| Granite is an Effective Helium Source Rock: Insights from the Helium Generation and Release Characteristics in Granites from the North Qinling Orogen, China |
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| 作者姓名： | ZHANG Wen LI Yuhong ZHAO Fenghu ZHOU Zheng HAN Wei ZHOU Junlin ZHANG Qiao |
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| 作者单位： | 1 Key Laboratory of Deep-Earth Dynamics of Ministry of Natural Resources, Laboratory of Isotope Thermochronology, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China,2 Xi’an Center, China Geological Survey, Xi’an 710054, China,3 College of Geoscience and Surveying Engineering, China University of Mining and Technology, Beijing 100083, China,4 Lancaster Environment Centre, Lancaster University, Lancaster, LA1 4YQ, UK,2 Xi’an Center, China Geological Survey, Xi’an 710054, China,2 Xi’an Center, China Geological Survey, Xi’an 710054, China and 2 Xi’an Center, China Geological Survey, Xi’an 710054, China |
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| 摘 要： | Global helium(He) shortage is a challenging problem; however, the types of helium source rock and the mechanisms of He generation and release therein remain still poorly understood. In this study, in order to evaluate the potential of granite as an effective helium source rock, we collected granitic samples from the North Qinling Orogen, Central China, in the south of the helium-rich Weihe Basin. The helium generation and release behaviors in granite were studied through analysis of U and Th concentrations, EMPA images, and He and Ar concentrations and isotopic ratios extracted by crushing and stepwise heating. The results indicate that Ar has a better retention and a lower mobility than He. 3 He/4 He ratios released by crushing and stepwise heating are 0.016–0.056 RA and 0.003–0.572 RA, respectively, where RA is the atmospheric 3 He/4 He of 1.4×10-6, reflecting a crustal and radiogenic source. Helium concentrations extracted by the two ways are 0.13–0.95 ucm3 STP/g and 7.82–115.62 ucm3 STP/g, respectively, suggesting that matrix-sited He accounts for more than 98% of total helium preserved in granite. In addition, the total generated He amounts in granites are calculated based on the measured U and Th concentrations in granitic samples. Dividing the preserved He quantities by the generated He amounts, it turned out that less than 10% of He produced since the formation of the granite is preserved in the rock over geological time, suggesting that more than 90% generated He can be transferred to the Weihe Basin. Temperature and fracture are the two critical factors controlling He release. Based on the relationship between He diffusivity of granites and temperature and the He closure temperatures of a variety of U-and Th-rich minerals(27–250°C), we estimate that He can be partially released out of granite at the depths 400 m and totally released at the depths 7800 m. Fractures provide effective transfer of free He from deep source rocks to shallow reservoirs. Finally, a model on granite as an effective helium source rock is established. We suggest exploring He resources in hydrocarbon basins with granitic basement(or adjacent to granite bodies), high geothermal field, and young active fractures.
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| 收稿时间： | 2018/10/20 0:00:00 |
| 修稿时间： | 2019/5/8 0:00:00 |
Granite is an Effective Helium Source Rock: Insights from the Helium Generation and Release Characteristics in Granites from the North Qinling Orogen,China |
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| Authors: | ZHANG Wen LI Yuhong ZHAO Fenghu ZHOU Zheng HAN Wei ZHOU Junlin and ZHANG Qiao |
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| Abstract: | Global helium (He) shortage is a challenging problem; however, the types of helium source rock and the mechanisms of He generation and release therein remain still poorly understood. In this study, in order to evaluate the potential of granite as an effective helium source rock, we collected granitic samples from the North Qinling Orogen, Central China, in the south of the helium‐rich Weihe Basin. The helium generation and release behaviors in granite were studied through analysis of U and Th concentrations, EMPA images, and He and Ar concentrations and isotopic ratios extracted by crushing and stepwise heating. The results indicate that Ar has a better retention and a lower mobility than He. 3He/4He ratios released by crushing and stepwise heating are 0.016–0.056 RA and 0.003–0.572 RA, respectively, where RA is the atmospheric 3He/4He of 1.4×10‐6, reflecting a crustal and radiogenic source. Helium concentrations extracted by the two ways are 0.13–0.95 ucm3 STP/g and 7.82–115.62 ucm3 STP/g, respectively, suggesting that matrix‐sited He accounts for more than 98% of total helium preserved in granite. In addition, the total generated He amounts in granites are calculated based on the measured U and Th concentrations in granitic samples. Dividing the preserved He quantities by the generated He amounts, it turned out that less than 10% of He produced since the formation of the granite is preserved in the rock over geological time, suggesting that more than 90% generated He can be transferred to the Weihe Basin. Temperature and fracture are the two critical factors controlling He release. Based on the relationship between He diffusivity of granites and temperature and the He closure temperatures of a variety of U‐ and Th‐rich minerals (27–250°C), we estimate that He can be partially released out of granite at the depths <400 m and totally released at the depths >7800 m. Fractures provide effective transfer of free He from deep source rocks to shallow reservoirs. Finally, a model on granite as an effective helium source rock is established. We suggest exploring He resources in hydrocarbon basins with granitic basement (or adjacent to granite bodies), high geothermal field, and young active fractures. |
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| Keywords: | granite helium source rock helium diffusivity crushing stepwise heating |
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