SUN Zhanxue, ZHANG Wen, HU Baoqun,. LI Wenjuan & PAN Tianyou. East China Institute of Technology, Fuzhou 344000, China. Correspondence should be ...
ARTICLES Chinese Science Bulletin 2005 Vol. 50 Supp. 111ü117
Geothermal field and its relation with coalbed methane distribution of the Qinshui Basin SUN Zhanxue, ZHANG Wen, HU Baoqun, LI Wenjuan & PAN Tianyou East China Institute of Technology, Fuzhou 344000, China Correspondence should be addressed to Sun Zhanxue (email: zhxsun@ ecit.edu.cn)
Abstract The average geothermal gradient in the Qinshui Basin, Shanxi Province, North China, estimated from temperature logging data of 20 boreholes is 28.2±1.03ć/km. The thermal conductivities of 39 rock samples are measured and 20 heat flow values are obtained. The estimated heat flow ranges from 44.75 mW/m2 to 101.81 mW/m2, with a mean of 62.69±15.20 mW/m2. The thermal history reconstruction from the inversion of vitrinite data, using Thermodel for Windows 2004, reveals that the average paleo-heat flow at the time of maximum burial in late Jurassic to early Cretaceous is 158.41 mW/m2 for the north part, 119.57 mW/m2 for the central part and 169.43 mW/m2 for the south part of the basin respectively. The reconstruction of the buried history of the strata indicates that the age for the end of sedimentation and the beginning of erosion for the basin is 108̣156 Ma, and that the eroded thickness of the strata is 2603 m in the north, 2291 m in the central, and 2528.9 m in the south of the basin respectively. The “higher in the north and the south, lower in the central” distribution pattern of the paleo-heat flow coincides with the distribution of the coal-bed methane spatially and temporally, which shows that the coal-bed methane is controlled by the paleo-geotemperature field in the basin. Keywords: Qinshui Basin, heat flow, vitrinite reflectance, thermal history reconstruction, coal-bed methane. DOI: 10.1360/98zk0016
The Qinshui Basin is located in the southeast of Shanxi Province, North China. The area of the basin is about 30000 km2. Rich in coal resource, the basin is one of the most important coal bases and the most prospective regions for the coalbed methane development in China. The main coal-bearing layers with average thickness of 146 m and around 10 coal beds are the Taiyuan Formation of the Upper Carboniferous and the Shanxi Formation of the Lower Permian. The Taiyuan Formation and the Shanxi Formation referred as the upper main coal layer and the lower main coal layer respectively, which widely distribute in the basin, are the main target coal-layers for the coalbed methane exploration and exploitation[1]. The present heat status and thermal history of a basin Chinese Science Bulletin
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December 2005
are not only the important parameters of distribution, transportation and storage for the coalbed methane, but also the objective reflection of the structural variation of the basin. In recent years, some progresses have been made on heat flow, tectonic-thermal events and paleogeotemperature reconstruction and so on in the Qinshui Basin by different researchers[1̣4]. However, it is necessary to discuss more intensively the relationship of the coal-bed methane and present- and paleo-geotemperature field with the progress of the Project “Basic Studies on Formation Mechanism and Economic Exploitation of Coal-bed methane Reservoir in China (Grant No. 2002CB211704)” supported by National Basic Research Program (973 Program) and the continuous accumulation of new data in the basin. Based on the research of the present heat flow in the basin, the paleo-thermal evolution and coal-containing strata buried history from vitrinite reflectance (R0) using Thermodel for Windows 2004 are reconstructed, and the relationship between pale-geothermal field and the distribution of the coal-bed methane are discussed in this paper. 1 Present-day temperature field The present-day temperature field is studied in the Qinshui Basin based on the work made by some researchers[4,5]. The temperature data are selected from bores in which drilling has been accomplished about 72 h to one year before temperatures are measured. The temperature profiles are linear with depth and belong to the conductive type. Except for four data from some relevant literatures[4,5], the geothermal gradients in the basin are calculated by the “Least Square” method according to the temperature logging data. The results listed in Table 1 shows that the geothermal gradient of the Qinshui Basin varies from 20.9 to 47.ć/km with a mean value of (28.2±1.03)ć/km. Considering the geologic characteristics and convenience for study, the basin is divided into three parts: the south part is the south from 36°0cN, the central part is the region between 36°0cN and 36°45cN, and the north part is the area from 36°45cN to 38°0cN. The regional geothermal gradient (Fig. 1) shows that the mean value is 35.3ć/km in the south, 22.4ć/km in the central, and 27.0ć/km in the north respectively in the Qinshui Basin. Generally, the geothermal gradient values are higher in the south and noth parts, and lower in the central part of the basin. Assuming the geo-temperature profile is linear with depth, the temperature at any depth can be obtained from eq. (1): T G ( H Hc ) Tc , (1) where G is geothermal gradient, Hc is the depth of the constant temperature zone, and Tc is the temperature at the constant temperature zone. The calculations of the geothermal gradient and the 111
ARTICLES Table 1 Heat flow values and related parameters in the Qinshui Basin No.
