Discovery of the Metal Trace Elements in Natural Gas and Its Ecological Environment Significance

EARTH SCIENCE FRONTIERS Volume 15, Issue 6, November 2008 Online English edition of the Chinese language journal Cite this article as: Earth Science Frontiers, 2008, 15(6): 124–132.

RESEARCH PAPER

Discovery of the Metal Trace Elements in Natural Gas and Its Ecological Environment Significance Wang Duoyi1, , Deng Meizhou2, Liu Yinghan3, Liu Yawei4, Li Xingyun1, Lu Renqi1 1 College of Energy Resources, Chengdu University of Technology, Chengdu 610059, China 2 Engineering Technology Institute of Southwest Petroleum Branch, SINOPEC, Deyang 618000, China 3 Institute of Geophysical and Geochemical Exploration, Chinese Academy of Geological Sciences, Langfang 065000, China 4 Exploration and Development Research Institution in Jianghan Oil Field, Qianjiang 433124, China

Abstract:

In order to conduct the ecological geochemical evaluation and the investigation of unconventional geochemical

exploration in natural gas reservoirs, the authors introduced the method of air-extracting for geogas survey for the first time and successfully collected the trace elements in natural gas. Fifty-one types of trace elements in the natural gas of west Sichuan are discovered by the method of plasma spectral analysis. These trace elements mainly appear in periods 4, 5, 6 of the Elements Periodic Table, and the attribute of them are alkali metals, alkaline-earth metals, rare metals, transition metals, main group elements, halogen, and nonmetals. The contents of metal trace elements (such as Zn, Ca, K, Na, P, Cu, Mg, Ba, Pb, Fe, and so on) make up 95% of the total. Among them, the content of Zn and Ca are the highest, making up 56.7% of the total; K, Na, P, and Cu are the secondary. This discovery not only plays an important role in the investigation of the origin, hydrocarbon source, reservoir-forming process, and utility of natural gas but also is of important significance for studying the ecological environment above the gas field. Key Words: natural gas; metal trace element; air-extracting geogas; plasma spectral analysis; agricultural ecological environment

1

Introduction

Natural gas is the general title of the gases with hydrocarbon-bearing and nonhydrocarbon in geological bodies, including C2-C5 hydrocarbon-bearing gases, N2, CO, H2S, He, Ar, etc. Finding out the compositions of natural gas can provide important basic data for determining the origin, migration, gas source contrast, and comprehensive utilization of natural gas. At present, an effective physicochemical separation technique for detecting natural gas compositions by gas chromatography is widely used[1]. According to the national standard(GB/T 13610–2003) of P.R.China stipulats that using GC as an method of detecting natural gas and the chemical compositions of analogous gas mixtures. Analysing and testing component of natural gas: helium, hydrogen, oxygen, nitrogen, carbon dioxide, methane, ethane, propane, butane, pentane and other 9 kinds of alkanes, sulfides, and the restructuring of sub-heptane, but collecting and detecting the

trace elements in natural gas cannot be achieved owing to the limits of sampling and detection methods. When conducting the ecological geochemistry evaluation of the important natural gas fields in Chengdu Economic Region, adsorption collection method for geogas survey is introduced by the authors for the first time to collect the trace elements in the natural gas, and 21 types of trace elements are successfully found out; thus, it achieves an important breakthrough[2]. Then, air-extracting collection method for geogas survey is introduced and 48 types of trace elements are found out. Altogether 51 types of trace elements are found out during the two detections in which 48 types are metal trace elements. At the same time, it is found out that the near-surface agricultural environment above the gas fields is affected to some extent. Applying the investigation results can provide a new way and important basis for determining the origin of natural gas, studying the migration, gas source contrast, and comprehensive utilization of natural gas and ecological environment.

Received date: 29-May-2008; Accepted date: 05-Sep-2008.

Corresponding author. E-mail: [email protected] Foundation item: Supported by the Project of China National Geology Survey Bureau (No. 200314200017). Copyright © 2008, China University of Geosciences (Beijing) and Peking University, Published by Elsevier B.V. All rights reserved.

