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Genesis and distribution of natural gas in the foreland basins of China
Journal of Petroleum Science and Engineering 41 (2004) 21 – 29 www.elsevier.com/locate/petrol
Genesis and distribution of natural gas in the foreland basins of China Yan Song *, Jing-Xing Dai, Xin-Yu Xia, Sheng-Fei Qin Geology Department, Research Institute of Petroleum Exploration and Development, P.O. Box 910, Beijing 100083, PR China
Abstract This article discusses the geochemical characteristics of various types of natural gas and features of gas reservoirs in the foreland basins in China. We conclude that there are four types of natural gas in origin: coal-formed cracking gas, coalformed thermal gas, oil-type thermal gas and mixed gas. Coal-formed gases are generated from three sets of coal measures: Permo-Carboniferous Formations in the Ordos Basin, Upper Triassic Formations in the Qaidam Basin, and Triassic Formations in northwest China. Source rocks for the oil-type gas include Permian Formations in the Junggar Basin, Triassic Formations in the Qaidam Basin, and lacustrine source rocks in basins along the Hexi Corridor. Abnormally high pressures commonly exist in the gas reservoirs in foreland basins where multiple sets of source – reservoir – seal combinations are present, and the major trap type is the anticlinal trap type and faults are the major hydrocarbon migration pathways. D 2003 Elsevier B.V. All rights reserved. Keywords: China foreland basin; Natural gas; Genesis; Geological features
1. Introduction Tens of foreland basins developed in central and western China (Fig. 1). These foreland basins are filled with Mesozoic –Cenozoic non-marine deposits that have resulted from Mesozoic – Cenozoic orogenic movements and most of them are mainly gas-prone. The ultimate recoverable reserves could be as high as 10 trillion cubic meters in these basins. The multiple orogenic histories resulted in that these basins have not only multi-sets of source rocks and multi-genetic types of natural gas, but also good reservoir, seal and
* Corresponding author. E-mail address: [email protected] (Y. Song). 0920-4105/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0920-4105(03)00140-2
trap conditions, which are favorable for the accumulation of natural gases.
2. Geochemical features and genetic types of natural gas The components and isotopic compositions of the different natural gases in present foreland basins vary widely, with methane content ranging from 67% to 98%, heavier hydrocarbons from 0.4% to 29% and non-hydrocarbons from 0.1% to 17%. These compositional characteristics indicate that accumulation of natural gas occurring in the foreland basins include both wet and dry gases. Carbon isotopic compositions of methane and ethane vary from 25xto 50x and 18x to 36x PDB, respectively, which
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Y. Song et al. / Journal of Petroleum Science and Engineering 41 (2004) 21–29
Fig. 1. The distribution of foreland basins in central and northwestern China.
indicate that these gases were generated by different types of source rocks. Helium isotopic composition of these gases is at 10 8 orders which represents the characteristics of crustal helium (Dai et al., 2000a,b) (Table 1), and indicates that deep source gases contribute very little, if at all, to the natural gas in foreland basins. According to certain geochemical features, characteristics of corresponding source rocks and thermal maturity, natural gas in foreland basins in China can be divided into four types: coal-formed thermal gas, coal-formed cracking gas, oil-type thermal gas and mixed gas. Fig. 2 illustrates that carbon isotopic compositions are the major criteria to distinguish the different geneses of the four natural gas type. Generally, carbon isotopic compositions of the coal-formed gas are heavier than those of oil-type gases (Dai,
1992). However, carbon isotopic compositions of methane are influenced highly by thermal maturity. For example, carbon isotopic values of methane increase with thermal maturity, which could result in that carbon isotopic values of coal-formed gas from lower thermally mature source rocks, have the same range as those of oil-type gas from higher thermally mature source rocks. Carbon isotopic compositions of ethane are more source rock oriented and are impacted by thermal evolution of source rock to less extent than carbon isotopic compositions of methane. Thus, so isotopic compositions of ethane are an effective index to distinguish coal-formed gas from oil-type gas. According to the characteristics of carbon isotopic compositions, the natural gases in the foreland basin can be divided into four types in genesis, which are described as follows (Fig. 2).
