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The characteristic and evolution of coal-forming swamp in Hanshuiquan district, Santanghu Coalfield, Xinjiang, NW China, during the Middle Jurassic: evidence from coal petrography, coal facies and sporopollen

  • Shuo Feng
  • Jing He
  • Jijun TianEmail author
  • Xingyu Lu
  • Bo Yang
Open Access
Research Article
  • 330 Downloads

Abstract

Santanghu Coalfield is the largest integrated coalfield exploration area in China. The major coal seams developing in Xishanyao Formation (Middle Jurassic) are the high-quality steam coals characterized by large thickness, favorable horizontal continuity and high coal quality. In this paper, twenty-two samples were collected from the three typical boreholes in Hanshuiquan district, representing the 11 coal seam sequences (7#, 8#, 9#, 13#, 14#, 15#, 17#, 18#, 19#, 20#, 22#), respectively. The petrographic characteristics of the coal-bearing sequence in Xishanyao Formation were firstly summarized systematicly, and then the coal-forming swamp characteristics and succession mechanism of the coal seam in Xishanyao Formation were defined by analyzing the samples. The maceral composition, structure, geochemical and geophysical characteristics of coal are included in original genetic criteria of coal-forming swamp analysis. And the composition of coal petrography, maceral and microlithotype are the most frequently used parameters. Coal is composed of microscopic constituents and inorganic substances. The Xishanyao Formation maceral mainly consists of vitrinite (65.74%–97.01%), inertinite (1.93%–34%), and the exinite shows the mode of regular change. The coal-forming swamp in Xishanyao Formation possesses the characteristics of mainly marsh, wet forest swamp facies, and shallow water covered forest swamp facies, and a few of coal seams distribute in the deep water covered forest swamp facies. In addition, the sporopollens in Xishanyao Formation are mainly Pinaceae evergreen broad leaf and needle-leaved plants, Osmundaceae, Cyatheaceae and Lygodiaceae, indicating that the warm and humid tropic-subtropical climate conductive to the persistent growth of coal-forming plants in the Middle Jurassic. The coal-forming swamp shows the characteristics of vertically upward fluctuation through the periodic transition. It indicates a shallow-deep-shallow change process of the water covered depth in the swamp. This is the principal factor for the formation of the high-quality and continuous coal seam in Hanshuiquan district, Santanghu Coalfield.

Keywords

Santanghu Coalfield Xishanyao Formation Coal-forming swamp Sporopollen 

1 Introduction

The contents of ash and sulphur in high-quality coal developing in Jurassic System of northwest China are low (Huang et al. 2010), which is conducive to the development of industrial economy and ecological environment, and its most significant coal petrography characteristic is the abundance of inertinite. For instance, in eastern Junggar Coalfield, southern Junggar Coalfield, Ili Coalfield, the average content of the inertinite is more than 40% and even 70% in Middle Jurassic of Xishanyao Formation (Zhou et al. 2010; Li et al. 2012a, b, 2014; Dai et al. 2015a, b; Fu et al. 2016). The content of inertinite varies from 35% to 90% in the Jurassic coal-bearing stratum, Muli Coalfield, Qaidam Basin (Dai et al. 2015a, b). The Jurassic coal in Erdos Basin is mainly rich-inertinite coal, and the average content of the inertinite in fluvial facies is above 40% (Du et al. 2009; Dai et al. 2012; Ao et al. 2012). However, this research found that the maceral of the coal petrography is mainly characterized by the abundant vitrinite in Hanshuiquan district, Santanghu Coalfield, Northwest China. Its average content is more than 80%, while the content of inertinite is about 10% (Fig. 1). The cognition of the Jurassic coal in Northwest China is enriched by the vitrinite-rich coal in Santanghu Coalfield.
Fig. 1

The research area location, the lithotype distribution and the change tendency of samples macerals in Xishanyao Formation, Hanshuiquan district, Santanghu Coalfield, NW China

In this area, previous scholars have focused on the tectonic evolution, sequence stratigraphy, provenance, sedimentary, coal derived oils and tight oil of the basin, and their research areas is mainly located in Tiaohu sag and Malang sag (Wang et al. 2015, 2016; Liang et al. 2014; Liu et al. 2015). The coal-derived oils were discovered in Xishanyao Formation in the Well Tangcan-1 (Zhang et al. 2000). In term of the provenance and sedimentary environment, the Jurassic sedimentary system of Santanghu Coalfield was systematically studied by Jiang (2003). He found that the Jurassic provenance of Santanghu Coalfield is characterized by near source, short flow and multiple sources, developing delta-lacustrine depositional system. Huang (2002) carried out a detailed analysis on the sporopollen of Xishanyao Formation at Well Tangqian-3 in Santanghu Coalfield, and recognized the palynological assemblage characteristics of Cyathidites minor- Osmundacidites- Cycadopites- Quadraeculin, which reflectes the temperate or subtropical climate in middle Jurassic.

