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Review

Reservoir Characteristics and Resource Potential of Marine Shale in South China: A Review

by
Zhiyao Zhang
1,
Shang Xu
2,*,†,
Qiyang Gou
1 and
Qiqi Li
1
1
Key Laboratory of Tectonics and Petroleum Resources, Ministry of Education, China University of Geosciences, Wuhan 430074, China
2
Shandong Provincial Key Laboratory of Deep Oil & Gas, China University of Petroleum (East China), Qingdao 266580, China
*
Author to whom correspondence should be addressed.
†
Current address: School of Geosciences, China University of Petroleum, Changjiangxi Road No. 66, Qingdao 266580, China.
Energies 2022, 15(22), 8696; https://0-doi-org.brum.beds.ac.uk/10.3390/en15228696
Submission received: 13 October 2022 / Revised: 12 November 2022 / Accepted: 13 November 2022 / Published: 19 November 2022
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Abstract

:
Many sets of Paleozoic marine organic-rich shale strata have developed in South China. However, the exploration and development results of these shale formations are quite different. Based on the data of core experiment analysis, drilling, fracturing test of typical wells, the reservoir differences and controlling factors of four sets of typical marine organic-rich shale in southern China are investigated. The four sets of shale have obvious differences in reservoir characteristics. Ordovician–Silurian shale mainly develops siliceous shale, mixed shale and argillaceous shale, with large pore diameter, high porosity, moderate thermal maturity, large pore volume and specific surface area. Cambrian shale mainly develops siliceous shale and mixed shale, with small pore diameter, low porosity, high thermal maturity and smaller pore volume and specific surface area than Ordovician–Silurian shale. Devonian–Carboniferous shale has similar mineral composition to Ordovician–Silurian shale, with small pore diameter, low porosity, moderate thermal maturity and similar pore volume and specific surface area to that of Cambrian shale. Permian shale has very complex mineral composition, with large pore diameter, low to medium thermal maturity and small specific surface area. Mineral composition, thermal maturity and tectonic preservation conditions are the main factors controlling shale reservoir development. Siliceous minerals in Cambrian shale and Ordovician–Silurian shale are mainly of biological origin, which make the support capacity better than Devonian–Carboniferous shale and Permian shale (siliceous minerals are mainly of terrigenous origin and biological origin). Thermal maturity of Ordovician–Silurian shale and Devonian–Carboniferous shale is moderate, with a large number of organic pores developed. Thermal maturity of Cambrian shale and Permian shale is respectively too high and too low, the development of organic pores is significantly weaker than the two sets of shale above. There are obvious differences in tectonic preservation conditions inside and outside the Sichuan Basin. Shale reservoirs inside the Sichuan Basin are characterized by overpressure due to stable tectonic activities, while shale reservoirs outside the Sichuan Basin are generally normal–pressure. Four sets of marine shale in South China all have certain resource potentials, but the exploration and development of shale gas is still constrained by complicated geological conditions, single economic shale formation, high exploration and development costs and other aspects. It is necessary for further research on shale gas accumulation theory, exploration and development technology and related policies to promote the development of China’s shale gas industry.

1. Introduction

Global shale gas resources are widely distributed, mainly in China, Argentina, Algeria, the United States, Canada, Mexico, Australia, South Africa, Russia and Brazil and other countries and regions [1]. According to the report by the United Nations Conference on Trade and Development (UNCTAD), the world’s economically recoverable shale gas reserves are about 214.5 × 1012 m3, of which the top five countries are China (31.6 × 1012 m3), Argentina (22.7 × 1012 m3), Algeria (20 × 1012 m3), the USA (17.7 × 1012 m3) and Canada (16.2 × 1012 m3) [2]. During the past decade, shale gas has become the main field of global oil and gas exploration and the production of shale gas has grown rapidly. In 2020, global shale gas production amounted to 768.8 × 109 m3, including 733 × 109 m3 in the United States, 20 × 109 m3 in China, 10.3 × 109 m3 in Argentina and 5.5 × 109 m3 in Canada; the world’s shale gas development is dominated by the United States [3].
Due to the development of horizontal well and fracturing technology, shale gas exploration and development in the United States has been successful and energy independence has been realized. At present, the commercially exploited shale in North America is mainly marine shale (mainly Devonian shale, Carboniferous shale, Permian shale and Jurassic shale), such as Devonian Marcellus shale in the Appalachian basin, Devonian Antrim shale in the Michigan basin, Devonian New Albany shale in the Illinois basin, Carboniferous Barnett in the Fort Worth basin, and Jurassic Haynesville shale in the East Texas basin and North Louisiana basin [4,5,6,7,8,9,10,11,12]. Following the United States and Canada, China has also made breakthroughs in shale gas exploration and development. China’s shale gas exploration is mainly aimed at marine organic-rich shale which is well developed in southern China, including Cambrian shale, Ordovician–Silurian shale, Devonian–Carboniferous shale and Permian shale [13,14,15,16,17,18,19,20]. However, although high-quality shale is well–developed in these marine strata, the exploration results of different shale are obviously different [21,22]. Therefore, it is necessary to evaluate the characteristics and influencing factors of the four sets of organic-rich marine shale reservoirs.

2. Geological Background

South China experienced multiple tectonic movements during the Caledonian–Himalayan period, which caused the uplift and subsidence of plates, resulting in the discontinuous deposition of multiple sets of marine organic-rich shale in southern China, mainly including Cambrian shale, Ordovician–Silurian shale, Devonian–Carboniferous shale and Permian shale (Figure 1) [23,24,25,26,27,28,29,30]. The basic geological characteristics of each shale are described as follows.