Y. Song et al. / Journal of Petroleum Science and Engineering 41 (2004) 21–29
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Table 1 Gas contents, isotopic, and helium isotopic compositions of natural gas in foreland basins, China Area
Foreland Well basin
CoalTarim Kuche formed gas Junggar S. Junggar Tuha Qaidam N. margin Sichuan W. Sichuan Ordos W. margin Oil-type Junggar N.W. gas margin Qaidam Caixi Mixture gas
Tarim
Kela 2 Dawan 1 Hu 2
Formation Content (%) C1
C2
C+3
CO2 N2
Carbon isotopic vale (d13Cx)
Helium isotope
C1
3
C2
C3
E N E3
98.05 88.32 93.58
0.40 0.94 0.60 2.96 1.18 0.03 7.51 3.88 1.15 1.39
27.3 33.3 37.84
19.4 21.6 22.96
26.5 21.00
Wen 1 J2 Xianshi 5 N
84.40 88.5
9.65 5.94 5.4 6.0
39.4 28.18
26.9 24.42
25.0 22.76
Zhong 39 T3
87.82
6.36 5.91 0.32 0.03
36.86
25.61
23.20
Ren 61
P1
93.78
3.36 1.57 0.09 1.19
35.13
26.69
24.82
Xiazijie Fengcheng Shazhong 180 Ke 333
T2 P N2
80.7
45.1 46.49 32.43
34.9 36.16 27.75
32.82 34.22 25.58
N1
83.49
8.11 4.01 0.02 3.38
37.69
26.36
24.72
N1
83.25
8.07 4.33
46.46
26.54
20.23
S.W. Trim Qaidam S. Du 230 Junggar
19.3
2.1. Coal-formed thermal gas Coal-formed thermal gas is the most widely distributed, because the coal measures of mature to high mature stage are widely developed in foreland basins in China. Coal-formed gas occurs mainly as wet gases, with carbon isotopic values of methane varying from 45x to 28x and ethane heavier than 26x , respectively. 2.2. Coal-formed cracking gas Coal-formed cracking gas is generated from highly mature coal measures and is characterized by high methane content and heavier carbon isotopic compositions. The natural gas from Well Kela 2 in the Kuche area of the Tarim Basin is a good example. Its carbon isotopic values of methane and ethane are > 26x and > 20x , respectively. 2.3. Oil-type thermal gas Compared with coal-formed gas, oil-type thermal gas is wetter and its carbon isotopic value is lighter. The
0.1
0.74
He/4He ( 10 8)
C4
R/ Ra
4.23 F 0.26 0.03 23.7 21.17 2.42 F 0.14 0.017 24.9
4.1 F 0.3
0.02
25.93
carbon isotopic values of methane range from to 35x , while ethane is less than 29x .
50x
2.4. Mixed gas Under certain thermal evolution conditions, carbon isotopic values of methane of coal-formed gas and oiltype gas can fall into the same range. Isotopic values of ethane for both types of natural gas range from 29xto 26x . When both types-I and -III source rocks occur in the same area, mixed gas can be generated and its carbon isotopic values will fall into the same range too.
3. Formation and distribution of different genetic natural gas The distributions of different genetic types of natural gas in foreland basins in China are controlled by the type and maturity of the source rocks. Two types of source rocks in these basins: coal measures that are rich in type-III organic matter and, thus, mainly generate gas; lacustrine mudstones, which
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Y. Song et al. / Journal of Petroleum Science and Engineering 41 (2004) 21–29
Fig. 2. Classification of natural gas in foreland basins, central and western China. I—coal-formed cracking gas; II—coal-formed thermal gas; III—oil-type thermal gas; and IV—mixture gases.
are rich in type-I organic matter and mainly generate oil. In gas-prone basins, natural gas occurs most commonly as coal-formed thermal gas containing small amounts of highly mature coal-formed cracking gas and mixed gas, while in oil-prone basins, natural gas occurs mainly oil-type gas. 3.1. Generation and distribution of coal-formed thermal and cracking gas Three sets of coal measures developed in foreland basins in Central and Western China. They are PermoCarboniferous, Upper Triassic and Middle Jurassic coal measures, of which, Middle Jurassic coal measures are the most widespread. They are of limnetic facies origin and have a thickness of approximately 1500– 2000 m (Dai et al., 1997a). The coal measures are distributed mainly in the Tarim, Junggar, Qaidam and Turpan-Hami Basins. These basins form parts of the Tethys belt (Jia et al., 2001). The depositional– structural evolution and formation conditions for natural gas are comparable to those in the Falkan and Karakumu Basins, which are located in the western part of the Tethys belt. Permo-Carboniferous and Upper Triassic coal measures are distributed along the western margin of the Ordos Basin and the western margin of Sichuan Basin (in central China), respectively. All these basins received depo-
sition of both marine and non-marine sediments. The formation and distribution of coal-formed gas are mainly controlled by the following factors. 3.1.1. Distribution of coal measure source rocks Source rocks are widely distributed in both major depressions and slope areas in foreland basins where coal measures developed. Gas generation rates in foreland basins in central and western China are usually above 30 108 m3/km2, and can be up to 100 108 m3/km2 in the Kuche, Southwest Tarim, South Junggar, and West Sichuan areas. These rates are much higher than those in other types of basins in China. Based on the statistics for gas pools and fields that contain gas in place of over 100 108 m3, the gas generation rates in these basins are higher than 20 108 m3/km2 (Dai et al., 1997a,b; Song et al., 1998). Thus, it can be concluded that foreland basins with well-developed coal measures tend to have abundant gas resources. These gas pools and fields are distributed in frontal thrust belts, depressions and slope areas, especially within and around gas kitchens. 3.1.2. Characteristics of macerals Organic matter in coal measures is composed of different kinds of macerals and the hydrocarbon generation capacities of different kinds of macerals
Y. Song et al. / Journal of Petroleum Science and Engineering 41 (2004) 21–29
vary dramatically. A study of thermal simulation (Fig. 3) of coals from the Kuche foreland in the Tarim Basin revealed that only liptinite could have capacity to generate large amount of liquid hydrocarbons, while vitrinite and inertinite mainly generated gas. Clearly, the organic components of coal measures determine in large part whether oil or gas will be generated. A study of macerals and oil and gas accumulations in coal-bearing basins throughout the world concluded that when coal measures contain more than 15% exinite and less than 80% vitrinite, liquid hydrocarbon will make up over 45% of the
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hydrocarbon products generated. In contrast, when liptinite content in coal measures is less than 5% and inertinite content is relatively high (about 30%), hydrocarbon products will be dominated by gas. The Permo-Carboniferous, Upper Triassic and Jurassic are the major periods for coal formation in China. During these periods, coal-bearing basins in central and western China were located in humid temperate to subtropical zones where plants were abundant (Shang, 1997; Wang et al., 1994). The source materials for coal formation are dominated by higher plants and coalification, which usually
Fig. 3. Results of thermal simulation of coal marcerals in Kuche Basin, Tarim area.
