common type of layered structure within beds. Laminae and beds typically reflect distinct or subtle variations in grain size, composition, and color, resulting from changes in depositional processes, sediment supply, or depositional environments ^[36]. Lazar et al. suggested that bedding records sediment input and accumulation, basal energy conditions, and bioturbation, and encompasses laminae, lamina sets, and beds ^[69].

Biddle et al. systematically clarified the relationship between bedding and laminae, noting that organic-rich mudstones are commonly described as thin-bedded, laminated, or fissile ^[12]. Analogous to the distinction between clay minerals and clay-sized particles, the application of the term “bed” in mudstones often leads to conceptual ambiguity. In the literature, “bed” is frequently used in a dual sense: first, as a depositional unit with a thickness greater than 1 cm, and second, as a genetic depositional unit representing a single or discrete depositional event ^[70]. Although both meanings are applicable to coarse-grained siliciclastic rocks, in fine-grained siliciclastic rocks—particularly clay-mineral-dominated sediments—many depositional units are thinner than 1 cm, with bed thicknesses predominantly in the range of 1 to 4 mm ^[69]. Therefore, when describing mudstones, it is recommended that the term “bed” be used to denote an individual event bed, whereas “laminae” should refer to the internal fissile subdivisions within these units, irrespective of absolute thickness.

In summary, the concepts of bedding and laminae are generally well defined and broadly consistent in either international and Chinese textbooks or monographs. However, differences exist in the criteria used to classify the thickness of strata or bedding ^{[2,13,20 -21,25,29,33,36,71 -74]}. International textbooks commonly adopt a thickness-based criterion, in which bedding is defined as having a thickness greater than 1 cm, whereas lamination is defined as having a thickness less than 1 cm ^[2,25]. In contrast, Chinese textbooks tend to emphasize that bedding and layers/strata belong to two distinct conceptual systems, resulting in inconsistencies in the standards used for thickness classification ^[13,33] (Table 6).

The concept of “bedding” is clearly and consistently defined in both international and Chinese textbooks and literature, whereas some scholars use “stratification” as the same meaning ^{[2,25 -26,36]}, except a slight difference in thickness - bedding refers to layers thicker than 1 cm, and lamination refers to layers thinner than 1 cm. HAN Shuhe’s translation of The Field Description of Sedimentary Rocks indicates that stratification in a broad sense includes both bedding (thicker than 1 cm) and lamination (thinner than 1 cm) ^[65]. ZHAO Chenglin et al. defined horizontal fine layers with thicknesses less than 1 mm as lamination ^[29]. SUN Honglie et al. defined lamination as the stratified nature of sediments composed of laminae, commonly manifested by variations in composition, grain size, and color, reflecting weak periodic fluctuations in depositional environments ^[70]. ZHAO Wenzhi et al. proposed that a lamina represents the smallest unit formed by stratified variations in sediment input as well as biological and biogeochemical processes within depositional environments; frequent occurrence of laminae results in lamination, and when lamina boundaries coincide with bedding planes, lamination is equivalent to bedding ^[71]. Notably, some Chinese authors regard lamination as the same as lamina ^[37].

Based on the discussion above, the following terminology is recommended: lamina or laminae for “Wenceng”, lamination for “Wenli”, and laminated for “Wencengzhuang”.

Fissility is a widely used term in the description of fine-grained sedimentology. The 1920 publication a Brief Account of China’s Mineral Resources by WENG Wenhao represents a landmark work that first systematically and definitively incorporated the concept of shale into the Chinese geological lexicon. In Chapter 2 Classification of Strata, the term fissility was explicitly introduced and defined for the first time. When describing Sinian system siliceous limestone, WENG noted that it also contains thin-bedded limestone with well-developed fissility. More importantly, in his description of Paleozoic strata, he provided a definitional statement for shale: Shale is consolidated clay; its fabric resembles the leaves of a book, hence its name. This statement clearly established that shale derives its name from its characteristic fissility (a leaf- or book- like planar fabric or layering). This marked the formal establishment of fissility as a specialized term describing a rock’s physical property in the Chinese publications ^[75]. Subsequent textbooks and monographs have consistently included descriptions of fissility and shale (Table 7).

It should be noted that some Chinese textbooks published from the 1960s to the 1990s cited Flawn’s classification scheme for argillaceous rocks based on recrystallization and rock-fabric characteristics, in which “parting” was translated as an equivalent to bedding or fissility ^[76]. An early Chinese bilingual paper entitled Some Aspects of the Palaeoecology of Non-marine Faunas and Rates of Sedimentation in the Lancashire Coal Measures used fissile and fissility ^[77], and other terms such as lamellae and foliation were presented as similar concepts ^[78]. However, the latter two words belongs to metamorphic petrology. By around 2017, with the rapid advancement of shale oil and gas exploration and development, Chinese publications showed increasingly diverse English equivalents for bedding, bed, laminate, lamination, and fissile. In the Chinese translation of Pettijohn’s Sedimentary Rocks by LI Hanyu et al., fissility was translated as “Pikaixing” and fissile as “Boliezhuang” ^[64]. Some textbooks and monographs also mention lamellation as an equivalent for “Yeli” ^[34,72].

