The surface geochemical exploration of oil and gas in the Gangbatong-Ya anxiang and the Dongqiao-Nam Co of the Qinghai-Tibet region

Science in China Series D: Earth Sciences 2009 SCIENCE IN CHINA PRESS Springer www.scichina.com earth.scichina.com www.springerlink.com The surface geochemical exploration of oil and gas in the Gangbatong-Ya anxiang and the Dongqiao-Nam Co of the Qinghai-Tibet region XIONG Bo 1,2, LI XianQing 3,4, LI YiBin 5 & TAN Qing 6 1 Department of Geochemistry, Yangtze University, Jingzhou 434023, China; 2 Key Laboratory of Exploration Technologies for Oil and Gas Resources (Yangtze University), Ministry of Education, Jingzhou 434023, China; 3 State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology, Beijing 100083, China; 4 Department of Resources and Earth Sciences, School of Resources and Safety Engineering, China University of Mining and Technology, Beijing 100083, China; 5 Institute of Geosciences, Yangtze University, Jingzhou 434023, China; 6 Jinzhou Research Branch of New Region Exploration, Research Institute of Petroleum Exploration and Development, SINOPEC, Jingzhou 434100, China Due to the complexity of geological background and the adverseness of natural geographical environment in the Qinghai-Tibet Plateau, it is very difficult to carry out petroleum geological study in this region. The Qinghai-Tibet Plateau is the only blank space of petroleum exploration on a large scale. The authors carried out the surface geochemical exploration of oil and gas in the Gangbatong-Ya anxiang and Dongqiao-Nam Co of the Qinghai-Tibet region. Based on the data of actual detection and experiments in the studied area, the characteristics of concentration and profile distribution of the main geochemical exploration indicators (total adsorbed hydrocarbon, altered carbonate, thermal released mercury, etc.) were discussed. The origin of hydrocarbon was also discussed. Moreover, the oil and gas potential of different sections in the Qiangtang Basin and the Biru Basin were evaluated, which could provide references for the exploration and prediction of oil and gas in this region. The concentrations of total adsorbed hydrocarbons in the Ya anxiang of Suoxian County-Zadong of Baqing County and the Jiecha-Dangxiong of Anduo County in the Qinghai-Tibet region are high, averaging 312.64 μl kg 1 and 164.36 μl kg 1. Their altered carbonate concentrations are relatively low, averaging 0.11% and 0.56%. Their concentrations of thermal released mercury are relatively high, averaging 13.09 10 9 and 1.94 10 9. Geochemical exploration results in fact reflect hydrocarbon information in the underground, which are mainly petroleum associated gas and over-matured cracking gas. The Jiecha-Charenluma and the Xiamaya ertong-suoba sections in the Qiangtang Basin are the most favorable ones for oil and gas and the Ekou-Zadong section is relatively favorable one, while the Nam Co and the Zigetangcuo-Dongqiao sections in the Biru Basin are the favorable ones for oil and gas, and the Jiang Co-Nading Co and the Ya anxiang-ya ertong sections are relatively favorable ones. geochemical exploration, adsorbed hydrocarbon, indicator, hydrocarbon anomaly, evaluation of oil and gas, Qinghai-Tibet region Received December 4, 2008; accepted March 1, 2009 doi: 10.1007/s11430-009-5001-0 Corresponding author (email: xiongbo@yangtzeu.edu.cn; lixq@gig.ac.cn) Supported by New-century Excellent Talent Program of Ministry of Education of China (Grant No. NCET-06-0204), National Natural Science Foundation of China (Grant No. 40572085), and the 9th Five-Year Plan of Scientific and Technological Program of CNPC (Grant No. BR95YZ2-02) Citation: Xiong B, Li X Q, Li Y B, et al. The surface geochemical exploration of oil and gas in the Gangbatong-Ya anxiang and the Dongqiao-Nam Co of the Qinghai-Tibet region. Sci China Ser D-Earth Sci, 2009, 52(Supp. I): 69-76, doi: 10.1007/s11430-009-5001-0