2.1. Cambrian Shale

From the Ediacaran to the Early Cambrian (about 550–530 Ma), the breakup of the Rodinia supercontinent ended and Gondwana was finally merged. The global blocks mainly included the Gondwana, Lauren, Baltic and Siberian plates [33,34]. South China (Yangtze, Cathay) and North China were independent cratons [35,36]. The South China block was located in the temperate zone of the Northern Hemisphere and northwest to Australia at that time (Figure 2a) [37,38,39]. Due to the fully developed interior cratonic rift basins in the Ediacaran, the South China block began to split and expand to form an ocean basin during the break-up of Rodinia in the Middle Neoproterozoic, and evolved into a passive continental margin basin during the Ediacaran–Early Cambrian [35,40,41,42].
The maximum flood of the South China block occurred during the Early Cambrian. During this period, the sedimentary environment formed by bodies of water that were rich in nutrients with high productivity associated with anoxic bottom-water conditions led to the development of a rich organic matter shale deposit. These Cambrian shales were deposited and buried on the top of the South China block’s carbonate platform. [36,39,44,45]. Several deep regions were distributed throughout the South China block, mainly in the west of Hubei Province, the southeast of Chongqing Province and the north of Guizhou Province. These regions are characterized by shallow-deep shelf sedimentary environment, in which black mudstone and shale are mainly deposited (Figure 3a) [46,47]. The sedimentary thickness is about 50–250 m (Figure 4a).

2.2. Ordovician–Silurian Shale

The Ordovician–Silurian (about 439–444 Ma) inherited the Cambrian paleogeographic pattern. At that time, the South China block was located near the equator and southwest to Australia (Figure 2b) [34]. During the Late Ordovician–Early Silurian, the compression and collision between the Yangtze block and the Cathaysian block intensified due to the Caledonian tectonic movement, making the southern part of the Central Yangtze near the Cathaysian plate rise to land. The South China Sea Basin continued to migrate to the northwest and the basement around the Yangtze plate rose, resulting in the formation of the surrounding areas (central Guizhou uplift, Xuefeng uplift and central Sichuan uplift) which isolated the upper Yangtze basin from the high sea, the South China block evolved from a shallow carbonate platform into a semi-isolated cratonic basin dominated by siliciclastic [57,58,59].
Two large-scale global transgression events in the South China block during the Late Ordovician–Early Silurian led to sea level rise, providing low-energy and anoxic deep-water conditions [60,61,62]. Ordovician–Silurian shale was then widely deposited and buried in the South China block. At this time, the sedimentary center of the South China block was mainly in the east and south of Sichuan Province. These areas are characterized by shallow-water shelf and deep shelf sedimentary environment, in which black siliceous shale and gray silty mudstone are mainly deposited (Figure 3b) [63,64,65]. The sedimentary thickness is about 50–150 m, which can reach more than 150 m in some areas (Figure 4b) [53].

2.3. Devonian–Carboniferous Shale

From the Devonian to the Carboniferous (about 416.2–295 Ma), Gondwana moved clockwise, resulting in collision with Lauren. The South China block was located near the equator at that time, the position moved slightly southward compared with Ordovician–Silurian (Figure 2c) [43]. During this period, the middle and upper Yangtze regions were uplifted and denuded due to the influence of large-scale tectonic uplift from the Caledonian movement. Owing to a 40-million-year long history of rifting in the eastern Yunnan, southern Guizhou, Guangxi, southern Hunan and lower Yangtze region, the South China craton margin developed a complex facies architecture of shallow platforms separated by deep-water basins [43].
The South China block experienced a transgression from south to north during Devonian, resulting in the deposition of Devonian–Carboniferous shale in the southern rift zone of the South China block. These areas are characterized by carbonate platform, platform margin slope and inter-platform basin sedimentary environment, in which the main sediments are gray–black mudstone, siliceous mudstone and shale, mixed with dark–gray arenaceous limestone and bioclastic limestone (Figure 3c). The sedimentary thickness is about 20–400 m (Figure 4c) [55].

2.4. Permian Shale

During the Permian (295–250 Ma), all the global plates gradually moved northward and rotated counterclockwise. The South China block was located near the equator and eastern to the Paleotethys Ocean at that time (Figure 2d) [34,66]. During this period, the seawater mainly intruded into the mainland from the south and west sides, and the middle and upper Yangtze regions were almost submerged by seawater, which were mainly open platform facies deposits. A few tidal flat deposits were developed near the ancient lands in some areas. The Yunnan–Guizhou–Guangxi region is characterized by the pattern of platform basin alternation, and the sedimentary environment is mainly deep shelf, platform margin slope and isolated platforms. The sedimentary environment of the lower Yangtze region is mainly platform and deep shelf, and the sedimentary characteristics are similar to those of the middle and upper Yangtze regions (Figure 3d) [50]. The lithology of Permian shale is dominated by thin gray–black siliceous mudstone and argillaceous shale, containing siliceous rock, carbonate and calcareous mudstone. Moreover, biofossils and coal formation are relatively developed, and the sedimentary thickness is about 40–100 m (Figure 4d) [56,67,68].

