Subscriber access provided by Georgetown University | Lauinger and Blommer Libraries
Fossil Fuels
Source, age and evolution of coal measures water in Central-South Qinshui Basin, China Haichao Wang, Xuehai Fu, Xiaoyang Zhang, Qinghe Niu, Yanyan Ge, Jijun Tian, Xiaoqian Cheng, Ning Chen, Xiaolin Hou, and Hua Du Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00701 • Publication Date (Web): 04 Jun 2018 Downloaded from http://pubs.acs.org on June 4, 2018
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
1
Source, age and evolution of coal measures water in
2
Central-South Qinshui Basin, China
3
Haichao Wanga, Xuehai Fu*b, Xiaoyang Zhangb, Qinghe Niub, Yanyan Gea, Jijun Tiana, Xiaoqian Chenga,
4
Ning Chenc, Xiaolin Houc, Hua Duc
5
a Institute of Geology and Mining Engineering, Xinjiang University, Urumqi, Xinjiang 830047,
6
China;
7
b Key Laboratory of Coalbed Methane Resources and Reservoir Formation Process, Ministry of
8
Education, China University of Mining and Technology, Xuzhou, Jiangsu 221008, China
9
c State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese
10
Academy of Sciences, Xi’an 710075, China
11
Abstract: Groundwater is one of the important factors to control the accumulation and
12
exploitation of coal measures gas. In this work, the water source identification method based on
13
hydrochemistry, stable isotope,
14
evolution of coal measures water in Central-South Qinshui Basin were clarified. The results reveal
15
that the hydro-geological environment of coal measures water in Carboniferous-Permian is
16
between semi-close and open, with free water exchanging. The coal measures water in Guxian
17
block and Shizhuangnan block are Ca-HCO3 and Na-HCO3 types, respectively, while the closed
18
coefficients are 1.77 and 322.75, respectively. Therefore, the water is attributed to river water or
19
shallow groundwater in Guxian block and deep groundwater in Shizhuangnan block. The age of
20
coal measures water is 1.51~20.61 Ma, which indicates that the water in coal measures at the
21
present stage is the mixture of a litter paleo sedimentary water and massive modern meteoric water,
22
and the modern meteoric water recharge is lasted until 1950. Above achievements deepen the
23
understanding of the coal measures reservoir type, and also guide the optimal selection and the
24
co-exploration and co-exploitation of coal measures gas.
25
Keywords: coal measures gas, Qinshui Basin, source of groundwater,
26
evolution
27
1. Introduction
28
129
I and 14C dating was first established, then the source, age and
129
I,
14
C, hydrochemistry,
Coal measures gas mainly includes coalbed methane, shale gas, tight sandstone gas, etc. 1.
ACS Paragon Plus Environment
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 2 of 36
29
The migration of coal measures water is closely related to that of coal measures gas. Specially, the
30
source, age and evolution of coal measures water directly reflect the preservation and loss of coal
31
measures gas, which is significance to the enrichment and accumulation of coal measures gas.
32
Based on the characteristics of groundwater chemistry and stable isotope, a series of studies
33
have been conducted on the water quality type, source and storage condition of groundwater.
34
According to the investigation about chemical reactions controlling the chemistry of groundwater
35
in an isthmus lying between Crystal Lake and Big Muskellunge Lake, northern Wisconsin, Kim 2
36
revealed that other reactions or processes such as cation exchange can also regulate groundwater
37
chemistry characteristic besides of mineral dissolution. Fynn et al.
38
evolution of groundwater in parts of the Nabogo catchment of the White Volta Basin in Ghana,
39
and suggested an evolutionary model and the mode of fluid fluxes. Huang et al.
40
sources of groundwater recharge in an arid area in northwest China, aiding in water resources
41
management and groundwater inrush prevention in the coalfield and at other coal mines. Now the
42
approaches to measure the groundwater age mainly include simulating groundwater age by solute
43
transport model 5, tracer technology of particles with opposite directions 6, CFCS and SF6 dating 7
44
and radioisotope dating 8. Groundwater radioisotope dating begins in the 1950s. With the
45
development of isotope analysis technology, radioisotope dating has gradually become an
46
important method to measure the groundwater age. Comparing with other dating methods, it
47
possesses a larger measuring range, longer application time and wider application field. In this
48
paper, 129I and 14C radioisotope dating are mainly adopted to judge the age of coal measures water.