However, the researches on coal petrography, coal facies and coal-forming swamp in Santanghu Coalfield are inadequate. In term of coal petrography and coal quality, the suberinite was first discovered by He et al. (1995) in Santanghu Coalfield through the research of coal petrography. An and Ma (2013) studied the No. 9 coal seam at Hanshuiquan and Tiaohu sags in Santanhu, and judged that it is the long flame coal characterized by low or extremely low ash, high volatiles, extremely low sulfur, high calorific value, high thermo stability, easy selection, high content of arsenic, extremely low chloride, low phosphorous, low fluorine, and high content of oil. A preliminary study on the arsenic in Santanghu coal field was carried out by Huang (2014), and pointed out that arsenic is abundant in the pyrite. In term of the coal-forming swamp, Chinese coal geology scholars studied the coal-forming swamps of the Jurassic coal accumulating basins in the north of Xinjiang since 2010. For instance, Nie et al. (2016) pointed out that the coal-forming swamp in Turpan-Hami Basin is mainly comprise of wetland marsh facies and wetland forest swamp facies. Li et al. (2012a, b) deemed that the coal-forming swamp mainly consists of dry forest swamp facies and wetland forest swamp facies in eastern Junggar coalfield. Alimujiang et al. (2014) considered that the coal-forming swamp is mainly composed of wetland swamp facies and wet forest swamp facies in southern Junggar coalfield. Zhang et al. (2015) studied the Jurassic coal-forming swamp in Santanghu, and believed that the major coal seam in Xishanyao Formation is transited from wide water marsh to wet forest swamp, and through the research of its coal petrography and coal-forming swamp, he found that exinite content is zero. This might be resulted from the small quantity of sample, which fails to accurately reflect the evolutionary characteristics of the coal-forming swamp in this area. For this reason, the major coal seams in Hanshuiquan district, Santanghu Coalfield are selected to study the coal-forming swamp characteristics and evolution rules. Moreover, this paper makes a comparison with the previously studied on coal-forming swamp of Xishanyao Formation in Xinjiang to study the type and characteristic of coal-forming swamp and its evolution characteristic in Xishanyao Formation (Teichmüller and Teichmüller 1982).

2 Geological setting

Santanghu Coalfield is located at Barkol County and Yiwu County, Northeast Xinjiang Province, with a strip shape oriented NW–SE between Moqinwula and Suhaitu Mountains (Liu et al. 2012a, b; Song et al. 2013; Ge et al. 2015; Hackleya et al. 2016). Santanghu Coalfield is the largest integrated Coalfield prospecting area in Xinjiang and even in China. Before 2012, 37,392,727,400 tons of coal reserves buried in the exploration depth of < 1000 m, has been prospected in Xishanyao Formation (No. 2-20 coal seams) of middle Jurassic, and the coal reserves buried in the exploration depth of < 2000 m is 120 billion tons. It provides a sound energy resource support to the implementation of “Transport coal from Xinjiang to Eastern China” and “Transport electricity from Xinjiang to Eastern China” (Huang and Tian 2013).

The Hanshuiquan district is located in the northwest of the Central depression belt (Fig. 1), north of the Baruntala anticline, extending to the Shaheba fault, and it is the largest two-stage tectonic unit in the basin (Xiao et al. 2004a, b; Ma et al. 2016a, b). The acreage of Hanshuiquan district is about 750 km2, with length of 60 km in east–west direction, width of 10 km in south-north direction. The overall terrain is high in the northwest and low in the southeast. The surface covers the Quaternary gravel and sandy loam. The outcrop of Xishanyao Formation sporadically can be discovered in the southwest of the Hanshuiquan district. The major lithology of Xishanyao Formation is mainly in kelly, celadon mudstone, siltstone mixed up with sandstone, coal seam and spathic iron. The bottom is mainly in large-coarse conglomerate and medium-coarse sandstone. There are a total of twenty-two coal seams in Xishanyao Formation, in the upper three coal seams are sporadically developed and their thickness is small. The lower coal seams distribute in the whole area and the thickness of No. 9 coal seam is more than 30 m in particular, the thicknesses of other coal seams are less than 5 m (Fig. 2).
Fig. 2

Photomicrographs in white reflected light (oil immersion) (ah) and fluorescing-inducing blue light (i) coal seams in Santanghu Coalfield. a Telinite showing well-preserved cell structure; b Telocollinite with inertodetrinite; c Desmocollinite with semifusinite; d Corpocollinite with pyrite; e the cell cavity of Fusinite filled with vitrodetrinite; f Semifusinite with desmocollinite; g Vitrodetrinite and inertodetrinite; h Resinite in the shape of ellipsoid; i Sporinite of the shape flat ring