3. Reservoir Characteristics and Control Factors of Four Sets of Marine Shale

3.1. Petrology and Mineralogy

According to the relevant measured and collected data, there are certain differences in mineral components of four sets organic-rich shale (Figure 5). The Cambrian shale mainly developed siliceous shale and mixed shale, and the average content of siliceous minerals is 31.0–76.9%. Furthermore, the clay minerals in Cambrian shale are relatively developed, with an average content of 15.3–54.9%, and the average content of the carbonate minerals is generally below 15% (Table 1).
Ordovician–Silurian shale mainly develop siliceous shale, mixed shale and argillaceous shale, which are rich in graptolite fossils (Figure 5). The siliceous minerals are slightly lower than Cambrian shale, with an average content of 40.0–63.3%. The average content of the clay minerals is 9.16–47.6%, and the carbonate minerals are relatively developed, with an average content of 5.1–39.1% (Table 2).
Lithofacies characteristics of Devonian–Carboniferous shale are similar to those of Ordovician–Silurian shale, which consist of mainly siliceous shale, mixed shale and argillaceous shale. The average content of the siliceous minerals is 42.6–48.9%, the average content of the clay minerals is 30.6–34.2%, and the average content of the carbonate minerals is 16.1–18.4% (Table 3).
The mineral composition of Permian shale is complex, with mixed shale, argillaceous shale and siliceous shale developed (Figure 5). The average contents of the siliceous minerals, clay minerals and carbonate minerals are 22.2–53.5%, 28.0–43.6% and 8.1–48.3%, respectively (Table 4).
Four sets of shale have certain differences in their mineral composition, which reflects the differences in sedimentary environments and parent rock properties. The content of the siliceous minerals and clay minerals in Ordovician–Silurian shale is relatively high, and the siliceous minerals are mainly of biogenic origin [22,99]. A large number of biogenic siliceous minerals can form a stable rigid framework, which can make the shale have a strong anti-compaction capacity and is conducive to the preservation of organic pores. Therefore, the pore diameter (tens to hundreds of nm) [100], porosity (1.3–4.7%), pore volume (average 27.73 cm3/g) and specific surface area (average 35.18 m2/g) of the Ordovician–Silurian shale are relatively large (Table 2, Figure 6). The pore types are mainly micropores (pore diameter < 2 nm) and mesopores (pore diameter 2–50 nm), according to the pore classification standard of IUPAC [101].
Cambrian shale is characterized by high content of siliceous minerals which are mainly of biological and hydrothermal origin [22,104]. The development of hydrogenic siliceous minerals is controlled by the intense hydrothermal activity caused by the strong continental internal extension during the Early Cambrian [39]. Therefore, the anti-compaction ability of Cambrian shale is weaker than that of Ordovician–Silurian shale. The pore types are dominated by micropores and mesopores (pore classification standard of IUPAC) [101], and the pore diameter (less than 100 nm) [105], porosity (0.85–2.8%), pore volume (average 18.46 cm3/g) and specific surface area (average 23.27 m2/g) are smaller than those of Ordovician–Silurian shale (Table 1, Figure 6).
The siliceous minerals in Devonian–Carboniferous shale are less than those of the two sets of shale above and are mainly of biological origin and terrigenous origin [30]. The pore types are dominated by micropores and mesopores (pore classification standard of IUPAC) [101], and the pore diameter is generally less than 100 nm. The porosity (1.0–2.1%), pore volume (average 15.68 cm3/g) and specific surface area (average 18.81 m2/g) are similar to those of Cambrian shale, which may be related to the well-developed clay minerals (Table 3, Figure 6).
The content of the mineral components in Permian shale are complex, which may be caused by the differences of the sedimentary environments across different regions [106,107]. Carbonate minerals and clay minerals are well developed in Permian shale. The numbers of organic pores are small, and the pore diameter is large (tens to hundreds of nm) [108]. Pore volume (average 17.21 cm3/g) is similar to Cambrian shale and Devonian–Carboniferous shale, and the specific surface area (average 13.41 m2/g) is obviously smaller than that of the three sets of shale above (Table 4, Figure 6).

3.2. Total Organic Carbon

Total organic carbon (TOC) of the four sets of organic shale widely developed in South China is significantly different (Figure 7). The TOC of Cambrian shale, Ordovician–Silurian shale and Permian shale is generally higher than 2.5%, while that of Devonian–Carboniferous shale is low and only shows high values in some areas [22,48,51,109,110].
Cambrian and Ordovician–Silurian shale are mainly developed in and around the Sichuan Basin. The TOC of Ordovician–Silurian shale is slightly higher than that of Cambrian shale, and Cambrian shale has abnormally high values in some areas (Figure 7) [22,111]. Inside the Sichuan Basin, the TOC of Ordovician–Silurian shale is about 3.6%, and that of Cambrian shale is about 2.9% (Table 1 and Table 2) [22]. Outside the Sichuan Basin, the TOC of Ordovician–Silurian shale is lower than that inside the basin (Figure 7). The average TOC of Well YY1 in southeast Chongqing and Well EYY2 in west Hubei are 2.46% and 2.47%, respectively (Table 2) [92,93]. The TOC of Cambrian shale inside and outside the basin is close. The TOC of Well DS1 in southeast Chongqing and Well Z103 in north Guizhou are similar to those inside the basin, and the average TOC are 2.3% and 2.3%, respectively (Table 1) [22]. However, the TOC of Cambrian shale is abnormally high in a few drilling wells (about 5.4% for YouK1 and 4.2% for CenY1) (Table 1) [22,84], which may be related to the local enrichment of organic matter. Devonian–Carboniferous shale is mainly developed in the Yunnan–Guizhou–Guangxi region, and the TOC is relatively low (generally lower than 2%) (Table 3, Figure 7) [95]. The Permian shale is mainly developed in western Hubei, northern Guizhou and southern Anhui. The average TOC is about 4.36%, and some intervals are lower than 1.5%. The higher TOC may be related to the commonly developed coal strata (Table 4, Figure 7) [50,112].
There is a strong positive correlation between TOC, pore volume and specific surface area for the four sets of shale in South China, indicating that TOC has a certain positive contribution to the pore development (Figure 8). However, previous studies also showed that the porosity in mature shale increased first and then decreased with the increase of TOC [112,113]. Through the study of the Devonian Marcellus shale in the United States, it was found that the porosity increases with the increase of TOC when the TOC is less than 5.6%; while there is an obvious negative correlation between the porosity and TOC when the TOC is more than 5.6% [114]. Previous studies have shown that this phenomenon can also be observed in Cambrian shale and Ordovician–Silurian shale, the size and development of organic pores decreased significantly with the increase of TOC when TOC is greater than 5% [112].
Therefore, TOC has two impacts on the pore development of marine organic-rich shale in South China. Moderate TOC can promote the formation of organic pores, which are not easily destroyed under the protection of the rigid framework, thus increasing the shale pore volume and enhancing the shale reservoir capacity. Too high TOC will make shale have high plasticity and more vulnerable to compaction, thereby resulting in the compression, deformation, and even closure of organic pores and some inorganic pores.

