Article pubs.acs.org/EF
The Isotopic Tracer and Resource Value of Microbial Gas Production in CoalbedsA Case Study of Coalbed Gas in Enhong, China Mingxin Tao,*,† Yuzhen Ma,† Zhongping Li,‡ Jing Li,† Pengyang Liu,† Yanlong Wang,§ Xiangrui Chen,† and Aihua Zhang† †
College of Resources Science and Technology/Key Laboratory of Environmental Change and Natural Disaster, Ministry of Education, Beijing Normal University, Beijing 100875, China ‡ Lanzhou Petroleum Resources Research Center, Institute of Geology and Geophysics, Chinese Academy of Sciences, Lanzhou 730000, China § Xi’an Research Institute, CCTEG, Xi’ an 710054, China ABSTRACT: From the new perspective of the dynamic isotopic fractionation of microbes and the mixture of secondary biogenic gas and thermogenic gas in coalbeds, this study evaluates the geochemical characteristics of mixed coalbed gas in Enhong. Additionally, by using isotopes, the study traces the microbial activities and the results of those activities, such as the methane production of bacteria, and reveals the formation mechanism and genetic types of coalbed gas. The measured δ13C1 values of the coalbed gas samples are from −54.5‰ to −32.0‰. Based on the values of Ro and δ13C2, by calculation, the δ13C1 values of thermogenic methane range from −30.6‰ to −28.1‰ and from −30.7‰ to −28.3‰, respectively. The results of the two calculations are highly consistent, and both are approximately 20‰ higher than the measured values. The measured δDCH4 values are from −217‰ to −196‰, being between thermogenic methane and microbiogenic methane; the δ13CCO2 values are from −30.5‰ to −23.9‰, growing heavier with respect to the original thermogenic gas; the measured δ13C2 values are from −25.7‰ to −22.6‰, and the estimated δ13C2 values are from −21.8‰ to −21.2‰, also indicating growing heavier. The δ13C1 and δ13C2 values are negatively correlated; both the Δδ13CC2−C1 and Δδ13CCO2−C1 values are increasing. All of the above characteristics indicate that CO2 is reduced into microbial genetic methane by methane-producing bacteria and it mixes with thermogenic gas, which is new evidence showing the existence of secondary biogenic gas. Via calculations using a variety of data, such as the values of δ13C1, δ13C2 and Ro, it is found that thermogenic methane accounts for approximately 38% to 58% of the total amount, and microbial genetic methane accounts for approximately 42% to 62%. The proportion of micro-biogenic methane reduced from the top down, which occupied more than 50% of that in the coalbed buried within 1000 m deep. It increased the content of coalbed gas by more than 1 times. Within 1000 m deep, coalbed temperatures are generally lower than 40 °C, which is the most appropriate section for methanogenic bacteria activity and secondary biogenic gas generation. Coalbed uplift to the shallow parts in the late stage is the basic geological condition for the formation of secondary biogenic gas, which has significant resource value.
1. INTRODUCTION Coalbed gas, featuring self-generating and self-storage in a coal seam, is an unconventional gas. The main component of it is methane. Against the background of the dwindling conventional oil and gas resources worldwide, coalbed gas is not only a practical supplementary resource, but it also has major significance in coal mine disaster reduction and environmental protection. It has become a global research focus. Generally, based on theories related to conventional gas, early scholars believed that the evolution degree of coal had exceeded the microbial gas generation stage. It was impossible to generate biogas, and the biogas generated before coal formation was difficult to retain and escaped. Therefore, the gas in the coalbed was basically thermogenic gas.1−4 In 1994, Scott et al.2 found the spatial distribution of wet gas and dry gas in the San Juan Basin (United States) coalbed gas. According to the features of the isotopic composition of dissolved inorganic carbon in the coalbed water, Scott et al. found that the dry gas was formed by microorganisms when coal was uplifted to the near surface.2 The coalbed gas, produced after the coalbed enters the thermal evolution gas © 2015 American Chemical Society
