Next Article in Journal
Wavelet-Based, Blur-Aware Decoupled Network for Video Deblurring
Previous Article in Journal
Analyzing Drill Core Logging Using Rock Quality Designation–60 Years’ Experience from Modifications to Applications
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Applied Sciences
Volume 15
Issue 3
10.3390/app15031310
applsci-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Microscopic Characteristics and Formation of Various Types of Organic Matter in High-Overmature Marine Shale via SEM

by
Meng Zhao
1,2,
Hongyan Wang
2,3,*,
Zhensheng Shi
1,2,
Qun Zhao
1,2,
Tianqi Zhou
1,2 and
Ling Qi
1,2
1
Research Institute of Petroleum Exploration & Development, PetroChina, Beijing 100083, China
2
National Energy Shale Gas Research and Development Experimental Center, Beijing 100083, China
3
National Elite Institute of Engineering, CNPC, Beijing 100096, China
*
Author to whom correspondence should be addressed.
Appl. Sci. 2025, 15(3), 1310; https://doi.org/10.3390/app15031310
Submission received: 22 December 2024 / Revised: 22 January 2025 / Accepted: 26 January 2025 / Published: 27 January 2025
Browse Figures
Versions Notes

Abstract

:
Organic matter exhibits significant heterogeneity and complexity, with varying pore structures across different types influenced by multiple interacting factors. This paper introduces a “two categories, six subcategories” classification scheme based on morphological observations using a combination of argon ion polishing and scanning electron microscopy (SEM). Organic matter is classified into two main categories: depositional organic matter and migrated organic matter, based on whether migration has occurred. Depositional organic matter is further subdivided into three types based on microscopic characteristics: bioclasts, compacted kerogen, and in situ remnants from post-hydrocarbon generation. Migrated organic matter is categorized into three types: organic matter in intragranular pores, organic matter in intergranular pores, and bitumen in microfractures. Bioclasts can be further classified into alginite, zooclasts, acritarchs, and encapsulated organic matter based on maceral type. Zooclasts, acritarchs, encapsulated organic matter, and compacted kerogen—types of depositional organic matter—have few or no pores. This is primarily related to the nature of the hydrocarbon-generating precursor materials, with compacted kerogen being influenced by low thermal maturity and diagenetic compaction. In contrast, pores are more developed in alginite, in situ remnants from post-hydrocarbon generation, and all forms of migrated organic matter, largely due to the expulsion of gaseous hydrocarbons during thermal evolution. The pores in alginite reflect both the original structural properties of the hydrocarbon-generating precursor materials and the thermal evolution process. Depositional organic matter exhibits a stronger oil-generating potential and a higher gas-generating potential, while migrated organic matter primarily possesses a stronger gas-generating capability. Specifically, organic matter enriched in alginite, in situ remnants from post-hydrocarbon generation, as well as migrated organic matter in intragranular pore and intergranular pore, exhibit a higher hydrocarbon-generation potential.

1. Introduction

Organic matter is the fundamental source material for petroleum and natural gas [1] and plays a vital role in the formation, migration, accumulation, and production of shale oil and gas [2]. The optical properties, structural characteristics, and occurrence states of organic matter record various geological processes from deposition to hydrocarbon generation, offering essential insights into the evolution pathways and hydrocarbon generation processes of shale reservoirs [3,4,5]. Organic matter is highly heterogeneous [6,7], and current research primarily focuses on its variations in abundance, maturity, and pore structure across vertical, horizontal, and planar dimensions [8,9,10,11]. The microscopic characteristics of organic matter significantly influence its hydrocarbon generation and evolution. Organic matter pores result from the generation, diffusion, and expulsion of gases during the thermal evolution of source rocks. These pores serve as key reservoirs for free gas in shale and act as primary adsorption sites for adsorbed gas [12,13,14,15,16]. Organic matter pore development is largely controlled by the properties of the organic matter itself [17,18]. During thermal evolution, organic matter pores develop within the organic material itself [19,20,21,22,23,24], significantly impacting shale gas enrichment and production.
Previous studies have classified organic matter based on factors such as macerals, distribution patterns, occurrence forms, and genetic dynamics to identify shale organic matter components and their pore development characteristics [25,26,27,28] (Table 1). But the current research faces the issue of oversimplification regarding the organic matter as a hydrocarbon source. Furthermore, marine shale contains a relatively low amount of vitrinite and inertinite, with smaller particle sizes, making it difficult to classify macerals. There is still no unified classification scheme based on the distribution patterns of organic matter, and it remains challenging to identify the hydrocarbon source material. From a genetic perspective, the classification of shale organic matter remains underdeveloped, with no comprehensive classification system established.
Organic matter pores are typically smaller than 1 μm [30]. Classification of organic matter pores often focuses on genesis, distinguishing between primary pores, secondary pores, and microfractures [31,32,33,34], or is based on pore size, permeability, morphology, and degree of opening/closure [35,36,37,38,39] (Table 2). It is evident that the majority of research on pore classification has focused on pore occurrence. However, studies on organic pores remain relatively underdeveloped. While the source of organic matter is known to influence the development of organic pores, the mechanisms underlying their interaction are not yet fully understood. Therefore, systematically summarizing the pore characteristics of different organic matter components and understanding the differential mechanisms of pore development are crucial for shale gas resource assessment and the evaluation of shale reservoirs. This knowledge also provides valuable guidance for decision-making and well placement in oil and gas exploration and development.
The Wufeng–Longmaxi Formation in the Sichuan Basin of China is a primary source of shale gas, characterized by high to over-maturity [43]. However, there is currently a lack of consensus regarding the identification and classification of organic matter components within shale. Current research generally agrees on the identification of solid bitumen in the Wufeng and Longmaxi Formation, but the recognition of alginite remains controversial. Amorphous organic matter is broadly considered to be secondary organic matter, although the source of certain structural organic materials remains unclear. Additionally, the pore development characteristics of these components are not yet fully understood. Despite their importance, comprehensive studies on the preservation states, distribution patterns, and pore development characteristics of organic matter are limited. In particular, research on how different organic matter components control pore development is scarce [44,45,46,47]. Therefore, it is crucial to elucidate the genetic mechanisms underlying both organic matter and organic pores, as well as the multifactorial controls that govern pore development.
This study, grounded in organic petrology theory, examines core samples from the Wufeng-Longmaxi formations (Ro > 2%) in the southern Sichuan Basin, collected from two wells: Well Y206 (Y206#) and Well Y101H3-8 (Y101H3-8#). Using argon ion polishing combined with scanning electron microscopy (SEM), the study investigates micro-scale characteristics of organic matter, including source types, morphological features, distribution patterns, and microscopic components. Based on these observations, a “two major and six minor” classification scheme for organic matter in high-overmature marine shales is proposed. Additionally, the study explores the associated pore development characteristics and examines the differential control mechanisms of various organic matter types in the formation of organic pores in these shales.

2. Research Methods

This study primarily employs the experimental technique of argon ion polishing combined with field emission scanning electron microscopy (FESEM) to observe and identify organic matter. Prior to SEM observation, irregular shale samples are prepared into a regular shape (1 cm × 1 cm × 1 cm) suitable for analysis. The observation surface, perpendicular to the shale layers, is polished using argon ion beams. Apreo LoVac 30 kV double-probe low-vacuum field emission scanning electron microscope (FESEM) was used for imaging, with an A+B composition mode in backscattered electron (BSE) mode. High-resolution FESEM is characterized by its high resolution, large depth of field, stable signal, simple sample preparation, wide field of view, and clear imaging [22,48]. In organic-rich shales, mineral particles are typically small, and nanometer-scale pores are widely developed [49]. FESEM allows for the clear and direct observation of morphological features and pore development in organic matter. However, the rough surface of fresh natural fractures in shale samples can obscure the observation of primary pores. Therefore, argon ion polishing is commonly applied to shale samples to smooth and flatten the surface. High-speed ion beams bombard the sample surface, reducing roughness and improving the quality of the SEM images [50,51]. Compared to mechanical polishing, argon ion polishing causes less sample damage and enhances imaging, making it an indispensable tool for characterizing shale reservoirs [52,53,54,55]. In addition, focused ion beam scanning electron microscopy (FIB-SEM) was employed to analyze pore distribution in Tuscaloosa Marine shale and reconstruct pore connectivity in three dimensions. This method is crucial for studying pore networks and fluid flow within reservoirs.

3. Classification of Organic Matter in Shale and Its Morphological Characteristics

This study, utilizing high-resolution organic matter imaging after argon ion polishing and considering micro-scale features such as source, morphology, distribution, and maceral types, proposes a “two major, six minor” classification scheme for organic matter in highly to over-mature marine shales. Organic matter is first divided into two major categories: depositional organic matter and migrated organic matter, based on whether the organic matter has undergone migration. According to the microscopic occurrence characteristics of organic matter, depositional organic matter is further divided into four types: bioclasts, compacted kerogen, in situ remnants from post-hydrocarbon generation, and migrated organic matter into three types: migrated organic matter in intragranular pore, migrated organic matter in intergranular pore, and bitumen in microfractures. Based on macerals, bioclasts can be subdivided into alginite, zooclasts, acritarchs and encapsulated organic matter.

3.1. Depositional Organic Matter

Depositional organic matter primarily consists of original organic material and its alteration products, which remain enriched in situ and have not undergone migration, preserving their sedimentary characteristics. This includes kerogen, bitumen generated during thermal evolution, solid bitumen, and pyrobitumen [26]. The boundaries between depositional organic matter and surrounding inorganic minerals are distinct, with close contact but no associated authigenic minerals [56]. In this study, based on the identification of organic matter sources and pore characteristics, we classified bioclasts and in situ remnants from post-hydrocarbon generation within the depositional organic matter. Based on maceral characteristics, we identified alginite, zooclasts, and acritarchs as subtypes of bioclasts. Additionally, through microscopic morphology and pore characteristics, we identified compacted kerogen. As a result, by integrating organic matter sources and their microscopic occurrences, we classified the depositional organic matter in highly to over-mature marine shales into three categories: bioclasts, compacted kerogen, and in situ remnants from post-hydrocarbon generation. The bioclasts can be further subdivided into alginite, zooclasts, acritarchs, and encapsulated organic matter.