3.3. Thermal Maturity

As one of the most important parameters for shale reservoir evaluation, the thermal maturity not only affects the generation of shale gas, but also has a huge impact on the pore development of the shale reservoir [118,119,120,121,122]. The thermal maturity of shale is generally characterized by vitrinite reflectance (Ro).
As one of the few countries in the world to achieve commercial development of shale gas, the shale gas production formations in the United States are mainly Devonian, Carboniferous, Jurassic and Permian—all having a relatively low thermal maturity. For example, the Ro of Mississippi Barnet shale in the Fort Worth basin is between 1% and 1.3%, the Ro of Devonian Marcellus shale in the Appalachian basin is between 1.2% and 3.5%, the Ro of Devonian Woodford shale in the Anadarko basin is between 1.2% and 4%, and the Ro of Devonian Antrim shale in the Michigan basin ranges from 0.4% to 1.6% [4,5,6,7,8,9,10,11,12,123]. Compared with the United States, the four sets of organic-rich shale in southern China have a relatively high thermal maturity. From older to younger, the thermal maturity gradually decreases (Figure 9). The thermal maturity of Cambrian shale is higher than that of the other three sets of shale (the Ro is 2.2–4.4%, generally 2.7–4.2%), which is in the over mature stage (Table 1, Figure 9) [22,84,111]. The thermal maturity of Ordovician–Silurian shale is moderate (the Ro is 1.9–3.7%, generally 2.0–3.3%), and is in the high maturity stage (Table 2, Figure 9) [22,48,92,94,124]. The thermal maturity of Devonian–Carboniferous shale is slightly lower than that of Ordovician–Silurian shale (the Ro is generally 2.0–2.5%), which is also in the high maturity stage (Table 3, Figure 9) [95,124]. The thermal maturity of Permian shale is low–moderate (the Ro of 1.2–3.03%, generally 1.4–2.4%), which is in the lower maturity stage (Table 4, Figure 9) [22,111].
The difference of thermal maturity also has a significant impact on the development of organic pores. Figure 10 shows that the influence of thermal maturity on the development of organic pores can be divided into three stages: formation stage (0.6% < Ro < 2.0%), development stage (2.0% < Ro < 3.5%) and transformation and destruction stage (Ro > 3.5%) [112,125]. When Ro < 2.0% (Permian shale), the organic matter is in the lower maturity stage, in which organic pores begin to form. At this time, the porosity, pore volume and specific surface area are small. When 2.0% < Ro < 3.5% (Ordovician–Silurian shale and Devonian–Carboniferous shale), the organic matter is in the high maturity stage, in which a large number of organic pores begin to form, and the pore volume increases with the increase of thermal maturity. When Ro > 3.5% (Cambrian shale), the organic matter is in the over mature stage, in which the organic pores are destroyed, collapsed or even closed due to the carbonization of organic matter. The porosity, pore volume and specific surface area are significantly reduced with the increase of thermal maturity at this stage.

3.4. Tectonic Preservation Conditions

Previous studies have shown that tectonic preservation conditions are one of the key factors controlling shale gas enrichment. Better tectonic preservation conditions can enhance the sealing capacity of shale and prevent pores from being destroyed by compaction, which is conducive to shale gas enrichment and preservation [16,114,126].
Cambrian shale was formed early, experienced long-term thermal evolution and multiple tectonic activities, the tectonic movement resulted in a strong transformation on the shale. Macroscopically, the preservation conditions of Cambrian shale were worse than the other three sets of shale [69,111]. The tectonic activity of Cambrian shale in the Sichuan Basin is stable, the faults are basically undeveloped and the relatively good preservation conditions are conducive to the preservation of shale gas. For example, the Cambrian shale of Jinye1 Well in the Weiyuan Gas Field can produce 86,000 m3/d of shale gas after fracturing [22]. The tectonic deformation intensity of the Cambrian shale outside the Sichuan Basin is relatively large, with well-developed faults and poor preservation conditions. The Cambrian shale in western Hubei and northern Guizhou show the characteristics of normal-pressure shale gas reservoirs, the pressure coefficients of Cambrian shale in Z103 Well (northern Guizhou) and CY1 Well (northwestern Hunan) are 0.85 and 0.9, respectively [22,127].
Ordovician–Silurian shale was deposited later than the Cambrian shale. The tectonic transformation intensity is relatively low, and the sealing conditions of the bottom and top layers are relatively good [111,128]. At present, the highest degree of exploration and development area of Ordovician–Silurian shale gas is Jiaoshiba area, which is located in the eastern Sichuan Basin ejective fold belt. The formation pressure is generally greater than 1.2 (overpressure shale gas reservoir). For example, the Ordovician–Silurian shale of JiaoYe1 Well in Jiaoshiba area has a pressure coefficient of 1.55, and the shale gas production can reach 203,000 m3/d after fracturing [111]. Outside Sichuan Basin, Ordovician–Silurian shale is generally manifested as normal-pressure shale gas reservoirs due to the strong tectonism and poor preservation conditions, the pressure coefficients in AY1 Well in North Guizhou and PY1 Well in Southeast Chongqing are 1.1 and 0.9, respectively [27,129].
Devonian–Carboniferous shale is mainly confined in the Yunnan–Guizhou–Guangxi region. Due to the Yanshan–Himalayan tectonism, the Yunnan–Guizhou–Guangxi region has experienced strong tectonic transformation, uplift and denudation. Devonian–Carboniferous shale gas is mainly concentrated in the Nanpanjiang Depression, Guizhong Depression and other areas with good preservation conditions in the east of the Yunnan–Guizhou–Guangxi region, which is controlled by the development of caprocks [86]. The gas content of Devonian–Carboniferous shale in DT1 Well in the Guizhong Depression is 1.4–2.17 m3/t, which only reaches the minimum standards of commercial development for shale gas (2.0 m3/t) in North America [130].
Permian shale was deposited the latest and is still within the gas generation window. The distribution of Permian shale is mainly confined in northeast Sichuan, central Guangxi, central Hunan and the Lower Yangtze region. Among them, the tectonic deformation inside the Sichuan Basin is relatively weak and the preservation conditions are good. The existing exploration and development data revealed that Permian shale have good gas-bearing properties. The gas content of DYS1 well in southern Sichuan is 0.56–8.78 m3/t [130]. Outside Sichuan Basin, the tectonic transformation is intensive and the anticline area has been completely denuded, resulting in Permian shale only being residual in some synclines and depressions [111]. It is worth mentioning that the degree of exploration and development in the Lower Yangtze region is also relatively high. Although the continuity of shale is damaged due to the Indosinian–Yanshan movement, the well-preserved areas still have relatively good preservation conditions [97].