In fact, international literature presents three distinct formulations for defining and describing shale and fissility. (1) Laminated clayey rock ^[79] - this concept was used in the earliest descriptions of shale. (2) Fissile-fissility ^{[23,47,67,77,80 -81]} - Ingram discussed fissility in mudrocks as the tendency to split along approximately parallel surfaces, attributed to the preferred parallel alignment of clay crystallites within the rock ^[81]. O’Brien further noted that fissility refers to the ease with which a rock separates into thin sheets along parting surfaces that are parallel or subparallel to bedding (e.g., bedding partings in shale or cleavage partings in schist), and that it is a key criterion distinguishing shale from mudstone ^[67]. Some studies suggest that fissility develops through the long-term combined effects of stress release, moisture loss, and weathering ^[80-84]. Moreover, fissility in mudstone is often misinterpreted as being caused by macroscopically visible bedding or lamination ^[83-84]. Fissile mudstones may be laminated or thinly bedded; in other cases, primary fabrics can be completely homogenized by bioturbation, whereas lamination may also become apparent only after later compaction ^{[11,85 -87]}. Weaver emphasized that the shale definition incorporates the structural term fissile, the development of which is closely related to the preferred alignment of laminae, platy minerals, and organic matter. He argued that fissility is governed not only by particle orientation and bedding but also by weathering, temperature, and moisture content. Notably, he proposed that shale does not exist at depth but only as potential shale; that is, fine-grained rocks in the subsurface may develop fissility and meet the shale definition only after overburden reduction and subsequent processes such as weathering or dehydration ^[80]. Therefore, fissility should be used strictly as a descriptive term and applied only to mudstones that exhibit evidence of splitting (or weathering-related parting) along bedding-parallel planes of weakness ^[69,82,84]. (3) Laminated or fissile ^[14] and laminated and fissile ^[22,25]. Factors promoting fissility include an increase in total clay-mineral content, which allows clay particles to rotate and align parallel under overburden pressure; settling of clay particles parallel to the sediment-water interface; preferred orientation of clay minerals under weak electrical currents; penecontemporaneous growth and surface growth of clay minerals parallel to bedding; unloading following erosion, with clay expansion inducing parting; and relatively low carbonate content, high sulfur content, and high carbon content.

Based on the above analysis, fissile, fissility, or lamellation may correspond to the same connotation, denoting the tendency of mudrocks to split into thin sheets or flakes along bedding planes (Fig. 2).

As previously explained, this section outlines the earliest usage and meaning of the term shale in Chinese. In the 1920s, during the formative stage of Chinese geology led by WENG Wenhao, geologists commonly used shale or claystone as collective terms for consolidated argillaceous rocks, with emphasis placed on material composition ^[75]. In 1929, TAN Xichou et al. from the Geological Survey of the Ministry of Agriculture and Commerce of the Nationalist Government proposed the name Nenjiang Formation, whose strata mainly include mudstone, silty mudstone, oil shale, and related lithologies ^[88]. However, mudstone appeared and became established relatively late in Chinese geological literature as an independent rock type parallel to shale. In sedimentology monographs and textbooks published in the early 1950s to early 1960s (e.g., the Lectures on Sedimentary Petrology of the Beijing Geological Institute ^[59]), shale was described as having well-developed thin-bedded fissility and as splitting readily into sheets along bedding, whereas mudstone was characterized as massive, lacking fissility or exhibiting only very weak fissility, with conchoidal or irregular fracture surfaces ^[21]. By the late 1970s to the 1980s, Sedimentary Petrology edited by LIU Baojun provided formal definitions, in which mudstone was defined as a clay rock lacking fissility or with poorly developed fissility, whereas shale was defined as a clay rock with fissility or schistosity ^[24].