49
In all iodine isotopes, 129I is the only one with a long-lived radioisotope. It possesses a
50
half-life 15.7 Ma and a maximum dating value of ~ 80 Ma 9. There are three main sources of 129I:
51
(1) cosmogenic
52
fissiogenic
53
129
54
129
3
evaluated the source and
4
studied the
129
I is produced by the spallation of Xe isotopes into the atmosphere; (2)
I is produced by spontaneous fission of
238
U in the Earth's crust; (3) anthropogenic
I originated from nuclear weapons testing and fuel processing since the 1950s 10. Since 1980s, as a tool for dating and tracing, 129I is used in studying the origin and evolution
55
of formation water. Fabryka-Martin et al. 11 analyzed the
129 127
56
granite, Sweden, and put forward the correction method of I age. Fehn et al. 9 measured 129I/127I of
57
the pore water at Blake Ridge in the Atlantic Ocean and determined the source and age of the pore
I/ I of groundwater in the Stripa
ACS Paragon Plus Environment
Page 3 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
12
water. Snyder et al.
59
of water in the main coal seam of San Juan Basin. Chen et al. 10 studied the age and source of brine
60
in an Ordovician paleokarst reservoir in the Tarim Basin to reveal the source of hydrocarbons. Ma
61
et al.
62
seam of Permo-Carboniferous in Hancheng coalbed methane field of Ordos Basin and southern
63
Qinshui Basin by combining hydrochemical and stable isotope characteristics, respectively. Ge
64
discriminated the I isotope age of the coal measures water in Zhuzang syncline of western
65
Guizhou province as 17.28 Ma, which is far younger than the reservoir age.
13
14
66
applied
129
58
and Wei and Ju
14
I to coalbed methane field, and identified three different sources
discussed the origin, age and evolution process of water in main coal
15
C, as a radioactive atom of carbon, is mainly generated by nuclear reactions between the
67
thermal neutron (n) and 14N that produced when cosmic rays enter into atmosphere (see Eq. 1). It
68
possesses a short half-life of 5730 ± 40 years and a determination upper limit of 5 × 104 years. The
69
production rate of 14C, is influenced by geomagnetic field, solar activity and CO2 concentration 16. 14
70
7N
+ 10n = 146C + 11H
(1)
In 1949, for the first time, 14C dating method was established by W. F. Libby 17. Currently, it
71
18-22
. In 1957, 14C dating was employed to
72
is widely used in geology, archaeology and art fields
73
determine the groundwater age. Since then,
74
widely used and mature methods in ancient groundwater dating. Bath et al. 23 studied the Triassic
75
Bunter sandstone aquifer in the eastern England, using radioactive carbon for determining the age
76
of groundwater in a relatively simple geochemical condition. Iwatsuki et al. 24 determined the
77
source of groundwater in the sedimentary rocks at the Tono study area, central Japan by
78
2001, and evaluated its hydraulic conditions. Huang et al. 25 used 14C residence time estimates for
79
determining the sources of groundwater recharge in the Jiaozuo coal-mining district, China.
80
14
C dating gradually has become one of the most
14
C in
Previous researches mainly identified the source, origin and evolution laws by ion 2-4, 26-30
81
characteristic, stable isotopic feature and hydrogen radioisotope dating
82
accurately ascertain the origin and age of coal measures water because the half-life of hydrogen
83
radioisotopes is quite short (only 12.43 years). In this study, by combining water types, salinities,
84
ions with stable isotopes characteristic of coal measures water, and simultaneously introducing a
85
long half-life radioisotope
86
14
, but it cannot
129
I (dating range from 0 to 80 Ma) and a short half-life radioisotope
C (dating range from 0 to 5×104 a), the source, age and evolution of coal measures water in
ACS Paragon Plus Environment
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 4 of 36
87
study area were investigated more precisely. The age of coal measures water can directly reflect its
88
storage condition, which is closely related to preservation of coal measures gas. Therefore, the
89
investigation in this work will strengthen the understanding of enrichment law and enrichment
90
process of coal measures gas, and provide guidance for the exploration and development of coal
91
measures gas.
