Geological and geochemical characteristics of the secondary biogenic gas

in coalbed gases, Huainan coalfield

Zhang Xiaojun1,2 Cao Zhenglin1 Tao Mingxin2,3 Wang Wanchun2 Ma Jinlong4

(1. Northwest Branch,Research Institute of Petroleum Exploration and Development,PetroChina,Gansu Lanzhou 730020; 2. Key Laboratory of Gas Geochemistry, Institute of Geology and Geophysics, Chinese Academy of Sciences, Gansu Lanzhou 730000); 3. College of Resources Sciences and Technology,Beijing Normal University,Beijing 100875; 4. School of Resources and Environmental Sciences,Lanzhou University, Gansu Lanzhou 730000) Abstract The research on coalbed gases genetic type of the different mining area is of great significance for exploration and development of the coalbed gases in Huainan coalfield. Through a combination of geological and geochemical method, we have conducted an in-depth study on the coalbed gases genetic type of main mining area in Panji and Zhangji. The research results show that the compositions of coalbed gases in Huainan coalfield have high content methane, low content heavy hydrocarbons and carbon dioxide, and special dry gas. This characteristic is accordant with coalbed gases composition of secondary biogenic gas in the domestic and oversea. 13 The evolution coal is at the stage of generation of thermogenic gases, but the δ C1 values within 13 the range of biogenic gas (the range of δ C1 values from -56.7‰ to –67.9‰). Comparative 13 studies show thatδ C1 values of coalbed gases in Huainan coalfield was lower than that of the 13 coal type gases in China and coalbed gases of main area in the worldm, and it was similar toδ C1 13 values of secondary biogenic gas reported in the domestic and oversea.The δ C2 value of coalbed gases in Huainan coalfield shows not only the features of the thermogenic ethane, but also the mixed features of the biogenic methane and thermogenic ethane. In geological characteristics, Huainan coalfield has favorable conditions of generation of secondary biogenic gas, such as suitable coal rank, shallow coalbed result from the strong tectonic uplift, the fierce infiltration of surface water, the best temperature of generation of biogenic gas. Thus, favorable geological conditions and geochemical composition of coalbed gases proved the existence of secondary biogenic gas in Huainan coalfield. Key words coalbed gases; secondary biogenic gas; geochemistry; geology; characteristic Different genetic types of coalbed gas have different formation mechanism. And their composition and isotopic geochemistry characteristic have large differences. Both of them are of great significance for research on production, storage, transportation, exploration and theoretical study of coalbed gas [1] [2]. Therefore, in the study of coalbed gas exploration and utilization, people pay more attention to the causes of the coalbed gas. In 90s 20th century, Scott put forward the concept of secondary biogenic gas when he studied the coalbed gas in San Juan Basin in America. He believes that the coalbed gas was produced by moisture, n-alkanes and other organic

1 compounds generating from the coal-generated process. Other geochemical composition is similar to the native biogenic gas. But the main difference is that the thermal evolution of coal over passes the formation of the native bio-gas phase, and Coal is generally uplifted to shallow[3]. Due to the proposing and development of the cause theory, the United States lignite coal successfully done the commercial exploitation of low-level coalbed gas in the Powder River Basin in 1998,and thus made their production of caolbed gas jump to 32 billion [4] [5]. Chinese Huainan coalfield locates in the southern coal-gathering basin of Carboniferous– Permian, north China which is a coalbed gas high enrichment zone [6]. The past research towards to coalbed gas in Huainan coalfield was mainly focused on the applied geological problems. In contrast, the study of geochemical characteristics and genetic types is still relatively weak. In recent years, there was a first demonstration of generating type of the secondary biogenic gas in Huainan coalfield by Tao mingxin and also the changes of their isotopic characteristics was studied [7]. On this basis, we have done more detailed study on the isotopic characteristics of coalbed gas in Pan Ⅰ coal field, Pan Ⅲ coal field and Zhangji coal field, Huainan coalfield. Combined with geological data, we did a series of study on generating types of coalbed gas in each mining in order to provide some scientific basis for the comprehensive development. 1. Geological condition The study area is composed of, Taiyuan formation of Upper Carboniferous System, Shanxi Formation and Xiashihezi formation of Lower Permian System and, Shangshihezi formation of Upper Permian System. The coal in Taiyuan formation of Upper Carboniferous system is poorly developed, it is thin unstable coal bed with no production value. Shanxi formation, lower Shihezi formation and upper Shihezi formation of Permian System are all with a characteristic of multi-bedding coal, large thickness, and relative stability. There are generally 30 coal-bearing lays and the main coal reservoirs concentrating in lower Permian are 1, 8, 11-2 and 13-1 coal beds. The average thickness of 13-1coal bed is 4.33m, and 3.31m for 11-2 coal bed. They are the most stable coal bed and also the main coal bed for methane resource production in this area. Huainan coal field is a complex syncline with EW axis. Tan-Lu fault and Fuyang deep fault which are with NNE distribution control the eastern and western borders respectively. Synclinorium within the fold and faults are developed. Secondary anticline and syncline includes Xieqiao-Gugou syncline, Chenqiao anticline and Panji anticline. There are two groups of faults: one is striking overthrust distributing in the two wings of the synclinorium, the other is NNE cross-cutting normal faults parallel to Tan-Lu fault. Thus the ladder structure from east to west is formed. The coal metamorphism of Huainan coal field is mainly plutonic metamorphism and the metamorphic stage is mostly gas coal, and some are fat coal and coking coal[8].