WANG Duoyi et al. / Earth Science Frontiers, 2008, 15(6): 124–132

2

Geology of study areas

Luodai gas field in Longquanyi District of Chengdu City, Xinchang gas field in Deyang City, and Zhongba gas field in Jiangyou City are chosen to conduct the detection as 3 typical natural gas fields in west Sichuan. Each specimen is obtained from the different well when sampling. The specimens are distributed over the gas fields, i.e. 3 specimens are from Luodai gas field, 4 specimens are from Xinchang gas field, and 3 specimens are from Zhongba gas field. By now Xinchang gas field in Deyang City is the biggest one discovered from tight sandstone in China. It is composed of a gentle nose anticline in the west member of NEE uplifted zone in the middle part of west Sichuan depression. Xinchang gas field is of 8 reservoirs distributed vertically and superposed laterally, its basic characteristics are abundant hydrocarbon resources, high reservoir pressure, proper channels, good reservoirs, good traps, and shallow burial depth. The main production formation of Xinchang gas field is Penglaichen Formatoin of Upper Jurassic; Shaximiao Qianfoyan Formation and Xujiahe Formation are the secondary, all of them are continental clastic rock reservoirs. The two main production formations of Zhongba gas field are the member 3 of Leikoupo Formation of Middle Triassic System and the member 2 of Upper Xujiahe Formation. With the lithology of dolomite carrying gypsum dolomite and thin layer limestone, Leikoupo Formation is a set of carbonate deposit with limit shallow beach facies and lagoonal facies. With the pseudoassemblage covering above Leikoupo Formation, Xujiahe Formation is a set of coal formations mainly with sandy shale carrying thin coal layer and with marine-continental transition facies-continental facies, and its member 2 is mainly sandstone. With abundant reserve in its shallow layer and about 200 km2, Luodai gas field is composed of a burial anticline structure in the gently slope zone in east part of west Sichuan depression. Being fluvial lake facies deposition, the main gas-bearing bed of Luodai gas field is the red clastic rock of Penglaichen Formation of Upper Jurassic.

3 Techniques of collecting and detecting the trace elements in natural gas The trace elements in natural gas appear as nano-particles[3] and dissociate in natural gas. This is the main reason why it is difficult to capture them. Geogas survey technique developed at the end of last century is a method of capturing and detecting free nano-matters in soil. Geogas survey techniques of capturing free nano-matters in soil are air-extracting collection and adsorption collection[4,5]. At first, adsorption collection method is introduced by the authors to collect the natural gas sample from Xinchang gas field in Deyang City, Sichuan, and 21 types of nano-particle

metal elements are found out[2]. Geogas adsorption collection method is burying a collection unit into underground for some time (generally for a month), then taking out the adsorption plate and conducting analysis and detection by neutron activation method. It only takes 24 hours to collect the natural gas sample owing to great gas quantity and centralization, a favorable result is obtained. One shortcoming of adsorption collection method is that only the nature can be determined but not quantities. Another shortcoming is that some elements such as Cd, Pb, Hg cannot be detected. Hence, air-extracting collection method is adopted to collect the samples from several natural gas fields and detect them, and a great progress is obtained. By geogas air-extracting collection method, extracting and capturing the nano-matters drifted in soil gaps are carried out rapidly with a pressure-reducing unit method is rapidly extracting and capturing the nano-matters drifted in soil gaps with a pressure-reducing unit. Collection installation consists of spiral collecting sample unit, filter, catcher, and pressure-reducing unit. When collecting the nanomatters in natural gas, polythene tube replaces the spiral collecting sample unit (Fig. 1). Three percentage of nitric acid (mini2Q water + new activated high pure acid) is used as capture medium (adsorption liquid), and each unit links with high-quality gum tubes. The constant-quality elements in liquid sample are detected with plasma spectrum (ICP-OEC); and the trace elements are done with plasma mass spectrum (ICP-MS). It is verified that the detection result is reliable by this method for a long time.

