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Volume 32 Issue 4
Aug.  2021
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Yufang Wang, Gangyi Zhai, Guoheng Liu, Wanzhong Shi, Yongchao Lu, Juan Li, Yunxiao Zhang. Geological Characteristics of Shale Gas in Different Strata of Marine Facies in South China. Journal of Earth Science, 2021, 32(4): 725-741. doi: 10.1007/s12583-020-1104-5
Citation: Yufang Wang, Gangyi Zhai, Guoheng Liu, Wanzhong Shi, Yongchao Lu, Juan Li, Yunxiao Zhang. Geological Characteristics of Shale Gas in Different Strata of Marine Facies in South China. Journal of Earth Science, 2021, 32(4): 725-741. doi: 10.1007/s12583-020-1104-5

Geological Characteristics of Shale Gas in Different Strata of Marine Facies in South China

doi: 10.1007/s12583-020-1104-5
More Information
  • In the Middle and Upper Yangtze region of South China, there are well developed sets of marine shale strata. Currently only Wufeng Longmaxi shale gas has been developed in scale, while the Sinian and Cambrian shale gas are still under exploration. The various shale strata show different characteristics in lithological features, such as lithofacies types and reservoir physical properties, which are due to the influence of tectonic pattern, sedimentary environment, and diagenesis caused by tectonic subsidence. This will affect the later fracturing technology and fracturing effect. The shale reservoirs of Sinian doushantuo, Cambrian Niutitang, Upper Ordovician Wufeng, and the Lower Silurian Longmaxi Formation shale were evaluated and compared with each other with respect to their sedimentary environments, lithofacies, minerals compositions, and micro pore characteristics. The reservoir characteristics of the shale and the main control factors of shale gas enrichment were summarized in this study.
  • Electronic Supplementary Material: Supplementary material (Fig. S1) is available in the online version of this article at https://doi.org/10.1007/s12583-020-1104-5.
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Geological Characteristics of Shale Gas in Different Strata of Marine Facies in South China

doi: 10.1007/s12583-020-1104-5

Abstract: In the Middle and Upper Yangtze region of South China, there are well developed sets of marine shale strata. Currently only Wufeng Longmaxi shale gas has been developed in scale, while the Sinian and Cambrian shale gas are still under exploration. The various shale strata show different characteristics in lithological features, such as lithofacies types and reservoir physical properties, which are due to the influence of tectonic pattern, sedimentary environment, and diagenesis caused by tectonic subsidence. This will affect the later fracturing technology and fracturing effect. The shale reservoirs of Sinian doushantuo, Cambrian Niutitang, Upper Ordovician Wufeng, and the Lower Silurian Longmaxi Formation shale were evaluated and compared with each other with respect to their sedimentary environments, lithofacies, minerals compositions, and micro pore characteristics. The reservoir characteristics of the shale and the main control factors of shale gas enrichment were summarized in this study.

Electronic Supplementary Material: Supplementary material (Fig. S1) is available in the online version of this article at https://doi.org/10.1007/s12583-020-1104-5.
Yufang Wang, Gangyi Zhai, Guoheng Liu, Wanzhong Shi, Yongchao Lu, Juan Li, Yunxiao Zhang. Geological Characteristics of Shale Gas in Different Strata of Marine Facies in South China. Journal of Earth Science, 2021, 32(4): 725-741. doi: 10.1007/s12583-020-1104-5
Citation: Yufang Wang, Gangyi Zhai, Guoheng Liu, Wanzhong Shi, Yongchao Lu, Juan Li, Yunxiao Zhang. Geological Characteristics of Shale Gas in Different Strata of Marine Facies in South China. Journal of Earth Science, 2021, 32(4): 725-741. doi: 10.1007/s12583-020-1104-5
  • The marine organic-rich shale strata of the Sinian Doushantuo, Cambrian Niutitang, Upper Ordovician Wufeng, and Lower Silurian Longmaxi Formation developed in the Middle and Upper Yangtze region of South China (Liu G H et al., 2019; Luo et al., 2018, 2017; Zou et al., 2015; Liu S G et al., 2014; Mou et al., 2011; Liang et al., 2009; Wen et al., 2001). Since 2009, commercialization development of shale gas from the Upper Ordovician Wufeng Formation and the Lower Silurian Longmaxi Formation shale happed in the Jiaoshiba area in Chongqing, Changning-Weiyuan area, and the Zhaotong area in Yunnan, in addition to other regions (Jin et al., 2016; Zhao W Z et al., 2016). However, for the Cambrian Niutitang Formation shale, there are only a few exploration wells in the Weiyuan area that have achieved production breakthrough through horizontal fracturing (Dong and Xiong, 2016; Liu and Fu, 2016; Xie et al., 2015). In the second-round bidding blocks in the periphery of the Sichuan Basin, several wells were drilled for shale gas exploration in the Cambrian Niutitang Formation shale gas, but failed to discover shale gas of commercial scale. It was found that the Cambrian Niutitang Formation shale had extremely high thermal maturity degree and poor gas bearing capacity (Liang et al., 2014; Han et al., 2013; Hu et al., 2012; Huang et al., 2012). After many years of oil and gas survey by Oil & Gas Survey in South China, China Geological Survey. A summary of shale gas enrichment was proposed that the shale reservoir with relatively lower thermal maturity and weaker geological deformation was favorable for the Sinian Doushantuo Formation and Cambrian Niutitang Formation shale. Hence, the "shale gas accumulating around the edges of paleo-uplifts" model was proposed (Zhai et al., 2019; Zhai et al., 2017a). On this basis, several survey wells and parameter wells for shale gas were deployed and drilled in the peripheral areas of Huangling, Hannan and Xuefeng paleo-uplifts, and succeeded in the discovery of commercial-scale shale gas enrichment. The Eyangye 1 Well deployed in the southern edge of Huangling paleo-uplift even drilled in the Sinian Doushantuo Formation shale. Then, the horizontal wells Eyangye 1HF and Eyangye 2HF succeeded in commercial-scale shale gas development and exploitation in the Sinian Doushantuo Formation and Cambrian Niutitang Formation shale after staged fracturing, respectively, which means there are great shale gas exploration prospects for these two sets of shale strata (Wang et al., 2019, 2017; Zhai et al., 2018a).

