Scholarly article on topic 'The Late Paleozoic relative gas fields of coal measure in China and their significances on the natural gas industry'

The Late Paleozoic relative gas fields of coal measure in China and their significances on the natural gas industry Academic research paper on "Earth and related environmental sciences"

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Abstract of research paper on Earth and related environmental sciences, author of scientific article — Chenchen Fang, Jinxing Dai, Wei Wu, Dan Liu, Ziqi Feng

Abstract The coal measure gas sources of coal-derived gas fields in the Late Paleozoic China are the Lower Carboniferous Dishuiquan Formation, the Upper Carboniferous Batamayineishan Formation and Benxi Formation, the Lower Permian Taiyuan Formation and Shanxi Formation, and the Upper Permian Longtan Formation. The coal-derived gas accumulates in Ordovician, Carboniferous, Permian, and Paleocene reservoirs and are distributed in Ordos Basin, Bohai Bay Basin, Junggar Basin, and Sichuan Basin. There are 16 gas fields and 12 of them are large gas fields such as the Sulige large gas field which is China's largest reserve with the highest annual output. According to component and alkane carbon isotope data of 99 gas samples, they are distinguished to be coal-derived gas from coal-derived gas with δ13C2 > −28.5‰ and δ13C1 -δ13C2 -δ13C3 identification chart. The Late Paleozoic relative gas fields of coal measure are significant for the Chinese natural gas industry: proven natural gas geological reserves and annual output of them account for 1/3 in China, and the gas source of three significant large gas fields is coal-derived, which of five significant large gas fields supporting China to be a great gas producing country. The average reserves of the gas fields and the large gas fields formed from the late Paleozoic coal measure are 5.3 and 1.7 times that of the gas fields and the large gas fields in China.

Academic research paper on topic "The Late Paleozoic relative gas fields of coal measure in China and their significances on the natural gas industry"

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ADVANCING RESEARCH EVOLVING SCIENCE

Available online at www.sciencedirect.com

ScienceDirect

Journal of Natural Gas Geoscience xx (2016) 1—13

http://www.keaipublishing.com/jnggs

Original research paper

The Late Paleozoic relative gas fields of coal measure in China and their

significances on the natural gas industry*

Chenchen Fang a, Jinxing Daia *, Wei Wu b, Dan Liu a, Ziqi Feng a

a Research Institute of Petroleum Exploration & Development, PetroChina, Beijing 100083, China ' Exploration and Development Research Institute of Southwest Oil & Gas Company, PetroChina, Chengdu 610056, China

Received 29 September 2016; revised 8 November 2016 Available online ■ ■ ■

Abstract

The coal measure gas sources of coal-derived gas fields in the Late Paleozoic China are the Lower Carboniferous Dishuiquan Formation, the Upper Carboniferous Batamayineishan Formation and Benxi Formation, the Lower Permian Taiyuan Formation and Shanxi Formation, and the Upper Permian Longtan Formation. The coal-derived gas accumulates in Ordovician, Carboniferous, Permian, and Paleocene reservoirs and are distributed in Ordos Basin, Bohai Bay Basin, Junggar Basin, and Sichuan Basin. There are 16 gas fields and 12 of them are large gas fields such as the Sulige large gas field which is China's largest reserve with the highest annual output. According to component and alkane carbon isotope data of 99 gas samples, they are distinguished to be coal-derived gas from coal-derived gas with S13C2 > —28.5%o and 813C1 -S13C2 -S13C3 identification chart. The Late Paleozoic relative gas fields of coal measure are significant for the Chinese natural gas industry: proven natural gas geological reserves and annual output of them account for 1/3 in China, and the gas source of three significant large gas fields is coal-derived, which of five significant large gas fields supporting China to be a great gas producing country. The average reserves of the gas fields and the large gas fields formed from the late Paleozoic coal measure are 5.3 and 1.7 times that of the gas fields and the large gas fields in China. Copyright © 2016, Lanzhou Literature and Information Center, Chinese Academy of Sciences AND Langfang Branch of Research Institute of Petroleum Exploration and Development, PetroChina. Publishing services by Elsevier B.V. on behalf of KeAi Communications Co. Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Keywords: China; The Late Paleozoic; Coal-derived gas; Natural gas industry

1. Introduction

Coal is mainly composed of humic coal and sapropelic coal. Sapropelic coal is formed from shallow sea algae, mainly in the lower organisms. The Cambrian, Ordovician, and Silurian had coal measures appeared. At present, the coal in China are anthracite in places where the proportion of coal is

* This is English translational work of an article originally published in Natural Gas Geoscience (in Chinese).The original article can be found at: 10. 11764/j.issn.1672—1926.2016.06.0960.

* Corresponding author.

E-mail address: djx@petrochina.com.cn (J. Dai). Peer review under responsibility of Editorial office of Journal of Natural Gas Geoscience.

very small and the geographical distribution was limited. Humic coal is formed from swamps and onshore higher plants and is a major component of coal measures. The earliest known terrestrial flora in the world evolved in the Late Silurian to Early Devonian period. On such flora in Altay in Xinjiang, Shanglin in Guangxi, Fengkai in Guangdong, Luquan in Yunnan formed Early and Middle Devonian coal line or thin coal seam. Early Devonian flora has begun to appear on the earth, but the distribution is sparse and have not discovered valuable coal seam yet [1]. Due to the early Paleozoic sapropel and Devonian system's humic coal restricted distribution area's inability to form scale coal measures, so far it has not been found in the world of coal-derived gas field.

http://dx.doi.org/10.1016/j.jnggs.2016.11.012

2468-256X/Copyright © 2016, Lanzhou Literature and Information Center, Chinese Academy of Sciences AND Langfang Branch of Research Institute of Petroleum Exploration and Development, PetroChina. Publishing services by Elsevier B.V. on behalf of KeAi Communications Co. Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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2. The coal measures associated with coal-derived gas in the Late Paleozoic

There are eight main coal accumulating periods in China from Early Cambrian to Tertiary. The Late Paleozoic, the Late Carboniferous to the Early Permian, and Late Permian to coal accumulating period, are two of China's four biggest coal accumulating effect phase [1]. The Late Paleozoic coal-bearing strata is distributed widely in China and developed as well. Associated with the currently known coal-derived gas field of coal-bearing strata, it is mainly distributed in the Upper Carboniferous Benxi Formation, the Lower Permian Taiyuan Formation and Shanxi Formation of the Eastern North and Northwest of China, the Lower Carboniferous Dishuiquan Formation and the Upper Carboniferous Batamayineishan Formation in Junggar Basin of Xinjiang, and the Upper Permian Longtan Formation of south China.