3.1.1. Bioclasts

In highly overmature marine shale, bioclastic organic matter can be classified into four main types: alginite, animal fragments, uncertain sources, and encapsulated matter.
  • Alginite: Alginite typically exhibits irregular shapes and is often closely associated with inorganic minerals, with well-defined boundaries and no secondary minerals in between (Figure 1). Microscopic examination reveals two main characteristics: one where algal debris contains clay minerals arranged in a fibrous distribution, showing some structure and orientation (Figure 1a), and another where the algal mat is pure, free from clay minerals (Figure 1b–d). During burial, alginite fragments can develop channels and fissures, where fluids interact with the rock to form fibrous illite [46], resulting in alginite containing fibrous illite.
  • The identification features of alginite under scanning electron microscopy (SEM) include three key aspects:
  • Large organic matter particle diameters: in the diagenetic stage, mineral particles are typically around 5 μm, insufficient to support interparticle pores larger than tens of micrometers, indicating that alginite is a type of depositional organic matter, rather than migrated organic matter.
  • Absence of secondary mineral enlargement at organic matter edges: secondary enlargement of quartz at pore edges is common during diagenesis, which further confirms that alginite is depositional organic matter.
  • The pores are mostly angular and vary in size.
2.
Zooclasts: Zooclasts primarily include graptolite periderm, chitinozoan vesicles, and scolecodonts, with graptolite periderm being the most common, and chitinozoan vesicles occasionally present. Graptolite periderm fragments typically appear as flat carbonaceous films parallel to bedding, exhibiting a banded or vein-like distribution with strong anisotropy (Figure 2a–c). These fragments show clear contact interfaces with the surrounding matrix, sometimes developing microfractures at these interfaces (Figure 2b,c). The cavities within the graptolite periderm often display segmented structures (Figure 2a–c) and are characterized by a “cortical bandage” structure (Figure 2d). Graptolites were primarily planktonic, though some were benthic and sessile [57], feeding on fungi, algae, and other microorganisms. After death, they were buried in sediments, forming flattened carbonaceous films preserved in black shales [58]. High graptolite abundance in shales often indicates deep-water, reducing environments [59]. Graptolite periderm has distinct morphological characteristics, and its microscopic identification features mainly include the following:
  • Banded or Vein-Like Habit: the organic matter exhibits a banded or vein-like structure, likely derived from graptolite fossil fragments.
  • Segmented Structure: the periderm develops a segmented structure resembling complex skeletal morphology.
  • “Cortical bandage” Structure: the periderm is composed of layered structures with cortical fibrils.
  • Poorly Developed Pores and Marginal Microfractures: under SEM, graptolite periderm displays poorly developed pores, with microfractures often present at the contact interface with matrix minerals.
Chitinozoan vesicles are typically vase-shaped but can also appear conical, cylindrical, or in other forms (Figure 2e,f). Some vesicles are connected in chains and usually exhibit symmetrical structures (Figure 2f). The cavities within these vesicles are often filled with inorganic minerals. In high-overmature marine shales, chitinozoan vesicles show strong anisotropy and generally range from several tens of micrometers to over 100 μm in length. Key microscopic features of chitinozoan vesicles include the following:
  • Typical Vase-Shaped Morphology.
  • Symmetrical Structure.
  • Length Ranging from several tens to hundreds of µm.
3.
Acritarchs: Acritarchs are microfossils with organic walls, whose phylogenetic relationships remain uncertain. These fossils may represent a polyphyletic assemblage, consisting of organisms with different evolutionary lineages [60,61]. Under scanning electron microscopy (SEM), acritarchs typically appear as circular or elliptical grains, often closely associated with surrounding minerals, and exhibit distinct boundaries. Apatite within the organic matter (Figure 3) suggests that acritarchs likely originated from plankton.
4.
Encapsulated organic matter: Encapsulated organic matter refers to organic material trapped within the siliceous shells of organisms during sedimentation. Under scanning electron microscopy (SEM), distinct boundaries are visible, typically showing a two-layer structure: the outer layer consists of a siliceous shell, which may be circular, elliptical, or semicircular in shape, while the inner layer is composed of pure organic matter. The morphology of the organic matter is shaped by the shell, preserving its overall form in accordance with the shape of the siliceous shell. Encapsulated organic matter generally exhibits little to no porosity. Microfractures can be observed between the siliceous shell and the organic matter (Figure 4). This type of organic matter primarily originates from cross-sections of sponge spicules and radiolarians, although identifying specific biological species under SEM is challenging. Studies, however, suggest that the organic matter encapsulated within these siliceous shells is in situ depositional organic matter, rather than migrated organic matter [62].

3.1.2. Compacted Kerogen

Kerogen demonstrates ductile behavior. Compacted kerogen refers to the deformation and elongation of organic matter, which occurs as kerogen and is subjected to compaction during burial, leading to its distribution between mineral particles and facilitating close contact with the surrounding minerals. Microscopically, the kerogen appears as amorphous organic matter, with rigid mineral particles, such as quartz, closely associated with it. The organic matter is in direct contact with the mineral particles, exhibiting limited porosity and no indications of cementation (Figure 5).

3.1.3. In Situ Remnants from Post-Hydrocarbon Generation

In situ remnants from post-hydrocarbon generation refer to bitumen, solid bitumen, or pyrobitumen that form and accumulate in place from depositional kerogen during the hydrocarbon generation process, as well as kerogen that remains in situ following hydrocarbon generation. Microscopically, the organic matter typically displays a range of distribution patterns, including granular or banded structures, with distinct boundaries separating the organic matter from adjacent minerals. Notably, these remnants exhibit no evidence of crystal growth, and the pores developed within the organic matter are clearly visible (Figure 6). The morphology of in situ remnants from post-hydrocarbon generation is primarily influenced by the pore structures defined by surrounding minerals.
The key distinctions between in situ remnants from post-hydrocarbon generation and compacted kerogen lie in their thermal maturation and physical properties. In situ remnants primarily represent the residual organic matter that remains during and after the “oil window” phase, whereas compacted kerogen refers to kerogen that has undergone deformation and filled intergranular pores due to compaction prior to entering the “oil window.” Morphologically, compacted kerogen typically lacks porosity, whereas in situ remnants from post-hydrocarbon generation exhibit significant pore development.

3.2. Migrated Organic Matter

Migrated organic matter refers to petroleum and bitumen formed during the thermal maturation of kerogen, which subsequently migrate into adjacent mineral pores or fractures, along with their byproducts, such as solid bitumen and pyrobitumen. The primary distinction between migrated organic matter and depositional organic matter lies in their temporal formation: migrated organic matter forms later in the maturation process and accumulates at the target site after migration, often accompanied by the development of authigenic minerals. A key characteristic of migrated organic matter is the early crystallization of surrounding minerals, with organic matter later filling the voids within the crystal frameworks of matrix minerals. The morphology of this organic matter is largely dictated by the spatial constraints of the host mineral structures. Based on its location and occurrence, migrated organic matter can be classified into three main types: intergranular pore, intragranular pore, and microfracture.

3.2.1. Migrated Organic Matter in Intragranular Pore

Migrated organic matter in intragranular pores refers to petroleum, bitumen, and other substances that are generated through the thermal maturation of kerogen and subsequently migrate to fill the pores within mineral particles. These mineral particles primarily consist of cavities formed by biological organisms. Siliceous radiolarians and sponge spicules are typical biological structures where migrated organic matter accumulates. Authigenic quartz can be observed replacing pyrite in siliceous radiolarians or sponge spicules, with the migrated organic matter filling the pore spaces, where organic matter is particularly abundant and well-developed (Figure 7).

3.2.2. Migrated Organic Matter in Intergranular Pore

The bitumen or oil that migrates and fills the pores between mineral grains, along with the resulting solid bitumen and pyrobitumen byproducts formed during their diagenesis, are classified as migrated organic matter within intergranular pores. This migrated organic matter typically exhibits irregular shapes and uneven size distribution, often coexisting with authigenic minerals. Consequently, indistinct boundaries are observed between the organic matter and the surrounding minerals, with the development of organic matter-associated pores (Figure 8).
In this type of migrated organic matter, three distinct distribution patterns can be identified. First, organic matter occupies the interstitial spaces between the platy crystals of clay minerals, where authigenic pyrite and quartz are also present. This arrangement provides a structural framework for the organic matter and facilitates the development of organic pores (Figure 8a,b). Second, migrated organic matter fills the intercrystalline pores within framboidal pyrite, where the organic pores are relatively well-developed (Figure 8c,d). Third, the migrated organic matter infiltrates the voids within microcrystalline quartz and its aggregates. Authigenic microcrystalline quartz typically manifests as cryptocrystalline and microcrystalline aggregates, with the intergranular pores between the quartz particles being filled by migrated organic matter, thereby forming an organic matter network (Figure 8e,f). This process primarily occurs during the early to early-middle diagenetic stages.

3.2.3. Bitumen in Microfractures

Bitumen within microfractures is typically distributed in a banded pattern, forming a conformable contact with sedimentary clastic particles, and displaying well-defined contact boundaries with matrix minerals. It is devoid of pores and is frequently interspersed with fragments of surrounding rock or framboidal pyrite. Microfractures predominantly form between organic matter and adjacent rock minerals, often extending along bedding planes, bypassing particle edges, and rarely penetrating through mineral grains or organic material (Figure 9).

4. Pore Characteristics of Different Types of Organic Matter

The development of pores in shale reservoirs creates storage spaces and migration pathways for hydrocarbons [63]. This section, based on the organic matter classification presented in the previous section, discusses the pore development characteristics of both depositional organic matter and migrated organic matter.

4.1. Pore Development Characteristics of Depositional Organic Matter

There are two primary mechanisms responsible for the development of organic matter pores in depositional organic matter. The first mechanism involves the pyrolysis of kerogen, during which bitumen is generated and expelled into adjacent pores, leading to the formation of organic matter pores within the kerogen itself [22]. The second mechanism pertains to the formation of organic matter pores through secondary cracking of organic matter that remains in situ after the oil generation stage [55,64]. Among the various categories of organic matter, compacted kerogen typically exhibits minimal porosity. In contrast, the in situ remnants left after the oil generation phase tend to display a greater degree of pore development. Bioclasts, with the exception of alginite—which forms angular pores—generally exhibit limited or no porosity, as is the case with zooclasts, acritarchs, and encapsulated organic matter. Both types of alginite, however, exhibit well-developed porosity. In alginite with a pure interior, the pores are predominantly irregular angular or a combination of irregular angular and circular or elliptical forms (Figure 1a). These pores range in size from tens to hundreds of nanometers and possess relatively high porosity (Figure 10a). In alginite that contains fibrous clay minerals, pores are developed between the organic matter and the fibrous clay minerals, displaying irregular angular or circular shapes. These pores are relatively uniform in size, ranging from approximately 10 to 50 nm, and the porosity is about 20% (Figure 1a). The formation of organic pores is believed to be influenced by the intrinsic biological structure and genetic characteristics of algae. The morphology, distribution, and size of these pores are primarily determined by the type and original structure of the hydrocarbon-generating organisms. In low-maturity organic matter, pores are mainly interstitial spaces between single-celled algae. Circular pores are thought to result from the early stages of petroleum generation and expulsion from algal-rich material [34,65,66]. The regular arrangement and elongation of elliptical pores are likely the consequence of compaction following hydrocarbon expulsion. In zooclasts, the internal structure of chitinozoan vesicles generally exhibits minimal porosity. Few pores or microfractures, typically around 50 nm in width, are observed at the interface between the vesicles and surrounding minerals (Figure 10b). Graptolite periderms also show little to no porosity, although occasional nano-sized round organic pores are observed locally, with a porosity much less than 1%. Microfractures ranging in width from 10 to 20 nm are visible along the edges (Figure 10c). The graptolite periderm features a “cortical bandage” structure, within which elongated nano-pores or microfractures develop between cortical fibrils, typically ranging from 20 to 70 nm in width (Figure 10d). These microfractures interconnect the pores, forming a network that serves as both hydrocarbon storage spaces and migration pathways [67].
Acritarchs and encapsulated organic matter exhibit no internal pore development. However, microfractures, typically 2–10 nm in width, are commonly observed at the interfaces between acritarchs and surrounding minerals, as well as between the organic matter and the siliceous shells of encapsulated organisms (Figure 10e,f). Compacted kerogen shows minimal porosity due to diagenetic compaction (Figure 10g). In contrast, in situ remnants from post-hydrocarbon generation display a significantly higher degree of pore development, often appearing honeycomb-like and/or alveolar upon polished examination. The pore diameters predominantly range from 300 to 800 nm, with some pores reaching micron-scale dimensions, and the associated porosity varies between 30% and 80%. Smaller pores are generally round or oval in shape, while larger pores tend to be elongated (Figure 10h). Most of the organic pores in the in situ remnants from post-hydrocarbon generation are formed by the nucleation and coalescence of uniform gas bubbles. In some instances, the smaller pores result from the inhibition of further rupture and connectivity of organic pores due to surrounding rigid minerals. Larger pores are formed when the friction between gas bubbles and the organic matter walls is sufficiently high to prevent the bubbles from migrating through the organic matter [55,68].