    Among researchers there are many differences in the classification of shale lithofacies. In summary, there are two types of shale lithofacies classification. The first divides lithofacies according to sedimentary characteristics (Wang and Carr, 2013; Abouelresh and Slatt, 2012; Hickey and Henk, 2007; Loucks and Ruppel, 2007; Doyle and Sweet, 1995), and the other divides lithofacies according to mineral composition (Milliken et al., 2012; Wang and Carr, 2012; Mitra et al., 2010). However, the study of shale facies mainly focuses on Wufeng and Longmaxi Formation. The lithofacies are divided by the principle of mineral triad and sedimentary microfacies. However, there are few studies on shale facies of the Niutitang Formation throughout the Cambrian. In this paper, we document the Triassic shale mineral lithofacies of the Wufeng Longmaxi Formation, in the Jiaoshiba area of the Fuling and Sichuan Basin, as well as the lithofacies of the Niutiatang Formation and the Doushantuo Formation of Cambrian in Yichang, Hubei Province are also carried out, and the lithofacies characteristics of three sets of shale series are compared and evaluated.

    Numerous shale gas enrichment theories have been proposed after continuous summary about the shale gas enrichment and unstopped breakthroughs of exploration and development, such as the "source-preservation theory", "source-cap control theory", and the "source-diagenesis-accumulation theory" (Zhai et al., 2019, 2018b, 2017b; Jin et al., 2016; Li et al., 2016; Nie et al., 2016; Guo, 2014; Guo and Zhang, 2014; Hu et al., 2012; Meng and Hou, 2012; Nie et al., 2009). It was found that the shale depositional environment and diagenetic evolution determines the lithology types in the shale strata, lithofacies, and reservoir property, thus became key geological factors for shale gas accumulation. In this study, systematic comparison and analysis among the four formations, highlight various aspects of tectonic-sedimentary environments, organic geochemical features, and reservoir properties, as well as two types of shale gas enrichment and accumulation models.

  • During the Nanhua Period, the Yangzi-Cathaysia Block that was previously pieced together was divided (Shu and Wang, 2019; Shu, 2012), resulting in the formation of fractured small blocks and rift basins of different sizes. In the South China Block, a series of south-north oriented taphrogenic troughs developed, forming a distinct tectonic pattern of platform-trough-basin (Figs. 1 and 2). From west to east, the troughs included the Mianyang-Changning taphrogenic troughs in Central Sichuan Province, the Xiang'e west taphrogenic trough and the Lower Yangtze taphrogenic trough, which are both distributed in the border area of Hunan, Hubei and Chongqing provinces. The development of the Sinian Doushantuo Formation and the Cambrian Niutitang Formation was controlled by these taphrogenic troughs.

    Figure 1.  Lithofacies paleogeographic map of Doushantuo Formation in South China.

    Figure 2.  Paleogeography map of Early Cambrian facies in South China.

    The Mianyang-Changning taphrogenic trough, covering an area of approximately 54×103 km2, is distributed in a quasi south-north orientation, and characterized with segmentation and variations for each segment. The narrowest place, is situated at the boundary of the south and north segmentation, located in the east of Weiyuan region, the west of Dazu region and the north of Longchang region and is merely 30 km wide. The north segment of Mianyang-Changning taphrogenic trough gradually becomes wider to the northwest, and the largest width exceeds 100 km. The Tensile strength of the south segment of Mianyang-Changning taphrogenic trough is weaker than that of the north segment, and presents non-obvious boundary feature. It opens to the southwest with a width between 30-90 km (Fig. 2). The boundary faults do not appear in both the east and west sides of the Mianyang-Changning taphrogenic trough border. The east side shows the characteristic of scarp, inducing the Lower Cambrian strata overlapping on the Sinian Dengying Formation strata and growing strata thickness (Fig. 3). The southern part of Mianyang-Changning taphrogenic trough shows the characteristic of a gentle slope, where the tensile feature was recognized through the thickness variation of the Lower Cambrian strata (Fig. 4).

