(1. School of Resources and Earth Sciences, China University of Mining and Technology, Xuzhou, Jiangsu 22116; 2. Key Laboratory of Coalbed Methane Resources and Accumulation Process in Xuzhou, Jiangsu Province 22 1008)

Generally speaking, the concentration of heavy hydrocarbons in coalbed methane is lower than 3% ~ 5%. However, in some areas, the concentration of heavy hydrocarbons in coalbed methane is higher than normal, which is abnormal. The chemical composition of coalbed methane contains abundant genetic information, and the study on the causes of heavy hydrocarbon anomalies can deepen the understanding of the causes of coalbed methane and promote the perfect development of basic research on coalbed methane geochemistry. This paper summarizes the distribution and characteristics of heavy hydrocarbon anomalies in coalbed methane at home and abroad, and the various explanations of the causes of heavy hydrocarbon anomalies by scholars at present. The author puts forward his own views on these explanations, which provides ideas and breakthrough points for the in-depth study of the causes of heavy hydrocarbon anomalies, and believes that further research should be carried out in combination with various factors in specific areas.

Keywords: causes of abnormal distribution characteristics of heavy hydrocarbons

Fund Project: Supported by the Key Project of National Natural Science Foundation (40730422).

Introduction to the first author: Lan, born in 1986, female, doctoral student, coalbed methane geology,13151981375, LanfJ1986 @/kloc-.

Distribution characteristics and genesis of abnormal heavy hydrocarbons in coalbed methane

Lan Fengjuan 1 Telly 1, 2 Chang Huizhen 1 Guo Chen 1 Zhang Fei 1

(1. School of Resources and Earth Sciences, China University of Mining and Technology, Xuzhou, Jiangsu 22116. Key Laboratory of Coalbed Methane Resources and Accumulation Process, Xuzhou, Jiangsu 22 1008)

Abstract: Generally speaking, the content of heavy hydrocarbons in coalbed methane is between 3% and 5%, but it will exceed the normal value in some places. The chemical composition of coalbed methane contains abundant genetic information. The study of its genesis will deepen our understanding of the genesis and geochemistry of coalbed methane. This paper summarizes the distribution characteristics of abnormal heavy hydrocarbons at home and abroad and scholars' explanations of their causes, and puts forward their own views, which provides a starting point for further study of their causes. It is considered that in a specific field, considering many factors, further research is still needed.

Keywords: abnormal heavy hydrocarbons; Distribution characteristics; cause

introduce

The main component of coalbed methane is CH4, the secondary components are heavy hydrocarbons (C2+), N2 and CO2, and the trace components are Ar, He, SO2 and CO (Tao Mingxin, 2005). According to Scott's statistics on the gas composition of 400 coalbed methane production wells in the United States/KLOC-0, the average composition of coalbed methane is CH4, 93%; Carbon dioxide, 3%; C2+，3%； N2， 1%； Dry-wet index (c 1/c 1 ~ 5), 0.77 ~ 1.0 (Scott, 1993). Although the coalbed methane in China is generally characterized by dry gas, many examples of "moisture" have been found. In these examples, the concentration of heavy hydrocarbons in coalbed methane is usually between 5% and 25%, and even the phenomenon that the concentration of heavy hydrocarbons is greater than that of methane appears (Wu Jun, 1994). As far as Longtan Formation in Yunnan, Guizhou and Sichuan is concerned, the concentration of heavy hydrocarbons in coalbed methane in Hong En mining area in Yunnan is often higher, followed by Qianxi and Chongqing. In Hong En syncline, the ethane concentration in coalbed methane reaches 4.38% ~ 33.90%, generally around 16%; Propane concentration is 0.7% ~ 5.88%, generally less than 3% (Wu Guoqiang et al., 2003). Heavy hydrocarbon anomalies occur not only in Hong En, but also in other areas. For example, the concentration of C2H6-C4H 10 in the upper Permian coking coalbed methane in Tianfu mining area of Chongqing is as high as 30.45%, which is 1.98 times that of CH4. The proportion of heavy hydrocarbons in coalbed methane in Nantong mining area is as high as 6% ~ 15% (Liu Mingxin, 1986).

