Chemical characteristics of fountains in Rejuechaka-Zangxiahe area of the Qiangtang Basin and their geological implications

Recent years, the chemical characteristics of fountains, which can reflect the redox environment and hydrodynamic conditions, have been widely used in prospecting for petroleum, natural gas and natural gas hydrate. In this paper, 37 samples of fountains and 13 samples of surface water have been collected from Rejuechaka-Zangxiahe areas of the Qiangtang Basin to analyze their chemical compositions, isotopic compositions of hydrogen and oxygen, and contents of aqueous hydrocarbon gases. The results show that the water types are mainly Na₂SO₄ type and MgCl₂ type, with subordinate NaHCO₃ type and CaCl₂ type. The general high mineralized degrees of the fountains reveal a good preservation condition in the area. The average content of the ammonium ion is 0.895 μg/g. In Zangxiahe area, the ammonium ion content is up to 7.381μg/g, which is consistent with the widespread organic-rich shale in the area, indicating the relatively high organic matter content. The isotopic results of hydrogen and oxygen show that the H and O isotopes of the water are near the precipitation line, indicating that the fountains originate from meteoric water. In addition, a certain content of aqueous methane has been detected in nine fountains, and the content is relatively high in Baiyun Lake area, showing a good hydrocarbon potential. The comprehensive study reveals that the Zangxiahe and Baiyun Lake areas in the Qiangtang Basin have good source rocks and good preservation conditions, showing a good prospect for oil, gas and gas hydrate resources.