Coordinate
Sub-area
Thermal conductivity Geothermal Gradient/ć·km1 /Wm1·K1
Borehole
Heat flow /mW·m2
longitude
latitude
1
112°37ƍ58Ǝ
35°37ƍ21Ǝ
TL011
29.5
2.11
62.02
2
112°33ƍ40Ǝ
35°39ƍ55Ǝ
TL007
32.8
2.11
69.14
3
112°38ƍ10Ǝ
35°46ƍ42Ǝ
TL004
34.7
2.11
73.08
4
112°40ƍ52Ǝ
35°50ƍ15Ǝ
TL003
35.5
2.11
74.66
South
5
112°42ƍ27Ǝ
35°54ƍ43Ǝ
TL009
44.2
2.11
93.13
6a)
112°14ƍ01Ǝ
36°01ƍ58Ǝ
Anyi, Shanxi
21.7
3.17
68.79
7a)
112°17ƍ55Ǝ
36°02ƍ05Ǝ
Anyi, Shanxi
23.2
2.79
64.73
8b)
112°33ƍ00Ǝ
36°34ƍ38Ǝ
Qincan 1
24.0
2.03
48.72
9a)
112°20ƍ08Ǝ
36°40ƍ07Ǝ
Qinyuan, Shanxi
20.5
3.26
66.83
10
112°13ƍ
36°49ƍ30Ǝ
Well Xishangzhuang
21.3
2.14
45.54
11
113°4ƍ36Ǝ
37°18ƍ6Ǝ
sy001
26.1
2.14
55.76
12
113°14ƍ49Ǝ
37°50ƍ36Ǝ
9̣6
26.0
2.06
52.91
13
113°11ƍ46Ǝ
37°51ƍ56Ǝ
13̣4
22.3
2.12
47.21
14
113°5ƍ41Ǝ
37°52ƍ36Ǝ
15
113°17ƍ54Ǝ
37°53ƍ47Ǝ
Central
North
sy002
47.6
2.14
101.81
5̣5
20.9
2.17
45.42
16
113°13ƍ35Ǝ
37°54ƍ45Ǝ
11̣3
27.5
2.12
58.41
17
113°16ƍ41Ǝ
37°55ƍ18Ǝ
6̣5
27.8
2.24
62.22
18
113°13ƍ31Ǝ
37°55ƍ47Ǝ
11̣6
31.2
2.06
64.12
19
113°11ƍ22Ǝ
37°55ƍ55Ǝ
13—1
25.4
2.14
54.46
20 113°14ƍ30Ǝ 37°56ƍ16Ǝ 20.9 2.14 44.75 9̣7 a) The data were cited from Wu Qianfan (1991)[5]; b) the data obtained by recalculation according to data reported by Ren Zhanli (1998)[4].
temperature at depth get started from the temperature and depth of the constant temperature zone. Generally, the temperature of the constant temperature zone is approximately equal to the local annual average air temperature. The temperature and the depth of the constant temperature zone are estimated to be 9ć and 20 m respectively based on the statistical analysis of the annual average air temperature in counties from south to north of the basin and the literature[6]. The variations of the buried depth of the upper main coal bed and the lower coal bed are taken into account. The geo-temperature values at the depth of 500 and 1000 m calculated respectively by the eq. (1) show that the temperature at depth of 500m varies from 18.84 to 31.86ć and the temperature at 1000 m deep ranges from 29.09 to 31.86ć. The geo-temperature distribution pattern generally is “higher in the south and the north, and lower in the central”. 2
Fig. 1. Distribution of geothermal gradient (ć/km) in Qinshui Basin.