    Figure 3.  Stratigraphic correlation section of the Mianyang-Changning Trough.

    Figure 4.  Seismic profile of the Mianyang-Changning Trough.

    The taphrogenic trough played a controlling function in the development and distribution of black shale. The taphrogenic trough were in a deep and low-energy hydrodynamic depositional conditions, which is conducive to source rocks development. The Lower Cambrian Qiongzhusi (also called "Niutitang" in other areas of South China) Formation black shale of large thickness developed in Mianyang-Changning taphrogenic trough. However, the black shale strata in the areas away from the trough is much thinner, and the granularity becomes larger. It is revealed through description of the drilling core that the Maidiping Formation deposit in the initial stage of tensile period and the Qiongzhusi Formation black shale of large thickness were deposited in the peak stage of tensile period. The thickness of Qiongzhusi Formation black shale of Gaoshi 17 and Zi 4 wells reached up to 492 and 390 m, respectively.

    The Xiang'e west taphrogenic trough is 400 km in length and 160 km in width, it is also distributed in a nearly north-south oriented, and nearly parallel to the Mianyang-Changning taphrogenic trough (Fig. 2). The Xiang'e west taphrogenic trough can also be divided into south and north segments with the Yichang-Wanxian area as the boundary. The north segment became wider at Chengkou-Wuxi area, and the border can extend to the Hannan paleo-uplift.

    In the late stage of Neoproterozoic Period, regional tensile and rift caused the subsidence of shallow water platform in Yichang area. In the Sinian Period (Early Ediacaran), a rift in platform developed at the Huangling paleo-uplift edge. During the Lower Doushantuo Formation sedimentary period (the Early Sinian Period), the water became deeper, and became a deep-water sedimentary environment when the second member of Doushantuo Formation was deposited. The ascending ocean currents bring a wealth of nutrients, inducing the lower aquatic organisms to flourish. At the bottom of the ocean, the water was in an oxygen-deficient condition (Fig. 5), and a set of organic-rich black shale formed. During the Early Cambrian Period, the Yichang region inherited the tectonic paleogeography pattern of a platform and rift. In the shallow-water zone of the platform, it was rich in nutrients and organisms thrived, which provided an abundant source of organic matter into the marine sediments. In the seawater rift zone, the water has the characteristics of stratification, vulcanization, and oxygen-deficiency, which was conducive to the preservation of sedimentary organic matter. These conditions were beneficial to the formation of black shale with large thickness and abundant organic matter, which lays a good material foundation for the shale gas formation and enrichment.

    Figure 5.  Continental shelf deposition pattern at the edge of rifting trough for the Sinian Doushantuo Formation black shale.

    The Xiang'e west taphrogenic trough has important control significance to the development of black shale in the west area of Hunan and Hubei provinces. At present, the Eyangye 1 Well drilled in the northeast of the trough show a shale thickness of about 200 m. Zidi 1 Well and Zidi 2 Well with thin shale thickness is in the platform edge located in the northeast part of the trough. Furthermore, the Liye 1 Well is located on a platform, showing a much thinner formation thickness, and the thickness of Niutitang Formation shale is less than 70 m (Fig. S1).

    Because the south part of Xiang'e west taphrogenic trough was closed to the Southern Ocean Basin, it was obviously influenced by volcanic hydrothermal eruption in the ocean. Hence, the silica content is higher in shale in the south part, generally around 80%. However, the middle and north part of Xiang'e west taphrogenic trough, the shale deposit was mainly affected by the biological effect due to far away from the ocean. Therefore, the silica in shale was mainly biogenic in origin. The Sinian Doushantuo Formation and the Cambrian Niutitang Formation in western Hubei Province were characterized by short burial time and low thermal evolution because the two sets of strata have been slowly uplifted since the Cambrian Period.

  • After the middle Ordovician Period, the paleogeography environment of the Central and Upper Yangtze region changed a lot because of the Caledonian Movement. Restricted by the Xuefeng paleo-uplift and the paleo-uplifts in Central Guizhou and Sichuan provinces, the main part of the Central and Upper Yangtze region was under a semi-closed and restricted marine conditions opening to the north, forming a large area of low-energy, under-compensated and anoxic sedimentary environment (Mou et al., 2011; Guo et al., 2004; Chen et al., 2001). At the end of Ordovician and the beginning of Silurian, two global transgression occurred, forming the Wufeng-Longmaxi Formation shale strata. The deep-water shelf condition controlled the development of organic-rich shale, which was widely distributed in the southern and eastern parts of the Sichuan Basin and the areas from northern Guizhou to western Hunan. Two sedimentary centers were formed and the strata wedged out to the edge of the paleo-uplifts (Fig. 6).