4 Preliminary analysis of the detection result of trace elements in natural gas 4.1 Trace element detection result of natural gas in west Sichuan The trace element samples from 11 wells in 3 natural gas fields of west Sichuan are collected by air-extracting collection method. Through detection, 49 types of nano-particle trace elements are found out (Table 1). Among the 21 types of elements detected from the samples by adsorption collection method, except As and Br, the others (19

Fig. 1

Schematic diagram of collection installation of natural gas

WANG Duoyi et al. / Earth Science Frontiers, 2008, 15(6): 124–132

types of elements) are all the same as those detected from the samples collected by air-extracting collection method. Both methods not only confirm each other but also complement each other. Thus, including As and Br, by now totally 51 types of trace elements are detected from natural gas. 4.2 Component and distribution of the trace elements in natural gas Most of the trace elements detected from the natural gas in west Sichuan are in the 6th period of the Element Periodic Table, including the 21 types of elements of lanthanide, making up over 41% of the total amount. The content of the elements in 4th period is the secondary, making up over 27% of the total amount (Table 2). The chemical attribute of the trace elements in natural gas is mainly metals; but most of them are rare metals and transition metals, about 65% in the total amount. The content of alkali metals, alkaline-earth metals and main group elements is the secondary, about 10% for each. The rest are quasi-metals, halogen, and nonmetals(Table 3). Zn and Ca in the natural gas of west Sichuan are very abundant; their contents make up 56.7% of the total amount. The contents of K, Na, P, and Cu is the secondary, which make up 28.4%. The contents of Mg, Ba, Pb, and Fe make up 11%. About 20% of the total elements that are mentioned above approximately make up 95% of the total amount in natural gas. The distribution of trace elements in the natural gas of west Sichuan is uneven. The natural gas in Luodai gas field is of the richest trace elements. The amount of trace element in natural gas from the 3 wells in Luodai gas field makes up 56% of the total amount from all 11 wells in west Sichuan. The content of Pb, Sb, and Sc from the natural gas of Zhongba gas field in Jiangyou City is the highest in west Sichuan. The content of V from the natural gas of Xinchang gas field in Deyang City is of the highest value in west Sichuan. The highest values of other 45 types of trace elements in west Sichuan are from the natural gas of Luodai gas field. The average values of the trace elements from the natural gas of 3 wells in Luodai gas field are higher than the highest values of the trace elements from other natural gas fields (Table 1). The distributive differences among the trace elements in natural gas are probably relating to their gas sources. The natural gas of Xinchang gas field in Deyang city belongs to coal-formed gas and with almost no H2S. Its gas source is mainly Upper Triassic Xujiahe Formation[6]. With a high H2S content, although the natural gas reservoirs of Zhongba gas field in Jiangyou city belong to coaled gas reservoirs[7,8], the gas components are very different from those of Xinchang gas field. Owing to the developed Permian source rocks in the deep zones of the area and with a high evolution degree, the gas sources probably are related to Permian hydrocarbon sources. Luodai structure, controlled by buried fault, is generally a NE