1 Abnormal distribution of heavy hydrocarbons in coal seams at home and abroad

Heavy hydrocarbon anomalies appear in Yunnan, Guizhou, Chongqing, Zhejiang, Hunan, Jiangsu, Anhui, Henan, Shaanxi, Liaoning, Hebei, Inner Mongolia and Heilongjiang from south to north (see table 1). The abnormal time of heavy hydrocarbons is concentrated in Carboniferous, Permian and Jurassic, of which Permian is the main one. The degree of coalification is in the stages of gas coal, fat coal and coking coal, and it also appears in long-flame coal. The heavy hydrocarbon concentration is between 0. 1%-48.7%. Coal seams with heavy hydrocarbon anomalies are usually related to oil and natural gas. Some have liquid oil in the coal seam or its roof and floor, and some have obvious oil and gas display.

Table 1 Abnormal Distribution of Heavy Hydrocarbons in Coalbed Methane in China

sequential

According to the consulting data, the abnormal heavy hydrocarbons in foreign coal seams are the United States, Russia and Germany. The degree of coal metamorphism is mainly in the stage of gas-fertilizer coal, and the highest concentration of heavy hydrocarbons is greater than 43%. Interestingly, there is a natural gas field or oil field related to coal-formed gas near many coal fields with abnormal heavy hydrocarbons, and some coal seams also have liquid oil or good gas-oil display. Therefore, some scholars use the composition of LPG to explain the concentration of heavy hydrocarbons, and think that it is related to the deep oil-bearing nature of the basin, and its migration may be along the deep fault (аикраа).

Table 2 Abnormal distribution of heavy hydrocarbons in coalbed methane abroad

sequential

2 causes of abnormal heavy hydrocarbons in coal seam

There are different opinions on the causes of heavy hydrocarbon anomalies in coalbed methane, including gas-generating parent material theory, oil-gas permeability theory, contact metamorphism theory and coalification stage theory. The following lists various hypotheses about the causes of heavy hydrocarbon anomalies.

2. 1 gas parent material

The composition characteristics of hydrocarbon-generating parent materials of source rocks affect the hydrocarbon-generating quality and potential of source rocks, which is an important content of source rock research. There are two main research methods: one is coal petrology, and the other is kerogen method. The coal petrology method retains the original state and structure of organic matter, which is beneficial to the study of genesis and the vitrinite reflectance is more reliable. Kerogen method enriches that part of organic matter in mineral asphalt matrix, which is beneficial to the determination of kerogen type (Hande

The maceral of coal and rock largely determines the hydrocarbon generation capacity of coal seam. Generally speaking, coal seams rich in crusts tend to produce oil, while coal seams rich in vitrinite tend to produce gas. Petrographic and geochemical studies show that in high volatile or medium volatile bituminous coal, sapropelic coal with shell component mainly produces water and liquid hydrocarbons, while sapropelic coal with vitrinite component mainly produces dry gas (Rice D D d, 1993). However, some vitrinite components also have the ability to generate gas with higher hydrocarbon concentration (BertrandP, 1984). For example, the vitrinite content of hydrogen-rich coal seams in New Zealand is above 80%, but it has a high oil production capacity (kill ops d et al.,1998); It is found that there is no obvious relationship between crust content and oil production capacity in middle Jurassic humic coal in Beihai, Norway. Gentzis et al. believe that the water content (ethane and propane concentration of 5%) in MedicineRiver coal seam in Alberta, Canada comes from a large number of hydrogen-rich vitrinite components in coal (Gentsist et al., 2008). Generally speaking, because of its high aromatization and oxidation degree and extremely low hydrogen content, the inert group can not only generate oil, but also produce less gas, so it is usually not used as oil and gas parent material. However, in recent years, after in-depth study by coal petrologists, it is found that some inert components are not completely inert, such as the discovery of "active hemifilament" (RSF) in southern hemisphere coal and the division of fluorescent and non-fluorescent inerts (Huang Difan et al., 1992), which provides organic petrological evidence for hydrocarbon generation by inerts. Xu Yongchang et al. heated the inert components, and the oil yield of the residue was 2.94kg/t (Xu Yongchang, 2005).