Internationally, the term shale has an even earlier origin ^{[79-82,89 -92]}. As early as 1960, Tourtelot reviewed the origin and usage of shale ^[79]. In general, “shale” has three principal connotations. (1) The earliest usage arose in the British mining industry. William Hooson defined shale as consolidated laminated clayey rocks ^[89]. Since the 1980s, this concept has been widely adopted in Chinese textbooks published by comprehensive universities and by the geological-mineral, petroleum, and coal sectors, where shale is commonly described as a special rock type composed mainly of clay minerals and fine clastics with particle sizes less than 0.003 9 mm (content higher than 50%), and characterized by fissility and/or lamination. (2) Shale may also denote a lithostratigraphic unit or a stratigraphic interval, such as the Barnett Shale and Woodford Shale in the United States, or the Wufeng Shale in the Sichuan Basin, China ^[93-96]. In 1924, Lee and Chao proposed the lithostratigraphic unit name Longma Shale, which represents one of the earliest known Chinese naming examples related to shale ^[90]. (3) In addition, shale is used as a collective term for fine-grained sedimentary rocks composed predominantly of particles less than 0.062 5 mm ^{[23,25,36,45,47]}. In this broad sense, shale spans the clay-silt boundary; consequently, many rocks classified as siltstone may also be termed shale, and vice versa.

From the above discussion, it is evident that the usage of the terms shale, mudstone, and mudrock differs between Chinese and international literature (Table 8). HE Qixiang et al. noted that in Europe and North America, mudrocks is commonly used as an umbrella term encompassing claystone and siltstone ^[13]. ZENG Yunfu et al. and other scholars further pointed out that the definition and usage of claystone remain inconsistent in international sedimentology. In the UK and US literature, rocks in which the content of the part with a particle size range less than 0.003 9 mm is greater than 2/3 are classified as claystone, whereas those in which the content of the part with a particle size range from 0.003 9 mm to 0.062 5 mm is greater than 2/3 are classified as siltstone, transitional lithologies between these end members are referred to as mudstone and shale. Collectively, these fine-grained rocks are termed mudrocks or argillaceous rocks ^[29-31]. Authors from the former Soviet Union classified the mud-sized particles (size less than 0.01 mm) content reaches 50% as argillaceous rocks, and further required that particles size greater than 0.001 mm account for no less than 25%.

In recent years, some Chinese textbooks have used mudstone as an umbrella term for the broad category of fine-grained terrigenous clastic rocks ^[37,42]. SUN Honglie et al. defined claystone as a clastic rock with grain size less than 0.003 9 mm, composed predominantly of clay minerals and commonly containing a certain proportion of silt ^[70]. Mudstone was defined as a clastic sedimentary rock in which the combined silt and clay content exceeds 90%, encompassing siltstone, claystone, and transitional lithologies between the two ^[70]. Shale was defined as a mudrock characterized by fissility or interlayer parting, formed from argillaceous sediments in which clay-mineral and silt fractions together exceed 90% through relatively strong and/or prolonged diagenetic compaction; shale accounts for approximately 40%-60% of sedimentary rocks by volume ^[70].

Biddle noted that the term shale is highly contentious ^[12]. Some authors use it as a generic category term for all fine-grained argillaceous sediments, broadly equivalent in rank to sandstone and limestone ^[2,79]. Others use shale and mudstone interchangeably ^[1,35]. Most authors define shale as a mudrock exhibiting fissility (or bedding-parallel parting), whereas mudstone is defined as a massive, non-fissile mudrock ^[2-3,22]. In addition, some authors restrict shale strictly to a descriptive textural term ^[82-83,91]. Selley further argued that shale should be abandoned by geologists and reserved for communication among engineers or managers ^[92].

Mudstone-shale is frequently used in Chinese literature but lacks a clear definition. In most cases, it is employed as a catch-all term when mudstone and shale cannot be unambiguously distinguished. However, the term is generally absent from published textbooks. One exception is Sedimentary Petrology by YU Bingsong et al., in which mudstone-shale is mentioned and interpreted as mudstone with a higher degree of induration ^[37]. Internationally, terms such as mudshale and mud-shale have also been used to denote indurated fine-grained sedimentary rocks with 33%-65% clay and bed thicknesses less than 10 mm ^{[2-3,23,46,50,52,81]}. Nevertheless, these terms are not widely adopted, and therefore the authors recommend avoiding their use.

In some Chinese publications, felsic shale, silty shale or clay quartz is usually referred to as the same Chinese expression (长英质页岩, or Changyingzhiyeyan in Chinese pinyin) ^[93-96]. Notably, felsic was originally established within the terminology of igneous and metamorphic rocks in earlier literature. The term felsic shale is rare within the Euro-American geological literature. It is believed to be comparable with the usage and relevant standards in conventional textbooks, such as silty shale and siliceous shale. The Chinese translation of “siliceous shale” is a standardized geological term in the Chinese framework, referring to shale composed mainly of clay minerals but enriched in siliceous components ^[34].

