Source, age and evolution of coal measures water in Central-South

c State Key Laboratory of Loess and Quaternary Geology, Institute of Earth ...... 238U when the quality is 129 (3×10-4) 62; ε is escape efficiency f...
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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

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Energy & Fuels

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Source, age and evolution of coal measures water in

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Central-South Qinshui Basin, China

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Haichao Wanga, Xuehai Fu*b, Xiaoyang Zhangb, Qinghe Niub, Yanyan Gea, Jijun Tiana, Xiaoqian Chenga,

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Ning Chenc, Xiaolin Houc, Hua Duc

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a Institute of Geology and Mining Engineering, Xinjiang University, Urumqi, Xinjiang 830047,

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China;

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b Key Laboratory of Coalbed Methane Resources and Reservoir Formation Process, Ministry of

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Education, China University of Mining and Technology, Xuzhou, Jiangsu 221008, China

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c State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese

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Academy of Sciences, Xi’an 710075, China

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Abstract: Groundwater is one of the important factors to control the accumulation and

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exploitation of coal measures gas. In this work, the water source identification method based on

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hydrochemistry, stable isotope,

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evolution of coal measures water in Central-South Qinshui Basin were clarified. The results reveal

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that the hydro-geological environment of coal measures water in Carboniferous-Permian is

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between semi-close and open, with free water exchanging. The coal measures water in Guxian

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block and Shizhuangnan block are Ca-HCO3 and Na-HCO3 types, respectively, while the closed

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coefficients are 1.77 and 322.75, respectively. Therefore, the water is attributed to river water or

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shallow groundwater in Guxian block and deep groundwater in Shizhuangnan block. The age of

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coal measures water is 1.51~20.61 Ma, which indicates that the water in coal measures at the

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present stage is the mixture of a litter paleo sedimentary water and massive modern meteoric water,

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and the modern meteoric water recharge is lasted until 1950. Above achievements deepen the

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understanding of the coal measures reservoir type, and also guide the optimal selection and the

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co-exploration and co-exploitation of coal measures gas.

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Keywords: coal measures gas, Qinshui Basin, source of groundwater,

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evolution

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1. Introduction

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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.

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The migration of coal measures water is closely related to that of coal measures gas. Specially, the

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source, age and evolution of coal measures water directly reflect the preservation and loss of coal

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measures gas, which is significance to the enrichment and accumulation of coal measures gas.

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Based on the characteristics of groundwater chemistry and stable isotope, a series of studies

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have been conducted on the water quality type, source and storage condition of groundwater.

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According to the investigation about chemical reactions controlling the chemistry of groundwater

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in an isthmus lying between Crystal Lake and Big Muskellunge Lake, northern Wisconsin, Kim 2

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revealed that other reactions or processes such as cation exchange can also regulate groundwater

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chemistry characteristic besides of mineral dissolution. Fynn et al.

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evolution of groundwater in parts of the Nabogo catchment of the White Volta Basin in Ghana,

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and suggested an evolutionary model and the mode of fluid fluxes. Huang et al.

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sources of groundwater recharge in an arid area in northwest China, aiding in water resources

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management and groundwater inrush prevention in the coalfield and at other coal mines. Now the

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approaches to measure the groundwater age mainly include simulating groundwater age by solute

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transport model 5, tracer technology of particles with opposite directions 6, CFCS and SF6 dating 7

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and radioisotope dating 8. Groundwater radioisotope dating begins in the 1950s. With the

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development of isotope analysis technology, radioisotope dating has gradually become an

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important method to measure the groundwater age. Comparing with other dating methods, it

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possesses a larger measuring range, longer application time and wider application field. In this

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paper, 129I and 14C radioisotope dating are mainly adopted to judge the age of coal measures water.

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In all iodine isotopes, 129I is the only one with a long-lived radioisotope. It possesses a

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half-life 15.7 Ma and a maximum dating value of ~ 80 Ma 9. There are three main sources of 129I:

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(1) cosmogenic

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fissiogenic

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129

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129

3

evaluated the source and

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studied the

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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

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of formation water. Fabryka-Martin et al. 11 analyzed the

129 127

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granite, Sweden, and put forward the correction method of I age. Fehn et al. 9 measured 129I/127I of

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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

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water. Snyder et al.

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of water in the main coal seam of San Juan Basin. Chen et al. 10 studied the age and source of brine

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in an Ordovician paleokarst reservoir in the Tarim Basin to reveal the source of hydrocarbons. Ma

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et al.

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seam of Permo-Carboniferous in Hancheng coalbed methane field of Ordos Basin and southern

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Qinshui Basin by combining hydrochemical and stable isotope characteristics, respectively. Ge

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discriminated the I isotope age of the coal measures water in Zhuzang syncline of western

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Guizhou province as 17.28 Ma, which is far younger than the reservoir age.