2.1. The coal measures associated with coal-derived gas in north China coal basin

Late Paleozoic coal accumulating basin of North China, or North China coal basin, is an important coal-bearing area in China. The original scope of the coal basin, north from south of Yinshan Mountain, south to north of Qinling Mountain and Dabie Mountain, west to the east of Helan Mountain, east facing Japan sea, an area of 120 x 104 km2, is a very large coal-bearing basin. Basin, in the main part of the formation from bottom to top are: Hutian Formation (Tielvyan Formation), Benxi Formation, Taiyuan Formation, Shanxi Formation, Shihezi Formation, and Shiqianfeng Formation [2]. The Benxi Formation, Taiyuan Formation and Shanxi Formation are coal-bearing strata in the north. The Shihezi Formation in southern basin, the south of the north latitude 35° of Ping-dingshan and Huainan regions, has coal and buried shallow. It is now an important coal mining area, in addition to the coal bed methane, not found coal-derived gas field.

North China coal basins, which are suffering the influence of the Cenozoic tectonic movement, have significant changes. It formed an uplift tectonic belt of the NNE trend Taihang Mountain—Lvliang Mountain in the middle. The rise of the late Paleozoic coal, with shallow buried depth, becomes China's major coal-producing area. And Qinshui area in its south became the coal bed of gas production areas in China. However, coal-derived gas field in the uplift belt haven't been found so far. In the Bohai Bay Basin of the eastern uplift belt, Mesozoic—Cenozoic tectonic is strongly active, it fractures more and is becoming a rift type. It makes many areas in the original coal continuous distribution by denudation because of the rising, just to be saved in deep sag coal measures, and found the relevant coal-derived gas field. In the west of Tai-hang Mountain—Lvliang Mountain uplift belt area mainly is Ordos Basin and is a craton-type structure region [3], the continuous Late Paleozoic coal measures preserved well in the interior.

2.1.1. The accumulation conditions of Late Paleozoic coal into gas in Ordos Basin

Ordos Basin is the earliest basin (in 1907) in mainland China to use mechanic well drilling (Well Yan 1) to explore oil and gas. But since then, because of the traditional industry think coal measures is not hydrocarbon source rock, it doesn't put the Carboniferous-Permian coal as exploration targets. Until 1978 it has no progress for natural gas exploration. In 1979, after the birth of the theory of coal-derived gas [4], many scholars have pointed out that coal-bearing strata of Benxi Formation, Taiyuan Formation, and Shanxi Formation in the basin are good gas source rocks since 1980, and should strengthen the coal-derived gas exploration [5—10].

Coal and mudstone of late Paleozoic in Ordos Basin are all kerogen III. Coal seams are mainly distributed in Taiyuan Formation and Shanxi Formation, the thickness 2—20 m in general, dark mudstone in the western basin generally 140—150 m, 70—140 m for east, and 20—50 m for north and

Table 1

Geochemical parameters of Upper Paleozoic source rocks in Ordos Basin [12,13].

47 48 49 Type Organic carbon Chloroform bitumen A/% Total hydrocarbon Maceral/%

/ppm Vitrinite Fusinoid Inertinite

50 Maximum/minimum

51 Average

52 Shanxi Formation Coal 89.17/49.28 2.45/0.1 6699.93/519.9 90.2/43.8 54/6.3 12.3/0

53 73.6 0.8 2539.8 73.6 24 4.6

54 Mudstone 19.29/0.07 0.5/0.0024 524.96/519.85 47/8 87/51.8 20.3/0

55 2.25 0.04 163.8 20.5 72 7.4

56 Taiyuan Formation Coal 83.2/3.83 1.96/0.03 4463/222 98.8/21.2 63.7/1.3 15.1/0

57 74.7 0.61 1757.1 64.2 32.1 3.7

58 Mudstone 23.38/0.1 2.95/0.003 1904.64/15 82/8.3 89.3/15.3 34.5/0.3

59 3.33 0.12 361.6 38 53.3 8.4

60 Limestone 6.29/0.11 0.43/0.0026 2194.53/88.92

61 1.41 0.08 493.2

62 Benxi Formation Coal 80.26/55.38 0.97/0.41 93.3/72 25.2/6.7 2.8/0

63 70.8 0.77 87.2 16 1.4

64 Mudstone 11.71/0.05 0.44/0.0024 1466.34/12.51 47.8/12.3 59.8/12.3 39.5/0.3

65 2.54 0.065 322.73 24.5 44 18.2

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south [11]. The geochemical parameters of the coal and dark mudstone in Benxi Formation, Taiyuan Formation, and Shanxi Formation [12,13] are shown in Table 1. Three sets of coal in basin can be seen in Table 1 as good gas source rock.

Shanxi Formation and Xiashihezi Formation in basin-developed sandstone reservoir, with sandstone porosity less than 8% accounted for 63.71%, 8%—12% accounted for 28.58%, larger than 12% accounted for only 7.70%. With permeability less than 1 x 10—3 mm2 accounted for 86.38%, it is a typical tight sandstone reservoir. The reservoir plane continuous distribution, distribution range is extensive. Vertically superimposed layers sand body, the sand layer thickness generally 30—100 m, the main reservoir sand mud ratio greater than 60%, provides good reservoir space for the large area of tight sandstone gas [14,15].