4.2. Pore Development Characteristics of Migrated Organic Matter

The bitumen that migrates outside of kerogen can undergo secondary cracking, resulting in the formation of organic matter pores. Migrated organic matter typically develops nanoscale pores and microfractures, with the pores often exhibiting ellipsoidal or irregular shapes. Organic matter pores within the migrated organic matter in intragranular pores are well-developed. Under scanning electron microscopy (SEM), organic matter filling the interior of fossilized skeletons—such as radiolarians or sponge spicules—replaced by pyrite, predominantly displays a spongy morphology. These pores are generally circular or elliptical, with diameters ranging from tens to hundreds of nanometers, and the porosity is approximately 50% (Figure 11a). Pyrite, which forms concurrently with bacterial sulfate reduction, can replace the siliceous skeletons of radiolarians. This material is subsequently transported to the sedimentary basin, where its internal cavities become filled with migrated organic matter. The confinement of the migrated organic matter within these biological cavities results in a distinct microstructural appearance under microscopy. In intergranular pores, nanopores are well-developed within the migrated organic matter (Figure 11b–e). Pores within the organic matter located between clay mineral lamellae are predominantly elliptical, with directional elongation, and exhibit diameters ranging from approximately 500 nm to 1 μm. Contraction fractures are also evident. The porosity in these areas is approximately 5–10% (Figure 11b,c). Within the intergranular pores of framboidal pyrite, the organic matter primarily forms granular pores, most of which have diameters in the tens of nanometers. Occasionally, moldic pores are observed, with diameters ranging between 1 μm and 2 μm. The porosity of the migrated organic matter in these intergranular pores is estimated to be around 10–30% (Figure 11d). Organic matter within authigenic microcrystalline quartz primarily develops spongy or honeycomb-like pores, with diameters ranging from tens to hundreds of nanometers. Additionally, a small number of relatively large, randomly shaped organic matter pores are observed, which are primarily alveolar in shape and range in diameter from 1 μm to 2 μm. The porosity in these areas ranges from 50% to 70% (Figure 11e). During the late stages of burial, cracking of the migrated organic matter generates significant quantities of dry gas, leaving behind spongy organic matter pores. The organic components found between euhedral mineral particles consist of solid bitumen or pyrobitumen, the result of further cracking of the oil generated within the formation.
Bitumen within microfractures along bedding planes typically does not form pores but often exhibits contraction fractures (Figure 11f), which play a significant role in the transport system. During the thermal maturation of kerogen, bitumen migrates into microfractures, filling them in the direction of extension. This bitumen is frequently associated with fragments of the surrounding rock or framboidal pyrite, suggesting that the microfractures primarily develop during the kerogen degradation process, which is linked to overpressure generated during hydrocarbon generation.
This study demonstrates that alginite, in situ remnant from post-hydrocarbon generation, along with migrated organic matter in intragranular and intergranular pores, generally exhibits well-developed porosity. This is indicative of favorable hydrocarbon potential and suggests enhanced reservoir quality. In contrast, zooclasts, acritarchs, encapsulated organic matter, and compacted kerogen typically exhibit poor porosity development. Reservoirs enriched with these materials may exhibit reduced hydrocarbon generation capacity, thus potentially lacking favorable prospects for hydrocarbon accumulation.

5. Key Determinants of Organic Matter Formation and Pore Development in Shale

The formation of organic matter and the associated pores in highly over-mature marine shales is influenced by a range of interrelated factors, including the hydrocarbon-generating parent material, depositional environment, compaction, degree of thermal evolution, mineral associations, and formation duration [69,70]. This study synthesizes a comprehensive body of research to investigate the underlying mechanisms (Table 3) and the key factors that govern the formation of different types of organic matter and their associated pore structures.

5.1. Key Determinants of Depositional Organic Matter Development

The diversity of depositional organic matter is primarily attributed to variations in hydrocarbon-generating precursors, compaction processes, as well as the properties, evolutionary stages, and characteristics of the surrounding minerals.
  • Hydrocarbon-Generation Precursors
All organic matter originates from biological organisms, and precursors play a fundamental role in shaping the development characteristics and hydrocarbon generation potential of organic matter [71,72]. The influence of hydrocarbon-generation precursors on depositional organic matter is particularly evident in zooclasts, which can be further subdivided based on their preserved biological structural characteristics. Organic matter in marine shale is primarily derived from marine planktonic algae and bacteria. The diversity of organisms and their degradation states are key factors contributing to variations in organic matter.
Alginite results from the humification of planktonic algae in marine environments and inherits the soft tissue structure of planktonic algae, exhibiting smooth edges and loose surface structures, which lead to irregular angular pores. It appears as amorphous particles due to degradation, and maintains high hydrogen and heteroatom content (e.g., N, S) lowers activation energy. After biological degradation, it develops sponge-like or flocculent characteristics [73], with well-developed pores, enhancing its hydrocarbon-generating potential. Zooclasts primarily originate from graptolites and chitinozoans, most of which are planktonic animals, with a few benthic organisms. The fibrous “cortical bandage” structure of graptolites retains the periderm tissue, which is characterized by low aliphatic content and high aromaticity. As a result, the graptolite periderm, perpendicular to bedding, has little or no porosity, though nanometer-scale pores have been observed in the cortical fibrils at the edges of graptolites resulting in low hydrocarbon-generating capacity, contributing to better preservation. Acritarchs likely originate from certain algae and unicellular organisms. Encapsulated organic matter, particularly from radiolarians and sponge spicules, is characterized by siliceous shells, with organic matter filling the cavities. These shells are resistant to dissolution providing physical protection, isolating the organic matter from microbial decay [62,74,75]. Depositional organic matter precursors primarily include four sources: planktonic plant fragments, zooplankton remains, zooplankton excreta, and bacteria (Figure 12).
2.
Compaction and Adjacent Mineral Properties
The impact of diagenetic compaction on depositional organic matter is most evident in compacted kerogen. During shallow burial, organic matter remains in an immature state, where ductile kerogen, under pressure from overlying strata, is compressed and squeezed into the spaces between rigid particles. This leads to a direct reduction in intergranular pores, causing a decrease in porosity and triggering dehydration. As compacted kerogen deforms under stress, it comes into closer contact with surrounding detrital particles, increasing the contact area. The contact transitions from point contact to line contact and, ultimately, to concave–convex contact. Due to the low thermal maturity at this stage, organic pores are typically underdeveloped, with only a few primary biological tissue structure pores remaining. At this point, compaction dominates as the primary diagenetic process, and intergranular pores between inorganic minerals continue to decrease as compaction intensifies (Figure 13). Non-siliceous alginite and encapsulated organic matter are also affected by compaction. Non-siliceous alginite, like compacted kerogen, undergo compressive compaction during burial, resulting in plastic deformation as they are squeezed between rigid mineral grains. Meanwhile, the disappearance of intergranular porosity prevents the minerals from undergoing cementation.
3.
Thermal Evolution
Organic matter pores are generally believed to result from gas expansion during thermal maturation [76,77]. The development and evolution of kerogen and organic matter pores are both driven by thermal maturation processes. The impact of thermal evolution on depositional organic matter is most apparent in the in situ remnants from post-hydrocarbon generation. During thermal maturation, as kerogen reaches the quasi-metamorphic stage, the liquid hydrocarbons and residual kerogen that generated within the “oil window” and remained in situ go through cracking, producing bitumen and dry gas [78,79]. Thus, the in situ remnants from post-hydrocarbon generation primarily consist of residual kerogen and oil retained after the gas generation phase. As these remnants undergo thermal cracking to form gaseous hydrocarbons, the organic matter decomposes from the edges inward, leading to a decrease in density. Gas is expelled from the kerogen, facilitating the development of organic matter pores, which are easily identified by the escape of methane (Figure 14). At the mesocatagenesis stage, increased burial depth raises the organic matter’s thermal maturity, moving it into the oil window. A portion of the oil generated by pyrolysis is expelled from the source rock, while another portion remains in situ. The hydrocarbon generation process also produces large quantities of organic acids, which dissolve minerals such as feldspars and carbonates, forming inorganic dissolution pores. These pores help counteract some of the porosity loss caused by hydrocarbon expulsion. As thermal evolution continues, higher temperatures further increase the thermal maturity of the organic matter, pushing it into a quasi-metamorphic stage, the retained liquid hydrocarbons mature further through cracking, producing bitumen and dry gas.