3 Sampling and experimental methods

Twenty-two samples from eleven coal seams were systematically collected from three typical boreholes (78-2, 82-2 and 460-1) in Hanshuiquan district, Santanghu Coalfield. Three pieces of samples (No. 3, No. 7, No. 9) in the 78-2 borehole were collected from the No. 7 and No. 9 coal seams. Six pieces of samples (No. 2, No. 5, No. 10, No. 12, No. 15, No. 18) in 460-1 borehole were collected from No. 7, No. 8, No. 9, No. 14 and No. 17 coal seams. Thirteen pieces of samples (No. 1, No. 4, No. 6, No. 8, No. 11, No. 13, No. 14, No. 16, No. 17, No. 19, No. 20, No. 21, No. 22) in 82-2 borehole were collected from the eleven coal seam sequences (7, 8, 9, 13, 14, 15, 17, 18, 19, 20, 22) (Fig. 3).
Fig. 3

The connecting-well section of 78-2, 82-2, 460-1 and the location of samples collection

The samples of each coal seam were divided into two parts. Smash one part and divide by the screen drum with the diameter of 1.0 mm, create into the briquette polished section for the maceral authentication and the vitrinite reflectance measurement. The coal polished section was prepared according to the ISO 7404-2 (2009). The classification and the quantitative analysis of maceral were carried out according to the ISO 7404-1 (1994). In the recognition and quantitative statistics of maceral and minerals, the observed effective points should be more than 500. The other parts of samples were sent to State Key Laboratory of Palaeobiology and Stratigraphy in Nanjing Institute of Geology and Palaeonotology for the identification of sporopollen and the statistics of the number.

4 Characteristics of coal petrography

4.1 Characteristics of coal macrolithotype and petrography

The macrolithotype of coal in Xishanyao Formation is mainly made up of bright coal, semibright coal and a little dull coal. The fracture is in the shape of conchoid and irregularity. No. 8, 13, 15, 20 and 22 coal seams are made up of bright coal, No. 7, 18 and 19 coal seams are made up of dull coal, No. 9, 14 and 17 coal seams are made up of bright coal and semibright coal together. The bright coal is developed in the coal seams with banded and lentoid shape, and poor horizontal continuity. The banded structure of the semibright coal is more significant, showing the interbedding between the bright part and the dark part. Siderite, pyrite, calcite, clay and other inorganic minerals, which are developed in the coal seam or filled in the fracture and bedding surface with the shape of granulous, lentoid and concretion.

4.2 Characteristics of microscopic coal petrography

The results of maceral analysis had been summarized in Table 1. Vitrinite is the most significant maceral in the twenty-two samples with averaging 88.21%, varies from 65.74% to 97.01%. And the content of the vitrinite in most samples varies from 70% to 90%. The vitrinite is mainly composed of telocollinite and desmocollinite, showing the vertically upward fluctuation trend. The content of the inertinite varies from 1.93% to 34%, which only inferior to the vitrinite in the coal samples. The content of the inertinite in most samples varies from 10% to 30%, which is mainly made up of semifusinite. Particularly, the contents of the inertinite at the bottom of No. 7 coal seam (No. 3 sample) and the top of No. 14 coal seam (No. 14 sample) are up to 30%, which can be attributed to the sufficient oxygen supply inducing by the shallow water coverage and be exposed in air. The exinite is distributed sporadically in local samples, and the content varies from 0.11% to 0.92%. It is characterized by obvious fluorescence. The exinite shows dark grey-black in the reflected light of the oil immersion and yellow fluorescence in blue excitation light source. Sporophore and resinite have been discovered in the exinite in this research area. The former is in the shape of helminth and flat ring, and the latter shows ellipsoid distribution (Fig. 2). In general, the content of vitrinite in bright coal is more than 80%, and in semibright coal is more than 60% (Fig. 2).
Table 1

Maceral analysis in Hanshuiquan district, Santanghu Coalfield, NW China

Sample

Coal seam

Vitrinite (%)

Inertinite (%)

Exinite (%)