The stable lateral distribution of lacustrine mudstones of the Upper Paleozoic Shangshihezi Formation and Shiqianfeng

Formation are important regional cap rocks. Shangshihezi Formation lacustrine argillaceous rocks are mainly sandy mudstone and silty mudstone, with thickness generally 150—200 m, mudstone gas absolute permeability commonly

(10~4—10~5)

103 mm2, gas breakthrough pressure

1.5—2.0 MPa, and have a strong ability to block.

Ordos Basin's Late Paleozoic large area has continuous stable distribution of coal-derived gas source rock at the bottom, middle plane shape wide distribution, vertically multilayer sand body overlaying large thickness tight sandstone, Shangshihezi Formation and Shiqianfeng Formation stable lateral distribution above, large thickness, and good lake mudstone seal forming good vertical source-reservoir-cap combinations, so it is good for large coal-derived gas accumulated in Xiashihezi Formation, Shanxi Formation, and Taiyuan Formation. Thus, many coal-derived gas fields have been found (Sulige, Daniudi, Yulin, Shenmu, Wushenqi, Zizhou, Mizhi, Liuyangbao, Dongsheng,

Fig. 1. Continuous distribution of Late Paleozoic coal measure relative gas fields.

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Shenglijing) (Fig. 1). In addition, there is the Jingbian gas field with the coal-derived gas and oil-type gas gathered in the Ordovician Majiagou Formation carbonate rocks karst weathering crust. Many scholars agree about coal-derived gas in Jingbian gas field generated by late Paleozoic coal measures and migrated by Majiagou Formation channeling [16—19]. But there are two views for the gas source rocks of the oil-type gas in the gas field. Some of them thought they are from limestone interlayer with gas generation condition in Taiyuan Formation and Shanxi Formation (Table 1) [11,17—19], and others considered gas source rock as the Ordovician Majiagou Formation carbonate rocks [20—22].

2.1.2. The accumulation conditions of late Paleozoic coal into gas in Bohai Bay Basin

Bohai Bay Basin is a part of north China coal basin, Late Paleozoic coal measures forming environment, strata group, sedimentary condition, tectonic setting, superior gas generation condition, basically consistent with Ordos Basin. But due to the Cenozoic tectonic movement to be reformed to rift basin, the original contiguous wide distribution coal measure of Upper Paleozoic Benxi, Taiyuan and Shanxi Formation are lifted by strong reform, and most of the coal measure were eroded and

preserved only in depression (fault). For example, coal measure of the Jidong Depression only distributed in southeast depression, the distribution area is less than 1/3 of the depression. So the coal accumulation region is much less, leading to accumulation scale small and scattered [23], and gas reservoir closely related to fracture (Fig. 2). Now discovered production gas reservoir are Suqiao condensate gas field, Wenliu gas field, Hubuzhai gas field, and many small gas fields, such as Chen-ghai, Wangguantun, Gubei, and so on [24—26].

2.2. The accumulation of Carboniferous coal into gas in Junggar Basin

Carboniferous in Junggar Basin tectonic activity is strong, and because of the frequent volcanic eruptions resulting in the same period stratigraphic lithology changing considerably and district named differently. Just analyze gas generation and accumulation function of the Dishuiquan Formation (Cid) and Batamayineishan Formation in Zhundong area with which the Kelameili large gas field is in. Many scholars have pointed out that the Carboniferous coal measures is effective hydrocarbon source rocks [27—32]: the main hydrocarbon source rocks are the middle of Batamayineishan Formation, followed by the

Fig. 2. Relationship between coal measure derived gas reservoir of Late Paleozoic and fracture in Bohai Bay Basin. (a) Chenghai; (b) Wen23; (c) Suqiao; (d) Bogu 4 buried hill.

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Table 2

Organic matter abundance of Carboniferous source rocks in Ludong-Wucaiwan Area [33].

Stratum Lithology TOC/% S1 + S2/ (mg/g)

Batamayineishan Formation C2b Dishuiquan Formation C1d Tuff and tuffite Mudstone and carbonaceous mudstone Coal Tuff and tuffite Mudstone and carbonaceous mudstone 0.4—8.36, average 1.75 0.46—19.26, average 4.07 15.95—37.59, average 21.96 0.46—2.43, average 1.01 0.4—2.51, average 1.06 0.2—49.12, average 4.27 0.05—27.2, average 3.56 0.55—53.27, average 18.5 0.1—0.74, average 0.34 0.07—10.71, average 0.65

Dishuiquan Formation, and both of them are the coal-bearing strata. Dishuiquan Formation are coastal — shoreland transitional facies sedimentary environment, mainly terrigenous clastic and volcanic debris, development in Dishuiquan fault and Dongdaohaizi—Wucaiwan fault of north and south Kela-meili gas field in the central basin, with thickness 49—623 m in Well Caican 1, hydrocarbon source rock 49—291 m, mainly dark mudstone, and a small amount of carbonaceous mudstone and coal. In Ludong-Wucaiwan area, Batamayineishan Formation is 124—3060 m thick, hydrocarbon source rocks 200—520 m, mainly dark mudstone, secondly carbonaceous mudstone and coal, then tuffite. In the Well Zhang 3, source rocks are 140.5 m thick, dark mudstone 106 m, carbonaceous mudstone 21.5 m, coal seam 13 m. Organic matter types of two formations are II2 and III, and the organic matter abundance [33] are shown in Table 2.

Batamayineishan Formation is the main hydrocarbon source rocks, the volcanic rock the main reservoir. Bata-mayineishan Formation volcanic rocks can be divided into two segments, the middle for sedimentary space, upper segment volcanic rocks with serious erosion later and limited distribution, lower segment volcanic rocks as the products of the volcanic strong eruption, intermediate-acid volcanic rocks mainly of eruption facies and flooding facies, reservoir space dominated by secondary dissolution pores and fractures. In the Kelameili gas field, the porosity is 0.80%—28.80% with an average of 8.85%, the permeability (0.01—522.00) x 10—3 mm2 with an average of 0.618 x 10—3 mm2. Regional cap rock is Permian mudstone. The Kelameili large gas filed is found in this source reservoir cap rock assemblage above. Due to changeable facies and bad continuous volcanic rock as the reservoir, this large gas field is composed of multiple gas reservoirs (Fig. 3) [34].