    Figure 6.  Paleogeography map of Upper Ordovician-Lower Silurian sedimentary facies in Sichuan Basin and its periphery.

    The Wufeng Formation deposited under a deep-water continental shelf, and mainly consists of black carbonaceous shale, silty shale, siliceous shale and siliceous rock. The Longmaxi Formation was mainly a set of sediments formed under deep water to shallow continental shelf, and composed of the dark grey shale, black silty shale, rich organic carbonaceous shale, siliceous shale and argillaceous siltstone. From the top to bottom, the color gradually became darker, content of sand reduced, and organic matter content increased. The Longmaxi Formation shale contains abundant graptolite fossils and pyrite framboids.

    The Wufeng-Longmaxi Formation shale is mainly developed in the southeast and southwest rather than the Middle and West of Sichuan Province. The thickness of Wufeng-Longmaxi Formation shale in Wuzhishan-Meigu-Zigong area of Southwest Sichuan is 100-178 m, with a buried depth of 1 000-3 500 m. The shale thickness in Yibin-Leibo area is more than 140 m. The shale thickness in the Qijiang area in Southeast Sichuan is 10-90 m and the buried depth is 1 000-3 500 m. The shale thickness in Southeast Fuling area is 40-100 m and the buried depth is 500-3 500 m. Shale in the Zhenba area has a thickness of 40-80 m and a buried depth of 3 000-6 500 m.

  • Mineral triterminal method was used in lithological classification of Doushantuo Formation shale from Eyangye 1 Well and Zidi 1 Well. The Doushantuo Formation shale strata are mainly composed of five types of rock (Fig. 7), which are calcareous-rich siliceous shale (S-1), calcareous-siliceous mixed shale (M-1), limestone (C), siliceous-rich calcareous shale (C-1), calcareous shale (C-2). The calcareous-siliceous mixed shale (M-1) was dominant, followed by siliceous-rich calcareous shale (C-1) and calcareous shale (C-2). Taking the Eyangye 1 Well as an example, the calcareous-siliceous mixed shale (M-1) accounts for 60%, and followed by siliceous-rich calcareous shale (C-1), which accounts for 15%. The proportion of limestone (C) and calcareous shale (C-2) is 10%. The lowest content is the calcareous-rich siliceous shale (S-1), accounts for only 5%. From the vertical distribution (Fig. 8), the calcareous-siliceous mixed shale (M-1) is more developed at the bottom and top. The siliceous-rich calcareous shale (C-1) and calcareous-rich siliceous shale (S-1) are mainly developed in the upper middle part. Limestone (C) and calcareous shale (C-2) are mainly developed below the middle.

    Figure 7.  Lithofacies division of different shale strata (N. Niutitang Formation; D. Doushantuo Formation).

    Figure 8.  Shale lithofacies sequence of Doushantuo Formation of Eyangye 1 Well.

  • Liu et al. (2018) divided the Niutitang Formation shale strata in the Sichuan Basin into 6 sequences and 5 middle sedimentary cycles according to sequence stratigraphy. Zhao et al. (2013) studied the sedimentary environment of Niutitang Formation from the perspective of paleontology. Wu et al. (2017) divided the Niutitang Formation into 6 types of lithofacies only from the perspective of sedimentation. In this research, the mineral triterminal method was used to classify the rock types of the Niutitang Formation shale strata. According to coring samples analysis of Eyangye 1 Well, Zidi 1 Well, Zidi 2 Well, Wudi 1 Well and Wudi 2 Well in the western Hubei Province (Fig. 7), the Niutitang Formation shale strata mainly consist of 11 rock types, which are siliceous rock (S), calcareous-rich siliceous shale (S-1), siliceous shale (S-2), argillaceous-rich siliceous shale (S-3), the calcareous-siliceous mixed shale (M-1), the argillaceous-siliceous mixed shale (M-2), the calcareous-argillaceous-siliceous mixed shale (M-3), siliceous-rich argillaceous shale (CM-1), limestone (C), siliceous-rich calcareous shale (C-1) and calcareous shale (C-2). The siliceous shale (S-2) is the dominant rock type, followed by the calcareous-siliceous mixed shale (M-1), the argillaceous-siliceous mixed shale (M-2) and argillaceous-rich siliceous shale (S-3).

    Taking the Eyangye 1 well as an example, there are five rock types in Niutitang Formation shale strata, including siliceous rock (S), calcareous-rich siliceous shale (S-1), siliceous shale (S-2), the calcareous-siliceous mixed shale (M-1) and calcareous shale (C-2). Among them, siliceous shale (S-2) accounts for 52%, and were followed by the calcareous-siliceous mixed shale (M-1) and siliceous rock (S), which account for 22% and 17%, respectively. The siliceous shale (S-2) and calcareous shale (C-2) were the least abundant rock types, both accounting for 4%, respectively. From the perspective of the vertical distribution of rock types (Fig. 9), the lower section mainly develops the calcareous-siliceous mixed shale (M-1). The shale in the middle section mainly develops siliceous rock (S), and interspersed with calcareous shale (C-2). The siliceous shale (S-2) is mainly developed in the upper part, and interbedded with calcareous-rich siliceous shale (S-1).