5.2. Key Determinants of Migrated Organic Matter Development

The formation and preservation of migrated organic matter are primarily controlled by the degree of thermal evolution, mineral composition, and the rigidity of the surrounding framework.
  • Thermal Evolution and Hydrocarbon Generation
The source of the migrated organic matter is the liquid hydrocarbons generated during the thermal evolution of organic matter through pyrolysis. Some of the produced liquid hydrocarbons, such as oil, may in situ fill the pores and fractures formed during hydrocarbon generation, while others are expelled from the kerogen, migrating into adjacent pores where they undergo conversion into solid bitumen, thus forming migrated organic matter. Therefore, reaching the oil window during kerogen thermal evolution is a key prerequisite for the formation of migrated organic matter.
Migrated organic matter also undergoes thermal evolution, and its transformation from bitumen to solid bitumen or pyrobitumen is associated with the creation of organic pores. After secondary cracking, significant quantities of dry gas are generated, leading to the formation and preservation of numerous organic pores. These pores connect isolated voids within the depositional organic matter, creating a continuous three-dimensional pore network. This network facilitates more efficient and continuous gas migration [5,80].
2.
Mineral Types and Rigid Framework
Migrated organic matter preferentially fills pre-existing intergranular pores during migration. Due to its high plasticity, the morphology of organic matter is shaped by surrounding minerals. The influence mechanisms can be divided into two types. First, they delineate the range of organic matter’s plastic flow, thereby controlling its morphological distribution. Second, the types of pores and fractures in the surrounding minerals qualitatively determine where migrated organic matter will accumulate. Migrated organic matter can be classified into three types based on the mineral pore types: (1) migrated organic matter in intragranular pores, which forms when organic matter migrates into and fills intragranular pores; (2) migrated organic matter in intergranular pores, which typically has blurred boundaries and is the most common form of migrated organic matter; and (3) bitumen in microfractures, which forms when organic matter fills microfractures (Figure 15).
The main minerals influencing the occurrence of organic matter in shale are quartz, pyrite, and clay minerals. Quartz can be biogenic or detrital in origin, creating a rigid, three-dimensional, grain-supported framework that preserves primary pores and generates numerous intergranular pores. These pores provide storage space for organic matter [81,82,83]. The accumulation locations associated with pyrite include two types: the radiolarian silica shells replaced by pyrite and the intercrystalline spaces within the pyrite crystals. Both pyrite and quartz, being brittle, form a framework with high resistance to compaction, helping to preserve organic matter and organic pores by minimizing damage during burial. Organic matter can also be stored within the platelets of clay minerals, where organic matter pores can develop. During diagenesis, clay minerals undergo dehydration, forming microfractures that are filled with migrated organic matter. However, because clay minerals are less resistant to compaction, pore development is limited, with pores concentrated in the central region and elongated along the bedding plane. Clay minerals and organic matter co-evolve during diagenesis. As maturity increases, smectite illitization causes spatial collapse, releasing adsorbed organic matter [84,85]. However, the micro-interface interactions between clay minerals and organic matter are crucial to their catalytic role in hydrocarbon generation [86].
Overall, depositional organic matter forms earlier than migrated organic matter, with the parent material source being a fundamental control on the hydrocarbon generation potential. Mineral types and rigid frameworks significantly influence the occurrence characteristics of organic matter during sedimentation and burial. Maturity and hydrocarbon generation evolution primarily affect the microscopic morphology and pore development of organic matter. The source material, compaction, and maturity control the development of depositional organic matter and its pores, while the development of migrated organic matter and its pores is mainly influenced by mineral types and maturity. The depositional organic matter exhibits a stronger oil-generating potential and a higher gas-generating potential, while migrated organic matter primarily possesses a stronger gas-generating capability. Organic matter enriched in alginite, in situ remnants from post-hydrocarbon generation, as well as migrated organic matter in intragranular pore and intergranular pore, exhibit a higher hydrocarbon-generation potential.

6. Practical Application or Future Recommendation

This paper aims to enhance our understanding of the hydrocarbon generation potential of organic matter in high-overmature marine shales, offering valuable insights for optimizing shale gas exploration by identifying favorable areas and sweet spots. Currently, the classification scheme is based solely on morphological features observed under scanning electron microscopy (SEM). Future research should expand on this by integrating optical characteristics with SEM-based morphological observations and obtaining structural parameter data to further refine and improve the classification framework.

7. Conclusions

Based on microscopic characteristics and pore development of organic matter under SEM, this paper systematically summarizes and proposes a classification scheme for the organic matter in high-overmature marine shale. It also examines and analyzes the pore development characteristics of different types of organic matter, leading to the following conclusions.
  • Organic matter is classified into depositional organic matter and migrated organic matter, based on whether migration has occurred. Depositional organic matter is further subdivided into three types based on microscopic characteristics: bioclasts, compacted kerogen, and in situ remnants from post-hydrocarbon generation. Migrated organic matter is categorized into three types: organic matter in intragranular pores, organic matter in intergranular pores, and bitumen in microfractures. Bioclasts can be further classified into alginite, zooclasts, acritarchs, and encapsulated organic matter based on maceral type.
  • Depositional organic matter, alginite, in situ remnants from post-hydrocarbon generation, and migrated organic matter in intragranular and intergranular pores generally exhibit well-developed pores. In contrast, bitumen in microfractures is characterized by contraction cracks. Zooclasts, acritarchs, encapsulated organic matter, and compacted kerogen, all types of depositional organic matter, typically have fewer or underdeveloped pores.
  • The source material, compaction, and maturity control the development of depositional organic matter and its pores, while the development of migrated organic matter and its pores is mainly influenced by mineral types and maturity. The depositional organic matter exhibits a stronger oil-generating potential and a higher gas-generating potential, while migrated organic matter primarily possesses a stronger gas-generating capability. Organic matter enriched in alginite, in situ remnants from post-hydrocarbon generation, as well as migrated organic matter in intragranular pore and intergranular pore, exhibit a higher hydrocarbon-generation potential.

Author Contributions

Conceptualization, M.Z., H.W. and Z.S.; methodology, M.Z.; software, M.Z. and T.Z.; validation, M.Z., and L.Q.; formal analysis, M.Z.; investigation, M.Z.; resources, H.W. and Z.S.; data curation, M.Z.; writing—original draft preparation, M.Z.; writing—review and editing, M.Z.; supervision, H.W., Z.S. and Q.Z.; project administration, Q.Z.; funding acquisition, H.W. and Z.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the 14th Five-Year Plan of the Ministry of Science and Technology of PetroChina, grant number 2021DJ1901.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Conflicts of Interest

The authors declare that they have no known competing financial interest or personal relation-ships that could have appeared to influence the work reported in this paper. Meng Zhao, Zhensheng Shi, Qun Zhao, Tianqi Zhou and Ling Qi are employees of PetroChina; Hongyan Wang is employee of CNPC. The authors declare that this study received funding from PetroChina. The funder had no role in the design of the study; in the collection, analysis, or interpretation of data, in the writing of the manuscript, or in the decision to publish the results.