2.3. The accumulation of the late Permian Longtan Formation coal into gas in Sichuan Basin

South China's Upper Permian Longtan Formation coal zones are mainly distributed in the Yangtze region. Yangtze region horizontally distributes two formations with the same period and different facies of late Permian: Longtan Formation and Wujiaping Formation. Wujiaping Formation, dominated by marine carbonate rocks with mud shale, are mainly distributed in the eastern Sichuan Basin and middle Yangtze region, but the discussion for that has nothing to do with the subject of this paper. Longtan Formation mainly formed in gulf lagoon facies and delta plain swamp, the

upper and middle dominated by black mudstone, shale, silty mudstone and coal, part with powder sandstone, the main distribution in Lower Yangtze area and the upper Yangtze area of the midwest Sichuan Basin [35,36]. Many oil and gas shows have been found in Longtan Formation of the Lower Yangtze region [4], but there are no related coal-derived gas fields yet, and that will not be discussed here. Longtan Formation in Sichuan Basin of the Upper-Yangtze area is generally 20—250 m thick, some coal reaching 2—10 layers [35], coal seam 3 m thick near Yilong [37]. Well, Yunan 19 in northeast Sichuan develops typical Longtan stratum, mudstone TOC content from 1% to 10%, mostly more than 2%, average 5.04%, source rocks 170 m thick. Hydrocarbon source rocks around Chongqing, Zigong, and Ziyang areas are 80—120 m thick, TOC content 1%—4%. Kerogen types are III and II, average original hydrocarbon potential of coal and coal shale 5—12 mg/g, up to 46 mg/g, RO 1.3%—3.4% [35,36]. Changxing Formation reef facies dolomite is Longtan Formation reservoir, in which solution pore residual raw dolomite, solution pore dolomite as the main reservoir in Yuanba area, mainly with mesopore and medium-low permeability reservoir, porosity with a maximum of 6.28%, the minimum value 2.36%, the average 3.76%, the permeability with a maximum of 0.73 x 10—3 mm2, the

10 3 mm2. Gypsum of Jialingjiang Formation and

minimum value 0.01 0.21

Leikoupo Formation is the regional cap rock, the fourth member of Feixianguan Formation gypsum and marl as direct cap rocks. Currently, two large gas fields, Yuanba (Fig. 4) [38] and Longgang, have been found in Sichuan Basin Changxing Formation.

3. Correlation between listed gas field and gas source

Listed gas field refers to the gas field published by China and related to the late Paleozoic coal (until the end of 2013).

3.1. Ordos Basin

Ordos Basin Benxi Formation, Taiyuan Formation, and Shanxi Formation coal measure form 12 gas fields (Fig. 5), the general exploration and development situation of these gas fields are shown in Table 3. From Table 3, it can be concluded that Ordos Basin coal-derived gas fields contain the largest coal-derived gas field (Sulige), the smallest coal-derived gas field (Liujiazhuang), and the highest annual output coal-derived gas field (Sulige).

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Fig. 3. Profile of Kelameili gas field (Ref. [34], 2011, supplement).

Fig. 4. Profile of reservoir in Changxing Formation, Yuanba gas field [38].

Natural gas geochemical parameters [11,18,39—44] of Ordos Basin which are being produced and already in production gas fields are shown in Table 4, including a lot of information of gas source correlation for listed gas fields. Many scholars pointed out that the S13C2 value is a good indicator to identify the coal-derived gas and oil-type gas. Zhang Shiya pointed out that the S13C2 is influenced by the hydrocarbon source rock maturity less than the S13C1, and —29%o may be a boundary to distinguish oil-type gas and coal-derived gas, the S13C2 of coal-derived gas generally heavier than —29%, while the S13C2 value of oil-type gas generally lower than —29% [45]. Wang Shiqian pointed out that the S13C2 > —29% for coal-derived gas [46]. Dai pointed out that the S13C2 value of coal-derived gas is basically higher than —28%, and the S13C2 value of oil-type gas basically lower than —28.5%, the S13C2 value between —28% and —28.5% for the mixture of these two types of gases and dominated by coal-derived gas [47]. According to the above, except Well Shan 5 and Well Shan 17 in Jingbian gas field for oil-type gas, all gases have the characteristics of coal-derived gas, shown in Table 4. Well Shan 5 and Well Shan 17 as oil-type gas are generated by intercalation limestone in Taiyuan Formation and Shanxi Formation [11,17—19].

Put the values of S13C1, S13C2, S13C3 in Table 4 to S13C1 -S13C2 -S13C3 identification chart (Fig. 6) [48], it indicates

that, except Well Shan 5 and Well Shan 17, all gases in the area of coal-derived gas. Carbon isotope of benzene and toluene [49,50], and light hydrocarbon research [51] also proved they are coal-derived gas.

3.2. Bohai Bay Basin

The original deposition of the late Paleozoic coal measures in Bohai Bay Basin is similar to Ordos Basin. But due to the strong Mesozoic—Cenozoic tectonic movement, gas generation and accumulation is less than the Ordos Basin, for just a small area, small reserves, and low production medium-small gas reservoir (Table 5).

Table 6 shows the gas geochemical parameters from Carboniferous-Permian coal measure related gas wells of Bohai Bay Basin Dagang oil field, Shengli oil field, Huabei oil field, and Zhongyuan oil field [24,52—54]. According to S13C2 value indicator and S13C1 -S13C2 -S13C3 identification chart (Fig. 6) to identify, all natural gases in Table 6 are coal-derived gases, consistent with the conclusion from related research [23,55—57].

3.3. Junggar Basin

Two coal strata from Lower Carboniferous Dishuiquan Formation and the Upper Carboniferous Batamayineishan

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ARTICLE IN PRESS

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Fig. 5. Distribution of coal measure derived gas fields (reservoirs) of Late Paleozoic in China.