The author thinks that the influence of maceral of coal and rock on the generation of heavy hydrocarbons is only inferred through microscopic observation and comparison of measured gas components, and the influence of specific maceral on the generation of heavy hydrocarbons has not been verified by experiments, especially the exploration and verification of the particularity of coal chemical structure, and the influence of kerogen of the same type in different regions on the generation of heavy hydrocarbons needs further study.

2.2 microorganisms

Microorganisms can affect the concentration of heavy hydrocarbons in two ways. One is that heavy hydrocarbon bacteria help to produce heavy hydrocarbons in coal seams, and the other is that microorganisms can consume heavy hydrocarbons (such as methanogens) and produce secondary biogas, which is not conducive to the preservation of heavy hydrocarbons.

One explanation is that there are heavy hydrocarbon bacteria in nature, and a small amount of heavy hydrocarbons in biogas is the contribution of heavy hydrocarbon bacteria, that is, biogenesis theory. However, to prove that biological action can form heavy hydrocarbons, there must be the following evidence: under certain geological background, biogenic gas can contain a small amount of heavy hydrocarbon components (0.1%~ 0.2%); The carbon isotope of ethane is relatively light (the carbon isotope values reported at present are all between-70 ‰ and-55 ‰) (Mattavelli L and Martinenghic, 1992), and there is sufficient evidence to prove that there is no other reason for ethane mixing; Another important condition is that heavy hydrocarbon-producing bacteria can be cultivated in the laboratory. Xu Yongchang et al. (2005) measured that the carbon isotope composition δ 13C2 of ethane in Luliang natural gas was -66.0 ‰ ~-6 1.2 ‰. Combined with the analysis of its single geological background, the possibility of thermogenic ethane is basically ruled out, which clearly shows that it is biogenic and gives a positive answer to whether ethane can be generated by biological action for a long time (Xu Yongchang, 2000).

The author believes that the factors that contribute to the generation of heavy hydrocarbons in coal seams, such as heavy hydrocarbon bacteria and bacteria sources, need to be further verified; The premise of secondary biogas affecting the preservation of heavy hydrocarbons to explain the anomaly of heavy hydrocarbons is that a large number of heavy hydrocarbons are produced in the whole syncline coal seam, but some mine fields are well preserved and not affected by microorganisms, so it is necessary to prove the existence of secondary biogas in the normal area of heavy hydrocarbons.

2.3 catalysis

In recent years, more and more scholars began to pay attention to the influence of catalysis on the formation of coalbed methane. Inorganic substances that can play a catalytic role in geological processes are mainly clay minerals, carbonate minerals, oxide minerals and transition metal elements (Wu Yanyan and Telly, 2009). There are also some hypotheses about the influence of catalysts on the formation of heavy hydrocarbons:

In some works, the hypothesis that heavy hydrocarbons in coal seams are caused by the chemical interaction of methane, coal ash and formation water is put forward. According to the conclusion ееворищй, the oxidation of methane contained in rock pores will lead to the formation of polymer homologues, which are as follows:

Fe2O3+2CH4→2FeO+C2H6+H2O and 2Fe(OH)3+2CH4→2FeO+C2H6+4H2O.

However, this assumption may not be correct. We should also study the distribution of heavy hydrocarbons subordinate to mineral impurities in coal seams (аикравцов, 1983).