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Y. Song et al. / Journal of Petroleum Science and Engineering 41 (2004) 21–29
occurs in weak-oxidizing to weak-reducing environments in which marcerals are characterized by higher vitrinite, higher inertinite and lower liptinite content (Fig. 4) (Dai et al., 2000a,b). In coal-bearing basins in central and western China, coal measures contain >50% vitrinite, 10 –40% inertinite and < 5% exinite on average, although the exinite content exceeds 10% in some cases, which causes these basins to be gasprone. The hydrocarbon producing capacities of these basins vary depending on the organic components of the coal measures. For instance, among these basins, the Tuha Basin, which is the only oil-prone coalbearing basin, has the highest liptinite content in the coal measures, and thus contains more hydrogen-rich components.
3.1.3. Maturation of organic matter and gas accumulation phases Source rocks in the coal-bearing foreland basins in China are all mature. In the central source kitchens of the Kuche, South Junggar and west Sichuan areas, source rocks are highly mature to over mature. Natural gas in these areas occurs mainly as coal-formed thermal gas. Coal-formed cracking gas has accumulated in some areas, such as the Kuche area of the Northern Tarim Basin, where the Kela 2 Gas Field was found. This field is located close to a highly mature to over mature source kitchen. The Kuche foreland basin developed during the Mesozoic, but its Jurassic source rocks were not deeply buried until thick, Tertiary – Quaternary sediments were deposited.
Fig. 4. Components of coal in foreland basins, central-west China. I—Liptinite, II—Vitrinite, III—Inertinite. (a) Jurassic Formation in Kuche area of Tarim Basin; (b) Jurassic Formation in Tuha Basin; (c) Upper Triassic SW Tarim Basin; (d) Perm-Carboniferous Formations in Ordos Basin.
Y. Song et al. / Journal of Petroleum Science and Engineering 41 (2004) 21–29
Therefore, the cracking gas was not generated and did not accumulated until very late. Since source rocks were deeply buried, the gas was only able to accumulate in area where faults connected the deep gas source rocks to shallow traps. Several foreland basins in central and western China contain over mature source rocks, but very few coal-formed gas fields have been discovered in them. This could be attributed to a lack of migration pathways from deep gas source rock to shallow traps. Because of the maturity of the source rocks and post-accumulation preservation conditions, the wetness of natural gas decreases towards from the margins to the basin centers in these foreland basins. 3.2. Formation and distribution of oil-type thermal gas Oil-type thermal gas is distributed mainly in the area of foreland basins where type-I source rocks developed. Since type-I source rocks mainly generate oil, these basins are oil-prone, and oil-type thermal gas usually occurs along with oil accumulations (i.e. associated gas); only small sized gas pools have developed independently. Type-I source rocks in the foreland basins of central and western China are mainly lacustrine mudstones, such as the Permian Formations along the western margin of the Junggar Basin, and the Tertiary Formations along the northern margin of the Qaidam Basin, and Cretaceous Formation in Hexi corridor basins.
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in the Lower Tertiary Anjihai Formation. Maturities of these source rocks range from sub-mature to mature. The underlying Jurassic coal measure formations are well developed as well, and the gas generated by them migrated upwards along faults and accumulated in shallow traps in which coal type gas usually mixed with oil-type gas generated by Tertiary source rocks.
4. Characteristics of gas pools in the foreland basins of China The foreland basins of central and western China are characterized by intensive compression, rapid subsidence and in filling with non-marine clastic rocks. Thus, the gas pools occurring in these basins have the following features. 4.1. Abnormally high fluid pressures Abnormal high pressures are widely distributed in the foreland basins of central and western China; the pressure coefficients can reach 2.0. Many studies have investigated the origin of abnormally high pressures in China (Song et al., 2000; Xia et al., 2001; Song et al., 2002; Wang and Hu, 2002), and have generally concluded that the conclusion is that undercompaction, structural compression and gas generation are the major factors involved in overpressuring. The Kela 2 gas field in the Kuche foreland basin of Tarim Basin is an abnormally high-pressure gas field with a pressure coefficient of 2.2.