wide-gentle nose arch located in the NE end of a NEE faulted zone of the middle member in west Sichuan depression. Although the natural gas of Luodai gas field is basically related to the coal-formed gas of Upper Triassic Xujiahe Formation[2], the special geological situation of deep developed fault is probably the important reason why the trace elements of the natural gas are so abundant. Hence, the diversities among hydrocarbon source, gas-generating conditions, and migration process probably cause the differences among the trace element content, even the attribution of natural gas. Especially, when gas-bearing structure is controlled by fault or is closely related to the fault; the trace element content of natural gas probably increases sharply. 4.3 Comparing the trace elements in natural gas with those in oil By now, over 60 types of trace elements in oil are found out, among which 45 types are metal trace elements. According to theirs chemical attribution, the trace elements can be divided into 3 types: (1) Valence-changing metals, such as V, Ni, Fe, Mo, Co, W, Cr, Cu, Mn, Pb, Ga, Hg, and Ti; (2) Alkali metals and alkali-earth metals, such as Na, K, Ba, Ca, Sr, and Mg; (3) Halogen and others, such as Cl, Br, I, Si, Al, and As. The trace elements of the natural gas in west Sichuan are similar to those of oil in the respects of quantity and attribution. Some trace elements in oil (like B, Ga, Ge, Se, Zr, In, Sm, Te, Hf, Ta, Re, Hg, etc) are not discovered in natural gas for the moment. The reason may be related to detection technique or capture liquid. Along with the improvements of detection technique and capture liquid, it is possible to find out more elements. But up to now, the trace element Sc and P found in natural gas have not been detected out from the oil (ash) in China. Judged by the range of the world, V content is the highest of all the trace elements in oil, with Ni content the secondary. The content of trace element V and Ni in the great majority oil of China makes up 50%–70% of the total trace element content, and the content of Ni is higher than that of V[9]. But the content of trace element Zn and Ca in the natural gas of west Sichuan is the highest, making up 56% of the total trace element content. Although the content of V and Ni is only about 10% of the total, the content of V is higher than that of Ni. At present, organic origin of oil and gas is universally recognized, i.e. oil and gas result from life matters. According to statistical data, the chemical elements kept in life matters are over 70 types. It is possible that all chemical elements in natural world are kept in life matters[10]. And life matters are the main source of the trace elements in oil and natural gas. The chemical element kinds and quantities of certain life matter are closely related to the geological and natural conditions. The trace element content difference between oil and gas is probably related to the origins and forming processes of oil and gas. The discovery of numerous trace

WANG Duoyi et al. / Earth Science Frontiers, 2008, 15(6): 124–132

Table 1 Trace elements of natural gas in part of gas fields in the west Sichuan ȡB/(ng·Lí1)

Location and numerical value types

Ag

Au

Ba

Be

Bi

Cd

Ce

Co

Cr

Cs

Cu

Dy

Total Average

0.018

0.001

18.23

0.009

0.065

0.087

0.322

0.148

0.995

0.035

30.25

0.027

Maximum

0.117

0.004

105.89

0.045

0.336

0.207

2.004

0.569

4.610

0.236

159.60

0.180

Minimum

0.000

0.000

0.84

0.001

0.005

0.018

0.029

0.019

0.000

0.005

1.38

0.003

Average

0.051

0.002

51.87

0.022

0.142

0.117

0.857

0.331

2.533

0.096

13.07

0.070

Maximum

0.117

0.004

105.89

0.045

0.336

0.207

2.004

0.569

4.610

0.236

20.79

0.180

Minimum

0.014

0.000

9.32

0.003

0.044

0.048

0.095

0.061

1.330

0.024

6.98

0.006

Luodai gas field

Zhongba gas field Average

0.002

3.45

0.003

0.022

0.056

0.064

0.045

0.337

0.008

4.51

0.006

Maximum

0.005

7.54

0.007

0.052

0.121

0.112

0.073

0.530

0.013

10.44

0.010

Minimum

0.000

0.84

0.001

0.005

0.018

0.029

0.019

0.000

0.005

1.38

0.003

Xinchang gas field Average

0.004

0.000

4.08

0.005

0.040

0.088

0.114

0.088

0.335

0.010

62.43

0.010

Maximum

0.013

0.001

5.18

0.008

0.065

0.117

0.141

0.109

1.340

0.012

159.60

0.012

Minimum

0.000

0.000

1.74

0.003

0.021

0.056

0.080

0.077

0.000

0.008

14.40

0.008

ȡB/(ng·Lí1)