13

14

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applied

129

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and Wei and Ju

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I to coalbed methane field, and identified three different sources

discussed the origin, age and evolution process of water in main coal

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C, as a radioactive atom of carbon, is mainly generated by nuclear reactions between the

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thermal neutron (n) and 14N that produced when cosmic rays enter into atmosphere (see Eq. 1). It

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possesses a short half-life of 5730 ± 40 years and a determination upper limit of 5 × 104 years. The

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production rate of 14C, is influenced by geomagnetic field, solar activity and CO2 concentration 16. 14

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7N

+ 10n = 146C + 11H

(1)

In 1949, for the first time, 14C dating method was established by W. F. Libby 17. Currently, it

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18-22

. In 1957, 14C dating was employed to

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is widely used in geology, archaeology and art fields

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determine the groundwater age. Since then,

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widely used and mature methods in ancient groundwater dating. Bath et al. 23 studied the Triassic

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Bunter sandstone aquifer in the eastern England, using radioactive carbon for determining the age

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of groundwater in a relatively simple geochemical condition. Iwatsuki et al. 24 determined the

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source of groundwater in the sedimentary rocks at the Tono study area, central Japan by

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2001, and evaluated its hydraulic conditions. Huang et al. 25 used 14C residence time estimates for

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determining the sources of groundwater recharge in the Jiaozuo coal-mining district, China.

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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

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characteristic, stable isotopic feature and hydrogen radioisotope dating

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accurately ascertain the origin and age of coal measures water because the half-life of hydrogen

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radioisotopes is quite short (only 12.43 years). In this study, by combining water types, salinities,

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ions with stable isotopes characteristic of coal measures water, and simultaneously introducing a

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long half-life radioisotope

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14

, but it cannot

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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

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study area were investigated more precisely. The age of coal measures water can directly reflect its

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storage condition, which is closely related to preservation of coal measures gas. Therefore, the

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investigation in this work will strengthen the understanding of enrichment law and enrichment

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process of coal measures gas, and provide guidance for the exploration and development of coal

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measures gas.

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2 Geological background

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Qinshui Basin, located in the southeastern Shanxi Province, is one of largest coalbed methane

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reservoirs and the first commercial CBM-producing basin in China 31. The study area is situated in

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the central and southern Qinshui Basin, bordered to the west by Huoshan uplift, to the east by

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Jin-huo major fault, to the south by Henghe fault, and to the north by Xiangyuan - Huozhou. It

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tectonically belongs to the southern zone of Qinshui synclinorium and is monoclinal with

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northwest inclination. The regional structure and the evolution of coal seam are controlled by

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Jin-huo major fault, the eastern boundary of the study area 32, 33. The middle Sitou faults are closed 34

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faults, which take an important role on the accumulation of coalbed methane

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structures in the study area are a series of low angle secondary folds, bordered to Sitou faults. In

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the east, the secondary folds are developed with axis of SN, contrarily, in the west, folds are

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mainly growth with axis of NNE (Fig. 1).

. The main

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Fig. 1 Map of structual outlines in Qinshui basin (modified from Qin et al. 35 and Zhang et al. 36).

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The strata of the Carboniferous and Permian include the Benxi, Taiyuan, Shanxi Formations

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in ascending order, among which the Taiyuan and Shanxi Formations are main coal measures

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strata with average thickness of 150 m (Fig. 2). The upper main coal seam (No. 2 coal seam in

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Guxian block and No. 3 coal seam in Shizhuangnan block) and lower main coal seam (No. 9+10

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coal seam in Guxian block and No. 15 coal seam in Shizhuangnan block) are developed in the

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Shanxi and Taiyuan Formations, respectively. The upper main coal seam and lower main coal

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seam are developed with considerable thickness in entire region. The upper main coal seam is

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grown in the early Permian, which thickness ranges from 2.15 to 8.66 m, with an average of 5.79

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m. It is sandwiched by 1~3 layers kaolinite or carbonaceous mudstone, and the roof and floor of

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the coal seam are always composed of mudstone or silty mudstone. The lower main coal seam is

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developed in the late Carboniferous, which thickness varies from 1.10~9.87 m, with an average

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thickness of 3.26 m. It is sandwiched by 1~6 layers mudstone and carbonaceous mudstone. The

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roof of the coal seam is K2 limestone, which is developed in the whole area, and the floor of the

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coal seam is mainly composed of mudstone and carbonaceous mudstone.