92
2 Geological background
93
Qinshui Basin, located in the southeastern Shanxi Province, is one of largest coalbed methane
94
reservoirs and the first commercial CBM-producing basin in China 31. The study area is situated in
95
the central and southern Qinshui Basin, bordered to the west by Huoshan uplift, to the east by
96
Jin-huo major fault, to the south by Henghe fault, and to the north by Xiangyuan - Huozhou. It
97
tectonically belongs to the southern zone of Qinshui synclinorium and is monoclinal with
98
northwest inclination. The regional structure and the evolution of coal seam are controlled by
99
Jin-huo major fault, the eastern boundary of the study area 32, 33. The middle Sitou faults are closed 34
100
faults, which take an important role on the accumulation of coalbed methane
101
structures in the study area are a series of low angle secondary folds, bordered to Sitou faults. In
102
the east, the secondary folds are developed with axis of SN, contrarily, in the west, folds are
103
mainly growth with axis of NNE (Fig. 1).
. The main
104 105
Fig. 1 Map of structual outlines in Qinshui basin (modified from Qin et al. 35 and Zhang et al. 36).
106 107
The strata of the Carboniferous and Permian include the Benxi, Taiyuan, Shanxi Formations
108
in ascending order, among which the Taiyuan and Shanxi Formations are main coal measures
109
strata with average thickness of 150 m (Fig. 2). The upper main coal seam (No. 2 coal seam in
110
Guxian block and No. 3 coal seam in Shizhuangnan block) and lower main coal seam (No. 9+10
111
coal seam in Guxian block and No. 15 coal seam in Shizhuangnan block) are developed in the
112
Shanxi and Taiyuan Formations, respectively. The upper main coal seam and lower main coal
113
seam are developed with considerable thickness in entire region. The upper main coal seam is
114
grown in the early Permian, which thickness ranges from 2.15 to 8.66 m, with an average of 5.79
115
m. It is sandwiched by 1~3 layers kaolinite or carbonaceous mudstone, and the roof and floor of
ACS Paragon Plus Environment
Page 5 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
116
the coal seam are always composed of mudstone or silty mudstone. The lower main coal seam is
117
developed in the late Carboniferous, which thickness varies from 1.10~9.87 m, with an average
118
thickness of 3.26 m. It is sandwiched by 1~6 layers mudstone and carbonaceous mudstone. The
119
roof of the coal seam is K2 limestone, which is developed in the whole area, and the floor of the
120
coal seam is mainly composed of mudstone and carbonaceous mudstone.
121
According to the aqueous medium characteristics, bottom-up main aquifers are divided into
122
four types: Ordovician fracture-karst aquifer, Carboniferous fracture-karst aquifer, Permian
123
clasolite fracture-karst aquifer and Quaternary loose sediment pore aquifer, among which
124
Ordovician fracture-karst aquifer is the main aquifer in study area, the water-abundance of
125
Permo-Carboniferous aquifer and Quaternary loose sediment pore aquifer are weak-medium and
126
medium, respectively. On the basin scale, there is no hydraulic connection among the aquifers
127
vertically. The main aquifuges from top to bottom are aluminum mudstone of Benxi Formation,
128
sandy mudstone, mudstone and coal seam of Taiyuan and Shanxi Formation and sandy mudstone
129
and mudstone of the middle-lower part of upper Shihezi Formation and lower Shihezi Formation.
130
Each aquiclude is tight and fractures are undeveloped, which blocks the vertical hydraulic
131
connection because of the poor permeability and thus makes the aquifers relatively independent 34,
132
35
.
133 134
Fig. 2 Stratigraphic column of coal measures strata in Central-South Qinshui Basin (modified from Zhang
135
et al. 36).
136 137
3 Methods and samples tests
138
3.1. Samples and tests
139
In this study, the water samples were collected from main coal seams of Guxian block and
140
Shizhuangnan block (CBM wells produced at least 2 years of the upper main coal seam and lower
141
main coal seam), and performed conventional ion tests (16 samples), hydrogen and oxygen isotope
142
tests (16 samples), 129I radioactive isotope samples (6 samples) and 14C radioactive isotope tests (6
143
samples) respectively. 500 mL clean plastic containers were used for sampling. Before sample
ACS Paragon Plus Environment
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 6 of 36
144
collecting, the plastic containers were flushed three times with target water. The containers were
145
filled in the whole plastic container to remove air. The container caps were carefully tightened.