Location and numerical value types

Er

Eu

Gd

Ho

La

Li

Lu

Mo

Nb

Nd

Ni

Average

0.016

0.014

0.035

Maximum

0.108

0.086

0.230

Minimum

0.002

0.001

Average

0.042

Maximum

0.108

Minimum

P

0.007

0.211

0.293

0.002

0.057

0.002

0.177

2.79

48.8

0.050

1.454

0.833

0.015

0.245

0.006

1.214

9.73

401.4

0.004

0.001

0.015

0.039

0.000

0.000

0.000

0.017

0.35

0.0

0.037

0.091

0.019

0.582

0.513

0.006

0.128

0.004

0.476

5.12

146.0

0.086

0.230

0.050

1.454

0.833

0.015

0.245

0.006

1.214

9.73

401.4

0.004

0.005

0.008

0.001

0.054

0.042

0.001

0.047

0.001

0.048

1.84

5.5

Average

0.003

0.003

0.007

0.002

0.035

0.147

0.001

0.005

0.001

0.033

1.05

5.4

Maximum

0.006

0.005

0.013

0.002

0.065

0.303

0.001

0.015

0.003

0.060

2.18

11.9

Minimum

0.002

0.001

0.004

0.001

0.015

0.039

0.000

0.000

0.000

0.017

0.35

0.0

Average

0.007

0.004

0.013

0.003

0.065

0.237

0.001

0.042

0.001

0.059

2.35

8.5

Maximum

0.008

0.005

0.016

0.003

0.084

0.316

0.001

0.057

0.003

0.073

3.70

16.0

Minimum

0.005

0.003

0.009

0.002

0.041

0.131

0.000

0.020

0.000

0.041

1.60

1.0

Sr

Tb

Th

Tl

Tm

U

Total

Luodai gas field

Zhongba gas field

Xinchang gas field

ȡB/(ng·Lí1)

Location and numerical value types

Pb

Pr

Rb

Sb

Sc

Sm

Average

18.97

0.049

0.383

0.047

0.265

0.033

4.40

0.006

0.005

0.002

0.002

0.018

Maximum

81.70

0.318

1.164

0.112

0.461

0.228

18.84

0.042

0.039

0.013

0.016

0.047

Minimum

0.96

0.004

0.072

0.006

0.025

0.004

0.63

0.001

0.000

0.000

0.000

0.005

Average

15.08

0.134

0.758

0.058

0.138

0.089

10.17

0.016

0.015

0.006

0.006

0.034

Maximum

25.59

0.318

1.164

0.084

0.153

0.228

18.84

0.042

0.039

0.013

0.016

0.047

Minimum

5.53

0.014

0.325

0.026

0.124

0.011

1.26

0.002

0.001

0.000

0.001

0.009

Average

2.90

0.009

0.193

0.012

0.172

0.006

2.51

0.001

0.000

0.000

0.000

0.015

Maximum

5.88

0.016

0.379

0.024

0.431

0.010

5.68

0.002

0.000

0.001

0.001

0.028

Minimum

0.96

0.004

0.072

0.006

0.025

0.004

0.63

0.001

0.000

0.000

0.000

0.008

Average

33.95

0.015

0.245

0.065

0.430

0.011

1.47

0.002

0.002

0.001

0.001

0.008

Maximum

81.70

0.019

0.310

0.112

0.461

0.014

1.63

0.003

0.004

0.001

0.001

0.011

Minimum

9.50

0.010

0.200

0.031

0.352

0.008

1.33

0.001

0.000

0.000

0.001

0.005

Total

Luodai gas field

Zhongba gas field

Xinchang gas field

WANG Duoyi et al. / Earth Science Frontiers, 2008, 15(6): 124–132 Continued ȡB/(ng·Lí1)