2 2. Sample collection and testing methods Study samples mainly come from Pan Ⅰ coal field, Pan Ⅲ coal field and 11 and 13 coal layers of Zhangji coal field with a depth of 500～700m. And the samples are mainly underground pumping gas. Gas sample is generally collected by drainage method, a few of them are collected using air bags and cylinders. Sample test was completed in Key Laboratory of Gas Geochemistry, Chinese Academy of Sciences. Carbon and hydrogen isotope are tested by Delta Plus XP Mass Spectrometers. The analysis accuracy of carbon isotope is ≤±0.25‰ using the international PDB standard. The analysis precision of hydrogen isotope is ≤±1.5‰ using international SMOW standard. Gas composition was tested by Mat-271 Mass Spectrometer. For low content components （C2H6、CO2）, we did the testing with the methods of gradually increased the sample amount and repeated measurements in order to ensure the accuracy of the test data. 3. Geochemistry characteristic 3.1 Components of Coalbed Gas Characteristic components of coalbed gas are the main basis for identifying the causes. Research has shown [7] [9] [10] that the component of secondary biogenic coalbed gas is composed of methane, heavy hydrocarbons are very low, and CO2 is generally less than 5%, they are typical dry air and extra dry air. Table 1 The geochemical composition of secondary biogenic coalbed gas

Coalbed Isotopic composition Basin/ Coal field Ro（%） Component age (‰,PDB) 13 CH 4 ：97.44～99.23% δ C 1 ：-64.4～-67.9‰ Pan Ⅰ coal field P 0.84～0.93 C 1 /C 1-3 >0.99 δDCH4 ：-211～-217‰ （China） 13 CO20.48% δ C 2 ：-27.9～-29.1‰ 13 CH 4 ：94.61～97.83% δ C 1 ：-56.7～-57.8‰ Pan Ⅲ coal field P 0.81～1.09 C 1 /C 1-3 >0.99 δDCH4 ：-151～-265‰ （China） 13 CO24.51% δ C 2 ：-22.2～-24.5‰ 13 CH 4 ：82.05～99.37% δ C 1 ：-59.9～-61.2‰ Zhangji coal field P 0.81～1.09 C 1 /C 1-3 >0.99 δDCH4 ：-188～-221‰ （China） 13 CO21.28% δ C 2 ：-21.4～-32.1‰ 13 [7] CH 4 ：60～90% δ C 1 ：-50.7～-61.3‰ Xinji coal field C,P 0.88～0.91 C 1 /C 1-5 >0.99 δDCH4 ：-219～-243‰ （China） 13 CO22% δ C 2 ：-15.9～-26.7‰

Sydney and 13 CH 4 /C 2 H6 ≥1000； δ C 1 ：-60±10‰ Bowen basin P 0.8～1.2 [9] CO 2 <5%； δDCH4 ：-217±17‰ （Australian） 13 δ C 1 ： -44.5～-79.9‰ Upper Silesian CH 4 >90% C 2 δDCH4 ： -153～-202‰ basin 0.59～1.17 C 1 /(C 2 +C3 )：122～10000 13 [10] δ C 2 ：-22.3～-24.6‰ （Poland） CDMI：0.0～21.0% 13 δ C CO2 ： -2.8～-27.2‰ Analysis results (table 1) from Pan Ⅰ coal field, Pan Ⅲ coal field and Zhangji coal field show that: components in each mine are mostly similar. The content of methane in coalbed gas is 82.05～99.37%, and with an average content of 94.79%. The content of heavy hydrocarbons is very low. The average contents of ethane and propane are 0.06% and 0.04% respectively.