146
After checking that the containers are not leaked, the containers were tagged sampling well.
147
Hereafter, they were immediately sent to the laboratory to ensure the accuracy and reliability of
148
the test results.
149
The conventional ion and hydrogen and oxygen stable isotope tests of water samples were
150
measured in State Key Laboratory of Environmental Geochemistry, China. All of the samples
151
were filtered via glass fiber filter membranes and stored at 4 oC in a refrigerator before analysis.
152
For cation analysis, the samples were acidided to a pH of < 3. The concentrations of Ca2+, Mg2+,
153
Na+ and K+ were analyzed by Vista MPX inductive coupled plasma emission spectrometer
154
(American Varia Company) and measured using standard methods
155
carbonate and bicarbonate were titrated with a Metrohm automatic titration apparatus and the
156
detection methods are according to People's Republic of China Geology and Mineral Industry
157
Standards: determination of carbonate, bicarbonate and hydroxide by titration (DZ/T
158
0064.49-1993). The concentrations of Cl-, SO42- were analyzed by ICS-90 ion chromatograph
159
(American Dionex Company) and the detection methods are according to People's Republic of
160
China Geology and Mineral Industry Standards: determination of chloride, fluoride, bromide,
161
nitrate, and sulfate by ion chromatography (DZ/T 0064.51-1993). Stable isotope compositions
162
were measured by the Liquid Water Isotopes Analyzer (Model: 912-0026, American Los Gatos
163
Research Company) with analytical precision of < ±0.1‰ for δ18O and < ±0.3‰ for δD. The δ18O
164
and δD values are expressed with respect to standard mean ocean water (VSMOW). The detection
165
methods of δD and δ18O are according to People's Republic of China Geology and Mineral
166
Industry Standards: determination of hydrogen isotope by zinc reduction method (DZ/T
167
0184.19-1997) and determination of oxygen isotopes in natural water by carbon dioxide - water
168
balance method (DZ/T 0184.21-1997), respectively. Water temperature, pH and dissolved oxygen
169
(DO) were determined by a thermometer, digital pH and dissolved oxygen meter.
170
129
I and
14
37
. The concentrations of
C radioactive isotopes of coal measures water were measured in the Xi’an
171
Accelerator Mass Spectrometry (AMS) Center, used a 3 MV Tandetron AMS (HEVV, The
172
Netherlands). The precision of this instrument for measuring 129I standard sample is 1.7% and the
ACS Paragon Plus Environment
Page 7 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
129 127
I/ I is 2×10-14. The detection accuracy of 14C/12C can reach 0.2%, and
173
detection accuracy of
174
the sensitivity is up to 10-12 38, 39.
175
3.2 Identification methods
176
The procedures of 129I radioactive isotope test are as follows: (1) Before the test, the water
177
samples are filtered through a cellulose filter to remove the suspended particle matter; (2) The
178
prepared water samples are weighted to a beaker, 2.5 mL of 1 mol/L NaHSO3 solution and 200Bq
179
of 125I solution, as a tracer, are added; (3) HNO3 is added to adjust PH to 2; (4) Transfer the water
180
sample to a separation funnel, 30 mL CCl4 and 4 mL of 1 mol/L NaNO2 solution are added to
181
oxidize iodide to I2; (5) Shake the separation funnel is shook and make the iodide ion extract into
182
pink or purple CCl4 organic phase; (6) After transferred it to a new beaker, 15 mL CCl4 was added
183
to extract the remaining I2; (7) Combining the CCl4 organic phase, and transferred it into a new
184
separation funnel, 10 mL water and 1 mL of 1 mol/L NaHSO3 solution are added to reversely
185
extract I2 to iodide; (8) 1 mL of 1mol/L AgNO3 is added to precipitate iodide as AgI, and then the
186
AgI is separated by Centrifugation and the separated AgI is dried at 60 oC for 2-3 h; (9) 3 times
187
(by weight) niobium powder is weighted and mixed with the prepared AgI powder, and then the
188
mixture is pressed into a copper holder for testing.