3.3. Formation and distribution of mixed gas As mentioned above, a mixed gas refers to both gases generated by different source rocks, or a mixture of coal-formed gas and oil-type gas. Mixed gas developed mainly in two types of foreland basins: (1) those containing both coaly formations and Palaeozoic marine sediments, and (2) those containing both coaly formations and type-III lacustrine source rocks. The first basin type includes the Tarim, Ordos, and Sichuan Basins, in which foreland basins developed over cratonic and coal-formed gas and highly mature oil-type gas usually occurred as mixed gas. The type of basins includes the south margin of the Junggar Basin. In this foreland basin, lacustrine dark mudstone source rocks with types-I and -II kerogens developed
4.2. Multiple sets of source rock – reservoir – seal combinations Jia et al. (2000) divided the source rock – reservoir –seal combinations in foreland basins into three categories: (1) a pre-foreland combination, in which source rock –reservoir – seal formations deposited in negative continental margin, cratonic, or rift basins prior to the formation of the foreland basin; (2) a foreland basin combination, which deposited during the development of the foreland basin; (3) a complex combination, in which the reservoir and seal rocks formed during the foreland developing stage and the source rocks were deposited during the pre-foreland stage.
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Y. Song et al. / Journal of Petroleum Science and Engineering 41 (2004) 21–29
These three types of combinations all developed in basins in central and western China, although the last two types are more widely distributed. The development of a particular source rock – reservoir –seal combination is closely related to the evolutionary history of a particular basin. In central and west China, foreland basins are characterized by two stages of evolution. In central China, foreland basins developed during the Mesozoic; the most common source rock – reservoir –seal combination in this area is the foreland basin combination. In western China, foreland basins are mainly Cenozoic-renewed ones and are mainly the complex source rock – reservoir – seal combination. Since the three categories discussed above usually developed in non-marine sediments represented by interbedded sandstone and mudstone, each may contain several sets of secondary source rock – reservoir – seal combinations. 4.3. Major trap type Local structural zones usually developed in rows parallel to orogenic belts in the foreland basins. Anticlines and faulted anticlines are the major trap types. Fig. 5 illustrates their characteristics. 4.4. Vertical migration pathways of natural gases There are different parthways of migrations for oil and gas in foreland basins. Along the slope area, oil and gas migrated along an unconformity surface. In
thrust belt or depression, faults were the major vertical migration pathways for oil and gas. Exploration activities have indicated that these foreland basins are characterized by a short oil and gas migration distance and source rocks underlying reservoirs and faults play an important role in oil and gas migration and accumulation.
5. Conclusions (1) Natural gas accumulations occurring in foreland basins of China can be divided into four genetic types: coal-formed thermal gas, coal-formed cracking gas, oil-type thermal gas and mixed gas. (2) Coal-formed thermal gas and coal-formed cracking gas in the foreland basins are generated by three sets of coal formations: Permo-Carboniferous Formations in the Ordos Basin, Upper Triassic Formations in the West Sichuan Basin and Jurassic Formations in northwest China. Source rocks for oil-type thermal gas include Permian Formations in the Junggar Basin, Tertiary Formations in the Qaidam Basin, and lacustrine sediments in Hexi corridor basins. (3) Abnormally high pressures are widely developed in these foreland basins. Natural gas accumulations are characterized by multiple source rock – reservoir – seal combinations. Anticline traps predominate, and faults generally act as the major vertical migration pathways.
Fig. 5. Cross-section of oil and gas pools in south margin of Junggar Basin.
Y. Song et al. / Journal of Petroleum Science and Engineering 41 (2004) 21–29
References Dai, J.-X., 1992. Identification and distinction of various alkane gases. Science in China. Series B 35 (10), 1246 – 1257. Dai, J.-X., Song, Y., Zhang, H.-H., 1997a. Natural Gas Accumulation Belts in China. Science Press, Beijing, pp. 110 – 123. In Chinese. Dai, J.-X., Song, Y., Zhang, H.-F., 1997b. Main factors controlling the foundation of medium-giant gas fields in China. Science in China. Series D 40 (1), 1 – 10. Dai, J.-X., Song, Y., Dai, C.-S., 2000a. Conditions Governing the Formation of Biogenic Gas and Gas Pools in Eastern China. Science Press, Beijing, pp. 65 – 72. Dai, J.-X., Zhong, N.-N., Liu, D.-H., Xia, X.-Y., Yang, J.-Y., Tang, D.-Z., 2000b. Geological Features and Control Factors of Large – Medium Sized Coal-Formed Gas Reservoirs in China. Petroleum Industry Press, Beijing. In Chinese. Jia, C.-Z., He, D.-F., Lei, Z.-Y., Zhou, L., Jia, J.-D., Wang, G.-H., 2000. Oil Exploration in Foreland Thrust Belts. Petroleum Industry Press, Beijing, pp. 185 – 187. Jia, C.-Z., Yang, S.-F., Chen, H.-L., 2001. Structural Geology and Natural Gas in Northern Tythes Basins. Petroleum Industry Press, Beijing. In Chinese.