References

  1. Liu, B. Organic Matter in Shales: Types, Thermal Evolution, and Organic Pores. Earth Sci. 2023, 48, 4641–4657, (In Chinese with English abstract). [Google Scholar]
  2. Yang, J.; Hatcherian, J.; Hackley, P.C.; Pomerantz, A.E. Nanoscale geochemical and geomechanical characterization of organic matter in shale. Nat. Commun. 2017, 8, 2179. [Google Scholar] [CrossRef]
  3. Mastalerz, M.; Drobniak, A.; Stankiewicz, A.B. Origin, properties, and implications of solid bitumen in source–rock reservoirs: A review. Int. J. Coal Geol. 2018, 195, 14–36. [Google Scholar] [CrossRef]
  4. Misch, D.; Groß, D.; Hawranek, G.; Horsfield, B.; Klaver, J.; Mendez–Martin, F.; Urai, J.L.; Vranjes–Wessely, S.; Sachsenhofer, R.F.; Schmatz, J.; et al. Solid bitumen in shales: Petrographic characteristics and implications for reservoir characterization. Int. J. Coal Geol. 2019, 205, 14–31. [Google Scholar] [CrossRef]
  5. Cardott, B.J.; Landis, C.R.; Curtis, M.E. Post–oil solid bitumen network in the Woodford Shale, USA—A potential primary migration pathway. Int. J. Coal Geol. 2015, 139, 106–113. [Google Scholar] [CrossRef]
  6. Slatt, R.M.; Rodriguez, N.D. Comparative sequence stratigraphy and organic geochemistry of gas shales: Commonality or coincidence? J. Nat. Gas Sci. Eng. 2012, 8, 68–84. [Google Scholar] [CrossRef]
  7. Hammes, U.; Frébourg, G. Haynesville and Bossier mudrocks:a facies and sequence stratigraphic investigation, East Texas and Louisiana, USA. Mar. Pet. Geol. 2012, 31, 8–26. [Google Scholar] [CrossRef]
  8. Wu, J.; Liang, C.; Jiang, Z.; Zhang, C. Shale reservoir characterization and control factors on gas accumulation of the Lower Cambrian Niutitang shale, Sichuan Basin, South China. Geol. J. 2019, 54, 1604–1616. [Google Scholar] [CrossRef]
  9. Zou, C.; Zhu, R.; Chen, Z.; Ogg, J.G.; Wu, S.; Dong, D.; Qiu, Z.; Wang, Y.; Wang, L.; Lin, S.; et al. Organic–matter–rich shales of China. Earth–Sci. Rev. 2019, 189, 51–78. [Google Scholar] [CrossRef]
  10. Bakshi, T.; Vishal, V. A review on the role of organic matter in gas adsorption in shale. Energy Fuels 2021, 35, 15249–15264. [Google Scholar] [CrossRef]
  11. Wang, Y.; Liu, L.; Cheng, H. Gas adsorption characterization of pore structure of organic–rich shale: Insights into contribution of organic matter to shale pore network. Nat. Resour. Res. 2021, 30, 2377–2395. [Google Scholar] [CrossRef]
  12. Zhang, J.; Li, Z.; Wang, D.; Xu, L.; Li, Z.; Niu, J.; Chen, L.; Sun, Y.; Li, Q.; Yang, Z.; et al. Shale gas accumulation patterns in China. Nat. Gas Ind. 2022, 42, 78–95, (In Chinese with English abstract). [Google Scholar] [CrossRef]
  13. Guo, X.; Borjigin, T.; Wei, X.; Yu, L.; Lu, X.; Sun, L.; Wei, F. Occurrence mechanism and exploration potential of deep marine shale gas in Sichuan Basin. Acta Pet. Sin. 2022, 43, 453–468, (In Chinese with English abstract). [Google Scholar]
  14. Ma, Y.; Cai, X.; Zhao, P. China’s shale gas exploration and development: Understanding and practice. Pet. Explor. Dev. 2018, 45, 561–574, (In Chinese with English abstract). [Google Scholar] [CrossRef]
  15. Chalmers, G.R.; Bustin, R.M.; Power, I.M. Characterization of gas shale pore systems by porosimetry, pycnometry, surface area, and field emission scanning electron microscopy/transmission electron microscopy image analyses: Examples from the Barnett, Woodford, Haynesville, Marcellus, and Doig units. AAPG Bull. 2012, 96, 1099–1119. [Google Scholar]
  16. Curtis, J.B. Fractured shale–gas systems. AAPG Bull. 2002, 86, 1921–1938. [Google Scholar]
  17. Schieber, J. Common themes in the formation and preservation of intrinsic porosity in shales and mudstones–illustrated with examples across the Phanerozoic. In Proceedings of the SPE Unconventional Resources Conference/Gas Technology Symposium, Pittsburgh, PA, USA, 23–25 February 2010; p. SPE–132370. [Google Scholar]
  18. Ko, L.T.; Loucks, R.G.; Zhang, T.; Ruppel, S.C.; Shao, D. Pore and pore network evolution of Upper Cretaceous Boquillas (Eagle Ford–equivalent) mudrocks:Results from gold tube pyrolysis experiments. AAPG Bull. 2016, 100, 1693–1722. [Google Scholar] [CrossRef]
  19. Reed, R.M.; Loucks, R.G. Imaging nanoscale pores in the Mississippian Barnett Shale of the northern Fort Worth Basin. AAPG Annu. Conv. Abstr. 2007, 16, 115. [Google Scholar]
  20. Jarvie, D.M.; Hill, R.J.; Ruble, T.E.; Pollastro, R.M. Unconventional shale–gas systems: The Mississippian Barnett Shale of northcentral Texas as one model for thermogenic shale–gas assessment. AAPG Bull. 2007, 91, 475–499. [Google Scholar] [CrossRef]
  21. Loucks, R.G.; Reed, R.M.; Ruppel, S.C.; Jarvie, D.M. Morphology, genesis, and distribution of nanometer–scale pores in siliceous mudstones of the Mississippian Barnett Shale. J. Sediment. Res. 2009, 79, 848–861. [Google Scholar] [CrossRef]
  22. Loucks, R.G.; Reed, R.M.; Ruppel, S.C.; Hammes, U. Spectrum of pore types and networks in mudrocks and a descriptive classification for matrix–related mudrock pores. AAPG Bull. 2012, 96, 1071–1098. [Google Scholar] [CrossRef]
  23. Borjigin, T.; Lu, L.; Yu, L.; Zhang, W.; Pan, A.; Shen, B.; Wang, Y.; Yang, Y.; Gao, Z. Formation, preservation and connectivity control of organic pores in shale. Pet. Explor. Dev. 2021, 48, 687–699. [Google Scholar] [CrossRef]
  24. Guo, Q.; Chen, X.; Song, H.; Zheng, M.; Huang, J.; Chen, N.; Gao, R. Evolution and models of shale porosity during burial process. Nat Gas Geosci 2013, 24, 439–449. [Google Scholar]
  25. Liu, B.; Mastalerz, M.; Schieber, J. SEM petrography of dispersed organic matter in black shales: A review. Earth–Sci. Rev. 2022, 224, 103874. [Google Scholar] [CrossRef]
  26. Loucks, R.G.; Reed, R.M. Scanning–electron–microscope petrographic evidence for distinguishing organic–matter pores associated with depositional organic matter versus migrated organic matter in mudrock. GCAGS J. 2014, 3, 51–60. [Google Scholar]
  27. Liu, Z.; Bai, M.; Yang, Y.; Wang, X.; Chen, J.; Xie, T.; Fang, L.; Qin, L. Discussion on the Genesis and Exploration Potential of Different Microscopic Forms of Organic Matters in the Longmaxi Formation Shale. Rock Miner. Anal. 2020, 39, 199–207, (In Chinese with English abstract). [Google Scholar]
  28. Yu, Y.; Xia, P.; Wang, Y.; Ning, S.; Zhong, Y.; Mou, Y.; Li, K. Occurrence state and characteristics of organic matter in over–mature marine shale:a case study for the Lower Cambrian Niutitang Formation in Guizhou Province. J. Northeast Pet. Univ. 2022, 46, 48–61+76+84–85, (In Chinese with English abstract). [Google Scholar]
  29. Belin, S. Application of backscattered electron imaging to the study of source rocks microtextures. Org. Geochem. 1992, 18, 333–346. [Google Scholar] [CrossRef]
  30. Bousige, C.; Ghimbeu, C.M.; Vix-Guterl, C.; Pomerantz, A.E.; Suleimenova, A.; Vaughan, G.; Garbarino, G.; Feygenson, M.; Wildgruber, C.; Ulm, F.; et al. Realistic Molecular Model of Kerogen’s Nanostructure. Nat. Mater. 2016, 15, 576–582. [Google Scholar] [CrossRef]
  31. He, J.; Ding, W.; Fu, J.; Li, A.; Dai, P. Study on genetic type of micropore in shale reservoir. Lithol. Reserv. 2014, 26, 30–35. [Google Scholar]
  32. Liu, B.; Schieber, J.; Mastalerz, M. Combined SEM and reflected light petrography of organic matter in the New Albany Shale (Devonian–Mississippian) in the Illinois Basin: A perspective on organic pore development with thermal maturation. Int. J. Coal Geol. 2017, 184, 57–72. [Google Scholar] [CrossRef]
  33. Ji, W.; Song, Y.; Jiang, Z.; Meng, M.; Liu, Q.; Gao, F. Micron–to nano–pore characteristics in the shale of Longmaxi Formation, southeast Sichuan Basin. Pet. Res. 2017, 2, 156–168. [Google Scholar] [CrossRef]
  34. Löhr, S.C.; Baruch, E.T.; Hall, P.A.; Kennedy, M.J. Is organic pore development in gas shales influenced by the primary porosity and structure of thermally immature organic matter? Org. Geochem. 2015, 87, 119–132. [Google Scholar] [CrossRef]
  35. IUPAC (International Union of Pure and Applied Chemistry). Physical chemistry division commission on colloid and surface chemistry, subcommittee on characterization of porous solids: Recommendations for the characterization of porous solids (Technical Report). Pure Appl. Chem. 1994, 66, 1739–1758. [Google Scholar] [CrossRef]
  36. Dai, J.; Lin, L. “Seepage” classification scheme of reservoir pores and its significance. Pet. Geol. Oilfield Dev. Daqing 2022, 41, 43–50, (In Chinese with English abstract). [Google Scholar]
  37. Cui, J.; Zou, C.; Zhu, R.; Bai, B.; Wu, S.; Wang, T. New advances in shale porosity research. Adv. Earth Sci. 2012, 27, 1319–1325, (In Chinese with English abstract). [Google Scholar]
  38. Zhang, P.; Liu, X.; Wang, Y.; Sun, X. Research progress in shale nanopores. Adv. Earth Sci. 2014, 29, 1242–1249, (In Chinese with English abstract). [Google Scholar]
  39. Sun, L.; Tuo, J.; Zhang, M.; Wu, C.; Wang, Z.; Zheng, Y. Formation and development of the pore structure in Chang 7 member oil–shale from Ordos Basin during organic matter evolution induced by hydrous pyrolysis. Fuel 2015, 158, 549–557. [Google Scholar] [CrossRef]
  40. Katz, B.J.; Arango, I. Organic porosity: A geochemist’s view of the current state of understanding. Org. Geochem. 2018, 123, 183192–183271. [Google Scholar] [CrossRef]
  41. Bloch, S.; Lander, R.H.; Bonnell, L. Anomalously high porosity and permeability in deeply buried sandstone reservoirs: Origin and predictability. AAPG Bull. 2002, 86, 301–328. [Google Scholar]
  42. Wang, L.; Cao, H. A possible mechanism of organic pores evolution in shale: A case from Dalong Formation, Lower Yangtze area. Nat. Gas Geosci. 2016, 27, 520–523, (In Chinese with English abstract). [Google Scholar]
  43. Jiang, P.; Wu, J.; Zhu, Y.; Zhang, D.; Wu, W.; Zhang, R.; Wu, Z.; Wang, Q.; Yang, Y.; Yang, X.; et al. Enrichment conditions and favorable areas for exploration and development of marine shale gas in Sichuan Basin. Acta Pet. Sin. 2023, 44, 91–109, (In Chinese with English abstract). [Google Scholar]
  44. Nie, H.; Jin, Z.; Zhang, J. Characteristics of three organic matter pore types in the Wufeng–Longmaxi Shale of the Sichuan Basin, Southwest China. Sci. Rep. 2018, 8, 7014. [Google Scholar] [CrossRef]
  45. Nie, H.; Jin, Z.; Sun, C.; He, Z.; Liu, G.; Liu, Q. Organic matter types of the Wufeng and Longmaxi Formations in the Sichuan Basin, South China: Implications for the formation of organic matter pores. Energy Fuels 2019, 33, 8076–8100. [Google Scholar] [CrossRef]
  46. Zhang, W.; Hu, W.; Borjigin, T.; Zhu, F. Pore characteristics of different organic matter in black shale: A case study of the Wufeng–Longmaxi Formation in the Southeast Sichuan Basin, China. Mar. Pet. Geol. 2020, 111, 33–43. [Google Scholar] [CrossRef]
  47. Wang, P.; Liu, Z.; Zhang, D.; Li, X.; Du, W.; Liu, H.; Li, P.; Wang, R. Control of organic matter enrichment on organic pore development in the Permian marine organic–rich shale, eastern Sichuan Basin. Oil Gas Geol. 2023, 44, 379–392, (In Chinese with English abstract). [Google Scholar]
  48. Zhang, H.; Jiao, S.; Pang, Q.; Li, N.; Lin, B. SEM observation of organic matters in the Eopaleozoic shale in South China. Oil Gas Geol. 2015, 36, 675–680, (In Chinese with English abstract). [Google Scholar]
  49. Zhou, S.; Yan, G.; Xue, H.; Guo, W.; Li, X. 2D and 3D nanopore characterization of gas shale in Longmaxi Formation based on FIB–SEM. Mar. Pet. Geol. 2016, 73, 174–180. [Google Scholar] [CrossRef]
