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Genetic types and distribution of shallow-buried natural gases

Genetic types and distribution of shallow-buried natural gases of biogases and low-mature gases have been found in the Mesozoic-Cenozoic sedimentary basins in China. Many shallow gas reservoirs in China are characterized by coexistence of biogas and low-mature gas, so identifying the genetic types of shallow gases is important for exploration and development in sedimentary basins. In this paper, we study the gas geochemistry characteristics and distribution in different basins, and classify the shallow gas into two genetic types, biogas and low-mature gas. The biogases are subdivided further into two subtypes by their sources, the source rock-derived biogas and hydrocarbon-derived biogas. Based on the burial history of the source rocks, the source rock-derived biogases are divided into primary and secondary biogas. The former is generated from the source rocks in the primary burial stage, and the latter is from uplifted source rocks or those in a secondary burial stage. In addition, the identifying parameters of each type of shallow gas are given. Based on the analysis above, the distributions of each type of shallow gas are studied. The primary biogases generated from source rocks are mostly distributed in Quaternary basins or modern deltas. Most of them migrate in water- soluble or diffused mode, and their migration distance is short. Reservoir and caprock assemblages play an important role in primary biogas accumulation. The secondary biogases are distributed in a basin with secondary burial history. The oil-degraded biogases are distributed near heavy oil pools. The low-mature gases are widely distributed in shallow-buried reservoirs in the Meso-Cenozoic basins. Primary biogas, secondary biogas, oil-degraded biogas, low-mature gas, genetic type, gas Key words: identifi cation, gas accumulation, distribution biogas can be generated from trapped oil by biodegradation. 1 Introduction Moreover, the organic matter in shallow-buried sediments can In oil and gas exploration, the natural gas which occurs also generate gas rich in methane before thermal degradation at depths of less than 2,000 meters is often known as shallow (Xu et al, 1990; Xu, 1994; Wang et al, 1988; 2003b). This is gas. It is widely distributed in the Meso-Cenozoic basins in called low-mature gas. The biogases are formed by anaerobic China, such as the Qaidam, Junggar, Songliao, and Bohai Bay bacteria, while the low-mature gases are generated in low basins, and is mainly composed of biogas and low-mature gas. temperature and low maturity conditions (Song and Xu, The biogas is also distributed in the sediments of deltas, such 2005). as the Yangtze River Delta. By the end of 2006, the proven Many shallow gas reservoirs in China are characterized geological reserves of biogas are estimated to be 2,860×10 by coexistence of biogas and low-mature gas with different m , and the total geological reserves are estimated to be genetic types. Examples include the gas reservoirs in the 8 3 38,629×10 m , which means that the proven reserves account Liuquan structure of the Langgu Sag and in the Jinhu for only 7.4% of the estimated biogas resource (Zhang et al, Sag, northern Jiangsu. Because different genetic types of 2005). shallow-buried gases have different accumulation processes, Rice and Claypool (1981) proposed that biogas was identifying the genetic types is signifi cant for understanding generated from organic matter in shallow-buried deposits in the gas accumulation. a reducing environment. They stated that it is rich in methane Based on the geochemical characteristics of shallow- and there are two ways for biogas generation, acetic acid buried gases in different basins, we defi ne the genetic types fermentation and CO reduction. Pallasser (2000) found that of gases according to their source materials and develop an identifi cation standard. The accumulation and distribution of the shallow gases is discussed with examples, which helps *Corresponding author. email: jinqiang@upc.edu.cn exploration for shallow gases in different basins. Received January 7, 2009 348 Pet.Sci.(2010)7:347-354 Hami, and Yinggehai-Qiongnan basins, the shallow-buried 2 Genetic types of shallow-buried gases gases are divided into biogases and low-mature gases, and There is not a uniform classifi cation for biogases and low- then the biogases are subdivided into two types according mature gases. In foreign countries, little attention is focused to their sources, one is from source rocks, and the other is on the genetic types of shallow-buried gases and research from hydrocarbon reservoirs. The biogases derived from has concentrated mainly on coalbed methane (CBM). Based source rocks are further classifi ed into primary biogases and on the study of coal bed biogenic methane in the Powder secondary biogases. Table 1 gives the identifying parameters River Basin, Flores et al (2008) suggested that biogases for each type of the gases. could be divided into CO reduction biogas and acetic acid 2.1 Primary biogas fermentation biogas. In China, Dai (1992) believed that the The methane content of primary biogases is as high as shallow-buried gases consisted of biogases and sub-biogases. 98%, the ethane and higher alkane (C ) content is very low Xu (1994) suggested that the shallow-buried gases had two 2+ (about 0.5%), the C /C ratio is greater than 0.99, and the types, biogases and bio-thermal-catalytic transitional gases. 