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Thermal conductivities of rocks and heat flow
The thermal conductivity is one of the basic parameters for calculation of heat flow. The heat flow can be obtained by the following formula: Q K ·ǻT/ǻZ , (2) 1 1 where, K is thermal conductivity of rock Wm ·K , ǻT/ǻZ is geothermal gradient (ć·km1). The thermal conductivities of 39 rock samples were Chinese Science Bulletin
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ARTICLES measuresed in this study. In order to ensure the representative of thermal conductivity data, the thicknessweighted average thermal conductivity was used to calculate the heat flow values. The heat flow (Q) is the quantity of heat from unit area at unit time from the inner part to the surface of the Earth, which is an indirect physical parameter calculated by the thermal conductivity of the rock and temperature gradient. Here Borehole 9-6 is taken as an example to explain how to calculate the heat flow. The total depth of the borehole is 972.95 m of which the sandstone takes 225.97 m, the mudstone is 718.88 m, the coal bed is 2.6 m, and the carbonate is 25.5 m respectively. The thermal conductivities of sandstone, mudstone rock, coal bed, and carbonate are 2.5, 1.9, 0.8, and 3.07 Wm1·K1 respectively. The thickness-weighted average thermal conductivity can be calculated by eq. (3): n
¦ ki hi K
i 0 n
¦ hi
.
(3)
i 0
Where, ki is the thermal conductivity of rock i, hi is the thickness of rock i, and n is the number of rock types. The result is K = 2.06 Wm1·K1 for Borehole 9-6. Substituting the temperature gradient of the borehole (26.0ć·km1) and the thermal conductivity K in eq. (2) gives the heat flow = 52.91 mW/m2. Table 1 shows the calculated heat flow data. The values in the Qinshui Basin varies from 44.75 to 101.81 mW/m2 with a mean of 62.69±15.20 mW/m2 which is slightly higher than that of the continental heat flow in China (61±15.5 mW/m2)[7], and obviously higher than the value of Mt Taihang Uplifted Area (46.89 mW/m2)[6], but slightly lower than the heat flow of the Ordos Basin (65.04 mW/m2)[9] and the Bohai Basin (65.8 mW/m2)[9], and significantly lower than that of Shanxi Rift (76.19 mW/m2)[5]. Of all the heat flow values in the Qinshui Basin, the highest one (101.81 mW/m2) occurs in Borehole sy002 of Shouyang in the north of the basin, and the second highest one (93.13 mW/m2) is found in Borehole TL009, south part of the basin. Generally, the regional heat flow pattern in the basin shows higher in the south and the north, and lower in the central part (Fig. 2). 3
Paleo-geotemperature field
The coalbed methane formation is closely related to heat and time of coal seams undergone. The thermal history of coal seams can be reflected by its buried history under the condition of no special heat source (such as magma activity). At the same time, the buried and thermal history can also reflect the formation, transportation and storage conditions of the coal-bed methane. The formation of the coal-bed methane was considered as the result of deep metamorphism. That is, Mesozoic thermal events Chinese Science Bulletin
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December 2005
Fig. 2. Heat flow contour in Qinshui Bain.