  50. Jiao, S.; Han, H.; Weng, Q.; Yang, F.; Jiang, D.; Cui, L. Scanning electron microscope analysis of porosity in shale. J. Chin. Electron Microsc. Soc. 2012, 31, 432–436, (In Chinese with English abstract). [Google Scholar]
  51. Desbois, G.; Urai, J.L.; Peter, A.; Jan, K.; Claudia, B. High resolution 3D fabric and porosity model in a tight gas sandstone reservoir: A new approach to investigate microstructures from mm–to nm–scale combining argon beam cross–sectioning and SEM imaging. J. Pet. Sci. Eng. 2011, 78, 243–251. [Google Scholar] [CrossRef]
  52. Guo, H.; He, R.; Jia, W.; Peng, P.; Lei, Y.; Luo, X.; Wang, X.; Zhang, L.; Jiang, C. Pore characteristics of lacustrine shale within the oil window in the Upper Triassic Yanchang Formation, southeastern Ordos Basin, China. Mar. Pet. Geol. 2018, 91, 279–296. [Google Scholar] [CrossRef]
  53. Jiao, S.; Zhang, H.; Xue, D.; Huang, Z.; Liu, G. Morphological structure and identify method of organic macerals of shale with SEM. J. Chin. Electron Microsc. Soc. 2018, 37, 137–144, (In Chinese with English abstract). [Google Scholar]
  54. Fishman, N.; Hackley, P.; Lowers, H.; Hill, R.; Egenhoff, S.; Eberl, D.; Blum, A. The nature of porosity in organic–rich mudstones of the Upper Jurassic Kimmeridge Clay Formation, North Sea, offshore United Kingdom. Int. J. Coal Geol. 2012, 103, 32–50. [Google Scholar] [CrossRef]
  55. Milliken, K.L.; Rudnicki, M.; Awwiller, D.N.; Zhang, T. Organic matter–hosted pore system, Marcellus formation (Devonian), Pennsylvania. AAPG Bull. 2013, 97, 177–200. [Google Scholar] [CrossRef]
  56. Zhao, J.; Jin, Z.; Jin, K.; Du, W.; Wen, X.; Geng, Y. Petrographic methods to distinguish organic matter type in shale. Pet. Geol. Exp. 2016, 38, 514–520+527, (In Chinese with English abstract). [Google Scholar]
  57. Cooper, R.A.; Maletz, J.; Taylor, L.; Zalasiewicz, J.A. Graptolites: Patterns of diversity across paleolatitudes. In The Great Ordovician Biodiversification Event; Columbia University Press: New York, NY, USA, 2004; pp. 281–293. [Google Scholar]
  58. Chen, X. Graptolite depth zonation. Acta Palaeontol. Sin. 1990, 29, 507–526, (In Chinese with English abstract). [Google Scholar]
  59. Zou, C.; Gong, J.; Wang, H.; Shi, Z. Importance of graptolite evolution and biostratigraphic calibration on shale gas exploration. China Pet. Explor. 2019, 24, 147–164. [Google Scholar]
  60. Obermajer, M.; Stasiuk, L.D.; Fowler, M.G.; Osadetz, K.G. Application of acritarch fluorescence in thermal maturity studies. Int. J. Coal Geol. 1999, 39, 185–204. [Google Scholar] [CrossRef]
  61. Yin, M.; Yuan, X.; Meng, F. Illustrated Book of Organic–Walled Microfossils; BEIJING BOOK CO., Inc.: Linden, NJ, USA, 2018; (In Chinese with English abstract). [Google Scholar]
  62. Zhou, X.; Guo, W.; Li, X.; Zhang, X.; Liang, P.; Yu, J. Mutual relation between organic matter types and pores with petrological evidence of radiolarian siliceous shale in Wufeng–Longmaxi Formation, Sichuan Basin. J. China Univ. Pet. 2022, 46, 12–22, (In Chinese with English abstract). [Google Scholar]
  63. Milliken, K.L.; Curtis, M.E. Imaging pores in sedimentary rocks: Foundation of porosity prediction. Mar. Pet. Geol. 2016, 73, 590–608. [Google Scholar] [CrossRef]
  64. Schieber, J. SEM Observations on Ion–milled Samples of Devonian Black Shales from Indiana and New York: The Petrographic Context of Multiple Pore Types. AAPG Mem. 2013, 102, 153–171. [Google Scholar]
  65. He, Z.; Nie, H.; Zhao, J.; Liu, W.; Bao, F.; Zhang, W. Types and origin of nanoscale pores and fractures in Wufeng and Longmaxi shale in Sichuan Basin and its periphery. J. Nanosci. Nanotechnol. 2017, 17, 6626–6633. [Google Scholar] [CrossRef]
  66. Valenza, J.J.; Drenzek, N.; Marques, F.; Pagels, M.; Mastalerz, M. Geochemical controls on shale microstructure. Geology 2013, 41, 611–614. [Google Scholar] [CrossRef]
  67. Ma, Y.; Zhong, N.; Cheng, L.; Pan, Z.; Dai, N.; Zhang, Y.; Yang, L. Pore structure of the graptolite–derived OM in the Longmaxi Shale, southeastern Upper Yangtze Region, China. Mar. Pet. Geol. 2016, 72, 1–11. [Google Scholar] [CrossRef]
  68. Brown, A. Evaluation of possible gas microseepage mechanisms. AAPG Bull. 2000, 84, 1775–1789. [Google Scholar]
  69. Zou, C.; Dong, D.; Wang, S.; Li, J.; Li, X.; Wang, Y.; Li, D.; Cheng, K. Geological characteristics, formation mechanism and resource potential of shale gas in China. Pet. Explor. Dev. 2010, 37, 641–653, (In Chinese with English abstract). [Google Scholar] [CrossRef]
  70. Kuang, L.; Hou, L.; Wu, S.; Cui, J.; Tian, H.; Zhang, L.; Zhao, Z.; Luo, X.; Jiang, X. Organic matter occurrence and pore–forming mechanisms in lacustrine shales in China. Pet. Sci. 2022, 19, 1460–1472. [Google Scholar] [CrossRef]
  71. Xiao, X.; Jin, K. Hydrocarbon formation mode of maceral. Chin. Sci. Bull. 1991, 36, 208–211, (In Chinese with English abstract). [Google Scholar]
  72. Cai, J.; Zeng, X.; Wei, H.; Song, M.; Wang, X.; Liu, Q. From water body to sediments: Exploring the depositional processes of organic matter and their implications. J. Palaeogeogr. 2019, 21, 49–66, (In Chinese with English abstract). [Google Scholar]
  73. Xie, G.; Liu, S.; Jiao, K.; Deng, B.; Ye, Y.; Sun, W.; Li, Z.; Liu, W.; Luo, C.; Li, Z. Organic pores in deep shale controlled by macerals: Classification and pore characteristics of organic matter components in Wufeng Formation–Longmaxi Formation of the Sichuan Basin. Nat. Gas Ind. 2021, 41, 23–34, (In Chinese with English abstract). [Google Scholar]
  74. Boltovskoy, D.; Correa, N. Biogeography of Radiolaria Polycystina (Protista) in the world ocean. Prog. Oceanogr. 2016, 149, 82–105. [Google Scholar] [CrossRef]
  75. Hemingway, J.D.; Rothman, D.H.; Grant, K.E.; Rosengard, S.Z.; Eglinton, T.I.; Derry, L.A.; Galy, V.V. Mineral protection regulates long–term global preservation of natural organic carbon. Nature 2019, 570, 228–231. [Google Scholar] [CrossRef]
  76. Yang, C.; Zhang, J.; Han, S.; Wang, L.; Yu, W. The Nature and Classification of Organic–Associated Pores Based on In–situ Organic Petrology Through Comparative Study on Marine, Transitional, and Lacustrine Gas Shales in Typical Areas, China. In Proceedings of the AAPG/SEG International Conference & Exhibition, Cancun, Mexico, 6–9 September 2016. [Google Scholar]
  77. Gai, H.; Xiao, X.; Cheng, P.; Tian, H.; Fu, J. Gas generation of shale organic matter with different contents of residual oil based on a pyrolysis experiment. Org. Geochem. 2015, 78, 69–78. [Google Scholar] [CrossRef]
  78. Hunt, J.M. Petroleum geochemistry and geology (textbook). In Petroleum Geochemistry and Geology (Textbook), 2nd ed.; WH Freeman Company: New York, NY, USA, 1995. [Google Scholar]
  79. Welte, D.H.; Horsfield, B.; Baker, D.R. Petroleum and Basin Evolution: Insights from Petroleum Geochemistry, Geology and Basin Modeling; Springer Science & Business Media: Berlin, Germany, 2012. [Google Scholar]
  80. Li, X.; Chen, G.; Chen, Z.; Wang, L.; Wang, Y.; Dong, D.; Lv, Z.; Lv, W.; Wang, S.; Huang, J.; et al. An insight into the mechanism and evolution of shale reservoir characteristics with over–high maturity. J. Nat. Gas Geosci. 2016, 1, 373–382. [Google Scholar] [CrossRef]
  81. Nie, H.; He, Z.; Liu, G.; Du, W.; Wang, R.; Zhang, G. Genetic mechanism of high–quality shale gas reservoirs in the Wufeng–Longmaxi Fms in the Sichuan Basin. Nat. Gas Ind. 2020, 40, 31–41, (In Chinese with English abstract). [Google Scholar]
  82. Sun, C.; Nie, H.; Liu, G.; Zhang, G.; Du, W.; Wang, R. Quartz type and its control on shale gas enrichment and production: A case study of the Wufeng–Longmaxi Formations in the Sichuan Basin and its surrounding areas, China. Earth Sci. 2019, 44, 3692–3704, (In Chinese with English abstract). [Google Scholar]
  83. Qiu, Z.; Liu, B.; Dong, D.; Lu, B.; Yawar, Z.; Chen, Z.; Schieber, J. Silica diagenesis in the Lower Paleozoic Wufeng and Longmaxi Formations in the Sichuan Basin, South China: Implications for reservoir properties and paleoproductivity. Mar. Pet. Geol. 2020, 121, 104594. [Google Scholar] [CrossRef]
  84. Cai, C.; Cai, J.; Liu, H.; Wang, X.; Zeng, X.; Wang, Y. Occurrence of organic matter in argillaceous sediments and rocks and its geological significance: A review. Chem. Geol. 2023, 639, 121737. [Google Scholar] [CrossRef]
  85. Zhao, T.; Xu, S.; Hao, F. Differential adsorption of clay minerals: Implications for organic matter enrichment. Earth–Sci. Rev. 2023, 246, 104598. [Google Scholar] [CrossRef]
  86. Yuan, P.; Liu, H.; Liu, D.; Wu, D. The catalytic effect of clay minerals in the process of oil and gas formation: Some thoughts. Acta Mineral. Sin. 2012, 32, 70–71, (In Chinese with English abstract). [Google Scholar]
Applsci 15 01310 g001
Figure 1. SEM image of alginite. (a) Alginite containing fibrous clay minerals, Y206#, 3851.82 m; (b) Alginite with a pure interior, Y206#, 3864.75 m; (c) Alginite with a pure interior, Y206#, 3861.36 m; (d) Alginite with a pure interior, Y206#, 3861 m.
Figure 1. SEM image of alginite. (a) Alginite containing fibrous clay minerals, Y206#, 3851.82 m; (b) Alginite with a pure interior, Y206#, 3864.75 m; (c) Alginite with a pure interior, Y206#, 3861.36 m; (d) Alginite with a pure interior, Y206#, 3861 m.
Applsci 15 01310 g001
Applsci 15 01310 g002
Figure 2. SEM image of zooclasts. (a) Graptolite periderm showing a banded or vein-like structure, Y101H3-8#, 3788.24 m; (b) Graptolite periderm with microcracks and segmented structures, Y101H3-8#, 3784.36 m; (c) Graptolite periderm exhibiting microcracks and segmented structures, Y101H3-8#, 3787.78 m; (d) Graptolite periderm cavity displaying a “cortical bandage” structure, Y101H3-8#, 3792.11 m; (e) Chitinozoan vesicles showing a vase-like shape, Y206#, 3869.84 m; (f) Chitinozoan vesicles with a symmetrical structure, Y206#, 3875.5 m.
Figure 2. SEM image of zooclasts. (a) Graptolite periderm showing a banded or vein-like structure, Y101H3-8#, 3788.24 m; (b) Graptolite periderm with microcracks and segmented structures, Y101H3-8#, 3784.36 m; (c) Graptolite periderm exhibiting microcracks and segmented structures, Y101H3-8#, 3787.78 m; (d) Graptolite periderm cavity displaying a “cortical bandage” structure, Y101H3-8#, 3792.11 m; (e) Chitinozoan vesicles showing a vase-like shape, Y206#, 3869.84 m; (f) Chitinozoan vesicles with a symmetrical structure, Y206#, 3875.5 m.
Applsci 15 01310 g002
Applsci 15 01310 g003
Figure 3. SEM image of acritarchs. (a) Acritarchs, Y101H3–8#, 3754.94 m; (b) Acritarchs, Y101H3–8#, 3780.05 m; (c) Acritarchs, Y101H3–8#, 3783.94 m; (d) Acritarchs, Y206#, 3861 m; (e) Acritarchs, Y206#, 3876.75 m; (f) Acritarchs, Y206#, 3877.75 m.
Figure 3. SEM image of acritarchs. (a) Acritarchs, Y101H3–8#, 3754.94 m; (b) Acritarchs, Y101H3–8#, 3780.05 m; (c) Acritarchs, Y101H3–8#, 3783.94 m; (d) Acritarchs, Y206#, 3861 m; (e) Acritarchs, Y206#, 3876.75 m; (f) Acritarchs, Y206#, 3877.75 m.
Applsci 15 01310 g003
Applsci 15 01310 g004
Figure 4. SEM image of encapsulated organic matter. (a) Encapsulated organic matter, Y101H3–8#, 3782.95 m; (b) Encapsulated organic matter, Y101H3–8#, 3782.95 m; (c) Encapsulated organic matter, Y206#, 3853.13 m; (d) Encapsulated organic matter, Y206#, 3851 m; (e) Encapsulated organic matter, Y206#, 3864.83 m; (f) Encapsulated organic matter, Y206#, 3864.75 m.