generation stage, is called secondary biogas (Secondary Biogenic Gases), which is also known as late stage biogas.5 Since then, many such coalbeds containing coalbed methane have been discovered in basins globally, for example, the Polish Upper Silesian basin5 and Lower Silesian basin,6 Canada’s Elk Valley Coal,3 Sydney and the Bowen Basin in Australia,7,8 and China’s Huainan and Liyazhuang et al. coalfield area.9−11 Although there are some reports of secondary biogenic gas (SBG), there are still different understandings of how it develops. There are two reasons for these differences: the first reason is the influence of traditional theory, which doubts that coalbed gas in the evolution stage can still generate biogenic gas; the second reason is that the general method of identifying genetic types of natural gas uses the carbon isotopes of components such as methane, but the carbon isotopic compositions of coalbed methane (CBM) and CO2 usually Received: November 16, 2014 Revised: February 21, 2015 Published: March 3, 2015 2134
DOI: 10.1021/ef502565g Energy Fuels 2015, 29, 2134−2142
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Energy & Fuels change greatly and in a complicated manner, which is difficult to explain. In most of the previous literature, when the δ13C1 value is as low as −55‰ or −60‰ (Ro value ≥0.5%) and the components are drier, it is generally thought that the coalbed gas contains SBG. That is, the academic field identifies SBG mainly based on the original geochemical characteristics of biogas.4 However, this type of identification should only be applicable to purer SBG. Under actual geological conditions, coalbed gas is mainly absorbed in the coal seam and cannot completely escape. The SBG that is generated later is often mixed with residual thermogenic gas. The mixing of two coalbed gases inevitably produces a new geochemical additive effect. That is, the characteristics of the geochemical composition of the new mixture are different from both those of pure SBG and those of thermogenic gas.12 Therefore, for this type of coalbed gas, it is inappropriate to identify its genetic type using the isotopic tracers of pure biogenic gas or thermogenic gas. In the fields of oil and gas, the research of hydrocarbon isotopes is not only commonly used to trace source rocks and kerogen types of oil and gas, but has also been used to estimate the maturity degree, formation temperature, and formation time of natural gas,13 showing better tracer results. From the above perspectives, this paper takes the coalbed gas in Enhong as an example, based on the theory of dynamic isotopic fractionation of microbes, and it studies the geochemical additive effect of such kinds of mixed coalbed gas. The study extracts various information or signs which identify the existence of SBG, providing new evidence for the identification of this kind of coalbed gas. Meanwhile, using the isotopic variations of various substance compositions involved in the microbial activity, it conducts isotopic tracing of such microbial activities as those of methane-producing bacteria in the coal seam, and reveals the formation mechanism and genetic types of coalbed gas; it further conducts quantitative studies on the proportion of SBG, evaluates its resource value, and studies such environmental conditions as the formation temperature and main distribution depth.
Figure 1. Location of Enhong Basin.
coal-bearing strata in the area show N−S trending (Figure 2).14,15
2. COALFIELD GEOLOGICAL SETTINGS AND SAMPLE TESTING 2.1. Coalfield Geological Settings. Enhong basin is located near Fuyuan and Qujing, Yunnan Province (approximately E103°53′−104°45′, N25°03′−25°40′), showing a NE− SW trending about 53 km long, with a width of 9 km to 20 km, being a priority coalbed gas exploration and development region in Yunnan Province (Figure 1, Figure 2). The exposed basement rocks of Proterozoic (Kunyang Group) are metamorphic rocks, on which is the Upper Paleozoic, namely Devonian, Carboniferous and Permian. In the mid-Permian, the area was mainly in the marine environment, deposited in marine carbonate deposition Qixia Fm (P2q) and Maokou Fm (P2m). In the early Late Permian, widespread Emeishan basalt formed in the area. After that, from the Kangdian Oldland in the west area of this region to the east, it in turn evolved into the Piedmont alluvial plain, the Littoral alluvial plain, and the Littoral plain, developing a good coalforming environment and having deposited a coal-bearing sedimentary strata. Late Permian is the most important coal forming period in South China. Enhong and Yunnan region are important parts of the entire South China coal region. The
Figure 2. Depositional environments during Late Permian in Enhong and adjacent area:14 I, Kangdian Oldland; 1, Piedmont alluvial plain; 2, Littoral alluvial plain (basal clinoform); 3, Littoral plain; 4, Epicontinental littoral clinoform; 5, Luxi-Luoping submarine trench.