189
The procedures of 14C radioactive isotope test are as follows: (1) Before the test, the water
190
samples are filtered through a cellulose filter to remove the impurities, and then the filtrate is
191
collected directly in the conical flask; (2) About 25 mL concentrated phosphoric acid is added in
192
the upper erlenmeyer flask (over-dose), and the erlenmeyer flask is linked to vacuum and then
193
close the valve, allowing the concentrated phosphoric acid to drop into the erlenmeyer flask and
194
respond fully with the water samples for several minutes. (3) Cold trap method is used to purified
195
in the purified vacuum system, and CO2, which is produced by the reaction, is collected
196
quantitatively, i.e., DIC (dissolved inorganic carbon) in water. (4) The collected CO2 is reduced to
197
graphite by Zn/Fe method in the graphite target preparation vacuum system, and then the graphite
198
was pressed to a target sample for testing.
199
4 Results and discussion
ACS Paragon Plus Environment
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
200
Page 8 of 36
4.1 Water type and TDS
201
Piper trilinear diagram is an effective method to study groundwater composition and water
202
type. The main advantage is that the water samples from different areas are marked on the same
203
figure, which can be used to analyze the evolution of groundwater chemical composition. The
204
total dissolved solids (TDS) equivalent to the total mass concentration of the major ions (Ca2+,
205
Mg2+, Na+, K+, SO42-, Cl-, HCO3-, and CO32-) minus half of the bicarbonate concentration 40, which
206
can be utilized to judge the groundwater hydro-geological environment and the storage conditions
207
of CBM. The water types are classified into four distinct zones: I, II, III and IV. The main
208
groundwater type of zone I is Ca-HCO3. Ca-HCO3 type water is typical freshwater 41-43, which is
209
generally inducted as surface water (e.g. river water) 43, 44 or shallow groundwater 45. Precipitation
210
is the predominant source of recharge to the ground-water flow system 46. It penetrates strata via
211
the primary pores or fractures. During the groundwater cycling, the dissolution of calcite or
212
precipitation maybe the source of Ca-HCO3 type water 47, 48. The main groundwater type of zone II
213
is Na-HCO3, and it is constituted by most freshwater and little brackish water, belonging to deep
214
groundwater. This kind of water is formed in hypoxia environment by the degradation effect of
215
organic matter extensively occurred in the stratum of study area. Besides, it can also be generated
216
through the dissolution of CO2 induced by thermal degradation of coal 40, the dissolution of alkali
217
metal-carbonate minerals and cation interchange (water-rock interaction), etc.
218
groundwater type of zone III is Na-Cl. TDS > 3000 mg/L means that this kind of groundwater is
219
salt water. It is generated by the dissolution of salt rock or massive sea water intrusion. Most of the
220
water in this zone is sea water, salt water or hot water 50. The main groundwater type of zone IV is
221
Ca-Mg-SO4-Cl. With TDS of 1000 - 3000 mg/L, the groundwater in this area is mixed with sea
222
water and has no dominant cation and anion. Therefore, water in this zone is mainly mixed by
223
groundwater and seawater 51.
49
. The main
224
Fig. 3 shows that the water samples of Guxian block fall in zone I, which indicates that the
225
type of coal measures water is Ca-HCO3, meaning that it belongs to river water or shallow
226
groundwater. The water samples of Shizhuangnan block fall in zone II, which suggests that the
227
type of coal measures water is Na-HCO3, meaning it belongs to deep groundwater. The TDS of
228
groundwater in closed storage condition is higher because it is well-protected, otherwise, the TDS
ACS Paragon Plus Environment
Page 9 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
229
of formation water decreases once it is infiltrated by atmospheric precipitation or surface water. In
230
the study area, the TDS of coal measures water ranges from 288.81 to 859.41 mg/L (with an
231
average value of 658.37 mg/L), which is far lower than that of seawater (35000 mg/L). This
232
indicates that in geological evolution process, because of the infiltration effect of surface water
233
induced by fault opening or overlying strata denudation, coal measures water lives in the semi
234
closed - open hydro-geological environment, which is between the open and closed storage
235
condition and contains a given mass of free alternate water 52.
236 237
Fig. 3 The Piper diagram of hydrochemical composition in coal measures water (G1-G3 are from Guxian
238
block; S1-S11 are from Shizhuangnan block; DB1-DB2 are from the surface of Shizhuangnan block).