4. Resource Potential

By the end of 2018, China’s cumulative proved shale gas geological reserves were 10,455.67 × 108 m3 [131]. As the focus of shale gas exploration in China, marine organic-rich shale in southern China has huge resource potential. The Ordovician–Silurian shale has the best conditions for shale gas enrichment and high production, which is the key formation for shale gas exploration and development. Shale is widely distributed with high TOC and moderate thermal maturity [14,16,22,111]. Inside Sichuan Basin, the tectonic is relatively stable, South and East Sichuan are significant shale gas enrichment areas. Industrial production has been achieved in Fuling, Changning-Weiyuan, Zhaotong and Fushun-Yongchuan, with the favorable area of 2.43 × 104 km2 and the geological resources of 16.96 × 1012 m3 [22]. Outside Sichuan Basin, Ordovician–Silurian shale is exposed, denuded and seriously broken due to the strong tectonic transformation, which is unfavorable to the preservation of shale gas. Only a few relatively stable areas have certain resource prospects, and the favorable area and the geological resources are 0.86 × 104 km2 and 3.08 × 1012 m3, respectively [22,131].
Cambrian shale has high thermal maturity, poor top and bottom layers conditions [132,133]. The shale gas enrichment and preservation conditions are weaker than those of Ordovician–Silurian shale [22]. Inside Sichuan Basin, Weiyuan area has relatively good preservation conditions, with the favorable area of 0.62 × 104 km2 and the geological resources of 2.24 × 1012 m3 [22]. Outside Sichuan Basin, there are certain shale gas shows only in western Hubei, northern Guizhou and other regions due to strong tectonic transformation. The formation pressure is generally less than 1.1, which is not conducive to large-scale exploration and development.
The distribution of Devonian–Carboniferous shale is limited (mainly being distributed in the Yunnan–Guizhou–Guangxi region), with large sedimentary thickness, low TOC, moderate thermal maturity and low degree of exploration and development. According to existing studies, Devonian–Carboniferous shale can form commercial shale gas reservoirs under good geological conditions. The gas content can reach the minimum standards of commercial development for shale gas in North America [130,134].
Permian shale is mainly distributed in northeast Sichuan, west Hubei, central Hunan and the Lower Yangtze region, with high TOC and low–moderate thermal maturity. The preservation conditions inside Sichuan Basin are relatively good, the preliminary estimation of the favorable area and the geological resources are 0.05 × 104 km2 and 0.13 × 1012 m3, respectively [114]. Outside Sichuan Basin, research has showed that central Hunan also has resource potential, with the favorable area of 1.4 × 104 km2 and the geological resources of 0.6 × 1012 m3 [135]. Western Hubei and Lower Yangtze region have been remade by tectonism, but there are still good shale gas shows that have been observed from the drilling in some tectonically favorable areas [124].
At the same time, the exploration and development conditions of shale gas are obviously different compared with North America. The main differences are described as follows.
(1) The geological conditions are complicated. The platform of North America is generally stable with relatively simple structure, moderate burial depth of shale reservoirs (between 700–4000 m), lower thermal maturity (about 1.5–2.5% on average) and high gas content (1.7–9.9 m3/t) [136,137,138,139]. Most of China’s shale gas basins have experienced multiple tectonic movements with complex structures and well-developed faults. Shale reservoirs are deeply buried (between 2500–6500 m), possessing relatively high thermal maturity (>2.0%) and low gas content (1.0–3.0 m3/t on average) [123,136,138]. The complexity of the geological conditions in China’s shale gas basins not only influence the enrichment and accumulation of shale gas, but also increase the difficulty of shale gas exploration and development.
(2) There is only one economic shale gas formation. At least 30 sets of economic shale gas formations have been found in 29 basins in the United States [4,5,6,7,8,9,10,11,12,136]. However, China has only found one shale formation (Ordovician–Silurian shale) with commercial exploitation value in one basin (the Sichuan Basin and its surrounding areas) [136]. The oneness of economic shale formations restricts the development of China’s shale gas industry.
(3) The surface environment is complicated. The terrain for shale gas production area in the United States is relatively flat, sparsely populated and rich in water resources, which is conducive to exploration and production [138]. China’s shale gas enrichment areas are mostly located in mountains and hills with complicated surfaces [123,136,139]. The complexity of the surface environment increases the difficulty of shale gas exploration and development, resulting in high costs of infrastructure construction, engineering operations and shale gas transportation.
Therefore, although the marine organic-rich shale in southern China all have a certain resource potential, China’s shale gas exploration and development cannot fully follow the experiences of the United States. It is necessary for further research on shale gas accumulation theory, exploration and development technology and related policies from a practical perspective to promote the development of China’s shale gas industry.