Formation formed Kelameili gas field and Wucaiwan gas field (Fig. 5, Table 7). They have two characteristics different from the coal-derived gas in China, one is the coal-derived gas field formed from China's oldest coal measure, and the second is the volcanic rock as a reservoir (Fig. 3).

Natural gases of the Kelameili gas field and the Wucaiwan gas field in Table 8 are formed by Upper and Lower Carboniferous coal measure, and natural gases of northern Kelamayi 5 Jiamuhe Formation gas reservoir (Pl/, Well Ke 82) and Wuerhe Formation gas reservoir (P2w, Well Ke 75) formed by the Jiamusihe Formation type III source rocks [30].

Table 3

Exploration and development synopsis of coal-derived gas field in Ordos Basin.

value indicator and S13Ci -S13C2

-S13C3

According to S C2 value indicator and o Ci -o C2 -o C3 identification chart (Fig. 6) to identify, all natural gases in Table 8 are coal-derived gases, consistent with the conclusion from many scholars [11,29,30,32].

3.4. Sichuan Basin

Sichuan Basin Longtan Formation coal-bearing strata forms Changxing Formation gas reservoir of Yuanba gas field and Longgang gas field (Fig. 5, Table 7), with two characteristics: first, it is different from Chinese coal-derived

52 Gas field Main production layer Discovered year Total proven geological Production in Cumulative production/ (x 108 m3)

53 reserves by 2013/ (x 108 m3) 2013/ (x108 m3)

54 Sulige P1X8, P1S1 2001 12725.9 212.20 771.82

55 Jingbian O2, P1X8 1992 5528.04 41.76 510.53

56 Daniudi P, C 2002 4545.63 34.34 236.31

57 Yulin P1S2 1997 1807.50 59.85 438.88

58 Zizhou P1X8, P1S2 2005 1151.97 13.87 51.63

59 Wushenqi P1X8, O2 1999 1012.1 6.95 46.75

60 Shenmu PlS, P1S2, C3 2007 934.99 Not produced 0

61 Liuyangbao C3t2 2012 549.65 Not produced 0

62 Mizhi P1x 1999 358.43 0.22 1.42

63 Dongsheng P1x3, P1x2 2010 162.87 0.10 0.10

64 Shenglijing P2s 1982 (Discovered) 18.25 0 0

65 Liujiazhuang P1x5 1969 (Discovered) 1.9 0 0

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Table 4

Geochemical parameters of natural gas related to Carboniferous-Permian coal measure in Ordos Basin.

Gas filed

Depth/m

Stratum Main components of natural gas/%

5 C/%o(VPDB)

Sulige

Daniudi

Jingbian

Dongsheng Shenglijing Wushenqi

Zizhou

Shenmu

CH4 C2H6 C3H8 C4H10 CO2 N2 CH4 C2H6 C3H8 C4H10

Su21 P1s,P2x 92.39 4.48 0.83 0.27 0.99 0.68 -33.4 -23.4 -23.8 -22.7 [11]

Su75 P2x 92.47 3.92 0.66 0.22 1.30 1.10 -33.2 -23.8 -23.4 -22.4

Su139 P1s,P2x 93.16 3.05 0.51 0.14 1.31 1.45 -30.4 -24.2 -26.8 -23.7

Su75-64-5X P2x 89.45 6.36 1.26 0.46 0.13 0.93 -33.5 -24.0 -23.3 -22.8

SU4-J1 3550.2 P1S 92.46 4.68 1.22 0.53 -32.9 -23.6 -22.9 -22.4 [39]

SUDONG37-44 3028.5 P2h8 94.18 3.36 0.54 0.19 -33.3 -24.3 -23.7 -22.5

Shan117 P1S 64 3.99 0.63 0.11 1.51 0.51 -32.2 -26.0 -24.9 -23.5 [40]

Shan215 P1S 93.60 3.79 0.55 0.15 0.76 0.64 -30.8 -25.8 -24.4 -23.1

Yu43-10 2781.4-2798.3 P1S 94.94 2.70 0.35 0.10 1.16 0.68 -31.9 -26.4 -23.0 -24.1 [41]

Yu45-10 2726.7-2736.0 P1S 94.26 3.39 0.51 0.15 0.99 0.54 -30.2 -26.1 -23.8 -21.9

Zhao4 Shihezi -31.3 -23.7 -23.0 -22.5 [42]

Qi2 P1Î -31.6 -25.2 -22.8 -21.4

D11 2600.5-2602.5 P 93.84 3.38 0.52 0.19 0.19 1.27 -34.5 -26.2 -24.7 -23.0 This

D13 2702-2731.5 P1S2 89.81 6.02 1.65 0.59 0.52 0.90 -36.0 -25.7 -24.5 -22.7

D16 2698-2703 Shihezi 2 94.24 3.43 0.54 0.21 0.33 0.84 -35.2 -27.1 -26.0 -23.9

D24 2659-2685 Shihezi 1 89.12 6.70 1.89 0.59 0.33 0.86 -37.2 -26.1 -25.3 -24.0

Shan5 3457-3484 O1m5 93.96 0.53 0.07 0.02 3.81 1.60 -32.2 -31.2 -25.7 [18]