1 1-5 nonhydrocarbon contents are less than 5% (Table 2). The Li et al (2005a) divided the biogases into primary biogases δ C of the primary biogas is less than -55‰ (Rice and and secondary biogases. Claypool, 1981; Dai, 1992). Based on the study in the Qaidam, Bohai Bay, Turpan- Genetic types and identifying parameters of biogases and low-mature gases Table 1 Biogas Derived from hydrocarbon Genetic types Derived from source rocks Low-mature gas reservoirs Primary biogas Secondary biogas Oil-degraded biogas Generated from immature Generated from source rocks Thermal degradation from Generation process source rocks in primary in uplift or secondary burial Oil biodegradation source rocks in the low burial history history maturity stage Mainly methane, a little C , Mainly methane, little C , C /C between 0.7 and 0.89, 2+ 2+ 1 1-5 Gas composition Mainly methane, little C 2+ CO and N more N than other biogas a little nonhydrocarbon 2 2 2 13 13 13 13 13 13 Carbon isotope composition δ C <-55‰, δ C <-40‰ δ C ≤-50‰, δ C > -40‰ -50‰<δ C <-75‰ -45‰<δ C <-55‰ 1 2 1 2 1 1 Qaidam, Yangtze River West slope of Songliao Basin, Turpan-Hami Basin Examples Langgu Sag Delta, Jiangsu Basin Jiyang Depression Liaohe Depression Primary biogases are generated by acetic acid less than 2% (C /C ratio > 0.98) and the nonhydrocarbon 1 1-5 fermentation or CO reduction. The latter is the main source contents are rather low. Carbon isotope compositions of of biogas. δD from acetic acid fermentation is less than methane in the secondary biogases are slightly heavier than CH -300‰, and δD from CO reduction ranges between those in the primary biogases. Meanwhile, the carbon isotope CH 2 -300‰ and -160‰. Whiticar et al (1986) analyzed 500 compositions of ethane and propane in the secondary biogas natural gas samples from different basins all over the world are also heavier than those in the primary biogas (Table 3). and suggested that the δD value of natural gases derived The sedimentary basins in east China experienced rifting CH from source rocks deposited in fresh-water environment is in the Tertiary, and were uplifted in the Himalayan movement. lower than that of natural gases derived from source rocks Before the movement, the basins were in the primary burial deposited in saline waters. Therefore, δD is not an effective stage and some source rocks were in the biogas generation CH indicator for biogas identifi cation. stage. After the movement, some source rocks returned to The primary biogases are distributed mainly in the the bacterial activity zone so that secondary biogas was Quaternary in the Qaidam Basin, Neocene-Quaternary in the generated. The secondary biogases are mainly distributed in Yinggehai-Qiongnan Basin, and the Yangtze River Delta. A the Bohai Bay Basin and small Tertiary basins, such as the few primary biogas reservoirs are found in the Tertiary. Baoshan and Qujing basins in the Yunnan Province. 2.2 Secondary biogas 2.3 Oil-degraded biogas Shi (2002) conducted biochemical experiments on source Anaerobic microorganisms generate biogas from rocks sampled from the Mesozoic in the Liaohe Depression oils in shallow-buried reservoirs (Zhu et al, 2005). In and from the Tertiary in the Langgu Sag, and demonstrated oil biodegradation, the oil loses its light hydrocarbon that the source rocks after thermal maturation could also components, so that its density, viscosity and sulfur content generate biogas. increase (Zhu et al, 2007). Secondary biogas is usually mixed with low-mature gas Because of the isotope fractionation which occurs during in a reservoir. The methane content in the secondary biogas the oil biodegradation, the carbon isotope value of methane is as high as 98%, while the heavy hydrocarbon contents are in the oil-degraded biogas is very low (-100‰ - -55‰), and Pet.Sci.(2010)7:347-354 349 Geochemical characteristics of primary biogas in China Table 2 Gas composition, % Structural belt Well No. Strata Depth, m δ C , ‰ C /C 1 1 1+ CH C H Nonhydrocarbon 4 2 6 Y1 Es 1504 98.1 0 1.91 -- 1 Y101 Es 1504 98.4 0.17 1.04 -60.6 0.9985 Y15 Es 1412 97.0 0.08 1.79 -60.9 0.9992 Yangxin Sub-sag Y16 Es 1309 97.8 0.18 2.06 -56.5 0.9982 Y21 Es 1520 95.0 0 5.00 -- 1 Average 97.3 0.14 2.36 -59.3 0.9986 TN Q -- 94.3 0 5.73 -69.9 1 1-2 SB1 Q -- 98.7 0.13 1.15 -66.8 0.9986 1-2 SB2 Q -- 98.4 0 1.6 -65.4 1 1-2 Qaidam Basin YH Q -- 95.2 0.28 4.52 -65.0 0.9971 1-2 TFS Q -- 99.9 0 0.15 -68.8 1 1-2 Average 97.3 0.08 2.63 -67.18 0.9992 J1 Q -- 96.2 -- 3.12 -74.2 -- L1 Q -- 94.2 -- 2.08 -- -- D1 Q -- 92.3 -- 2.03 -- -- Southeast Littoral T1 Q -- 93.9 -- 2.04 -- -- F6 Q -- 91.4 -- 1.47 -84.5 -- Average 93.6 -- 2.15 -79.4 -- Geochemical characteristics of Es biogas reservoir in the Liuquan structure Table 3 Gas composition, % Well No. Strata Depth, m δ C , ‰ C /C 1 1 1+ CH C H N CO 4 2 6 2 2 G26 Es 1530 98.5 1.09 0.32 0.1 -55.7 0.99 G131 Es 1168 98.6 0.05 1.36 0.05 -57.2 0.99 G13 Es 1612 97.2 1.62 0.63 0.19 -55.9 0.98 G131 Es 1597 96.8 2.11 0.81 0.12 -54.3 -- Q36 Es 1001 98.8 0.63 0.167 0.32 -51.1 0.99 XQ2 Es 1135 99.0 0 1.17 -- -55.6 0.99 Average 98.2 0.91 0.74 0.15 -55.0 0.988 δ C of CO is abnormally high (up to +2‰) (Table 4). Oil- the oil generation peak (Xu, 1994; 1999). degraded biogases are often distributed with heavy oils, and Low-mature gases are characterized by lower C /C ratio 1 1-5 usually accumulate with other shallow-buried gases. (0.7-0.9) and heavier carbon isotope (δ C between -55‰ The oil-degraded biogases have been found in the west and -42‰) compared with those of biogases. slope of the Songliao Basin, Bohai Bay and Junggar basins. The identification of low-mature gas is a disputed The potential reserves of oil-degraded biogases in the issue. Galimov (1988) believed that R of the source rocks 8 3 Songliao Basin are estimated to be about 240×10 m . generating low-mature gas ranged from 0.5% to 0.7% based on the study of Siberian gas. Wang et al (2003b) concluded 2.4 Low-mature gas that the low-mature gases are generated from the source rocks with R from 0.4% to 0.55%, even up to 0.7%, and the burial The low-mature gas is also called early thermal genetic o depth of the source rocks is 1,000-2,500 m, even up to 3,000 gas or bio-thermal-catalytic transitional gas (Xu et al, 1990). m. Xu et al (2008) suggested that source rocks generated low- Actually, it is formed after the biogas generation and before 350 Pet.Sci.