The coal-bearing strata formed in Enhong basin and adjacent areas in the Late Permian are Upper Permian Xuanwei Fm (P2x). Xuanwei Fm in the area is a coal strata composed of sandstone, mudstone and shale strata, with a large thickness variation, with the general change being about 250 m. Dozens of coal layers developed in Xuanwei Fm, with large lateral variations, among which only about ten large coal layers are minable, being concentrated in the middle and lower parts of the coal-bearing strata, with a layer thickness up to 2.7 m (Figure 3).16,17 The coal strata in Enhong basin were subject to a certain degree of tectonism, forming a synclinorium nearly N−S trending, with fault development. Coal seams are hosted in the synclinorium (specifically described later). Therefore, the depth of each seam also shows a greater variation. According to Dai, for the mineral constituents of the coals in Xuanwei Fm, the quartz and chamosite were mainly from hydrothermal 2135
DOI: 10.1021/ef502565g Energy Fuels 2015, 29, 2134−2142
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Energy & Fuels
fluids; namely, there was an invasion of hydrothermal fluid in the late stage.14 2.2. Composition and Types of Coal. Take the coal sample data from Enhong basin EH−01 for example: the contents of organic material of the 7 coal samples are from 81.2% to 95.6%, most of which are vitrinite; except for one sample being 56.3%, the other 6 samples are from 73.6% to 84.7%; inertinite contents follow, being from 10.8% to 24.9%; exinite content is very low or almost not included; the contents of inorganic matter are from 4.4% to 18.8%, mainly clay minerals, followed by quartz (SiO2) (Table 1). The coal quality analysis results of major coal seams in the area are listed in Table 2, with no further discussion. The vitrinite reflectances (Ro values) of the coal samples in the study area are approximately 1.2% to 1.4%, being medium-volatile bituminous. Table 2. Results of Proximate Analysis of Coal Samples from Well EH-01 (%)17 Samples
Mad
Ad
Vdaf
St.d
CRC
TD (g/cm3)
coal coal coal coal
0.76 0.89 0.85 0.8
21.38 26.94 16.24 11.64
24.02 21.97 21.82 20.6
0.11 2.28 0.24 1.51
7 6 6 7
1.54 1.4 1.4
10 15 16 19
2.3. Sample and Test Methods. The coalbed gas samples studied in this paper are collected from two coalbed gas drillings from Enhong Coalfield in China: Well EH-01, well coordinates: X = 2802050, Y = 18412825, with the depth of 636.15 m; Well EH-02, well coordinates: X = 2806595, Y = 18417302, with the depth of 667 m.17 The desorption and collection of coalbed gas samples are in accordance with the relevant standards and unified operating procedures: to elevate the drill pipe carrying coal core to the wellhead, put the coal core into the coalbed gas desorption canister (10 cm × 30 cm) quickly and fill it and close and seal the lid. After about 1 to 2 min, open the valve to discharge a small amount of gas, mainly to discharge the residual atmosphere in the canister, and then close the valve. After placing the canister (desorption) for 24 h, open the valve and collect the coalbed gas by using a collection process with displacement of water until the gas stream stops; then close the valve. After placing the canister (desorption) for another 48 h, open the valve for the second coalbed gas collection. The first collected coalbed gas samples are generally used for the sample testing. Testing and studies have shown that by using this method and procedure of desorption/collection coal core samples, it can basically rule out or significantly reduce atmosphere
Figure 3. Comprehensive coal measurement column of the Enhong coalfield.16
Table 1. Results of Petrographic Analyses of Main Coal Seams in Well EH-0117 Organic Material (%)
Mineral (%)
Samples
Vitrinite
Inertinite
Exinite
Total
Clays
9−1 10−1 10−2 16−1 16−2 16−3 19−1
73.6 78 73.9 56.3 84.7 81.2 83.8
18.1 14.1 15.5 24.9 10.9 12.9 10.8
0.7 0.7
92.4 92.8 89.4 81.2 95.6 94.1 94.6
3.8 3.8 7 7.6 1.8 4.5 2.4
2136
Pyrite
Carbonates
SiO2
Total
0.2
0.2 0.2
3.6 3 3.6 9 2.2 1.2 1.2
7.6 7.2 10.6 18.8 4.4 5.9 5.4
1.6
1
0.6 0.4 0.2 0.8
DOI: 10.1021/ef502565g Energy Fuels 2015, 29, 2134−2142
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Energy & Fuels pollution.11 In addition, the contents of CH4 and CO2 in the atmosphere are extremely low, being only 1.75 × 10−6 and 364 × 10−6, respectively, and the δ13C value of atmospheric methane (about−46‰) is also within the range of the δ13C value of CBM. Therefore, even if a limited amount of air interfused into the samples, it cannot cause significant influence on the carbon isotopic testing results of coalbed gas samples.18−20 After gas desorption, collect and seal the coal samples from the desorption canister and send them to the laboratory for analysis. The analysis of the gas samples was conducted in the Key Laboratory of Gas Geochemistry (Lanzhou), Institute of Geology and Geophysics, Chinese Academy of Sciences. Molecular components of the gas samples were analyzed on a MAT-271 trace gas mass spectrometer and compared with the recognized atmosphere value. The spectrometer shows a very high precision, being suitable for the test of components of gas samples. The atmosphere sample measured on the apparatus had a slight difference of