Surface geochemical exploration of oil and gas has experienced tortuous development for more than 50 years [1 6]. It has gradually been attracted more attention by petroleum geologists. New analysis techniques have been developed especially in the recent 20 years, which promote the interpretation level of geochemical exploration anomaly and the accuracy and reliability of hydrocarbon prediction [7 20]. Surface geochemical anomalies of hydrocarbon result from hydrocarbon escaped from the underground oil and gas reservoir, which is the modern manifestation of hydrocarbon generation, migration, accumulation, diffusion and oxidation processes in the geological history. The final objective of surface geochemical exploration of hydrocarbon is to reconstruct the process of hydrocarbon generation, migration and accumulation by using geochemical anomalies, and to judge whether the oil and gas reservoirs exist or not. The Qinghai-Tibet Plateau is located in the middle part of Tethyan structure, to the west of which are the famous Middle East Persian Gulf giant oilfields and to the east is the Southeast Asia petroliferous basin. However, the Qinghai-Tibet Plateau has a complicated geological background. Its natural geographical environment is also bad. It is very difficult to carry out petroleum geological study in this region. The Qinghai-Tibet Plateau is the only blank space of petroleum exploration on a large scale. Therefore, the authors carried out the surface geochemical exploration of oil and gas in this region. On the basis of the data of actual detection and experiments, the characteristics of main geochemical indicators and origins of hydrocarbons in the Gangbatong-Ya anxiang and Dongqiao-Nam Co surface geochemical profiles were discussed, and the potential of oil and gas was evaluated, which may provide references for the evaluation, exploration and prediction of oil and gas resources. 1 Geological setting Most of the surface geochemical exploration of oil and gas in the Gangbatong-Ya anxiang and the Dongqiao- Nam Co of the Qinghai-Tibet region is located in the Biru Basin, some of which is in the southeast part of the Qiangtang Basin (Figure 1). The Biru Basin is bounded on the north by the Bangong Lake-Nujiang River suture belt and the Qiangtang-Sanjiang area. On its south is the Dangxiong-Jiali fault separated from the Lhasa area. On its west is the Bange-Dangxiong line adjacent to the Qiling area. and its east of the basin bends southward into Burma. The north of the Gangbatong-Ya anxiang belongs to the Tanggula Range with a high altitude (over 4700 m on average). The range is covered with snow all the year round, and with a lot of glaciers. The annual average temperature is about 3. No people can survive there for lack of basic living conditions. Middle part (averaging about 5000 m a.s.l.) is the drainage area of the Nujiang River, and the Qinghai-Tibet Highway crosses this area, in Suoxian and Biru counties of the Tibet Autonomous Region where inhabitants are sparsely distributed. The southern part is the Nyainqentanglha Range, in Biru and Jiali counties of the Tibet Autonomous Region. Its geographical environment is similar to that in the middle part. But the altitude is higher, and the relatively lower point is