were the prerequisites for the formation of the coalbed methane in the basin. Therefore, the reconstruction of the thermal history plays a very important role in the study of formation mechanism of the coal-bed methane. 3.1
Simulation parameters of paleo-geotemperature
The main parameters used in the paleo-geotemperature simulation by Thermodel for Windows 2004 for the study are selected from related literature and measured data. These parameters are lithology (Table 2), stratigraphical parameters and vitrinite reflectance (Table 3) etc. The lithology is obtained from the average data of measured results of rock samples at the relevant part of the basin, and the stratigraphical parameters are determined by the geological columnar sections of Borehole TL003 in the south, Borehole Qincen 1 in the central, and Borehole 5-5 in the north of the basin. 3.2
Buried history of strata
Thermodel for Windows 2004 applied in this study is revised from Dr. Kerry Gallagher’s Monte Trax (A Fission Track Thermal History Modeling Program for the Macintosh) by Dr. Hu Shenbiao, Institute of Geology and Geophysics, Chinese Academy of Sciences. It is an internationally used simulation software using apatite and zircon fission track, which is run on Windows 9x or Windows 113
ARTICLES Lithology
Density /g·cm3 0.90
Coal
Table 2 Statistics of lithology in the Qinshui Basin Borehole 5-5 Borehole Qincen 1 Thermal conductivity Density Thermal conductivity /Wm1·K1 /g·cm3 /Wm1·K1 0.37 0.90 0.37
Density /g·cm3 0.90
Borehole TL003 Thermal conductivity /Wm1·K1 0.80
Mudstone
3.17
2.10
2.73
1.75
2.6
2.02
Limestone
2.72
3.04
2.72
3.25
2.72
3.04
Fine sandstone
2.65
2.10
2.65
1.74
2.65
2.41
Medium sandstone
2.71
2.14
2.63
2.76
2.68
2.13
Table 3 Data of vitrinite reflectance (R0) in the Qinshui Basin Borehole Qincen 1a)
Borehole 5-5 Sampling depth/m 751
R0 (%)
Sampling depth/m
2.17
715
R0 (%)
Sampling depth/m
Borehole TL003
R0 (%)
Sampling depth/m
R0 (%)
1.25
1023.16
2.19
418.8
2.47 2.62
755
2.40
775
0.99
1029.71
2.02
420.9
761.5
2.35
838
1.30
1038.85
2.22
468.1
2.85
768
2.08
867
1.36
1053
1.91
478
3.01
772
2.57
907.5
1.28
1062.75
2.19
520.6
3.23
798
2.04
949.5
1.51
1083
2
835.5
2.61
979.5
1.47
1096.02
2.21
1009.5
1.81
1099.55
2.24
1022
1.91
1115
2.14
1022.36 2.09 1137 2.01 a) Data for Borehole Qincen 1 are provided by Prof. Zhanli Ren from Northwest University of China.
NT. The software focuses on the reconstruction of thermal history of basins. Structurally, it is composed of forward and inverse modeling of the thermal history by geothermometers on the basin scale and thermo-tectonic evolution simulation on the lithospheric scale. The whole system consists of five simulators, including stratigraphical, geo-temperature historical, inverse thermal historical, maturity historical, and thermo-tectonic simulators. Of these five simulators, the inverse thermal historical simulator is composed of six daughter simulators: vitrinite reflectance (Ro), apatite fission track (AFT), coupled inverse and clay mineral (I/S), Ar-Ar age spectrometric geothermics, and Raman spectrometers. Firstly, the eroded depth of strata is simulated by using the inverse Ro simulator of Thermodel for Windows 2004. Then the buried history of strata is simulated by the stratigraphical simulator. The results are shown in Figs. 3̣5. The greatest buried depths are from 3322.1 to 3506.0 m in the north (Borehole 5-5), from 2828.5 to 3063.5 m in the central (Borehole Qincen 1), and from 2961.8 to 3047.4 m in the south (Borehole TL003) respectively. Generally, the eroded depth in the north or the south is greater than that in the central of the Qinshui Basin. The evidence reveals that the pale-geo-temperatures of the upper and the lower main coal beds in the south and the north are higher than those in the central of the basin. On the other hand, the previous research work[1̣3] suggests that Yanshanian magmatic activities exsiting in 114
the south and the north part of the basin and such thermotectonic event occurred in the Late Mesozoic Period (from Late Jurassic to Early Cretaceous). The abnormalously high paleo-heat flow during the Late Jurassic to Early Cretaceous was favorable to the development of coalification. Chinese Qingling Orogenic Belt was re-active after the orogenic process. The reactivitiy was evidenced by obvious magmatic activities at about 100 Ma ago. Several Mesozoic magmatic bodies were found around the marginal areas of the Qinshui Basin[2]. All these facts indicate that North China Block is already in the obviously active period at the Late Mesozoic Era[10]. Regional geological investigations show that the K-Ar age of magmatic rocks in Mt. Taer and Mt. Erfengshan in the southwest part of the basin varies from 91 to 138 Ma ago, and mainly distributes around 130ma ago which is the Mid-Yanshanian Period, i.e. Early Cretaceous Era[11]. In the area, the ages of the magmatic bodies mostly range from 108 to 156 Ma ago that are selected as the ages of sedimentation ended and erosion started. Since the magmatic activities were the manifestations of the deep geothermal activities, the additional magmatic heat sources surely existed in the south and the north of the basin during the Late Jurassic to the Early Cretaceous. Furthermore, the buried depth of the upper and the lower coal beds in the south and the north was greater than that in the central. Obviously, the south and the north parts of the basin were in the abnormal geothermal belts at that Chinese Science Bulletin
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ARTICLES
Fig. 3.