(2010)7:347-354 Table 4 Geochemical characteristics of oil-degraded biogas in the Songliao Basin Gas composition, % 13 13 13 Well C /C δ C , ‰ δ C , ‰ δ C , ‰ 1 1+ 1 2 CO CH C CO N 4 2+ 2 2 D2-2 94.4 0.08 0.059 5.405 0.9992 -56.32 -44.2 -2.69 D5-3 93.93 0.06 0.062 5.898 0.9994 -57.55 -46.2 0.82 D6 90.63 0.03 0.099 9.162 0.9997 -59.06 -46.7 -1.89 D602 91.14 0.05 0.072 8.665 0.9995 -58.93 -47.6 0.95 D603 91.66 0.12 0.115 8.031 0.9987 -58.45 -51.3 -3.87 D610 91.26 0.06 0.087 8.511 0.9993 -58.75 -47.4 -6.9 D621 89.86 0.13 0.091 9.328 0.9986 -59.45 -47.3 -18.78 D2-2 94.4 0.078 0.059 5.405 0.9992 -56.32 -44.2 -2.69 D5-3 93.93 0.058 0.062 5.898 0.9994 -57.55 -46.2 0.82 D610 91.26 0.059 0.087 8.511 0.9994 -58.75 -47.4 -6.9 D602 91.14 0.051 0.072 8.665 0.9996 -58.93 -47.6 0.95 D6 90.63 0.032 0.099 9.162 0.9986 -59.06 -46.7 -1.89 D603 91.66 0.115 0.115 8.031 0.9987 -58.45 -51.3 -3.87 mature gas when R was less than 0.8%. 11.80%. The source rocks have been in the biogas generation stage since Ng. The reservoirs are mainly bioclastic 3 Distribution of various genetic types of limestones and dolomites deposited in lakeshore or sand sheet facies with high porosity and permeability with a caprock of shallow-buried gases lacustrine mudstones. The distributions of shallow-buried gases are controlled 02 46km by the sources. Biogas generation needs conditions suitable for microorganism metabolism, such as temperature between Y 23 Y 101 Y 1 Y 18 Y 15 Y 16 Y 24 35°C and 75°C and appropriate pH, Eh and salinity (Chen Ng Ng et al, 1994). The distribution of shallow-buried gases is also Ed Ed controlled by structural aspects, sedimentary facies and seal Es qualities. Legend Es Igneous rock Es Biogas reservoir 3.1 Distribution of primary biogases Migration direction The primary biogases are mainly distributed in the Central uplift zone Drape structure anticlinal trap structural-lithologic trap Quaternary basins or modern delta deposits, such as the eastern Qaidam Basin and the Yangtze River Delta. The Fig. 1 Es primary biogas accumulation in the Yangxin Sub-sag primary biogases migrate in water-soluble or diffused mode. The migration distance is short, so most of the biogas The biogases in the Yangxin Sub-sag migrated in water- reservoirs are distributed near the source kitchens. soluble or diffused mode. The migration distance is short The primary biogas reservoirs have high porosity and because the biogases diffuse easily in rocks with weak permeability due to their weak diagenesis. The dynamic diagenesis. Biogases mostly accumulated in the central uplift balance of charging and dissipating of biogases in the near the sub-sags. In the central uplift, many traps developed, reservoirs plays an important role in the biogas accumulation. which were suitable for gas capturing (Fig. 1). Biogases from In general, the biogas charging quantity is closely controlled the west sub-sag accumulated in the eastern drape structure by gas generating intensity. If the biogas source is suffi cient, and fi nally structural-lithologic gas pools were formed. The the cap-rock controls the enrichment of biogas in the charging biogases in the Yangxin Sub-sag were preserved because of area. good cap-rocks and a lack of faulting. Taking the Es primary biogas reservoir in the Yangxin Sub-sag for example, a series of traps are developed on the 3.2 Distribution of secondary biogases slope and structural highs, just updip on the source kitchen, and are charged fully with the biogas (Fig. 1). Its source rocks Study of secondary biogas has been mainly about whether are grey mudstones and dark-grey oil shales with R between the source rocks are able to generate biogas after thermal 0.27% and 0.35%, and TOC contents between 0.22% and evolution, but the distribution of secondary biogas is less Depth, m Pet.Sci.(2010)7:347-354 351 studied. caprocks. The source rocks of the secondary biogas experienced The Liuquan structure has experienced complex tectonic thermal evolution, and were then uplifted to the bacterial activities, and many faults are developed. The distributions activity zone. These source rocks are widely distributed in of biogases are controlled by the source conditions and fault basins. During the Mesozoic-Tertiary, some basins were in a activities. The evolution of source rocks can be divided into rifting stage and in deep-water environments, so that source three stages, I: primary subsidence stage, II: uplift stage, III: rocks were deposited. In the Himalayan movement, most of secondary subsidence stage (Fig. 2). the basins were uplifted, and the source rocks went back into During the first two stages, almost all faults are active, the biological activity zone. Thus, the secondary biogases and the sediments are strongly uplifted by the structural could be generated. movement so biogases were not easily preserved. Taking the Faults are well developed in the rift basins. The faults not Q2 well as an example, the distance between the top of the only provide good migration pathways for biogases when Es formation and the bottom of the Ng formation is only 100 they are active, but also act as a good seal for the biogases m (Fig. 3). After Ng, the secondary biogases are easy to be when they are not active. preserved because of weak fault activities. Taking the Es secondary biogas in the Liuquan Therefore, secondary biogas accumulation occurs in structure as an example, fault activities are signifi cant in gas basins or depressions with uplifting or secondary burial accumulation (Fig. 2). history, and is controlled by fault activities. 