more than 4700 m a.s.l. The Dongqiao-Nam Co area is located in Anduo and Dangxiong counties of the Tibet Autonomous Region, most of which belongs to the northern Tibet geographically. The southern part is in the Nyainqentanglha Range, with flat topography, many lakes and swamps all over the ground. The altitude is relatively high, commonly about 4700 m a.s.l. at a low point. May to June is the dry season, while July to August is the rainy season. It is easy to fall into mud. 2 Samples and experimental The localities of sample collection in the field were determined mainly by GPS, with the 1:100000 topographic map and geological compass. The sample depth was 0.5 1.0 m. The sample spacing was about 1000 m. The 301 samples (including 200 sub-sandy soil, 36 sub-clay, 56 sandy soil, 8 clay, and 1 contrast sample) were collected in the Gangbatong-Ya anxiang and Dongqiao- Nam Co profiles. The 301 samples were analyzed for adsorbed hydrocarbon, altered carbonate, thermal released mercury, fluorescence spectra, ultraviolet spectra, and methane carbon isotope according to the standard methods [7,11,15]. Figure 2 shows the flow chart of sample analysis procedures. 3 The concentration characteristics of geochemical exploration indicators The concentration characteristics of surface geochemical exploration indicators in the Ya anxiang of Suoxian County-Zadong of Baqing County and the Jiecha-Dangxiong of Anduo County in the Qinghai-Tibet region are 70 Xiong B et al. Sci China Ser D-Earth Sci Jun. 2009 vol. 52 Supp. I 69-76

Figure 1 Location map of the studied area. 1, Provincial boundary; 2, regional boundary; 3, road; 4, county; 5, measured section point; 6, Suoxiayaan-Baqingzadong profile; 7, Anduojiecha-Dangxiong profile. Figure 2 The flow chart of sample analysis procedures. shown in Table 1. The average concentration of total adsorbed hydrocarbon in the Ya anxiang of Suoxian County- Zadong of Baqing County is 312.64 μl kg 1. The concentrations of methane, ethane and propane are relatively high, the total of which accounts for 97.22% of total absorbed hydrocarbon. Especially, the concentration of methane (averaging 271.28 μl kg 1 ) is very high, accounting for 86.93% of total hydrocarbon, and its humidity coefficient is 0.18. The mean concentration of total adsorbed hydrocarbon in the Jiecha-Dangxiong of Anduo County is 164.36 μl kg 1. The average concentrations of methane, ethane and propane are relatively high, which accounts for 98.07% of total hydrocarbon. Especially, the concentration of methane (averaging 146 μl kg 1 ) is also high, accounting for 88.83% of total hydrocarbon, and its humidity coefficient is 0.28. The altered carbonate concentrations of the two geochemical exploration profiles are relatively low. Their average concentrations of altered carbonate are 0.11% and 0.56%, respectively. The concentrations of thermal released mercury are relatively high, especially in the Ya anxiang of Suoxian County-Zadong of Baqing County. The mean concentration is up to 13.09 10 9. The concentrations of fluorescence spectra F380 nm, F355 nm and F304 nm in the Ya anxiang of Suoxian County-Zadong of Baqing County are relatively high. Xiong B et al. Sci China Ser D-Earth Sci Jun. 2009 vol. 52 Supp. I 69-76 71