Figure 4. SEM image of encapsulated organic matter. (a) Encapsulated organic matter, Y101H3–8#, 3782.95 m; (b) Encapsulated organic matter, Y101H3–8#, 3782.95 m; (c) Encapsulated organic matter, Y206#, 3853.13 m; (d) Encapsulated organic matter, Y206#, 3851 m; (e) Encapsulated organic matter, Y206#, 3864.83 m; (f) Encapsulated organic matter, Y206#, 3864.75 m.
Applsci 15 01310 g004
Applsci 15 01310 g005
Figure 5. SEM image of compacted kerogen. (a) Compacted kerogen, Y206#, 3853.13 m; (b) Compacted kerogen, Y206#, 3859 m; (c) Compacted kerogen, Y206#, 3868.06 m; (d) Compacted kerogen, Y206#, 3876.75 m; (e) Compacted kerogen, Y206#, 3876.26 m; (f) Compacted kerogen, Y206#, 3880.17 m.
Figure 5. SEM image of compacted kerogen. (a) Compacted kerogen, Y206#, 3853.13 m; (b) Compacted kerogen, Y206#, 3859 m; (c) Compacted kerogen, Y206#, 3868.06 m; (d) Compacted kerogen, Y206#, 3876.75 m; (e) Compacted kerogen, Y206#, 3876.26 m; (f) Compacted kerogen, Y206#, 3880.17 m.
Applsci 15 01310 g005
Applsci 15 01310 g006
Figure 6. SEM image of in situ remnants from post–hydrocarbon generation. (a) In situ remnants from post–hydrocarbon generation, Y206#, 3860.75 m; (b) In situ remnants from post–hydrocarbon generation, Y206#, 3859 m; (c) In situ remnants from post–hydrocarbon generation, Y206#, 3865.5 m; (d) In situ remnants from post–hydrocarbon generation, Y206#, 3873.7 m; (e) In situ remnants from post–hydrocarbon generation, Y206#, 3872 m; (f) In situ remnants from post–hydrocarbon generation, Y206#, 3872 m.
Figure 6. SEM image of in situ remnants from post–hydrocarbon generation. (a) In situ remnants from post–hydrocarbon generation, Y206#, 3860.75 m; (b) In situ remnants from post–hydrocarbon generation, Y206#, 3859 m; (c) In situ remnants from post–hydrocarbon generation, Y206#, 3865.5 m; (d) In situ remnants from post–hydrocarbon generation, Y206#, 3873.7 m; (e) In situ remnants from post–hydrocarbon generation, Y206#, 3872 m; (f) In situ remnants from post–hydrocarbon generation, Y206#, 3872 m.
Applsci 15 01310 g006
Applsci 15 01310 g007
Figure 7. SEM image of migrated organic matter in intragranular pore. (a) Migrated organic matter in intragranular pore, Y101H3–8#, 3780.05 m; (b) Migrated organic matter in intragranular pore, Y101H3–8#, 3780.05 m; (c) Migrated organic matter in intragranular pore, Y101H3–8#, 3771.32 m; (d) Migrated organic matter in intragranular pore, Y206#, 3864.75 m; (e) Migrated organic matter in intragranular pore, Y206#, 3864.75 m; (f) Migrated organic matter in intragranular pore, Y206#, 3867.5 m.
Figure 7. SEM image of migrated organic matter in intragranular pore. (a) Migrated organic matter in intragranular pore, Y101H3–8#, 3780.05 m; (b) Migrated organic matter in intragranular pore, Y101H3–8#, 3780.05 m; (c) Migrated organic matter in intragranular pore, Y101H3–8#, 3771.32 m; (d) Migrated organic matter in intragranular pore, Y206#, 3864.75 m; (e) Migrated organic matter in intragranular pore, Y206#, 3864.75 m; (f) Migrated organic matter in intragranular pore, Y206#, 3867.5 m.
Applsci 15 01310 g007
Applsci 15 01310 g008
Figure 8. SEM image of migrated organic matter in intergranular pore. (a) Migrated organic matter infilling between clay mineral flakes, Y101H3–8#, 3780.05 m; (b) Migrated organic matter infilling between clay mineral flakes, Y101H3–8#, 3780.05 m; (c) Migrated organic matter infilling between pyrite crystals, Y101H3–8#, 3789.3 m; (d) Migrated organic matter infilling between pyrite crystals, Y101H3–8#, 3789.3 m; (e) Migrated organic matter infilling between microcrystalline quartz, Y101H3–8#, 3780.05 m; (f) Migrated organic matter infilling between microcrystalline quartz, Y101H3–8#, 3784.36 m.
Figure 8. SEM image of migrated organic matter in intergranular pore. (a) Migrated organic matter infilling between clay mineral flakes, Y101H3–8#, 3780.05 m; (b) Migrated organic matter infilling between clay mineral flakes, Y101H3–8#, 3780.05 m; (c) Migrated organic matter infilling between pyrite crystals, Y101H3–8#, 3789.3 m; (d) Migrated organic matter infilling between pyrite crystals, Y101H3–8#, 3789.3 m; (e) Migrated organic matter infilling between microcrystalline quartz, Y101H3–8#, 3780.05 m; (f) Migrated organic matter infilling between microcrystalline quartz, Y101H3–8#, 3784.36 m.
Applsci 15 01310 g008
Applsci 15 01310 g009
Figure 9. SEM image of bitumen in microfractures. (a) Bitumen in microfractures, Y101H3–8#, 3754.94 m; (b) Bitumen in microfractures, Y101H3–8#, 3754.94 m; (c) Bitumen in microfractures, Y101H3–8#, 3754.94 m; (d) Bitumen in microfractures, Y101H3–8#, 3759.27 m; (e) Bitumen in microfractures, Y101H3–8#, 3785.51 m; (f) Bitumen in microfractures, Y101H3–8#, 3787.78 m.
Figure 9. SEM image of bitumen in microfractures. (a) Bitumen in microfractures, Y101H3–8#, 3754.94 m; (b) Bitumen in microfractures, Y101H3–8#, 3754.94 m; (c) Bitumen in microfractures, Y101H3–8#, 3754.94 m; (d) Bitumen in microfractures, Y101H3–8#, 3759.27 m; (e) Bitumen in microfractures, Y101H3–8#, 3785.51 m; (f) Bitumen in microfractures, Y101H3–8#, 3787.78 m.
Applsci 15 01310 g009
Applsci 15 01310 g010
Figure 10. SEM image of pore development characteristics of depositional organic matter. (a) Alginite with angular pores, Y206#, 3767.73 m; (b) Chitinozoan vesicles with no pore, Y206#, 3788.69 m; (c) Graptolite periderm fragments with no pore, Y101H3–8#, 3784.36 m; (d) “Cortical bandage” structure of the graptolite periderm with spindle–shaped pores between layers, Y206#, 3864.83 m; (e) Acritarch with no pore, Y101H3–8#, 3780.05 m; (f) Encapsulated organic matter with no pore, Y206#, 3864.75 m; (g) Compacted kerogen with no pore, Y206#, 3871.62 m; (h) In situ remnants from post–hydrocarbon generation with honeycomb–like pores, Y206#, 3861.25 m.
Figure 10. SEM image of pore development characteristics of depositional organic matter. (a) Alginite with angular pores, Y206#, 3767.73 m; (b) Chitinozoan vesicles with no pore, Y206#, 3788.69 m; (c) Graptolite periderm fragments with no pore, Y101H3–8#, 3784.36 m; (d) “Cortical bandage” structure of the graptolite periderm with spindle–shaped pores between layers, Y206#, 3864.83 m; (e) Acritarch with no pore, Y101H3–8#, 3780.05 m; (f) Encapsulated organic matter with no pore, Y206#, 3864.75 m; (g) Compacted kerogen with no pore, Y206#, 3871.62 m; (h) In situ remnants from post–hydrocarbon generation with honeycomb–like pores, Y206#, 3861.25 m.
Applsci 15 01310 g010
Applsci 15 01310 g011
Figure 11. SEM image of pore development characteristics of migrated organic matter. (a) Migrated organic matter within intragranular pores exhibiting a sponge-like texture, Y101H3-8#, 3780.05 m; (b) Migrated organic matter infilling the interlamellar spaces of clay mineral flakes, with nanopores, Y101H3-8#, 3785.51 m; (c) Migrated organic matter infilling the interlamellar spaces of clay mineral flakes, with nanopores, Y101H3-8#, 3780.05 m; (d) Migrated organic matter infilling the spaces between pyrite crystals, characterized by globular pores, Y206#, 3864.83 m; (e) Migrated organic matter infilling the interstitial spaces between microcrystalline quartz, with well-developed pores, Y101H3-8#, 3783.94 m; (f) Bitumen filling microfractures, with no observable pores, Y101H3-8#, 3754.94 m.
Figure 11. SEM image of pore development characteristics of migrated organic matter. (a) Migrated organic matter within intragranular pores exhibiting a sponge-like texture, Y101H3-8#, 3780.05 m; (b) Migrated organic matter infilling the interlamellar spaces of clay mineral flakes, with nanopores, Y101H3-8#, 3785.51 m; (c) Migrated organic matter infilling the interlamellar spaces of clay mineral flakes, with nanopores, Y101H3-8#, 3780.05 m; (d) Migrated organic matter infilling the spaces between pyrite crystals, characterized by globular pores, Y206#, 3864.83 m; (e) Migrated organic matter infilling the interstitial spaces between microcrystalline quartz, with well-developed pores, Y101H3-8#, 3783.94 m; (f) Bitumen filling microfractures, with no observable pores, Y101H3-8#, 3754.94 m.
Applsci 15 01310 g011
Applsci 15 01310 g012
Figure 12. Schematic diagram of the origin of depositional organic matter.
Figure 12. Schematic diagram of the origin of depositional organic matter.
Applsci 15 01310 g012
Applsci 15 01310 g013
Figure 13. Schematic diagram of compaction.
Figure 13. Schematic diagram of compaction.
Applsci 15 01310 g013
Applsci 15 01310 g014
Figure 14. Schematic diagram of in situ remnants from post–hydrocarbon generation.
Figure 14. Schematic diagram of in situ remnants from post–hydrocarbon generation.
Applsci 15 01310 g014
Applsci 15 01310 g015
Figure 15. Schematic diagram of the types of migrated organic matter in highly-over mature marine shale.
Figure 15. Schematic diagram of the types of migrated organic matter in highly-over mature marine shale.
Applsci 15 01310 g015
Table 1. Summary of organic matter classification.
Table 1. Summary of organic matter classification.
Classification CriteriaClassification of Organic MatterCharacteristics
Maceral groups [12]VitriniteOriginating from higher plants, these particles are dispersed within shale matrices and generally lack cellular structure.
InertinieOriginating from higher plants, these particles are dispersed within shale matrices and retain the cellular structure, including plant cell lumens.
LiptiniteThe highest concentration in shale occurs in fragmental forms, primarily consisting of alginite, bituminite, and liptodetrinite. Alginite is mainly derived from planktonic algae and is classified into telalginite and lamalginite. Bituminite originates from plankton, zooplankton, and bacteria that have undergone bacterial degradation, lacking a fixed structure.
ZooclastsDerived from fragments of planktonic or benthic organisms, distinguishable by their microscopic morphology.
Secondary organic matterDerived from fragments of planktonic or benthic organisms, distinguishable by their microscopic morphology.
Mode of occurrence [28]Banding organic matterAppearing as straight or curved elongated strips or lenticular shapes, distributed parallel to the bedding, influenced by compaction and surrounding matrix minerals.
Agglomerate organic matterExhibiting irregular blocky shapes or regular round (or elliptical) forms, with distinct boundaries separating them from the matrix minerals, often oriented in a specific direction.
Infilling organic matterFilling clay-sized pores, bedding planes, or interlaminar fissures, serving as a cement for detrital grains, with indistinct boundaries between the organic matter and surrounding minerals.
Interwrapped organic matterEncapsulating or being encapsulated by authigenic minerals, with no fixed morphology.
Distribution pattern [29]Laminated organic matterIncludes two types: one characterized by interbedded organic-rich layers with mineral layers, and the other by elongated organic matter dispersed within a mineral matrix.
Particulate organic matterIncludes two types: one characterized by interbedded organic-rich layers with mineral layers, and the other by elongated organic matter dispersed within a mineral matrix.
Transitional organic matterThe shape is irregular or constrained by surrounding minerals, with indistinct boundaries between the organic matter and the minerals.