3 Regulations on one of hydrocarbon gases, the average content of methane reached no less than 99.89%. It can clearly be seen the dominating situation of methane in hydrocarbon gases. In non-hydrocarbon components, the average content of N2 is 3.89%, CO2 is 0.92%, and C1/C1-3 is no less than 0.99. They are dry gases of high content of methane, heavy hydrocarbons and low content of CO2. They are very similar with the components characteristic of secondary biogenic coalbed gas abroad in table 1, and also clearly show a components characteristic of secondary biogenic coalbed gas. 3.2 Carbon Isotope Composition of Coalbed Gas Carbon isotopic composition is an important indicator for identifying the causes of 13 conventional natural gas. In research of how to divide genetic types of coalbed gas, δ C1 value is [11] [12] [13] 13 also used as an important geochemical indicator . We can see δ C1 and δDCH4 value of 13 coalbed gas from Pan Ⅰ coal field, Pan Ⅲ coal field and Zhangji coal field in table 1. theδ C1 13 value overall -56.7‰～-67.9‰, and with an average value of 61.3‰. If we choose δ C1 = -55‰ as the boundary value for dividing thermogenic cause and microbial cause [12] [13], then it is very 13 clear that the δ C1 value of each coal field is no more than -55‰ which indicates a biogenic cause. The thermal evolution of the study coal rocks ( Ro = 0.81～1.09%, table 1 ) is just come to [3] 13 a stage when the intense heat methane starts to be produced . Therefore, theδ C1 value of colabed gas in this area is not match the thermal evolution of coal rocks.

From figure 1, we can know that most samples are in the scope of biogas resulting from CO2 deoxygenation and a small number of them are in the mixed gas scope[14]. It clearly shows that the coalbed gas from Pan Ⅰ coal field, Pan Ⅲ coal field and Zhangji coal field is mostly biogenic gas produced by CO2 deoxygenation.

N IO T C U D -80 E NE R METHA 2 IAL O MICROB C TRA NSITION

-60 FERMENTATION MIXING C(‰,PDB) 13 δ -40 T HER MOG ENI C GA SES

-20 -400 -300 -200 -100

δDCH4(‰,SMOW) Pan Ⅰ coal field Pan Ⅲ coal field Zhangji coal field Fig. 1 Genetic classification of carbon and hydrogen isotope of methane (According to Whiticar, etc. 1999 Diagram) In addition to test data in this study, we also collected a number of carbon isotope data of

4 methane and ethane coming from thermal genetic coalbed gas (including secondary biogas from some typical region or basin) at home and abroad. They are used to study and compare the carbon isotopic composition characteristic in Huainan coal field (table 1, figure 2 and figure 3). 13 As shown in table 1 and figure 2. the δ C1 value of thermal genetic caolbed gas at home is significantly smaller than the one from other country, but they are basically in the same range with [7] [9] [10] 13 secondary biogenic coalbed gas at home and abroad . The δ C1 value from two study coal field in Huannan coal field are generally smaller than that from Xinji coal field, and the biogas 13 characteristic is more obvious. The δ C1 value shown above indicates that the biogas from Pan Ⅰ coal field, Pan Ⅲ coal field and Zhangji coal field is secondary biogas.

-90 Coal-type gas in China

-80 Coalbed gas in the world Secondary biogenic coalbed -70 Coalbed gas in China Huainan

-60

（ ‰ ） 1

C -50 13

δ -40

-30

-20 0 20 40 60 80 100 120 Sample number

13 Fig. 2 The distribution and variation of δ C1 value from Huainan coal field and other countries (Data of coal-type gas from the references[15]～[19]; Data of coalbed gas from the references[1],[9],[10],[13],[20]～[24]） The carbon isotopic composition of ethane from Pan Ⅰ coal field, Pan Ⅲ coal field and Zhangji coal field is entirely in a scope of -21.4‰～-32.1‰, and most of them is in the scope of -22～-29‰. They are obviously own a characteristic of thermal cause, the same with that from Xinji coal field and Upper Silesian Basin in Poland [10] (table 1). 13 This thermal genetic characteristic is more obvious in figure 3. The δ C2 value from the study area is significantly in the same range with the one resulted from thermal genetic coalbed gas inland, and also the same with coalbed gas from major countries in the world (including secondary-biogas-bearing coalbed gas from typical region or basin). 13 Thus, the δ C2 value of coalbed gas from Pan Ⅰ coal field, Pan Ⅲ coal field and Zhangji coal field not only reflects the thermal genetic characteristic of ethane, but also reflects the mixing which is generated by thermal genetic ethane and microbial genetic methane. Rice also believe 13 that the ethane with heavier carbon isotope (δ C2 value is -28‰～-24‰) and the methane with 13 lighter carbon isotope (δ C2 value is no more than -55‰) are coexist. They show the characteristic of mixed genetic coalbed gas[12].

5 In summary, the carbon isotopic composition from Pan Ⅰ coal field, Pan Ⅲ coal field and Zhangji coal field have demonstrated a characteristic of secondary biogenic gas.