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According to the aqueous medium characteristics, bottom-up main aquifers are divided into

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four types: Ordovician fracture-karst aquifer, Carboniferous fracture-karst aquifer, Permian

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clasolite fracture-karst aquifer and Quaternary loose sediment pore aquifer, among which

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Ordovician fracture-karst aquifer is the main aquifer in study area, the water-abundance of

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Permo-Carboniferous aquifer and Quaternary loose sediment pore aquifer are weak-medium and

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medium, respectively. On the basin scale, there is no hydraulic connection among the aquifers

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vertically. The main aquifuges from top to bottom are aluminum mudstone of Benxi Formation,

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sandy mudstone, mudstone and coal seam of Taiyuan and Shanxi Formation and sandy mudstone

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and mudstone of the middle-lower part of upper Shihezi Formation and lower Shihezi Formation.

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Each aquiclude is tight and fractures are undeveloped, which blocks the vertical hydraulic

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connection because of the poor permeability and thus makes the aquifers relatively independent 34,

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35

.

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Fig. 2 Stratigraphic column of coal measures strata in Central-South Qinshui Basin (modified from Zhang

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et al. 36).

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3 Methods and samples tests

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3.1. Samples and tests

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In this study, the water samples were collected from main coal seams of Guxian block and

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Shizhuangnan block (CBM wells produced at least 2 years of the upper main coal seam and lower

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main coal seam), and performed conventional ion tests (16 samples), hydrogen and oxygen isotope

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tests (16 samples), 129I radioactive isotope samples (6 samples) and 14C radioactive isotope tests (6

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samples) respectively. 500 mL clean plastic containers were used for sampling. Before sample

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collecting, the plastic containers were flushed three times with target water. The containers were

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filled in the whole plastic container to remove air. The container caps were carefully tightened.

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After checking that the containers are not leaked, the containers were tagged sampling well.

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Hereafter, they were immediately sent to the laboratory to ensure the accuracy and reliability of

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the test results.

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The conventional ion and hydrogen and oxygen stable isotope tests of water samples were

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measured in State Key Laboratory of Environmental Geochemistry, China. All of the samples

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were filtered via glass fiber filter membranes and stored at 4 oC in a refrigerator before analysis.

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For cation analysis, the samples were acidided to a pH of < 3. The concentrations of Ca2+, Mg2+,

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Na+ and K+ were analyzed by Vista MPX inductive coupled plasma emission spectrometer

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(American Varia Company) and measured using standard methods

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carbonate and bicarbonate were titrated with a Metrohm automatic titration apparatus and the

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detection methods are according to People's Republic of China Geology and Mineral Industry

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Standards: determination of carbonate, bicarbonate and hydroxide by titration (DZ/T

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0064.49-1993). The concentrations of Cl-, SO42- were analyzed by ICS-90 ion chromatograph

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(American Dionex Company) and the detection methods are according to People's Republic of

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China Geology and Mineral Industry Standards: determination of chloride, fluoride, bromide,

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nitrate, and sulfate by ion chromatography (DZ/T 0064.51-1993). Stable isotope compositions

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were measured by the Liquid Water Isotopes Analyzer (Model: 912-0026, American Los Gatos

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Research Company) with analytical precision of < ±0.1‰ for δ18O and < ±0.3‰ for δD. The δ18O

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and δD values are expressed with respect to standard mean ocean water (VSMOW). The detection

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methods of δD and δ18O are according to People's Republic of China Geology and Mineral

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Industry Standards: determination of hydrogen isotope by zinc reduction method (DZ/T

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0184.19-1997) and determination of oxygen isotopes in natural water by carbon dioxide - water

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balance method (DZ/T 0184.21-1997), respectively. Water temperature, pH and dissolved oxygen

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(DO) were determined by a thermometer, digital pH and dissolved oxygen meter.

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129

I and

14

37

. The concentrations of

C radioactive isotopes of coal measures water were measured in the Xi’an

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Accelerator Mass Spectrometry (AMS) Center, used a 3 MV Tandetron AMS (HEVV, The

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Netherlands). The precision of this instrument for measuring 129I standard sample is 1.7% and the

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I/ I is 2×10-14. The detection accuracy of 14C/12C can reach 0.2%, and

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detection accuracy of

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the sensitivity is up to 10-12 38, 39.

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3.2 Identification methods

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The procedures of 129I radioactive isotope test are as follows: (1) Before the test, the water

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samples are filtered through a cellulose filter to remove the suspended particle matter; (2) The

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prepared water samples are weighted to a beaker, 2.5 mL of 1 mol/L NaHSO3 solution and 200Bq

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of 125I solution, as a tracer, are added; (3) HNO3 is added to adjust PH to 2; (4) Transfer the water

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sample to a separation funnel, 30 mL CCl4 and 4 mL of 1 mol/L NaNO2 solution are added to

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oxidize iodide to I2; (5) Shake the separation funnel is shook and make the iodide ion extract into

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pink or purple CCl4 organic phase; (6) After transferred it to a new beaker, 15 mL CCl4 was added

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to extract the remaining I2; (7) Combining the CCl4 organic phase, and transferred it into a new

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separation funnel, 10 mL water and 1 mL of 1 mol/L NaHSO3 solution are added to reversely

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extract I2 to iodide; (8) 1 mL of 1mol/L AgNO3 is added to precipitate iodide as AgI, and then the

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AgI is separated by Centrifugation and the separated AgI is dried at 60 oC for 2-3 h; (9) 3 times

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(by weight) niobium powder is weighted and mixed with the prepared AgI powder, and then the

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mixture is pressed into a copper holder for testing.