Telinite

Telocollinite

Corpocollinite

Desmocollinite

Vitrodetrinite

Fusinite

Semifusinite

Inertodetrinite

1

7

0.73

22.91

0.14

35.53

11.75

5.30

6.16

17.18

0.30

2

7

4.70

33.55

0.84

28.49

9.44

3.19

5.75

13.36

0.68

3

7

2.90

28.98

1.44

24.57

8.63

8.73

12.69

11.90

0.14

4

8

0.98

52.93

1.18

15.88

13.14

2.16

3.73

9.41

0.59

5

8

1.71

38.63

1.22

29.44

18.48

1.86

4.72

3.66

0.27

6

9

1.73

57.36

2.78

12.31

8.67

2.78

5.89

7.97

0.52

7

9

1.39

35.43

1.79

30.06

10.18

3.48

5.26

11.52

0.90

8

9

2.65

24.13

4.09

34.56

23.11

1.64

2.05

7.36

0.41

9

9

1.37

19.09

1.95

56.73

18.32

0.00

0.78

1.17

0.59

10

9

1.51

20.67

1.13

49.63

24.44

0.00

1.88

0.56

0.19

11

9

2.48

40.71

1.03

23.89

15.57

1.13

6.92

8.03

0.23

12

9

11.99

37.03

2.68

30.41

14.31

0.18

1.07

2.15

0.18

13

13

4.40

50.28

1.93

24.13

8.72

0.59

4.41

5.42

0.12

14

14

26.46

14.15

1.85

5.54

19.48

13.95

15.59

2.05

0.94

15

14

1.90

37.29

2.37

28.98

12.70

1.67

6.39

8.35

0.36

16

15

30.34

20.16

1.83

13.03

21.79

2.24

3.05

7.13

0.42

17

17

1.89

39.49

0.44

29.16

4.73

6.79

4.42

12.60

0.47

18

17

1.84

15.98

2.12

41.36

22.59

4.25

2.13

9.56

0.16

19

18

0.87

45.16

1.26

25.31

8.28

1.25

8.91

8.84

0.11

20

19

2.34

35.88

2.37

23.73

9.33

6.90

8.48

10.76

0.23

21

20

1.89

30.93

0.99

38.86

11.38

2.36

6.92

6.54

0.12

22

22

1.25

26.02

1.27

46.46

16.07

0.89

2.45

5.30

0.28

The maceral components of coal samples in the research area are compared with the those of the coal in some large fault basins in China (such as Fushun, Meihekou, Xiaolongtan, and Fuxin basin). The results show that the maceral contents of the coal petrography in Xishanyao Formation at Hanshuiquan district in Santanghu Coalfield are closed to the maceral contents of those fault basins. Due to the relatively rapid basal deposition rate in the fault basin, the coal-forming swamp is covered by deep water and buried rapidly. This lead to a high vitrinite content (Pang et al. 2012).

There have differences in the macerals of Middle Jurassic coal seams. For instance, the average content of vitrinite is 50% in Heshituoluogai Coalfield, varies from 1.45% to 95.3% in eastern Junggar area, higher than 87% in Balikun, varies from 56.1% to 93.4% in southern Junggar area, varies from 21.5% to 49% in western Junggar, and varies from 26.9% to 35.7% in Fukang local district (Wei 2002).

The content of inertinite generally varies from 53.9% to 83.1% in eastern Junggar area, and the content is decreased to 2.9%–10.8% in Barkol area. In contrast, the content of inertinite from the Fukang Sangonghe area to Urumqi Liudaowan is up to 38.5%–66.9%. The contents vary from 1.1% to 6.1% in Liuhuanggou area (Wei 2002). Inorganic minerals such as siderite, pyrite, calcite and clay can be seen in the coal seam or filling the cracks and bedding with the shap of granular, lenticular and nodular morphology. Clay minerals mainly contain kaolinites and illites, which distribute in disseminated or thin stratified structure (Zhang et al. 2018) (Fig. 4).
Fig. 4

XRD analysis on samples from Xishanyao Formation, Hanshuiquan district, Santanghu Coalfield, China

There is low content of exinite in Xinjiang. The exinite was not be discovered by Li et al. (2012a, b) in eastern Junggar coalfield according to the mathematical statistics of coal petrography maceral. Compared with the major coal-producing area in Xinjiang, the coal petrography maceral of Xishanyao Formation at Hanshuiquan district in Santanghu coalfield is characterized by abundant vitrinite and a little content of inertinite and lower exinite.