    Figure 9.  Shale lithofacies sequence of Niutitang Formation of Eyangye 1 Well.

  • There were a variety of methods used to classify the Wufeng-Longmaxi Formation shale lithofacies. Zhu et al. (2016) used quartz content, quartz genesis and laminae type to distinguish shale lithofacies. Yang and Wang (2015) divided the shale strata of Longmaxi Formation into four main categories, namely siltstone, mud shale, stormstone and contourite, and then into nine lithofacies, according to the characteristics of rock minerals, sedimentary structures and paleontological characteristics. Ran et al. (2016) classified Wufeng-Longmaxi into 9 types of lithofacies according to the percentage of quartz and the degree of laminae development. Wang et al. (2014) classified 8 types of lithofacies from the perspective of shale deposition hydrodynamic origin. According to mineralogy, rock fabric, biological composition and sedimentary structure, Zhao J H et al. (2016) identified 7 types of lithofacies from the Wufeng-Longmaxi Formation. Lu et al. (2017) and Li and Quan (1992) classified the lithofacies according to the paleontological assemblages. The mineral triterminal method and sedimentary microfacies was also used to divide the Wufeng-Longmaxi Formation in Jiaoshiba and southern Sichuan (Che, 2018; Wang et al., 2018a, b; Jiang et al., 2016; Wang et al., 2016; Wu et al., 2016). Mineral triterminal method was used in this research to compare the Niutitang Formation shale and the Doushantuo Formation shale in Yichang area in Hubei Province with the Wufeng-Longmaxi Formation shale of Jiaoshiba area (Fig. 7). The Wufeng-Longmaxi Formation shale strata in Jiaoshiba area is mainly composed of the siliceous-rich argillaceous shale (CM-1), the argillaceous-siliceous mixed shale (M-2), argillaceous-rich siliceous shale (S-3), siliceous shale (S-2), and siliceous rock (S). In general, the Wufeng-Longmaxi Formation shale shows the characteristics of low carbonate mineral content, but high content of siliceous minerals and clay minerals.

  • The Doushantuo Formation and Niutitang Formation shale in Yichang area is obviously different from shale of Jiaoshiba area and North American (Fig. 7). The shale of North American mainly consists of siliceous-rich argillaceous shale (CM-1), the argillaceous-siliceous mixed shale (M-2), argillaceous-rich siliceous shale (S-3) and calcareous shale (C-2). Part of the North American shale is similar to the Jiaoshiba shale, both showing low carbonate mineral content, high siliceous minerals and clay mineral content. Another part of the North American shale contains high carbonate mineral content, and belongs to calcareous shale (C-2). The shale strata in Yichang area not only contains rock with high siliceous mineral content, such as siliceous rock (S), calcareous-rich siliceous shale (S-1), siliceous shale (S-2), argillaceous-rich siliceous shale (S-3), but also has rocks with relatively equal content of siliceous mineral, clay mineral and carbonate mineral, such as the calcareous-siliceous mixed shale (M-1), the argillaceous-siliceous mixed shale (M-2), the calcareous-argillaceous-siliceous mixed shale (M-3). Furthermore, the shale strata in Yichang area also contains rock with high carbonate mineral content, such as limestone (C), siliceous-rich calcareous shale (C-1) and calcareous shale (C-2). In general, the shale in Yichang area is more complex and diverse in rock types than the typical shale in Jiaoshi area and North America.

  • Clay minerals are an important part of shale, and their special physical and chemical properties play an important role in the formation and development of shale gas. The types and content of clay minerals can also reflect the water environment and evolution process since shale deposition and burial, such as acid-base property of formation water and diagenetic stages (Jin et al., 2015). In general, with the increase of buried depth, the formation pressure and formation temperature will continuously increase. During this process, and montmorillonite will gradually transform to illite/smectite mixed layers, and then to illite in alkaline water medium environment with the interlayer water constantly losing. In the transformation process of montmorillonite to illite, if there were abundant Fe2+ and Mg2+ ions, the transformation process would change to chlorite/smectite mixed layer, and then to chlorite, with the increase of the buried depth, the formation pressure and temperature. Therefore, the clay mineral types in shale reflect both the diagenesis stage and the water medium conditions in the diagenesis stage. By comparing and analysing the content of shale clay minerals in Longmaxi Formation, Niutitang Formation and Doushantuo Formation in Yichang area of Hubei Province (Fig. 10), it is found that the clay minerals in Longmaxi shale are dominated by the illite/smectite mixed layer (Fig. 10a), which reflects the characteristics of the early stage of mesodiagenesis. The clay minerals of Niutitang Formation shale are dominated by illite (Fig. 10b), reflecting the characteristics of the late stage of mesodiagenesis. The clay minerals of Doushantuo Formation shale are dominated by chlorite (Fig. 10c), reflecting the characteristics of the mesogenesis-late diagenesis stage.