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The procedures of 14C radioactive isotope test are as follows: (1) Before the test, the water

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samples are filtered through a cellulose filter to remove the impurities, and then the filtrate is

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collected directly in the conical flask; (2) About 25 mL concentrated phosphoric acid is added in

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the upper erlenmeyer flask (over-dose), and the erlenmeyer flask is linked to vacuum and then

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close the valve, allowing the concentrated phosphoric acid to drop into the erlenmeyer flask and

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respond fully with the water samples for several minutes. (3) Cold trap method is used to purified

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in the purified vacuum system, and CO2, which is produced by the reaction, is collected

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quantitatively, i.e., DIC (dissolved inorganic carbon) in water. (4) The collected CO2 is reduced to

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graphite by Zn/Fe method in the graphite target preparation vacuum system, and then the graphite

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was pressed to a target sample for testing.

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4 Results and discussion

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4.1 Water type and TDS

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Piper trilinear diagram is an effective method to study groundwater composition and water

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type. The main advantage is that the water samples from different areas are marked on the same

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figure, which can be used to analyze the evolution of groundwater chemical composition. The

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total dissolved solids (TDS) equivalent to the total mass concentration of the major ions (Ca2+,

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Mg2+, Na+, K+, SO42-, Cl-, HCO3-, and CO32-) minus half of the bicarbonate concentration 40, which

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can be utilized to judge the groundwater hydro-geological environment and the storage conditions

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of CBM. The water types are classified into four distinct zones: I, II, III and IV. The main

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groundwater type of zone I is Ca-HCO3. Ca-HCO3 type water is typical freshwater 41-43, which is

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generally inducted as surface water (e.g. river water) 43, 44 or shallow groundwater 45. Precipitation

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is the predominant source of recharge to the ground-water flow system 46. It penetrates strata via

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the primary pores or fractures. During the groundwater cycling, the dissolution of calcite or

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precipitation maybe the source of Ca-HCO3 type water 47, 48. The main groundwater type of zone II

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is Na-HCO3, and it is constituted by most freshwater and little brackish water, belonging to deep

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groundwater. This kind of water is formed in hypoxia environment by the degradation effect of

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organic matter extensively occurred in the stratum of study area. Besides, it can also be generated

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through the dissolution of CO2 induced by thermal degradation of coal 40, the dissolution of alkali

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metal-carbonate minerals and cation interchange (water-rock interaction), etc.

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groundwater type of zone III is Na-Cl. TDS > 3000 mg/L means that this kind of groundwater is

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salt water. It is generated by the dissolution of salt rock or massive sea water intrusion. Most of the

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water in this zone is sea water, salt water or hot water 50. The main groundwater type of zone IV is

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Ca-Mg-SO4-Cl. With TDS of 1000 - 3000 mg/L, the groundwater in this area is mixed with sea

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water and has no dominant cation and anion. Therefore, water in this zone is mainly mixed by

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groundwater and seawater 51.

49

. The main

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Fig. 3 shows that the water samples of Guxian block fall in zone I, which indicates that the

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type of coal measures water is Ca-HCO3, meaning that it belongs to river water or shallow

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groundwater. The water samples of Shizhuangnan block fall in zone II, which suggests that the

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type of coal measures water is Na-HCO3, meaning it belongs to deep groundwater. The TDS of

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groundwater in closed storage condition is higher because it is well-protected, otherwise, the TDS

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of formation water decreases once it is infiltrated by atmospheric precipitation or surface water. In

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the study area, the TDS of coal measures water ranges from 288.81 to 859.41 mg/L (with an

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average value of 658.37 mg/L), which is far lower than that of seawater (35000 mg/L). This

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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

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closed - open hydro-geological environment, which is between the open and closed storage

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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

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oxygen-enriched environment, while Na+, K+, Cl- and HCO3- enrichment is often regarded as

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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.

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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

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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.

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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

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ratio

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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

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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

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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

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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

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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.

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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.

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40

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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‰

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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

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129

I decayed from the

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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

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129 127

I/ I ratio (Robs) (Fig.

(5)

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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

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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 (