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Shang, G.-X., 1997. Geology of Paleozoic Coal in North China Platform (in Chinese) Taiyuan. Shanxi Scientific and Technological Press. Song, Y., Dai, J.-X., Li, X.-Q., Hong, F., 1998. The major geochemical condition and geological features of large and medium sized gas fields in China. Acta Petrolei Sinica 19 (1), 1 – 5. Song, Y., Wang, Z.-L., Wang, Y., Fang, D.-Q., Meng, F.-Y., Zheng, J.-J., 2000. Natural Gas Accumulations in Junggar Basin. Science Press, Beijing, pp. 115 – 119. Song, Y., Xia, X.-Y., Hong, F., 2002. Formations of abnormal high pressures and gas reservoirs in foreland basins. Scientific Bulletin 47, 70 – 76 (extra edition). Xia, X.-Y., Song, Y., Fang, D.-Q., 2001. Impact of Structural lift to formation pressure and the origin of abnormal high pressure in Kela 2 gas field. Natural Gas Industry 21 (1), 30 – 34. Wang, H.-J., Hu, J.-Y., 2002. Cretaceous oil system in Kuche Depression and the formation of abnormal high pressures. Natural Gas Industry 22 (1), 5 – 8. Wang, S.-S., Zhang, Z.-C., Yao, P.Y., 1994. Jurassic – Cretaceous Coal-Bearing Formation and its Accumulation in China. Geological Press, Beijing.
Genesis and distribution of natural gas in the foreland basins of China Yan Song *, Jing-Xing Dai, Xin-Yu Xia, Sheng-Fei Qin Geology Department, Research Institute of Petroleum Exploration and Development, P.O. Box 910, Beijing 100083, PR China
Abstract This article discusses the geochemical characteristics of various types of natural gas and features of gas reservoirs in the foreland basins in China. We conclude that there are four types of natural gas in origin: coal-formed cracking gas, coalformed thermal gas, oil-type thermal gas and mixed gas. Coal-formed gases are generated from three sets of coal measures: Permo-Carboniferous Formations in the Ordos Basin, Upper Triassic Formations in the Qaidam Basin, and Triassic Formations in northwest China. Source rocks for the oil-type gas include Permian Formations in the Junggar Basin, Triassic Formations in the Qaidam Basin, and lacustrine source rocks in basins along the Hexi Corridor. Abnormally high pressures commonly exist in the gas reservoirs in foreland basins where multiple sets of source – reservoir – seal combinations are present, and the major trap type is the anticlinal trap type and faults are the major hydrocarbon migration pathways. D 2003 Elsevier B.V. All rights reserved. Keywords: China foreland basin; Natural gas; Genesis; Geological features
1. Introduction Tens of foreland basins developed in central and western China (Fig. 1). These foreland basins are filled with Mesozoic –Cenozoic non-marine deposits that have resulted from Mesozoic – Cenozoic orogenic movements and most of them are mainly gas-prone. The ultimate recoverable reserves could be as high as 10 trillion cubic meters in these basins. The multiple orogenic histories resulted in that these basins have not only multi-sets of source rocks and multi-genetic types of natural gas, but also good reservoir, seal and
* Corresponding author. E-mail address: [email protected] (Y. Song). 0920-4105/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0920-4105(03)00140-2
trap conditions, which are favorable for the accumulation of natural gases.
2. Geochemical features and genetic types of natural gas The components and isotopic compositions of the different natural gases in present foreland basins vary widely, with methane content ranging from 67% to 98%, heavier hydrocarbons from 0.4% to 29% and non-hydrocarbons from 0.1% to 17%. These compositional characteristics indicate that accumulation of natural gas occurring in the foreland basins include both wet and dry gases. Carbon isotopic compositions of methane and ethane vary from 25xto 50x and 18x to 36x PDB, respectively, which
22
Y. Song et al. / Journal of Petroleum Science and Engineering 41 (2004) 21–29
Fig. 1. The distribution of foreland basins in central and northwestern China.