5. Conclusions

(1) The reservoir characteristics of the four sets of marine organic-rich shale in South China are obviously different. Ordovician–Silurian shale has the best reservoir quality (high TOC and porosity, large pore diameter and moderate thermal maturity), followed by Cambrian shale (high TOC, small porosity and pore diameter) and Devonian–Carboniferous shale (low TOC and porosity, small pore diameter). Permian shale has poor reservoir quality, with low–moderate thermal maturity.
(2) Lithology, thermal maturity and tectonic preservation conditions are the main factors controlling shale reservoir development. Siliceous minerals of biogenic origin can form a stable rigid framework which increase the supporting capacity of shale, and facilitate the preservation of pores. Moderate thermal maturity is conducive to the formation of organic pores, and organic matter can form a large number of organic pores when Ro is between 2% and 3.5%. Tectonic preservation conditions are the important factor for pore preservation in complicated areas, the shale reservoirs inside the Sichuan Basin are generally characterized by overpressure due to good preservation conditions, while shale reservoirs outside Sichuan Basin is generally manifested as normal-pressure shale gas reservoirs.
(3) Four sets of marine organic-rich shale in South China all have a certain resource potential, of which the Ordovician–Silurian shale is the most favorable exploration horizon at present. At the same time, the exploration and development of shale gas in China is still constrained by complicated geological conditions, the oneness of economic shale formation, high exploration and development costs and other aspects. Research on shale gas accumulation theory, exploration and development technology and related policies are necessary, and can promote the development of China’s shale gas industry.

Author Contributions

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

Funding

This research was supported by the National Natural Science Foundation of China (42122017, 41821002), the Shandong Provincial Key Research and Development Program (2020ZLYS08), and the Independent innovation research program of China University of Petroleum (East China) (21CX06001A).

Data Availability Statement

The data used to support the findings of this study are included within the article.

Conflicts of Interest

The authors declare that they have no conflict of interest.