Table 1 The concentration characteristics of surface geochemical exploration indicators in the Qinghai-Tibet region Ya anxiang of Suoxian County-Zadong of Geochemical exploration indicator Baqing County Jiecha-Dangxiong of Anduo County Range Average Standard error Range Average Standard error Methane C 1 (μl kg 1 ) 5.76 943.64 271.78 266.63 2.25 2345.62 146.00 300.23 Ethane C 2 (μl kg 1 ) 1.46 100.95 22.00 22.59 0.00 184.86 10.74 18.15 Propane C 3 (μl kg 1 ) 0.74 40.22 10.17 10.41 0.00 61.06 4.45 7.31 Iso-butane ic 4 (μl kg 1 ) 0.05 13.28 2.62 3.06 0.04 23.51 1.28 2.43 Normal butane nc 4 (μl kg 1 ) 0.11 12.21 3.12 3.30 0.06 21.77 1.78 2.88 Iso-pentane ic 5 (μl kg 1 ) 0.03 7.99 1.94 2.05 0.04 22.28 1.01 2.17 Normal pentane nc 5 (μl kg 1 ) 0.08 5.16 1.24 1.29 0.06 7.39 0.85 1.11 Heavy hydrocarbon C + 2 (μl kg 1 ) 2.74 177.70 40.86 42.41 0.00 317.97 18.57 32.54 Total absorbed hydrocarbon C(μL kg 1 ) 8.50 1092.73 312.64 307.19 2.39 2558.94 164.36 327.20 Humidity coefficient W h 0.04 0.57 0.18 0.11 0.01 1.97 0.28 0.24 Altered carbonate ΔC(%) 0.02 2.48 0.11 0.06 0.01 4.28 0.56 0.72 Thermal released mercury Hg(10 9 ) 1.20 48.62 13.09 9.12 0.06 8.41 1.94 1.42 F405 nm (10 3 ) 0.00 24.00 5.57 4.37 1.00 201.00 11.70 20.85 F390 nm (10 3 ) 1.00 31.00 7.74 5.86 3.00 240.00 16.26 22.80 Fluorescence spectrum F380 nm (10 3 ) 2.00 34.00 10.02 6.90 5.00 288.00 20.62 25.25 F355 nm (10 3 ) 2.00 44.00 13.13 9.05 6.00 316.00 24.07 26.61 F330 nm (10 3 ) 6.00 32.00 6.92 15.87 4.00 126.00 18.10 12.71 F304 nm (10 3 ) 6.00 28.00 13.28 5.60 0.00 24.00 2.45 2.60 U316 nm (10 3 ) 0.00 9.00 1.74 1.52 0.00 12.00 1.72 1.08 U260 nm (10 3 ) 2.00 17.00 6.67 2.72 1.00 59.00 6.47 5.74 Ultraviolet spectrum U254 nm (10 3 ) 3.00 20.00 8.09 3.15 2.00 57.00 8.51 5.95 U222 nm (10 3 ) 6.00 47.00 24.00 9.69 4.00 109.00 19.27 11.37 U216 nm (10 3 ) 9.00 58.00 28.11 10.64 8.00 108.00 23.59 11.55 They are 10.02 10 3, 13.13 10 3 and 13.28 10 3, respectively. They account for 64.30% of average concentration of six wave bands. The concentrations of ultraviolet spectra U216 nm and U222 nm are relatively high. They are 28.11 10 3 and 24.00 10 3, respectively, accounting for 75.95% of average concentration of five wave bands. The concentration of fluorescence spectrum F304 nm in the Jiecha-Dangxiong of Anduo County is relatively low. The mean concentrations of other wave bands (F405 nm, F390 nm, F380 nm, F355 nm, F330 nm) are between 11.70 10 3 24.07 10 3. The mean concentrations of ultraviolet spectra U216 nm and U222 nm are relatively high. They are 23.59 10 3 and 19.27 10 3, respectively, which account for 71.96% of mean concentration of five wave bands. Compared with the concentrations of adsorbed hydrocarbons in other main petroliferous basins in China [2 4], the mean concentrations of methane (271.78 μl kg 1 ) and heavy hydrocarbon (40.86 μl kg 1 ) in Ya anxiang of Suoxian County-Zadong of Baqing County are larger than those of Jianghan Basin and southern Songliao Basin. They are similiar to those of Jiyang Depression and Junggar Basin. They are less than those of eastern Tarim Basin and Dongpu Depression [2 4]. The mean concentration of methane (146.00 μl kg 1 ) in Jiecha-Dangxiong of Anduo County is generally similar to that of Jianghan Basin and southern Songliao Basin. The mean concentration of heavy hydrocarbon (18.57 μl kg 1 ) is higher than that of the Jianghan Basin and southern Songliao Basin [2 4]. Therefore, this area has a prospect of finding underground oil and gas reservoirs. 4 Origins of hydrocarbons The genetic type determination of adsorbed hydrocarbon from near surface samples by using carbon isotope of methane (δ 13 C 1 ) is reliable. Many researches have demonstrated that hydrocarbon measured in near-surface soil mainly comes from the micro-leakage of underground hydrocarbon and is less influenced by biological activities. Therefore, it can actually reflect the information of underground oil and gas [7,8,10,11]. It is generally accepted that the δ 13 C 1 value of biogenic gas is < 54, the δ 13 C 1 value of oil associated gas is 54 40, and the δ 13 C 1 value of over-matured cracking gas or coal-formed gas is > 40 [7,8,10,21 23]. 72 Xiong B et al. Sci China Ser D-Earth Sci Jun. 2009 vol. 52 Supp. I 69-76