3.3 Distribution of oil-degraded biogas Time, Ma 50 40 30 20 10 0 Oil-degraded biogases accumulated near heavy oil pools. 0 Q Ed Nm In the biogas generation, water activity is important for Es Ng Es microorganism propagation so that the biogas reservoirs are Es often located near the oil-water contact. The biogas reserves Es 1500 are determined by the quantity of biodegraded oils. 3 Oil-degraded biogases are distributed in shallow 2000 Es reservoirs in the Songliao, Bohai Bay (Fig. 4) and Junggar basins, because many heavy oil reservoirs are distributed in these basins. The Alaxin lithologic-structural gas reservoir is located in the west slope of the Songliao Basin. It is less than 700 m Fig. 2 Burial history of Es -Ed source rocks of the Q35 well 9 3 deep. The reserves of biogases are 1.74×10 m , and the daily 6 3 gas production is 0.115×10 m . It is a shallow-buried, low The source rocks of Liuquan biogases are mainly dark abundance, high production small gas field. The gases are lacustrine mudstones with thicknesses of 1,100 m. Their characterized by high methane content and relatively light TOC contents are 1.24%-1.98%, and R value is generally δ C with the content from -83.88‰ to -58.25‰, showing 0.5. The kerogen type is mainly II -II . The source rocks are 1 2 microorganism degradation characteristics. less than 2,000 m deep because of tectonic movements, and The sources of Alaxin biogases are oil pools (reserves are favorable for biogas generation. The reservoirs consist are 3.15 million ton) around the biogases. The oil pools are of delta, fan delta and sublacustrine fan sandstones. The commonly degraded and most of the n-alkane in oil has been porosity is from 11.2% to 32.6% with an average of 26.8%, degraded by microorganisms(Fig. 5). The Alaxin biogas and the permeability is 0.1-876 md with an average of 193.6 reservoirs are grey green mudstones and siltstones deposited md. The lacustrine mudstones cover the reservoirs directly as 00.6km Q107 Xq2 G33x Q46-11 Q177 Q58-5 Q46-65 Q50 Xq181 Q36 Nm 900 3 Es 4-Ư 4-Ư Es Ư Es Ư 2 3 1100 1100 Es 3 Nm Es 4-Ư Es ư Es 3 3 1300 4-Ư Es ư 5 -Ư 3 Es Ư 5 -Ư 4-2 Es ư Es Ư 3 3 5 -3 Es Ư 1500 4-2 4-3 3 Es ư Es Ư 4-2 3 3 Es ư Es Es 1700 3 Es Es Oil Gas Water-bearing Faults oil reservoirs Fig. 3 Oil and gas accumulation in the Liuquan structure in the Langgu Sag Depth, m Depth, m 352 Pet.Sci.(2010)7:347-354 Pet.Sci.(2010)7:347-354 353 Table 5 Activation energy of gas production from source rocks in different basins Activation energy, KJ/mol Source rocks Data source Methane Heavy hydrocarbon Jurassic coal in the Kuche Sag 235 269 Li et al, 2003 Jurassic mudstone in the Kuche Sag 206 223 Li et al, 2003 Tertiary oil shale in the Dongying Sag 272 259 Wang et al, 2003a Jurassic coal in the southwest Tarim Basin 227 281 Li et al, 2004 Jurassic mudstone in the southwest Tarim Basin 223 281 Li et al, 2004 Yancheng Formation kerogen in the Qiongdongnan Basin 231 -- Li et al, 2005b Shanxi Formation coal in the Ordos Basin 273 -- Mi et al, 2005 Coal in the Turpan-Hami Basin 209 169 Xu et al, 2008 Mudstone in the Turpan-Hami Basin 191 147 Xu et al, 2008 Dai J X. Identifi cation of different kinds of alkane gases. Science in of biogas and low-mature gas. The biogas is divided into two China, Ser. B. 1992. 22(2): 185-193 (in Chinese) types according to their sources, one is from source rocks, Flor es R M, Rice C A, Stricker G D, et al. Methanogenic pathways and the other is from hydrocarbon reservoirs. The biogases of coal-bed gas in the Powder River Basin, United States: The derived from source rocks are classifi ed further into primary geologic factor. International Journal of Coal Geology. 2008. biogas and secondary biogas. The former is generated from 76(1-2): 52-75 immature source rocks in primary burial stage, and the latter Gal imov E M. Sources and mechanisms of formation of gaseous is generated from the source rocks in uplift or secondary hydrocarbons in sedimentary rocks. Chemical Geology. 1988. burial stage. 71(1-3): 77-95 The identifying parameters of shallow-buried gas are Li X Q, Xiao X M, Tang Y, et al. Kinetic study of carbon isotopes in given. Choosing suitable identifying parameters combined humic gas methane in the Kuqa Depression. Oil & Gas Geology. with the analysis of accumulation conditions can distinguish 2004. 25(1): 21-25 (in Chinese) Li X Q, Xiao X M, Tang Y C, et al. The origin evaluation of natural the genetic types of shallow-buried gases effectively. The gas using carbon isotope kinetic modeling. China Petroleum heavy hydrocarbon content of biogas is lower than that of Exploration. 2003. 8(4): 50-55 (in Chinese) low-mature gas. The former is lower than 0.5%, and the latter Li X Q, Zhang S C, Zhu G Y, et al. Types and research direction of is about 2%-30%. The δ C of biogas is less than -55‰, but biogenic gas in China. Natural Gas Geoscience. 2005a. 16(4): that of low-mature gas is about -50‰ to -42‰. The difference 477-484 (in Chinese) between biogas derived from source rocks and biogas derived Li X S, Xiao X M, Huang B J, et al. Hydrocarbon-generating from hydrocarbon reservoirs is mainly demonstrated in the dynamics and carbon isotope dynamics of the source rocks in δ C of CO . The latter is much heavier than the former. Yanan Sag. Natural Gas Industry. 