Fig. 4.
The buried history of coal beds of Borehole 5-5.
The buried history of coal beds of Borehole Qincan 1.
time. Such high paleo-geothermal conditions are favorable to the formation of coal-bed methane. 3.3
Evolution of paleo-geothermal field
The thermal histories of strata can be reflected by their buried histories. The greater the buried depth of a stratum is, the higher the paleo-temperature is. The high geothermal background results in high thermal maturity of Carboniferous and Permian coal beds. The zonation and difference of metamorphism for the Carboniferous and Permian coal beds results from deep metamorphism and regional magmatic therm-metamorphism under conditions of the Mesozoic abnormal geothermal settings. Chinese Science Bulletin
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Fig. 5.
The buried history of coal beds of Borehole TL003.
Figs. 6̣8 show the paleo-temperature profiles of the upper and the lower main coal seams in the north (Borehole 5-5), the central (Borehole Qincen 1) and the south (Borehole TL003) of the basin respectively during the last thermal event (Late Jurassic to Early Cretaceous). These profiles suggest that the temperatures of the main coal seams are from 252.28 to 263.34ć in the north, from 182.32 to 188.26ć in the central, and from 246.68 to 252.03ć in the south respectively. The mean paleo-geothermal gradients are 73.0ć/km in the north, 58.9ć/km in the central and 80.3ć/km in the south of the basin. In comparison, the present-day geothermal gradients are 27.0ć/km in the north, 22.4ć/km in the central and 35.3ć/km in the south respectively. It goes without saying that the paleo-geothermal gradients during the Late Jurassic to Early Cretaceous were much higher than those of the present. If the present thermal conductivities of rocks are used in the calculation, the paleo-heat flow values can be estimated to be 158.41 mW/m2 in the north, 119.57 mW/m2 in the central and 169.43 mW/m2 in the south of the Qinshui Basin. Obviously, the paleo-geotemperature and paleo-heat flows show that they are higher in the south and the north, and lower in the central part of the basin. Anthracites with high-grade metamorphism are found in the present shallowly-buried Carbniferous-Permian coal beds in the area around latitude 35°N in the south and around latitude 38°N in the north of the basin, which cannot be explained by deep metamorphism although these shallowly-buried zones had ever subjected to some erosion process during the late sedimentation period. Therefore, the phenomenon can only resonably be attributed to the Late Jurassic and Early Cretaceous 115
ARTICLES
Fig. 6. 5-5.
The paleo-temperature profiles of main coal seams in Borehole
Fig. 7. The paleo-temperature profiles of main coal seams in Borehole Qincan 1.
thermal events. The magmatic invasion occuring in the north and the south parts of the basin resulted in the paleoheat flow distribution pattern “higher in the north and the south, lower in the central”. The anomalous heat caused by the hidden magmatic invasions has not completely disapeared due to the relatively great thickness of overlaying strata, which makes the present-day heat flow inherit the distribution pattern “higher in the north and the south, lower in the central” of the paleo-heat flow in the basin. 4 Paleo-geothermal field and its relations with coalbed methane distribution The formation of the coalbed methane is closely related with the geothermal evolution of coal seams. As men116
Fig. 8. The paleo-temperature profiles of main coal seams in Borehole TL003.