indicate that these gases were generated by different types of source rocks. Helium isotopic composition of these gases is at 10 8 orders which represents the characteristics of crustal helium (Dai et al., 2000a,b) (Table 1), and indicates that deep source gases contribute very little, if at all, to the natural gas in foreland basins. According to certain geochemical features, characteristics of corresponding source rocks and thermal maturity, natural gas in foreland basins in China can be divided into four types: coal-formed thermal gas, coal-formed cracking gas, oil-type thermal gas and mixed gas. Fig. 2 illustrates that carbon isotopic compositions are the major criteria to distinguish the different geneses of the four natural gas type. Generally, carbon isotopic compositions of the coal-formed gas are heavier than those of oil-type gases (Dai,
1992). However, carbon isotopic compositions of methane are influenced highly by thermal maturity. For example, carbon isotopic values of methane increase with thermal maturity, which could result in that carbon isotopic values of coal-formed gas from lower thermally mature source rocks, have the same range as those of oil-type gas from higher thermally mature source rocks. Carbon isotopic compositions of ethane are more source rock oriented and are impacted by thermal evolution of source rock to less extent than carbon isotopic compositions of methane. Thus, so isotopic compositions of ethane are an effective index to distinguish coal-formed gas from oil-type gas. According to the characteristics of carbon isotopic compositions, the natural gases in the foreland basin can be divided into four types in genesis, which are described as follows (Fig. 2).
Y. Song et al. / Journal of Petroleum Science and Engineering 41 (2004) 21–29
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Table 1 Gas contents, isotopic, and helium isotopic compositions of natural gas in foreland basins, China Area
Foreland Well basin
CoalTarim Kuche formed gas Junggar S. Junggar Tuha Qaidam N. margin Sichuan W. Sichuan Ordos W. margin Oil-type Junggar N.W. gas margin Qaidam Caixi Mixture gas
Tarim
Kela 2 Dawan 1 Hu 2
Formation Content (%) C1
C2
C+3
CO2 N2
Carbon isotopic vale (d13Cx)
Helium isotope
C1
3
C2
C3
E N E3
98.05 88.32 93.58
0.40 0.94 0.60 2.96 1.18 0.03 7.51 3.88 1.15 1.39
27.3 33.3 37.84
19.4 21.6 22.96
26.5 21.00
Wen 1 J2 Xianshi 5 N
84.40 88.5
9.65 5.94 5.4 6.0
39.4 28.18
26.9 24.42
25.0 22.76
Zhong 39 T3
87.82
6.36 5.91 0.32 0.03
36.86
25.61
23.20
Ren 61
P1
93.78
3.36 1.57 0.09 1.19
35.13
26.69
24.82
Xiazijie Fengcheng Shazhong 180 Ke 333
T2 P N2
80.7
45.1 46.49 32.43
34.9 36.16 27.75
32.82 34.22 25.58
N1
83.49
8.11 4.01 0.02 3.38
37.69
26.36
24.72
N1
83.25
8.07 4.33
46.46
26.54
20.23
S.W. Trim Qaidam S. Du 230 Junggar
19.3
2.1. Coal-formed thermal gas Coal-formed thermal gas is the most widely distributed, because the coal measures of mature to high mature stage are widely developed in foreland basins in China. Coal-formed gas occurs mainly as wet gases, with carbon isotopic values of methane varying from 45x to 28x and ethane heavier than 26x , respectively. 2.2. Coal-formed cracking gas Coal-formed cracking gas is generated from highly mature coal measures and is characterized by high methane content and heavier carbon isotopic compositions. The natural gas from Well Kela 2 in the Kuche area of the Tarim Basin is a good example. Its carbon isotopic values of methane and ethane are > 26x and > 20x , respectively. 2.3. Oil-type thermal gas Compared with coal-formed gas, oil-type thermal gas is wetter and its carbon isotopic value is lighter. The
0.1
0.74
He/4He ( 10 8)
C4
R/ Ra
4.23 F 0.26 0.03 23.7 21.17 2.42 F 0.14 0.017 24.9
4.1 F 0.3
0.02
25.93
carbon isotopic values of methane range from to 35x , while ethane is less than 29x .
50x
2.4. Mixed gas Under certain thermal evolution conditions, carbon isotopic values of methane of coal-formed gas and oiltype gas can fall into the same range. Isotopic values of ethane for both types of natural gas range from 29xto 26x . When both types-I and -III source rocks occur in the same area, mixed gas can be generated and its carbon isotopic values will fall into the same range too.
3. Formation and distribution of different genetic natural gas The distributions of different genetic types of natural gas in foreland basins in China are controlled by the type and maturity of the source rocks. Two types of source rocks in these basins: coal measures that are rich in type-III organic matter and, thus, mainly generate gas; lacustrine mudstones, which
24
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Fig. 2. Classification of natural gas in foreland basins, central and western China. I—coal-formed cracking gas; II—coal-formed thermal gas; III—oil-type thermal gas; and IV—mixture gases.