5 Characteristics of coal-forming swamp

5.1 The coal-forming swamp characteristics and analysis

The coal-forming swamp is generally analyzed through TPI (tissue preservation index), GI (gelatification index) (Diessel 1982; Kalkreuth et al. 1991, 2000; Kalaitzidis et al. 2009), GWI (groundwater influence index), VI (vegetation index) (Calder et al. 1991; Gmur and Kwiecinska 2002; Dai et al. 2007) and T-D-F triangular diagram. In addition, V/I (vitrinite/inertinite), WI (woodland index), F/M (framewok/matrix), OI (oxidization index) (Gorbanenko and Ligouis 2015), TI (transfer index), BI (breaking index), MI (mobility index) were adopted to analyze the coal-forming swamp characteristics with the palynological assemblage characteristics in the research area (Table 2) (Diessel and Gammidge 1998). The latter parameters can reflect the vegetational forms of the coal-forming swamp, and the changes of the swamp water coverage depth and the strength of the hydrodynamic force in the research area, so that the conclusion can be more accurate and more reliable (Souvik et al. 2016).
Table 2

The parameters of coal-forming swamp in Hanshuiquan district, Santanghu Coalfield, NW China

Sample

Coal seam

TPI

GI

GWI

VI

WI

V/I

F/M

OI

TI

BI

MI

1

7

0.54

2.49

0.21

0.64

0.5

2.45

0.55

0.2

0.42

0.4

0.5

2

7

0.72

4.97

0.14

0.48

0.43

3.46

0.74

0.14

0.17

0.23

0.16

3

7

0.85

1.86

0.09

0.54

0.49

2

0.88

0.04

0.1

0.18

0.11

4

8

1.51

5.56

0.22

1.99

1.86

5.49

1.56

0.1

0.31

0.28

0.33

5

8

1.16

8.63

0.25

1.71

1.68

8.74

1.17

0.08

0.27

0.16

0.26

6

9

2.14

5

0.17

2.84

2.82

5

2.34

0.09

0.25

0.2

0.25

7

9

1.53

3.89

0.29

0.25

0.23

3.89

1.55

0.02

0.3

0.23

0.31

8

9

0.44

8.19

0.48

0.51

0.47

8.04

0.47

0.08

0.54

0.41

0.53

9

9

0.27

50.11

0.27

0.28

0.27

50.11

0.28

0.01

0.28

0.24

0.26

10

9

0.32

39.92

0.36

0.33

0.3

39.92

0.32

0.01

0.36

0.33

0.36

11

9

0.63

4.95

0.19

0.76

2.84

5.2

0.64

0.05

0.19

0.28

0.21

12

9

1.01

44.86

0.21

1.12

1.1

28.36

1.07

0.02

0.24

0.19

0.25

13

13

2.13

11.47

0.16

2.48

2.41

8.58

2.17

0.04

0.17

0.22

1.21

14

14

2.34

2.17

0.47

2.26

1.62

1.94

2.5

0.03

0.28

0.27

1.09

15

14

1.01

4.93

0.32

1.03

0.99

5.07

1.05

0.07

0.33

0.31

0.34

16

15

1.27

7.03

0.39

1.56

1.45

6.39

1.32

0.08

0.34

0.4

0.87

17

17

0.98

2.48

0.12

0.89

0.87

3.18

0.99

0.11

0.11

0.11

0.14

18

17

0.36

4.99

0.17

0.18

0.15

5.26

0.38

0.1

0.19

0.16

0.18

19

18

2.25

1.89

0.09

2.63

2.61

4.25

2.29

0.09

0.11

0.21

0.1

20

19

1.72

1.78

0.08

1.92

1.86

2.82

1.74

0.11

0.09

0.15

0.08

21

20

1.4

1.88

0.11

1.46

1.4

5.31

1.41

0.08

0.13

0.23

0.13

22

22

0.66

8.94

0.32

0.59

0.58

10.54

0.67

0.07

0.33

0.17

0.34

TPI structure preservation index, GI gelation index, GWI groundwater flow index, VI vegetation index, WI forest index, V/I mirror inert ratio, F/M bone base ratio, OI oxygenation index, TI transport index, BI shatter index, MI mobility index

5.1.1 Gelification of the coal-forming plants

According to Fig. 5, the woody plants have the dominant contribution to the coal-forming swamp during the coal-forming period of No. 8, 13, 14, 15, 18, 19 and 20 coal seams. The tissue of plants is well preserved. A large number of telinite, fusinites, and semifusinite can be observed in the microscopic observation. The woody plants and herbaceous plants are mixed in the coal-forming swamp of No. 9 coal seam during the coal-forming period, while the coal-forming swamps of No. 7, 17 and 22 coal seams are mainly made up of herbaceous plants. Compared with other coal-forming swamp consisted by woody plants with cellular structures are likely to break (Diessel and Gammidge 1998). The detrital components and desmocollinite are found in the microscopic observation. Meantime, the preservation degree of plant cells can be also reflected by BI (breaking index). However, the value of BI changes in small amplitude, which has little reference significance compared with TPI. In addition, the pH value of water body can be also indicated by the value of TPI. The activity of bacteria is weak in the water body with lower PH value, which plays a less important role in the decomposition of the remains of plants. In contrast, the activity of bacteria is intense in water body environment with higher PH value, and the cellular structure is seriously broke (Teichmüller 1989). Therefore, it can be inferred that the water environment in the coal-forming process is partial acid, and the structures of the plant cells are not well preserved on the whole through the TPI value of Xishanyao Formation at Hanshui district, Santanghu Coalfield (Fig. 5).
Fig. 5

Coa-forming swamp interpretation based on TPI and GI in Hanshuiquan district, Santanghu Coalfield, NW China.