The role of volcanic activity and deep fluid activity in the geological process of hydrocarbon generation of sedimentary organic matter has been paid more and more attention. Zhang Jinglian believes that the possible mode of crude oil in coal-bearing basins is that deep hydrogen and organic matter are hydrogenated and liquefied to generate hydrocarbons, or deep H2, CO2 and CO are combined in low-speed and high-conductivity layers in the middle crust to generate oil and gas (Zhang Jinglian, 200 1). Jin Zhijun and others believe that deep fluid affects hydrocarbon generation from at least three aspects: first, it directly participates in the hydrocarbon generation process in the form of matter, and hydrogen in deep fluid may react with sedimentary organic matter to increase hydrocarbon generation; Second, the thermal effect, a large amount of thermal energy carried by deep fluid is helpful to improve the maturity of organic matter and accelerate the hydrocarbon generation process of organic matter; The third is catalysis. Various elements carried by deep fluid may become catalysts for hydrocarbon generation from source rocks (Jin Zhijun et al., 2002). The experimental results show that alkanes can be synthesized from CO2 and H2 with molten iron as the medium. Basalt, olivine basalt and peridotite deep underground are similar to molten iron under laboratory conditions (Guo Zhanqian and Yang Haibo, 2005).

The author thinks that if volcanic activity and deep fluid activity play a catalytic role in the process of hydrocarbon generation in coal seam, the distribution characteristics of many abnormal points of heavy hydrocarbons can be well explained, so the influence of fluid activity on heavy hydrocarbons deserves further study.

2.4 coalification stage differences

In the middle stage of thermal genesis of coalbed methane, organic matter is mainly transformed from asphalt formed in the initial stage of degradation of stable components such as resin, spores and keratin, and hydrocarbon branches in aromatic nucleus structure are broken to form gas rich in heavy hydrocarbons. The initial stage of fat coal and coking coal is the peak period of organic matter oil generation, which is an important reason for the relative increase of heavy hydrocarbon concentration in coalbed methane. According to China's statistical data, in the whole coal rank sequence, the maximum reflectance of vitrinite is between 0.9%- 1.4%, and the concentration of heavy hydrocarbons in coalbed methane is obviously high (Wu Jun, 1994).

Although the author is the peak of heavy hydrocarbon production in fat coking coal stage, only a few coalbed methane in fat coking coal have heavy hydrocarbon anomalies, so the coalification stage is the influencing factor of heavy hydrocarbon anomalies, but it is not the only influencing factor.

2.5 different adsorption of coal on gas components

Because of the difference in adsorption potential, the adsorption capacity of coal for heavy hydrocarbon gas components is greater than that of methane. In coal micropores, heavy hydrocarbon gas molecules are mainly adsorbed on the surface of the pore wall, and methane molecules are mainly located in the adsorption layer of heavy hydrocarbon molecules. This difference in adsorption force makes methane molecules migrate easily, which leads to the relative enrichment of heavy hydrocarbon gas in coal seam (Wu Jun, 1994).

Some scholars have noticed the changes of hydrocarbon components in coal due to vitrinite adsorption. Given, Derbyshire and edman found that the oil produced in coal seams was adsorbed in vitrinite micropores (given P,1984; Derby-Charles F et al.,1989; Erdmann M and Horsfield B, 2006). Ritter studied the adsorption of micropores in vitrinite by using the concept of molecular diameter, and simulated the emission of high-content aromatic condensate gas based on the vitrinite adsorption model of Dubinin-Radushkevitch theory. It is considered that the distribution and crosslinking density of micropores in macerals may play a decisive role in the composition of hydrocarbons discharged from coal seams, and the adsorption and dissolution processes in kerogen affect the composition of hydrocarbons discharged from coal seams (RitterU, 2005).