2005b. 25(8): 9-11 (in Chinese) The carbon isotope of heavy hydrocarbons can be used to Liu W H and Xu Y C. Relationship between oil and gas from the identify the primary biogas and secondary biogas. The heavy bio-, thermo-catalytic transitional zone. Acta Sedimentologica hydrocarbons of secondary biogas show thermal genetic Sinica. 1995. 13(2): 4-13 (in Chinese) Liu W H and Xu Y C. Genetic indicators for natural gases. Acta characteristics and their carbon isotope of C is heavier than 2+ Sedimentologica Sinica. 1996. 14(1): 20-28 (in Chinese) that of the primary biogas, generally δ C >-40‰. Mi J K, Liu X H, Yang M D, et al. Discussion on the source of oil The distribution of different types of shallow-buried gas and gas using the dynamics of carbon isotope and hydrocarbon is controlled by different factors. Primary biogas generated generation. Acta Sedimentologica Sinica. 2005. 23(3): 537-541 from source rocks is mostly distributed in Quaternary basins (in Chinese) or modern deltas, and its accumulation is controlled by Pal lasser R J. Recognising biodegradation in gas/oil accumulations the properties of the reservoir and caprock assemblages. 13 through the δ C compositions of gas components. Organic Secondary biogas is mainly found in a basin with secondary Geochemistry. 2000. 31(12): 1363-1373 burial history, and the distribution is controlled by fault Ric e D D and Claypool G E. Generation, accumulation, and resource activity. The oil-degraded biogas is distributed near heavy oil potential of biogenic gas. 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Genetic types and distribution of shallow-buried natural gases

Gao, Yang; Jin, Qiang; Zhu, Guangyou
Petroleum Science , Volume 7 (3) – Aug 3, 2010
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Springer Journals
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Copyright © 2010 by China University of Petroleum (Beijing) and Springer-Verlag Berlin Heidelberg
Subject
Earth Sciences; Mineral Resources; Industrial Chemistry/Chemical Engineering; Industrial and Production Engineering; Energy Economics
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1672-5107
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1995-8226
DOI
10.1007/s12182-010-0076-y
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Abstract

of biogases and low-mature gases have been found in the Mesozoic-Cenozoic sedimentary basins in China. Many shallow gas reservoirs in China are characterized by coexistence of biogas and low-mature gas, so identifying the genetic types of shallow gases is important for exploration and development in sedimentary basins. In this paper, we study the gas geochemistry characteristics and distribution in different basins, and classify the shallow gas into two genetic types, biogas and low-mature gas. The biogases are subdivided further into two subtypes by their sources, the source rock-derived biogas and hydrocarbon-derived biogas. Based on the burial history of the source rocks, the source rock-derived biogases are divided into primary and secondary biogas. The former is generated from the source rocks in the primary burial stage, and the latter is from uplifted source rocks or those in a secondary burial stage. In addition, the identifying parameters of each type of shallow gas are given. Based on the analysis above, the distributions of each type of shallow gas are studied. The primary biogases generated from source rocks are mostly distributed in Quaternary basins or modern deltas. Most of them migrate in water- soluble or diffused mode, and their migration distance is short. Reservoir and caprock assemblages play an important role in primary biogas accumulation. The secondary biogases are distributed in a basin with secondary burial history. The oil-degraded biogases are distributed near heavy oil pools. The low-mature gases are widely distributed in shallow-buried reservoirs in the Meso-Cenozoic basins. Primary biogas, secondary biogas, oil-degraded biogas, low-mature gas, genetic type, gas Key words: identifi cation, gas accumulation, distribution biogas can be generated from trapped oil by biodegradation. 1 Introduction Moreover, the organic matter in shallow-buried sediments can In oil and gas exploration, the natural gas which occurs also generate gas rich in methane before thermal degradation at depths of less than 2,000 meters is often known as shallow (Xu et al, 1990; Xu, 1994; Wang et al, 1988; 2003b). This is gas. It is widely distributed in the Meso-Cenozoic basins in called low-mature gas. The biogases are formed by anaerobic China, such as the Qaidam, Junggar, Songliao, and Bohai Bay bacteria, while the low-mature gases are generated in low basins, and is mainly composed of biogas and low-mature gas. temperature and low maturity conditions (Song and Xu, The biogas is also distributed in the sediments of deltas, such 2005). as the Yangtze River Delta. By the end of 2006, the proven Many shallow gas reservoirs in China are characterized geological reserves of biogas are estimated to be 2,860×10 by coexistence of biogas and low-mature gas with different m , and the total geological reserves are estimated to be genetic types. Examples include the gas reservoirs in the 8 3 38,629×10 m , which means that the proven reserves account Liuquan structure of the Langgu Sag and in the Jinhu for only 7.4% of the estimated biogas resource (Zhang et al, Sag, northern Jiangsu. Because different genetic types of 2005). shallow-buried gases have different accumulation processes, Rice and Claypool (1981) proposed that biogas was identifying the genetic types is signifi cant for understanding generated from organic matter in shallow-buried deposits in the gas accumulation. a reducing environment. They stated that it is rich in methane Based on the geochemical characteristics of shallow- and there are two ways for biogas generation, acetic acid buried gases in different basins, we defi ne the genetic types fermentation and CO reduction. Pallasser (2000) found that of gases according to their source materials and develop an identifi cation standard. The accumulation and distribution of the shallow gases is discussed with examples, which helps *Corresponding author. email: jinqiang@upc.edu.cn exploration for shallow gases in different basins. Received January 7, 2009 348 Pet.Sci.