Shan2 3364.4-3369.4 O1mJ 96.09 1.09 0.13 0.04 2.60 -35.3 -26.2 -25.5 -23.2

Shan17 3176.9-3182 O1m5 93.89 0.69 0.08 0.01 4.55 0.62 -33.3 -30.2 -27.8 -22.3

Shan21 3226-3230 O1m5 95.87 1.28 0.17 0.04 2.83 0.21 -34.9 -24.5 -24.7 -23.0

Shan34 3410-3413 O1m5~2 94.02 1.28 0.15 0.06 0.36 4.11 -35.3 -25.5 -24.4 -21.9

Shan65 3149-3154 P1x 95.74 2.54 0.29 0.07 0.13 1.10 -29.1 -23.5 -25.5 -24.1

Shan85 3266.6-3287 O1m5 95.27 0.47 0.05 0.02 3.56 1.46 -33.1 -26.7 -20.9 -19.0

Yishen1 93.96 3.62 0.87 0.37 0.20 0.81 -33.5 -25.1 -24.6 This

ESP2 93.74 3.64 0.85 0.29 1.32 -33.2 -25.3 -24.9

Jin11 93.69 3.57 0.87 0.34 1.34 -33.8 -25.0 -24.5

Ren4 2299-2303 Shihezi 3 91.09 4.79 0.70 0.19 3.23 -33.8 -26.4 -24.1

Ren9 2240-2243 Shihezi 91.84 3.86 1.21 0.51 2.40 -35.2 -26.6 -24.7

Ren11 2534-2537 Shihezi 4 93.78 3.36 1.07 0.43 0.09 1.19 -35.1 -26.7 -24.8

YU22-7 3119.8-3142.0 P1x 92.51 4.10 0.69 0.22 0.55 1.67 -32.6 -23.7 -24.2 -21.9

G01-9 3038.0-3053.2 P1x 93.46 3.92 0.54 0.14 0.45 1.38 -33.7 -23.1 -24.8 -22.7

Shan165 3103.2-3133.7 P1x 93.17 3.46 0.60 0.19 0.65 1.67 -33.0 -24.0 -24.5 -22.3

Shan243 3042.2-3080.2 P1x 90.85 5.46 1.03 0.35 0.54 1.55 -35.0 -24.0 -23.6 -22.5

Zhou16-19 2712.5 P1S 91.53 5.22 1.16 0.39 -34.5 -24.3 -21.7 -21.7 [39]

Zhou17-20 2644.45 P1S 91.55 5.07 1.13 0.40 -33.0 -24.5 -22.0 -21.7

Zhou19-22 2635 P1S 93.00 4.43 0.84 0.31 -33.3 -24.7 -21.9 -21.6

Zhou22-18 2592 P1S 93.12 4.22 0.76 0.27 -31.1 -25.7 -24.3 -23.1

Mi4 2208 P2h8 93.73 4.44 0.09 0.02 -28.1 -22.0 -22.7 -21.6 [43]

Mi17 2544 P2h8 92.75 4.39 0.86 0.33 -34.0 -23.7 -22.4 -21.2

Mi18 2303 "P2^8 93.32 5.09 0.19 0.08 -34.1 -23.5

Mi21 2303.5 P2h8 95.18 3.38 0.50 0.16 -35.1 -22.7

Shen1 P2x 92.86 4.69 1.23 0.34 0.73 -37.1 -24.7 -24.5 -23.9 [44]

Shuang15 2753.0-2756.5 P1S 93.65 3.59 0.75 0.29 1.45 0.42 -35.9 -23.6 -22.6 -22.3

Shuang20 P1S 93.06 3.22 0.56 0.21 2.47 0.82 -35.8 -25.6 -24.0 -23.0

gas fields (reservoir) with marine reef and beach facies carbonate rocks as reservoirs (Fig. 4) [11,38,58,59], and second, it is the natural gas containing more H2S (Table 9) [37,38,60].

Table 9 shows gas geochemical parameters of Yuanba gas field and Longgang gas field Changxing Formation [37,38,60]. According to S13C2 value indicator and S13C1 -S13C2 -S13C3 identification chart (Fig. 6) to identify, all natural gases in Table 9 are coal-derived gases. Some scholars have also pointed out that Changxing Formation of Longgang gas field and Yuanba gas field belongs to coal-derived gas [11,37,59]. But also, scholars think that the Yuanba gas field Changxing Formation gas reservoir belongs to oil-type gas [58].

4. The significance of late Paleozoic coal-related gas field in natural gas industry

4.1. The geological reserves and the annual production of Late Paleozoic coal-related gas fields accounting for one-third of the national

Fig. 7 and Fig. 8 show China's total geological reserves and annual production, and the change of the late Paleozoic, Mesozoic and Cenozoic coal-derived gas proportion this decade. In 2013, the late Paleozoic coal-derived gas reserves and annual production account for the country's 33.9% and 31.94% respectively, indicating that the late Paleozoic coal

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Fig. 6. 513C1 -513C2 -513C3 identification chart of alkane gas type in Ordos, Bohai Bay, Junggar, and Sichuan Basin.

Table 5

Exploration and development synopsis of coal-derived gas (oil) field in Bohai Bay Basin.

28 29 Gas field Main production layer Discovered year Total proven geological reserves by 2013/ (x108m3) Production in 2013/ (x 108m3) Cumulative production/ (x 108m3)

30 Suqiao O,P 1982 108.81 0.01 33.34

31 Wen'an P2s 1979 20.61 0 7.67

32 Guxinzhuang O 1977 9.71 0.12 3.52

33 Wen23 (Wenliu) E3s4 1977 154.12 0.64 108.28

34 Baimiao E3s3 1980 126.23 0.30 8.18

related gas fields playing an important role in reserves and production in China.

4.2. The average reserves in coal related gas fields and large gas fields in late Paleozoic higher than the national gas fields and large gas fields

Sixteen coal-related gas fields of late Paleozoic have been found at the end of 2013, and 12 of them are large gas fields, which proved total geologic reserves of 32,773.77 x 108 m3 and 32,590.73 x 108 m3 respectively, the average geologic reserves of gas fields and large gas fields 2048.4 x 108 m3 and 2715.9 x 108 m3 respectively. At the end of 2013, China has found 253 gas fields, including 51 large gas fields, proved total geologic reserves 98,006.64 x 108 m3 and 81,683.77 x 108 m3 respectively, average geologic reserves of gas fields and large gas fields 387.4 x 108 m3 and 1601.6 x 108 m3 respectively. Thus, average geologic reserves of late Paleozoic coal-related gas fields and large gas fields, are 5.3 times and 1.7 times of the national gas fields and large gas fields respectively. After our country becoming a gas power with annual production of 1000 x 108 m3, only the Late Paleozoic related gas fields and large gas fields with large reserves, make a greater contribution to the gas industry continued development.