are rich in type-I organic matter and mainly generate oil. In gas-prone basins, natural gas occurs most commonly as coal-formed thermal gas containing small amounts of highly mature coal-formed cracking gas and mixed gas, while in oil-prone basins, natural gas occurs mainly oil-type gas. 3.1. Generation and distribution of coal-formed thermal and cracking gas Three sets of coal measures developed in foreland basins in Central and Western China. They are PermoCarboniferous, Upper Triassic and Middle Jurassic coal measures, of which, Middle Jurassic coal measures are the most widespread. They are of limnetic facies origin and have a thickness of approximately 1500– 2000 m (Dai et al., 1997a). The coal measures are distributed mainly in the Tarim, Junggar, Qaidam and Turpan-Hami Basins. These basins form parts of the Tethys belt (Jia et al., 2001). The depositional– structural evolution and formation conditions for natural gas are comparable to those in the Falkan and Karakumu Basins, which are located in the western part of the Tethys belt. Permo-Carboniferous and Upper Triassic coal measures are distributed along the western margin of the Ordos Basin and the western margin of Sichuan Basin (in central China), respectively. All these basins received depo-
sition of both marine and non-marine sediments. The formation and distribution of coal-formed gas are mainly controlled by the following factors. 3.1.1. Distribution of coal measure source rocks Source rocks are widely distributed in both major depressions and slope areas in foreland basins where coal measures developed. Gas generation rates in foreland basins in central and western China are usually above 30 108 m3/km2, and can be up to 100 108 m3/km2 in the Kuche, Southwest Tarim, South Junggar, and West Sichuan areas. These rates are much higher than those in other types of basins in China. Based on the statistics for gas pools and fields that contain gas in place of over 100 108 m3, the gas generation rates in these basins are higher than 20 108 m3/km2 (Dai et al., 1997a,b; Song et al., 1998). Thus, it can be concluded that foreland basins with well-developed coal measures tend to have abundant gas resources. These gas pools and fields are distributed in frontal thrust belts, depressions and slope areas, especially within and around gas kitchens. 3.1.2. Characteristics of macerals Organic matter in coal measures is composed of different kinds of macerals and the hydrocarbon generation capacities of different kinds of macerals
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vary dramatically. A study of thermal simulation (Fig. 3) of coals from the Kuche foreland in the Tarim Basin revealed that only liptinite could have capacity to generate large amount of liquid hydrocarbons, while vitrinite and inertinite mainly generated gas. Clearly, the organic components of coal measures determine in large part whether oil or gas will be generated. A study of macerals and oil and gas accumulations in coal-bearing basins throughout the world concluded that when coal measures contain more than 15% exinite and less than 80% vitrinite, liquid hydrocarbon will make up over 45% of the
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hydrocarbon products generated. In contrast, when liptinite content in coal measures is less than 5% and inertinite content is relatively high (about 30%), hydrocarbon products will be dominated by gas. The Permo-Carboniferous, Upper Triassic and Jurassic are the major periods for coal formation in China. During these periods, coal-bearing basins in central and western China were located in humid temperate to subtropical zones where plants were abundant (Shang, 1997; Wang et al., 1994). The source materials for coal formation are dominated by higher plants and coalification, which usually
Fig. 3. Results of thermal simulation of coal marcerals in Kuche Basin, Tarim area.
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occurs in weak-oxidizing to weak-reducing environments in which marcerals are characterized by higher vitrinite, higher inertinite and lower liptinite content (Fig. 4) (Dai et al., 2000a,b). In coal-bearing basins in central and western China, coal measures contain >50% vitrinite, 10 –40% inertinite and < 5% exinite on average, although the exinite content exceeds 10% in some cases, which causes these basins to be gasprone. The hydrocarbon producing capacities of these basins vary depending on the organic components of the coal measures. For instance, among these basins, the Tuha Basin, which is the only oil-prone coalbearing basin, has the highest liptinite content in the coal measures, and thus contains more hydrogen-rich components.
3.1.3. Maturation of organic matter and gas accumulation phases Source rocks in the coal-bearing foreland basins in China are all mature. In the central source kitchens of the Kuche, South Junggar and west Sichuan areas, source rocks are highly mature to over mature. Natural gas in these areas occurs mainly as coal-formed thermal gas. Coal-formed cracking gas has accumulated in some areas, such as the Kuche area of the Northern Tarim Basin, where the Kela 2 Gas Field was found. This field is located close to a highly mature to over mature source kitchen. The Kuche foreland basin developed during the Mesozoic, but its Jurassic source rocks were not deeply buried until thick, Tertiary – Quaternary sediments were deposited.
Fig. 4. Components of coal in foreland basins, central-west China. I—Liptinite, II—Vitrinite, III—Inertinite. (a) Jurassic Formation in Kuche area of Tarim Basin; (b) Jurassic Formation in Tuha Basin; (c) Upper Triassic SW Tarim Basin; (d) Perm-Carboniferous Formations in Ordos Basin.
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Therefore, the cracking gas was not generated and did not accumulated until very late. Since source rocks were deeply buried, the gas was only able to accumulate in area where faults connected the deep gas source rocks to shallow traps. Several foreland basins in central and western China contain over mature source rocks, but very few coal-formed gas fields have been discovered in them. This could be attributed to a lack of migration pathways from deep gas source rock to shallow traps. Because of the maturity of the source rocks and post-accumulation preservation conditions, the wetness of natural gas decreases towards from the margins to the basin centers in these foreland basins. 3.2. Formation and distribution of oil-type thermal gas Oil-type thermal gas is distributed mainly in the area of foreland basins where type-I source rocks developed. Since type-I source rocks mainly generate oil, these basins are oil-prone, and oil-type thermal gas usually occurs along with oil accumulations (i.e. associated gas); only small sized gas pools have developed independently. Type-I source rocks in the foreland basins of central and western China are mainly lacustrine mudstones, such as the Permian Formations along the western margin of the Junggar Basin, and the Tertiary Formations along the northern margin of the Qaidam Basin, and Cretaceous Formation in Hexi corridor basins.