(2010)7:347-354 Hami, and Yinggehai-Qiongnan basins, the shallow-buried 2 Genetic types of shallow-buried gases gases are divided into biogases and low-mature gases, and There is not a uniform classifi cation for biogases and low- then the biogases are subdivided into two types according mature gases. In foreign countries, little attention is focused to their sources, one is from source rocks, and the other is on the genetic types of shallow-buried gases and research from hydrocarbon reservoirs. The biogases derived from has concentrated mainly on coalbed methane (CBM). Based source rocks are further classifi ed into primary biogases and on the study of coal bed biogenic methane in the Powder secondary biogases. Table 1 gives the identifying parameters River Basin, Flores et al (2008) suggested that biogases for each type of the gases. could be divided into CO reduction biogas and acetic acid 2.1 Primary biogas fermentation biogas. In China, Dai (1992) believed that the The methane content of primary biogases is as high as shallow-buried gases consisted of biogases and sub-biogases. 98%, the ethane and higher alkane (C ) content is very low Xu (1994) suggested that the shallow-buried gases had two 2+ (about 0.5%), the C /C ratio is greater than 0.99, and the types, biogases and bio-thermal-catalytic transitional gases. 1 1-5 nonhydrocarbon contents are less than 5% (Table 2). The Li et al (2005a) divided the biogases into primary biogases δ C of the primary biogas is less than -55‰ (Rice and and secondary biogases. Claypool, 1981; Dai, 1992). Based on the study in the Qaidam, Bohai Bay, Turpan- Genetic types and identifying parameters of biogases and low-mature gases Table 1 Biogas Derived from hydrocarbon Genetic types Derived from source rocks Low-mature gas reservoirs Primary biogas Secondary biogas Oil-degraded biogas Generated from immature Generated from source rocks Thermal degradation from Generation process source rocks in primary in uplift or secondary burial Oil biodegradation source rocks in the low burial history history maturity stage Mainly methane, a little C , Mainly methane, little C , C /C between 0.7 and 0.89, 2+ 2+ 1 1-5 Gas composition Mainly methane, little C 2+ CO and N more N than other biogas a little nonhydrocarbon 2 2 2 13 13 13 13 13 13 Carbon isotope composition δ C <-55‰, δ C <-40‰ δ C ≤-50‰, δ C > -40‰ -50‰<δ C <-75‰ -45‰<δ C <-55‰ 1 2 1 2 1 1 Qaidam, Yangtze River West slope of Songliao Basin, Turpan-Hami Basin Examples Langgu Sag Delta, Jiangsu Basin Jiyang Depression Liaohe Depression Primary biogases are generated by acetic acid less than 2% (C /C ratio > 0.98) and the nonhydrocarbon 1 1-5 fermentation or CO reduction. The latter is the main source contents are rather low. Carbon isotope compositions of of biogas. δD from acetic acid fermentation is less than methane in the secondary biogases are slightly heavier than CH -300‰, and δD from CO reduction ranges between those in the primary biogases. Meanwhile, the carbon isotope CH 2 -300‰ and -160‰. Whiticar et al (1986) analyzed 500 compositions of ethane and propane in the secondary biogas natural gas samples from different basins all over the world are also heavier than those in the primary biogas (Table 3). and suggested that the δD value of natural gases derived The sedimentary basins in east China experienced rifting CH from source rocks deposited in fresh-water environment is in the Tertiary, and were uplifted in the Himalayan movement. lower than that of natural gases derived from source rocks Before the movement, the basins were in the primary burial deposited in saline waters. Therefore, δD is not an effective stage and some source rocks were in the biogas generation CH indicator for biogas identifi cation. stage. After the movement, some source rocks returned to The primary biogases are distributed mainly in the the bacterial activity zone so that secondary biogas was Quaternary in the Qaidam Basin, Neocene-Quaternary in the generated. The secondary biogases are mainly distributed in Yinggehai-Qiongnan Basin, and the Yangtze River Delta. A the Bohai Bay Basin and small Tertiary basins, such as the few primary biogas reservoirs are found in the Tertiary. Baoshan and Qujing basins in the Yunnan Province. 2.2 Secondary biogas 2.3 Oil-degraded biogas Shi (2002) conducted biochemical experiments on source Anaerobic microorganisms generate biogas from rocks sampled from the Mesozoic in the Liaohe Depression oils in shallow-buried reservoirs (Zhu et al, 2005). In and from the Tertiary in the Langgu Sag, and demonstrated oil biodegradation, the oil loses its light hydrocarbon that the source rocks after thermal maturation could also components, so that its density, viscosity and sulfur content generate biogas. increase (Zhu et al, 2007). Secondary biogas is usually mixed with low-mature gas Because of the isotope fractionation which occurs during in a reservoir. The methane content in the secondary biogas the oil biodegradation, the carbon isotope value of methane is as high as 98%, while the heavy hydrocarbon contents are in the oil-degraded biogas is very low (-100‰ - -55‰), and Pet.Sci.