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Article

Reservoir Characteristics and Influencing Factors of Organic-Rich Siliceous Shale of the Upper Permian Dalong Formation in Western Hubei

by
Yang Wang
1,2,
Luheng Bai
1,*,
Yanlin Zhang
3,*,
Xiaoming Zhang
1,
Bowei Yang
1,
Ke Duan
3,
Yi Wang
3 and
Tong Xie
3
1
School of Earth Resources, China University of Geosciences, Wuhan 430074, China
2
Hubei Geological Bureau, Wuhan 430074, China
3
Hubei Geological Survey, Wuhan 430074, China
*
Authors to whom correspondence should be addressed.
Energies 2023, 16(13), 5130; https://doi.org/10.3390/en16135130
Submission received: 11 June 2023 / Revised: 27 June 2023 / Accepted: 29 June 2023 / Published: 3 July 2023
(This article belongs to the Special Issue Unconventional Oil and Gas: Reservoir Evaluation and Accumulation Mechanism Research)
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Abstract

:
To elucidate the reservoir characteristics of organic-rich siliceous shale of the Upper Permian Dalong Formation in western Hubei, this study focused on the drilling cores of Well ED-2. Various techniques, including a mineral composition analysis, an organic carbon content analysis, a vitrinite reflectance measurement, a total porosity determination, field emission scanning electron microscopy (FE-SEM), and low-pressure CO2 and N2 physical adsorption tests, were employed to analyze the mineralogy, organic geochemistry, total porosity, and pore structure characteristics. Additionally, the factors influencing the reservoir performance of the Dalong Formation shale were investigated. The results indicated that the Dalong Formation’s shale was characterized as an organic-rich siliceous shale. Organic matter was mainly of sapropelic type, with a relatively high thermal evolution degree and Ro ranging from 2.59% to 2.76%. The total porosity of the Dalong Formation’s siliceous shale was low, indicating poor reservoir properties. Organic matter pores were highly developed, mainly the ones formed after the hydrocarbon generation of solid asphalt. Micropores and mesopores were the dominant pore types in the shale, with macropores being significantly less abundant. The study further revealed that the pore volume and specific surface area exhibited a significantly positive correlation with total organic carbon (TOC) content and clay minerals, while demonstrating a weak negative correlation with quartz content. The comprehensive analysis revealed that there were two factors contributing to the poor physical properties of organic-rich siliceous shale in the Dalong Formation. Firstly, in siliceous shale with a high quartz content, the siliceous component was partly derived from the siliceous cementation of hydrothermal fluids. This process led to the formation of secondary quartz that filled intergranular pores, resulting in a decrease in macropore volume, total porosity, and a weak negative correlation with quartz content. Secondly, in siliceous shale with a relatively high clay mineral content, the organic matter was subjected to stronger compaction due to the relatively low content of brittle minerals. This compaction caused the destruction of most macropores, leaving behind primarily micropores and mesopores. Consequently, the average pore size decreased, leading to poor physical properties.

1. Introduction

Shale gas is a clean energy resource and hydraulic fracturing has made its exploration much easier [1,2,3]. Following the successful commercial exploitation of shale gas in North America, China has achieved a significant milestone by attaining commercial viability in the Longmaxi Formation’s shale gas reservoir within the Sichuan Basin. This achievement has positioned China’s shale gas field as the largest outside of North America [4,5,6,7]. Exploration for shale oil and gas in the Yangtze region has identified the Dalong Formation of the Upper Permian as a significant target, as part of the ongoing efforts to discover additional shale gas fields in China [4,5].
The shale of the Upper Permian Dalong Formation in western Hubei, located in the middle Yangtze region, is mainly of rifted platform basin facies, with a wide distribution, a high content of organic matter and brittle minerals, a relatively shallow burial depth, and a moderate thermal evolution degree [8,9,10,11]. Moreover, good gas logging has been found in Well Baye 1 in the Badong area and Well Endi 1 in the Enshi area of western Hubei, showing good shale gas exploration prospects [12,13]. The organic-rich siliceous shale of the Dalong Formation in western Hubei has excellent hydrocarbon generation potential [9,13], but its shale gas exploration and development is still in the preliminary stage mainly due to a lack of understanding of the reservoir characteristics of shale gas.
In recent years, China has made significant progress in the exploration and development of shale gas in the Sichun Basin and in shale reservoir research of the Longmaxi Formation of the Lower Silurian located at the periphery of the Sichuan Basin [14,15,16,17,18]. The Longmaxi Formation’s shale is mainly argillaceous-rich siliceous shale, and the average TOC content and quartz content are 35% and 43.7%, respectively. The siliceous shale of the Dalong Formation in western Hubei has good similarities in terms of mineral composition and organic matter content [8,14,15]. The total porosity of the Longmaxi Formation’s shale is relatively high (average = 4.78%) with good physical properties, and the organic matter content has a good positive correlation with the quartz content, which is related to the fact that organic matter is derived from siliceous organisms [16,17,18]. In addition, the Longmaxi Formation’s shale has a relatively high total porosity and a good positive correlation with the quartz content, which is manifested as a high content of brittle minerals being beneficial for pore resistance to compaction [19]. However, according to current research on shale reservoir in the Dalong Formation in western Hubei, the total porosity of the Dalong Formation’s shale is low, with poor physical properties, which is significantly different from the high porosity of the Longmaxi Formation’s shale [20,21]. The organic matter content of the Dalong Formation’s siliceous shale has a positive correlation with the clay mineral content and a negative correlation with the quartz content, which is also quite different from that of the Longmaxi Formation’s shale and contrary to usual understanding [8,18]. The lack of clear understanding of the reservoir characteristics, pore structure types, and control factors leading to the low total porosity of the Dalong Formation’s siliceous shale in western Hubei has seriously constrained the exploration and development process of the organic-rich shale in Dalong Formation, which is in urgent need of solution.
In this study, we take the Dalong Formation’s siliceous shale in Western Hubei as the research object. By analyzing the mineral composition and organic geochemical characteristics of the shale, conducting total porosity tests, performing field emission scanning electron microscope (FE-SEM) observations, and carrying out CO2 and N2 adsorption tests, this paper analyzes the microscopic characteristics and reservoir physical properties of the organic-rich siliceous shale of the Dalong Formation, qualitatively evaluates the shale pore type, pore morphology and size, and quantitatively characterizes the pore structure parameters such as pore volume and specific surface area, to explore the reservoir characteristics and influencing factors of organic-rich siliceous shale in the Dalong Formation and clarify the cause of its pore development. This research provides essential knowledge for assessing the shale gas potential in the Upper Permian Dalong Formation in western Hubei.

2. Geological Setting

During the Late Permian period, the South China plate occupied a position in close proximity to the ancient equator. It was situated adjacent to the Paleo-Tethys Ocean in the western direction, while Panthalassa lay to the east. Many microplates were developed around the plate, and the overall structure was a multi-island ocean structure [22,23] (Figure 1a). The central Yangtze region in South China was predominantly characterized as a carbonate platform. However, the Dalong Formation, which consists mainly of siliceous shale, represented a deep-water facies that was distributed along the northern and southern margins of the Yangtze platform [24]. During the Late Permian period, Western Hubei was situated within an intraplatform, secondary, rifted basin on the northern margin of the Middle Yangtze region. The sedimentary basin was bordered by the platform on three sides, while in the north, it connected to a vast ocean, resulting in a semiclosed sedimentary environment (Figure 1b). The Dalong Formation’s shale representing a deep-water basin facies is primarily found within the rift trough, which is transformed into Changxing Formation limestone from the center of the rift trough to the surrounding platform [8,9,25].
Currently, a significant portion of western Hubei lies within the western section of the Hunan–Hubei fold belt, situated in the central region of the South China plate’s Yangtze platform, which has undergone multiple tectonic movements and mainly develops two sets of basin-controlling faults in the NNE and NW directions. It is a residual basin formed by the reconstruction and superposition of various types of prototype basins [11,26]. The western Hubei area studied in this research is adjacent to the Sichuan Basin, with the Qiyueshan fault as the boundary in the northwest, and connected to the Jiangnan Xuefeng nappe uplift with the Tianyangping-Jianli fault and Huangling anticline–Yichang slope as the boundary in the northeast, and the Cili–Baojing fault as the boundary in the southeast, mainly including four substructural units: Lichuan and Huaguoping synclinoria, central and Yidu–Hefeng anticlinoria (Figure 1c).
The Well ED-2 was located in a rifted trough of the western Hubei platform in late Permian (Figure 1) and experienced frequent sea level changes. The thickness, mineral composition, and sedimentary environment of the Dalong formation are mainly controlled by the platform basin facies, with a dominant development of siliceous mudstone, carbonaceous mudstone, calcareous mudstone, siliceous rock, and bioclastic limestone (Figure 2).

3. Samples and Experiments

3.1. Sampling

A total of twenty-six samples were collected from Well ED-2 at depths ranging from 1215.9 m to 1252.95 m. These samples were subjected to various analyses including X-ray diffraction, organic geochemistry, total porosity measurements, field emission scanning electron microscopy, as well as CO2 and N2 physical adsorption analyses (Figure 2). The analyses were conducted at the Laboratory of Wuhan Xinshengji Technology Co., Ltd. (Wuhan, China).

3.2. XRD and Organic Geochemistry

The whole-rock X-ray diffraction (XRD) analysis of the 26 samples was performed using a Rigaku SmartLab SE X-ray diffractometer, operating at 40 kV and 40 mA. The analysis followed the guidelines outlined in the national standard SY/T5163-2018 [27]. The TOC analysis of the same 26 samples was carried out using a Shimadzu TOC-LCPH SM5000 analyzer, following the specifications stated in the national standard GB/T19145-2003 [28]. For the Ro analysis, 20 out of the 26 samples were examined using a Leica MPM-80 stereomicroscope, in accordance with the national standard SY/T5124-2012 [29]. Additionally, the microstructure of kerogen in all 26 samples was investigated using a Leica DMLD microscope, following the guidelines outlined in the national standard SY/T 5125-1996 [30].

3.3. Scanning Electron Microscope (SEM)

We selected 6 samples to observe and analyze shale pore type and morphology by the EVO LS 15 scanning electron microscope system. Before conducting the experiment, the shale samples were prepared by undergoing mechanical grinding to achieve a polished and flat surface. Subsequently, ion milling was performed on the polished surface, and a carbon coating was applied to ensure conductivity. The prepared samples were then subjected to secondary electron imaging for surface observation.

3.4. Total Porosity and CO2 and N2 Adsorption Experiments

For the analysis of the total porosity, 18 shale samples were selected and tested by the KXD-II porosity and permeability joint tester. We selected 7 samples for the carbon dioxide and nitrogen adsorption tests to quantitatively analyze the shale pore structure characteristics. The specific surface adsorption instrument Autosorb IQ3 from Quantachrome Instruments was utilized to conduct adsorption tests on carbon dioxide (CO2) and nitrogen (N2) at low temperature and low pressure. For the carbon dioxide adsorption test, different relative pressure conditions ranging from 0 to 0.03 were examined to determine the corresponding gas adsorption quantity using CO2 as the adsorbate. The obtained data were processed using the density functional theory (DFT) model, with a focus on analyzing the pore volume and specific surface area of micropores. In the nitrogen adsorption test, measurements of the nitrogen’s adsorption and desorption capacity were performed at the temperature of liquid nitrogen (77.4 K) under various pressures. The acquired data were then used to calculate relevant parameters, such as pore volume and specific surface area, by employing the BJH model.

4. Results

4.1. Mineral Compositions and Lithofacies Classification

The X-ray diffraction (XRD) analysis of the Dalong Formation’s shale obtained from Well ED-2 revealed a significant abundance of brittle minerals, accounting for an average of 85.5% of the composition. The brittle minerals identified included quartz, feldspar, calcite, dolomite, and pyrite (Figure 3). Among these minerals, quartz exhibited the highest content, ranging from 36.4% to 87.2% and averaging at 62.2%. Carbonate minerals, such as calcite and dolomite, comprised a moderate portion, ranging from 2.4% to 37.6% and averaging at 13.8%. Clay minerals and feldspar also contributed to the composition, with averages of 13.7% and 6.5%, respectively. Additionally, the shale samples contained minor amounts of pyrite and hematite (Figure 3).
The shale lithofacies of the Dalong Formation in Well ED-2 was classified based on the “three end-elements” lithofacies classification method. This method categorizes the shale lithofacies based on the mineral compositions of three primary components: clay minerals, calcite + dolomite, and quartz + feldspar [31,32]. The Dalong Formation’s shale in western Hubei included siliceous rock (S), carbonate-rich siliceous shale (S-1), siliceous shale (S-2), and clay-rich siliceous shale (S-3) (Figure 4). Compared with other sets of Paleozoic marine shale from the Niutitang Formation from the Lower Cambrian in western Hubei, the Longmaxi Formation from the Lower Silurian in eastern Sichuan and the Luzhai Formation/Dawuba Formation from the Lower Carboniferous in central Guangxi [33,34,35], the marine shale found in the Dalong Formation in western Hubei exhibited a significant abundance of siliceous minerals, particularly quartz and feldspar (Table 1).

4.2. Organic Geochemistry Characters

4.2.1. Organic Matter Content

Among the 26 samples selected from Well ED-2 in the Upper Permian Dalong Formation, 25 siliceous shale samples exhibited a total organic carbon (TOC) content ranging from 2.6% to 14.27%, with an average value of 6.66%. However, there was one limestone sample with a relatively low TOC content of 0.25%. It is noteworthy that the shale in the lower section of the Dalong Formation displayed a high organic carbon content, and the organic content of some siliceous carbonaceous shale was higher than 10% (Figure 2). The TOC content of the Dalong Formation’s siliceous shale in western Hubei is significantly higher compared to other Paleozoic marine shales in South China, so it is an organic-rich shale (Table 1).
The Dalong Formation’s siliceous shale had a high organic matter content. The analysis of the Dalong Formation’s shale revealed a correlation between the total organic carbon (TOC) content and the mineral composition content. The clay mineral content in the Dalong Formation’s shale exhibited a strong positive correlation with the organic matter content, while the quartz and carbonate mineral contents displayed weak negative correlations with the organic matter content (Figure 5). The TOC content of the Lower Silurian Longmaxi Formation’s shale in the Sichuan Basin, which has made a commercial breakthrough in shale gas, displays a positive correlation with quartz content and a negative correlation with clay mineral content, which is due to the fact that the organic matter of the Longmaxi Formation is mainly derived from siliceous organisms [14,15]. The presence of clay minerals plays a significant role in the reservoir characteristics of the organic matter in the Dalong Formation’s siliceous shale in western Hubei, distinguishing it from the shale found in the Longmaxi Formation, which is significantly different from that in the Longmaxi Formation’s shale.

4.2.2. Organic Matter Maturity and Types

According to the measured solid asphalt reflectance of 20 shale samples, the organic matter maturity of the Dalong Formation’s shale ranged from 2.56% to 2.79%, and the thermal evolution of the organic matter in the Dalong Formation’s shale has progressed to the overmature dry gas stage (Figure 2). The microscopic identification of the macerals of kerogen in the Dalong Formation’s shale showed that the macerals of organic matter were mainly amorphous solids of sapropelic formation and a small amount of vitrinite (Figure 6a–c). The dominant organic matter found in the Dalong Formation’s shale was sapropelic, with a minor fraction being classified as humic–sapropelic. The primary source of organic matter in the Dalong Formation’s shale is derived from marine organisms, primarily algae [36,37].

4.3. Porosity

The physical property test of 21 shale cores from Well ED-2 revealed that the porosity of the Dalong Formation’s siliceous shale was 0.79~5.6%, with two samples containing microfractures over 5.0% and the main body located at 2.0~4.0%, with an average of 3.13% (Figure 7a). Generally speaking, the organic-rich siliceous shale of the Dalong Formation contains high brittle minerals content and organic matter content, and its reservoir performance should be very good. However, based on the analysis of the mineral composition content and porosity in the Dalong Formation of Well ED-2, it was found that the porosity exhibited a weak positive correlation with the clay mineral content, a weak negative correlation with the quartz content, and no significant correlation with the carbonate mineral content. (Figure 7b). A high quartz content does not correspond to good reservoir performance.

4.4. Pore Types

11 samples are selected for pore observation and pore structure parameters characterization (Table 2). The primary factors influencing the reservoir’s physical properties of the shale are the shale pore types and their structural characteristics. Loucks [38,39] conducted a comprehensive analysis of the pore types in Barnett shale. By examining the relationship between mineral matrix pores and particles, shale reservoir pores were categorized into three distinct types: intergranular pores, intragranular pores, and organic matter pores. Later, researchers included microfractures in the classification of shale pore types [16,17]. Through a microscopic observation, the pores within the Dalong Formation’s shale in Well ED-2 can be categorized into three types: organic matter pores, inorganic matter pores, and microfractures. Due to differences in mineral composition, organic matter content, and organic matter type, notable variations were observed in the types of pores, their morphology and size, as well as the connectivity within the Dalong Formation’s shale.

4.4.1. Organic Matter Pores

Nanoscale pores in organic matter are formed within the particles of organic material subsequent to hydrocarbon generation [40,41,42]. A scanning electron microscope observation revealed the siliceous shale samples from the Dalong Formation exhibited a significant abundance of organic matter pores, with shapes of bubbles, ellipses, and narrow slits, and significant differences in pore size. We used a large number of SEM photos to calculate the approximate aperture range of organic pores on scanning electron microscope operating software. The most common type was organic matter pores developed after hydrocarbon generation of solid asphalt filled mineral intergranular pores (Figure 8a), with a high density and pore size mostly less than 50 nm. Some organic matter pores were elliptical or vermicular (Figure 8b), with a pore size between 50 and 600 nm, and a few between 20 and 50 nm. As the organic matter reached an overmature stage, a significant increase in the number of organic matter pores occurred following extensive hydrocarbon generation. Some pores were interconnected or collapsed, forming vermicular and large elliptical pores. Supported by siliceous mineral particles in shale, organic matter pores were well preserved and remained elliptical without being squashed. In addition, due to the different types of kerogens, the macerals with different hydrocarbon generation abilities were mixed with each other, resulting in significant variations in the development of organic matter pores and an uneven distribution of them within different organic components (Figure 8c). Organic matter pores did not develop inside the humic-type kerogen but developed at the edge of the kerogen (Figure 8d).

4.4.2. Inorganic Matter Pores

In contrast to the highly developed pores observed in the organic matter, the inorganic matter pores of the shale samples of the Dalong Formation were not developed, and the recognizable types included intragranular pores, intercrystalline pores, and intergranular pores. The Dalong Formation’s shale contained a small number of carbonate minerals, and plenty of organic acid fluid generated during the hydrocarbon generation of organic matter dissolved calcite particles to form plenty of corrosion pores (Figure 8e). The intragranular pores were mainly circular and quadrangular, with a pore size generally within 300 nm. The majority of intercrystalline pores formed within unoccupied pore spaces between framboidal pyrite crystals or recrystallized minerals after diagenesis (Figure 8f). The pores were irregularly polygonal or striped, with a pore size mostly ranging between 100 and 500 nm. Intergranular pores were primarily unfilled ones due to a mutual support of particles, including mineral intergranular pores (Figure 8g) and slit pores between clay minerals (Figure 8h). They were mainly irregularly polygonal, triangular, and parallel-plate-shaped, with pore sizes ranging between 50 and 500 nm, and formed in the pores between organic matter and rigid mineral particles, as well as in the pores and gaps between clay minerals, with a good connectivity.

4.4.3. Microfractures

Shale microfractures were formed during multiple periods such as shale sedimentation, diagenesis, hydrocarbon generation, organic matter thermal evolution, and later tectonic processes [16,17]. Shale samples from the Dalong Formation exhibited the presence of microfractures with diverse origins and varying scales, including shale diagenetic fractures, structural fractures, and organic matter edge contraction fractures. Organic matter edge contraction fractures were banded nanoscale fractures formed along the contact edge of organic matter and mineral particles throughout the thermal evolution of the organic matter and hydrocarbon generation process. The length of the fractures was influenced by the size of the organic matter and the surrounding mineral aggregates, and the fracture width was generally within 200 nm (Figure 8i). Diagenetic fractures were formed during the processes of compaction, clay mineral dehydration, and recrystallization. Under the scanning electron microscope, these microfractures were developed along the edges of mineral particles and with a relatively short width, generally within 500 nm (Figure 8j). Structural fractures were formed by the mutual compression and fragmentation of brittle mineral particles under tectonic stress, such as the penetrating intragranular fractures formed by compression within framboidal pyrite particles (Figure 8k), and the “triangular fracture”-like intragranular fractures formed by quartz particles under the stress (Figure 8l). The intragranular fractures were generally short in extension, with a straight fracture surface, and a width of 100–300 nm. The development of brittle minerals in the Dalong Formation’s siliceous shale was beneficial for resisting compaction and later tectonic stress, preventing organic matter pores from being squashed and collapsing.

4.5. Quantitative Analyses of Pore Structure

Figure 9 displays the CO2 and N2 adsorption isotherms of seven selected shale samples, which were utilized to quantify the pore structure of the Dalong Formation’s siliceous shale.
The carbon dioxide adsorption isotherm showed that the adsorption isotherms of all samples increased rapidly with the increase in the relative pressure (P/P0) from 0 to 0.03 (Figure 9a). The results showed that the Dalong Formation’s siliceous shale had a strong adsorption capacity for CO2, with the maximum adsorption capacity between 1.52 and 3.53 cm3/g with an average value of 2.26 cm3/g. The carbon dioxide adsorption experiment was primarily utilized to analyze the pore structure parameters of micropores in shale with size less than 1.5 nm. Based on the NLDFT model, the pore volume of micropores (pore size < 2 nm) of the Dalong Formation’s siliceous shale was 3.58 × 10−3 cm3/g–11.85 × 10−3 cm3/g, with an average value of 7.31 × 10−3 cm3/g. The specific surface area of micropores was 11.87–37.89 m2/g, with an average value of 23.4 m2/g (Table 3). According to the increase in pore volume and the relationship between pore size distribution and cumulative pore volume (Figure 9b,c), there were three peak ranges of micropore distribution in the Dalong Formation’s shale, namely 0.3 nm, 0.4–0.7 nm, and 0.8 nm.
The nitrogen adsorption–desorption isotherm of the Dalong Formation’s shale samples showed that the N2 adsorption capacity gradually increased with the increase in pressure. When the relative pressure (P/P0) was greater than 0.45, a desorption hysteresis existed in the desorption curves of all shale samples, and the desorption curves and adsorption curves were separated, forming an obvious hysteresis loop (Figure 9d). The hysteresis loop opening of the shale samples was narrow, close to Type H4 in the hysteresis loop classification, and had the characteristics of Type H3, which is mainly characterized by a slow rise of the adsorption curve when the relative pressure is less than 0.9 and a rapid rise when the relative pressure is 0.9~1.0, indicating that there were more slit pores and a small number of parallel-plate pores in the Dalong Formation’s shale [43,44]. The Dalong Formation’s shale had a strong adsorption capacity for N2, with a maximum adsorption capacity of 14.39–28.91 cm3/g and an average value of 21.35 cm3/g (Figure 9d). The nitrogen adsorption experiment was mainly used to characterize the pore structure parameters of shale pores larger than 1.5 nm. According to the BJH model, the pore volume and pore specific surface area of mesopores (pore size = 2–50 nm) and macropores (pore size > 50 nm) were calculated. The mesopore volume of the Dalong Formation’s siliceous shale was 15.8 × 10−3 cm3/g–28.2 × 10−3 cm3/g, with an average of 21.4 × 10−3 cm3/g, and the specific surface area was 10.05–20.83 m2/g, with an average of 14.75 m2/g; the macropore volume was 1.7 × 10−3 cm3/g–4.0 × 10−3 cm3/g, with an average of 2.4 × 10−3 cm3/g, and the specific surface area was 0.07–0.18 m2/g, with an average of 0.1 m2/g (Table 3). According to the increase in pore volume and the relationship between the cumulative pore volume and the pore size distribution (Figure 9e,f), the pores of the Dalong Formation’s shale were mainly distributed at 2–50 nm, with a relatively high proportion of mesopore volume. When the pore size was larger than 50 nm, the cumulative pore volume increased slowly, indicating that the proportion of macropore volume was relatively low. The combination of CO2 and N2 adsorption test results shows (Figure 10) that mesopores constituted the primary component of the pore volume in the Dalong Formation, accounting for 68.9%, followed by micropores, accounting for 23.5%, and macropores, accounting for 7.6%. The pore specific surface area was dominated by micropores, accounting for 61.2%, followed by mesopores, accounting for 38.6%. The proportion of macropore specific surface area was very small, only accounting for 0.3%. The proportion of macropore volume in the Dalong Formation’s siliceous shale was relatively low, which was obviously different from that of the Longmaxi Formation’s siliceous shale [11,12]. Meanwhile, the higher the TOC content of the Dalong Formation’s shale, the higher the pore specific surface area and pore volume.

5. Discussion

5.1. Relationships between Pore Structure Parameters and Organic Matter

Numerous nanoscale pores in the organic matter were generated during the thermal evolution and hydrocarbon generation process in the siliceous shale of the Dalong Formation (Figure 8), which controlled the pore structure development of organic matter and provided an important pore volume and specific surface area for the shale. The organic matter pores’ development is related to the total organic carbon content, organic matter type, and maturity. Organic matter in the Dalong Formation’s shale of Well ED-2 was mainly of sapropelic type, with maturity ranging between 2.56 and 2.79% (Table 2). There was little difference in the type and maturity of organic matter, and here, we only discuss the influence of organic matter content on the organic matter pores’ formation in the Dalong Formation.
Scanning electron microscope observation revealed that there was a large amount of spotted organic matter and a small amount of striped organic matter present in the Dalong Formation’s shale (Figure 11a). The spotted organic matter was mainly composed of migrating organic matter filled in intergranular pores and surrounded by idiomorphic minerals (Figure 11b–f), without a fixed structure. The solid asphalt generated after the oil generation peak (Ro 0.8–1.0%) becomes the main type of organic matter in shale after overmaturity [45,46]. Under the scanning electron microscope, it could be seen that the solid asphalt filled the quartz intergranular pores (Figure 11b–d), carbonate intergranular pores (Figure 11e), and framboidal pyrite intercrystalline pores (Figure 11f), with most pore sizes less than 50 nm, and a few more than 50 nm. Due to the support of brittle mineral particles, organic matter pores of this kind of solid asphalt were well preserved and remained elliptical in shape without being squashed. Some spotted and striped organic matter was mainly kerogen and humic organic matter retained from the siliceous shale of the Dalong Formation after diagenesis (Figure 11g–i). The organic matter pores were relatively undeveloped, with pore size mostly less than 100 nm. In addition, in some shale with a high content of organic matter, it could be observed under the scanning electron microscope that a large amount of organic matter was mixed with quartz, clay minerals, etc., with a directional arrangement of debris particles in a streamline shape (Figure 11i), reflecting that this type of shale was affected by compaction or a tectonic process, which can have a certain impact on organic matter pores of shale.
According to the correlation between the shale’s pore structure parameters and total organic carbon (TOC) content of the Dalong Formation’s siliceous shale, there was a weak correlation between the total porosity and organic matter content (Figure 12a), significantly positive between the micropores and mesopores’ volume and the specific surface area and TOC content (Figure 12b,c), and weak between macropores and organic matter content (Figure 12d). Combined with the scanning electron microscope observation, it revealed that a significant proportion of the pores in the Dalong Formation’s siliceous shale were attributed to organic matter. Therefore, the micropores and mesopores in the Dalong Formation’s siliceous shale were mainly provided by organic matter pores, and the pore structure of the Dalong Formation’s shale was primarily influenced by the content of organic matter.

5.2. Relationships between Pore Structure Parameters and Mineral Composition

A different mineral composition will affect the shale’s pore structure. According to the correlations between the pore volume and specific surface area and the quartz, clay mineral, and carbonate mineral contents of the Dalong Formation’s siliceous shale (Figure 13), there was an excellent positive correlation between the micropore volume and specific surface area and the clay mineral content, a good negative correlation between them and the quartz content, and a weak negative correlation between them and the carbonate mineral content; there was a good positive correlation between the clay mineral content and the mesopore volume and specific surface area, a weak negative correlation between them and the quartz content, and a weak negative correlation between them and the carbonate mineral content. However, there was no obvious correlation between the mineral content and the macropore volume and specific surface.
The Dalong Formation’s siliceous shale in western Hubei has a complex source of siliceous material, mainly consisting of authigenic biogenic silicon, terrigenous detrital silicon, and hydrothermal silicon [8,9,14]. Through the identification of hydrothermal activity using Zn-Ni-Co trace element plates [47,48,49], the Dalong Formation’s siliceous shale of Well ED-2 was significantly affected by hydrothermal activity (Figure 14). During the sedimentary period of the Dalong Formation’s shale, silicon-rich hydrothermal fluid filled the mineral intergranular pores. In the early diagenesis stage, siliceous material precipitated from the hydrothermal fluid and produced secondary quartz around quartz particles [50,51,52]. On the one hand, this led to an increase in quartz content in the Dalong Formation’s shale, and on the other hand, a siliceous cementation filled the intergranular pores of siliceous shale, reducing the total porosity of the siliceous shale and leading to a weak negative correlation with the quartz content (Figure 7b). A comprehensive analysis of the correlation between the pore volume and specific surface area and mineral contents showed that the presence of abundant clay minerals promoted the development of the pore volume and specific surface area, whereas the content of quartz and carbonate minerals did not have the same effect.

5.3. Control Factors of Pore Development in the Dalong Formation’s Siliceous Shale

On the basis of the above-mentioned pore structure parameters of the Dalong Formation’s siliceous shale and their correlation with organic matter and mineral content, the organic matter pores generated by the hydrocarbon generation of solid asphalt after maturity were the main pore type of the Dalong Formation’s siliceous shale. The shale exhibited a relatively low total porosity, and it was inversely correlated with the content of quartz, which may be related to the filling of intergranular pores by hydrothermal siliceous cementation. The correlations among the diverse mineral contents and total organic carbon of the Dalong Formation’s shales showed that there was a strong positive correlation between the content of organic matter and clay minerals, indicating their close association. On the other hand, a weak negative correlation existed between the organic matter content and the presence of quartz and carbonate minerals. This suggested that higher levels of organic matter were generally accompanied by an increased clay mineral content and reduced amounts of quartz and carbonate minerals in the shale formation (Figure 15a). At the same time, as the organic matter content increased, the average pore size of the Dalong Formation’s shale decreased (Figure 15b). SEM observation showed that the presence of clay minerals in the Dalong Formation’s shale facilitated the adsorption of a significant quantity of organic matter during sedimentation. Therefore, the pore volume and specific surface area of the shale were positively correlated with the clay mineral content. Essentially, the pore structure was still influenced by the organic matter content. Meanwhile, the shale with a relatively high organic matter content may have been affected by a relatively strong compaction due to its relatively low brittle mineral content, resulting in a decrease in the size of organic matter pores. To sum up, there were two types of pore structure development of the Dalong Formation’s siliceous shale in western Hubei as follows: (1) A TOC of the Dalong Formation less than 5%, representing the common organic-rich siliceous shale with a relatively low organic matter content. It had a relatively high quartz content. Some early diagenetic hydrothermal silicon filled the intergranular pores, resulting in a rather low total porosity and macropore volume. The low clay mineral content resulted in a relatively low organic matter content adsorbed during sedimentation. Organic matter underwent a high superheat maturity and hydrocarbon expulsion, resulting in organic matter pores with a large size. Due to the presence of a large number of brittle minerals, which is beneficial for resisting compaction, the organic matter pores were not squashed and collapsed (Figure 15c,d). However, the low overall content of organic matter in the Dalong Formation’s shale resulted in a relatively low pore volume and pore specific surface area. (2) A TOC of the Dalong Formation larger than 5%, and some TOC greater than 10%, representing the high organic-rich siliceous shale with relatively high organic matter content. It had a relatively high clay mineral content and a relatively low quartz content, leading to a higher organic matter content adsorbed during sedimentation. The organic matter pores generated by hydrocarbon expulsion after the thermal maturation of organic matter may have been affected by compaction, resulting in relatively small but dense organic matter pores inside the organic matter, while the pore size of organic matter mixed with clay minerals was relatively large (Figure 15e,f). However, despite the poor physical properties, the organic-rich siliceous shale in the Dalong Formation exhibited a relatively high pore volume and pore specific surface area, which could be attributed to its overall high organic matter content.

6. Conclusions

The Upper Permian Dalong Formation’s shale in Well ED-2, western Hubei, was predominantly organic-rich siliceous shale with a significant quartz content (averaging 62.2%) and a high organic matter content (averaging 6.66%). The organic matter content exhibited a positive correlation with clay minerals and a weak negative correlation with quartz. The dominant types of organic matter present were sapropelic and humic–sapropelic, with a high thermal maturity (average of 2.56–2.79%).
The total porosity of the Dalong Formation’s siliceous shale was relatively low and weakly negatively correlated with quartz. Pores in the shale included well-developed organic matter pores, primarily in the form of micropores and mesopores (<50 nm), while inorganic matter pores and microfractures were less developed. Mesopores contributed the most to pore volume (68.9%), followed by micropores (23.5%), with macropores having a minimal proportion (7.6%). Micropores contributed the most to the specific surface area of pores (61.2%), followed by mesopores (38.6%), while the proportion of macropores was extremely low.
The pore volume and specific surface area in the Dalong Formation’s siliceous shale showed positive correlations with clay minerals and organic matter, while they were weakly negatively correlated with quartz. The poor physical properties of the organic-rich siliceous shale in the Dalong Formation may be attributed to the cementation of siliceous material from hydrothermal fluids, leading to the formation of secondary quartz that fills intergranular pores and reduces macropore volume and total porosity. Additionally, shale with a high clay mineral content fails to provide sufficient support against compaction, resulting in the destruction of macropores within solid asphalt, leaving predominantly micropores and mesopores, and ultimately resulting in poor physical properties.

Author Contributions

Conceptualization, Y.W. (Yang Wang) and L.B.; methodology, Y.Z.; formal analysis, X.Z. and B.Y.; investigation, Y.W. (Yi Wang); resources, T.X.; writing—original draft preparation, Y.W. (Yang Wang); writing—review and editing, Y.Z. and L.B.; funding acquisition, K.D. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Data are available within the article.

Acknowledgments

The data acquired in the research process are primarily attributed to Hubei Geological Survey, Wuhan, China.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (c) Present-day tectonic zoning map of western Hubei. (a) The global paleogeographic pattern during the Late Permian, with modifications made from references [22,23]. (b) The Late Permian lithofacies paleogeography specific to the South China plate, with modifications derived from reference [24]. (c) The current tectonic zoning of western Hubei and the location of Well ED-2.
Figure 1. (c) Present-day tectonic zoning map of western Hubei. (a) The global paleogeographic pattern during the Late Permian, with modifications made from references [22,23]. (b) The Late Permian lithofacies paleogeography specific to the South China plate, with modifications derived from reference [24]. (c) The current tectonic zoning of western Hubei and the location of Well ED-2.
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Figure 2. The natural gamma-ray curves, lithology, sampling location, TOC, Ro, and mineral composition of the Dalong Formation’s Well ED-2.
Figure 2. The natural gamma-ray curves, lithology, sampling location, TOC, Ro, and mineral composition of the Dalong Formation’s Well ED-2.
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Figure 3. Mineral composition of the Upper Permian Dalong Formation in Well ED-2.
Figure 3. Mineral composition of the Upper Permian Dalong Formation in Well ED-2.
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Figure 4. Lithofacies classification of the Upper Permian Dalong Formation in Western Hubei and other Paleozoic shales in South China.
Figure 4. Lithofacies classification of the Upper Permian Dalong Formation in Western Hubei and other Paleozoic shales in South China.
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Figure 5. Correlation between total organic carbon (TOC) and quartz, clay minerals, and carbonate minerals of the Dalong Formation’s shale.
Figure 5. Correlation between total organic carbon (TOC) and quartz, clay minerals, and carbonate minerals of the Dalong Formation’s shale.
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Figure 6. (a–c) Microscopic photos of kerogen macerals in the Dalong Formation’s shale of Well ED-2 in western Hubei.
Figure 6. (a–c) Microscopic photos of kerogen macerals in the Dalong Formation’s shale of Well ED-2 in western Hubei.
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Figure 7. (a) Porosity distribution of the Dalong Formation’s shale and (b) correlation between porosity and mineral composition.
Figure 7. (a) Porosity distribution of the Dalong Formation’s shale and (b) correlation between porosity and mineral composition.
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Figure 8. Pore types of the Dalong Formation’s shale of Well ED-2. (a) Organic matter pores developed by solid asphalt filled in intergranular pores, 1234.2 m. (b) Elliptical–vermicular organic matter pores, 1237.2 m. (c) Organic matter pores with small pore size and uniform distribution, 1244.2 m. (d) Organic matter pores developed at the edge of organic matter, 1252.9 m. (e) Intragranular corrosion pores developed in calcite, 1244.2 m. (f) Intercrystalline pores developed between recrystallized dolomite particles, 1234.2 m. (g) Intergranular pores developed between mineral particles, 1229.6 m. (h) Slit pores located between clay minerals, 1244.2 m. (i) Contraction fractures located at the edge of organic matter, 1229.6 m. (j) Diagenetic fractures located between clay mineral particles and rigid mineral particles, 1234.2 m. (k) Intragranular structural fractures developed within framboidal pyrite particles, 1244.2 m. (l) Intragranular structural fractures developed within quartz mineral particles, 1237.2 m.
Figure 8. Pore types of the Dalong Formation’s shale of Well ED-2. (a) Organic matter pores developed by solid asphalt filled in intergranular pores, 1234.2 m. (b) Elliptical–vermicular organic matter pores, 1237.2 m. (c) Organic matter pores with small pore size and uniform distribution, 1244.2 m. (d) Organic matter pores developed at the edge of organic matter, 1252.9 m. (e) Intragranular corrosion pores developed in calcite, 1244.2 m. (f) Intercrystalline pores developed between recrystallized dolomite particles, 1234.2 m. (g) Intergranular pores developed between mineral particles, 1229.6 m. (h) Slit pores located between clay minerals, 1244.2 m. (i) Contraction fractures located at the edge of organic matter, 1229.6 m. (j) Diagenetic fractures located between clay mineral particles and rigid mineral particles, 1234.2 m. (k) Intragranular structural fractures developed within framboidal pyrite particles, 1244.2 m. (l) Intragranular structural fractures developed within quartz mineral particles, 1237.2 m.
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Figure 9. (a–c) CO2 adsorption curves, pore size distribution, and (d–f) N2 adsorption–desorption curves, pore size distribution of the Dalong Formation’s shale in Well ED-2.
Figure 9. (a–c) CO2 adsorption curves, pore size distribution, and (d–f) N2 adsorption–desorption curves, pore size distribution of the Dalong Formation’s shale in Well ED-2.
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Figure 10. (a) Distribution of pore volume, and (b) pore specific surface area of the siliceous shale samples of the Dalong Formation in Well ED-2.
Figure 10. (a) Distribution of pore volume, and (b) pore specific surface area of the siliceous shale samples of the Dalong Formation in Well ED-2.
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Figure 11. Organic matter occurrence characteristics and organic matter pores’ development characteristics in the Dalong Formation. (a) Spotted and striped organic matter observed under scanning electron microscope, 1229.6 m. (b) Solid asphalt filled between idiomorphic and authigenic minerals, 1228.1 m. (c) Solid asphalt filled between idiomorphic and authigenic minerals, 1228.1 m. (d) Organic matter pores with a uniform distribution and small pore size, 1244.2 m. (e) Collapse and connectivity of organic matter pores, 1237.2 m. (f) Intercrystalline pores of framboidal pyrite filled with solid asphalt, 1229.6 m. (g) Primary striped organic matter, 1229.6 m. (h) Organic matter pores developed from organic matter mixed with clay minerals, 1249.2 m. (i) Organic matter and mineral particles of directional arrangement in a streamline shape, 1249.2 m.
Figure 11. Organic matter occurrence characteristics and organic matter pores’ development characteristics in the Dalong Formation. (a) Spotted and striped organic matter observed under scanning electron microscope, 1229.6 m. (b) Solid asphalt filled between idiomorphic and authigenic minerals, 1228.1 m. (c) Solid asphalt filled between idiomorphic and authigenic minerals, 1228.1 m. (d) Organic matter pores with a uniform distribution and small pore size, 1244.2 m. (e) Collapse and connectivity of organic matter pores, 1237.2 m. (f) Intercrystalline pores of framboidal pyrite filled with solid asphalt, 1229.6 m. (g) Primary striped organic matter, 1229.6 m. (h) Organic matter pores developed from organic matter mixed with clay minerals, 1249.2 m. (i) Organic matter and mineral particles of directional arrangement in a streamline shape, 1249.2 m.
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Figure 12. Correlation between (a) porosity, (b) micropore structure parameters, (c) mesopore structure parameters, (d) macropore structure parameters and total organic carbon (TOC) content of shale samples from the Dalong Formation of Well ED-2.
Figure 12. Correlation between (a) porosity, (b) micropore structure parameters, (c) mesopore structure parameters, (d) macropore structure parameters and total organic carbon (TOC) content of shale samples from the Dalong Formation of Well ED-2.
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Figure 13. Correlation between (a–d) pore volume, (e–h) pore specific surface area and quartz, clay minerals, and carbonate minerals of the Dalong Formation’s siliceous shale of Well ED-2.
Figure 13. Correlation between (a–d) pore volume, (e–h) pore specific surface area and quartz, clay minerals, and carbonate minerals of the Dalong Formation’s siliceous shale of Well ED-2.
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Figure 14. Identification of the source of siliceous material in the Dalong Formation’s siliceous shale of Well ED-2.
Figure 14. Identification of the source of siliceous material in the Dalong Formation’s siliceous shale of Well ED-2.
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Figure 15. (a) Correlation between total organic carbon (TOC) content and quartz, clay mineral, and carbonate mineral content in the Dalong Formation’s shale of Well ED-2, (b) correlation between average pore size and TOC, (c,d) organic matter pore development pattern of siliceous shale with a relatively low organic matter content, and (e,f) organic matter pore development pattern of siliceous shale with a relatively high organic matter content.
Figure 15. (a) Correlation between total organic carbon (TOC) content and quartz, clay mineral, and carbonate mineral content in the Dalong Formation’s shale of Well ED-2, (b) correlation between average pore size and TOC, (c,d) organic matter pore development pattern of siliceous shale with a relatively low organic matter content, and (e,f) organic matter pore development pattern of siliceous shale with a relatively high organic matter content.
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Table
Table 1. Comparison of mineral content and organic geochemical characteristics between typical shales of the Dalong Formation and other formations in South China.
Table 1. Comparison of mineral content and organic geochemical characteristics between typical shales of the Dalong Formation and other formations in South China.
Sample
Location		StrataDepth
(m)		TOC
(wt.%)		Ro
(%)		Mineral Content (wt.%)
QuartzClayCarbonate
Niutitang Formation in western Hubei [34]Lower Cambrian30002.053.0437.723.528.3
Longmaxi Formation in eastern Sichuan [34]Lower
Silurian		23943052.9143.735.79.1
Luzhai Formation in central Guangxi [35]Lower Carboniferous16001.862.6743.030.021.1
Dalong Formation in western HubeiUpper Permian12336.72.6862..213.713.8
Table
Table 2. Basic information of the shale samples of the Dalong Formation in Well ED-2.
Table 2. Basic information of the shale samples of the Dalong Formation in Well ED-2.
Sample
No.		Depth
(m)		TOC
(wt.%)		Ro
(%)		Kerogen
Type		Mineral Content (wt.%)
QuartzFeldsparClayCarbonatePyrite
P3d-11215.94.942.56I81.31.48.85.02.4
P3d-21222.57.492.64I62.42.78.623.72.0
P3d-31222.93.462.62I54.94.35.833.41.1
P3d-41228.16.172.67I58.15.813.917.93.6
P3d-51229.65.482.65I74.73.610.18.52.5
P3d-61234..25.322.67I57.55.226.45.44.4
P3d-71237.24.602.69I80.73.77.45.81.7
P3d-81244.213.412.71I55.67.522.45.96.6
P3d-91245.65.242.75I57.87.610.621.02.9
P3d-101249.214.272.77I43.710.536.44.34.4
P3d-111252.93.892.79II182.71.39.92.62.6
Table
Table 3. Average pore size, pore volume, and specific surface area of the Dalong Formation’s shale samples of Well ED-2 in western Hubei.
Table 3. Average pore size, pore volume, and specific surface area of the Dalong Formation’s shale samples of Well ED-2 in western Hubei.
Sample
No.		Depth
(m)		Average Pore Size (nm)Pore Volume (10−3 cm3/g)Specific Surface Area (m2/g)
MicroporeMesoporeMacroporeMicroporeMesoporeMacropore
P3d-11215.93.6973.5815.762.4711.87311.0850.106
P3d-21222.53.7877.2514.591.7123.82716.4260.073
P3d-31222.94.4415.1615.951.8516.06310.0500.080
P3d-41228.14.3067.0021.463.9921.97313.6630.175
P3d-81244.23.35410.4327.242.0234.39520.8280.084
P3d-91245.64.1485.9116.842.1417.77011.4220.092
P3d-101249.23.49511.8528.242.4337.88819.7550.105
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Wang, Y.; Bai, L.; Zhang, Y.; Zhang, X.; Yang, B.; Duan, K.; Wang, Y.; Xie, T. Reservoir Characteristics and Influencing Factors of Organic-Rich Siliceous Shale of the Upper Permian Dalong Formation in Western Hubei. Energies 2023, 16, 5130. https://doi.org/10.3390/en16135130

AMA Style

Wang Y, Bai L, Zhang Y, Zhang X, Yang B, Duan K, Wang Y, Xie T. Reservoir Characteristics and Influencing Factors of Organic-Rich Siliceous Shale of the Upper Permian Dalong Formation in Western Hubei. Energies. 2023; 16(13):5130. https://doi.org/10.3390/en16135130

Chicago/Turabian Style

Wang, Yang, Luheng Bai, Yanlin Zhang, Xiaoming Zhang, Bowei Yang, Ke Duan, Yi Wang, and Tong Xie. 2023. "Reservoir Characteristics and Influencing Factors of Organic-Rich Siliceous Shale of the Upper Permian Dalong Formation in Western Hubei" Energies 16, no. 13: 5130. https://doi.org/10.3390/en16135130

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