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in the Lower Tertiary Anjihai Formation. Maturities of these source rocks range from sub-mature to mature. The underlying Jurassic coal measure formations are well developed as well, and the gas generated by them migrated upwards along faults and accumulated in shallow traps in which coal type gas usually mixed with oil-type gas generated by Tertiary source rocks.
4. Characteristics of gas pools in the foreland basins of China The foreland basins of central and western China are characterized by intensive compression, rapid subsidence and in filling with non-marine clastic rocks. Thus, the gas pools occurring in these basins have the following features. 4.1. Abnormally high fluid pressures Abnormal high pressures are widely distributed in the foreland basins of central and western China; the pressure coefficients can reach 2.0. Many studies have investigated the origin of abnormally high pressures in China (Song et al., 2000; Xia et al., 2001; Song et al., 2002; Wang and Hu, 2002), and have generally concluded that the conclusion is that undercompaction, structural compression and gas generation are the major factors involved in overpressuring. The Kela 2 gas field in the Kuche foreland basin of Tarim Basin is an abnormally high-pressure gas field with a pressure coefficient of 2.2.
3.3. Formation and distribution of mixed gas As mentioned above, a mixed gas refers to both gases generated by different source rocks, or a mixture of coal-formed gas and oil-type gas. Mixed gas developed mainly in two types of foreland basins: (1) those containing both coaly formations and Palaeozoic marine sediments, and (2) those containing both coaly formations and type-III lacustrine source rocks. The first basin type includes the Tarim, Ordos, and Sichuan Basins, in which foreland basins developed over cratonic and coal-formed gas and highly mature oil-type gas usually occurred as mixed gas. The type of basins includes the south margin of the Junggar Basin. In this foreland basin, lacustrine dark mudstone source rocks with types-I and -II kerogens developed
4.2. Multiple sets of source rock – reservoir – seal combinations Jia et al. (2000) divided the source rock – reservoir –seal combinations in foreland basins into three categories: (1) a pre-foreland combination, in which source rock –reservoir – seal formations deposited in negative continental margin, cratonic, or rift basins prior to the formation of the foreland basin; (2) a foreland basin combination, which deposited during the development of the foreland basin; (3) a complex combination, in which the reservoir and seal rocks formed during the foreland developing stage and the source rocks were deposited during the pre-foreland stage.
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These three types of combinations all developed in basins in central and western China, although the last two types are more widely distributed. The development of a particular source rock – reservoir –seal combination is closely related to the evolutionary history of a particular basin. In central and west China, foreland basins are characterized by two stages of evolution. In central China, foreland basins developed during the Mesozoic; the most common source rock – reservoir –seal combination in this area is the foreland basin combination. In western China, foreland basins are mainly Cenozoic-renewed ones and are mainly the complex source rock – reservoir – seal combination. Since the three categories discussed above usually developed in non-marine sediments represented by interbedded sandstone and mudstone, each may contain several sets of secondary source rock – reservoir – seal combinations. 4.3. Major trap type Local structural zones usually developed in rows parallel to orogenic belts in the foreland basins. Anticlines and faulted anticlines are the major trap types. Fig. 5 illustrates their characteristics. 4.4. Vertical migration pathways of natural gases There are different parthways of migrations for oil and gas in foreland basins. Along the slope area, oil and gas migrated along an unconformity surface. In
thrust belt or depression, faults were the major vertical migration pathways for oil and gas. Exploration activities have indicated that these foreland basins are characterized by a short oil and gas migration distance and source rocks underlying reservoirs and faults play an important role in oil and gas migration and accumulation.
5. Conclusions (1) Natural gas accumulations occurring in foreland basins of China can be divided into four genetic types: coal-formed thermal gas, coal-formed cracking gas, oil-type thermal gas and mixed gas. (2) Coal-formed thermal gas and coal-formed cracking gas in the foreland basins are generated by three sets of coal formations: Permo-Carboniferous Formations in the Ordos Basin, Upper Triassic Formations in the West Sichuan Basin and Jurassic Formations in northwest China. Source rocks for oil-type thermal gas include Permian Formations in the Junggar Basin, Tertiary Formations in the Qaidam Basin, and lacustrine sediments in Hexi corridor basins. (3) Abnormally high pressures are widely developed in these foreland basins. Natural gas accumulations are characterized by multiple source rock – reservoir – seal combinations. Anticline traps predominate, and faults generally act as the major vertical migration pathways.
Fig. 5. Cross-section of oil and gas pools in south margin of Junggar Basin.
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