Table 2 is a list of the δ 13 C 1 results of 16 samples from the Ya anxiang of Suoxian County-Zadong of Baqing County and the Jiecha-Dangxiong of Anduo County. The δ 13 C 1 values of 9 samples are > 40. They account for 56.25% of total samples, with the maximum of 32.46, the minimum of 39.84 and averaging 36.97, which indicates over-matured cracking gas or coal-formed gas. The δ 13 C 1 values of 7 samples are between 54 and 40. They account for 43.75% of total samples, with the maximum of 40.18, the minimum of 50.02 and averaging 42.81, which indicates oil-associated gas. It is demonstrated that hydrocarbons in this area are oil-associated gas and over-matured cracking gas. The compositional characteristics of adsorbed hydrocarbons also can provide genetic information about hydrocarbons. The common indicators including methane content, heavy hydrocarbon concentration, humidity coefficient mainly reflect the source of hydrocarbon. For instance, when methane content is particularly high, the gas property can be determined by methane carbon isotopic composition. Excluding biochemical origin, relatively low C + 2 content indicates that hydrocarbon is mainly related to gas reservoir. While C + 2 content is comparatively high and methane content is relatively low, hydrocarbon is mainly related to oil reservoir. The adsorbed hydrocarbon in the samples of the Qinghai-Tibet region is predominantly methane with little heavy hydrocarbon. The methane concentration of the Ya anxiang-zadong profile is high (averaging 271.28 μl kg 1 ). It accounts for 86.93% of total adsorbed hydrocarbon and its humidity coefficient is 0.18. The methane concentration of the Jiecha-Dangxiong profile is also high (averaging 146 μl kg 1 ), and accounts for 88.83% of total adsorbed hydrocarbon. Its humidity coefficient is 0.28 (Table 1). It also indicates that the adsorbed hydrocarbons are mainly derived from underground oil and gas, most of which are oil-associated gas or over-matured cracking gas. 5 The characteristics of geochemical profile and the evaluation of hydrocarbon potential 5.1 Division and characteristic of geochemical subsegment The Ya anxiang of Suoxian County-Zadong of Baqing County profile is the geochemical segment I and spans 46 km, crossing the Bangong Lake-Nujiang River suture belt. To the north it is adjacent to the Qiangtang Basin, and to the south it extends to the Biru Basin. Based on the distribution characteristics of adsorbed hydrocarbon, altered carbonate, and thermal released mercury, this profile is divided into three geochemical sub-segments, i.e. I 1, I 2, I 3 (Table 3). The Jiecha-Dangxiong of Anduo County profile is the geochemical segment II and it spans about 254 km, crossing the Bangong Lake-Nujiang River suture belt. To the north it is adjacent to the Qiangtang Basin, to the Table 2 The measured results of methane carbon isotope in the Qinghai-Tibet samples Sample No. δ 13 C 1 ( ) Genetic type Sample No. δ 13 C 1 ( ) Genetic type 007 36.13 622 32.64 Over-matured cracking gas 033 37.72 037 45.16 303 38.44 505 40.23 409 36.06 550 40.94 Over-matured cracking gas 432 36.24 590 40.94 Oil-associated gas 456 38.91 605 42.18 479 39.84 403 40.18 515 36.75 417 50.02 Table 3 Division of geochemical sub-segments in the Ya anxiang of Suoxian County-Zadong of Baqing County and the Jiecha-Dangxiong of Anduo County profiles Geochemical segment Geochemical sub-segment Tectonic belt Sample No. Length (km) I 1 Qiangtang Basin 007 037 31 I I 2 Bangong Lake-Nujiang River suture belt 001 006, 309 7 I 3 Biru Basin 301 308 8 II 1 Qiangtang Basin 401 429 29 II II 2 Bangong Lake-Nujiang River suture belt 430 442 13 II 3 Biru Basin 443 654 212 Xiong B et al. Sci China Ser D-Earth Sci Jun. 2009 vol. 52 Supp. I 69-76 73