239 240
Ca2+, Mg2+ and SO42- enrichment means that the water is close to the recharge area of
241
oxygen-enriched environment, while Na+, K+, Cl- and HCO3- enrichment is often regarded as
242
reducing environment, which is far away from recharge area, and the residence degree is increased
243
53
244
storage condition:
. Based on the ion composition, Guo 54 proposed a closed coefficient to evaluate the groundwater
Closed coefficient = (n Na++ n K++ n HCO3-)/(n Ca2++ n Mg2++ n SO42-)
245 246
(2)
where nX is the mass concentration, with the unit of mg/L.
247
The higher the closed coefficient is, the better the closed storage condition of groundwater is.
248
The closed coefficients of coal measures water are generally high in Shizhuangnan block, most of
249
which are above 110, with an average of 322.75. It indicates that the sealing degree of the
250
groundwater storage condition is high. The closed coefficients of coal measures water in Guxian
251
block are low (0.97~3.14, with an average of 1.77). It means that the sealing degree of the
252
groundwater storage condition is lower.
253
Ratio of Ca2+ to Mg2+ (ρCa2+/ρMg2+, where ρX is the equivalent concentration of material)
254
represents the metamorphic degree of groundwater. Groundwater metamorphoses more adequately
255
with a longer sealing time, which signifies a better sealing performance and can be reflected in this
256
ratio
257
3.31~65.96, with an average of 14.12; Ca2+/Mg2+ ratios of the coal measures water in Guxian
55
. Ca2+/Mg2+ ratios of the coal measures water in Shizhuangnan block are between
ACS Paragon Plus Environment
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 10 of 36
258
block are between 0.90~4.42, with an average of 2.39. Apparently, the sealing performance of
259
groundwater storage condition in Shizhuangnan block is better than that in Guxian block.
260
Closed coefficient is computed by the ionic concentration of groundwater, which in this paper
261
is adopted to characterize the closed degree of groundwater storage condition. Closed coefficient
262
of Shizhuangnan block > Guxian block approves that the sealing degree of coal measures water in
263
Shizhuangnan block exceeds that in Guxian block. Ca2+/Mg2+ ratio depicts the inspissation and
264
exchange-adsorption effects of cations, the high-strength and long-term dolomitisation reduces the
265
Mg2+ content of formation water and therewith increases the Ca2+/Mg2+ ratio, which thus advances
266
the closure property and prefers to accumulate oil and gas
267
Shizhuangnan block than Guxian block reconfirms the fact that Shizhuangnan block possesses a
268
stronger closure property. In brief, based on the results of closed coefficient and Ca2+/Mg2+ ratio,
269
coal measures water in Guxian block is in open storage condition, contrarily, coal measures water
270
in Shizhuangnan block is in closed storage condition. And the coal measures water in the study
271
area is between the closed and open hydro-geological environment.
272
4.2 Ion characteristics
55, 56
. The higher Ca2+/Mg2+ ratio of
273
In study area, coal measures water mainly contains Na+, HCO3- and Cl-, followed by Ca2+,
274
Mg2+, K+, SO42- and F- (Table 1). Besides, other trace elements (Li, Ga, Rb, Sr, Ba, etc.) are also
275
included in it.
276
The further analysis of ionic concentration indicates that the coal measures water type of
277
Guxian block is mainly Ca-HCO3, and the concentrations of Ca2+, HCO3- and SO42- exceed that of
278
Na+, Cl- and Mg2+. The average concentrations of Ca2+ and HCO3- are respectively 112.11 mg/L
279
and 388.33 mg/L, respectively. The higher Na+ and SO42- concentrations of Guxian block may be
280
related to the evaporation and dissolution of sulfate minerals 40. The coal measures water type of
281
Shizhuangnan block is Na-HCO3, and the average concentrations of Na+ and HCO3- are 294.59
282
mg/L and 647.76 mg/L, respectively. The additional high HCO3- concentration may be the
283
dissolution of CO2 produced by the thermal degradation or biodegradation of organic matter
284
(Fig. 4).
285 286
Fig. 4 The box chart of ion concentration in coal measures water of study area.