Genetic mechanism [26]Depositional organic matterThe shape is irregular or constrained by surrounding minerals, with indistinct boundaries between the organic matter and the minerals.
Migrated organic matterPetroleum and bitumen, along with their evolved derivatives, are formed through the thermal evolution of organic matter and migrate to adjacent pores/fractures. This process is characterized by the sequence where surrounding minerals form first, followed by the filling and injection of organic matter.
Micromorphology [27]Structural sedimentary organic matterExhibiting distinct structural morphology with clear boundaries separating it from the matrix minerals.
Differentiated syngenetic organic matterIntergrown with clay minerals or authigenic quartz, forming a structure resembling a ’granitoid texture’, and distributed in a dispersed manner.
Interstitially transported organic matterIntergrown with clay minerals or authigenic quartz, forming a structure resembling a ’granitoid texture’, and distributed in a dispersed manner.
Table 2. Summary of pore classification.
Table 2. Summary of pore classification.
CriteriaClassification of PoresCharacteristics
Genesis and occurrence location [40]Organic matter poresPores are formed during the thermal cracking of organic matter to generate hydrocarbons, primarily developing between and within organic matter.
Inorganic poresPores are formed during sedimentation or diagenesis through processes such as crystallization, cementation, and dissolution, and are distributed between or within mineral grains.
MicrofracturesIncluding bedding-parallel fractures formed by sedimentation processes, and non-bedding-parallel fractures formed during diagenesis. Non-bedding-parallel fractures include hydrocarbon generation overpressure fractures, diagenetic shrinkage fractures, and dissolution fractures.
Formation stage [41]Primary poresBelonging to intergranular residual pores, controlled by compaction and cementation.
Secondary poresPores formed through diagenetic modification based on primary pores, primarily controlled by dissolution processes.
Mixed poresIncluding partial primary pores and partial secondary pores.
Percolation characteristics [36]Total poresThe total sum of connected and non-connected pores within a reservoir, also referred to as absolute porosity.
Connected poresSuper-capillary poresPore diameter > 500 μm. The liquid within the pores can flow freely under the influence of gravity.
Capillary poresPore diameter from 0.2 μm to 500 μm. Fluid particles in the pores can only flow under the influence of driving pressure.
Microcapillary poresPore diameter < 0.2 μm. The liquid within the pores is immobile and exists in an adsorbed state.
Effective connected pores The total of the reservoir’s ultra-micropores and micropores, also known as theoretical flow pores.
Flowing pores, and The total of supra-capillary and capillary pores that can participate in fluid flow under reservoir production conditions, also referred to as production flow pores.
Charged poresThe sum of capillary and super-capillary pores that can participate in fluid flow under reservoir formation conditions, also referred to as charge flow pores.
Pore size [35]MacroporesPore diameter > 50 nm
MesoporesPore diameter from 2 to 50 nm
MicroporesPore diameter < 2 nm
Developmental morphology [42]Zonal poresThe morphology is variable, constrained by the shape of organic matter particles, and is formed during hydrocarbons generation from the degradation of organic matter.
Globular poresAt initial stage of pore formation resulting from the cracking of residual oil into gas.
Spongy poresForming through gas generation during pyrolysis at high evolution of the late diagenetic stage.
Degree of opening and Connectivity [37]Open poresConnected to the external environment
Closed poresIsolated from external connections
Table 3. Classification of organic matter and its pore characteristics and genesis in high-overmature marine shale.
Table 3. Classification of organic matter and its pore characteristics and genesis in high-overmature marine shale.
Type of Organic MatterMorphologyDistribution CharacteristicsPore Development CharacteristicsGenetic Mechanism
Organic MatterOrganic Matter Pores
Depositional organic matterBioclastsAlginiteirregularIsolated granular distributed with a clear boundary with surrounding minerals. It is internally pure, with no clay minerals present, or less clay minerals exhibiting a fibrous distribution within the algal detritus.The pores are well-developed, exhibiting irregular angular or rounded shapes, and are uniformly sized. The organic matter pore size between fibrous clay minerals ranges from approximately 10 to 50 nm, while the pore size within pure alginite ranges from several tens to several hundreds of nm.Products of the diagenesis of planktonic algae. During the burial period, the formation fluids in some channels and fractures of the alginite react with rocks to form fibrous illite, resulting in alginite containing fibrous illite.In low-mature organic matter, pores are often gaps between single-cell algae arrangements. Spherical pores result from the early stages of oil generation and the expulsion from algal-rich material. Pores that are regular in arrangement and have elongated, narrow, elliptical shapes may be attributed to compaction effects following hydrocarbon expulsion.
ZooclastsGraptolite peridermbanding or veiningIt predominantly exists in the form of planar carbon films parallel to the bedding, with the cavities developing a segmented structure resulting in anisotropy.The graptolite body exhibits either an absence or minimal development of pores, with occasional occurrences of localized nanoscale organic matter pores. Elongated nanoscale pores or microfractures are developed between the cortical fibrils.Graptolite periderm is primarily composed of collagenous skeleton secreted by the conodont organism, typically preserved in black shale, which indicates a tranquil, anoxic, and stagnant reduction environment.The biological tissue structure of conodonts reveals pores between cortical fibrils, with an increase in porosity corresponding to the evolution degree.
Chitinozoan vesiclesvase-shaped, conical, and cylindrical etc.Some chitinous vesicles are connected in a chain-like manner, with most exhibiting a symmetrical structure. Shell cavities are often filled with inorganic minerals.No poreIndividual microbial fossil./
Acritarchsround or elliptical granulesIsolated and sporadically distributed, with close contact with surrounding minerals, clear boundaries, and visible apatite within the organic matter.No poreDerived from certain algae and unicellular organisms./
Encapsulated organic matterspherical, ellipsoidal, or concentric structuresIsolated granular distributed with the development of siliceous shells.No poreThe original organic matter is filled in siliceous shells primarily derived from radiolarians and sponges spicules./
Compacted kerogenamorphous massiveRigid mineral grains, such as quartz, are present in the surrounding environment, with no mineral cementation, allowing for direct contact between it and mineral grains.Almost no poreUnder shallow burial or pressure, mineral particles are compressed and compacted, leading to plastic deformation of organic matter and subsequently the closure of adjacent pore spaces./
In situ remnants from post-hydrocarbon generationamorphous massiveNo crystal growth between it and adjacent minerals, with clear boundaries. The morphology includes granular and banded forms.The pores are significantly developed, exhibiting round or elliptical shapes, resembling bubble-like or sponge-like structures.Primary including the residual kerogen after the generation of oil and gas, and the in situ retained oil following the gas generation.The formation of organic pores can be attributed to two primary mechanisms: first, during the pyrolysis of kerogen, a portion of the generated oil and gas is expelled from the source rock, resulting in the formation of pores; second, some in situ liquid hydrocarbons retained within the source rock undergo further thermal maturation through cracking, generating natural gas and consequently leading to the formation of organic pores.
Migrated organic matterMigrated organic matter in intragranular poreamorphous or constrained by mineral particlesFilling in the radiolarian siliceous shells replaced by pyrite, authigenic minerals are typically observed in the surrounding areas.The pores are well-developed, exhibiting a vesicular morphology with irregular sizes and its diameters ranging from 30 to 300 nm.Migrated organic matter is subsequently filled in these siliceous radiolarian shells replaced by pyrite, which forms concomitantly with bacterial sulfate reduction.A significant amount of dry gas, generated from the secondary cracking of migrated organic matter, escapes from the organic matter, leading to the formation of organic matter pores.
Migrated organic matter in intergranular poreamorphousFilling in the intergranular pores of minerals with uneven size, the coexistence with authigenic minerals, and accumulation in the clay mineral flakes, pyrite crystals and microcrystalline quartz.The pores are well-developed, with pores exhibiting a sponge-like structure and pore diameters generally ranging from 10 to 80 nm.The organic matter that has migrated preferentially fills the pre-existing pores among mineral grains.
Bitumen in microfracturesbandingIn conformable contact with sedimentary clasts, It often interspersed with fragments of the surrounding rock or framboidal pyrite, with distinct boundary with matrix minerals.Commonly, shrinkage cracks are well-developed, while pores are underdeveloped.Organic-rich shale fractures along bedding planes under overburden pressure, forming microfractures, due to the lower cohesion and internal friction angle, increased porosity and reduced rock strength after hydrocarbon generation. The bitumen generated after the oil window migrates and fills these microfractures along their extension direction./
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Zhao, M.; Wang, H.; Shi, Z.; Zhao, Q.; Zhou, T.; Qi, L. Microscopic Characteristics and Formation of Various Types of Organic Matter in High-Overmature Marine Shale via SEM. Appl. Sci. 2025, 15, 1310. https://doi.org/10.3390/app15031310

AMA Style

Zhao M, Wang H, Shi Z, Zhao Q, Zhou T, Qi L. Microscopic Characteristics and Formation of Various Types of Organic Matter in High-Overmature Marine Shale via SEM. Applied Sciences. 2025; 15(3):1310. https://doi.org/10.3390/app15031310

Chicago/Turabian Style

Zhao, Meng, Hongyan Wang, Zhensheng Shi, Qun Zhao, Tianqi Zhou, and Ling Qi. 2025. "Microscopic Characteristics and Formation of Various Types of Organic Matter in High-Overmature Marine Shale via SEM" Applied Sciences 15, no. 3: 1310. https://doi.org/10.3390/app15031310

APA Style

Zhao, M., Wang, H., Shi, Z., Zhao, Q., Zhou, T., & Qi, L. (2025). Microscopic Characteristics and Formation of Various Types of Organic Matter in High-Overmature Marine Shale via SEM. Applied Sciences, 15(3), 1310. https://doi.org/10.3390/app15031310

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop