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Article

Comparative Study of the Characteristics of Lower Cambrian Marine Shale and Their Gas-Bearing Controlling Factors in the Middle and Lower Yangtze Areas, South China

by
Hezheng Dong
1,2,
Dongsheng Zhou
1,2,*,
Ziyan Deng
1,2 and
Xiaowei Huang
3
1
School of Ocean Sciences, China University of Geosciences (Beijing), Beijing 100083, China
2
Key Laboratory of Polar Geology and Marine Mineral Resources, China University of Geosciences (Beijing), Beijing 100083, China
3
Institute of Energy, Peking University, Beijing 100871, China
*
Author to whom correspondence should be addressed.
Minerals 2024, 14(1), 31; https://doi.org/10.3390/min14010031
Submission received: 17 November 2023 / Revised: 15 December 2023 / Accepted: 26 December 2023 / Published: 28 December 2023
(This article belongs to the Topic Hydrocarbon Generation and Accumulation in Unconventional Shale Reservoir: Up-to-Date Advances in Theory, Experiment, Method and Application, Volume II)
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Abstract

:
This study comparatively analyzed the geological, geochemical, reservoir, and gas-bearing characteristics of the lower Cambrian marine shale in the Middle and Lower Yangtze regions. The main factors controlling the gas-bearing properties of the shales were identified, and the favorable and unfavorable conditions for shale gas accumulation are discussed. The results show that the organic carbon contents and thermal evolution degree of the organic matter in the lower Cambrian marine shale in the Lower Yangtze area were higher than those generally found in the Middle Yangtze area. The brittle mineral composition of the Middle Yangtze area was typically low silicon and high calcium, whereas the Lower Yangtze area was characterized by high silicon and low calcium. The development of micropores in the Lower Yangtze area was poorer than in the Middle Yangtze area, with the organic pores being particularly underdeveloped. The adsorption capacity of shale in the Lower Yangtze area was obviously higher than in the Middle Yangtze area. It was considered that the organic carbon content, thermal evolution degree, and molecular structure of kerogen were the main factors that controlled the adsorption properties of the shale. In addition, the Lower Yangtze area suffered a stronger tectonic transformation and frequent magmatic activity, and the preservation conditions were inferior to those in the Middle Yangtze area.

1. Introduction

Shale gas refers to unconventional natural gas resources that accumulate in black shales and interbedded sandy and silty shales through adsorption, dissolution, or as free gas [1,2]. Over the past two decades, the successful examples of shale gas exploration and development in North America have set off a global shale gas revolution [3,4,5,6]. China has been carrying out extensive shale gas exploration and development work since 2009 [7,8]. In recent years, marine shale gas exploration in southern China has achieved tremendous results [9,10]. The Wufeng–Longmaxi Formation shale in the Upper Yangtze area has achieved large-scale industrial gas production [11], while the Niutitang Formation in the lower Cambrian in the south of Sichuan province has also made breakthroughs in shale gas development [12,13]. The Shuijingtuo Formation in the Yichang area in the Middle Yangtze area also shows industrial-grade shale gas potential [14]. However, exploration of the lower Cambrian marine shale gas in the Lower Yangtze area has yet to achieve a significant breakthrough [15].
Shale contains various elements, including silicon, aluminum, calcium, potassium, uranium, thorium, iron, and manganese [16,17]. The significant features of the lower Cambrian marine shales in southern China are their relatively high silicon content, the widespread development of siliceous shales, and the high organic richness of the siliceous shales [18,19]. Silicon is an important nutrient element in seawater [20] and has three currently understood sources: hydrothermal fluids [21,22], biogenesis (for example, from silicon-secreting organisms like sponges and radiolarians) [23,24], and terrestrial input [25]. Pioneering studies, notably that of Holdaway and Clayton (1982), introduced the concept of “excess silicon”, which refers to the proportion of siliceous minerals that exceeds the usual terrigenous clastic sources [26]. Wedepohl (1971), Adachi et al. (1986), and Yamamoto (1987) proposed a technique to ascertain whether silicate minerals have a hydrothermal or biogenic origin by utilizing an Al-Fe-Mn ternary diagram [16,17,27]. Previous research on the mechanisms of organic matter enrichment in lower Cambrian shales in southern China has shown that sources of silicon or conditions for organic matter enrichment vary in different regions and sedimentary environments. Numerous studies suggested that hydrothermal processes played a significant role in the genesis of silicon and organic matter enrichment in the lower Cambrian shales in southern China [28,29,30,31]. However, some argued that hydrothermal activity made little contribution to siliceous enrichment in the intra-platform basins of the Yangtze Block [32].
The lower Cambrian siliceous shales in southern China, which are rich in quartz and susceptible to hydraulic fracturing [1,33], were identified as prime targets for shale gas exploration [9]. However, shale gas accumulation is often the result of a combination of many factors [34,35]. Mineral composition, thermal evolution, pore structure, and tectonic activity all have an important influence on the gas-bearing properties of shale, especially on the adsorbed gas content [36,37,38,39,40,41]. However, the interplay between these factors is, as yet, poorly understood, and further research is needed to investigate their combined controlling effect. Research on the mechanisms of organic matter enrichment, silicon sources, and gas-bearing properties in the Lower and Middle Yangtze regions has mainly focused on individual wells or sections, and thus, there is a lack of comprehensive regional studies [41,42,43,44,45,46]. This study aimed to combine individual-well and regional-scale comparative research based on individual-well data and regional characteristics. This approach has both theoretical and practical significance for evaluating the mechanisms of organic matter enrichment and the gas-bearing properties of shales and will be a valuable aid in the assessment of shale gas exploration potential in the Lower and Middle Yangtze shales.
In this study, the factors controlling shale gas accumulation in the lower Cambrian marine shale in the Middle and Lower Yangtze regions were compared by analyzing their organic geochemistry, reservoir characteristics, pore types, and gas-bearing characteristics. The differences and relative advantages of shale gas accumulation conditions in the two regions are discussed. The objectives of this study were (1) to enhance the understanding of the lateral variations in geochemistry, petrophysics, and shale gas storage capacity within time-equivalent stratigraphic units spanning the early Cambrian period in the Middle and Lower Yangtze regions; (2) to discuss the spatial disparities observed in these elements and their controlling factors; and (3) to establish the correlations between these disparities and the potential for shale gas resources.

2. Geological Setting

The Yangtze platform is one of the three oldest in China and is divided into three tectonic units: the Upper, Middle, and Lower Yangtze regions [47] (see Figure 1). The Upper Yangtze Platform is bounded by the Qinling–Dabie orogenic belt to the north, the Red River Fault Zone to the southwest, the Xianshuihe Fault Zone to the west, and the Cathaysia Platform to the southeast [48]. The Middle Yangtze area is located in the middle reaches of the Yangtze River, bounded by the Baojing–Cili fault in the southeast and separated from the Jiangnan–Xuefeng paleo-uplift, with the Qiyueshan fault in the northwest being separated from the rest of the structure in eastern Sichuan. The area is blocked by the Huangling paleo-uplift in the north and separated from the arc structure of Dabashan Mountain and connects with the structure of Middle Guizhou in the south. The Lower Yangtze area lies in the lower reaches of the Yangtze River, bounded by the Tancheng–Lujiang fault zone and separated from the Qinling–Dabie orogenic belt in the west, the Sulu orogenic belt in the north, and the Wuyi orogenic belt in the south [49].
Against the background of accelerated fragmentation of the Rodinia supercontinent after the Neoproterozoic [50], the South China plate initially developed an intracontinental rift [51] and then evolved into a passive continental margin under extensional tectonic movement [52]. The stratigraphic framework of the Yangtze platform near the Ediacaran-Cambrian boundary exhibited a pattern of alternating uplifts and depressions, forming two basins and a platform [53]. A large-scale marine transgression occurred in the early Cambrian, leading to widespread marine sedimentation on the platform, and the water depth gradually increased from the NW to the SE, with the Upper Yangtze developing carbonate platforms and intrashelf basin (or lagoon) sedimentation and Middle Yangtze areas primarily developing an intrashelf basin, shallow shelf, and slop [54,55]. The Lower Yangtze area developed various sedimentation types, including carbonate platforms, deep shelves, and deep basins [56,57].
The Cambrian strata in the Yangtze regions are rich in organic-rich shale, particularly in the middle and lower strata (see Figure 2). Due to frequent sea-level fluctuations in the Yangtze platform, the multi-cyclic simultaneous development of calcareous shale, marl, calcareous mudstone, siliceous rock, and limestone occurred (see Figure 2). These rock layers effectively prevented vertical migration and loss of hydrocarbons, forming an effective seal [19,45,58,59]. The lower Cambrian marine shales in the Yangtze Platform therefore have favorable preliminary conditions for shale gas accumulation. This study focused on the lower Cambrian marine shale research in the Middle and Lower Yangtze regions. Due to the different sedimentary and tectonic evolution settings in the two regions, there are certain differences in the shales, and these differences significantly affect shale gas enrichment and subsequent exploration and development in the two regions.
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Figure 1. The location of the study area and the lithofacies paleogeography of the Yangtze Block during the early Cambrian (~530 Ma), modified from references [56,57]. The lower Cambrian organic-rich marine shale has been primarily developed in intrashelf basins, shelves, and basins.
Figure 1. The location of the study area and the lithofacies paleogeography of the Yangtze Block during the early Cambrian (~530 Ma), modified from references [56,57]. The lower Cambrian organic-rich marine shale has been primarily developed in intrashelf basins, shelves, and basins.
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Figure 2. Correlation of the stratigraphic column of the lower Cambrian in different parts of the Middle and Lower Yangtze areas. Wells ZK03, TD-1, CY-1, and JY-1 were modified from references [19,40,45,59], respectively. The lower Cambrian organic-rich marine shale is mostly found in the middle and lower parts of the Shuijingtuo Formation and the Hetang Formation (known as the Wangyinpu Formation in northwest Jiangxi). These are coeval deposits. In the Middle Yangtze area, the Shuijingtuo Formation contains higher proportions of calcareous and argillaceous intercalations. In the Lower Yangtze area, siliceous rocks and siliceous shales have developed in the Hetang Formation.
Figure 2. Correlation of the stratigraphic column of the lower Cambrian in different parts of the Middle and Lower Yangtze areas. Wells ZK03, TD-1, CY-1, and JY-1 were modified from references [19,40,45,59], respectively. The lower Cambrian organic-rich marine shale is mostly found in the middle and lower parts of the Shuijingtuo Formation and the Hetang Formation (known as the Wangyinpu Formation in northwest Jiangxi). These are coeval deposits. In the Middle Yangtze area, the Shuijingtuo Formation contains higher proportions of calcareous and argillaceous intercalations. In the Lower Yangtze area, siliceous rocks and siliceous shales have developed in the Hetang Formation.
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3. Materials and Methods

3.1. Samples

Fresh outcrop samples of lower Cambrian marine shale were collected in the Middle and Lower Yangtze regions, including 8 samples from the Niutitang Formation in the northwest of the Middle Yangtze area, 18 samples from the Shuijingtuo Formation in western Hubei Province, and 7 samples from the Shuijingtuo Formation in eastern Hubei province. Furthermore, 22 samples were collected from the Hetang Formation in southern Anhui province in the Lower Yangtze area, 53 samples from the Hetang Formation in western Zhejiang province, 20 samples from the Mufushan Formation in northern Jiangsu province, and 9 samples from the Wangyinpu Formation in northwest Jiangxi province. Previous well data (well locations are shown in Figure 1) were also collected to investigate the factors that control the enrichment and accumulation of lower Cambrian shale gas in the Middle and Lower Yangtze River regions.

3.2. Methods

For each sample, about 100 g of fresh rock was carefully crushed into gravel-sized particles using a sturdy steel jaw crusher. The particles were then pulverized to a fine powder with a mesh size finer than 200 in an agate mortar, after which the powdered samples were divided into several fractions for geochemical and mineralogical analyses.
Total organic carbon (TOC) analyses were carried out in the Experimental Research Center (ERC) of Yangtze University. For each experiment, 200 mg of powder was treated with 10% hydrochloric acid (HCl) at a temperature of 60 °C to eliminate carbonates. The sample was then washed with distilled water to ensure the complete removal of any residual HCl. The residue was dried and the TOC was analyzed using a Leco CS-230 carbon and sulfur analyzer with an accuracy of ±0.5%. The consistency was confirmed by repeating the experiments in accordance with the Chinese national standard [60]. Vitrinite reflectance (Ro) tests were carried out at the ERC on an MPV-SP microphotometer under oil immersion, with more than ten readings for each sample.
Mineralogical analyses were conducted at the ERC on randomly oriented rock powder samples using a HUBER MC 9300 X-ray diffraction (XRD) instrument. The analysis procedure and data evaluation methodologies followed those of [61].
Stable carbon isotope analyses were conducted using a Finnigan MAT-253 instrument at the ERC of Yangtze University. The stable carbon isotope data were expressed in δ-notation (δ13C, ‰) and referenced against the V-PDB standard with a precision of ± 0.5‰.
Field-emission scanning electron microscopy (FE-SEM) was carried out at the Experimental Center, iRock Technology Co., Ltd., Beijing, China, using a Thermo Fisher Helios G5CX (FE-SEM) with secondary electron imaging and electron backscatter diffraction. The samples were prepared via argon ion milling prior to the pore structure observation using FE-SEM.
Isothermal adsorption was carried out at the Research Institute of Exploration and Development of the North China Oilfield. An ISO 300 isothermal adsorption instrument was used, with an experimental temperature of 30 °C and a methane gas concentration of 99.999%.
In addition, 192 samples were collected for major element analysis, with the source of silicon being determined by calculating the excess siliceous content (Siex) (the content of siliceous minerals other than regular terrigenous clastic deposits), which was determined according to the following formula:
Siex = Sis − (Si/Al)bg × Als
where Sis represents the elemental silicon content in the sample, and Als is the elemental aluminum content. The obtained value for (Si/Al)bg was 3.11, which is the average content observed in shales [26].

4. Results

4.1. Total Organic Carbon (TOC) and Vitrinite Reflectance (Ro)

The TOC and Ro of fifty samples from the Lower Yangtze region were tested. The TOC of the samples from HTC in the Lower Yangtze region was 0.11–17.72 wt.% (average 7.67 wt.%), with an Ro of 3.84%–5.08% (average 4.62%); the TOC of the samples from LY was 0.16–14.30 wt.% (average 4.01 wt.%), with an Ro of 2.57%–4.41% (average 3.73%); the TOC of the samples from DB was 0.19–15.10 wt.% (average 4.29 wt.%), with an Ro of 2.76%–3.76% (average 3.13%); the TOC of the samples from XY-1 was 3.13–17.18 wt.% (average 11.0 wt.%), with an Ro of 3.20%–4.66% (average 4.19%); the TOC of the samples from JJD was 8.29–25.10 wt. % (average 14.02 wt. %), with an Ro of 3.06%–3.85% (average 3.47%); see Table S1.
The TOC and Ro data for the lower Cambrian marine shale in the Middle and Lower Yangtze regions from previous studies are summarized in Table S1. As shown in Figure 3a, the average TOC values for both regions were greater than 1.5%, but the TOC value in the Lower Yangtze region was significantly higher than that in the Middle Yangtze region (see Figure 3a). The Ro of the lower Cambrian marine shale in both the Middle and Lower Yangtze regions was greater than 2%, indicating that the organic matter entered the overmature stage. However, the degree of thermal evolution in the Lower Yangtze region was significantly greater than in the Middle Yangtze region (see Figure 3b).

4.2. Carbon Isotope and Organic Matter Types

The δ13C values for six shale samples from the lower Cambrian Shuijingtuo Formation in the Central Yangtze region ranged from −32.71‰ to −29.23‰, confirming that they belonged to type I and type II1 kerogens. The δ13C values for nine shale samples from the lower Cambrian Hetang Formation in the Lower Yangtze region ranged from −36.72‰ to −30.72‰, which all belonged to type I kerogens (Table S1).

4.3. Mineralogy

In the Lower Yangtze region, the HTC and TLL samples from the Hetang Formation shale showed high clay mineral contents, ranging from 35.0 to 41.0% (average 38.4%) and 41.0 to 51.0% (average 44.8%), respectively. The quartz contents of these samples were comparatively low, ranging from 31.0 to 48.0% (average 39.4%) and 31.0 to 37.0% (average 34.8%), respectively. However, the Hetang Formation shale in DT and LY had higher quartz contents—ranging from 71.7 to 85.9% (average 75.8%) and 53.6 to 86.3% (average 76.1%), respectively—but lower clay mineral contents, ranging from 7.4 to 13.5% (average 11.2%) and 6.4 to 20.5% (average 11.7%), respectively. The carbonate contents of all these samples were low. Previous research data on the mineral composition of the lower Cambrian marine shales are summarized in Table S2, with the data indicating that the lower Cambrian marine shales in the Middle Yangtze region had high calcium and low silicon contents, while the shales in the Lower Yangtze region had high silicon and low calcium contents (Figure 4).

4.4. Isothermal Adsorption

4.4.1. Nitrogen Isothermal Adsorption

The nitrogen adsorption hysteresis regression curves of the samples from the lower Cambrian Hetang Formation in the Lower Yangtze region were all H3-type hysteresis loops according to the IUPAC classification, corresponding to parallel plate-like pores [69,70]. The shale samples from the Hetang Formation showed small adsorption amounts in the low-pressure region, indicating a small number of micropores. The absence of a saturation trend in P/P0 suggests the presence of large pores that had not been filled [69]. The BJH total pore volumes of the mud shale samples from well XY-1, DT, LY, and FC sections were 0.012~0.015 cm3·g−1 (average 0.013 cm3·g−1), 0.011~0.014 cm3·g−1 (average 0.013 cm3·g−1), 0.012~0.015 cm3·g−1 (average 0.014 cm3·g−1), and 0.014 cm3·g−1, respectively; the BET specific surface areas were 3.62~4.69 m2·g−1 (average 4.16 m2·g−1), 2.67~3.98 m2·g−1 (average 3.33 m2·g−1), 3.62~5.95 m2·g−1 (average 4.79 m2·g−1), and 4.66 m2·g−1, respectively; and the BJH average pore sizes were 13.07~13.78 nm (average 13.43 nm), 14.68~20.28 nm (average 17.48 nm), 9.01~13.07 nm (average 11.04 nm), and 10.98 nm, respectively. The results of previous studies of the mineral composition of lower Cambrian marine shale in the middle and lower Yangtze regions are summarized in Table 1.
The principal differences in the pore traits of the lower Cambrian marine shale across the Middle and Lower Yangtze regions lay in the pore size, volume, and specific surface area. A comparison with data from previous studies indicates that the lower Cambrian marine shale in the Middle Yangtze region had a greater total BJH pore volume and BET-specific surface area, while the average BJH pore size was smaller than that in the Lower Yangtze region (Table 1).

4.4.2. Methane Isothermal Adsorption

Linear regression was applied to compare the adsorption capacities of the lower Cambrian marine shales in the middle and lower Yangtze regions [73]. Their Langmuir volumes (VL) were calculated based on the isothermal adsorption curve of methane (see Table 2). The range of VL for the samples from the middle Yangtze region was 1.16~8.19 m3/t (average 2.73 m3/t); the range of VL for the samples in the lower Yangtze region was 0.50~16.87 m3/t (average 5.42 m3/t).

4.5. Pore Types Identified by FE-SEM

Shale pores can be divided into intergranular pores, intragranular pores, organic matter pores, and microfractures [74], with the organic components of micropores being the main controlling factors for gas adsorption [75]. Figure 5 shows the pore characteristics of the marine shale of the Shuijingtuo Formation in the Luojiacun (LJC) section, Zigui County, in the Yichang area of the Middle Yangtze area, under a field emission scanning electron microscope (FE-SEM). Both the organic and inorganic pores were generally well developed, with the inorganic pores being predominantly dissolution pores in carbonate minerals. Spongy micropores had developed in the organic matter, and microfractures occurred in the interiors and at the edges of the organic matter. Figure 6 shows the FE-SEM pore characteristics of marine shale in the Hetang Formation in the Hongtaocun (HTC) Section in Shitai County, southern Anhui Province. Under the microscope, large amounts of interstitial and bulk organic matter could be observed, but organic and inorganic pores were less developed. A small number of isolated organic pores occurred sporadically in organic matter, mostly flat and irregular in shape, with obviously high long-axis/short-axis ratios and collapsed edges. Inorganic pores occurred as clay mineral intercrystalline pores, potash feldspar intragranular solution pores, and other types. Organic pores and intragranular carbonate solution pores were well developed in the Middle Yangtze area, while inorganic pores were the main pore types in the Lower Yangtze area, where organic pores were comparatively underdeveloped.

5. Discussion

5.1. The Source of Silicon and Its Controlling Effect on the Quality of Early Cambrian Marine Shale

The hydrocarbon-generating capacities of different types of organic matter vary. Type I kerogens have greater hydrocarbon-generation potentials [76,77]. The kerogen type of the Lower Yangtze shale is type I, and thus, the hydrocarbon generation potential of the lower Cambrian marine shale in that area is generally high. In addition, the abundance of organic matter is the material basis for the accumulation of shale gas [55,56]. Both areas had relatively high TOC contents, with average values exceeding 1.50 wt. % (see Figure 3a), providing the material basis for shale gas generation. TOC contents in the Lower Yangtze area were significantly higher than in the Middle Yangtze area (see Figure 3a), indicating that the Lower Yangtze area had a better material basis for hydrocarbon generation. Greater organic richness facilitates the generation of more shale gas and is also more effective in generating porosity during the cracking process [77].
Analysis of mineral composition is an indispensable element in shale reservoir research and an important basis for the study of shale gas adsorption and storage, fracture evaluation, percolation and migration, fracturing, and technological properties [34,35,78,79]. Contents of brittle minerals (such as quartz, feldspar, and calcite) are important factors in controlling the degree of fracture development, which directly affects the reservoir space and percolation channels [80]. It also forms a rigid supporting framework, which is beneficial for the preservation of organic and inter-crystalline pores in clay minerals [43]. In addition, an increase in brittle mineral contents makes it more likely that network cracks will form, which is conducive to fracturing [81].
The brittle mineral contents of the Barnett and Woodford shales, which meet industrial development conditions abroad, are generally more than 40% [82]. The brittle mineral content of the organic-rich marine shale in the early Cambrian in the Middle and Lower Yangtze areas was more than 40%, and the average content was more than 60% (see Figure 4), indicating high brittleness and good fracturability [83,84]. However, there were some differences in the proportions and types of brittle minerals in the early Cambrian shales in different areas. The brittle minerals in the shales in the Middle Yangtze area were higher in calcium and lower in silicon than in the Lower Yangtze area (Figure 4). Across the Middle and Lower Yangtze areas, carbonate minerals generally decreased while siliceous minerals increased (see Figure 7a).
In the early Cambrian, the water depth of the Yangtze Plate increased from the northwest to the southeast [28,29]. The water body in the Middle Yangtze region was relatively shallow (Figure 1), and thus, there were more calcareous components in the sedimentary rocks. However, the sources of siliceous material in the sediment were mainly terrigenous clastic, hydrothermal fluid, and biogenic [25,90,91,92]. Excessive silica (SiO2 ex) is often used to characterize the SiO2 content above that of normal clastic sedimentation [25], which is caused by hydrothermal fluids and biogenesis [93,94].
The Al-Fe-Mn ternary diagram is often used to indicate the origin of additional silicon [17]. Figure 7b shows that the sample points in the Middle Yangtze region almost all fell in areas with non-hydrothermal origins, while the sample points in the Lower Yangtze region mostly fell in areas with hydrothermal origins. This indicates that the Siex in the Lower Yangtze region was more significantly affected by hydrothermal fluids, and it also indicates that hydrothermal activity in the Lower Yangtze region was more widespread and frequent than in the Middle Yangtze region during the early Cambrian. Figure 8a shows a good positive correlation between Siex and TOC in the Middle Yangtze region (R2YD-2 = 0.53, R2ZK03 = 0.61), reflecting a biogenic origin for Siex. Figure 8b shows no significant correlation between Siex and TOC in the Lower Yangtze region, indicating a non-biogenic origin for Siex, which supports the results shown in the Al-Fe-Mn ternary diagram, which indicate that the Siex in the Lower Yangtze shale came from hydrothermal fluids.
Hydrothermal fluid activity was, therefore, the main factor that caused the higher silica content in the shale of the Lower Yangtze region. The large amounts of nutrients carried by hydrothermal fluids lead to increased marine productivity [79,80] and increased organic carbon deposition flux, which is more favorable for organic matter enrichment [95]. This explains the higher TOC content in the shale in the Lower Yangtze region, where hydrothermal activity during deposition was the main factor in causing greater enrichment of the organic matter, as well as a higher brittle mineral quartz content.

5.2. Reservoir Physical Property Differences and Its Control Factors

The total pore volume and BET specific surface area of the lower Cambrian marine shale in the Middle Yangtze region were comparatively large (Figure 1), which theoretically provided more adsorption sites for the shale. However, comparing the TOC and the BET specific surface areas and total pore volumes in the Middle and Lower Yangtze regions (Figure 9), it was found that there was no significant linear relationship between the TOC content and the specific surface area and total pore volume in the shale of the Middle Yangtze region (Figure 9a,b), indicating that organic matter pores were not the main contributor to the micropore volume and specific surface area. However, the TOC content of the shale in the Lower Yangtze region did show a significant positive correlation with the specific surface area and total pore volume, indicating that the micropore volume and specific surface area were mainly provided by organic matter pores (Figure 9c,d). As shown in Figure 5, a large number of carbonate intragranular dissolution pores developed in the shale of the Middle Yangtze region, and thus, the micropore volume and specific surface area in that region were mainly provided by organic matter pores and carbonate intragranular dissolution pores. However, shale gas is generally adsorbed in organic matter pores [1]. The pores of the lower Cambrian marine shale in the Middle Yangtze region were more developed and possessed substantially larger specific surface areas, which suggests that the “effective” specific surface area of pores—the area with the ability to adsorb shale gas—was smaller in the Middle Yangtze region than in the Lower Yangtze region.
From the perspective of sedimentary–tectonic evolution, the Middle and Lower Yangtze regions experienced strong tectonic deformation from the end of the Paleozoic to the Mesozoic [96]. However, the tectonic activity and magma intrusion in the Lower Yangtze region were more intense than in the Middle Yangtze region [84,85], and the thickness of the Mesozoic strata in the Lower Yangtze region is greater than in the Middle Yangtze region, especially in the southern Anhui region, where it can reach 20,090 m [96]. Strong tectonic deformation and deep burial depths significantly affect the development and preservation of shale pores [97,98].
Strong tectonic and magmatic activity generates large numbers of fractures in shale, which leads to a deterioration in the sealing ability of the hydrocarbon generation system and strengthening of the organic matter condensation reactions, resulting in dense shale with reduced porosity. At the same time, the fluid that originally filled the pores will escape and dissipate in large quantities due to fractures and magmatic channels [98]. However, the formation pressure generated by the thick upper overburden layer will compress or collapse organic pores without fluid filling and compress or compact the interlayer pores of clay minerals [98]. In addition, it was found in exploration practice that under deep, high-temperature, and high-pressure conditions, the brittle–ductile transition zone of shale gradually merges into the ductile zone, and the plasticity of the shale increases while its compressive strength decreases, which is extremely detrimental to the development and preservation of pores [99,100,101]. These factors have resulted in reduced pore development in the lower Cambrian marine shale in the Lower Yangtze region compared with the Middle Yangtze region.

5.3. Differences in Shale Gas-Bearing Properties and Controlling Factors

The cross-plot of VL and TOC of the lower Cambrian marine shale in the Middle and Lower Yangtze regions (Figure 10) shows that both TOC and VL show good positive correlations in the Middle and Lower Yangtze regions, indicating that TOC was an important factor in controlling the adsorption capacity of shale. In addition, when the TOC values are similar, the VL of the lower Cambrian marine shale in the Lower Yangtze region was larger than that of the Middle Cambrian marine shale in the Middle Yangtze region (in Figure 10, the slope of the fitting curve for the Lower Yangtze samples was significantly greater than that of the Middle Yangtze samples), indicating that the adsorption capacity of the lower Cambrian marine shale in the Lower Yangtze region was greater.
The pores of the lower Cambrian marine shale in the Middle Yangtze area were more developed (see Figure 5 and Figure 6), and their specific surface areas were significantly larger than those in the Lower Yangtze area (see Table 1). However, the adsorption capacity (VL) of mud shale was significantly inferior to that of the Lower Yangtze region. This indicates that the “effective” specific surface area of the pores capable of adsorbing shale gas was smaller in the Middle Yangtze area than in the Lower Yangtze area. The thermal evolution degree of the organic matter in the shale in the Lower Yangtze area was higher than that in the Middle Yangtze area (see Table S1 and Figure 3b). As the degree of thermal evolution increases, the lipid structure decreases and the aromatic structure increases due to the aromatization of kerogen [98]. However, the aromatic kerogen has a stronger affinity for methane [98], which increases the “effective” specific surface area for methane adsorption, resulting in increased methane adsorption. Changes in the chemical structure of kerogen at high degrees of thermal evolution were therefore among the main factors that controlled the methane adsorption intensity in the lower Cambrian marine shale in the Middle and Lower Yangtze areas.
An increase in carbonate content will also lead to more complex pore structures [43]. The lower Cambrian marine shale in the Middle Yangtze area had a higher carbonate content (see Table S2, Figure 4 and Figure 7a), and thus, it had a more complex pore structure. Previous nitrogen isothermal adsorption experiments on the lower Cambrian marine shale in the Middle and Lower Yangtze areas showed that the nitrogen isothermal adsorption hysteresis loop of the Lower Yangtze area was mostly the H3 type, while that of the Middle Yangtze area was mostly a combination of the H2 and H3 types [36,41,43,72,75,86,102,103]. The shale in the Middle Yangtze area, therefore, had more fine-neck and ink-bottle-like pores, corresponding to organic and dissolution pores [104] and showing poor pore connectivity. Shale with complex pore structures and poor connectivity also has poor methane adsorption performance [105], and thus, the complexity and connectivity of pores were clearly other important factors that controlled the adsorption performance of the lower Cambrian marine shale in the Middle and Lower Yangtze areas.
Although the shale in the Lower Yangtze area has better hydrocarbon generation and reservoir qualities, exploration for shale gas in the area has not made great progress, with exploration and investigation wells typically showing low gas content, low methane content, or no gas content at all [14,43,45,46,106]. In comparison, the shale of the lower Cambrian Shuijingtuo Formation in the Yichang area of the Middle Yangtze area has produced industrial gas flow [14]. Given that the accumulation of shale gas is controlled by both the material nature of the shale and the preservation conditions for shale gas [107], poor preservation conditions are likely to be an important factor that restricted the accumulation of shale gas in the lower Cambrian in the Lower Yangtze area.

5.4. The Influence of Tectonics and Magmatic Activity on the Preservation of Shale Gas

Shale gas reservoirs are generally capable of self-generation and self-storage, and shales that generate shale gas are also reservoirs [35]. Consequently, the sealing ability of the later hydrocarbon generation system will directly affect whether shale gas can be enriched and preserved [35]. The Middle and Lower Yangtze areas are situated within a larger area of strong tectonic deformation [96] (see Figure 11). The stages of tectonic evolution can be divided into a steady subsidence stage before the Indosinian movement, a compressional nappe stage from the Indosinian to the Middle Yanshanian, and an extensional rift stage from the late Yanshanian to the Himalayan.
During the compression and thrusting stage of the Indosinian–Yanshanian period, strong uplift and thrusting caused faulting, strike-slip, folding, and erosion in the Paleozoic strata in the Middle and Lower Yangtze areas. In most areas, the Paleozoic strata were eroded almost completely, with large expanses of Silurian strata exposed. The integrity of the regional cover layers was disrupted, the continuity of the strata was broken, and the development of structural fractures led to the deterioration of reservoir sealing, resulting in the significant loss of shale gas. However, the shielding effect of the Huangling uplift and the ancient basement of Shennongjia meant that there was less structural deformation of the lower Cambrian strata in the Middle Yangtze area than in the Lower Yangtze area [109,110], and therefore, comparatively little destruction of shale gas reservoir traps. Tectonic inversion occurred in the Middle and Lower Yangtze areas during the extensional rift stage from the late Yanshanian to the Himalayan, with the whole area experiencing considerable extension. Most of the reverse faults were reversed into normal faults, and many new normal faults formed. The sealing ability of normal faults is poor [111], and thus, they are not conducive to the preservation of shale gas. However, due to obstruction by the Huangling paleo-uplift, the extension from east to west in the Middle Yangtze area gradually weakened and finally disappeared. In this area, reverse faults turned into normal faults, but new normal faults were relatively few, and shale gas preservation conditions were, therefore, relatively good.
Magmatic activity in the Middle and Lower Yangtze areas primarily occurred during the Yanshan and Himalayan periods and was concentrated in the Lower Yangtze area, particularly in southern Anhui (see Figure 11). Magmatic activity can enhance the permeability of shale to some extent, and, due to the formation of special high-pressure strata, it can enhance its adsorption capacity for shale gas [77,112]. However, in the later stage of reservoir formation, magmatic activity can significantly inhibit the accumulation of shale gas. It can destroy the cap rock, allowing for the intrusive rock pipeline to become a path for shale gas to escape [113]. The intruding high-temperature magma will also bake the surrounding organic matter, resulting in carbonization or alteration of the mineral composition of the shale, reducing its adsorption capacity for gas [33,114,115,116]. Due to the particularly intense magmatic activity in the Lower Yangtze region in the later stage of reservoir formation, the damage to the shale gas reservoir and its cap rock was more significant, which is the main reason why exploration for shale gas in this region is particularly challenging.
Generally, strong tectonic deformation and volcanic activity play important roles in the formation and accumulation of shale gas [117]. The development of structural fractures is conducive to shale gas migration, providing more pore space for shale gas attachment, and later, fracturing [107]. However, the destruction of caprock integrity, stratigraphic continuity, and hydrocarbon generation system sealing led to shale gas loss and the excessive thermal evolution of organic matter. Compared with the Middle Yangtze area, stronger tectonic deformation and magmatic activity in the Lower Yangtze area mean that the preservation conditions for shale gas are worse than in the Middle Yangtze area. This is the fundamental reason why shale gas has generally not been preserved in the lower Cambrian strata in the Lower Yangtze area. Finding small-scale, structurally stable areas in the fragmented environment of the Lower Yangtze area is, therefore, the priority for lower Cambrian shale gas exploration in the area.

6. Conclusions

The findings were as follows:
(1)
The organic carbon content of lower Cambrian marine shale in the Lower Yangtze area was markedly higher than that of the Middle Yangtze area, and the thermal evolution degree of organic matter in the Lower Yangtze area was higher than in the Middle Yangtze area.
(2)
The brittle mineral contents of lower Cambrian marine shales in the Middle and Lower Yangtze regions were more than 60%, and they all showed good fracturability. From NW to SE, the types of brittle minerals decreased while siliceous minerals increased. The early Paleozoic marine shale in the Lower Yangtze area had high silicon contents and low calcium contents.
(3)
The adsorption properties of lower Cambrian marine shale in the Lower Yangtze area were better than those in the Middle Yangtze area. However, micropores in the Middle Yangtze shale were more developed than in the Lower Yangtze, indicating that the degree of pore development was not the main factor controlling the adsorption properties of the lower Cambrian marine shale. However, the thermal evolution of organic matter was higher in the Lower Yangtze area, and thus, the proportion of aromatic components in organic matter was higher, and shale gas was therefore more easily absorbed. The particular molecular structure of kerogen caused by high thermal evolution was, therefore, an important factor that controlled the adsorption properties of lower Cambrian marine shale in the Middle and Lower Yangtze regions.
(4)
Strong tectonic deformation and frequent magmatic activity caused the integrity of the regional caprock and the sealing of the hydrocarbon generation system in the Lower Yangtze area to be much poorer than those in the Middle Yangtze area, and thus, the preservation conditions for shale gas were relatively poor. This was the main factor that restricted the accumulation and preservation of lower Cambrian shale gas in the Lower Yangtze area.

Supplementary Materials

The following supporting information can be downloaded from https://www.mdpi.com/article/10.3390/min14010031/s1—Table S1: Organic geochemical parameters of the lower Cambrian marine shale in the middle and lower Yangtze region (including organic matter types, TOC (%), and Ro (%)); Table S2: Main mineral component content (%) of the lower Cambrian marine shale in the middle and lower Yangtze region. Refs. [118,119,120,121] are cited in Supplementary Materials.

Author Contributions

Conceptualization, D.Z. and H.D.; methodology, H.D.; validation, D.Z., H.D. and Z.D.; investigation, H.D., X.H. and Z.D.; resources, H.D. and Z.D.; writing—original draft preparation, H.D.; writing—review and editing, D.Z. and X.H.; supervision, D.Z. and X.H.; project administration, D.Z.; funding acquisition, D.Z. All authors have read and agreed to the published version of the manuscript.

Funding

The authors wish to acknowledge the financial support of the National Natural Science Foundation of China (grant no. 42272359) and the Ministry of Natural Resources, P.R.C. (project: evaluation of shale gas resources and selection of favorable areas in the Middle and Lower Yangtze areas).

Data Availability Statement

The original contributions presented in the study are included in the article/supplementary material; further inquiries can be directed to the corresponding authors.

Acknowledgments

We would like to thank Zhanhong Liu, School of Marine Science, China University of Geosciences (Wuhan), for providing samples from the Middle Yangtze area. The authors are grateful for constructive comments by two anonymous reviewers on an earlier draft of this manuscript. We also want to express our gratitude to Shiping Wei from the School of Ocean Sciences of China University of Geosciences (Beijing) for his invaluable support and assistance throughout our research and manuscript revision process. We sincerely appreciate all anonymous reviewers and the handling editor for their critical comments and constructive suggestions.

Conflicts of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as potential conflicts of interest.

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Figure 3. Comparison of the organic geochemical characteristics of the lower Cambrian marine shale in the Middle and Lower Yangtze areas. (a) TOC; (b) Ro. The average values of TOC and Ro in the organic-rich shale of the lower Cambrian in the Lower Yangtze area were higher than those in the Middle Yangtze area. The data for wells CY-1, HY-1, YD-2, YY-1, TD-1, XY, and the JSL section were taken from references [46,58,59,62,63,64,65], respectively.
Figure 3. Comparison of the organic geochemical characteristics of the lower Cambrian marine shale in the Middle and Lower Yangtze areas. (a) TOC; (b) Ro. The average values of TOC and Ro in the organic-rich shale of the lower Cambrian in the Lower Yangtze area were higher than those in the Middle Yangtze area. The data for wells CY-1, HY-1, YD-2, YY-1, TD-1, XY, and the JSL section were taken from references [46,58,59,62,63,64,65], respectively.
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Figure 4. Comparison of the bulk rock mineral compositions of the lower Cambrian marine shales in the Middle and Lower Yangtze areas. The carbonate content in the lower Cambrian shale in the Middle Yangtze region was higher than that in the Lower Yangtze region, whereas the quartz content was significantly lower. The data for wells CY-1 and HY-1 were from reference [58], the data for wells WD-1 and WD-2 were from reference [43], the data for the JSL and ZZK sections were from reference [66], the data for well WY-1 were from reference [46], the data for the JJ section were from reference [67], and the data for the XS section were from reference [68].
Figure 4. Comparison of the bulk rock mineral compositions of the lower Cambrian marine shales in the Middle and Lower Yangtze areas. The carbonate content in the lower Cambrian shale in the Middle Yangtze region was higher than that in the Lower Yangtze region, whereas the quartz content was significantly lower. The data for wells CY-1 and HY-1 were from reference [58], the data for wells WD-1 and WD-2 were from reference [43], the data for the JSL and ZZK sections were from reference [66], the data for well WY-1 were from reference [46], the data for the JJ section were from reference [67], and the data for the XS section were from reference [68].
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Figure 5. Pore characteristics of marine shale in the lower Cambrian Shuijingtuo Formation in the Luojiacun section (LJC), Zigui County, Yichang City, Middle Yangtze. The lithology of the test sample was black shale, TOC was 4.07 wt. % and Ro was 3.16%, with observation using FE-SEM. (a) The block-like organic matter shows visible shrinkage fissures, which were filled with calcite. (b) The block-like organic matter contains many sponge-like organic micropores internally, with the edges showing shrinkage fissures. (c) The organic matter was mixed with phosphorite, and many sponge-like organic micropores were developed. (d) The organic matter was mixed with clay minerals, with a small number of isolated organic micropores having developed. (e) Interlayer pores of clay minerals. (f) Pores between strawberry-like goethite grains. (g) Pores between mineral grains. (h) Calcite grains show a large number of dissolution pores internally. (i) Dolomite grains show a large number of dissolution pores internally. OM—organic matter, Qtz—quartz, Cal—calcite, Dol—dolomite, Py—pyrite, Ap—apatite, Kfs—K-feldspar.
Figure 5. Pore characteristics of marine shale in the lower Cambrian Shuijingtuo Formation in the Luojiacun section (LJC), Zigui County, Yichang City, Middle Yangtze. The lithology of the test sample was black shale, TOC was 4.07 wt. % and Ro was 3.16%, with observation using FE-SEM. (a) The block-like organic matter shows visible shrinkage fissures, which were filled with calcite. (b) The block-like organic matter contains many sponge-like organic micropores internally, with the edges showing shrinkage fissures. (c) The organic matter was mixed with phosphorite, and many sponge-like organic micropores were developed. (d) The organic matter was mixed with clay minerals, with a small number of isolated organic micropores having developed. (e) Interlayer pores of clay minerals. (f) Pores between strawberry-like goethite grains. (g) Pores between mineral grains. (h) Calcite grains show a large number of dissolution pores internally. (i) Dolomite grains show a large number of dissolution pores internally. OM—organic matter, Qtz—quartz, Cal—calcite, Dol—dolomite, Py—pyrite, Ap—apatite, Kfs—K-feldspar.
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Figure 6. Pore characteristics of lower Cambrian Hetang Formation Marine Shale in the Hongtaocun Section (HTC), Shitai County, Southern Anhui Province, Lower Yangtze Region. The lithology of the test sample was carbonaceous shale, TOC was 7.59 wt. % and Ro was 3.86%, with observation using FE-SEM. (a–c) Dispersed blocky organic matter and infilled organic matter developed sporadic pores, which were relatively large. The developed organic matter pores mostly exhibited a flattened irregular shape, with a significantly higher long-axis/short-axis ratio and collapsed edges. (d) Interlayer pores of clay minerals. (e,f) Intergranular pores between strawberry-like goethite particles. (g) Intergranular pores between mineral grains. (h,i) Alteration of potassium feldspar along the edges, with the development of intragranular dissolution pores. OM—organic matter, Qtz—quartz, Cal—calcite, Dol—dolomite, Py—pyrite, Ap—apatite, Kfs—K-feldspar.
Figure 6. Pore characteristics of lower Cambrian Hetang Formation Marine Shale in the Hongtaocun Section (HTC), Shitai County, Southern Anhui Province, Lower Yangtze Region. The lithology of the test sample was carbonaceous shale, TOC was 7.59 wt. % and Ro was 3.86%, with observation using FE-SEM. (a–c) Dispersed blocky organic matter and infilled organic matter developed sporadic pores, which were relatively large. The developed organic matter pores mostly exhibited a flattened irregular shape, with a significantly higher long-axis/short-axis ratio and collapsed edges. (d) Interlayer pores of clay minerals. (e,f) Intergranular pores between strawberry-like goethite particles. (g) Intergranular pores between mineral grains. (h,i) Alteration of potassium feldspar along the edges, with the development of intragranular dissolution pores. OM—organic matter, Qtz—quartz, Cal—calcite, Dol—dolomite, Py—pyrite, Ap—apatite, Kfs—K-feldspar.
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Figure 7. (a) Ternary diagram of the mineral components of the lower Cambrian marine shales in the Middle and Lower Yangtze areas, with the base map drawn from [85]. The data for wells WD-1 and WD-2 were from reference [43], the data for the ZZK and JSL sections were from reference [66], and the data for wells CY-1 and WY-1 were from references [46,58], respectively. The data for the JR and PM1 sections were from references [67,86], respectively. (b) Ternary diagram of Al-Fe-Mn for the lower Cambrian marine shales in the Middle and Lower Yangtze areas reflects the sources of silica. The base map was from reference [16,17]. The data for wells YD-2, ZK03, XY-1, and XN-1 were from references [19,30,44,87], respectively. The data for the ZJJ and TS sections were from references [88,89], respectively.
Figure 7. (a) Ternary diagram of the mineral components of the lower Cambrian marine shales in the Middle and Lower Yangtze areas, with the base map drawn from [85]. The data for wells WD-1 and WD-2 were from reference [43], the data for the ZZK and JSL sections were from reference [66], and the data for wells CY-1 and WY-1 were from references [46,58], respectively. The data for the JR and PM1 sections were from references [67,86], respectively. (b) Ternary diagram of Al-Fe-Mn for the lower Cambrian marine shales in the Middle and Lower Yangtze areas reflects the sources of silica. The base map was from reference [16,17]. The data for wells YD-2, ZK03, XY-1, and XN-1 were from references [19,30,44,87], respectively. The data for the ZJJ and TS sections were from references [88,89], respectively.
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Figure 8. Cross-plot of Siex and TOC in the lower Cambrian marine shales in the Middle and Lower Yangtze areas, with (a) representing the Middle Yangtze area and (b) representing the Lower Yangtze area. The data for wells YD-2 and ZK03 were from references [19,44], respectively, while the data for the DB and LWD sections were from reference [28]. In the Middle Yangtze area, there was a strong positive correlation between Siex and TOC, indicating that the silica was primarily of biogenic origin. However, in the Lower Yangtze area, there was no significant linear correlation between Siex and TOC, suggesting that the silica was primarily derived from hydrothermal fluids.
Figure 8. Cross-plot of Siex and TOC in the lower Cambrian marine shales in the Middle and Lower Yangtze areas, with (a) representing the Middle Yangtze area and (b) representing the Lower Yangtze area. The data for wells YD-2 and ZK03 were from references [19,44], respectively, while the data for the DB and LWD sections were from reference [28]. In the Middle Yangtze area, there was a strong positive correlation between Siex and TOC, indicating that the silica was primarily of biogenic origin. However, in the Lower Yangtze area, there was no significant linear correlation between Siex and TOC, suggesting that the silica was primarily derived from hydrothermal fluids.
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Figure 9. Cross-plot of TOC, BET specific surface area, and BJH pore volume for the lower Cambrian marine shales in the Middle and Lower Yangtze areas. There was no significant correlation between TOC and BET specific surface area (a) or between TOC and BJH pore volume (b) in the Middle Yangtze area. However, a significant positive correlation was observed between TOC and BET specific surface area (c) and between TOC and BJH pore volume (d) in the shales of the Lower Yangtze area. The data for wells WD-1 and WD-2 were from reference [43], the data for the WN section were from reference [39], and the data for well RDZ01 were from reference [72].
Figure 9. Cross-plot of TOC, BET specific surface area, and BJH pore volume for the lower Cambrian marine shales in the Middle and Lower Yangtze areas. There was no significant correlation between TOC and BET specific surface area (a) or between TOC and BJH pore volume (b) in the Middle Yangtze area. However, a significant positive correlation was observed between TOC and BET specific surface area (c) and between TOC and BJH pore volume (d) in the shales of the Lower Yangtze area. The data for wells WD-1 and WD-2 were from reference [43], the data for the WN section were from reference [39], and the data for well RDZ01 were from reference [72].
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Figure 10. Cross-plot of VL versus TOC data for the lower Cambrian marine shale in the Middle and Lower Yangtze areas. (a) represents the Middle Yangtze area and (b) represents the Lower Yangtze area. The TOC and VL in both regions showed good positive correlations, but there were still differences between them. The variation in VL with TOC in the Lower Yangtze area was more significant than in the Middle Yangtze area, indicating that the content of TOC exerted stronger control over VL (adsorption capacity). Specifically, the data for ZD-2, BGP, YY-1, WN, GXB, and RDZ01 were from references [36,37,38,39,40,41], respectively. The other data were from this study.
Figure 10. Cross-plot of VL versus TOC data for the lower Cambrian marine shale in the Middle and Lower Yangtze areas. (a) represents the Middle Yangtze area and (b) represents the Lower Yangtze area. The TOC and VL in both regions showed good positive correlations, but there were still differences between them. The variation in VL with TOC in the Lower Yangtze area was more significant than in the Middle Yangtze area, indicating that the content of TOC exerted stronger control over VL (adsorption capacity). Specifically, the data for ZD-2, BGP, YY-1, WN, GXB, and RDZ01 were from references [36,37,38,39,40,41], respectively. The other data were from this study.
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Figure 11. Tectonic division and distribution of intrusive rocks in the Middle and Lower Yangtze areas, modified from references [96,108]. The distribution of intrusive rocks in the Lower Yangtze region is more extensive than in the Middle Yangtze region, where intrusive rocks are mainly confined to the eastern margin close to the Lower Yangtze region.
Figure 11. Tectonic division and distribution of intrusive rocks in the Middle and Lower Yangtze areas, modified from references [96,108]. The distribution of intrusive rocks in the Lower Yangtze region is more extensive than in the Middle Yangtze region, where intrusive rocks are mainly confined to the eastern margin close to the Lower Yangtze region.
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Table 1. Comparison of the reservoir physical properties (including BJH total pore volume (cm3·g−1), BET specific surface area (m2·g−1), and BJH average pore diameter (nm)) of the lower Cambrian marine shale in the Middle and Lower Yangtze regions.
Table 1. Comparison of the reservoir physical properties (including BJH total pore volume (cm3·g−1), BET specific surface area (m2·g−1), and BJH average pore diameter (nm)) of the lower Cambrian marine shale in the Middle and Lower Yangtze regions.
RegionSectionFormationPhysical Characteristics of Shale ReservoirsData Sources
BJH Total Pore Volume/(cm3·g−1)BET Specific Surface Area/(m2·g−1)BJH Average Pore Diameter/(nm)
Middle
Yangtze		WD-1Shuijingtuo0.007~0.044/0.017 *5.94~25.72/11.085.42~7.86/6.85[43]
WD-2Shuijingtuo0.011~0.039/0.02110.67~19.8/14.756.14~7.71/7.07[43]
HY-1Suijingtuo0.001~0.020/0.0080.80~27.54/8.113.84~8.80/5.32[71]
Lower
Yangtze		XCHetang0.006~0.010/0.0082.15~8.89/5.654.21~14.3/7.63[39]
XY-1Hetang0.006~0.0124.30~7.50No data[46]
DTHetang0.012~0.015/0.0133.62~4.69/4.1613.07~13.78/13.43This study
LYHetang0.011~0.014/0.0132.67~3.98/3.3314.68~20.28/17.48This study
FCHetang0.012~0.015/0.0143.62~5.95/4.799.01~13.07/11.04This study
XY-1Wangyinpu0.0144.6610.98This study
RDZ01Wangyinpu0.005~0.018/0.0081.84~9.19/5.234.76~21.21/9.63[72]
Note: * number 1~number 2/number 3 represent min~max/mean, respectively.
Table 2. The adsorption capacity (VL (m3/t)) of lower Cambrian marine shale in the Middle and Lower Yangtze regions.
Table 2. The adsorption capacity (VL (m3/t)) of lower Cambrian marine shale in the Middle and Lower Yangtze regions.
RegionSampleTOC (%)VL (m3/t)RegionSampleTOC (%)VL (m3/t)
Middle YangtzeZZK-10.581.16Lower YangtzeLWD-12.492.86
ZZK-21.731.84AR-11.191.22
MR-10.812.77YZJ-13.984.47
MR-21.491.72YZJ-25.375.69
XLC-10.881.74FC-10.090.62
XLC-21.022.00FC-27.358.6
XLC-31.651.89DT-10.240.5
MXK-10.972.03DT-20.090.92
MXK-213.138.19DT-33.034.06
JSL-11.361.72DT-42.012.03
JSL-22.362.47LY-114.315.95
JSL-31.542.46LY-25.266.65
WJP-11.452.01LY-38.9416.87
WJP-22.992.90
SFJ-110.966.09
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Dong, H.; Zhou, D.; Deng, Z.; Huang, X. Comparative Study of the Characteristics of Lower Cambrian Marine Shale and Their Gas-Bearing Controlling Factors in the Middle and Lower Yangtze Areas, South China. Minerals 2024, 14, 31. https://doi.org/10.3390/min14010031

AMA Style

Dong H, Zhou D, Deng Z, Huang X. Comparative Study of the Characteristics of Lower Cambrian Marine Shale and Their Gas-Bearing Controlling Factors in the Middle and Lower Yangtze Areas, South China. Minerals. 2024; 14(1):31. https://doi.org/10.3390/min14010031

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Dong, Hezheng, Dongsheng Zhou, Ziyan Deng, and Xiaowei Huang. 2024. "Comparative Study of the Characteristics of Lower Cambrian Marine Shale and Their Gas-Bearing Controlling Factors in the Middle and Lower Yangtze Areas, South China" Minerals 14, no. 1: 31. https://doi.org/10.3390/min14010031

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