    Figure 10.  Clay mineral content of different shale strata.

  • The pore structure of shale is an important basis for evaluating the porosity, permeability, and effectiveness of shale reservoirs. According to the research on the pore structure of different lithofacies of Longmaxi Formation shale in Fuling area (Wang et al., 2018b), argillaceous-rich siliceous shale of Longmaxi Formation contains a large amount of organic matter pores, but rare intrgranular and intergranular pores. Moreover, the organic matter pores are mostly round or elliptical shaped, with the pore diameter ranging from nanoscale to micron and good connectivity.

    The calcareous-argillaceous-siliceous mixed shale lithofacies contains various types of pores. The diameter of organic matter pore is relatively smaller, and the inorganic pores are mainly clay mineral intercrystalline pores and primary intergranular pores among clastic particles. The pores of siliceous-rich argillaceous shale are dominated by intergranular pores of clay minerals, and the organic matter pores are not much.

    According to the scanning electron microscopy (SEM) observation on pore structures of 25 samples from different lithofacies in Sinian Doushantuo Formation and Cambrian Niutiang Formation of Eyangye 1 Well, it was found that organic pores in siliceous rock (S) were flock-like distribution, and some pores were connected to form long strip-shaped pore fractures, which can be up to 120 nm in length (Fig. 11a). In the siliceous shale (S-2), the organic pores are densely developed, and the number of organic pores is the most among all the shale lithofacies (Fig. 11b), and then the calcareous-rich siliceous shale (S-1) (Fig. 11c). However, organic matter in argillaceous-rich siliceous shale (S-3) often forms intergranular crack with mineral particles, but the organic pores are hardly developed (Fig. 11d).

    Figure 11.  Organic pore characteristics of siliceous shale lithofacies of Eyangye 1 Well. (a) Organic pores, organic matter filling between pyrite crystals, Eyangye 1 Well, S lithofacies, 3 041.98 m; (b) organic pores, Eyangye 1 Well, S-1 lithofacies, 3 313.3 m; (c) organic pores, densely distributed, Eyangye 1 Well, S-2 lithofacies, 3 037.69 m; (d) no organic pores developed, Eyangye 1 Well, S-3 lithofacies, 3 016.88 m.

    Cloddy shaped organic matter with a large number of organic pores was vastly distributed in the calcareous-siliceous mixed shale (M-1). These organic pores were nearly round in shape and the pore diameter was between 10-100 nm (Figs. 12a, 12b). The organic pores on the surface of the organic matter filled between flake or flock-like clay minerals are irregular in shape and have a large pore size ranging from 50 to 200 nm (Figs. 12c, 12d). In limestone facies (C), a small amount of organic matter in long strips is distributed among inorganic minerals. A small number of near-circular holes on their surface, with a large pore size of about 100 nm (Fig. 13a). Siliceous-rich calcareous shale (C-1) has a relatively high content of organic matter, which develops in wedge-shaped pores between inorganic mineral particles. A large number of organic pores can be seen on their surface with a pore diameter of about 30 nm (Fig. 13b). However, the shale samples in the calcareous shale (C-2) and argillaceous-rich calcareous shale (C-3) have very little organic matter content, and basically no organic pores (Figs. 13c, 13d).

    Figure 12.  Organic pore characteristics of mixed shale lithofacies of Eyangye 1 Well. (a) Organic pore, Eyangye 1 Well, M-1 lithofacies, 3 309.37 m; (b) organic pore, Eyangye 1 Well, M-1 lithofacies, 3 307.32 m; (c) organic pores, irregular in shape, distributed among flake-shaped clay minerals, Eyangye 1 Well, M-1 lithofacies, 3 307.32 m; (d) organic pores, distributed among flock-shaped clay minerals, Eyangye 1 Well, M-1 lithofacies, 3 308.09 m.

    Figure 13.  Organic pore characteristics of calcareous shale lithofacies of Eyangye 1 Well. (a) Organic pore, nearly circular, Eyangye 1 Well, C lithofacies, 3 309.36 m; (b) organic pore, Eyangye 1 Well, C-1 lithofacies, 3 311.96 m; (c) no organic pores developed, Eyangye 1 Well, C-2 lithofacies, 3 320.19 m; (d) no organic pores developed, Eyangye 1 Well, C-3 lithofacies, 3 324.30 m.