south it extends to the Biru Basin and even the Dangxiong Yangbajing rift belt. Based on the distribution characteristics of adsorbed hydrocarbon, altered carbonate and thermal released mercury, this profile is also divided into three sub-segments, i.e. II 1, II 2, II 3 (Table 3). Table 4 shows the concentration distribution characteristics of main geochemical indicators of different sub-segments from the two profiles. The sub-segment I 1 is consistent with the boundary of southern Qiangtang Basin. The concentration of adsorbed hydrocarbon presents high in the south and low in the north. The sub-segment I 2 is consistent with the Bangong Lake-Nujiang River suture belt. The concentrations of all geochemical indicators are relatively high. Especially, the concentrations of adsorbed hydrocarbon and altered carbonate are obviously high. The sub-segment I 3 is consistent with the boundary of northern Biru Basin. The concentrations of all geochemical indicators present high and low values alternately with a large magnitude. The sub-segment II 1 is consistent with the margin of Southern Qiangtang Basin, which has high concentrations of methane and heavy hydrocarbon. The values of F405 nm and U222 nm fluctuate around their average. The concentration of thermal released mercury has a few high values. The concentration of altered carbonate has a medium-high value in samples 410 418, corresponding with the concentrations of methane and heavy hydrocarbon. The sub-segment II 2 is consistent with the boundary of Bangong Lake-Nujiang River suture belt. The concentrations of methane and heavy hydrocarbon present medium-high values. The sub-segment II 3 is consistent with the boundary of northern Biru Basin, extending to the Dangxiong Yabajing rift belt. The concentrations of methane and heavy hydrocarbon have a few medium-high values only at eastern Nam Co, neighboring Cuolongque and eastern Jiangcuo, while there are low-medium values in other areas. 5.2 Evaluation of oil and gas potential Based on the concentration characteristics of adsorbed hydrocarbon, altered carbonate, thermal released mercury, fluorescence spectrum and ultraviolet spectrum, oil and gas potential of geochemical sub-segments are evaluated by factor analysis method, which is commonly used in mathematical geology. By analyzing the indicators and consulting the related literature [7 11], the main factors for the evaluation include methane and heavy hydrocarbon in adsorbed hydrocarbon. The minor factors contain fluorescence spectrum F405 nm, ultraviolet spectrum U222 nm, altered carbonate and thermal released mercury. The evaluation of hydrocarbon potential is based on the error contribution and the loaded factor in the factor analysis. The average concentrations of all the evaluation factors are divided into different ranks and endowed with s according to the endowing criteria (Table 5). The evaluation factors are noted with a, b, c, d, e, f, respectively, and H presents the product of six s of evaluation factors. Then, the advantages of geochemical sub-segments are determined according to H. For convenience, we usually convert