Modified by Diessel (1986)

GI can be taken as the index to measure the degree of moisture in coal-forming swamp, the depth of water coverage and the strength of gelification, which is the specific value between the gelified components and the sum of the fusainisation and semi-fusainisation (Lamberson et al. 1991; Alkande et al. 1992; Diessel et al. 2000; Diessel 1992). The GI value of the major coal seams in the research area is highter than one, and the maximum value is up to fifty. The GI value in the middle and lower segments of No. 9 coal seam is relatively high, correspondingly, the content of vitrinite is higher than 80%. This indicates that there develops a moist and closed swamp environment with deep water covered. The cellular structures were severely damaged and even disappeared in the case of intense gelification. The change of the water coverage depth in the coal-forming swamp can be refected by VI, which is the specific value between the contents of vitrinite and inertinite in maceral. The V/I values in the middle and lower segments of No. 9 coal seam and No. 13 coal seam are higher than 10, and the water coverage in the swamp is relatively deep during the coal-forming period. The V/I values of No. 8, 15, 22 coal seams and the top of No. 9 coal seam are varied from 5 to 10. The coal-forming environment is moist-shallow water coverage. The V/I values of No. 7, 14, 17, 18, 19 and 20 coal seams vary from 1 to 5, which signify the moist environment. This indicates that the coal-forming environment of Xishanyao Formation in Hanshuiquan district, Santanghu Coalfield is partial reducing environment with moist-shallow water covered on the whole.

According to the TPI-GI diagram (Fig. 5), it can be indicated that the changes of coal-forming swamp are relatively frequent in this research area. The coal-forming swamps are mainly in marsh, wet forest swamp facies, and shallow water forest swamp facies, followed by deep water covered forest swamp. The shallow water covered forest swamp facies only occurs in No. 9 coal seam (No. 12 sample) and No. 13 coal seam (No. 13 sample).

5.1.2 Swamp change of the coal forming vegetation

The GWI (Strobl et al. 2014) value of the test samples are all smaller than 0.5. It indicates that there is a light affection to the coal-forming swamp by the groundwater, and the coal-forming swamp water is covered by relatively shallow water. It is found that the telinite, telocollinite and desmocollinite in the samples with small GWI value are much more than the sum of gelocollinite, corpocollinite, vitrodetrinite and clay mineral through the microscopic observation. This indicates that the coal-forming swamp is less affected by the groundwater and has a weak hydrodynamic condition, which provides a good condition for the sustainable and stable development of the coal-forming swamp (Calder et al. 1991). MI, TI, and F/M can also reflect the strength of the water mobility in the coal-forming swamp. According to the data analysis, it can be known that the water mobility at the top of No. 7 and 8 coal seam and No. 18 coal seam was intense during the coal-forming period, and the remains of the plants had been damaged seriously. The number of the telinite is little (Silva et al. 2008). So, all these indicate that groundwater has less influence on the major coal seam of Xishanyao Formation in Hanshuiquan district, Santanghu Coalfield, which is not the decisive factor influencing the changes of coal-forming swamp.

VI shows the same rule to WI and TPI. The samples are uniformly distributed in the wetland swamp facies and wetland forest facies. Only a small number of samples are distributed in the wet swamp facies, wet forest facies and water covered forest facies. No. 7, 17 and 22 coal seams are made up of the herbaceous plants dominate in the coal-forming plants, while the woody plants dominate the coal-forming swamps of No. 8, 13, 14, 15, 18, 19 and 20 coal seams. No. 9 coal seam is the peat swamp mixed by woody plants and herbaceous plants. From bottom to top, the characteristics and evolution of coal-forming swamp is transited from forest facies to swamp facies (Fig. 6).
Fig. 6

Coal-forming swamp interpretation based on GWI and VI in Hanshuiquan district, Santanghu Coalfield, NW China.