    Intercrystalline pore, secondary dissolution pore and clay mineral pore are commonly developed in different lithofacies of Doushantuo Formation and Niutitang Formation shales. An intergranular pore is mainly the pore between pyrites, and pores among calcite particles and authigenic siliceous mineral particles formed by recrystallization. However, these pores mainly developed in the siliceous shale lithofacies (S) and mixed shale lithofacies (M), and rarely in the calcareous shale lithofacies (Fig. 14). The secondary dissolution pores mainly consist of intergranular dissolution pores and intragranular dissolution pores. The pore size of intergranular dissolution pores is relatively large, most of which are 1-10 micron, and the distribution is irregular and linear around the edge of the particles, with generally good connectivity. The pore size of intragranular dissolution pores is small, mainly less than 1 micron, and generally develops on the surface of mineral particles such as feldspar and calcite, showing a scattered distribution (Fig. 14). In the siliceous shale lithofacies, the dissolution pores are mainly in feldspar, while mainly in calcite in the calcareous shale lithofacies. In mixed shale lithofacies (M) dissolution pores both in feldspar and calcite are developed. The pores among clay minerals are mainly developed in the mixed shale lithofacies, but rare in the siliceous and calcareous shale lithofacies.

    Figure 14.  Inorganic pore characteristics of each lithofacies of Eyangye 1 Well shale. (a) Pyrite intercrystalline pore, Eyangye 1 Well, S-1 lithofacies, 3 029.15 m; (b) pyrite intercrystalline pore, Eyangye 1 Well, M-1 lithofacies, 3 309.37 m; (c) dissolution pores in feldspar, Eyangye 1 Well, M-1 lithofacies, 3 309.4 m; (d) dissolution pores in calcite grains, Eyangye 1 Well, M-1 lithofacies, 3 306.88 m; (e) quartz intergranular pore, Eyangye 1 Well, C-2 lithofacies, 3 052.12 m; (f) intergranular dissolution pores of calcite, Eyangye 1 Well, S-1 lithofacies, 3 029.15 m; (g) intergranular pore of clay minerals, Eyangye 1 Well, C-2 lithofacies, 3 319.9 m; (h) intergranular pore of clay minerals, Eyangye 1 Well, C-2 lithofacies, 3 319.9 m

    In general, the pore structure development characteristics of the three types of organic-rich shale lithofacies in the Doushantuo Formation and Niutitang Formation in Yichang area are as follows: organic matter and pyrite are developed in the siliceous shale lithofacies, and the pore types are dominated by organic matter and pyrite intercrystalline pores, followed by secondary dissolution pores, but pores among clay mineral are rare. The contents of organic matter and pyrite in mixed shale lithofacies are medium, and the pore types are mainly among clay minerals and secondary dissolution pores, followed by organic matter pores. The content of organic matter in the calcareous shale lithofacies is relatively small, and the pore types are mainly secondary dissolution pores of carbonate particles, followed by organic pores, but pores among clay mineral are rare (Table 1).

    Lithofacies types Organic pores Inorganic pores
    Intercrystalline pores Intergranular dissolution pores Intragranular dissolution pore Pores among clay mineral particles
    Siliceous shale lithofacies Vastly developed, Nearly spherical and irregular Vastly developed, in strawberry pyrites Vastly developed, among quartz or feldspar particles, leptosomatic shape Developed, mainly dissolution pores in feldspar Relatively developed, leptosomatic shape, parallel to clay minerals
    Mixed shale lithofacies Relatively developed, organic matter among clay mierals Relatively developed, Nearly spherical Rare Developed, dissolution pores in feldspar or calcite Developed, forming flocculent shaped network structure, good connectivity
    Calcareous shale lithofacies Developed, small diameter and less amount Developed, in small strawberry pyrites Vastly developed, among calcite particles Vastly developed, dissolution pores in calcite Rare

    Table 1.  Comparison of pore characteristics of different lithofacies of Doushantuo and Niutitang formations

  • Based on the comparison of porosity and permeability among the Niutitang Formation and Doushantuo Formation shale in Yichang area, the Wufeng-Longmaxi Formation shale in Jiaoshiba area and the shale in north American (Table 2), it is concluded that the Niutitang Formation and Doushantuo Formation shale in Yichang area have their unique porosity and permeability characteristics. The burial depth of Niutitang Formation and Doushantuo Formation shale in Yichang area is larger, which means larger overlying formation pressure. It leads to the relatively undeveloped pore system in Niutitang Formation and Doushantuo Formation shale. The shale in Jiaoshiba area has higher content of clay minerals than that in Yichang area. In the process of organic matter evolution and gas generation, the clay minerals are more prone to form local fracture, and then increase the porosity. Therefore, the porosity of Niutitang Formation and Doushantuo Formation shale in Yichang area is much lower.

    Formations Porosity (%) Permeability (10-6 μm2)
    Longmaxi 3.22-7.13 (4.48) 0.003-0.936 5
    Wufeng 3.01-7.08 (5.02) 0.001 6-0.545 1
    Niutitang 0.28-6.26 (2.25) 0.000 16-0.003 76
    Doushantuo 0.28-3.41 (1.83) 0.000 47-0.003 7
    Fayetteville 2-8 0.1-0.8
    Barnett 4-5 0.073-0.5
    Haynesville 8-9 0.05-0.8
    Marcellus 9-11 0.1-0.7
    Utica 3-6 0.8-3.5

    Table 2.  Summary table of the porosity and permeability from the different shale strata and typical shale in the United States

    Compared with the Wufeng-Longmaxi Formation shale in Jiaoshiba area, the Niutitang Formation and Doushantuo Formation shale in Yichang area have lower transverse permeability. Besides siliceous mineral content impact, the shale in Yichang area is also affected by carbonate mineral content, which is mainly formed by chemical precipitation and then late filling in shale. These carbonate minerals are greatly influenced by the environment and in irregular shape, blocking the relatively complete and well-connected pores. Hence, these carbonate minerals decrease horizontal permeability in the case of constant porosity.