it into a ten system, i.e. 10 H/H max. Table 6 is a list of the evaluation results of all the Table 4 The concentration distribution of main geochemical indicators of different sub-segments Geochemical Concentration distribution of main indicators C 1 (μl kg 1 ) C + 2 (μl kg 1 ) trum F405 nm (10 3 ) trum U222 nm (10 3 ) ate ΔC (%) mercury Hg (10 9 ) Methane Heavy hydrocarbon Fluorescence spec- Ultraviolet spec- Altered carbon- Thermal released sub-segment Range 5.76 943.64 2.74 177.70 0.00 14.00 6.00 46.00 0.03 2.48 1.20 48.62 I 1 Average 235.38 34.29 13.35 13.52 0.84 13.53 I 2 I 3 II 1 II 2 II 3 Range 32.67 527.26 18.02 104.22 0.00 24.00 13.00 46.00 0.60 2.00 11.55 27.30 Average 342.39 56.01 14.76 23.00 1.47 17.82 Range 37.16 869.13 6.82 109.42 2.00 10.00 19.00 47.00 0.02 1.28 3.66 15.18 Average 351.03 53.07 7.31 27.38 0.68 7.31 Range 10.77 2345.62 3.64 213.32 6.00 22.00 6.00 47.00 0.14 2.44 0.44 5.41 Average 508.66 49.34 2.09 24.34 0.97 2.09 Range 67.31 912.75 4.36 85.09 6.00 38.00 17.00 39.00 0.12 2.16 0.66 3.69 Average 291.23 17.33 1.68 25.54 0.84 1.68 Range 2.25 1776.53 0.00 317.97 1.00 185.00 4.00 109.00 0.01 4.28 0.06 8.41 Average 87.49 14.38 1.93 18.19 0.49 1.93 74 Xiong B et al. Sci China Ser D-Earth Sci Jun. 2009 vol. 52 Supp. I 69-76

Table 5 The endowing criteria of the evaluation factors Methane Heavy hydrocarbon Altered carbonate Ultraviolet spectrum Fluorescence spectrum Thermal released mercury C 1 (μl kg 1 ) Mark C + 2 (μl kg 1 ) Mark ΔC (%) Mark U222 nm (10 3 ) Mark F405 nm (10 3 ) Mark Hg (10 9 ) Mark 50 125 0.5 5 15 0.5 0.1 0.5 0.45 1 10 0.2 1 5 0.25 1 5 0.5 125 200 0.75 15 25 0.75 0.5 1.0 1.05 10 15 0.6 5 7.5 0.65 5 10 0.85 200 275 1.0 25 35 1.25 1.0 1.5 1.45 15 20 1.0 7.5 10 1.05 10 15 1.15 275 350 1.25 35 45 1.75 20 25 1.40 10 14 1.45 15 20 1.45 350 425 1.5 45 55 2.25 25 30 1.80 14 18 1.85 425 500 2.0 55 65 2.50 500 575 2.5 Table 6 Geochemical segment I II The evaluation results of the geochemical sub-segments Geochemical sub-segment Methane a Heavy hydrocarbon b Altered carbonate c Ultraviolet spectrum d Fluorescence spectrum e Thermal released mercury f H = a b c d e f Evaluation H 10/ H max I 1 1.0 1.75 1.45 1.4 0.65 1.15 2.66 4.4 relatively good I 2 1.25 2.5 1.45 1.4 0.65 1.45 5.98 10 very good I 3 1.5 2.25 1.05 1.8 0.65 0.85 3.52 5.9 relatively good II 1 2.5 2.25 1.05 1.4 1.45 0.5 5.99 10 very good II 2 1.0 0.75 1.05 1.8 1.85 0.5 1.31 2.2 relatively poor II 3 0.5 0.5 1.45 1.0 1.45 0.5 0.08 0.01 poor Grade geochemical sub-segments. The geochemical sub-segments I 2 and II 1 get the highest s (ten points), indicating the best oil and gas potential. The geochemical sub-segments I 1 and I 3 get the s of 4.4 and 5.9, respectively, indicating relatively better oil and gas potential. The geochemical sub-segment II 2 gets 2.2 point, indicating relatively poor oil and gas potential. The geochemical sub-segment II 3 gets the lowest (less than 1 point), indicating very poor oil and gas potential. Table 7 shows the comprehensive evaluation of oil and gas potential in the Qiangtang Basin and Biru Basin of the Qinghai-Tibet region. The Qiangtang Basin includes the Jiecha-Charenluma, the Xiamaya ertong-suoba, the Ekou-Zadong sections. The Jiecha-Charenluma section belongs to the geochemical sub-segment II 1, corresponding to samples 401 410 and 413 419, and spans about 19 km. It has very good oil and gas potential. This section is comprehensively evaluated as the best favorable section, which consists of a rank I and a rank II comprehensive anomalies. The Xiamaya ertong-suoba section belongs to the geochemical sub-segments