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Figure 1. (a) Typical well locations of marine organic-rich shale in South China; (b) Stratigraphic histogram of Paleozoic in South China (modified after Refs. [31,32]).
Figure 1. (a) Typical well locations of marine organic-rich shale in South China; (b) Stratigraphic histogram of Paleozoic in South China (modified after Refs. [31,32]).
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Figure 2. (a) Global paleogeography of Cambrian (modified after Ref. [31]); (b) Global paleogeography of Ordovician–Silurian (modified after Ref. [32]); (c) Global paleogeography of Devonian–Carboniferous (modified after Ref. [43]); (d) Global paleogeography of Permian (modified after Ref. [32]).
Figure 2. (a) Global paleogeography of Cambrian (modified after Ref. [31]); (b) Global paleogeography of Ordovician–Silurian (modified after Ref. [32]); (c) Global paleogeography of Devonian–Carboniferous (modified after Ref. [43]); (d) Global paleogeography of Permian (modified after Ref. [32]).
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Figure 3. (a) Sedimentary characteristics of the Cambrian shale in southern China (modified after Ref. [48]); (b) Sedimentary characteristics of the Ordovician–Silurian shale in southern China (modified after Ref. [49]); (c) Sedimentary characteristics of the Devonian–Carboniferous shale in southern China (modified after Ref. [43]); (d) Sedimentary characteristics of the Permian shale in southern China (modified after Ref. [50]).
Figure 3. (a) Sedimentary characteristics of the Cambrian shale in southern China (modified after Ref. [48]); (b) Sedimentary characteristics of the Ordovician–Silurian shale in southern China (modified after Ref. [49]); (c) Sedimentary characteristics of the Devonian–Carboniferous shale in southern China (modified after Ref. [43]); (d) Sedimentary characteristics of the Permian shale in southern China (modified after Ref. [50]).
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Figure 4. (a) Sedimentary thickness of the Cambrian shale in southern China (modified after Refs. [51,52]); (b) Sedimentary thickness of the Ordovician-Silurian shale in southern China (modified after Refs. [53,54]); (c) Sedimentary thickness of the Devonian-Carboniferous shale in southern China (modified after Ref. [55]); (d) Sedimentary thickness of the Permian shale in southern China (modified after Ref. [56]).
Figure 4. (a) Sedimentary thickness of the Cambrian shale in southern China (modified after Refs. [51,52]); (b) Sedimentary thickness of the Ordovician-Silurian shale in southern China (modified after Refs. [53,54]); (c) Sedimentary thickness of the Devonian-Carboniferous shale in southern China (modified after Ref. [55]); (d) Sedimentary thickness of the Permian shale in southern China (modified after Ref. [56]).
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Figure 5. Lithofacies triangle diagram of marine organic-rich shale in South China (data of Cambrian shale from Refs. [16,30,69,70,71,72,73,74,75,76,77,78,79,80,81,82]).
Figure 5. Lithofacies triangle diagram of marine organic-rich shale in South China (data of Cambrian shale from Refs. [16,30,69,70,71,72,73,74,75,76,77,78,79,80,81,82]).
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Figure 6. (a) Pore volume characteristics of the marine organic-rich shale in southern China (data from Refs. [16,84,86,102,103]); (b) Specific surface area characteristics of the marine organic-rich shale in southern China (data from Refs. [16,84,86,102,103]).
Figure 6. (a) Pore volume characteristics of the marine organic-rich shale in southern China (data from Refs. [16,84,86,102,103]); (b) Specific surface area characteristics of the marine organic-rich shale in southern China (data from Refs. [16,84,86,102,103]).
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Figure 7. TOC distribution characteristics of marine organic-rich shale in South China (data from Refs. [22,27,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98]).
Figure 7. TOC distribution characteristics of marine organic-rich shale in South China (data from Refs. [22,27,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98]).
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Figure 8. (a) Correlation between marine organic-rich shale TOC and pore volume (data from Refs. [16,82,85,102,103,114,115,116,117]); (b) Correlation between marine organic-rich shale TOC and specific surface area. (data from Refs. [16,82,85,102,103,114,115,116,117]).
Figure 8. (a) Correlation between marine organic-rich shale TOC and pore volume (data from Refs. [16,82,85,102,103,114,115,116,117]); (b) Correlation between marine organic-rich shale TOC and specific surface area. (data from Refs. [16,82,85,102,103,114,115,116,117]).
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Figure 9. Ro distribution characteristics of marine organic-rich shale in South China (data from Refs. [22,27,82,83,85,86,87,88,89,90,91,92,93,94,95,96,98]).
Figure 9. Ro distribution characteristics of marine organic-rich shale in South China (data from Refs. [22,27,82,83,85,86,87,88,89,90,91,92,93,94,95,96,98]).
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Figure 10. Evolution diagram of shale pore volume and specific surface area in South China (modified after Ref. [22], data from Refs. [22,72,82,85,115,116]).
Figure 10. Evolution diagram of shale pore volume and specific surface area in South China (modified after Ref. [22], data from Refs. [22,72,82,85,115,116]).