In the petroleum industry, shale oil and gas reservoirs in Europe and North America are not restricted to fine-grained shales that are clay-rich or silica-rich; many productive intervals are instead fine-grained carbonates or siltstones. Increasing proportions of siliceous and carbonate components generally enhance rock brittleness, which promotes the development of natural fractures and improves fracability during hydraulic fracturing, thereby increasing reservoir stimulability. In a carbonate-silica- clay ternary diagram (including a range of lithologies such as chert, diatomite, radiolarian earth, platy diatomaceous earth or silica, siliceous mudstones chalk columns, and calcareous mudstone on carbonate columns) ^[99], both the siliceous and carbonate end-members fall within the compositional domain of developed “unconventional” reservoirs (Fig. 3).

In recent years, with the steady progress of shale oil and gas exploration and development in China, several professional bodies have issued formal definitions of shale. In the Geological Evaluation Methods for Shale Gas, shale is defined as a fine-grained sedimentary rock composed of detrital particles (grain size less than 0.062 5 mm), clay, organic matter, and related components, and characterized by fissility and brittleness ^[100]. The Geological Evaluating Methods for Shale Oil defines shale as a fine-grained sedimentary rock with grain size less than 0.062 5 mm and with platy or flaky bedding ^[101]. The Shale Oil: Terminology defines shale as a fine-grained sedimentary rock dominated by clay minerals, featuring platy or flaky rhythmic lamination, with grain size less than or equal to 0.062 5 mm. It further defines a shale formation series as a stratigraphic succession consisting of shale together with adjacent, associated, or intercalated thin beds of siltstone, fine sandstone, carbonate rocks, mixed sediments, and related lithologies, in which individual interbeds are less than or equal to 5 m thick ^[102].

It is evident that, across the broader geoscience community and the energy industry, “shale” has been widely adopted as an umbrella term for fine-grained clastic sedimentary rocks. Moreover, descriptive derivative terms such as “shale oil” and “shale gas” have become firmly established in both academic and industrial usage.

As noted by Ulmer-Scholle, numerous classification schemes for fine-grained sedimentary rocks have been proposed historically; however, the schemes discussed above have received only 2-3 citations per year since publication. In contrast, the carbonate-rock classification schemes of Folk and Dunham have been widely adopted, with mean annual citation rates of 26 and 55, respectively ^[103-104]. These comparisons suggest that the development of robust classification schemes for fine-grained sedimentary rocks will require more detailed work and broader community discussion among sedimentologists.

In line with the definitions adopted in most historical Chinese textbooks and the views accepted by the majority of scholars, the authors recommend that, in academic usage, fine-grained sedimentary rocks be employed as an umbrella term for sedimentary rocks composed of particles less than 0.062 5 mm. This category includes claystone/mudrocks and siltstone, the latter being defined by a silt-sized fraction (0.003 9-0.062 5 mm) exceeding 50%. Claystone/mudrocks may be further subdivided into argillaceous mudstone/shale, calcareous mudstone/shale, siliceous mudstone/shale, silty mudstone/shale, and silt- containing mudstone/shale. In this framework, shale is characterized by fissility, whereas mudstone is massive. Argillaceous mudstone/shale is defined by the content of clay minerals or clay-sized particles (grain size less than 0.003 9 mm or less than 0.01 mm) greater than 50%, whereas the other mudstone/ shale types emphasize that the content of the part with a particle size range less than 0.062 5 mm exceeds 50%.

In industrial practice, relevant standards emphasize that shale should be fine grained (less than 0.062 5 mm) and should display fissility ^[100-102]. Nevertheless, in both academic and industrial usage, shale in its ordinary sense should not be taken to include siltstone.

As pointed out by WANG Chengshan et al. ^[105], advances in sedimentological theory have been closely coupled with changes in observational scale. In the early 20th century, research shifted from macroscopic description to micrometer-scale observation under the microscope, which laid the foundation for sedimentology as a discipline. Since the 1970s, the focus expanded from outcrop- and section-scale studies to basin-scale analysis. With the advent of the 21st century, sedimentological research has again moved toward higher-resolution observations at submicron to nanometer scales using scanning electron microscopy, and a six-level framework of multi-scale observational and descriptive techniques has been established ^[106]. More recently, SUN Longde et al. proposed a new classification scheme based on three end members—rigid grains, plastic components, and reservoir space ^[107]. Looking ahead, extensive analyses of large sample sets from shale successions in representative basins, together with the microfacies concept developed for carbonate rocks, are expected to enable integrated shale microfacies studies that link thin-section petrography and electron microscopy and incorporate palaeontological, sedimentological, and petrographic descriptions. Such an approach may ultimately yield a classification scheme and systematic framework for fine-grained sedimentary rocks with practical value for evaluating shale oil and gas enrichment zones/intervals, and may further support the development of an independent discipline of “fine-grained sedimentology”.