0

Coal-type gas in China Coalbed gas in the world -10 Coalbed gas in China Huainan

（ ‰ ）

2 -20

C 13

δ

-30

-40 0 1020304050 60 Sample number 13 Fig. 3 The distribution and variation of δ C2 value from Huainan coal field and other countries (Data of coal-type gas from the references[15]～[19]; Data of coalbed gas from the references[1],[9],[10],[20]～[24]） 4. Geological features Previous studies [25] [26] showed that: controlled by hypometamorphism, when reached a metamorphic degree of gas and fat coal stage (the Ro value is 0.81～1.09%, table 1), the Permian coal beds in Huainan coal field has experienced almost 100Ma strong uplift and erosion between 120Ma and 23Ma. Thus, it made the coal beds become buried in shallow, and some are exposed to the surface. Since 23Ma, there was only 50～700m thick Neogene and Quaternary sediments [25]. Strong and long-term regional uplift provide the conditions for surface water and bacteria to enter the coal beds. And at this time, the temperature of the main coal beds is basically lower than 45℃ [25], it is just the best biogas generation temperature [3]. The main aquifers are composed of Quaternary loose pervious aquifers, Permian sandstone fissured aquifers, and karst fissured aquifers in Taiyuan formation and Ordovician. They are stacked, each occurrence of hydraulic connection. Especially in coal beds covered by Cenozoic loose strata, coal measure aquifer near the shallow outcrops was controlled by Quaternary bottom loose aquifers. And pervious aquifers of Quaternary loose strata are mainly supplied by the atmospheric precipitation and the vertical infiltration of surface water [25] [27] [28]. Therefore, coal beds should be closely associated with surface water. The analysis result of hydrogen and oxygen isotope from groundwater in Huainan coal field shows that: the linear relationship of δD value and δ18O value is consistent with the relationship of Chinese atmospheric precipitation δD = 7.9δ18O + 8.2[29]. It indicates that the water supply of the ground hot water is precipitation water experiencing deep-cycle. It can clearly be seen that there is surface water infiltration in coal beds.

6 When talking about generation conditions of secondary biogenic gas, Scott etc consider that there are four main conditions[3]: firstly, the buried coal transform into lignite coal or higher rank

(R0 value is generally 0.30～1.50+%); secondly, there is regional uplift or coal beds are uplifted to shallow; thirdly, water along the basin edge recharges to coal beds; finally, coal temperature drops to 75℃ below, especially 35～42℃ in which it is most suitable for methane bacteria reproduction. From the above analysis, the geological conditions of Huainan coal field are fully complied with the generation conditions of secondary biogenic gas. These favorable geological conditions and isotopic and composition characteristics mutually reflect the existence of secondary biogenic gas in Huainan coal field. 5. Conclusions (1) The analysis result from Pan Ⅰ coal field, Pan Ⅲ coal field and Zhangji coal field shows that: components of each mine of Huainan coal field are basically similar. Methane whose average content is more than 99.89% is the dominating component in hydrocarbon gas. In non-hydrocarbon components, the average content of CO2 is 0.92% and C1/C1-3 value is more than 0.99 which show a characteristic of high content of methane, low content of heavy hydrocarbon and CO2. They are consistent with the components of secondary biogenic coalbed gas both at home and abroad. 13 (2) The δ C1 value in the study area is entirely -56.7‰～-67.9‰, and 61.3‰ in average. It mostly belongs to the distribution value of biogenic gas, obviously not consistent with the thermal evolution (Ro value is 0.81～1.09%) of the coal rock in this area. Genetic classification diagram shows that the coalbed gas is mainly composed of biogenic gas which generates from CO2 13 deoxidisation. Contrastively, the δ C1 value of coalbed gas from Pan Ⅰ coal field, Pan Ⅲ coal field and Zhangji coal field is clearly smaller than that from China and other countries, but it is 13 very similar with that of secondary biogenic gas. And compared with Xinji coal field, the δ C1 value from the three coal fields is obviously has a characteristic of biogenic gas. Thus, the coalbed gas in the study area shows a characteristic of secondary biogenic coalbed gas. 13 (3) The δ C2 value of coalbed gas from the study area not only reflects a characteristic of thermal genetic ethane, but also indicates that the coalbed gas in this area is a mixture of thermal genetic ethane and microbial genetic methane. (4) The geological analysis results show that: ① there is appropriate coal rank in Huainan coal field; ② there was strong uplift and the coal beds has been uplifted to shallow; ③ the surface water has infiltrated to coal beds; ④ there is favorable geological condition—the best temperature for the generation of secondary biogas (methane). References [1] Scott A R. Composition and origin of coalbed gases from selected basins in the United States[C]. International Coalbed Methane Symposium Proceedings. 1993, 207～222.

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