ACS Paragon Plus Environment
40
Page 11 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
287 288
Iodide concentrations of the coal measures water in Guxian block and Shizhuangnan block
289
are quite low (ranging from 0.28 µg/L to 9.21 µg/L, with an average of 3.25 µg/L), which is far
290
lower than the seawater (55.88 µg /L) 57. It is speculated that this area is generally influenced by
291
the infiltration of atmospheric precipitation 13. The change and reaction of formation water can be
292
deduced by the concentration relationship of Cl ion and I ion. I- concentration is increased by the
293
diagenesis in geological period without affecting the Cl- concentration. Then under the influence
294
of diagenesis, the concentration relationship diagram of I- and Cl- shows a vertical upward trend.
295
The decrease of both Cl- and I- concentration is caused by mixed-dilution effect through surface
296
water infiltration. However, at locally, there is no obvious evolution law of the ratio of I to Cl. As a
297
whole, concentration distribution shifts to the lower left direction along the I/Cl line in the Fig. 5.
298
Despite the concentrations of Cl- and I- in atmospheric precipitation are low, the I/Cl value is fixed.
299
Therefore, the mixed-dilution effect induced by surface water infiltration causes the decrease of
300
Cl- and I- concentration but does not change I/Cl value momentously
301
Shizhuangnan block, I/Cl ratios of coal measures water are between 8×10-7~1×10-4, dropping in
302
the mixed-dilution effect area, and exhibiting the shifting phenomenon to the lower left direction
303
along the I/Cl line. This shows that the coal measures water has experienced the mixing effect
304
with surface water (Fig. 5).
12, 13
. In Guxian block and
305 306
Table 1 The geochemistry analysis results of water from coal measure in study area.
307 308
Fig. 5 The concentration relationship of Cl ion and I ion of coal measures water (modified from Snyder et al. 12
309
).
310 311
4.3 Stable isotope characteristic
312
The stable hydrogen and oxygen isotopes of formation water are important means for
313
understanding the origin and formation of groundwater, and the change and migration law of
314
groundwater chemical composition in source field 52. δD of coal measures water in the study area
315
ranges from -83.69‰ to -66.42‰, with an average of -78.25‰, while δ18O varies from -11.78‰
ACS Paragon Plus Environment
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 12 of 36
316
to -8.91‰, with an average of -10.83‰ (Table 1). The values are all in the range of composition
317
range of hydrogen and oxygen isotopes in Chinese atmospheric precipitation.
318
The global meteoric water equation is δD=8δ18O+10, and the Chinese meteoric water
319
equation is δD=7.9δ18O+8.2. From the relationship charge of δD and δ18O (Fig. 6), the points of
320
δD and δ18O distribute near or slightly below the global meteoric water line (GMWL) and Chinese
321
meteoric water line (CMWL). It reflects that the original source of coal measures water is
322
atmospheric precipitation, namely, the coal measures water is recharged by atmospheric
323
precipitation. Experiencing the tectonic uplift in geologic history, surface water infiltrated the coal
324
seam along the fault or dredging layer, accompanied by the fault opening and formation
325
denudation. This effect facilitates the mixed-dilution reaction of and raw rock water and surface
326
water, which appearance is consistent with the analysis of halogen ion. Evaporation and the
327
mixture effect of fresh water with brine all incline to cause the slight deviation of Chinese and
328
global meteoric water lines 13, 58.
329
The hydrogen and oxygen isotopes of surface water samples distribute in the upper right of
330
Fig. 6 along meteoric water lines. This is because surface water has been subjected to strong
331
evaporation effect for a long time, the lighter δ16O is more easily evaporated than δ18O, and
332
thereby heavy 18O and D enrichment is appeared 27, 59.
333 Fig. 6 Relationship between δD and δ18O of coal measures water.
334 335 336 337
4.4 129I features and age During the deposition and burial process, the
129
I of the surface water carried in sinking
338
stratum decays gradually. The exact age of formation water can be obtained by the standard decay
339
equation of
340
equation of 129I can be expressed as:
129
I, using the radioactivity level of present
129
I in the formation water. The decay
Rreal =Ri e -λ129 t
341
(3)
342
where Rreal is the corrected 129I/127I ratio; Ri is the initial value of 129I/127I (Ri=1.50×10-12) 9; λ129 is
343
the decay constant of
344
beginning up to now, which is equal to the time of strata carrying surface water.