tioned above, the general trend of temperature, buried depth, geothermal gradient and paleo-heat flow for main coal seams from the north to the south is that paleo-geotemperatures and paleo-heat flow values are higher in the south and the north of the basin. These higher geo-temperature and heat flow conditions are favorable to the formation of the coal-bed methane. Meanwhile, the paleo-buried thickness of main coal seams is greater in the south and the north of the basin, which is favorable to the storage of the coal-bed methane. In comparison, the paleo-geo-temperature and paleo-heat flows are lower, and the paleo-buried depth of main coal seams is smaller in the central part of the basin, which is not favorable to the formation and storage of the coal-bed methane. These evidences are consistent with the distribution characteristics speculated by Liu Huanjie and others[13] that the coal-bed methane capacity in the central part is smaller than that in the south or the north of the basin. The main reason for the above-mentioned distribution characteristics is that the Late Jurassic-Early Cretaceous magmatic hydrothermal activities in the region around latitude 35°N in the south and around latitude 38°N in the north intensified metamorphism of coal seams and enlarged coal-bed methane capacity in the relevant parts of the basin. In fact, the evolution of the paleo-geotemperature field reflects the distribution of the coal-bed methane to some extent, and provides reference to the exploitation of the coal-bed methane. In summary, the distribution and formation of the coalbed methane are controlled by the paleo-geothermal filed and the Late Jurassic and Early Cretaceous thermal events in the Qinshui Basin. The period of the anomalous paleogeothermal field occurring is the period for the formation of the coalbed methane, and the area of the paleo-geoChinese Science Bulletin
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ARTICLES thermal anomalies distributed is the area for the creation of the coal-bed methane on a large scale. 5
Conclusions
(i) The average present-day geothermal gradient in the Qinshui Basin is (28.2±1.03)ć/km, but varies from 35.3 ć/km in the south, and 22.4ć/km in the central, to 27.0ć/km in the north. (ii) The estimated present-day heat flow ranges from 44.75 mW/m2 to 101.81 mW/m2, with a mean value of 62.69 mW/m2ˈwhich is obviously higher than that of Mt Taihang Uplifted Area, but slightly lower than that of the Ordos Basin and the Bohai Basin, and significantly lower than that of the Shanxi Rift. (iii) The Paleo-geothermal simulation indicates that the north, the central and the south of the Qinshui Basin have undergone different thermal histories. The period from the formation of coal seams to the last thermal event (110̣ 156 Ma) is the most important stage for the formation of coal-bed methane. During this period, the maximum paleo-heat flow is 158.41 mW/m2 in the north, 119.57 mW/m2 in the central and 169.38 mW/m2 in the south. (iv) The reconstruction of the buried history of the strata indicates that the eroded thickness of the strata is 2603 m in the north, 2291 m in the central, and 2528.9 m in the south of the basin respectively. The “higher in the north and the south, lower in the central” distribution pattern of the present-day heat flow is inherited from the pattern of paleo-heat flow and influenced by the difference in thickness of strata in different parts of the basin. (v) The formation of the coalbed methane is closely related to geothermal evolution, and the distribution of coal-bed methane is controlled by the paleo-geothermal field in the basin. There existed a strong thermo-tectonic event that made the Carboniferous-Permian coal beds have higher thermal maturity in the Late Mesozoic Period. During the period, the Paleo-heat flow and paleo-geothermal gradients were much higher than those of the present day that were favorable to the formation of the coalbed methane in the Qinshui Basin. Acknowledgements The authors thank Dr. Hu Shengbiao, Institute of Geology and Geophysics, Chinese Academy of Sciences, for providing his newly revised Thermodel software. Prof. Ren Zhanli at Chinese Northwest University is also thanked for providing vitrinite reflectance
Chinese Science Bulletin
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data of Borehole Qincen 1. This work is supported by National Basic Research Program (973 Program) under the Project “Basic Studies on Formation Mechanism and Economic Exploitation of Coal-bed methane Reservoir in China (Grant No. 2002CB211704)”.
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