Location and numerical value types

W

Y

Yb

Zn

Al

Ca

Fe

K

Mg

Average

0.048

0.228

0.013

Maximum

0.161

1.628

0.081

200.2

7.96

161.5

525.1

27.09

600.7

19.0

36.0

13.1

93.1

124.5

54.6

Minimum

0.002

0.021

0.001

31.3

1.12

39.8

4.9

3.3

1.8

Average

0.114

Maximum

0.161

0.612

0.032

305.8

1.628

0.081

525.1

14.19

300.8

41.3

75.0

27.09

600.7

93.1

124.5

Minimum

0.039

0.043

0.004

77.7

3.24

78.1

11.2

Average

0.006

0.042

Maximum

0.009

0.079

0.003

60.9

3.24

67.2

0.005

114.4

6.42

103.0

Minimum

0.002

0.021

0.001

31.3

1.12

Average Maximum

0.030

0.080

0.007

225.5

0.071

0.107

0.012

439.0

Minimum

0.013

0.056

0.004

88.0

Mn

Na

Ti

V

2.36

65.7

1.14

3.45

14.02

324.1

3.56

5.26

0.31

3.7

0.00

1.35

29.8

6.00

161.8

2.44

3.87

54.6

14.02

324.1

3.56

4.11

29.9

7.2

0.71

37.0

0.24

3.39

6.1

15.8

4.2

0.63

24.2

0.30

4.24

7.7

36.5

7.2

1.00

55.5

0.65

5.26

39.8

4.9

3.3

1.8

0.31

3.7

0.00

3.13

6.83

127.8

12.1

22.0

7.2

0.93

24.8

0.80

2.54

9.60

165.0

14.0

31.5

9.0

1.11

34.8

1.48

3.54

5.40

87.0

9.8

14.1

6.0

0.65

18.5

0.00

1.35

Total

Luodai gas field

Zhongba gas field

Xinchang gas field

\

Table 2 Composition of trace elements of natural gas in the west Sichuan Element period

Chemical element components

2 3 4 5 6

Li, Be Na, Mg, Al, P K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Br Rb, Sr, Y, Nb, Mo, Ag, Cd, Sb Cs, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, W, Au, Tl, Pb, Bi Th, U

7

Table 3 Chemical attributes of trace elements of natural gas in the west Sichuan Chemical attribute Alkali metals Alkaline-earth metals Rare metals Transition metals Main group elements

Chemical element components Cs, K, Li, Na, Rb Ba, Be, Ca, Mg, Sr Ce, Dy, Er, Eu, Gd, Ho, La, Lu, Nd, Pr, Sm, Tb, Th, Tm, U, Yb Ag, Au, Cd, Co, Cr, Cu, Fe, Mn, Mo, Nb, Ni, Sc, Ti, V, W, Y, Zn Al, Bi, Pb, Tl, Sb

Quasi-metal

As

Halogen

Br

Nonmetal

P

elements in natural gas will produce an important influence on the researches of origin, hydrocarbon source, reservoirforming process, and utilization of natural gas.

5

Ecological environment significance

The metal trace elements in natural gas influence the agricultural ecological environment above natural gas field to certain extent or sometimes to serious extent, which mainly results in the increase in the content of metal element, especially heavy metal element in the crops, and thus reduces the quality of the crops of certain influence, sometimes even

Fig. 2

Organic geochemistry section of Xingchang gas field

serious influence, on the agricultural ecological environment above natural gas field, which mainly appears content increase

WANG Duoyi et al. / Earth Science Frontiers, 2008, 15(6): 124–132

of the metal element, especially heavy metal element in the crops, thus reducing the quality of the crops[2]. The metal trace elements in natural gas are the important material source of the metal element content increase, especially heavy metal content increase in the crops above the natural gas field. Some metal element abnormalities in the soil above natural gas field had been discovered for a long time[10–13]. Some researchers believe that the abnormalities are indirectly related to the leakage of caprock-penetrating hydrocarbon in natural gas reservoirs; some believe that the trace leakages of the hydrocarbons in natural gas reservoirs make the methane content become high in soil, and the trace elements in soil migrate thus to cause the metal elements overcontent; and others believe that the weak acidities of H2S and CO2 in natural gas reduce the pH value in soil thus releasing the metal elements with poison such as Zr[14]. The authors believe that it is probably the metal overcontent in the soil above natural gas field directly relating to the leakage of natural gas, and the mechanism of forming is identical with that of geogas abnormality or is similar to that of geogas abnormality. Geogas detection technique now is mainly applied to exploring of metal ore deposits[15–22]. It is discovered that the geogas sample from abnormal district is with lead of deep ore sulfide by researching the geogas matter source of Banlongzhang