(2010)7:347-354 349 Geochemical characteristics of primary biogas in China Table 2 Gas composition, % Structural belt Well No. Strata Depth, m δ C , ‰ C /C 1 1 1+ CH C H Nonhydrocarbon 4 2 6 Y1 Es 1504 98.1 0 1.91 -- 1 Y101 Es 1504 98.4 0.17 1.04 -60.6 0.9985 Y15 Es 1412 97.0 0.08 1.79 -60.9 0.9992 Yangxin Sub-sag Y16 Es 1309 97.8 0.18 2.06 -56.5 0.9982 Y21 Es 1520 95.0 0 5.00 -- 1 Average 97.3 0.14 2.36 -59.3 0.9986 TN Q -- 94.3 0 5.73 -69.9 1 1-2 SB1 Q -- 98.7 0.13 1.15 -66.8 0.9986 1-2 SB2 Q -- 98.4 0 1.6 -65.4 1 1-2 Qaidam Basin YH Q -- 95.2 0.28 4.52 -65.0 0.9971 1-2 TFS Q -- 99.9 0 0.15 -68.8 1 1-2 Average 97.3 0.08 2.63 -67.18 0.9992 J1 Q -- 96.2 -- 3.12 -74.2 -- L1 Q -- 94.2 -- 2.08 -- -- D1 Q -- 92.3 -- 2.03 -- -- Southeast Littoral T1 Q -- 93.9 -- 2.04 -- -- F6 Q -- 91.4 -- 1.47 -84.5 -- Average 93.6 -- 2.15 -79.4 -- Geochemical characteristics of Es biogas reservoir in the Liuquan structure Table 3 Gas composition, % Well No. Strata Depth, m δ C , ‰ C /C 1 1 1+ CH C H N CO 4 2 6 2 2 G26 Es 1530 98.5 1.09 0.32 0.1 -55.7 0.99 G131 Es 1168 98.6 0.05 1.36 0.05 -57.2 0.99 G13 Es 1612 97.2 1.62 0.63 0.19 -55.9 0.98 G131 Es 1597 96.8 2.11 0.81 0.12 -54.3 -- Q36 Es 1001 98.8 0.63 0.167 0.32 -51.1 0.99 XQ2 Es 1135 99.0 0 1.17 -- -55.6 0.99 Average 98.2 0.91 0.74 0.15 -55.0 0.988 δ C of CO is abnormally high (up to +2‰) (Table 4). Oil- the oil generation peak (Xu, 1994; 1999). degraded biogases are often distributed with heavy oils, and Low-mature gases are characterized by lower C /C ratio 1 1-5 usually accumulate with other shallow-buried gases. (0.7-0.9) and heavier carbon isotope (δ C between -55‰ The oil-degraded biogases have been found in the west and -42‰) compared with those of biogases. slope of the Songliao Basin, Bohai Bay and Junggar basins. The identification of low-mature gas is a disputed The potential reserves of oil-degraded biogases in the issue. Galimov (1988) believed that R of the source rocks 8 3 Songliao Basin are estimated to be about 240×10 m . generating low-mature gas ranged from 0.5% to 0.7% based on the study of Siberian gas. Wang et al (2003b) concluded 2.4 Low-mature gas that the low-mature gases are generated from the source rocks with R from 0.4% to 0.55%, even up to 0.7%, and the burial The low-mature gas is also called early thermal genetic o depth of the source rocks is 1,000-2,500 m, even up to 3,000 gas or bio-thermal-catalytic transitional gas (Xu et al, 1990). m. Xu et al (2008) suggested that source rocks generated low- Actually, it is formed after the biogas generation and before 350 Pet.Sci.(2010)7:347-354 Table 4 Geochemical characteristics of oil-degraded biogas in the Songliao Basin Gas composition, % 13 13 13 Well C /C δ C , ‰ δ C , ‰ δ C , ‰ 1 1+ 1 2 CO CH C CO N 4 2+ 2 2 D2-2 94.4 0.08 0.059 5.405 0.9992 -56.32 -44.2 -2.69 D5-3 93.93 0.06 0.062 5.898 0.9994 -57.55 -46.2 0.82 D6 90.63 0.03 0.099 9.162 0.9997 -59.06 -46.7 -1.89 D602 91.14 0.05 0.072 8.665 0.9995 -58.93 -47.6 0.95 D603 91.66 0.12 0.115 8.031 0.9987 -58.45 -51.3 -3.87 D610 91.26 0.06 0.087 8.511 0.9993 -58.75 -47.4 -6.9 D621 89.86 0.13 0.091 9.328 0.9986 -59.45 -47.3 -18.78 D2-2 94.4 0.078 0.059 5.405 0.9992 -56.32 -44.2 -2.69 D5-3 93.93 0.058 0.062 5.898 0.9994 -57.55 -46.2 0.82 D610 91.26 0.059 0.087 8.511 0.9994 -58.75 -47.4 -6.9 D602 91.14 0.051 0.072 8.665 0.9996 -58.93 -47.6 0.95 D6 90.63 0.032 0.099 9.162 0.9986 -59.06 -46.7 -1.89 D603 91.66 0.115 0.115 8.031 0.9987 -58.45 -51.3 -3.87 mature gas when R was less than 0.8%. 11.80%. The source rocks have been in the biogas generation stage since Ng. The reservoirs are mainly bioclastic 3 Distribution of various genetic types of limestones and dolomites deposited in lakeshore or sand sheet facies with high porosity and permeability with a caprock of shallow-buried gases lacustrine mudstones. The distributions of shallow-buried gases are controlled 02 46km by the sources. Biogas generation needs conditions suitable for microorganism metabolism, such as temperature between Y 23 Y 101 Y 1 Y 18 Y 15 Y 16 Y 24 35°C and 75°C and appropriate pH, Eh and salinity (Chen Ng Ng et al, 1994). The distribution of shallow-buried gases is also Ed Ed controlled by structural aspects, sedimentary facies and seal Es qualities. Legend Es Igneous rock Es Biogas reservoir 3.1 Distribution of primary biogases Migration direction The primary biogases are mainly distributed in the Central uplift zone Drape structure anticlinal trap structural-lithologic trap Quaternary basins or modern delta deposits, such as the eastern Qaidam Basin and the Yangtze River Delta. The Fig. 1 Es primary biogas accumulation in the Yangxin Sub-sag primary biogases migrate in water-soluble or diffused mode. The migration distance is short, so most of the biogas The biogases in the Yangxin Sub-sag migrated in water- reservoirs are distributed near the source kitchens. soluble or diffused mode. The migration distance is short The primary biogas reservoirs have high porosity and because the biogases diffuse easily in rocks with weak permeability due to their weak diagenesis. The dynamic diagenesis. Biogases mostly accumulated in the central uplift balance of charging and dissipating of biogases in the near the sub-sags. In the central uplift, many traps developed, reservoirs plays an important role in the biogas accumulation. which were suitable for gas capturing (Fig. 1). Biogases from In general, the biogas charging quantity is closely controlled the west sub-sag accumulated in the eastern drape structure by gas generating intensity. If the biogas source is suffi cient, and fi nally structural-lithologic gas pools were formed. The the cap-rock controls the enrichment of biogas in the charging biogases in the Yangxin Sub-sag were preserved because of area. good cap-rocks and a lack of faulting. Taking the Es primary biogas reservoir in the Yangxin Sub-sag for example, a series of traps are developed on the 3.2 Distribution of secondary biogases slope and structural highs, just updip on the source kitchen, and are charged fully with the biogas (Fig. 1). Its source rocks Study of secondary biogas has been mainly about whether are grey mudstones and dark-grey oil shales with R between the source rocks are able to generate biogas after thermal 0.27% and 0.35%, and TOC contents between 0.22% and evolution, but the distribution of secondary biogas is less Depth, m Pet.Sci.