    The porosity and permeability of the Niutitang Formation and Doushantuo Formation in Yichang area are not correlated with each other, which was caused by the greater contribution of microcracks rather than the pores on permeability. Eventually, it leads to different pore connectivity in different shale samples, and samples with largest porosity do not necessarily has the characteristics of the largest content of connected pores.

    By comparison, the content of brittle minerals in Doushantuo Formation shale is similar to that in Barnett shale, but the content of clay minerals is relatively low. The results show that the high content of brittle minerals, such as quartz, is not only conducive to the formation of micro-fractures and the increase of storage space, but also beneficial to the reservoir fracturing and shale gas exploitation. The high calcite content of carbonate minerals is conducive to the development of dissolution pores. The mineral composition of shale is dominated by quartz. Quartz from different sources reflects different sedimentary environments and has different effects on the physical properties of shale reservoirs. Generally, the mineral components of shale are divided into organic and inorganic types. Based on the previous research results, two types of quartz are summarized, including detrital quartz and authigenic quartz. The authigenic quartz of biological origin show obvious correlation with organic matter content. The shale containing the highest content of authigenic quartz of biological origin shows the highest porosity and permeability, and the best physical properties, which are fully reflected by the specialty of Wufeng-Longmaxi Formation biogenic silica shale.

  • The analysis of the sedimentary environment from the main organic-rich shale, such as the Sinian Doushantuo Formation, the Cambrian Niutitang Formation, the Ordovician Wufeng Formation and the Silurian Longmaxi Formation, indicates that the Sinian-Cambrian shale is controlled by the Mianyang-Changning taphrogenic trough, Xiang'e west taphrogenic trough and the Lower Yangtze taphrogenic trough at the border area of Hunan, Hubei and Chongqing provinces, formed by the large scale rifting happened in Nanhua Period. The distribution of Ordovician Wufeng and the Silurian Longmaxi Formation organic-rich shale was confined by the Xuefeng paleo-uplift and the paleo-uplifts in Central Guizhou and Sichuan provinces, forming semi-closed sea environment with the main body open to the north. At the end of the Ordovician and the beginning Silurian Period, two global marine transgression happened, and the Ordovician-Silurian shale deposit was controlled by semi-closed and limited deep-water shelf condition.

    According to the study of shale lithofacies in different strata, the shale lithofacies is mainly controlled by sedimentary microfacies. The characteristics of lithology, minerals, paleontology, and sedimentary structure were all controlled by the sedimentary characteristics of the period. The main lithofacies types in Sinian Doushantuo Formation shale are calcareous-siliceous mixed shale (M-1), followed by siliceous-rich calcareous shale (C-1) and calcareous shale (C-2). The main lithofacies types in Cambrian Niutitang Formation shale are siliceous shale (S-2), followed by calcareous-siliceous mixed shale (M-1), argillaceous-siliceous mixed shale (M-2), and argillaceous-rich siliceous shale (S-3). The main lithofacies types in the Ordovician Wufeng Formation and Silurian Longmaxi Formation shale are siliceous-rich argillaceous shale (CM-1), followed by argillaceous-siliceous mixed shale (M-2), argillaceous-rich siliceous shale (S-3), and siliceous shale (S-2).

    The shale lithofacies type affects the microscopic pore structure, and the pore development in shale is an important place for the shale gas enrichment and preservation. The Doushantuo Formation and Niutitang Formation siliceous shale lithofacies contain a large content of organic matter and pyrites, and the pore types in the lithofacies were mainly organic matter pores and pyrite intercrystalline pores, and then the dissolution pores and clay mineral intergranular pores. The mixed shale lithofacies contain relatively high content of organic matter and pyrites, and the pore types are mainly clay mineral intergranular pores and secondary dissolution pores, and then the organic matter pores. The content of organic matter in calcareous shale lithofacies is small, and the pore types are mainly secondary dissolution pores of carbonate particles. But the organic matter pores and clay mineral intergranular pores are rare. In the argillaceous-rich siliceous shale lithofacies of the Wufeng-Longmaxi Formation, the organic matter pores are vastly developed, and then the intrgranular pores and intergranular pores. The pore types in the mixed shale lithofacies are in diverse. In general, all types of organic matter pore and inorganic pore can be observed. However, the pore types in siliceous-rich argillaceous shale are mainly clay minerals intergranular pores, and the organic matter pores are underdeveloped.

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