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Table
Table 1. Reservoir characteristics of Cambrian organic-rich shale in southern China.
Table 1. Reservoir characteristics of Cambrian organic-rich shale in southern China.
RegionWell IDTOCRoPorosityGas ContentMineral Composition/%Data Sources
%%%m3/tSiliceousCarbonateClay
Sichuan BasinJinYe10.2–11.5 (4.1)2.9–3.2 (3.0)0.52–6.92 (2.7)1.02–4.68 (2.03)28.0–62.0 (35.0)0.0–31.3 (10.1)8.0–72.0 (54.9)[83]
W2012.0–3.7 (2.8)3.2–3.6 (3.3)0.82–4.86 (2.2)1.10–3.51 (2.01)29.0–72.5 (38.5)2.1–15.6 (8.5) [22]
N2060.2–7.4 (3.9)3.5–4.2 (4.0)0.9–3.2 (2.3)0.00–0.80 (0.65)36.0–40.3 (38.4)6.0–10.9 (8.5) [22]
Southeast ChongqingDS12.0–4.0 (2.3)3.0–3.7 (3.3)0.71–0.93 (0.85) [22]
YouK11.4–9.8 (5.4)2.8–3.8 (3.2) 3.0–85.0 (51.3)0.0–92.0 (13.0)3.0–55.0 (32.7)[84]
YuK10.4–9.8 (4.8)2.7–3.5 (3.2) 51.0–85.0 (67.7)0.0–33.0 (10.9)5.0–32.0 (21.8)[85]
Western HunanXAD10.1–19.9 (6.9) 25.1–92.5 (55.9)0.0–75.53.2–55.4 (28.0)[86]
ChangY12.1–17.6 (9.8)2.2–3.1 (2.7)1.2–1.9 (1.5)0.06–2.10 (1.02)26.0–78.0 (56.7)0.0–12.0 (5.5) [22]
Northern GuizhouYX10.3–9.3 (3.6) 30.5–75.5 (43.5)0.0–32.8 (10.4)8.7–47.1 (29.5)[87]
Z1031.9–2.7 (2.3)3.8–4.4 (4.2)0.75–3.65 (1.79)0.06–0.62 (0.35)15.0–56.0 (44.0)23.0–50.0 (32.0) [22]
CenY10.8–7.8 (4.2)3.6–3.7 (3.6)0.8–6.2 (2.1)0.04–0.64 (0.33)33.0–86.0 (63.1)2.0–32.0 (8.5)11.0–46.0 (28.3)[88]
TX14.1–7.6 (5.6)2.8–2.92.3–3.2 (2.8)0.70–2.0 (1.69)66.0–86.9 (76.9)3.2–13.0 (7.8)7.3–26.6 (15.3)[88]
Western GuizhouFS12.0–8.0 (3.5)(4.5)0.74–0.77 [22]
Western HubeiYE11.0–9.2 (4.2)2.8–3.3 (3.1)(1.48)0.26–4.48 (2.30)2.0–68.0 (31.0)0.0–81.0 (14.0)0.0–59.0 (20.0)[89]
Note: Data in brackets are averaged values.
Table
Table 2. Reservoir characteristics of Ordovician–Silurian organic-rich shale in southern China.
Table 2. Reservoir characteristics of Ordovician–Silurian organic-rich shale in southern China.
RegionWell IDTOCRoPorosityGas ContentMineral Composition/%Data Sources
%%%m3/tSiliceousCarbonate Clay
Sichuan BasinJiaoY11.1–6.5 (2.8)2.9–3.7 (3.3)2.8–7.1 (4.7)4.4–8.2 (6.1)30.9–52.7 (40.2)6.5–21.6 (12.6)26.8–62.6 (39.7)[90]
W2012.6–5.3 (3.2)1.9–2.9 (2.5)3.9–4.7 (4.0)2.1–4.8 (2.4)16.7–72.8 (40.0)9.2–65.2 (21.8) [22]
W2020.1–4.5 (2.86) 0.3–2.4 (1.97) 13.0–69.0 (35.6)6.0–49.0 (23.3)15.0–56.0 (32.3)[91]
N2012.7–3.32.70–3.3 (3.1)2.5–6.8 (4.0)2.0–6.2 (4.8)25.8–67.6 (41.1)0.0–43.2 (20.5) [22]
N2090.7–4.3 (2.36)2.4–3.52.0–8.9 (4.4)(3.4)23.0–60.0 (40.0)23.0–55.0 (39.1)6.0–43.0 (16.8)[91]
YS1081.9–5.6 (3.1)2.8–3.3 (3.1)2.5–6.9 (3.6)2.2–6.5 (3.9)19.5–48.3 (37.3)10.7–62.0 (24.8) [22]
Southeast ChongqingPY12.6–4.2 (3.2)2.4–3. 1(2.8)1.8–4.5 (2.67)1.8–2.5 (1.99) [27]
YY10.3–6.6 (2.46)2.1–3.7 (2.68)0.8–4.7 (2.8)0.8–4.7 (2.8)30.9–74.5 (45.6)0.0–17.9 (4.27)24.7–65.7 (47.6)[92]
Northern GuizhouDY11.2–5.52.2–3.40.01–4.9 (2.1) 18.0–83.9 (58.6)4.2–26.7 (9.2)7.7–46.8 (27.6)[93]
AY13.8–4.9 (4.4)2.2–2.52.8–4.5 (3.7)2.6–6.2 (4.5)49.1–72.2 (63.3)19.0–45.0 (27.6)3.4–19.7 (9.16)
Western HubeiEYY20.3–5.5 (2.47)1.9–2.0 (1.98)1.01–1.8 (1.3)0.07–3.3 (1.1)23.4–96.1 (62.8)0.0–59.8 (5.1)8.5–42.4 (29.1)[94]
Note: Data in brackets are averaged values.
Table
Table 3. Reservoir characteristics of Devonian–Carboniferous organic-rich shale in southern China.
Table 3. Reservoir characteristics of Devonian–Carboniferous organic-rich shale in southern China.
RegionWell IDTOCRoPorosityGas ContentMineral Composition/%Data Sources
%%%m3/tSiliceousCarbonate Clay
Yunnan-Guizhou-Guizhou RegionGY10.2–3.96 (1.4)2.2–2.8 (2.6)0.1–2.6 (1.0)0.03–0.388.5–67.0 (42.6)(18.4)5.8–77.4 (34.2)[95]
GRY10.9–2.2 (1.5)(2.0)3.1–4.9 (3.43) 42.6–54.8 (47.2)7.7–32.4 (18.4)21.9–41.1 (30.6)[82]
GTD10.5–1.2 (0.8)(3.5)2.9–5.1 (2.1) 31.8–73.2 (48.9)0.5–40.3 (16.1)18.2–40.4 (32.3)[82]
Note: Data in brackets are averaged values.
Table
Table 4. Reservoir characteristics of Permian organic-rich shale in southern China.
Table 4. Reservoir characteristics of Permian organic-rich shale in southern China.
RegionWell IDTOCRoPorosityGas ContentMineral Composition/%Data Sources
%%%m3/tSiliceousCarbonate Clay
Sichuan BasinDYS10.6–18.4 (3.2)2.0–2.4 (2.2)1.13–9.0 (5.5)0.6–8.8 (2.0)0.0–71.9 (22.2)6.2–90.6 (48.3)0.2–82.0 (29.5)[96,97]
LB1(6.8)1.5–3.0 (2.4)(2.6) (45.0)(27.0)(28.0)[98]
Central HunanXiangY10.4–10.5 (3.9)1.5–1.7 (1.6)3.0–8.00.3–0.7 (0.4)23.7–63.1 (41.1)5.2–53.7 (21.3)11.0–43.5 (24.3)[97]
Southern AnhuiGD10.9–9.7 (3.8)1.0–1.3 (1.2) 0.5–1.5(53.5)1.7–28.8 (8.1)8.6–61.4 (38.8)[97]
XuanY11.7–9.3 (4.1)1.2–1.5 (1.4)0.5–1.2 29.0–45.4(41.6)(4.8)33.7–59.8 (43.6)[97]
Western HubeiHD11.4–14.7 (5.0)1.8–2.6 (2.4) 0.5–4.4 (1.1)23.9–68.6(46.8)10.3–40.0 (28.0)1.7–43.6 (28.0)[97]
Note: Data in brackets are averaged values.
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Zhang, Z.; Xu, S.; Gou, Q.; Li, Q. Reservoir Characteristics and Resource Potential of Marine Shale in South China: A Review. Energies 2022, 15, 8696. https://0-doi-org.brum.beds.ac.uk/10.3390/en15228696

AMA Style

Zhang Z, Xu S, Gou Q, Li Q. Reservoir Characteristics and Resource Potential of Marine Shale in South China: A Review. Energies. 2022; 15(22):8696. https://0-doi-org.brum.beds.ac.uk/10.3390/en15228696

Chicago/Turabian Style

Zhang, Zhiyao, Shang Xu, Qiyang Gou, and Qiqi Li. 2022. "Reservoir Characteristics and Resource Potential of Marine Shale in South China: A Review" Energies 15, no. 22: 8696. https://0-doi-org.brum.beds.ac.uk/10.3390/en15228696

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