(2010)7:347-354 351 studied. caprocks. The source rocks of the secondary biogas experienced The Liuquan structure has experienced complex tectonic thermal evolution, and were then uplifted to the bacterial activities, and many faults are developed. The distributions activity zone. These source rocks are widely distributed in of biogases are controlled by the source conditions and fault basins. During the Mesozoic-Tertiary, some basins were in a activities. The evolution of source rocks can be divided into rifting stage and in deep-water environments, so that source three stages, I: primary subsidence stage, II: uplift stage, III: rocks were deposited. In the Himalayan movement, most of secondary subsidence stage (Fig. 2). the basins were uplifted, and the source rocks went back into During the first two stages, almost all faults are active, the biological activity zone. Thus, the secondary biogases and the sediments are strongly uplifted by the structural could be generated. movement so biogases were not easily preserved. Taking the Faults are well developed in the rift basins. The faults not Q2 well as an example, the distance between the top of the only provide good migration pathways for biogases when Es formation and the bottom of the Ng formation is only 100 they are active, but also act as a good seal for the biogases m (Fig. 3). After Ng, the secondary biogases are easy to be when they are not active. preserved because of weak fault activities. Taking the Es secondary biogas in the Liuquan Therefore, secondary biogas accumulation occurs in structure as an example, fault activities are signifi cant in gas basins or depressions with uplifting or secondary burial accumulation (Fig. 2). history, and is controlled by fault activities. 3.3 Distribution of oil-degraded biogas Time, Ma 50 40 30 20 10 0 Oil-degraded biogases accumulated near heavy oil pools. 0 Q Ed Nm In the biogas generation, water activity is important for Es Ng Es microorganism propagation so that the biogas reservoirs are Es often located near the oil-water contact. The biogas reserves Es 1500 are determined by the quantity of biodegraded oils. 3 Oil-degraded biogases are distributed in shallow 2000 Es reservoirs in the Songliao, Bohai Bay (Fig. 4) and Junggar basins, because many heavy oil reservoirs are distributed in these basins. The Alaxin lithologic-structural gas reservoir is located in the west slope of the Songliao Basin. It is less than 700 m Fig. 2 Burial history of Es -Ed source rocks of the Q35 well 9 3 deep. The reserves of biogases are 1.74×10 m , and the daily 6 3 gas production is 0.115×10 m . It is a shallow-buried, low The source rocks of Liuquan biogases are mainly dark abundance, high production small gas field. The gases are lacustrine mudstones with thicknesses of 1,100 m. Their characterized by high methane content and relatively light TOC contents are 1.24%-1.98%, and R value is generally δ C with the content from -83.88‰ to -58.25‰, showing 0.5. The kerogen type is mainly II -II . The source rocks are 1 2 microorganism degradation characteristics. less than 2,000 m deep because of tectonic movements, and The sources of Alaxin biogases are oil pools (reserves are favorable for biogas generation. The reservoirs consist are 3.15 million ton) around the biogases. The oil pools are of delta, fan delta and sublacustrine fan sandstones. The commonly degraded and most of the n-alkane in oil has been porosity is from 11.2% to 32.6% with an average of 26.8%, degraded by microorganisms(Fig. 5). The Alaxin biogas and the permeability is 0.1-876 md with an average of 193.6 reservoirs are grey green mudstones and siltstones deposited md. The lacustrine mudstones cover the reservoirs directly as 00.6km Q107 Xq2 G33x Q46-11 Q177 Q58-5 Q46-65 Q50 Xq181 Q36 Nm 900 3 Es 4-Ư 4-Ư Es Ư Es Ư 2 3 1100 1100 Es 3 Nm Es 4-Ư Es ư Es 3 3 1300 4-Ư Es ư 5 -Ư 3 Es Ư 5 -Ư 4-2 Es ư Es Ư 3 3 5 -3 Es Ư 1500 4-2 4-3 3 Es ư Es Ư 4-2 3 3 Es ư Es Es 1700 3 Es Es Oil Gas Water-bearing Faults oil reservoirs Fig. 3 Oil and gas accumulation in the Liuquan structure in the Langgu Sag Depth, m Depth, m 352 Pet.Sci.(2010)7:347-354 Pet.Sci.(2010)7:347-354 353 Table 5 Activation energy of gas production from source rocks in different basins Activation energy, KJ/mol Source rocks Data source Methane Heavy hydrocarbon Jurassic coal in the Kuche Sag 235 269 Li et al, 2003 Jurassic mudstone in the Kuche Sag 206 223 Li et al, 2003 Tertiary oil shale in the Dongying Sag 272 259 Wang et al, 2003a Jurassic coal in the southwest Tarim Basin 227 281 Li et al, 2004 Jurassic mudstone in the southwest Tarim Basin 223 281 Li et al, 2004 Yancheng Formation kerogen in the Qiongdongnan Basin 231 -- Li et al, 2005b Shanxi Formation coal in the Ordos Basin 273 -- Mi et al, 2005 Coal in the Turpan-Hami Basin 209 169 Xu et al, 2008 Mudstone in the Turpan-Hami Basin 191 147 Xu et al, 2008 Dai J X. 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Published: Aug 3, 2010

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