Modified by Calder et al. (1991)

5.1.3 Triangular chart

The coal-forming swamp is divided into three categories, open swamp facies, wet forest forest swamp facies, and land forest swamp facies through the model constructed by calculating the values of T, D and F (Mukhopadhyay 1989). The higher value of T indicates the greater correlation between the coal-forming swamp type and the wet forest swamp facies. The land forest swamp facies is reflected by the high value of F. The great correlation between the coal-forming swamp and the open swamp facies is represented by the higher value of D.
$${\text{T}} = {\text{telinite}} + {\text{telocollinite}}$$
$${\text{F}} = {\text{fusinite}} + {\text{semifusinite}}$$
$${\text{D}} = {\text{vitrodetrinite}} + {\text{sporophore}} + {\text{liptodetrinite}} + {\text{cutinite}} + {\text{inertodetrinite}} + {\text{detrital}}\;{\text{mineral}}\,({\text{clay}}\;{\text{mineral}} + {\text{quartz}})$$
According to T-D-F triangular chart (Fig. 7), the samples are all near the wet forest swamp facies. Both F and D value are smaller than forty, while T value is relatively high. The F value of some samples is greater than T value. In other samples, the F value is greater than T value and D value. It indicates that the dry environment indication is not obvious in this research area. The coal-forming swamp type is closely related to the wet forest swamp and open swamp. The open swamp and wet forest swamp interactively occur from top to bottom in the area.
Fig. 7

Triangular chart of Coal-forming swamp in Hanshuiquan district, Santanghu Coalfield, NW China.

Modified by Mukhopadhyay (1989)

5.2 Characteristics of coal-forming swamp

It can be confirmed that the coal-forming swamp type is the forest swamp facies covered by shallow water and wet forest swamp facies dominated by woody plants and the coal-forming swamp dominated by herbaceous plants based on T-D-F triangular chart, TPI-GI relation diagram, and GWI-VI relation diagram. In contrast, the deep water covered forest swamp facies is relatively few.
  1. (1)

    Marsh facies: The maceral of coal-forming swamp in the research area is dominated by vitrinite, where the contents of desmocollinite, telocollinite, and vitrodetrinite are overwhelming. Therefore, it can be judged that the coal-forming plants are mainly in herbaceous plants like Osmundaceae and woody plants like Cyatheaceae. A great number of herbaceous plant sporopollens also occur in some coal seams. In this case, their cell structures are likely to be damaged in the coal-forming process. Therefore, there are relatively low contents of the telinite and the fusinite. In addition, the low peat swamp is supplied sufficiently by the surface water, which leads to huge ash content in the coal.

     
  2. (2)

    Shallow water covered forest swamp facies: The coal-forming plants are dominated by evergreen needle-leaved plants like pine and cypress, and broad-leaved woody plants like Cycadaceae. The remains of plants are subjected to severe gelification due to the intense decomposition of anaerobic bacteria, and a large number of desmocollinite and telocollinite are formed in the weak hydrodynamic force environment, the high value of T increase in the triangular chart accordingly.

     
  3. (3)

    Deep water covered forest swamp facies: It indicates that the peat swamp is less influenced by the oxidation and flow transfer through the high value of GI and the low value of GWI. The cellular structures of maceral are well preserved compared with other samples in the same coal seams.

     
  4. (4)

    Wet forest swamp facies: There are high value of TPI, VI, WI and low value of GI and GWI in this coal-forming swamp, which content of the inertinite is higher than the other kinds of forest swamp facies, and the coal-forming plants are mainly made up of Cyatheaceae and Pinaceae.

     

6 Vertical evolution characteristics

Three periodic cycles transiting from forest swamp to low herbaceous swamp from bottom to top are found in Hanshuiquan district, Santanghu Coalfield (Fig. 8). The rule progressively transiting from wet swamp to relatively shallow-deep water covered swamp is reflected from bottom to top in the forest swamp. The development of coal-forming swamp has the fluctuation characteristics. Three changes of shallow-deep-shallow of water are found in the coal-forming swamp from the bottom to the top. The gelification also went through a process of weak-intense-weak.
Fig. 8

The variation of coal-forming swamp parameters and sporopollens in Hanshuiquan district, Santanghu Coalfield, NW China

No. 9 coal seam is the most important coal seam in the Santanghu Coalfield. The coal-forming swamp type went through a process from herbaceous swamp to woody swamp, and the water body shows the change rule from shallow to deep then to shallow. The TPI develops from small to large, and the middle part of No. 9 coal seam has the great value of GWI. It indicates that the cellular structure of the coal petrography formed in the herbaceous plant swamp is easy to be destroyed by the strong power of groundwater.

In addition, the sporopollen number of woody plants in vertical direction shows three cycles changes firstly increasing and then decreasing (Fig. 8), which is totally identical with the rules reflected by the VI, WI and TPI index. The rule of the changes in herbaceous plant sporopollen is contrary to that of the woody plant sporopollen. The type of the original coal-forming swamp can be accurately characterized by the types and contents of different sporopollens. The contents of the woody plant sporopollen are great in the No. 8 coal seam, No. 13, No. 19 coal seam and the upper of No. 9 coal seam. It indicates that the woody plants occupy the leading position in the coal forming period. The large contents of herbaceous plant sporopollen occur in the No. 7 coal seam, the middle of No. 9 coal seam, No. 17 coal seam and No. 22 coalseam. It indicates that the coal-forming swamp is provided the main raw material by herbaceous plants. The other coal seams are mainly in the swamps mixed by herbaceous plants and woody plants.