Fig. 3

lead ore deposit in Gansu, which proves that the metal particles in the Earth’s crust can be carried to Earth’s surface by ascending air current to form the abnormality[23]. Geogas abnormality discovered by Tong Chunhan et al. Above Xuanhan gas field indirectly indicates the area with natural gas[24–26]. The geogas and metal trace element abnormalities in soil are discovered above Xinchang gas field in Sichuan (Fig. 2), and organic geochemical abnormality also exists in that area (Fig. 3). Geogas and metal trace element abnormalities in soil are basically identical with organic geochemical abnormality in the respects of shape and space distribution. Hydrocarbon leakages in oil and gas reservoirs and the organic geochemical abnormality formed above the reservoirs have been confirmed by several oil and gas geochemists[27–32], which is successfully applied to oil and gas exploration practices[33–37]. The identity of geogas abnormality, metal trace element abnormality in soil, and organic geochemical abnormality in the respects of shape and space distribution indicates that probably there is a close relationship among the abnormalities. The nano-particles produced continuously in the earth’s crust are carried to Earth’s surface by ascending air current thus to form the geogas abnormality in Earth’s surface[38–39]. The metal trace elements in oil and gas are nano-particles[3], whereas methane is the important composition of the

Characteristic of partial metallic trace elements of soil(left)and geogas(right) in Xingchang gas field

WANG Duoyi et al. / Earth Science Frontiers, 2008, 15(6): 124–132

ascending air current in the Earth’s crust. Hence, when the air current with hydrocarbons in deep oil and gas reservoirs are ascending, the metal trace element particles in the reservoirs is also being carried to the Earth surface. Furthermore, it is very difficult to avoid the leakages of natural gas in exploration wells and producing wells during gas exploration and development, which will directly carry the metal trace elements to the Earth’s surface[2]. After the metal trace elements are carried to the Earth’s surface, a part of them is at a standstill in soil crevices and pore (the part detected with geogas); a portion of them is adsorbed by clay[40]; some of them combines with the sulfur in natural gas to form new metal minerals and enrich in soil crevices[3]; the rest is probably dissolved in groundwater to increase the metal element content in soil (and groundwater) and reduce the quality of crops owing to absorbing overquantity metal elements. Hence, the discovery of the metal trace elements in natural gas is of important significance for studying the ecological environment, especially the agricultural ecological environment above natural gas field.

nano-size particle sulfur ores in the ground soil in Xinchang gas field and its significance. Natural Gas Industry, 2005, 25(11): 14–16. [4]

[5]

250–253. Tang J R, Yang Z F, Wang M Q, et al. The study of geogas anomal methods and its application. Geophysical and

[6]

Geochemical Exploration, 2004, 6(3): 193–197. Ye J, Wang L G. A discussion on the prospects for natural gas resources in west Sichuan in accordance with the geochemical properties of the bitumens found at Xinchang. Natural Gas Industry, 1999, 9(3): 18–32.

[7]

Conclusions

(1) The trace elements in natural gas can be collected by the methods of air-extracting collection and adsorption collection, but the former is better than the latter. (2) So far, 51 types of trace elements in the natural gas of west Sichuan have been discovered, and the attribute of them is alkali metals, alkaline-earth metals, rare metals, transition metals, main group elements, quasi-metal, halogen, and nonmetals. Most of them are rare metals and transition metals; alkali metals, alkaline-earth, and main group elements are the secondary. They mainly appear in the periods 4, 5, 6 of the Elements Periodic Table. (3) The contents of metal trace elements (such as Zn, Ca, K, Na, P, Cu, Mg, Ba, Pb, Fe) make up 95% of the total. Among them, the content of Zn and Ca are the highest, making up 56.7% of the total; K, Na, P, and Cu are the secondary. (4) Great differences are probably among the metal trace element contents in natural gas owing to the diversities of gas sources and geological conditions. (5) It is of important significance for studying the agricultural ecological environment above natural gas field with metal trace elements.

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