129
I (λ129=4.41×10-8/a); t is the time that the original
ACS Paragon Plus Environment
129
I decayed from the
Page 13 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
345
The observed ratios of 129I/127I (Robs) of coal measures water in the study area fall in the range
346
of 6.63~806.00×10-12 (Table 2), which are all greater than the initial value of 1.50×10-12. However,
347
they are 1 ~ 3 orders of magnitude lower than the
348
Apparently, the contributions of other causes (fission cause) for 129I in geological evolution cannot
349
be ignored. In this study, the influence of fission cause for 129I must be eliminated so as to obtain
350
an accurate outcome 13, 60.
129
I of current atmospheric precipitation.
351 Table 2 The observed values, corrected values and ages of 129I/127I in water from coal measures.
352 353
In the burial process of surface water,
354 355
rock, leads to the increase of measured
356
water 13.
129
I, produced by the fission of 238U in surrounding
129 127
I/ I. This will overestimate the age of the formation
Under natural condition, the amount of 129I produced by 238U fission is:
357
N129 = N238λsf Y129ερ[(1-φ)/φ](1- e-λ129 t) /λ129
358
(4)
359
where N129 is the amount of 129I produced by fission (atoms/L); N238 is the amount of 238U in rock
360
(atoms/kg); λsf is a fission constant of
238
361
238
62
362
density of coal (cm3/g); φ is effective porosity (%); λ129 is a decay constant of 129I (4.41×10-8/a); t
363
is the interaction time between fluid and coal, which is also the time of fluid being closed or
364
isolated.
U when the quality is 129 (3×10-4)
U (8.5×10-17/a) 61; Y129 is a spontaneous fission yield of
; ε is escape efficiency from mineral lattice to fluid; ρ is
365
The average amount of U in the coal is usually 2.8 ppm (7.08×1018 atoms/kg), and the
366
effective porosity is often 0.01 63. Escaping coefficient is the percentage of a particular radioactive
367
isotope releasing or entering the fluid in mineral or its maceral. Usually, the escaping coefficient
368
129
I in the coal is 0.006 63. The true 129I/127I ratio (Rreal) of the formation water is obtained by subtracting the contribution
369 370
amount of
371
7):
I generated by
238
U spontaneous fission from the observed
Rreal =(Robs N127-N129)/ N127
372 373
129
Let
ACS Paragon Plus Environment
129 127
I/ I ratio (Robs) (Fig.
(5)
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
374 375
K=N238λsfY129ερ[(1-φ)/φ]/λ129
(6)
Simultaneous equations (3), (4), (5) and (6): t=ln[(K-RobsN127)/ (K-RiN127)]/(- λ129)
376 377
Page 14 of 36
After correcting
(7)
129 127
I/ I and calculating the age of coal measures water in the study area 129 127
I/ I are between 0.60×10-12
378
according to the above method, we found that the true values of
379
and 1.40×10-12, which are all lower than the initial values of
380
ages of the coal measures water range from 1.51 Ma to 20.61 Ma, which is exactly in Himalayan
381
period.
129 127
I/ I (1.5×10-12). The calculated
382 Fig. 7 The constitution of 129I in formation water.
383 384 385
The age of coal measures water in the study area is generally much younger than the strata
386
age (Fig. 8). It indicates that this water is not the original depositional water formed in the same
387
period of stratum. Two situations can be concluded: ① The present coal measures water is
388
recharged by the younger ancient atmospheric precipitation (1.51~20.61 Ma); ② The present
389
coal measures water is constituted by a very small amount of primitive ancient water mixed with a
390
large amount of modern water (surface water after the human nuclear activity since 1950).
391 Fig. 8 The comparison of 129I age in water from coal measures and formation age.
392 393 394
129 127
I/ I ratio of original depositional water and connate diagenetic water are basically
395
concordant and their ages are similar. After diluting by the present water, I concentration of the
396
samples decreases while
397
coal measures water can be identified by the comparison of 129I and I concentration. 129I/127I of the
398
diagenetic water is stable, which is distributed along the same age line in the identification map;
399
the surface water possesses a lower I concentration but the higher
400
located at the top left of the identification map; I and
401
precipitation are low, which are distributed in the lower left of the identification map; the mixed
402
water formed by modern water dilution develops the low I (