The coal-forming swamps are mainly composed of herbaceous plants like Lygodiaceae (0%–28.6%) and Osmundaceae (7.7%–43.4%) as well as woody plants like Cyatheaceae (0%–14.3%), Cycadaceae (1.9%–14.3%) and Pinaceae (0%–38.6%) through the test results of the sporopollen (Fig. 8). The No. 7, 9 and 14 coal seams evolved from the coal-forming swamps dominated by Osmundaceae. No. 8, 13, 15, 18, 19 and 20 coal seams evolved from the coal-forming swamps dominated by pine and cypress. No. 17 coal seam evolved from the coal-forming swamp dominated by Lygodiaceae, and No. 22 coal seam evolved from the coal-forming swamp jointly dominated by Osmundaceae and Lygodiaceae.

The palynological assemblage type of the Xishanyao Formation is dominated by Leiotriletes-Cyathidites-Osmundacidites-Cycadopites-Pinuspollenites. Osmundacidite remains dominant in ferns, and it reached the progenitive peak in Mesozoic, especially in Early and Middle Jurassic. The content of Cyathidites significantly increases, which is the typical plant in the Jurassic (Liu 2008), and the content of Pinuspollenites dominated in gymnosperm (Deng 2007).

Cyatheaceae, Lycopodium, Selaginella and other herbaceous tree ferns have no secondary xylem tissue or poor developed, while the parenchyma and epidermal cells mainly develop. These tissues cannot be preserved because of the poor resistant ability to decomposition, so in the wet and reduction environment, they can easily evolve into to the desmocollinite and clastic maceral. The secondary xylem tissue stems of the woody gymnosperms like fir, ginkgo, pine and cypress as well as Cycadaceae are well developed, and they cannot be decomposed or destroyed. In moist and reducing environemnt, they can easily envolve into the telinite and telocollinite (Zhang and Wu 1995; Wang et al. 1997).

In the early period of Middle Jurassic, the warm and moist climate was suitable for the plant growth. The moist degree for coal-forming swamp varies from paludose to mesophyte. The plants in mesophyte environment are all Pinaceae, and they can resist the drought of short time (Du 2015). The tall needle-leaved plants such as Pinaceae and Podocarpaceae accompanied by Cycadaceae, Araucariaceae and other broad-leaved evergreen plants grew in the alpine zone. The scrubby Cyatheaceae widely grew at the shore of the lake (Deng 2007; Du 2015; Maria et al. 2015; Wang et al. 2005; Yang et al. 2006). All these shows an ancient ecosystem with alpine zone and vast lake.

7 Conclusion

  1. (1)

    The vitrinite is the most significant maceral in the major coal seam of Xishanyao Formation in Hanshuiquan district, Santanghu Coalfield, with higher content than 60%. Inertinite occupies the secondary position, and its content is less than 40%. A small amount of exinite can be discovered in most samples under fluorescence. The content of vitrinite in bright coal usually more than 80%.

     
  2. (2)

    Three periodic cycles transiting from forest swamp to low herbaceous swamp from bottom to top are found Hanshuiquan district, Santanghu Coalfield. The development of coal-forming swamp has the fluctuation characteristics. Three changes of shallow-deep-shallow of water are found in the coal-forming swamp from the bottom to the top.

     
  3. (3)

    The palynological assemblage type of the Xishanyao Formation is dominated by Leiotriletes-Cyathidites-Osmundacidites-Cycadopites-Pinuspollenites. The No. 7, 9 and 14 coal seams envolved from the coal-forming swamps dominated by Osmundaceae. No. 8, 13, 15, 18, 19 and 20 coal seams evolved from the coal-forming swamps dominated by pine and cypress. No. 17 coal seam evolved from the coal-forming swamp dominated by Lygodiaceae, and No. 22 coal seam evolved from the coal-forming swamp jointly dominated by Osmundaceae and Lygodiaceae. The environment of coal-forming swamp shows an ancient ecosystem with alpine zone and vast lake.

     

Notes

Funding

Funding was provided by National Natural Science Foundation Project (Grant No. 41662010) and Xinjiang Uygur Autonomous Region University Scientific Research Program For Young Teachers Research And Cultivation Fund Project (Grant No. XJEDU2016S038).

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Authors and Affiliations

  1. 1.Institute of Geology and Mining EngineeringUrumqiChina

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