4.3. Late Paleozoic coal measures forming the three key large gas fields to support our country becoming a big gas producer

The so-called key large gas field refers to the large gas fields supporting the national to be the gas power, they often with the reserves located in the forefront of the country. China has five key large gas fields from 2011 to 2013 (Sulige, Jingbian, Daniudi, Puguang and Kela 2). The production of these key large gas fields in 2013 accounted for 38.0% of the national natural gas production [61], as the foundation of our country to be a gas power. Among them, all the gas sources of Sulige, Jingbian, and Daniudi are the Late Paleozoic coal-derived gas. Sulige gas field, is the largest gas field, with an annual production of 212.2 counting for 17.6% of the total gas production.

108 m3 in 2013 (Table 3), ac-

5. Conclusions

China's gas source rock of the Late Paleozoic coal related gas fields are Lower Carboniferous Dishuiquan Formation, Upper Carboniferous Batamayineishan Formation, Benxi Formation, Lower Permian Taiyuan Formation and Shanxi Formation, and Upper Permian Longtan Formation, reservoirs mainly sandstone reservoir, secondly reef carbonate rocks, and

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Table 6

Geochemical parameters of natural gas related to Carboniferous-Permian coal measure in Bohai Bay Basin.

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3 4 5 Gas reservoir Well Depth/m Stratum Main components of natural gas/% 513C/%o(VPDB) Ref.

CH4 C2H6 C3H8 C4H10 H2S CO2 N2 CH4 C2H6 C3H8 C4H10

6 Chenghai Haigu1 4510—4587.7 O 50.43 0.22 0.03 0.02 11.79 33.6 3.86 -27.2 -18.7 -22.1 [24]

7 Haigu101 5110—5187 O 55.64 0.15 6.03 33.33 1.95 -26.8 -14.1

8 Kongxi Wanggu1 3830.2—3867 P 84.09 6.29 1.98 0.93 3.84 2.06 -35.5 -25.4 -23.5 - 24.7

9 Wumaying Wushen1 5456—5515 O 87.82 3.84 -38.5 -21.4 -22.1

10 5460—5496 88.86 2.54 -38.0 -22.4 -22.1

11 Gubei buried hill Yi132 3374.0—3387.0 C—P 82.10 8.10 3.43 1.79 1.87 -37.0 -25.4 -25.0 -25.5 [52]

12 Gubeigu1 4020.9—4139.5 P 86.67 5.44 1.28 0.40 5.45 -35.9 -23.1 -21.2 -21.2

13 Gubeigu 2 3689.0—3731.0 C—P -41.0 -25.8 -23.6 -23.6

14 15 16 17 18 Bo93 3230.0—3249.4 C—P 88.99 6.30 1.03 0.38 2.29 -38.1 -22.7 -21.3 -21.8

Bogu4 buried hill Bogu4 4375.0—4460.0 O 81.96 6.83 2.20 0.88 7.39 -38.2 -24.9 -22.5 -23.6

Suqiao-Wen'an Wen23 2710—2762.4 P 79.40 12.28 4.35 1.66 0.35 1.09 -36.9 -26.9 -25.5 This paper

Su20 3344.6—3392.4 P 79.50 10.40 4.32 2.14 1.68 1.08 -37.4 -26.8 -25.3 - 24.3

Su401 4848—4912.73 O 86.76 5.94 2.38 1.29 1.20 1.79 -36.5 -25.6 -23.7

Su402 4568—4700 O 86.02 7.25 2.28 0.94 1.37 1.88 -36.2 -26.2 -25.1

19 Su1-7 4145—4177 O2 82.02 10.00 4.05 1.39 1.71 0.53 -38.0 -27.0 -26.6 -26.8

20 Ba21 3390.67—3553.6 O 2.80 8.59 -37.0 -22.0 -22.5 -23.8

21 Shenxian Ze79 3658.7—3720 O 64.21 10.85 4.92 2.27 12.84 3.32 -35.2 -25.0 -24.0 -23.7

22 Ze85 3939.4—3941.1 O 87.66 3.70 1.33 1.06 3.27 0.98 -33.9 -25.1 -23.1

23 Wenliu Wen23 2813.2—3026.8 Es4 93.61 1.81 0.35 0.21 0.99 2.34 -27.8 -24.3 -24.1 -23.9

24 Wen23 2969.8—2987.0 Es4 95.20 2.39 0.64 0.67 0.46 0.19 -28.8 -25.7 -25.7 -26.1

25 Wen31 2968—2987 Es4 96.50 0.60 0.17 0.12 0.48 2.07 -27.7 -24.4 -25.1 -26.1

26 Wen105 2800—2890 Es4 -27.7 -24.6 -25.7 -26.0

27 Hubuzhai Wei112 2741—2807 Es3 81.56 6.51 2.32 1.79 1.19 1.05 -34.7 -25.8 -25.4 -25.7

28 Wei79-9 Es4 92.80 3.04 0.75 0.27 -30.2 -25.4 -25.8 [53]

29 Wei351-2 3342—3346 Es3 92.86 -20.9 -26.4 -27.8

30 Machang Kai33 3344—3346.5 Es4 95.70 0.83 0.13 0.05 0.38 2.93 -31.6 -22.1 -20.6 [54]

Table 7

Exploration and development synopsis of coal-derived gas fields in Junggar Basin and Sichuan Basin.

Basin Gas field Main production layer Discovered year Total proven geological Production in 2013 (x 108m3) Cumulative production

reserves by 2013 (x108m3) (x 108m3)

Junggar Kelameili C2b 2008 ^1053.34 6.96 29.87

Wucaiwan C2b 8.33 Not produced 0

Sichuan Yuanba P2ch, T1f 2011 2194.57 Not produced 0

Longgang P2ch, T1f 2010 720.33 9.15 55.54

Table 8

Geochemical parameters of coal-derived natural gas related to Carboniferous coal measure in Junggar Basin.