2.6 coal microporous molecular sieve function

The distribution of pores in coal is extremely uneven, which has obvious molecular sieve effect on hydrocarbon gases with different molecular diameters. Methane gas has the smallest molecular diameter and is the easiest to migrate in coal seams. The molecular diameter of heavy hydrocarbon gas is large, and it is often limited by the pore size and stays in the pores during the migration process, which makes the heavy hydrocarbon gas relatively rich and often exists at high pressure (Wu Jun, 1994).

2.7 hydrocarbon displacement effect

Many coal seams have the characteristics of coal, oil and gas generation, and more liquid hydrocarbons in high oil content coal seams occupy the effective pores in coal and replace gaseous hydrocarbons. The smaller the molecular weight, the more obvious the displacement effect. This differential displacement characteristic leads to the relative enrichment of heavy hydrocarbon gas above C2 in coal seam (Wu Jun, 1994).

The author thinks that differential adsorption, molecular sieve action and displacement effect are related to the enrichment and preservation of heavy hydrocarbons by gas fractionation, and the verification of this factor needs to rule out the possibility of differences in hydrocarbon-generating parent materials.

2.8 Oil and gas permeability theory

Advocates of oil and gas permeability believe that the existence of heavy hydrocarbons in coal seams is the result of oil, oil in gas reservoirs or natural gas infiltrating into coal seams (Yu, 198 1).

2.9 structure

Hydrocarbon gas preserved in coal seam now includes not only hydrocarbon gas produced by plutonic metamorphism, but also hydrocarbon gas produced by tectonic coal dynamic metamorphism superimposed on plutonic metamorphism.

Zhao Zhigen and others discussed the hydrocarbon generation of structural coal dynamic metamorphism, and considered that: ① hydrocarbon gas was formed in the process of structural coal dynamic metamorphism; ② Hydrocarbon gas formed by dynamic metamorphism plays an important role in the increase of gas content and pressure; ③ Heavy hydrocarbons were formed during the dynamic metamorphism of structural coal (Zhao Zhigen et al., 1998). Cao and others believe that there are two basic mechanisms by which tectonic stress affects chemical coalification → stress degradation and stress polycondensation. Stress degradation refers to the process that structural stress acts on coal organic macromolecules in the form of mechanical force or kinetic energy, so that chemical bonds with low decomposition energy such as side chains and functional groups on coal aromatic ring structure are broken, degraded into free groups with small molecular weight, and escaped in the form of fluid organic substances (hydrocarbons). Stress polycondensation refers to the process that under the action of anisotropic tectonic stress, the coal aromatic ring laminates are rotated, displaced and arranged in parallel, which improves the level of materialization, makes the basic structural units grow directionally and splice preferentially, and increases the aromatic condensed ring system. Tectonic stress has catalytic significance for coalification (Cao et al., 2006).

The author thinks that analyzing the generation of heavy hydrocarbons from the tectonic dynamic mechanism can explain the distribution characteristics of heavy hydrocarbon anomalies along faults in some areas, but why there are only heavy hydrocarbon anomalies on both sides of some faults needs further study.

3 Conclusion

(1) Heavy hydrocarbon anomalies have occurred in coalbed methane in many areas at home and abroad. The time of heavy hydrocarbon anomalies is concentrated in Carboniferous, Permian and Jurassic, with Permian as the main one. The degree of coalification is in the stages of gas coal, fat coal and coking coal, and it also appears in long-flame coal. Coal seams with heavy hydrocarbon anomalies are usually related to oil and natural gas. Some have liquid oil in the coal seam or its roof and floor, and some have obvious oil and gas display.

(2) The possible reasons for heavy hydrocarbon anomalies are summarized from the aspects of gas-generating parent material, microorganism, catalysis, different stages of coalification, differential adsorption, coal microporous molecular sieve, hydrocarbon expulsion effect, oil and gas permeability theory and tectonic action. And put forward the author's views respectively. It is considered that the study on the causes of heavy hydrocarbon anomalies is of great significance to the genesis, exploration and development of coalbed methane and safe production in coal mines, and various factors need to be further considered in combination with specific areas.

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