I 2 and I 1, corresponding to samples 309, 001 003 and 005 011, and spans about 12 km. It also has very good oil and gas potential. This section is composed of a rank I and a rank II comprehensive anomalies. It is comprehensively evaluated as the most favorable oil and gas section. The Ekou- Zadong section is the geochemical sub-segment I 1, corresponding to samples 030 037, and spans about 8 km. It has relatively good oil and gas potential. This section is composed of rank III comprehensive anomaly, and is comprehensively evaluated as a relatively favorable oil and gas section. The Biru Basin includes the Ya an-ya ertong, the Nam Co, the Zigetangcuo-Dongqiao, the Jiangcuo-Nading Co sections. The Ya an-ya ertong section belongs to the Table 7 The comprehensive evaluation of oil and gas potential in the Qiangtang Basin and Biru Basin of the Qinghai-Tibet region Basin Section Evaluation of geochemical sub-segment Comprehensive Comprehensive evaluation Geochemical Evaluation anomaly rank of oil and gas potential sub-segment Sample No. 001 003, 309, Xiamaya ertong-suoba I 2, I 1 good I, II most favorable 005 011 Qiangtang Jiecha-Charenluma II 1 good I, II most favorable 401 410, 413 419 Ekou-Zadong I 1 relatively good III relatively favorable 030 037 Ya an-ya ertong I 3 relatively good II, III relatively favorable 301 307 Zigetangcuo-Dongqiao II 2, II 3 relatively poor II favorable 438 446 Biru Jiangcuo-Nadingcuo II 3 relatively poor III relatively favorable 493 507 Nam Co II 3 relatively poor II, I favorable 603 610, 622, 623 Xiong B et al. Sci China Ser D-Earth Sci Jun. 2009 vol. 52 Supp. I 69-76 75

geochemical sub-segment I 3, corresponding to samples 301 307, and spans about 8 km. It has relatively good oil and gas potential. This section consists of a rank II and two rank III comprehensive anomalies, and it is comprehensively evaluated as a relatively favorable section. The Nam Co section is the geochemical sub-segment II 3, corresponding to samples 603 610, 622 and 623, and spans about 10 km. It is composed of a rank I and a rank II comprehensive anomalies, and it is comprehensively evaluated as the favorable oil and gas section. The Zigetangcuo-Dongqiao section belongs to the geochemical sub-segment II 2 and II 3, corresponding to samples 438 446, and spans about 9 km. This section is rank II comprehensive anomaly, and it is comprehensively evaluated as the favorable oil and gas section. The Jiangcuo-Nading Co section is the geochemical subsegment II 3, corresponding to samples 493 507, and spans about 15 km. It consists of two rank III comprehensive anomalies, and it is comprehensively evaluated as the relatively favorable oil and gas section. 6 Conclusion The concentrations of total adsorbed hydrocarbons in the Ya anxiang of Suoxian County-Zadong of Baqing County and the Jiecha-Dangxiong of Anduo County in the Qinghai-Tibet region are high, averaging 312.64 μl kg 1 and 164.36 μl kg 1, respeetivelg The two profiles are predominated by methane, ethane and propane, which account for 97.22% and 88.83% of total adsorbed hydrocarbon. Their altered carbonate concentrations are relatively low, averaging 0.11% and 0.56%, respectively. Their concentrations of thermal released mercury are relatively high, averaging 13.09 10 9 and 1.94 10 9. Geochemical exploration results in fact reflect hydrocarbon information in the underground, which are mainly oil-associated gas and over-matured cracking gas. It can provide references for the exploration and prediction of oil and gas in this region. 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