Gas field (Reservoir) Well

Depth/m

Stratum Main components of natural gas/%

ô13C/%o(VPDB)

CH4 C2H6 C3H8 C4H10 CO2 N2 CH4 C2H6 C3H8 C4H10

Kelameili Dixi 10 3024 C2b 90.97 2.48 0.73 0.41 0.38 4.05 -29.5 -26.6 -24.6 -24.5 [11]

Dixi 14 3582 C2b 92.32 3.51 1.07 0.54 0.09 2.14 -30.5 -27.6 -25.2 -25.3

Dixi 17 3662 C2b 85.44 6.12 2.10 1.36 0.28 3.48 -30.1 -26.4 -24.4 -24.8

Dixi 18 3510 C2b 83.95 6.31 2.55 1.51 0.02 3.74 -30.0 -27.1 -24.7 -24.7

Dixi 20 3313 C2b 81.91 4.79 1.94 1.18 0.06 9.45 -29.8 -26.7 -24.8 -25.1

Dixi 21 2849 C2b 86.08 3.24 1.52 0.74 0.11 8.11 -29.4 -27.1 -25.0 -24.4

Dixi 171 3670 C2b 90.07 4.17 1.41 0.86 0.05 0.81 -30.4 -26.1 -24.2 -24.2

Dixi 172 3552 C2b 88.14 4.56 1.40 1.14 0.21 3.90 -29.4 -25.9 -23.6 -24.0

Dixi 182 3635 C2b 84.83 5.84 2.45 1.59 0.04 4.45 -30.4 -26.5 -23.7 -23.7

Dixi 5 3650— 3665 C2b -29.2 -26.8 -25.3 -25.2 [30]

Wucaiwan Cai25 3028— 3080 C2b 94.37 2.13 0.46 0.11 2.60 -30.0 -24.4 -22.6 -22.3

Cai27 2778— 2790 C2b 73.71 6.89 4.33 0.03 11.53 -30.3 -25.0 -23.0 -22.6

Northern Kelamayi 5 Ke75 2604. —2672 P2w 93.20 3.70 0.90 0.37 -31.7 -26.5 -24.7 -24.5

Ke77 2763— 2768 P2w 91.50 4.30 1.10 0.49 -32.9 -26.4 -24.0 -24.8

Ke82 4070— 4084 Pi/ -29.7 -23.0 -20.1 -20.0

Ke82 4184— 4166 Pj -30.0 -24.2 -22.6 -20.0

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Table 9

Natural gas geochemical parameters of Yuanba gas field and Longgang gas field Changxing Formation.

Gas field

Depth/m

Stratum Main components of natural gas/%

ô13C/%o(VPDB)

Yuanba

Longgang

CH4 C2H6 C3H8 H2S CO2 N2 CH4 C2H6

YB1-1 7330—7367.6 P2ch2 86.72 0.04 0 6.61 6.25 0.28 -28.9 -25.3 [60]

YB27 7330.7—7367.6 P2ch2 90.71 0.04 0 5.14 3.12 0.83 -28.9 -26.6

Y104 6700—6726 P2ch2 87.09 0.04 0 7.04 5.23 0.52 -29.1 -25.6 [38]

Y204 6523—6590 P2ch2 91.23 0.04 0.005 2.36 4.32 1.54 -29.4 -26.0

Y205 6448—6480 P2ch2 89.14 0.05 0 5.33 5.03 0.00 -29.5 -27.5

Y27 6262—6319 P2ch2 89.03 0.09 0.002 4.08 5.06 1.22 -28.9 -26.6

YB1 7081—7150 P2ch2 53.25 0.09 0.09 13.33 30.20 3.04 -30.2 -27.6 [37]

YB11 6797—6917 P2ch2 80.55 0.05 0 11.80 0.23 7.37 -27.9 -25.2

LG1 6202—6204 P2ch2 92.33 0.07 0 2.50 4.40 0.70 -29.4 -22.7

LG2 6112—6132 P2ch2 89.03 0.06 0 4.53 6.07 0.31 -28.5 -21.7

LG9 6353—6373 P2ch2 63.50 0.26 0.04 6.19 30.00 0.01 -31.7 -22.7

LG11 6045—6143 P2ch2 84.56 0.07 0.01 9.11 6.08 0.17 -27.8 -27.0

LG27 4904—4953 P2ch2 95.28 0.27 0.01 0 3.90 0.54 -29.4 -26.1

Fig. 7. Reserves of natural gas and coal-derived gas of Late Paleozoic, Mesozoic and Cenozoic from 2004 to 2013 in China.

Fig. 8. Annual output of natural gas and coal-derived gas of Late Paleozoic, Mesozoic and Cenozoic from 2004 to 2013 in China.

volcanic rocks, forming the gas to accumulate in Ordovician, Carboniferous, Permian, and the Paleogene System, which are distributed in Ordos Basin, Bohai Bay Basin, Junggar Basin, and Sichuan Basin. A total of 16 gas fields have been found, and 12 of them for large gas fields.

Based on 99 samples of gas component and alkane gas carbon isotopic composition, using S13C2 > —28.5% and S13C1 -S13C2 -S13C3 identification chart to identify, the gas sources of gas fields above are all coal-derived gas.

Late Paleozoic coal related gas field is of great significance to China's gas industry rapid development: first, it is accounting for one-third of China's natural gas proven total geological reserves and annual production at the end of 2013; second is three (Sulige, Jingbian and Daniudi) of the five key large fields, which support China to be a gas power, with the gas source as coal-derived gas; third is the average reserves of the late Paleozoic coal related gas fields and large gas fields, being 5.3 times and 1.7 times of the national gas fields and large gas fields, indicating that the late Paleozoic coal-related gas field and large gas fields have a greater contribution to the gas industry's continued development.

Foundation item

Supported by Project of Major Projects of China National Petroleum Corporation (2014B-0608); China National Science & Technology Major Project (2016ZX05007-001).

Conflict of interest

The authors declare no conflict of interest. References

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